Decrease of endogenous vasopressin release necessary for expression of the circadian rise in plasma corticosterone: a reverse microdialysis study.

The mammalian suprachiasmatic nuclei (SCN) contain an endogenous pacemaker that generates daily rhythms in behavior and secretion of hormones. Previously we hypothesized that the SCN imposes its circadian rhythm on the rest of the brain through a rhythmic release of its transmitters in its target areas. In the present study we employed microdialysis-mediated intracerebral administration of vasopressin (VP) and its V(1) -antagonist to study the mechanisms underlying the circadian control of the release of the adrenal hormone corticosterone. Stress-free application of the VP V(1) -antagonist in the dorsomedial hypothalamus of freely moving, undisturbed animals during the middle of the light period (i.e. the trough of the corticosterone rhythm) caused an immediate increase of circulating plasma corticosterone levels. A similar administration of VP at the end of the light period completely prevented the diurnal rise in plasma corticosterone. These results indicate a pronounced inhibitory role for SCN-derived VP at the level of the dorsomedial hypothalamus with respect to the activity of the hypothalamo-pituitary-adrenal axis during the day period. Thus, the daily decline in VP release sets a specific time window for the occurrence of the daily corticosterone peak. On the other hand, during the dark period corticosterone levels are decreasing together with basal VP levels. Therefore, in addition to the inhibitory VP signal from the SCN, there is also the need for an excitatory SCN signal in order to accomplish the complete circadian profile of plasma corticosterone.