Symposium 37: Hibernation and daily torpor as the periodic expres- sion of genes. Chair: A. Makm (France) and L.C.H. Wang (Canada)

s37-1

Circadian heterothermy: from reptiles to mammals.

Heldmaier G Department of Biology, Philipps University, D-35032 Marburg, Germany

Mammals are able to control their body temperature (Tb) in the cold with an accuracy of 0.1 “C, due to thermoregulatory heat production. Despite this potential they display a circadian heterothermy decreasing their Tb by up to 2 “C during sleep or up to 25 “C during daily torpor. The periods with low Tb are accompanied by a suppression of metabolic rate instead of increasing metabolic rate for heat production. Daily heterothermy, and related phenoma like hibernation and estivation, were frequently considered as adaptations to save energy. Alternatively they could be considered as an ancestral trait of thermoregulation. This is supported by the observation that controlled heterothermy is common in monotremes, marsupials and all ancient eutherian mammals, and also found in birds. Furthermore, reptiles (e.g. green lizards) kept in a thermal gradient actively select high Tb’S of 28-32 “C during their circadian activity period and low Tb’S of 20 “C during inactivity. It supports the view that~heterothermy may reflect an ancestral trait of thermoregulation, which allows to switch between bradymetabolic states during rest and tachymetabolic states during activity.

s37-3

Seasonal and hibernation-specific proteins in mammalian hibernators. Kondo N Mitsubishi Kasei Inst. of Life Sci., Machida, Tokyo 194-85 11, Japan.

Mammalian hibernation brings about remarkable physiological adaptation by which animals survive a severely low body temperature. Such adaptation has been suggested to be due to readjustment of cells and organs to a new physiological state under control of a circannual hibernation rhythm. Recently, a novel protein complex (HP) which was found in the blood of chipmunks, a mammalian hibernator, was defined as a unique factor regulated by gene expression linked to the endogenous &annual rhythm. From biochemical studies of HP regulation, two hormones controlled through the hypothalamo- hypophyseal tract were identified as candidate molecules regulating HP in the liver. This suggests that this system plays an important role in physiological readjustment during hibernation. Furthermore, HP was detected in the cerebrospinal fluid (CSF) and its content was much less than that in the blood. HP level in CSF was up-regulated only during hibernation although that in the blood was down-regulated, suggesting substantial involvement of HP in the control of hibernation in the brain. These studies of the mechanism of regulation of HP have suggested a molecular system controlling hibernation.

s37-4

Hibernation-related genes : did temperature regulation and hibernation originate from circadian heterothermy ?

S37-2

A Malan Neurobiologie des Fonctions Rythmiques et Saisonni&es, CNRS and Universitk Louis Pasteur, Strasbourg, France

Mammalian Hibernation and Torpor: The Switching of Physiological States Wang LCH Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9

Mammalian hibernation and torpor represent nature’s triumphant capacity for survival when confronted by unfavourable external challenges. The occurrence of hibernation in mammals involves orchestration of a complex suite of various physiological and biochemical adaptations. Irrespective of the pattern of torpor (seasonal vs non-seasonal), there appears to be a physiological convergence in achieving a depressed metabolic state with only difference in quantitative aspects. Hibernation is not continuous but wnsists of a series of bouts, the transition between each state appears to be precisely controlled and reversible with only internally driven mechanisms. Despite years of extensive research, the exact underlying mechanism remains largely unknown. With the recent development of molecular biology, differential gene expression has been indicated to play a cruical role in mammalian hibernation.

Hibernation and diurnal torpor have been considered so far as phenotypic adaptations evolved independently in multiple mam- malian (and avian) phyla from homeothermic archetypes. Recent data show however that hibernation involves changes at gene expression level. Genes coding for various proteins are either under- or overexpressed in hibernation, while the gene of a hiber- nation-specific protein is present but unexpressed in a non-hiber- nating ground squirrel. Should these proteins be shared by several phyla, then they would have to be primitive. Are then hibernation and diurnal torpor primitive ? Daily heterothermy, with a higher body temperature Tb during periods of activity, is frequent in reptiles and insects. These animals alternatively benefit from higher performance or from energy economy. In a thermal choice setup, the green lizard is heterothermic on a 12:12 photoperiod. From this it switches to a constant high Tb or to a constant low Tb resp. on a long or short photoperiod (Rismiller and Heldmaier 1988). It is proposed that mammalian (and avian) homeothermy and hibernation originated in a similar way from a primitive heterothermy through an inhibition of the amplitude of the circadian rhythm ofTb. This is supported among other arguments by the persistence of circadian control in deep hibernation.