Concurrent decrease of vasopressin and protein kinase Calpha immunoreactivity during the light phase in the vole suprachiasmatic nucleus.

Vasopressin (AVP) is a major neuropeptide in the suprachiasmatic nucleus, the mammalian hypothalamic circadian pacemaker. Protein kinase Calpha is a putatively coupled intracellular messenger. Mean numbers of AVP- and protein kinase Calpha-immunoreactive neurons were determined in the suprachiasmatic nucleus of common voles, entrained to a 12:12 h light-dark (LD) cycle, at the beginning of the light period (zeitgeber time zero) and 6 h later (zeitgeber time six). At zeitgeber time zero, mean numbers of AVP- and protein kinase Calpha- immunoreactive neurons were 2194 and 9897, respectively. Both numbers decreased significantly with about 40% at zeitgeber time six. This concurrent decrease was most pronounced in the dorsomedial aspect of the suprachiasmatic nucleus. These findings are consistent with the findings of a peak of AVP release in rats during the early light phase.