Phase-specific central regulatory systems of hibernation in Syrian hamsters

The central body temperature (Tb) 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, Tb and respiratory rate. The hibernation of hamsters was classified into (i) entrance, (ii) maintenance and (iii) arousal phases according to Tb changes. In hibernating hamsters, Tb 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 Tb 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, Tb 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 Tb 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 Tb elevations during the arousal phase of hibernating hamsters.