endogenous opioids encode relative taste preference · endogenous opioids encode relative taste...

Endogenous opioids encode relative taste preference

Sharif A. Taha,1,2 Ebba Norsted,1,2 Lillian S. Lee,1,2 Penelope D. Lang,1,2 Brian S. Lee,1,2 Joshua D. Woolley1,2 andHoward L. Fields1,2

1Ernest Gallo Clinic and Research Center, University of California, San Francisco, Emeryville, CA 94608, USA2Departments of Neurology and Physiology, and the Wheeler Center for the Neurobiology of Addiction, University of California,San Francisco, San Francisco, CA 94143, USA

Keywords: food intake, nucleus accumbens, opioids, palatability, rat, ventral tegmental area

Abstract

Endogenous opioid signaling contributes to the neural control of food intake. Opioid signaling is thought to regulate palatability, thereward value of a food item as determined by orosensory cues such as taste and texture. The reward value of a food reflects not onlythese sensory properties but also the relative value of competing food choices. In the present experiment, we used a consummatorycontrast paradigm to manipulate the relative value of a sucrose solution for two groups of rats. Systemic injection of the nonspecificopioid antagonist naltrexone suppressed sucrose intake; for both groups, however, this suppression was selective, occurring only forthe relatively more valuable sucrose solution. Our results indicate that endogenous opioid signaling contributes to the encoding ofrelative reward value.

Introduction

A rich network of interrelated factors control feeding (Schwartz et al.,2000). These include the status of energy reserves, metabolic demandand the palatability of available foods. Of these, palatability is ofparticular interest because it plays an important role in driving caloricconsumption in excess of metabolic demand, promoting obesity(Raynor & Epstein, 2001). Rats offered ad libitum access to a variedand palatable ‘cafeteria diet’ ingest many more calories than animalsconsuming nutritionally balanced but bland rat chow; indeed, cafeteriadiets offer a well established paradigm for experimentally inducingobesity (Rogers & Blundell, 1984). Sensory attributes of palatablefoods are generally those which signal high energy density, such assweetness or fatty texture.Opioid signaling in the central nervous system plays a major role in

neural processing related to palatability. Opioid agonists such asmorphine potentiate food intake selectively, with the largest effectsseen in consumption of palatable sweet or high-fat foods (Evans &Vaccarino, 1990). Nonspecific opioid antagonists such as naltrexoneand naloxone suppress food intake, again with the largest effectsevident during consumption of highly palatable food items(Le Magnen et al., 1980). These effects do not occur throughmodulation of postingestive feedback, as they are apparent even insham-feeding rats (Rockwood & Reid, 1982). In addition, morphineadministration increases positive facial reactivity displays, measureswhich are tightly correlated with food preference (Pecina & Berridge,1995). These findings suggest a specific role for opioids in signalingthe reinforcing value of palatable food items.The nucleus accumbens (NAcc) is a critical site of opioid signaling

related to palatability. Infusion of naltrexone directly into the NAcccauses a selective decrease in sated rats’ sucrose intake, with minimaleffects upon consumption of (less preferred) chow (Kelley et al.,

1996). Conversely, infusion of the mu-opioid receptor (MOR) specificagonist [D-Ala2,N-Me-Phe4-Gly5-ol]-enkephalin (DAMGO) selec-tively increases intake of a variety of preferred substances, includingsucrose, noncaloric saccharin and a dilute saline solution (Zhang &Kelley, 2002). These findings demonstrate an important role for NAccopioid signaling in promoting consummatory behaviours related topreferred, highly palatable food items.The intrinsic sensory qualities of a particular food item are

important determinants of its reward value, as measured by the food’sability to motivate appetitive behaviour and to sustain consumption. Inaddition, when different foods are available, food-directed behaviours(both appetitive and consummatory) are typically guided by therelative value of competing food choices. A food item which is avidlyconsumed under normal conditions, for instance, may be rejectedwhen more preferable alternatives are available (Brosnan & De Waal,2003). Consummatory contrast paradigms provide well establishedbehavioural models for manipulating the relative value of foodrewards such as sucrose solutions (Flaherty, 1982). In anticipatorycontrast paradigms, consumption of a palatable solution is suppressedby subsequent presentation of a second, more reinforcing, solution.This contrast effect is thought to develop through learned anticipationof the second solution (Flaherty & Checke, 1982; Flaherty & Rowan,1985), and is dependent on the relative reward value of the contrastedsolutions (Flaherty et al., 1994).We recently reported that a subset of NAcc neurons encode

