Phase-specific central regulatory systems of hibernation in Syrian hamsters

Abstract

The central body temperature ( T b) regulation system during hibernation was investigated in Syrian hamsters of either sex. Hibernation induced in Syrian hamsters by housing them in a cold room under short day-light/dark cycle was confirmed by marked reductions in the heart rate, T b and respiratory rate. The hibernation of hamsters was classified into (i) entrance, (ii) maintenance and (iii) arousal phases according to T b changes. In hibernating hamsters, T b elevations were phase-selectively elicited by intracerebroventricular (ICV) injection of 8-cyclopenthyltheophylline (CPT; a selective A1-adenosine receptor antagonist) and naloxone (a non-selective opioid receptor antagonist) during the entrance and maintenance phases, respectively. Moreover, a similar T b elevation tendency during the maintenance phase was also induced by ICV naloxonazine, (a selective μ1-opioid receptor antagonist), although such was not the case for naltrindole (a selective δ-opioid receptor antagonist) or nor-binaltorphimine (nor-BNI, a selective κ-opioid receptor antagonist). Furthermore, T b elevations in hibernating hamsters were similarly induced with ICV thyrotropin-releasing hormone (TRH) during the entrance and maintenance phases. Furthermore, ICV injection of the anti-TRH antibody ameliorated the T b elevations induced by tactile stimulation. These results suggest that activation of the A1-receptor by adenosine is important for the generation of hypothermia in the entrance phase, and that activation of the μ1-opioid receptor by opioid peptides is required for perpetuation of hypothermia in the maintenance phase. In addition, TRH is a key endogenous substance involved in T b elevations during the arousal phase of hibernating hamsters.