Vasopressin-containing neurons of the suprachiasmatic nuclei inhibit corticosterone release.

The suprachiasmatic nucleus (SCN) is the major pacemaker in the central nervous system responsible for generating circadian rhythmicity in mammals. Tracer studies show limited projections of the SCN, mainly to the paraventricular nucleus of the thalamus and paraventricular and dorsomedial nuclei of the hypothalamus, suggesting that the latter two areas may be the target areas of the SCN for controlling corticosterone release. The present results show that when infused in the paraventricular/dorsomedial nucleus of the hypothalamus femtomolar concentrations of vasopressin (VP), but not vasoactive intestinal peptide (VIP), are able to suppress elevated levels of corticosterone in SCN-lesioned animals to basal daytime values. On the other hand, infusion of the VP antagonist in the same hypothalamic area induced a sevenfold increase of basal corticosterone levels in intact animals. The SCN origin of this VP input was established in SCN-lesioned animals where no difference between the effect of infusing the antagonist or Ringer could be detected. These results imply that the SCN can influence the daily corticosterone rhythm through its VP-containing projection to the paraventricular/dorsomedial nucleus of the hypothalamus.