Research ReportRegulatory mechanism of body temperature in the central nervous system during the maintenance phase of hibernation in Syrian hamsters: Involvement of β-endorphin

We have shown previously that intracerebroventricular (icv) injection of naloxone (a non-selective opioid receptor antagonist) or naloxonazine (a selective μ1-opioid receptor antagonist) at the maintenance phase of hibernation arouses Syrian hamsters from hibernation. This study was designed to clarify the role of β-endorphin (an endogenous μ-opioid receptor ligand) on regulation of body temperature (Tb) during the maintenance phase of hibernation. The number of c-Fos-positive cells and β-endorphin-like immunoreactivity increased in the arcuate nucleus (ARC) after hibernation onset. In contrast, endomorphin-1 (an endogenous μ-opioid receptor ligand)-like immunoreactivity observed on the anterior hypothalamus decreased after hibernation onset. In addition, hibernation was interrupted by icv injection of anti-β-endorphin antiserum at the maintenance phase of hibernation. The mRNA expression level of proopiomelanocortin (a precursor of β-endorphin) on ARC did not change throughout the hibernation phase. However, the mRNA expression level of prohormone convertase-1 increased after hibernation onset. [D-Ala2,N-MePhe4,Gly-ol5] enkephalin (DAMGO, a selective μ-opioid receptor agonist) microinjection into the dorsomedial hypothalamus (DMH) elicited the most marked Tb decrease than other sites such as the preoptic area (PO), anterior hypothalamus (AH), lateral hypothalamus (LH), ventromedial hypothalamus and posterior hypothalamus (PH). However, microinjected DAMGO into the medial septum indicated negligible changes in Tb. These results suggest that β-endorphin which synthesizes in ARC neurons regulates Tb during the maintenance phase of hibernation by activating μ-opioid receptors in PO, AH, VMH, DMH and PH.

► We identify the opioid peptide which regulates hibernation in Syrian hamsters. ► The beta-endorphin-like immunoreactivity increases after hibernation onset. ► Hibernation is interrupted by icv injection of beta-endorphin-antiserum. ► Dorsomedial hypothalamus is the most sensitive site in decreasing body temperature. ► The beta-endorphin regulates hibernation by activating mu-opioid receptors in DMH.