information related to the changing relative value of a sucrose solutionmodulated using a contrast paradigm (Taha & Fields, 2005). BecauseMOR agonists microinjected into the NAcc can increase consumptionof palatable foods, we hypothesized that endogenous opioidscontribute to this encoding and to consummatory behaviours guidedby relative value. In the present study, an anticipatory contrastparadigm was used to modulate the relative value of an otherwiseidentical 4% sucrose solution in two groups of rats. Systemicadministration of the nonspecific opioid antagonist naltrexone selec-tively reduced consumption of the relatively more palatable sucrose

Correspondence: Dr Sharif A. Taha, 1Ernest Gallo Clinic and Research Center, as above.E-mail: [email protected]

Received 30 June 2005, revised 6 June 2006, accepted 8 June 2006

European Journal of Neuroscience, Vol. 24, pp. 1220–1226, 2006 doi:10.1111/j.1460-9568.2006.04987.x

ª The Authors (2006). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd

solution for each group, demonstrating that endogenous opioidsparticipate in signaling learned relative value.

Materials and methods

All procedures used were approved by the University of California,San Francisco, Animal Care and Use Committee. Male Long-Evansrats (250–300 g) were randomly assigned to one of two experimentalgroups (4-0 or 4-20 group; see below) upon arrival in the animal carefacility. Rats were habituated to handling for 1 week, then, for thoseincluded in the NAcc or ventral tegmental area (VTA) microinfusionstudies, surgically implanted with bilateral cannulae prior to training inthe anticipatory contrast paradigm. Rats were allowed ad libitumaccess to food and water at all times throughout the experiments.

Training took place daily in operant chambers (Medical Associates,Georgia, VT, USA) equipped with a single lick spout with attachedphotobeam lickometer. Sessions lasted 30 min and consisted of twosuccessive 15-min periods. During the first 15 min of each session, ratsin both the 4-0 group and the 4-20 group received free access to anidentical 4% sucrose solution. During the second 15-min period, rats inthe 4-0 group received free access to a 0% sucrose solution. Rats in the4-20 group received free access to a 20% sucrose solution (Fig. 1A).The number of licks delivered to the spout was recorded in 1-min binsthroughout experimental sessions. Each lick resulted in consumption of� 5 lL solution (volume consumed per lick was calculated in

preliminary experiments by recording licks while rats consumed apremeasured volume of 4% sucrose). A masking white noise cue(90 dB) was present throughout training and experimental sessions.Daily training continued until the mean consumption of 4% sucrose

in the 4-0 group reached 2500 licks. This threshold value allowedsufficient time for a robust contrast effect (a large difference in 4%sucrose consumption between the two groups) to emerge. Trainingtime averaged 24 ± 2 days across all animals.For cannula implantation, surgical anaesthesia was induced with

ketamine and xylazine (50 ⁄ 10 mg ⁄ kg) and maintained with isofluo-rane (3% in O2). Cannulae were directed at the NAcc (targetcoordinates from bregma: anterioposterior, +1.0; lateral, ±1.1; ventral,7.5 mm) or the VTA (anterioposterior, )5.2; lateral, ±0.5; ventral,8.0 mm). Cannulae were implanted 2 mm above the target coordi-nates; injectors projected 2 mm beyond the end of the cannulae tips.Endogenous opioid encoding of relative value was tested with

administration of the nonspecific opioid antagonist naltrexone. Allnaltrexone treatments (both systemic and intracranial routes ofdelivery) were administered 10 min before the beginning of experi-mental sessions. During the 10-min interval between drug adminis-tration and testing, rats were returned to their home cages, with foodhoppers removed. Systemic naltrexone (1 mg ⁄ kg) was deliveredsubcutaneously. A total of 45 rats were used in systemic naltrexoneexperiments (21 in the 4-0 group, 24 in the 4-20 group). Each rat wasinjected with naltrexone and control saline, with injections taking

