the regulation of food intake in mammalian hibernators: a review
TRANSCRIPT
REVIEW
The regulation of food intake in mammalian hibernators:a review
Gregory L. Florant • Jessica E. Healy
Received: 6 August 2011 / Revised: 20 October 2011 / Accepted: 29 October 2011 / Published online: 12 November 2011
� Springer-Verlag 2011
Abstract One of the most profound hallmarks of mam-
malian hibernation is the dramatic reduction in food intake
during the winter months. Several species of hibernator
completely cease food intake (aphagia) for nearly 7 months
regardless of ambient temperature and in many cases,
whether or not food is available to them. Food intake reg-
ulation has been studied in mammals that hibernate for over
50 years and still little is known about the physiological
mechanisms that control this important behavior in hiber-
nators. It is well known from lesion experiments in non-
hibernators that the hypothalamus is the main brain region
controlling food intake and therefore body mass. In hiber-
nators, the regulation of food intake and body mass is pre-
sumably governed by a circannual rhythm since there is a
clear seasonal rhythm to food intake: animals increase food
intake in the summer and early autumn, food intake declines
in autumn and actually ceases in winter in many species,
and resumes again in spring as food becomes available in
the environment. Changes in circulating hormones (e.g.,
leptin, insulin, and ghrelin), nutrients (glucose, and free
fatty acids), and cellular enzymes such as AMP-activated
protein kinase (AMPK) have been shown to determine the
activity of neurons involved in the food intake pathway.
Thus, it appears likely that the food intake pathway is
controlled by a variety of inputs, but is also acted upon by
upstream regulators that are presumably rhythmic in nature.
Current research examining the molecular mechanisms and
integration of environmental signals (e.g., temperature and
light) with these molecular mechanisms will hopefully shed
light on how animals can turn off food intake and survive
without eating for months on end.
Keywords Food intake � Hibernation � Torpor � AMPK �Hypothalamus � Arcuate nucleus � Leptin
Abbreviations
ACC Acetyl CoA carboxylase
AgRP Agouti-related protein
AMPK AMP-activated protein kinase
ARC Arcuate nucleus of hypothalamus
BBB Blood–brain barrier
CART Cocaine-amphetamine regulated transcript
FFAs Free fatty acids
GMGS Golden-mantled ground squirrel
NPY Neuropeptide Y
POMC Pro-opiomelanocortin
Ta Ambient temperature
Tb Body temperature
UCP Uncoupling protein
WAT White adipose tissue
Introduction
One of the common difficulties facing all endotherms is
surviving periods of food dearth occurring during winter in
temperate climates. Certain mammalian species (hibernators)
Communicated by I.D. Hume.
G. L. Florant (&)
Department of Biology, Colorado State University,
Fort Collins, CO 80523, USA
e-mail: [email protected]
J. E. Healy
Department of Basic Medical Sciences, University of Arizona
College of Medicine-Phoenix, 425 N. 5th St. Building ABC1,
Phoenix, AZ 85004, USA
123
J Comp Physiol B (2012) 182:451–467
DOI 10.1007/s00360-011-0630-y
have evolved unique physiological adaptations that allow
them to cease feeding and decrease body temperature (Tb) to
extremely low levels in order to conserve energy when food
supplies are virtually non-existent and ambient temperatures
(Ta) are low (winter). Over 30 years ago, an excellent review
of this form of self-induced anorexia hypothesized that the
annual fluctuations of food intake in these animals were likely
the result of an endogenous ‘sliding set-point’ mechanism
which sets a different ‘ideal’ body mass for each season,
presumably regulated by hypothalamic control of food intake
(Mrosovsky and Powley 1977). The physiological/molecular
mechanism(s) that actually generates the set-point and thus
regulates the changing drive to feed is still unknown; how-
ever, recent studies are beginning to shed light on what might
produce this rhythm.
A part of the difficulty in defining the controls of food
intake in hibernators lies in the diversity of life-history
strategies employed by these heterothermic mammals.
Multi-day torpor bouts are utilized by mammals in orders
as diverse as Monotremata (echidnas), Diprotodontia
(pygmy possums), Erinaceomorpha (hedgehogs), Carniv-
ora (bears), Chiroptera (bats), Primates (fat-tailed dwarf
lemur), and Rodentia (ground squirrels, jumping mice,
etc.), and these torpor bouts can occur either during sum-
mer (estivation) or winter (hibernation). In marsupials, the
use of heterothermy is quite variable, and research has
revealed a number of spectacular instances of prolonged
torpor (in one case, a full year of continuous hibernation),
which seem to be limited by stores of body fat rather than
by a circannual cycle (Geiser 2007). Bears and some other
carnivores undergo ‘winter lethargy’, during which they
cease food and water intake, and their Tb and metabolic
rate are reduced for several months at a time, but not to the
extremes exhibited by smaller ‘true hibernators’ (Tøien
et al. 2011). Insectivorous bats are generally aphagic in
winter, and exhibit deep (Tb approximates Ta) and long
(several weeks between periodic arousals) torpor bouts in
winter, but also utilize daily torpor extensively in the
summer, unlike many other ‘true hibernators’ which are
fully euthermic in the summer (Matheson et al. 2010).
Many other species which employ multi-day torpor bouts
are facultative, rather than obligate hibernators, and as such
are not bound to hibernate based on a circannual rhythm,
but instead require an exogenous stressor to enter torpor,
such as extremes of Ta and food or water shortage (Geiser
2007; Harlow 1996).
Even among hibernators in the order Rodentia, there are
a variety of physiological and behavioral strategies utilized
to survive the winter. Some species [e.g., sciurids in the
genera Marmota and Callospermophilus (Helgen et al.
2009)] are completely aphagic during the hibernation sea-
son and survive entirely on endogenous fat stores. Other
species of ground squirrel (e.g., Cynomys and Eutamias)
rely partially on a cache of stored food with which they
supplement their fat reserves during the hibernation season,
and as such are not completely aphagic. Patterns of torpor
and food intake in a third group (hamsters such as Cricetus
and Phodopus) are regulated by photoperiod rather than the
endogenous circannual rhythm found in sciurid hiberna-
tors. This review focuses on food intake regulation in those
mammalian hibernators whose endogenous circannual
rhythm causes them to store enough fat to meet their entire
metabolic energy requirements during the winter months
(e.g., marmots and other ground squirrels). The regulation
of energy intake and expenditure in food-caching hiber-
nators (e.g., Eutamius) has been reviewed elsewhere
(Humphries et al. 2003) and will not be addressed here,
although both fat-storing and food-caching hibernators
share some common physiological life-history strategies.
In addition, although there have been many recent advan-
ces in research on the effects of caloric restriction in
mammals, we will not discuss this topic in the context of
hibernators, but instead refer readers to a recent compre-
hensive review on caloric restriction (Speakman and
Mitchell 2011).
Behavioral observations of food intake
During the euthermic season (usually mid-spring through
early autumn), hibernators meticulously regulate food
intake and metabolism to provide the maximum possible
body mass gain prior to hibernation (Lyman et al. 1982). A
careful investigation of the timing of these processes sug-
gests that, although food intake and body mass are posi-
tively correlated, a hibernator’s food consumption peaks
before peak body mass is reached, accompanied by a
decrease in metabolism as food intake slows in late sum-
mer, shifting the energy balance toward weight gain, pri-
marily in the form of fat (Fig. 1).
Many sciurid hibernators, which are primarily herbivo-
rous in the spring months, become preferentially granivo-
rous during the late summer when they are increasing body
mass and food intake extremely quickly (Hill and Florant
2000; Morton 1975). These hibernators, especially ground
squirrels, are particularly selecting those seeds that are high
in polyunsaturated fatty acids (PUFA) (Frank 1994, 1998)
which suggests that animals are feeding selectively to
acquire the necessary fatty acids for hibernation (Florant
et al. 1993; Frank et al. 2008; Geiser 1991; Geiser and
Kenagy 1987). Many studies on several species of hiber-
nator have demonstrated that fat-storing hibernators double
or triple their food intake as measured by grams eaten per
day during the late summer hyperphagic period of their
food intake cycle (Dark 2005; Lyman et al. 1982; Pen-
gelley et al. 1976). For example, analysis of feeding
452 J Comp Physiol B (2012) 182:451–467
123
patterns of the golden-mantled ground squirrel (GMGS,
Callospermophilus lateralis) during the hyperphagic period
demonstrated that these animals spent roughly 57% of their
active hours feeding, and that food consumption and meal
frequency increased roughly twofold when compared with
earlier summer levels (Mrosovsky and Boshes 1986).
Pengelley and Asmundson (1969) and Pengelley (1974)
have demonstrated that GMGS have a circannual rhythm of
food intake. When animals are kept at a high Ta that makes
entry into torpor impossible (Ta = 35�C), GMGS still
manifest a significant decline in food intake during the
winter hibernation period. This suggests that food intake
regulation is not affected by the Ta at which the animal is
maintained. However, the presence or absence of food did
influence the length of the hibernation period: when pro-
vided with ad libitum food during the hibernation season,
animals maintained at 0�C remained aphagic and hetero-
thermic for much longer than animals maintained at 22�C,
which ate the provided food when their body mass dropped
below a certain point, and returned to stable euthermia at
an earlier date than those animals kept at 0�C (Pengelley
1974; Pengelley et al. 1976).
Once an obligate hibernator has reached its peak body
mass, food intake ceases, and the animal begins to undergo
torpor. This has been the general dogma for sciurid hib-
ernators in the lab, however, evidence regarding the com-
plete cessation of feeding in the field is lacking for some
species (Millesi et al. 1998). Excavations of the burrow
systems of various sciurid hibernators have revealed food
caches in the hibernacula of even those species thought to
be completely aphagic during hibernation (Barnes et al.
1986). Barnes et al. (1986) suggest that male arctic ground
squirrels (which are demonstrably aphagic for the entire
hibernation season when kept at 5�C in a laboratory set-
ting) cache food and eat at the end of the hibernation period
in order to enhance their ability to support spermatogenesis
and testes growth when food resources in the environment
are still scarce. Males of many hibernating species have
been shown to end their hibernation season quite early in
the spring (when there is still significant snow cover) and
remain underground for several days to weeks at high Tb
before emerging, presumably feeding on stored food
caches and undergoing testicular recrudescence (Barnes
et al. 1986). Females of most hibernating species, on the
other hand, come above ground within 1–2 days of their
last torpor bout much later in the season when food
resources are more readily available, and as such have no
need for a food cache. Both male and female alpine mar-
mots (Marmota marmota) emerge from hibernation in the
spring with approximately 16% body fat; this species does
not cache food, so the remaining fat depot is necessary to
sustain reproduction in case there is no sufficient food in
the environment upon emergence (Hume et al. 2002).
Michener (1993) found that while wild Urocitellus
columbianus did not cache food in their winter burrows,
the hibernacula of males of the closely related species
U. richardsonii did contain food caches. However, Michener
(1993) suggests that these food caches are not consumed
during inter-bout arousals during the main part of the
hibernation season, particularly since the arousal time is
brief and most of the time during an inter-bout arousal is
spent in sleep (Daan et al. 1991).
The common theme to all of these reports of food intake
behavior in sciurid hibernators is that they essentially stop
eating during the autumn and resume eating some
5–8 months later, whether they are in the field or laboratory
setting. For some species, there may be a sex difference in
their feeding behavior at the end of the hibernation period
(Sheriff et al. 2011). However, many laboratory-kept spe-
cies of sciurid hibernators do not feed even with food
available in their cage. Thus, some very powerful physio-
logical mechanism must be working to shut off appetite
and regulate endogenous fuel levels so that all cellular
energy demands are met over this winter period.
