Neuropeptides and Neurotransmitters in the Suprachiasmatic Nucleus: Relationship with the Hibernation Process

Abstract

To elucidate the functional role of the suprachiasmatic nuclei (SCN) in control of hibernation, especially in timing of hibernation and arousal, the reactivity patterns of suprachiasmatic neuropeptide and neurotransmitter systems were studied in several species of mammalian hibernators by the use of histochemical and biochemical methods during different stages or hibernation and euthermia. Although the distribution patterns of the suprachiasmatic messenger systems of hibernating species did not differ from that in mammals not able to hibernate, their reactivity patter ns were dramatically changing in relation with the stages of hibernation, arousal or euthermia. The messenger systems directly or indirectly transmitting light signals to the SCN (glutamate [GLU], pituitary-adenylyl cyclase activating peptide [PACAP], neuropeptide Y [NPY]) were almost inactive, whereas the systems originating from other entities within the brain (serotonin [5HT], substance P [SP], somatostatin [SOM], enkephalin [ENK]) remain active throughout the entire hibernation phase. Among the messenger systems intrinsic to the SCN, the reactivity of the gamma-amino butyric acid (GABA) system remains almost stable between hibernation and euthermia, whereas the reactivity of optic input relays (vasoactive intestinal peptide [VIP], gastrin-releasing peptide [GRP]) was drastically decreased and that of output relays (arginine vasopressin [AVP]) was increased during hibernation. These data together with results on regulatory processes in hypothalamic and basal forebrain structures emphasize a prominent role of the SCN in timing of initiation and termination of hibernation.