Fig. 1. Consummatory contrast paradigm.(A) All rats were allowed 15 min free access to a4% sucrose solution. Immediately thereafter, onegroup of rats (4-0 group) was allowed 15 min freeaccess to a 0% sucrose solution. The second groupof rats (4-20 group) was allowed 15 min freeaccess to a highly palatable 20% sucrose solution.Rats were trained daily using this paradigm for� 3 weeks before testing drug effects.(B) The consummatory contrast paradigm createda marked difference in the two groups’ consump-tion of the 4% sucrose solution; 4% sucroseconsumption averaged 2514 ± 154 licks in the4-0 group (n ¼ 52 rats) but 1116 ± 126 licks in the4-20 group (n ¼ 54 rats). Graphs show meannumber of licks for each group in the absence ofany drug treatment. *P < 0.001. (C) Four percent sucrose consumption in the first half of theexperimental session diverged early in the sessionfor 4-0 and 4-20 groups, and this difference wasmaintained throughout the session. (D) Twentyper cent sucrose consumption was robust in thesecond half of the experimental session in the 4-20group, while 0% sucrose consumption remainedlow.

Opioids encode relative value 1221

ª The Authors (2006). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing LtdEuropean Journal of Neuroscience, 24, 1220–1226

place in a counterbalanced fashion, and a single rest day allowedbetween injections. In NAcc-cannulated rats, three doses of naltrexone(0.5 lL volume) were infused into each NAcc: 6.5, 20 and 40 lg; 49rats were used in intra-NAcc infusions (24 rats in the 4-0 group, 25 inthe 4-20 group). Each rat received all treatments (one saline and threenaltrexone infusions). Infusions were performed every other day,allowing a rest day in between injections, and the order of naltrexoneand saline infusions was randomized across days. In VTA-cannulatedrats the experimental design was identical, with three doses ofnaltrexone (0.5 lL volume) infused into each VTA: 1, 5 and 20 lg; 48rats were used in intra-VTA infusions (24 rats in the 4-0 group, 24 inthe 4-20 group). Data illustrating the contrast effect (Fig. 1B and C)were drawn from a total of 106 drug-naı̈ve rats on the final day oftraining in the contrast paradigm (52 in the 4-0 group, 54 in the 4-20group); these rats were subsequently used for systemic or intracranial(NAcc or VTA) naltrexone administration experiments.Statistical analyses of the contrast paradigm and the effects of

systemic naltrexone administration were performed using t-tests; whencomparing effects within subjects, paired t-tests were used. To analysedrug effects following intracranial naltrexone infusion, statisticalanalyses were performed using one-way repeated-measures anovas,with drug concentration as the dependent measure. Finally, t-tests wereused to compare naltrexone-induced suppression of 4% sucroseconsumption in the 4-0 and 4-20 groups. Naltrexone-induced suppres-sion was calculated for each rat as the difference in consumptionfollowing naltrexone administration relative to that following saline;these difference values were then compared across 4-0 and 4-20 groups.For the NAcc and VTA microinfusions, consumption after administra-tion of the highest dose of naltrexone was used for this calculation.To analyse cannula placements, rats were deeply anaesthetized and

cannula sites were marked by passing a 20-lA current for 20 sthrough an electrode cut to the same final length as the injectors. Ratswere perfused with a solution of 10% formaldehyde and 3% potassiumferricyanide to mark sites of iron deposition. Brains were cryopro-tected, and coronal sections cut and mounted. Cannula positions werelocated under a light microscope and recorded on atlas figures adaptedfrom Paxinos & Watson (1997).