An important change in circulating energy availability
as animals begin to undergo torpor is the increase in serum
Fig. 1 Schematic of circannual
cycle of body mass, food intake
and metabolism in a lab-kept
rodent hibernator (simplification
based on data from Armitage
and Shulenberger 1972; Ward
and Armitage 1981; Thorp et al.
1994; reviewed in Dark 2005).
A Enters hibernation, B exits
hibernation
J Comp Physiol B (2012) 182:451–467 453
123
free fatty acids, presumably due to the dramatic increase in
body fat that occurred during the summer months. Free
fatty acid (FFA) concentrations are elevated in blood dur-
ing the hyperphagic period of all species studied (Florant
et al. 1990; Galster and Morrison 1975; Nelson et al. 2010),
but there is no information about how FFAs change in the
brain (e.g., CSF). Dark and Miller (1997) suggest that
plasma FFAs are important as a metabolic fuel during
torpor, as ground squirrels are pharmacologically pre-
vented from oxidizing fatty acids interrupt torpor bouts
with early arousals. As such, circulating FFAs may be an
important energy gauge even while an animal is at low
tissue temperature. Recent reports on feeding in non-
hibernating rodents suggest that nutrients circulating in the
blood feed back to the hypothalamus and can alter energy
metabolism and food intake (Blouet and Schwartz 2010;
Lam 2011; Lam et al. 2005). Therefore, it is possible that
FFAs, which generally increase in late summer (Florant
et al. 1990; Nelson et al. 2010) as animals fatten, provide a
feedback signal to the brain that shuts down food intake
once a threshold fat mass is reached.
Several hormonal factors are also important energy
feedback signals that either increase (i.e., insulin and lep-
tin) or decrease (ghrelin) in the blood with changes in body
fat (Fig. 2; Table 1). These hormones provide important
feedback to the hypothalamus regarding endogenous
energy balance. Specifically, insulin and leptin increase in
the blood of hibernators during the autumn (Concannon
et al. 2001; Florant et al. 1985, 2004) and may be part of
the hypothalamic signal that terminates food intake along
with an increase in brain FFAs. Manipulations of body fat
levels in ground squirrels strongly suggest that leptin is
involved in the regulation of the food intake cycle in these
non-photoperiodic hibernators (Florant 1998).
The role of the hypothalamus in regulating feeding
in hibernators
Although there have been numerous studies on the regu-
lation of food intake in mammals (i.e., rats), the neuronal
mechanisms that regulate the hibernator’s food intake and
dramatic body fat cycle have received little attention con-
sidering recent advances in understanding food intake. The
endogenous physiological signals that increase feeding and
subsequently turn off feeding in hibernators are presently
unknown, although past studies have suggested that the
hypothalamus must be involved (Mrosovsky 1975; Mro-
sovsky and Powley 1977; Satinoff 1967, 1970). Most of
these past studies were based on experiments in which
lesions within the ventromedial and lateral hypothalamic
areas were performed in hibernators. These studies dem-
onstrated that ground squirrels (heterotherms) responded to
hypothalamic lesions in a similar manner to that of rats
(homeotherms); ventral medial hypothalamus (VMH)
lesions caused an increase in feeding and obesity, regard-
less of time of year, and lateral hypothalamus (LH) lesions
caused aphagia and extreme weight loss (Satinoff 1970).
Currently, it is well known that the hypothalamus and
brainstem are crucial in the regulation of feeding. The
hypothalamus, including the arcuate nucleus (ARC), VMH,
LH, paraventricular nucleus (PVN) and dorsomedial
Fig. 2 Seasonal changes in
enzymes, hormones, and
transcription factors in
hibernators. AMPK AMP-
activated protein kinase, CCKcholecystokinin, FAS fatty acid
synthase, MCT1 ketone
transporter monocarboxylic acid
transporter 1; 1 Andrews et al.
2009, 2 Bauman et al. 1988,
3 Concannon et al. 2001,
4 Florant et al. 1985, 5 Healy
et al. 2010, 6 Healy et al. 2011a,
7 Geiser and Kenagy 1987,
8 Nelson et al. 2010, 9 Sallmen
et al. 1999, 10 Strauss et al.
2009, 11 Wang et al. 1997
454 J Comp Physiol B (2012) 182:451–467
123
nucleus, plays a key role in the regulation of food intake
and sensing endogenous signals that influence feeding and
energy balance. However, the ARC may be the most
important and its close proximity to the PVN, VMH and
LH may explain why gross lesions of that area in early
studies of hibernators produced feeding and body mass
responses. In particular, the ARC is a vital area due to the
blood–brain barrier (BBB) being uniquely permeable in
this area (Peruzzo et al. 2000), and the close access to the
median eminence means that peripheral factors such as
hormones and nutrients are capable of affecting first order
neurons in the ARC (Minor et al. 2009). Furthermore, two
populations of food intake-related neurons have been
identified in the ARC: first, neurons expressing neuropep-
tide Y (NPY) and agouti-related protein (AgRP) and sec-
ondly, cocaine-amphetamine regulated transcript (CART)
and pro-opiomelanocortin (POMC) neurons (Schwartz
et al. 2000). The NPY and AgRP neurons promote energy
storage and increased feeding, while the CART and POMC
neurons promote energy utilization and decrease feeding. A
prior study on GMGS demonstrated that NPY injection into
the ARC caused a prolonged increase in feeding (Boswell
et al. 1993). These results support the hypothesis that non-
hibernating rodents and hibernators share similar food
intake pathways within the brain and that NPY in the ARC
may be involved in generating the body mass cycles of
these rodents. To our knowledge, there are no studies of
ARC lesions in non-photoperiodic hibernators.
In photoperiodic hibernators (hamsters), there is evi-
dence that the ARC is involved in mediating torpor, and
that changes in nutrients within the ARC may be involved
in the response (Pelz et al. 2008). Using monosodium
glutamate (MSG) to induce ARC neuronal damage, Pelz
et al. (2008) showed that MSG-treated animals did not
enter torpor as frequently as controls under a short photo-
period. Furthermore, food restriction-induced torpor was
also reduced in MSG-treated animals. Taken together, it
appears that the ARC is necessary for a torpor response to
short photoperiods. Interestingly, another study in Siberian
hamsters (Phodopus sungorus) suggests that neuronal
output from the ARC may not be necessary for food
deprivation-induced increases in food hoarding and forag-
ing, indicating that in this species, the ARC may not be as
important in regulating certain behaviors related to feeding
(Dailey and Bartness 2010). Given that animals in torpor
are unable to feed, it is assumed that photoperiodic animals
need to shut down the drive for food intake (hoarding etc.)
prior to entering torpor.
In the Siberian hamster, thyroid hormones have also
been shown to influence seasonal feeding behavior. Tri-
iodothyronine (T3) is the active form of the thyroid hor-
mone and it acts in the ARC and PVN as a short-term
signal of low energy during fasting (Boelen et al. 2008).
Furthermore, it appears that thyroid axis acts on tanycytes
within the third ventricle of the Siberian hamster via pho-
toperiod; specifically, short days appear to increase the
thyroid hormone transporter monocarboxylate transport 8
in tanycytes, while fasting downregulated expression of the
transporter (Herwig et al. 2009). Thus, it appears that in a
photoperiodic hibernator, the thyroid hormone system’s
response to fasting (i.e., energy balance) may be signifi-
cantly influenced by photoperiod (Murphy and Ebling
2011). Similar studies on non-photoperiodic hibernators
are necessary to help elucidate the role of the ARC on
torpor, food intake, and energy balance without the influ-
ence of photoperiod.
Hormone and nutrient regulation of neuronal activity
within the hypothalamus
The hypothalamus is crucial for integrating the regulation
of many physiological responses including energy balance
and reproduction. It is well known that 50-adenosine
monophosphate-activated protein kinase (AMPK) is an
important control of energy balance in all mammals studied
Table 1 Seasonal changes in
enzymes, hormones, and
transcription factors in
hibernators
AMPK AMP-activated protein
kinase, CCK cholecystokinin,
FAS fatty acid synthase, FFAsfree fatty acids, MCT1 ketone
transporter monocarboxylic acid
transporter 1
Spring Summer Autumn Winter
AMPK (Healy et al. 2011a, b) : ;
CCK (Bauman et al. 1988) : ;
Estradiol (Strauss et al., 2009) : : ; ;
FAS (Wang et al. 1997) : ;
FFAs (Nelson et al. 2010) :
Ghrelin (Healy et al. 2010) ; : : ;
Histamine (Sallmen et al. 1999) ; :
Insulin (Florant et al. 1985) : ;
Leptin (Concannon et al. 2001) ; ; : ;
MCT1 (Andrews et al. 2009) ; :
Testosterone (Geiser and Kenagy 1987) : ; ; ;
J Comp Physiol B (2012) 182:451–467 455
123
(Carling 2005; Hardie and Carling 1997; Minokoshi et al.
2004). Thus, how it acts within the hypothalamus to control
food intake is of great interest. Recent evidence suggests
that in one way AMPK may be acting is through inhibiting
mammalian target of rapamycin (mTOR) within the ARC
(Cota et al. 2006; Woods et al. 2008). It is well known that
hormones such as leptin, insulin, ghrelin, as well as cir-
culating nutrients (glucose, FFAs, and the AMP/ATP ratio
within the cell) all play a role in producing the signal to
feed or not by acting on or influencing the activity of
AMPK (Fig. 3). Consequently, there are many controls on
food intake behavior and the interaction of the pathways is
very complex.
Control of reproduction is regulated primarily by gon-
adotropin releasing hormone (GnRH) within the hypo-
thalamus. The ability to reproduce is highly correlated with
energy stores, as it has recently been shown that GnRH
neurons have energy sensing capabilities (Roland and
Moenter 2011). These two branches of life history, the
ability to increase energy stores and to regulate specific
reproductive hormones, are clearly crucial to whole-animal
survival, but specific interactions are poorly known, espe-
cially in hibernating species. The following sections will
describe in more detail the central actions of certain hor-
mones and nutrients known to be involved in the regulation
of food intake.
Leptin and its role in food intake
Leptin is a steroid hormone produced primarily by white
adipose tissue (WAT); as such, higher fat stores result in
higher circulating leptin, in effect signaling abundant
energy stores and acting as a satiety signal by
Fig. 3 Food intake is regulated by various humeral signals whose
message is transferred to hypothalamic neurons. Fasting results in
decreased circulating glucose, leptin, and insulin, increased ghrelin
and FFAs, and an increased AMP:ATP ratio, which can also be
mimicked by the synthetic AMPK activator AICAR. An increase in
the AMP:ATP ratio results in a conformational change of the csubunit of the AMPK molecule, which allows phosphorylation (and
activation) of the AMPK-a subunit. Activation of AMPK phosphor-
ylates and inactivates ACC, which leads to a downregulation of fatty
acid synthesis and an increase of fatty acid oxidation. Activation of
AMPK also upregulates expression of orexigenic neuropeptides NPY
and AgRP, and downregulates anorexigenic POMC, leading to an
increase in food intake. Under orexigenic conditions, the STAT3 and
mTOR pathways are downregulated, possibly through AMPK, and
their positive influence on POMC is eliminated, allowing increased
food intake. Testosterone (DHT) also inhibits POMC, allowing
increased food intake. ACC acetyl-CoA carboxylase, AgRP agouti-
related peptide, AICAR 5-Aminoimidazole-4-carboxyamide ribonu-
cleoside, AMPK AMP-activated protein kinase, AR androgen recep-
tor, DHT dihydrotestosterone, ERa estrogen receptor alpha, FFAs free
fatty acids, GHSR growth hormone secretagogue (ghrelin) receptor,
InsR insulin receptor, LepR leptin receptor, mTOR mammalian target
of rapamycin, NPY neuropeptide Y, POMC pro-opiomelanocortin,
STAT3 signal transducer and activator of transcription 3. Based on
data from: Cota et al. 2006; Dhillon and Belsham 2011; Dieguez et al.
