RESTRICTED DAYTIME FEEDING MODIFIES SUPRACHIASMATIC NUCLEUS VASOPRESSIN RELEASE IN RATS

The authors have shown previously that vasopressin (VP) release from s uprachiasmatic nucleus (SCN) efferents in rats is important for the ti ming of the circadian activity of the hypothalamo-pituitary-adrenal (H PA) axis, resulting in a circadian rise in corticosterone at dusk. Whe n meals are supplied at a fixed time during the light period, however, this normal circadian activity of the HPA axis is strongly modified. Under such a restricted feeding regimen, a corticosterone peak appears just before the daily meal in addition to the circadian corticosteron e peak at dusk. This feeding-associated rise in corticosterone is rega rded as an SCN-independent circadian rhythm because it is sustained in SCN-lesioned animals. Despite these previous results, the authors inv estigated a putative involvement of SCN-derived VP in the control of t he prefeeding corticosterone peak by measuring the intranuclear releas e of VP in the SCN and plasma corticosterone levels in rats in ad libi tum feeding conditions as well as in animals that were obliged to feed during a 2-h period in the middle of the light period. Restricted day time feeding caused clear changes in the daily release pattern of VP f rom SCN terminals. Both a delayed onset of the diurnal rise and a prem ature decline of the elevated daytime levels were observed, but the ac rophase of the VP rhythm was not phase shifted. Concerning the circadi an corticosterone peak, no phase shift of its acrophase was observed e ither. It is concluded that (1) restricted daytime feeding does affect SCN activity (2) intranuclear release of VP within the SCN is an impo rtant mechanism to amplify and synchronize the circadian rhythms as di ctated by the light/dark-entrained circadian pacemaker, and (3) VP rel ease observed in animals on restricted feeding is completely compatibl e with the previously proposed inhibitory action of SCN-derived VP on the HPA axis.