Results

The anticipatory contrast paradigm (Fig. 1A) created a robust differencebetween the two groups in the consumption of the 4% sucrosesolution; 4% sucrose consumption in the 4-0 group averaged 2514 ±154 (mean ± SEM) licks (Fig. 1B). In the 4-20 group consumptionwas 56% lower, averaging 1116 ± 126 licks. This reduction in totallicking was highly significant (t104 ¼ 7.0, P < 0.001).Within each group, consumption of different sucrose reinforcers

was also significantly different. In the 4-0 group, consumption of 0%sucrose during the second half of the experimental session averaged117 ± 47 licks, significantly less than occurred during 4% sucroseconsumption in the first half of the session (t51 ¼ 14.2, P < 0.001). Inthe 4-20 group, 20% sucrose consumption averaged 1893 ± 156 licks,significantly more than 4% sucrose consumption in the same group(t53 ¼ 3.9, P < 0.001). It is notable that, in this 4-20 group, thevolume of 20% sucrose consumed exceeded 4% sucrose consumptiondespite occurring in the second half of the session, when satiety effectsare more likely to have suppressed overall intake.Figure 1C and D shows cumulative consumption (in licks) for both

4-0 and 4-20 groups for the first half (Fig. 1C) and second half(Fig. 1D) of the experimental paradigm to illustrate the time course ofdifferent consumption rates. Differences in 4% sucrose consumptionbetween the two groups emerged rapidly and were maintained for the

duration of the first 15-min consumption period (Fig. 1C). In thesecond half of the session, 20% sucrose consumption was robust whileonly small amounts of 0% sucrose were consumed (Fig. 1D).To test for a potential role of endogenous opioids in encoding

relative value, we tested the effects of systemic naltrexone injection(1 mg ⁄ kg) on sucrose intake in each of the two groups. Naltrexonecaused a significant reduction in 4% sucrose consumption in the 4-0group (Fig. 2A). Rats injected with a control saline solution averaged2621 ± 206 licks. This declined to 1637 ± 123 licks followingsubcutaneous naltrexone administration (t20 ¼ 4.6, P < 0.001). Nal-trexone administration did not alter subsequent 0% sucrose consump-tion (Fig. 2B; t20 ¼ 0.8, P ¼ 0.45).Administration of systemic naltrexone to the 4-20 group caused

strikingly different results: 4% sucrose consumption remained robust,at levels very similar to those occurring following control salineinjections. Consumption of 4% sucrose averaged 1047 ± 159 licksfollowing naltrexone injection, relative to 1234 ± 203 licks followingsaline injection. This 15% reduction in consumption was notsignificant (Fig. 2C; t23 ¼ 1.9, P ¼ 0.07).While naltrexone had negligible effects upon 4% sucrose consump-

tion in this group, subsequent 20% sucrose consumption wassubstantially reduced; 20% sucrose consumption averaged1336 ± 135 licks following control saline administration, but declined51% to 656 ± 69 licks following systemic naltrexone administration(Fig. 2D; t23 ¼ 5.6, P < 0.001). Importantly, this finding alsodemonstrates that the absence of a naltrexone effect upon 4%consumption in the 4-20 group was not the result of a floor effect.Under control conditions, 4% and 20% sucrose consumption werequite similar in this 4-20 group; however, despite these similar levelsof baseline consumption, naltrexone significantly reduced consump-tion of the latter but not the former.Thus, while blocking endogenous opioid signaling selectively

reduced intake of a relatively more reinforcing 4% sucrose solution inthe 4-0 group, it did not alter consumption of a relatively lessreinforcing 4% sucrose solution in the 4-20 group. Direct comparisonof the magnitude of the naltrexone-induced reduction in consumptionin the two groups showed that this was significantly larger in the 4-0group than in the 4-20 group (Fig. 3; t43 ¼ 3.5, P < 0.001). Despitethe identical sensory properties of the 4% sucrose solution for the twogroups, endogenous opioid signaling (and its consequent effects uponbehaviour) were very different. In the 4-20 group of rats, consumptionof the 20% sucrose solution was dramatically reduced by naltrexoneadministration. For each group, then, systemic naltrexone administra-tion selectively reduced consumption of only the relatively mostreinforcing solution. These results demonstrate that opioid signalingrelated to palatability flexibly encodes the relative value of a sucrosereinforcer.Opioid agonists and antagonists injected into the NAcc have effects