2011; Minokoshi et al. 2004; Nohara et al. 2011
456 J Comp Physiol B (2012) 182:451–467
123
downregulating NPY and AgRP activity and upregulating
POMC (Schwartz et al. 2000; Zhang et al. 1994). Leptin
generally has an antagonistic relationship with (and oppo-
site physiological effects of) the orexigenic hormone
ghrelin. In rodents, high fat mass increases blood leptin
concentrations, which decreases circulating ghrelin in some
cases, but not all (Cummings and Foster 2003). Barazzoni
et al. (2003) found that exogenous leptin prevented the
typical fasting-induced increase in circulating ghrelin, but
other studies have shown that rodents treated with exoge-
nous leptin, creating artificially high circulating leptin
concentrations, had a lean body type and increased ghrelin
levels (Ariyasu et al. 2002; Toshinai et al. 2001).
Leptin binds to cell surface receptor LEPRb in various
nuclei of the hypothalamus (especially in the VMH) and
acts through the signal transducer and activator of tran-
scription 3 (STAT3) pathway (Ghilardi et al. 1996). Leptin
crosses the BBB in proportion to its circulating levels, and
as such acts as a proximal signal of energy balance in the
body. Once leptin is bound to the LEPRb receptor, it
activates STAT3, which is phosphorylated and eventually
leads to a decrease in appetite. There are six isoforms of the
leptin receptor, but only LEPRb has the necessary structure
for activation of the STAT3 pathway (Tartaglia 1997).
Leptin inhibits AMPK in the hypothalamus, specifically
in the ARC and PVN (Minokoshi et al. 2004), with the
effect of decreasing food intake. Peripherally, leptin stim-
ulates AMPK activation in skeletal muscle, with the effect
of suppressing ACC and increasing fatty acid oxidation
(Minokoshi et al. 2002). Leptin has been implicated in
thermoregulation in addition to its role in regulating food
intake and energy balance. Exogenous leptin upregulates
mitochondrial uncoupling proteins (UCPs) which modulate
proton gradient over the mitochondrial membrane and are
involved in heat production by brown adipose tissue (BAT,
UCP1) and protection against oxidative damage (UCP2,
expressed ubiquitously and UCP4&5, expressed in the
brain) (Ho et al. 2010; Kwok et al. 2010; Liu et al. 2006).
Leptin resistance (decreased sensitivity to the anorexi-
genic effects of leptin) occurs in most morbidly obese
animals, including humans. This state is often associated
with hyperleptinemia. The exact mechanism of leptin
resistance in diet-induced obesity is unknown, but has been
associated with inhibition of transport by triglycerides
(Banks et al. 2004) and suppression of the STAT3 pathway
(de Lartigue et al. 2011). Lou/C rats are a model of
spontaneous caloric restriction and are resistant to diet-
induced obesity (Veyrat-Durebex et al. 2011). When
exposed to a high-fat (HF) diet, these animals respond by
increasing density of noradrenergic fibers in WAT, which
leads to high sensitivity to sympathetic activation. These
rats have lower circulating concentrations of glucose,
insulin, leptin, and non-esterified fatty acids than Wistar
rats fed the same HF diet, and express UCP1 in WAT as
well as in BAT (Veyrat-Durebex et al. 2011). In photo-
periodic Siberian hamsters (Phodopus sungorus), reduced
leptin concentrations were required in order for the animal
to enter torpor (Freeman et al. 2004). In Arctic ground
squirrels (Urocitellus parryii), injected leptin was found to
reduce pre-hibernation hyperphagia (Boyer et al. 1997;
Ormseth et al. 1996). Recent experiments have shown that
decreased leptin levels are associated with initiation of
food intake in hibernating GMGS (Healy et al. 2008).
In most mammals (including some rodent hibernators),
leptin levels change concurrently with fat mass fluctuation
(Chen et al. 2008; Concannon et al. 2001; Florant et al.
2004), but Kronfeld-Schor et al. (2000) showed a dissoci-
ation between leptin and fat mass in pre-hibernatory little
brown bats (Myotis lucifugus), possibly to allow greater
amounts of WAT to be stored and bypassing leptin’s
satiety effect. Something similar may be occurring in pre-
hibernatory GMGS (Healy et al. 2008), suggesting that this
mechanism may be common to small hibernators, allowing
them to store maximum amounts of WAT for their body
size. This dissociation would not be necessary in large
hibernators (i.e., Marmota), as their large body size easily
accommodates sufficient fat storage to survive the winter
hibernation season.
Cholecystokinin (CCK) is released from endocrine cells
in the small intestine upon contact with lipid and protein
digestive products. CCK activates CCK1 receptors on
vagal nerve fibers in the gut wall that terminate in the
nucleus tractus solitaries (NTS) region of the brainstem to
stimulate the anorexigenic POMC/CART neurons in the
Arc. In non-hibernators, CCK acts synergistically with
leptin to potentiate its anorexigenic effect (Tache and
Stengel 2011). Activation of NTS neurons in response to
exogenous CCK varies seasonally in ground squirrels, with
a decrease in sensitivity from spring through summer (Otis
et al. 2011). This suggests that seasonal changes in sensi-
tivity of NTS neurons to CCK may influence appetite in the
active phase of the hibernation cycle.
Insulin and its role in food intake
In addition to leptin, the pancreatic hormone insulin is one
of the main central signals of adiposity. Insulin receptors
can be found in a wide variety of brain areas, including the
ARC (Havrankova et al. 1978). Endogenous insulin in
mammals facilitates uptake and utilization of glucose. In
non-hibernators, fasting leads to a decrease in circulating
insulin, accompanied by an increase in mRNA of the
orexigenic NPY, which is prevented by administration of
insulin and exogenous insulin causes a decrease in food
intake (Schwartz et al. 1992, 2000).
J Comp Physiol B (2012) 182:451–467 457
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Insulin’s role in food intake and fatting in hibernators
was examined extensively in the 1980s (Florant and Bau-
mann 1984; Melnyk et al. 1983; Mrosovsky and Sherry
1980). In male ground squirrels in the field, peak insulin
concentrations occur in the fall concomitant with peak
body mass, whereas in field females circulating insulin
remains high through pregnancy, lactation, and the body
mass gain phase of summer (Boswell et al. 1994). Infusion
or central injection of insulin into normothermic marmots
decreases food intake and body weight during the summer
feeding period, but has no effect during the winter aphagic
period (Florant et al. 1991). In fact, after peak body mass is
reached in the autumn, hibernators appear to become
insulin resistant, in that plasma insulin increases dramati-
cally after a glucose challenge in autumnal animals,
decreases very slowly to pre-challenge levels, and that an
insulin injection at this time fails to produce a significant
decrease in plasma glucose (Florant et al. 1985). This state
of resistance is reversed after hibernators have reached
peak body mass and are preparing to enter torpor, but the
mechanism by which this occurs is as yet unclear. In non-
hibernators, insulin resistance may driven by oxidative
stress, specifically by excess production of mitochondrial
superoxide, which occurs when an influx of nutrients
occurs in the absence of increased ATP consumption;
treatment of cells with mitochondrial antioxidants reversed
this insulin resistance (Hoehn et al. 2009). Similarly, a
recent study in murine embryonic stem cells found that a
transient exposure to elevated concentrations of reactive
oxygen species (ROS) led to induction of the insulin
resistance pathway (Mouzannar et al. 2011). As such, it is
interesting that ROS production in hibernators is generally
low during the hibernation season (Orr et al. 2009) and
antioxidant defenses are upregulated, particularly during
arousals when fatty acid metabolism is high (Buzadzic
et al. 1990). It is possible that a similar mechanism leads to
the reversible insulin resistance in hibernators, which
makes hibernators interesting animal models for further
study of this issue.
Ghrelin and its role in food intake
The discovery of the gut-produced hormone ghrelin in 1999
was accompanied by a great deal of interest, as a novel
orexigenic proximal signal that reacted quickly to changes
in food intake had been lacking in the literature for some
time. Originally described as an endogenous ligand for the
growth hormone receptor (Kojima et al. 1999), ghrelin also
acts as a potent orexigen in mammals. Ghrelin levels
increase with fasting (Toshinai et al. 2001) and exogenous
ghrelin causes an increase in food intake and adiposity,
whether injected centrally or peripherally (Keen-Rhinehart
and Bartness 2005; Tschop et al. 2000). Ghrelin is produced
primarily in the stomach, but is also produced in the
intestine, the pancreas, and in the hypothalamus (Kojima
et al. 1999). The ghrelin receptor (GHSR1) is found in
NPY/AgRP and in POMC/CART neurons in the ARC, as
well as in the LH (Hewson and Dickson 2000; Mondal et al.
2005). Ghrelin induces an orexigenic response by stimu-
lating NPY and AgRP secretion and downregulating
POMC/CART expression, as well as activating AMPK, a
key regulator of lipid metabolism and energy balance (Chen
et al. 2004; Lopez et al. 2008). In addition to its role in food
intake, ghrelin has also been implicated in regulation of
sleep (Szentirmai et al. 2007), in behavior (Jaszberenyi et al.
2006), and in thermoregulation (Gluck et al. 2006; Verty
et al. 2010). In Mus musculus, which responds to food
dearth and low Ta by becoming torpid, peripherally injected
ghrelin resulted in a deeper (lower Tb) and more robust
torpor bout; however, this response was eradicated by
ablation of the ARC(Gluck et al. 2006).
In hibernators, circulating ghrelin has been shown to
exhibit a daily cycle, and to increase with fasting in sum-
mer-acclimated animals (Healy et al. 2010). It also appears
to be regulated at different levels during different seasons,
low in the spring upon emergence from hibernation, grad-
ually increasing in the summer, high during the autumnal-
hyperphagic period, and extremely low during the winter
(Healy et al. 2010, 2011a). Peripheral injection of ghrelin
causes an increase in food intake during all seasons, even in
normally aphagic hibernators (Healy et al. 2011a).
AMPK and its role in food intake
AMPK is a heterotrimeric enzyme that has recently been
identified as an intracellular energy sensor and is evolu-
tionarily conserved from yeast to humans (Hardie and Car-
ling 1997). AMPK plays a significant role in regulating food
intake and energy metabolism in peripheral tissues (Fig. 3).
When activated by phosphorylation, AMPK decreases the
activities of anabolic pathways and increases the production
of ATP by stimulating catabolic pathways, thus acting to
maintain normal cellular energy balance (Hardie and Carling
1997; Kahn et al. 2005; Minokoshi et al. 2008). Centrally,
AMPK is sensitive to cellular AMP/ATP ratios within the
ARC and conveys information about energy status within the
animal. In the brain, AMPK is expressed in hypothalamic
ARC neurons (e.g., POMC, AgRP, NPY) that play a central
role in modulating food intake, torpor, and sensing cellular
energy levels (Claret et al. 2007). A decrease in metabolic
energy fuels (e.g., FFAs and glucose) initiates hypothalamic
AMPK activation, which in turn produces several behavioral
and physiological responses, including changes in food
intake and increases in fatty acid oxidation [primarily by
458 J Comp Physiol B (2012) 182:451–467
123
stimulating the phosphorylation and inactivation of ACC
(Kahn et al. 2005)].
