The effects of [Arg8]vasopressin and [Arg8]vasotocin on the firing rate of suprachiasmatic neurons in vitro

The excitatory effect of [Arg8]-vasopressin and its potential contribution to the circadian cycle of electrical activity in the suprachiasmatic nucleus of the rat was investigated using extracellular recordings from hypothalamic slices from virgin female rats. The majority of neurons tested for their responses to vasopressin and [Arg8]-vasotocin displayed coincident, dose-dependent excitation by both Peptides, although the relative efficacy varied between neurons, with some showing a highly preferential excitation by vasotocin. Perifusion with the Vasopressin Receptor antagonist d(CH2)5[Tyr(OEt)2,Val4,Cit8]-vasopressin was able to block the majority of responses to vasopressin or vasotocin (20/25), and similar excitation could be induced by the selective agonist [Phe2,Orn8]-vasotocin, indicating a mainly V1 receptor-mediated effect. Few neurons (3/27; 11%) responded to the oxytocin-specific agonist, [Thr4,Gly7]-oxytocin, suggesting a low occurrence of oxytocin receptors. In addition to blocking the action of exogenous vasopressin, the V1 antagonist caused a reversible suppression of spontaneous basal activity in 7/25 cases, consistent with the presence of an endogenous excitatory vasopressin tone. In agreement with previous reports, the activity of suprachiasmatic nucleus neurons showed a significant correlation between spontaneous activity and the light-dark cycle, with activity decreasing during the subjective dark phase. When neurons were divided on the basis of their response to vasopressin and/or vasotocin, the peptide-sensitive neurons continued to show a strong correlation (r = 0.513, P < 0.01) while the insensitive neurons showed no correlation (r = 0.136, P > 0.05). These data confirm the presence of V1 type receptors in the suprachiasmatic nucleus and also indicate a small number of neurons possessing additional classes of receptor selective for either oxytocin or vasotocin. Contrary to previous reports, they also demonstrate that endogenous vasopressin tonically excites suprachiasmatic nucleus neurons. The fact that vasopressin-sensitive (but not vasopressin-insensitive) neurons show a level of basal activity correlated with time, suggests that this tone may contribute to the circadian cycle of electrical activity in the suprachiasmatic nucleus.