similar to those seen after systemic routes of administration, suggest-ing that the NAcc is an important site of opioid signaling related topalatability. Therefore, we next studied the effects of direct injection ofnaltrexone into the NAcc to attempt to localize the critical site ofendogenous opioid signaling of relative value.Naltrexone microinfusion in the NAcc did not have significant

effects upon consumption in 4-0 or 4-20 groups. The largest effectswere seen for 4% consumption in the 4-0 group (Fig. 4A), wherenaltrexone administration produced a trend toward significantlyreduced consumption which did not reach significance (no effect ofdrug, F3,69 ¼ 2.5, P ¼ 0.06). Consumption during all other condi-tions was unaffected by naltrexone administration (Fig. 4B–D: allF < 1.3, all P >> 0.05). As a direct measure of naltrexone effectsrelated to relative value, we examined the magnitude of the highest

1222 S. A. Taha et al.


dose of naltrexone in reducing 4% sucrose consumption in the twogroups. The reduction in 4% sucrose consumption in the 4-0 groupcaused by naltrexone (mean reduction of 355 ± 187 licks) trendedtoward a larger effect than that observed in the 4-20 group (meanincrease of 37 ± 99 licks following naltrexone administration), but thisdifference did not reach significance (Fig. 4E; t47 ¼ 1.9, P ¼ 0.07).

Opioid modulation of food intake is not confined to the NAcc, butrather occurs in a number of other brain regions. Opioids acting in theVTA affect feeding behaviours, and an extensive literature implicatesVTA neurons in encoding reward information (Schultz, 2001;Roitman et al., 2004). Therefore, we extended our study to includemicroinfusion of naltrexone directly into the VTA.

Similar to results found for NAcc microinfusion, intra-VTAnaltrexone infusion had the largest effects upon 4% sucroseconsumption in the 4-0 group (Fig. 5A; significant effect of drug,F3,69 ¼ 3.1, P ¼ 0.03). Post hoc tests showed that only the highestnaltrexone dose (20 lg) significantly reduced 4% sucrose consump-tion relative to saline control (P < 0.05). Naltrexone did notsignificantly reduce sucrose consumption under other conditions(Fig. 5B–D; all F < 2, P >> 0.05). As for NAcc microinjections, thehighest dose of naltrexone caused a trend toward larger naltrexone-induced suppression of 4% sucrose consumption in the 4-0 grouprelative to the 4-20 group, but this did not reach significance (Fig. 5E;t46 ¼ 1.8, P ¼ 0.08).Cannula placements were confined to the NAcc (Fig. 6A) or the

VTA (Fig. 6B) for these two series of experiments. NAcc core andshell regions are known to subserve distinct functions (Kelley &Swanson, 1997; Floresco et al., 2006; Lecca et al., 2006), raising thepossibility that naltrexone effects might differ as a function of infusionsite within the NAcc. However, because few of our rats had cannulaeconfined to the NAcc shell (most implants were in the core, orincluded both core and shell in the two hemispheres), we were unableto draw conclusions about different naltrexone effects in the core andshell.

Discussion

Our data demonstrate that endogenous opioids carry informationrelated to a learned difference in the relative value of an otherwiseidentical 4% sucrose solution. These data show clearly that endog-enous opioid signaling is not related exclusively to the macronutrientcontent of food items. Endogenous opioid signaling can act to promoteconsumption of preferred food items, even when preference is entirely

Fig. 3. Naltrexone-induced reduction in 4% sucrose consumption was largerin the 4-0 group than the 4-20 group. Box plots (indicating 25th, median and75th percentile values) show naltrexone-induced reduction in 4% sucroseconsumption relative to saline control. The magnitude of the median reductionin the 4-0 group was significantly larger than that occurring in the 4-20 group(*P < 0.001).