Activation of AMPK in the hypothalamus by the AMPK
activator 5-aminoimidazole-4-carboxamide-1-b-D-ribofur-
anoside (AICAR) has been shown to stimulate phosphor-
ylation of AMPK and ACC as well as upregulating
expression of NPY and AgRP (Coyral-Castel et al. 2008;
Shimizu et al. 2008). Molecular and pharmacological
inhibition of hypothalamic AMPK decreases hepatic glu-
cose production (Yang et al. 2010). AICAR-induced acti-
vation of AMPK stimulates ketogenesis in cultured
astrocytes and in vivo, concurrent with a depression of
ACC activity, and a decrease in intracellular malonyl-CoA
and rate of fatty acid synthesis (Blazquez et al. 1999).
Astrocytes are the only brain cell population that can oxi-
dize fatty acids to ketone bodies, and preferentially
metabolize fatty acids over glucose as their primary fuel
(Edmond 1992). Activation of AMPK during hypoxia
sustains ketogenesis in astrocytes (Blazquez et al. 1999).
Central infusion of AICAR has been shown to induce
food intake and normothermia in aphagic, heterothermic
winter-state marmots, presumably through the activation of
AMPK (Florant et al. 2010). There is some evidence that
AMPK is involved in seasonal regulation of food intake in
hibernators as well, but that evidence is not conclusive
(Healy et al. 2011b; Horman et al. 2005). In ground
squirrels, AMPK appears to be differentially regulated by
physiological condition: a short-term fast in summer-
acclimated animals resulted in increased relative abun-
dance of the active form of the enzyme (pAMPK), and
torpid winter-acclimated animals had higher relative
abundance of pAMPK than did winter-acclimated euther-
mic animals (Healy et al. 2011b). However, the cellular
mechanisms behind these changes are unclear, and differ-
ences in cellular energy between torpor states (i.e., torpor
entry, early torpor, late torpor, and arousal) remain to be
elucidated. Interestingly, a recent study by Jinka et al.
(2011) suggests that adenosine may be required for the
initiation of torpor. This is an intriguing idea since aden-
osine, AMP, and ATP levels within the neurons of the
hypothalamus (Dunwiddie and Masino 2001) might be
interacting in such a way as to ‘‘sense’’ energy levels and
also initiate a torpor bout by metabolic suppression.
Nutrients and food intake regulation: the role of glucose
and FFAs
The two main nutrients that have been implicated in reg-
ulating food intake at the hypothalamic level are glucose
and fatty acids (FA). The glucostatic theory (which pur-
ports that glucose is the major signal) suggests that glucose,
and/or its metabolic products change the neuronal activity
of hypothalamic cells involved in regulating feeding
behavior (Levin 2002; Sokoloff et al. 1977). Glucose
sensing neurons are capable of sensing changes in external
glucose concentration and respond by changing their
activity (for reviews see Belgardt et al. 2009; Karnani and
Burdakov 2011; Mountjoy and Rutter 2007). The lipostatic
theory, which suggests that lipids are a major regulator of
food intake, has wide support from a number of investi-
gations as well (Obici et al. 2002; Wang et al. 2006).
Clearly, food intake is such an important behavioral event
that evolutionary processes would have not allowed this
behavior to be based on a single feedback signal, and
results from many investigations suggest that there are
multiple signals and feedback loops involved in the com-
plex regulation of feeding. In non-hibernating rodents,
there is strong evidence that the metabolism of glucose
(Migrenne et al. 2011) or FA (Lam 2011) can alter neu-
ronal activity within the hypothalamus, and specifically the
ARC (Kohno et al. 2011). The metabolism of energy-rich
molecules or lack thereof will alter AMP/ATP ratios with
in hypothalamic neurons and also alter calcium currents
(Kohno et al. 2011). Thus, these changes will potentially
alter neuronal activity and neurotransmitter release. For
example, if long-chain esterified FA accumulate within the
ARC there is a marked decrease in food intake (Lam et al.
2005; Obici et al. 2002), suggesting that cellular accumu-
lation and metabolism of FA may be a prominent signal
within the hypothalamus to decrease food intake. Further-
more, the infusion of oleic acid into the hypothalamus will
inhibit food intake and glucose production in rats (Wang
et al. 2006). The conclusion from several recent studies
suggests that FA can alter many hypothalamic activities
including calcium signaling (Le Foll et al. 2009b), FA
transporters (Migrenne et al. 2011), receptors, and potas-
sium channels (Le Foll et al. 2009a). The metabolism of
glucose and FA within hypothalamic neurons leads to
changes in the AMP/ATP ratio with these cells. This ratio
is extremely important for the activation of AMPK (Hardie
et al. 1989). It is interesting that virtually all of the FA
metabolic effect studies have used oleic acid rather than
polyunsaturated fatty acids (PUFA), which are known to be
very important in hibernators (for a review see Ruf and
Arnold 2008) as well as non-hibernators. It would be
interesting to repeat many of these experiments on hiber-
nators and also use different fatty acids to elicit responses
in hypothalamic neurons.
Interactions between sex hormones and food intake
Proximate control of food intake is primarily due to fluc-
tuations in orexigenic/anorexigenic neuropeptides, espe-
cially NPY, AgRP, POMC, and CART. All of these
J Comp Physiol B (2012) 182:451–467 459
123
neuropeptides are affected by upstream hormones such as
leptin, ghrelin, insulin, as previously discussed, and also
the sex hormone estradiol. The anorexigenic hormone
leptin has a stimulatory effect on POMC, and an inhibitory
effect on NPY and AgRP (Belsham et al. 2004; Cowley
et al. 2001). Estradiol has a similar anorexigenic effect
(Wade 1972; reviewed in Gao and Horvath 2008),
increasing POMC mRNA in the ARC of mice, and inhib-
iting NPY secretion in immortalized hypothalamic neurons
(Dhillon and Belsham 2011; Gao et al. 2007). Estrogen
receptor alpha (ERa) colocalizes with POMC in the ARC,
and may be regulating the expression of POMC in the
hypothalamus (de Souza et al. 2011). Similarly, estradiol’s
effects on NPY seem to be occurring through ERa rather
than ERb and this is mediated through the AMPK pathway
(Dhillon and Belsham 2011).
In general, male mammals tend to have higher food
intake than females, and this sex difference appears to be
initiated by prenatal exposure to testosterone, which acts
through the androgen receptor (AR) to decrease the gene
and protein expression of the anorexigenic neuropeptide
POMC in the ARC (Nohara et al. 2011). However,
understanding the sex differences in feeding in hibernators
is more difficult due to the highly seasonal nature of
reproduction and feeding in these animals.
Sciurid hibernators are typically thought of as mones-
trous—due to the rigid time constraints involved in repro-
duction and pre-hibernatory fattening, females only have
time to raise one litter prior to re-entering hibernation.
However, females of some species of hibernator appear to
undergo a second non-reproductive summer estrous cycle
during weaning (Strauss et al. 2009). The purpose of this
second estrous is unclear, but it may be involved in pre-
liminary follicle development (the follicle is ‘primed’ and
then arrested prior to entry into torpor). Alternatively, it has
been hypothesized that the presence of the reproductive
hormones progesterone and prolactin [which are typically
considered lipogenic (reviewed in Saleh et al. 2011)] allow
pre-hibernatory females to build fat stores rapidly following
the energetically expensive process of lactation (Strauss et al.
2009). Similarly, in the arctic ground squirrel (U. parryii)
androgen levels are elevated in the autumn during the pre-
hibernation period. This elevation in serum androgens may be
related to the ground squirrel’s ability to increase muscle
mass for gluconeogenesis later in the hibernation period when
glucose is needed and lipid stores may not supply sufficient
glucose (Boonstra et al. 2011). In some non-sciurid obligate
hibernators [e.g., dormice (Muscardinus avellanarius) and
hedgehogs (Erinaceus europaeus)], females occasionally
produce two litters per year, but typically these diestrous
hibernators are found in warmer climates, and as such have a
longer active season in which to raise multiple litters of young
(Buchner et al. 2003; Fowler 1988).
Future directions
There is a long history of using hibernators as laboratory
animals to examine physiology under extreme conditions,
but much of the effort to date has centered on simply
characterizing the physiology of these unique mammals.
As technology has advanced in recent years, however,
hibernation research has increasingly focused on the
molecular mechanisms and central controls underlying
these extreme shifts in physiology and behavior. Much
remains to be investigated, but the following questions
address areas of particular interest to those of us interested
in controls of food intake.
Does a common pathway suppress both food intake
and metabolism?
It has long been known that the decreased Tb exhibited by
torpid hibernators is accompanied by metabolic depression
(e.g., Lyman 1948), but clarifying the relative roles of
temperature effects versus specific physiological regulation
has taken more time (Buck and Barnes 2000; Geiser 1988;
Heldmaier and Ruf 1992; Ortmann and Heldmaier 2000;
Snapp and Heller 1981). The biochemical pathways
through which this metabolic suppression takes place are
still not fully understood, although recently it has been
shown that mitochondrial metabolic suppression (e.g.,
respiration) most likely occurs early during the entrance
into torpor and does not involve changes in membrane
phospholipid composition (Chung et al. 2011). Initial
adjustments to environmental stressors can be made
through behavioral or physiological modifications, but to
adapt to extreme conditions, animals reorganize at the
cellular level in order to facilitate hypometabolism and
which might increase long-term survival (Biggar and Sto-
rey 2011; Melvin and Andrews 2009). Some of the
mechanisms involved in this reorganization are suppression
of catabolic processes (protein synthesis, gene transcrip-
tion, etc.), and preferentially using ATP for basic cell
functions (Storey and Storey 2010). Studies on bears have
demonstrated that the metabolic depression observed in
hibernators is not due to temperature or body mass (Tøien
et al. 2011), suggesting that some reorganization of
metabolism has occurred. Recent efforts to understand the
mechanisms behind metabolic depression have focused on
the rapid and reversible transcriptome regulation provided
by microRNA molecules, which are short, non-coding
RNAs that regulate post-transcriptional expression of
mRNA transcripts (Biggar and Storey 2011). A significant
source of metabolic energy expenditure (20–30% of the
standard metabolic rate) appears to be mitochondrial pro-
ton leak (Brand et al. 1994). This proton leakage is actively
460 J Comp Physiol B (2012) 182:451–467
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depressed in liver (but not in skeletal muscle) during
hibernation via an upstream reduction in the substrate
oxidation system, rather than an alteration in membrane
proton permeability (Barger et al. 2003).
