A diurnal rhythm of stimulatory input to the hypothalamo–pituitary–adrenal system as revealed by timed intrahypothalamic administration of the vasopressin

Abstract

The mammalian suprachiasmatic nuclei (SCN) contain an en-dogenous pacemaker that generates daily rhythms in behavior and secretion of hormones. We hypothesized that the SCN imposes its circadian rhythm on the rest of the brain via a rhythmic release of its transmitters in its target areas. Previously , we demonstrated a pronounced inhibitory effect of va-sopressin (VP), released from SCN terminals in the dorsomedial hypothalamus, on the release of the adrenal hormone cortico-sterone. In the present study, microdialysis-mediated intrace-rebral administration of the VP V 1-receptor antagonist was used to pursue the study of the mechanisms underlying the circadian control of basal corticosterone release. Using timed administrations of the VP antagonist divided equally over the day/night cycle, we were able to uncover the existence of an additional stimulatory input from the SCN to the hypothalamo-pituitary-adrenal (HPA) axis. Peak activity of this stimulatory SCN input takes place during the second half of the light period, after the daily peak of VP secretion, with a delay of 4-6 hr. In all likelihood, the inhibitory and stimulatory circa-dian input via separate mechanisms affects corticosterone release. Together, these two opposing circadian control mechanisms of the HPA axis enable a precise timing of the circadian peak in corticosterone release.