Fig. 2. Systemic naltrexone reduced 4% sucrose consumption in the 4-0 group, but not the 4-20 group. (A) Following saline injections, rats consumed 2621 ± 206licks of 4% sucrose. Consumption was significantly reduced to 1637 ± 123 licks (37% reduction; P < 0.001) following subcutaneous administration of naltrexone(1 mg ⁄ kg). The left panel shows the total number of licks delivered under each condition. The right panel shows cumulative licks as a function of time in theexperimental session (shaded symbols, naltrexone administration; open symbols, saline). *P < 0.001 for A–D. (B) Subcutaneous administration of naltrexone didnot significantly alter 0% sucrose consumption relative to saline control. Insets show the same data with the y-axis rescaled. (C) Four per cent sucrose consumptionin the 4-20 group following saline (1234 ± 203) and naltrexone (1047 ± 159) administration were very similar. (D) Twenty per cent sucrose consumption wassignificantly reduced following naltrexone administration (656 ± 69) relative to saline injection (1336 ± 135; a 51% reduction).



dissociated from the gustatory properties of the reinforcer; nonethelessopioid signaling may also play a special role in promoting consump-tion of food items high in particular macronutrients (such as fat; seebelow). Our results broaden the scope of behaviours known to bemodulated by endogenous opioid signaling.The data that we report here are consistent with a number of

experimental results suggesting that opioids play an important role incontrolling food intake specifically through modulation of palatability.A consistent finding across many studies is that opioid antagonistsselectively reduce intake of preferred food items. Thus, consumptionof sweet saccharin or sucrose solutions is reduced relative to waterintake following naloxone administration (Le Magnen et al., 1980);naloxone suppresses sweet chow consumption more potently thannormal chow intake (Levine et al., 1995), and consumption of high-fatand ⁄ or high-sugar foods is more potently reduced than consumptionof normal chow following naloxone administration (Giraudo et al.,1993). Indeed, in a study where preferred and nonpreferred food itemswere offered concurrently, naltrexone decreased preferred food intakewhile slightly increasing nonpreferred food intake (Cooper & Turkish,1989). Opioid antagonist-induced reductions in preferred foodconsumption occur in both sham-feeding and normal animals,demonstrating that opioid antagonists act on orosensory processing

related to consumption itself rather than postingestional signaling suchas gastric distention (Rockwood & Reid, 1982).Consistent with this data, administration of opioid agonists such as

morphine increase preferred food intake. Morphine increasesconsumption of saccharin without affecting water intake (Calcagnetti& Reid, 1983), and more potently increases intake of sucrose orsweetened chow than standard chow (Evans & Vaccarino, 1990).Morphine effects can be dependent upon baseline preference, in thatsystemic morphine administration increases carbohydrate intake mostin carbohydrate-preferring rats and fat intake most in fat-preferringrats (Gosnell et al., 1990). However, other studies have shown thatsystemically administered morphine causes macronutrient-specificeffects, preferentially increasing fat intake (Welch et al., 1994);similar results have been reported for microinjection of opioidagonists directly into the NAcc (Zhang & Kelley, 2000). While ourresults do not rule out a specific role for opioid signaling inpromoting macronutrient-specific consumption, the contrast paradigmthat we have used in this study allows unambiguous dissociation ofthe sensory properties of a sucrose reinforcer from its relative value,and shows that endogenous opioid signaling can carry informationrelated exclusively to the degree to which a sucrose solution ispreferred.

Fig. 4. Infusion of naltrexone into the NAcc has minor effects upon sucroseconsumption.(A-D) Naltrexone infusion directly in the NAcc had few effectsupon sucrose intake; the largest effect was seen for 4% sucrose consumption inthe 4-0 group (A), where a trend toward decreased consumption followingnaltrexone was apparent (P ¼ 0.06).(E) Box plots (indicating 25th, median,and 75th percentile values) show the effects of the highest dose of naltrexone(40 lg) in reducing 4% sucrose consumption relative to saline control,comparing 4-0 and 4-20 groups. A trend toward a larger effect in the 4-0 group(P ¼ 0.07) was apparent.

Fig. 5. Infusion of naltrexone into the ventral tegmental area has small effectsupon sucrose consumption.(A-D) Naltrexone infusion directly in the ventraltegmental area caused a significant decrease in consumption only for 4%sucrose consumption in the 4-0 group; post hoc tests showed that the largestdose of naltrexone significantly reduced consumption relative to saline control(*P < 0.05). Naltrexone did not alter consumption in any other condition.(E) A trend toward a larger suppression of consumption in the 4-0 grouprelative to the 4-20 group (P ¼ 0.08) was apparent following administration ofthe highest dose of naltrexone.