Many heterothermic mammals utilize a lower metabolic
rate and increased food intake in order to facilitate maxi-
mum fat storage during their comparatively brief active
season (Armitage and Shulenberger 1972; Geiser 2004;
Thorp et al. 1994; Ward and Armitage 1981; reviewed in
Dark 2005). It is possible that the mechanism by which
metabolic rate is markedly slowed is also involved in the
suppression of food intake at the beginning of the hiber-
nation season. We hypothesize that the food intake path-
way and the process by which metabolism is suppressed
are linked and that to initiate a drop in Tb, the food intake
pathway (drive) must first be shut down. The typical
response of endotherms (not only hibernators) to food
dearth (fasting) is a decrease in Tb caused by metabolic
suppression (i.e., torpor). The long-term fast exhibited by
obligate hibernators is initiated ‘voluntarily’ and is so
robust that a hibernator will cease eating and enter torpor
even if food is still available to them in the environment,
but the initial response to fasting is similar to that under-
gone by non-hibernators. In rats (non-hibernators), blood
glucose is utilized first (over a period of 4–8 h); after this
time, low insulin and high glucagon facilitate use of liver
glycogen and gluconeogenesis (Parrilla 1978). Glycogen
stores are quickly depleted, after which fat becomes the
primary source of ATP, resulting in high circulating con-
centrations of FFAs, which are broken down into ketones
through liver ketogenesis; AMPK is high to allow fatty
acid oxidation. In non-hibernators, the fat stores are gen-
erally depleted quickly and animals metabolize protein in
the final stages of starvation. In animals that are able to
undergo torpor, however, the suppressed metabolic state
results in decreased endogenous heat production leading to
a decrease in Tb during the fat-burning stage, allowing
lipolysis to be sustained for a much longer period of time.
Metabolism is a balance between ATP-generating (cat-
abolic) processes and ATP-utilizing (anabolic) processes—
since there are more anabolic than catabolic processes, the
ability to carefully regulate catabolic processes is essential
to regulating metabolic depression. Excellent reviews by
Storey and Storey (1990), (2010) describe some of the
specific enzymatic changes that take place during torpor in
order to shift from carbohydrate metabolism to lipolysis
(e.g., inactivation of pyruvate dehydrogenase (PDH)
through reversible phosphorylation during entry into tor-
por); these changes are corroborated by recent studies of
gene expression in torpid versus euthermic ground squirrels
(e.g., upregulation of Pdk4, which phosphorylates and
inactivates PDH, effectively blocking carbohydrate
metabolism; Andrews et al. 1998; Yan et al. 2008).
However, a more in-depth examination of the torpor entry
stage and of the physiological condition immediately fol-
lowing the hyperphagic period and preceding the first tor-
por bout is needed to clarify the timing and magnitude of
changes in gene expression and post-translational modifi-
cation of gene products during these times of physiological
upheaval. Some progress is being made toward this goal
recently (e.g., Epperson et al. 2011), but the controls of
metabolic suppression and food intake are far from being
fully understood.
Does blood–brain barrier permeability change
seasonally to regulate physiological state?
The brain reacts to external stimuli primarily by what is
allowed entry through the blood–brain barrier (BBB),
which is a modified capillary bed with tight junctions that
eliminate intercellular spaces, in effect providing a barrier
that only certain molecules are allowed to cross. Many
hormones (e.g., insulin) have little or no central production,
and as such need to pass through the BBB in order to exert
their effects. This may occur via diffusion of small, lipid-
soluble molecules (Bradbury 1979) or by a saturable
transport mechanism (Davson and Segal 1996). These
transporter molecules in essence control the permeability
of the BBB, which may be made more or less ‘leaky’ by
up- or downregulation of receptors and transporter
molecules.
It is possible that permeability of the BBB changes with
season in hibernators in order to allow more of one type of
substance or less of another type under different physio-
logical conditions. In support of this hypothesis, certain
types of BBB transporter appear to be differentially regu-
lated in hibernators. Torpid hibernators appear to possess
much higher concentrations of the ketone transporter
monocarboxylic acid transporter 1 than rats, and this
transporter is upregulated as animals enter hibernation,
possibly allowing preferential entry and utilization of
ketones during torpor and arousal (Andrews et al. 2009).
Cerebral blood flow is partially regulated by histamine,
which is thought to increase BBB permeability through
histamine receptors (H1, 2, and 3) (Panula et al. 2000).
Both histamine and receptor H3 are upregulated during the
torpid stage of hibernation in GMGS (C. lateralis) (Sall-
men et al. 1999, 2003).
In non-hibernators, the rate of transport of certain mol-
ecules across the BBB depends on brain region and cir-
culating concentrations of those molecules. For instance,
leptin is transported most readily into the hypothalamus at
low serum concentrations (i.e., when animals are fasting)
and has an effect on food intake; when serum leptin con-
centrations are high, it is preferentially taken up by the
J Comp Physiol B (2012) 182:451–467 461
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hippocampus, which may explain leptin’s effects on
learning and memory (reviewed in Banks 2006; Farr et al.
2006). It is possible that the transporters in the BBB that
allow nutrient sensing are downregulated (making the BBB
less permeable) as animals make the autumnal switch from
homeothermy to heterothermy, causing the hibernating
hypothalamus to be less reactive to fluctuations in circu-
lating hormones and enzymes than it would be in a
euthermic animal. Similarly, a change in BBB permeability
to certain enzymes might mark the transitions between
euthermia and torpor during the hibernation season.
What are the effects of ambient temperature
and circadian clock on the regulation of torpor
and food intake?
Despite species differences in torpor bout length and other
characteristics of the hibernation season, all hibernators
demonstrate a clear and fairly regularly spaced rhythm of
torpor and arousal bouts, which indicates that some sort of
internal timing mechanism is involved in generating the
bouts. Recently, it has been shown that the circadian clock
is arrested at low body temperatures (Revel et al. 2007),
which begs the question: if the clock is arrested then what
regulates the arousal from torpor? Furthermore, it has been
shown that dormice (Glis glis) can estivate for extended
periods of time, regardless of the ambient temperature,
perhaps in an effort to avoid predation (Bieber and Ruf
2009). Whether there is a circannual clock that controls the
yearly hibernation cycle (Kondo 2007) and/or circadian
clocks controlled not by temperature but by a torpor-
arousal clock (Malan 2010) is subject to debate. It is
becoming clearer that the circadian system, metabolism,
and temperature are very intricately related (Bass and Ta-
kahashi 2010; Buhr et al. 2010). The exact mechanism that
initiates and controls a torpor bout is unknown, but it is
clear that environmental temperature can alter the fre-
quency of the torpor bouts (Florant et al. unpub. data;
Geiser and Kenagy 1988; Russell et al. 2010). With
increasing Ta, torpor bouts are more frequent and of shorter
duration, suggesting that an internal ‘‘energy supply’’
might be used up faster, leading to an arousal. This
hypothesis could be tested by infusing energy-rich nutrients
such as fatty acids into the hypothalamus.
We believe that mammalian hibernators are excellent
animal models to study the physiological mechanisms
involved with food intake and body mass regulation. These
animals are capable of illustrating the tremendous variation
that can occur within neural pathways that lead to a
behavioral response. By studying these animals, there is the
distinct possibility that we may better understand such
disorders as obesity, anorexias, and diabetes.
Acknowledgments We wish to thank Drs. Brian Barnes, Loren
Buck and Hannah Carey for their feedback and discussion of ideas in
this manuscript. We also thank the anonymous reviewers that helped
us to clarify and strengthen the manuscript.
References
Andrews MT, Squire TL, Bowen CM, Rollins MB (1998) Low-
temperature carbon utilization is regulated by novel gene activity
in the heart of a hibernating mammal. Physiology 95:8392–8397
Andrews MT, Russeth KP, Drewes LR, Henry PG (2009) Adaptive
mechanisms regulate preferred utilization of ketones in the heart
and brain of a hibernating mammal during arousal from torpor.
Am J Physiol Regul Integr Comp Physiol 296:R383–R393
Ariyasu H, Takaya K, Hosoda H, Iwakura H, Ebihara K, Mori K
(2002) Delayed short-term secretory regulation of ghrelin in
obese animals: evidenced by a specific RIA for the active form
of ghrelin. Endocrinology 143:3341–3350
Armitage KB, Shulenberger E (1972) Evidence for a circannual
metabolic cycle in Citellus tridecemlineatus, a hibernator. Comp
Biochem Physiol A Comp Physiol 42:667–688
Banks WA (2006) The blood–brain barrier as a regulatory interface in
the gut–brain axes. Physiol Behav 89:472–476
Banks WA, Coon AB, Robinson SM, Moinuddin A, Shultz JM,
Nakaoke R, Morley JE (2004) Triglycerides induce leptin
resistance at the blood–brain barrier. Diabetes 53:1253–1260
Barazzoni R, Zanetti M, Stebel M, Biolo G, Cattin L, Guarnieri G
(2003) Hyperleptinemia prevents increased plasma ghrelin
concentration during short-term moderate caloric restriction in
rats. Gastroenterology 124:1188–1192
Barger JL, Brand MD, Barnes BM, Boyer BB (2003) Tissue-specific
depression of mitochondrial proton leak and substrate oxidation
in hibernating arctic ground squirrels. Am J Physiol Regul Integr
Comp Physiol 284:R1306–R1313
Barnes BM, Kretzmann M, Licht P, Zucker I (1986) The influence of
hibernation on testis growth and spermatogenesis in the golden-
mantled ground squirrel, Spermophilus lateralis. Biol Reprod
35:1289–1297
Bass J, Takahashi JS (2010) Circadian integration of metabolism and
energetics. Science 330:1349–1354
Bauman WA, Meryn S, Florant GL (1988) Cholecystokinin (CCK)
and vasoactive intestinal peptide (VIP) in the cerebral cortex of
the non-hibernating and hibernating golden-mantled ground
squirrel. Comp Biochem Physiol A Comp Physiol 91(1):179–
181
Belgardt BF, Okamura T, Bruning JC (2009) Hormone and glucose
signaling in POMC and AgRP neurons. J Physiol 587:5305–
5314
Belsham DD, Cai F, Cui H, Smukler SR, Salapatek AM, Shkreta L
(2004) Generation of a phenotypic array of hypothalamic
neuronal cell models to study complex neuroendocrine disorders.
Endocrinology 145:393–400
Bieber C, Ruf T (2009) Summer dormancy in edible dormice (Glisglis) without energetic constraints. Naturwissenschaften 96:165–
171
Biggar KK, Storey KB (2011) The emerging roles of microRNAs in
the molecular responses of metabolic rate depression. J Mol Cell
Biol 3:167–175
Blazquez C, Woods A, de Ceballos ML, Carling D, Guzman M
(1999) The AMP-activated protein kinase is involved in the
regulation of ketone body production by astrocytes. J Neurochem
73:1674–1682
Blouet C, Schwartz GJ (2010) Hypothalamic nutrient sensing in the
control of energy homeostasis. Behav Brain Res 209:1–12
462 J Comp Physiol B (2012) 182:451–467
123
Boelen A, Wiersinga WM, Fliers E (2008) Fasting-induced changes
in the hypothalamus-pituitary-thyroid axis. Thyroid 18:123–129
Boonstra R, Bradley AJ, Delehanty B (2011) Preparing for hiberna-
tion in ground squirrels: adrenal androgen production in summer
linked to environmental severity in winter. Funct Ecol. doi:
10.1111/j.1365-2435.2011.01890.x
Boswell T, Richardson RD, Schwartz MW, D’Alessio DA, Woods
SC, Sipols AJ, Baskin DG, Kenagy GJ (1993) NPY and galanin
in a hibernator: hypothalamic gene expression and effects on
feeding. Brain Res Bull 32:379–384
Boswell T, Woods SC, Kenagy GJ (1994) Seasonal changes in body
mass, insulin, and glucocorticoids of free-living golden-mantled
ground squirrels. Gen Comp Endocrinol 96:339–346
Boyer BB, Ormseth OA, Buck L, Nicolson M, Pelleymounter MA,
Barnes BM (1997) Leptin prevents posthibernation weight gain
but does not reduce energy expenditure in Arctic GS. Comp
Biochem Physiol 118:405–412
Bradbury M (1979) The concept of a blood–brain barrier. Wiley, New
York
Brand MD, Chien LF, Ainscow EK, Rolfe DFS, Porter RK (1994)
The causes and functions of mitochondrial proton leak. Biochim
Biophys Acta 1187:132–139
Buchner S, Stubbe M, Striese D (2003) Breeding and biological data
for the common dormouse (Muscardinus avellanarius) in
Eastern Saxony (Germany). Acta Zool Acad Sci Hung 49(Suppl.
