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 pacem aker that generates daily rhythms in behavior and secretion of hormone s. Previously we hypothesized that the SCN imposes its circadian rhyth m on the rest of the brain through a rhythmic release of its transmitt ers in its target areas. In the present study we employed microdialysi s-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 app lication of the VP V-1-antagonist in the dorsomedial hypothalamus of f reely moving, undisturbed animals during the middle of the light perio d (i.e. the trough of the corticosterone rhythm) caused an immediate i ncrease of circulating plasma corticosterone levels. A similar adminis tration of VP at the end of the light period completely prevented the diurnal rise in plasma corticosterone. These results indicate a pronou nced inhibitory role for SCN-derived VP at the level of the dorsomedia l hypothalamus with respect to the activity of the hypothalamo-pituita ry-adrenal axis during the day period. Thus, the daily decline in VP r elease sets a specific time window for the occurrence of the daily cor ticosterone peak. On the other hand, during the dark period corticoste rone levels are decreasing together with basal VP levels. Therefore, i n 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.