In our study, neither intra-NAcc nor intra-VTA naltrexone admin-istration had large effects upon sucrose consumption, particularlyrelative to the robust suppression of consumption seen after systemicnaltrexone injection. It may be that neither the NAcc nor the VTA isthe critical site of opioid signaling mediating relative reward. Whilethis possibility cannot be excluded, there is ample evidence that opioidsignaling in each of these nuclei has potent effects upon food intake.Opioid agonists infused into the NAcc profoundly elevate food intake,with preferential effects upon palatable foods (Zhang & Kelley, 1997,2002), and opioid agonists in the VTA also potentiate food intake(Noel & Wise, 1995; Echo et al., 2002; MacDonald et al., 2003). Still,opioid signaling in a number of other brain regions, including thenucleus of the solitary tract, parabrachial nucleus, lateral hypothala-mus and amygdala (Stanley et al., 1988; Kotz et al., 1997; Giraudoet al., 1998; Wilson et al., 2003), also modulates food intake, and oneof these brain regions may critically mediate opioid signalingunderlying relative value. An alternative possibility is that opioidsignaling underlying relative value occurs through a widely distributednetwork, with multiple brain regions acting in parallel to promoteopioid-driven feeding. This hypothesis suggests that small effects seenwith single-site microinjections (either NAcc or VTA) might addi-tively suppress intake when both regions are injected simultaneously.

Our study leaves open the question of the specific opioid receptormediating effects on palatable food consumption, but there is ampleevidence from previous studies that signaling through the MOR ismost important in the regulation of palatable food intake. In both theNAcc and the VTA, the MOR-specific agonist DAMGO most potentlyelicits feeding (Zhang & Kelley, 1997; Echo et al., 2002), and MOR-specific antagonists most potently reduce sucrose intake (Kelley et al.,1996; Ward et al., 2006). Interestingly, signaling through a distinctopioid receptor appears to exert effects on feeding that oppose those

driven by MOR signaling: intra-NAcc administration of a selectivedelta opioid receptor (DOR) antagonist increases sucrose consumptionin sated animals (Kelley et al., 1996). This finding suggests a thirdpossibility for the minimal effects on feeding we observed followingintra-NAcc naltrexone administration. Naltrexone is a nonselectiveopioid antagonist. Concurrent blockade of both endogenous mu anddelta opioid signaling may produce competing effects upon foodintake, with MOR antagonism acting to decrease intake and DORantagonism acting to increase consumption, with the net result thatsucrose intake is largely unchanged from baseline conditions.Enkephalin, which is abundantly expressed in a subset of NAccmedium spiny neurons, is a ligand at both the MOR and DOR, and islikely to provide the tonic signaling at both receptors in the NAcc thatis inhibited by naltrexone administration. Further experiments withselective MOR and DOR antagonists will be useful in testing thishypothesis.

Acknowledgements

We thank P. M. Newton and L. H. Corbit for helpful discussions and commentson the manuscript and R. Van and V. N. Kharazia for histology assistance. Thiswork was supported by a grant from the State of California for research onalcohol and substance abuse through the University of California, SanFrancisco, and by funds provided by the Wheeler Center for the Neurobiologyof Addiction, the National Institute on Drug Abuse, and NARSAD.

Abbreviations

DAMGO, [D-Ala2, N-Me-Phe4-Gly5-ol]-enkephalin; DOR, delta opioidreceptor; MOR, mu opioid receptor; NAcc, nucleus accumbens; VTA, ventraltegmental area.

Fig. 6. Cannulae placements. (A) Reconstructed NAcc cannula positions ranged from anterioposterior +0.7 to +1.7 and mediolateral 0.5–1.4 mm. Cannulapositions spanned the shell and core subregions of the NAcc. (B) Reconstructed ventral tegmental area cannula positions ranged from anterioposterior )5.2 to)6.3 mm. For both A and B, anteroposterior position is relative to bregma.



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endogenous opioids encode relative taste preference · endogenous opioids encode relative taste...

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