1):19–26
Buck CL, Barnes BM (2000) Effects of ambient temperature on
metabolic rate, respiratory quotient, and torpor in an arctic
hibernator. Am J Physiol 279:R255–R262
Buhr ED, Yoo SH, Takahashi JS (2010) Temperature as a universal
resetting cue for mammalian circadian oscillators. Science
330:379–385
Buzadzic B, Spasic M, Saicic ZS, Radojicic R, Petrovic VM,
Halliwell B (1990) Antioxidant defenses in the ground squirrel
Citellus citellus. 2. The effect of hibernation. Free Radic Biol
Med 9:407–413
Carling D (2005) AMP-activated protein kinase: balancing the scales.
Biochimie 87:87–91
Chen HY, Trumbauer ME, Chen AS, Weingarth DT, Adams JR,
Frazier EG, Shen Z, Marsh DJ, Feighner SD, Guan XM, Ye Z,
Nargund RP, Smith RG, Van Der Ploeg LH, Howard AD,
MacNeil DJ, Qian S (2004) Orexigenic action of peripheral
ghrelin is mediated by neuropeptide Y (NPY) and Agouti-related
protein (AgRP). Endocrinol 145:2607–2612
Chen YJ, Wu CY, Shen JL, Chu SY, Chen CK, Chang YT, Chen CM
(2008) Psoriasis independently associated with hyperleptinemia
contributing to metabolic syndrome. Arch Dermatol 144:1571–
1575
Chung D, Lloyd GP, Thomas RH, Guglielmo CG, Staples JF (2011)
Mitochondrial respiration and succinate dehydrogenase are
suppressed early during entrance into a hibernation bout, but
membrane remodeling is only transient. J Comp Physiol B. doi:
10.1007/s00360-010-0547-x
Claret M, Smith MA, Batterham RL, Selman C, Choudhury AI, Fryer
LG, Clements M, Al-Qassab H, Heffron H, Xu AW, Speakman
JR, Barsh GS, Viollet B, Vaulont S, Ashford ML, Carling D,
Withers DJ (2007) AMPK is essential for energy homeostasis
regulation and glucose sensing by POMC and AgRP neurons.
J Clin Invest 117:2325–2336
Concannon P, Levac K, Rawson R, Tennant B, Bensadoun A (2001)
Seasonal changes in serum leptin, food intake, and body weight
in photoentrained woodchucks. Am J Physiol Regul Integr Comp
Physiol 281:R951–R959
Cota D, Proulx K, Blake Smith KA, Kozma SC, Thomas G, Woods
SC, Seeley RJ (2006) Hypothalamic mTOR signaling regulates
food intake. Science 312:927–930
Cowley MA, Smart JL, Rubinstein M, Cerdan MG, Diano S, Horvath
TL, Cone RD, Low MJ (2001) Leptin activates anorexigenic
POMC neurons through a neural network in the arcuate nucleus.
Nature 411:480–484
Coyral-Castel S, Tosca L, Ferreira G, Jeanpierre E, Rame C, Lomet
D, Caraty A, Monget P, Chabrolle C, Dupont J (2008) The effect
of AMP-activated kinase activation on gonadotropin-releasing
hormone secretion in GT1–7 cells and its potential role in
hypothalamic regulation of the oestrous cyclicity in rats.
J Neuroendocrinol 20:335–346
Cummings DE, Foster KE (2003) Ghrelin-leptin tango in body-weight
regulation. Gastroenterology 124:1532–1535
Daan S, Barnes BM, Strijkstra AM (1991) Warming up for sleep?–
Ground squirrels sleep during arousals from hibernation. Neu-
rosci Lett 12:265–268
Dailey MJ, Bartness TJ (2010) Arcuate nucleus destruction does not
block food deprivation-induced increases in food foraging and
hoarding. Brain Res 1323:94–108
Dark J (2005) Annual lipid cycles in hibernators: integration of
physiology and behavior. Annu Rev Nutr 25:469–497
Dark J, Miller DR (1997) Metabolic fuel privation in hibernating and
awake ground squirrels. Physiol Behav 63:59–65
Davson H, Segal MB (1996) Special aspects of the blood–brain
barrier. In: Physiology of the CSF and blood–brain barriers. CRC
Press, Boca Raton, pp 303–485
de Lartigue G, Barbier de la Serre C, Espero E, Lee J, Raybould HA
(2011) Diet-induced obesity leads to the development of leptin
resistance in vagal afferent neurons. Am J Physiol Endocrinol
Metab 301:E187–E195
de Souza FSJ, Nasif S, Lopez-Leal R, Levi DH, Low MJ, Rubinsten
M (2011) The estrogen receptor � colocalizes with proopiomel-
anocortin in hypothalamic neurons and binds to a conserved
motif present in the neuron-specific enhancer nPE2. Eur J
Pharmacol 660:181–187
Dhillon SS, Belsham DD (2011) Estrogen inhibits NPY secretion
through membrane-associated estrogen receptor (ER)-a in
clonal, immortalized hypothalamic neurons. Int J Obes
35:198–207
Dieguez C, Vazquez MJ, Romero A, Lopez M, Nogueiras R (2011)
Hypothalamic control of lipid metabolism: focus on leptin,
ghrelin and melanocortins. Neuroendocrinology 94:1–11
Dunwiddie TV, Masino SA (2001) The role and regulation of adenosine
in the central nervous system. Annu Rev Neurosci 24:31–55
Edmond J (1992) Energy metabolism in developing brain cells. Can J
Physiol Pharmacol 70:S118–S129
Epperson LE, Karimpour-Fard A, Hunter LE, Martin SL (2011)
Metabolic cycles in a circannual hibernator. Physiol Genomics
43:799–807
Farr SA, Banks WA, Morley JE (2006) Effects of leptin on memory
processing. Peptides 27:1420–1425
Florant GL (1998) Lipid metabolism in hibernators: the importance of
essential fatty acids. Am Zool 38:331–340
Florant GL, Baumann WA (1984) Seasonal variations in carbohydrate
metabolism in mammalian hibernators: insulin and body weight
changes. In: Van Itallie TB, Hirsch J (eds) Advances in obesity
research, vol 4. John Libbey, London, pp 57–64
Florant GL, Lawrence AK, Williams K, Bauman WA (1985)
Seasonal changes in pancreatic B-cell function in euthermic
yellow-bellied marmots. Am J Physiol 249:R159–R165
Florant GL, Nuttle LC, Mullinex DE, Rintoul DA (1990) Plasma and
white adipose tissue lipid composition in marmots. Am J Physiol
258:R1123–R1131
Florant GL, Singer L, Scheurink AJW, Park CR, Richardson RD,
Woods SC (1991) Intraventricular insulin reduces food intake
and body weight of marmots during the summer feeding period.
Phys Behav 49:335–338
J Comp Physiol B (2012) 182:451–467 463
123
Florant GL, Hester L, Ameenuddin S, Rintoul DA (1993) The effect
of a low essential fatty acid diet on hibernation in marmots. Am J
Physiol 264:R747–R753
Florant GL, Porst H, Peiffer A, Hudachek SF, Pittman C, Summers
SA, Rajala MW, Scherer PE (2004) Fat-cell mass, serum leptin
and adiponectin changes during weight gain and loss in yellow-
bellied marmots (Marmota flaviventris). J Comp Physiol B
174:633–639
Florant GL, Fenn AM, Healy JE, Wilkerson GK, Handa RJ (2010) To
eat or not to eat: the effect of AICAR on food intake regulation
in yellow-bellied marmots (Marmota flaviventris). J Exp Bio
213:2031–2037
Fowler PA (1988) Seasonal endocrine cycles in the European
hedgehog, Erinaceus europaeus. J Reprod Fert 84:259–272
Frank CL (1994) Polyunsaturate content and diet selection by ground
squirrels (Spermophilus lateralis). Ecology 75:458–463
Frank CL, Dierenfeld ES, Storey KB (1998) The relationship between
lipid peroxidation, hibernation, and food selection in mammals.
Am Zool 38:341–349
Frank CL, Karpovich S, Barnes BM (2008) Dietary fatty acid
composition and the hibernation patterns in free-ranging arctic
ground squirrels. Physiol Biochem Zool 81:486–495
Freeman DA, Lewis DA, Kauffman AS, Blum RM, Dark J (2004)
Reduced leptin concentrations are permissive for display of
torpor in Siberian hamsters. Am J Physiol Regul Integr Comp
Physiol 287:R97–R103
Galster W, Morrison PR (1975) Gluconeogenesis in arctic ground
squirrels between periods of hibernation. Am J Physiol 228:325–
330
Gao Q, Horvath TL (2008) Cross-talk between estrogen and leptin
signaling in the hypothalamus. Am J Physiol Endocrinol Metab
294:E817–E826
Gao Q, Mezei G, Nie Y, Rao Y, Choi CS, Bechmann I, Leranth C,
Toran-Allerand D, Priest CA, Roberts JL, Gao XB, Mobbs C,
Shulman GI, Diano S, Horvath TL (2007) Anorectic estrogen
mimics leptin’s effect on the rewiring of melanocortin cells and
Stat3 signaling in obese animals. Nat Med 13:89–94
Geiser F (1988) Reduction of metabolism during hibernation and
daily torpor in mammals and birds: temperature effect or
physiological inhibition? J Comp Physiol B 158:25
Geiser F (1991) The effect of unsaturated and saturated dietary lipids
on the pattern of daily torpor and the fatty acid composition of
tissues and membranes of the deer mouse Peromyscus manicul-atus. J Comp Physiol B 161:590–597
Geiser F (2004) Metabolic rate and body temperature reduction
during hibernation and daily torpor. Ann Rev Physiol 66:239–
274
Geiser F (2007) Yearlong hibernation in a marsupial mammal.
Naturwissenschaften 94:941–944
Geiser F, Kenagy GJ (1987) Polyunsaturated lipid diet lengthens
torpor and reduces body temperature in a hibernator. Am J
Physiol 252:R897–R901
Geiser F, Kenagy GJ (1988) Duration of torpor bouts in relation to
temperature and energy metabolism in hibernating ground
squirrels. Physiol Zool 61:442–449
Ghilardi N, Ziegler S, Wiestner A, Stoffel R, Heim MH, Skoda RC
(1996) Defective STAT signaling by the leptin receptor in
diabetic mice. Proc Natl Acad Sci USA 93:6231–6235
Gluck EF, Stephens N, Swoap SJ (2006) Peripheral ghrelin deepens
torpor bouts in mice through the arcuate nucleus neuropeptide Y
signaling pathway. Am J Physiol 291:R1303–R1309
Hardie DG, Carling D (1997) The AMP-activated protein kinase—
fuel gauge of the mammalian cell? Eur J Biochem 246:259–273
Hardie DG, Carling D, Sim ATR (1989) The AMP-activated protein
kinase–a multisubstrate regulator of lipid metabolism. Trends
Biochem Sci 14:20–23
Harlow HJ (1996) Winter body fat, food consumption and nonshi-
vering thermogenesis of representative spontaneous and facul-
tative hibernators: the white-tailed prairie dog and black-tailed
prairie dog. J Therm Biol 22:21–30
Havrankova J, Roth J, Brownstein M (1978) Insulin receptors are
widely distributed in the central nervous system of the rat.
