The suprachiasmatic nucleus (SCN) of the hypothalamus serves as the master circadian clock in mammals. Most SCN neurons express the inhibitory neurotransmitter GABA (gamma amino butyric acid) along with a peptide cotransmitter. Notably, the neuropeptides vasopressin (VP) and vasoactive intestinal peptide (VIP) define two prominent clusters within the SCN: those located in the ventral core (VIP) and those forming the dorsomedial "shell" of the nucleus (VP). Axons emerging from VP neurons in the shell are thought to mediate much of the SCN's output to other brain regions as well as VP release into the cerebrospinal fluid (CSF). Previous work has shown that VP release by SCN neurons is activity dependent and SCN VP neurons fire action potentials at a higher rate during the light phase. Accordingly, CSF VP levels are higher during daytime. Interestingly, the amplitude of the CSF VP rhythm is greater in males than females, suggesting the existence of sex differences in the electrical activity of SCN VP neurons. Here we investigated this hypothesis by performing cell-attached recordings from 1070 SCN VP neurons across the entire circadian cycle in both sexes of transgenic rats that express green fluorescent protein (GFP) driven by the VP gene promoter. Using an immunocytochemical approach we confirmed that >60% of SCN VP neurons display visible GFP. Recordings in acute coronal slices revealed that VP neurons display a striking circadian pattern of action potential firing, but the characteristics of this activity cycle differ in males and females. Specifically, neurons in males reached a significantly higher peak firing frequency during subjective daytime compared to females and the acrophase occurred ~1 h earlier in females. Peak firing rates in females were not significantly different at various phases of the estrous cycle.