Nature 272:827–829
Healy JE, Richter MM, Suu L, Fried SK, Florant GL (2008) Changes
in serum leptin concentrations with fat mass in golden-mantled
ground squirrels (Spermophilus lateralis). In: Lovegrove BG,
McKechnie AE (eds) Hypometabolism in animals: torpor,
hibernation and cryobiology. University of KwaZulu-Natal,
Pietermaritzburg
Healy JE, Ostrom CE, Wilkerson GK, Florant GL (2010) Plasma
ghrelin concentrations change with physiological state in a
sciurid hibernator (Spermophilus lateralis). Gen Comp Endocri-
nol 166:372–378
Healy JE, Bateman JL, Ostrom CE, Florant GL (2011a) Peripheral
ghrelin stimulates feeding behavior and positive energy balance
in a sciurid hibernator. Horm Behav 59:512–519
Healy JE, Gearhart CN, Bateman JL, Handa RJ, Florant GL (2011b)
AMPK and ACC change with fasting and physiological condi-
tion in euthermic and hibernating golden-mantled ground
squirrels (Callospermophilus lateralis). Comp Biochem Physiol
A Mol Integr Physiol 159:322–331
Heldmaier G, Ruf T (1992) Body temperature and metabolic rate during
natural hypothermia in endotherms. J Comp Physiol B 162:696
Helgen KM, Cole FR, Helgen LE, Wilson DE (2009) Generic
revision in the holarctic ground squirrel genus Spermophilus.
J Mamm 90:270–305
Herwig A, Wilson D, Logie TJ, Boelen A, Morgan PJ, Mercer JG,
Barrett P (2009) Photoperiod and acute energy deficits interact
on components of the thyroid hormone system in hypothalamic
tanycytes of the Siberian hamster. Am J Physiol Regul Integr
Comp Physiol 296:R1307–R1315
Hewson AK, Dickson SL (2000) Systemic administration of ghrelin
induces Fos and Egr-1 proteins in the hypothalamic arcuate
nucleus of fasted and fed rats. J Neuroendocrinol 12:1047–1049
Hill VL, Florant GL (2000) The effect of a linseed oil diet on
hibernation in yellow-bellied marmots (Marmota flaviventris).
Physiol Behav 68:431–437
Ho PWL, Liu HF, Ho JWM, Zhang WY, Chu ACY, Kwok KHH, Ge
X, Chan KH, Ramsden DB, Ho SL (2010) Mitochondrial
uncoupling protein-2 (UCP2) mediates leptin protection against
MPP? toxicity in neuronal cells. Neurotoxic Res 17:332–343
Hoehn KL, Salmon AB, Hohnen-Behrens C, Turner N, Hoy AJ,
Maghzal GJ, Stocker R, Van Remmen H, Kraegen EW, Cooney GJ,
Richardson AR, James DE (2009) Insulin resistance is a cellular
antioxidant defense mechanism. PNAS 106:17787–17792
Horman S, Hussain N, Dilworth SM, Storey KB, Rider MH (2005)
Evaluation of the role of AMP-activated protein kinase and its
downstream targets in mammalian hibernation. Comp Biochem
Physiol B Biochem Mol Biol 142:374–382
Hume ID, Beiglbock C, Ruf T, Frey-Roos F, Bruns U, Arnold W
(2002) Seasonal changes in morphology and function of the
gastrointestinal tract of free-living alpine marmots (Marmotamarmota). J Comp Physiol B 172:197–207
Humphries MM, Thomas DW, Kramer DL (2003) The role of energy
availability in mammalian hibernation: a cost-benefit approach.
Physiol Biochem Zool 76:165–179
Jaszberenyi M, Bujdoso E, Bagosi Z, Telegdy G (2006) Mediation of
the behavioral, endocrine and thermoregulatory actions of
ghrelin. Horm Behav 50:266–273
Jinka TR, Tøien Ø, Drew JK (2011) Season primes the brain in an
arctic hibernator to facilitate entrance into torpor mediated by
adenosine A1 receptors. J Neurosci 31:10752–10758
464 J Comp Physiol B (2012) 182:451–467
123
Kahn BB, Alquier T, Carling D, Hardie DG (2005) AMP-activated
protein kinase: ancient energy gauge provides clues to modern
understanding of metabolism. Cell Metab 1:15–25
Karnani M, Burdakov D (2011) Multiple hypothalamic circuits sense
and regulate glucose levels. Am J Physiol Regul Integr Comp
Physiol 300:R47–R55
Keen-Rhinehart E, Bartness TJ (2005) Peripheral ghrelin injections
stimulate food intake, foraging, and food hoarding in Siberian
hamsters. Am J Physiol Regul Integr Comp Physiol 288:R716–
R722
Kohno D, Sone H, Tanaka S, Kurita H, Gantulga D, Yada T (2011)
AMP-activated protein kinase activates neuropeptide Y neurons
in the hypothalamic arcuate nucleus to increase food intake in
rats. Neurosci Lett 499:194–198
Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K
(1999) Ghrelin is a growth-hormone-releasing acylated peptide
from stomach. Nature 402:656–660
Kondo K (2007) Endogenous circannual clock and HP complex in a
hibernation control system. Cold Spring Harbor Symp Quant
Biol LXXIL:607–613
Kronfeld-Schor N, Richardson C, Silvia BA, Kunz TH, Widmaier EP
(2000) Dissociation of leptin secretion and adiposity during
prehibernatory fattening in little brown bats. Am J Physiol
279:R1277–R1281
Kwok KH, Ho PW, Chu AC, Ho JW, Liu HF, Yiu DC, Chan KH,
Kung MH, Ramsden DB, Ho SL (2010) Mitochondrial UCP5 is
neuroprotective by preserving mitochondrial membrane poten-
tial, ATP levels, and reducing oxidative stress in MPP? and
dopamine toxicity. Free Radic Biol Med 49:1023–1035
Lam TK (2011) Neuronal regulation of homeostasis by nutrient
sensing. Nat Med 16:392–395
Lam TK, Schwartz GJ, Rossetti L (2005) Hypothalamic sensing of
fatty acids. Nat Neurosci 8:579–584
Le Foll C, Irani BG, Magnan C, Dunn-Meynell AA, Levin BE
(2009a) Characteristics and mechanisms of hypothalamic neu-
ronal fatty acid sensing. Am J Physiol Regul Integr Comp
Physiol 297:R655–R664
Le Foll C, Irani BG, Magnan C, Dunn-Meynell AA, Levin BE (2009b)
Effects of maternal genotype and diet on offspring glucose and fatty
acid-sensing ventromedial hypothalamic nucleus neurons. Am J
Physiol Regul Integr Comp Physiol 297:R1351–R1357
Levin BE (2002) Glucosensing neurons: the metabolic sensors of the
brain? Diabetes Nutr Metab 15:274–280
Liu D, Chan SL, de Souza-Pinto NC, Slevin JR, Wersto RP, Zhan M,
Mustafa K, de Cabo R, Mattson MP (2006) Mitochondrial UCP4
mediates an adaptive shift in energy metabolism and increases
the resistance of neurons to metabolic and oxidative stress.
Neuromolecular Med 8:389–414
Lopez M, Lage R, Saha AK, Perez-Tilve D, Vazquez MJ, Varela L,
Sangiao-Alvarellos S, Tovar S, Raghay K, Rodrıguez-Cuenca S,
Deoliveira RM, Castaneda T, Datta R, Dong JZ, Culler M,
Sleeman MW, Alvarez CV, Gallego R, Lelliott CJ, Carling D,
Tschop MH, Dieguez C, Vidal-Puig A (2008) Hypothalamic
fatty acid metabolism mediates the orexigenic action of ghrelin.
Cell Metab 7:389–399
Lyman CP (1948) The oxygen consumption and temperature
regulation of hibernating hamsters. J Exp Zool 109:55–78
Lyman CP, Willis JS, Malan A, Wang LCH (1982) Hibernation and
torpor in mammals and birds. Academic Press, New York
Malan A (2010) Is the torpor-arousal cycle of hibernation controlled
by a non-temperature compensated circadian clock? J Biol
Rhythm 25:166–175
Matheson AL, Campbell KL, Willis CKR (2010) Feeding, fasting and
freezing: energetic effects of meal size and temperature on torpor
expression by little brown bats Myotis lucifugus. J Exp Bio
213:2165–2173
Melnyk RB, Mrosovsky N, Martin JM (1983) Spontaneous obesity
and weight loss: insulin binding and lipogenesis in the dormouse.
Am J Physiol 245:R403–R407
Melvin RG, Andrews MT (2009) Torpor induction in mammals:
recent discoveries fueling new ideas. Trends Endocrinol Metab
20:490–498
Michener G (1993) Sexual differences in hibernaculum contents of
Richardson’s ground squirrels: males store food. In: Carey C,
Florant GL, Wunder BA, Horwitz B (eds) Life in the cold:
ecological, physiological, and molecular mechanisms. Westview
Press, Colorado, p 575
Migrenne S, Le Foll C, Levin BE, Magnan C (2011) Brain lipid
sensing and nervous control of energy balance. Diabetes Metab
37:83–88
Millesi EHS, Dittami JP, Hoffmann LE, Daan S (1998) Parameters of
mating effort and success in male European ground squirrels,
Spermophilus citellus. Ethology 104:298–313
Minokoshi Y, Kim YB, Peroni OD, Fryer LG, Muller C, Carling D,
Kahn BB (2002) Leptin stimulates fatty-acid oxidation by
activating AMP-activated protein kinase. Nature 415:339–343
Minokoshi Y, Alquier T, Furukawa N, Kim YB, Lee A, Xue B, Mu J,
Foufelle F, Ferre P, Birnbaum MJ, Stuck BJ, Kahn BB (2004)
AMP-kinase regulates food intake by responding to hormonal
and nutrient signals in the hypothalamus. Nature 428:569–574
Minokoshi Y, Shiuchi T, Lee S, Suzuki A, Okamoto S (2008) Role of
hypothalamic AMP-kinase in food intake regulation. Nutrition
24:786–790
Minor RK, Chang JW, de Cabo R (2009) Hungry for life: how the
arcuate nucleus and neuropeptide Y may play a critical role in
mediating the benefits of calorie restriction. Mol Cell Endocrinol
299:79–88
Mondal MS, Date Y, Yamaguchi H, Toshinai K, Tsuruta T, Kangawa
K, Nakazato M (2005) Identification of ghrelin and its receptor
in neurons of the rat arcuate nucleus. Regul Pept 126:55–59
Morton ML (1975) Seasonal cycles of body weights and lipid in
Belding ground squirrels. Bull S C Acad Sci 74:128–143
Mountjoy PD, Rutter GA (2007) Glucose sensing by hypothalamic
neurons and pancreatic islet cells: AMPle evidence for common
mechanisms? Exp Physiol 92:311–319
Mouzannar R, McCafferty J, Benedetto G, Richardson C (2011)
Transcriptional and phospho-proteomic screens reveal stem cell
activation of insulin-resistance and transformation pathways
following a single minimally toxic episode of ROS. Int J
Genomics Proteomics 2:34–49
Mrosovsky N (1975) The amplitude and period of circannual cycles
of body weight in golden-mantled ground squirrels with medial
hypothalamic lesions. Brain Res 99:97–116
Mrosovsky N, Boshes M (1986) Meal patterns and food intakes of
ground squirrels during circannual cycles. Appetite 7:163–175
Mrosovsky N, Powley TL (1977) Set points for body weight and fat.
Behav Biol 20:205–223
Mrosovsky N, Sherry DF (1980) Animal anorexias. Science
207:837–842
Murphy M, Ebling FJ (2011) The role of hypothalamic tri-iodothyr-
onine availability in seasonal regulation of energy balance and
body weight. J Thyroid Res. doi:10.4061/2011/387562
Nelson CJ, Otis JP, Carey HV (2010) Global analysis of circulating
metabolites in hibernating ground squirrels. Comp Biochem
Physiol Part D Genomics Proteomics 5:265–273
Nohara K, Zhang Y, Waraich RS, Laque A, Tiano JP, Tong J,
Munzberg H, Mauvais-Jarvis F (2011) Early-life exposure to
testosterone programs the hypothalamic melanocortin system.
Endocrinology 152:1661–1669
Obici S, Feng Z, Morgan K, Stein D, Karkanias G, Rossetti L (2002)
Central administration of oleic acid inhibits glucose production
and food intake. Diabetes 51:271–275
J Comp Physiol B (2012) 182:451–467 465
123
Ormseth OA, Nicolson M, Pelleymounter MA, Boyer BB (1996)
Leptin inhibits prehibernation hyperphagia and reduces body
weight in arctic ground squirrels. Am J Physiol 271:R1775–
R1779
Orr AL, Lohse LA, Drew KL, Hermes-Lima M (2009) Physiological
oxidative stress after arousal from hibernation in Arctic ground
squirrel. Comp Biochem Physiol A Mol Integr Physiol
153:213–221
Ortmann S, Heldmaier G (2000) Regulation of body temperature and
energy requirements of hibernating alpine marmots (Marmotamarmota). Am J Physiol Regul Integr Comp Physiol 278:R698–
R704
Otis JP, Raybould HE, Carey HV (2011) Cholecystokinin activation
of central satiety centers changes seasonally in a mammalian
hibernator. Gen Comp Endocrinol 171:269–274
Panula P, Karlstedt K, Sallmen T, Peitsaro N, Kaslin J, Michelsen
KA, Anichtchik O, Kukko-Lukjanov T, Lintunen M (2000) The
histaminergic system in the brain: structural characteristics and
changes in hibernation. J Chem Neuroanat 18:65–74
Parrilla R (1978) Flux of metabolic fuels during starvation in the rat.
Pflugers Arch Eur J Physiol 374:3–7
Pelz KM, Routman D, Driscoll JR, Kriegsfeld LJ, Dark J (2008)
Monosodium glutamate-induced arcuate nucleus damage affects
both natural torpor and 2DG-induced torpor-like hypothermia in
Siberian hamsters. Am J Physiol Regul Integr Comp Physiol
294:R255–R265
Pengelley ET (1974) Circannual Clocks. Academic Press, San
Francisco
Pengelley ET, Asmundson SM (1969) Free-running periods of
endogenous circannian rhythms in the golden-mantled ground
squirrel, Citellus lateralis. Comp Biochem Physiol 30:177–183
Pengelley ET, Asmundson SJ, Barnes B, Aloia RC (1976) Relation-
ship of light intensity and photoperiod to circannual rhythmicity
in the hibernating ground squirrel, Citellus lateralis. Comp
Biochem Physiol A Comp Physiol 53:273–277
Peruzzo B, Pastor FE, Blazquez JL, Schobitz K, Pelaez B, Amat P,
Rodriguez EM (2000) A second look at the barriers of the medial
basal hypothalamus. Exp Brain Res 132:10–26
Revel FG, Herwig A, Garidou ML, Dardente H, Menet JS, Masson-
Pevet M, Simonneaux V, Saboureau M, Pevet P (2007) The
circadian clock stops ticking during deep hibernation in the
European hamster. Proc Natl Acad Sci USA 104:13816–13820
Roland AV, Moenter SM (2011) Glucosensing by GnRH neurons:
inhibition by androgens and involvement of AMP-activated
protein kinase. Mol Endocrinol 25:847–858
Ruf T, Arnold W (2008) Effects of polyunsaturated fatty acids on
hibernation and torpor: a review and hypothesis. Am J Physiol
Regul Integr Comp Physiol 294:R1044–R1052
Russell RL, O’Neill PH, Epperson LE, Martin SL (2010) Extensive
use of torpor in 13-lined ground squirrels in the fall prior to cold
exposure. J Comp Physiol B 180:1165–1172
Saleh J, Al-Wardy N, Farhan H, Al-Khanbashi M, Cianflone K (2011)
Acylation stimulating protein: a female lipogenic factor? Obes
Rev 12:440–448
Sallmen T, Beckman AL, Stanton TL, Eriksson KS, Tarhanen J,
Tuomisto L, Panula P (1999) Major changes in the brain
histamine system of the ground squirrel Citellus lateralis during
hibernation. J Neurosci 19:1824–1835
Sallmen T, Lozada AF, Anichtchik OV, Beckman AL, Panula P
(2003) Increased brain histamine H3 receptor expression during
hibernation in golden-mantled ground squirrels. BMC Neurosci
4:24
Satinoff E (1967) Aberrations of regulation in ground squirrels
following hypothalamic lesions. Am J Physiol 212:215–220
Satinoff E (1970) Hibernation and the Central Nervous System. Prog
Physiol Psychol 3:201–236
Schwartz MW, Sipols AJ, Marks JL, Sanacora G, White JD,
Scheurink A, Kahn SE, Baskin DG, Woods SC, Figlewicz DP
(1992) Inhibition of hypothalamic neuropeptide Y gene expres-
sion by insulin. Endocrinology 130:3608–3616
Schwartz MW, Woods SC, Porte D Jr, Seeley RJ, Baskin DG (2000)
Central nervous system control of food intake. Nature
404:661–671
Sheriff MJ, Kenagy GJ, Richter M, Lee T, Tøien Ø, Kohl F, Buck CL,
Barnes BM (2011) Phenological variation in annual timing of
hibernation and breeding in nearby populations of Arctic ground
squirrels. Proc Biol Sci 278:2369–2375
Shimizu H, Arima H, Watanabe M, Goto M, Banno R, Sato I, Ozaki
N, Nagasaki H, Oiso Y (2008) Glucocorticoids increase neuro-
peptide Y and agouti-related peptide gene expression via
adenosine monophosphate-activated protein kinase signaling in
the arcuate nucleus of rats. Endocrinol 149:4544–4553
Snapp BD, Heller HC (1981) Suppression of metabolism during
hibernation in ground squirrels (Citellus lateralis). Physiol Zool
54:297
Sokoloff L, Reivich M, Kennedy C, Des Rosiers MH, Patlak CS,
Pettigrew KD, Sakurada O, Shinohara M (1977) The
[14C]deoxyglucose method for the measurement of local
cerebral glucose utilization: theory, procedure, and normal
values in the conscious and anesthetized albino rat. J Neurochem
28:897–916
Speakman JR, Mitchell SE (2011) Caloric restriction. Mol Aspects
Med. doi:10.1016/j.mam.2011.07.001
Storey KB, Storey JM (1990) Metabolic rate depression and
biochemical adaptation in anaerobiosis, hibernation and estiva-
tion. Q Rev Biol 65:145–174
Storey KB, Storey JM (2010) Metabolic rate depression: the biochem-
istry of mammalian hibernation. Adv Clin Chem 52:77–108
Strauss A, Hoffmann IE, Walzl M, Millesi E (2009) Vaginal oestrus
during the reproductive and non-reproductive period in European
ground squirrels. Animal Reprod Sci 112:362–370
Szentirmai E, Kapas L, Krueger JM (2007) Ghrelin microinjection
into forebrain sites induces wakefulness and feeding in rats. Am
J Physiol Regul Integr Comp Physiol 292:R575–R585
Tache Y, Stengel A (2011) Interaction between gastric and upper
small intestinal hormones in the regulation of hunger and satiety:
ghrelin and cholecystokinin take the central stage. Curr Protein
Pept Sci 12:293–304
Tartaglia LA (1997) The leptin receptor. J Biol Chem 272:6093–6096
Thorp CR, Ram PK, Florant GL (1994) Diet alters metabolic rate in
the yellow-bellied marmot (Marmota flaviventris) during hiber-
nation. Physiol Zool 67:1213–1229
Tøien Ø, Blake J, Edgar DM, Grahn DA, Heller HC, Barnes BM
(2011) Hibernation in black bears: independence of metabolic
suppression from body temperature. Science 331:906–909
Toshinai K, Mondal MS, Nakazato M, Date Y, Murakami N, Kojima
M, Kangawa K, Matsukura S (2001) Upregulation of ghrelin
expression in the stomach upon fasting, insulin-induced hypo-
glycemia, and leptin administration. Biochem Biophys Res
Commun 281:1220–1225
Tschop M, Smiley DL, Heiman ML (2000) Ghrelin induces adiposity
in rodents. Nature 407:908–913
Verty ANA, Allen AM, Oldfield BJ (2010) The endogenous actions
of hypothalamic peptides on brown adipose tissue thermogenesis
in the rat. Endocrinol 151:4236–4246
Veyrat-Durebex C, Poher AL, Caillon A, Montet X, Rohner-
Jeanrenaud F (2011) Alterations in lipid metabolism and
thermogenesis with emergence of brown adipocytes in white
adipose tissue in diet-induced obesity resistant Lou/C rats. Am J
Physiol Endocrinol Metab 300:E1146–E1157
Wade GN (1972) Gonadal hormones and behavioral regulation of
body weight. Physiol Behav 8:523–534
466 J Comp Physiol B (2012) 182:451–467
123
Wang P, Walter RD, Bhat BG, Florant GL, Coleman RA (1997)
Seasonal changes in enzymes of lipogenesis and triacylglycerol
synthesis in the golden-mantled ground squirrel (Spermophiluslateralis). Comp Biochem Physiol B Biochem Mol Biol
118(2):261–267
Wang R, Cruciani-Guglielmacci C, Migrenne S, Magnan C, Cotero
VE, Routh VH (2006) Effects of oleic acid on distinct
populations of neurons in the hypothalamic arcuate nucleus are
dependent on extracellular glucose levels. J Neurophysiol
95:1491–1498
Ward JM, Armitage KB (1981) Circannual rhythms of food
consumption, body mass, and metabolism in yellow-bellied
marmots. Comp Biochem Physiol A 69:621–626
Woods SC, Seeley RJ, Cota D (2008) Regulation of food intake
through hypothalamic signaling networks involving mTOR.
Annu Rev Nutr 28:295–311
Yan J, Barnes BM, Kohl F, Marr TG (2008) Modulation of gene
expression in hibernating arctic ground squirrels. Physiol
Genomics 32:170–181
Yang CS, Lam CKL, Chari M, Cheung GWC, Kokorovic A, Gao S,
Leclerc I, Rutter GA, Lam TKT (2010) Hypothalamic AMP-
activated protein kinase regulates glucose production. Diabetes
59:2435–2443
Zhang Y, Proenca R, Maffei M, Barone M, Leopold L, Friedman JM
(1994) Positional cloning of the mouse obese gene and its human
homologue. Nature 372:425–431
J Comp Physiol B (2012) 182:451–467 467
123