HYPOTHALAMIC VENTROMEDIAL NUCLEI AMPLIFY CIRCADIAN-RHYTHMS - DO THEY CONTAIN A FOOD-ENTRAINED ENDOGENOUS OSCILLATOR

Several endogenous oscillators determine circadian rhythms. One, light -entrained, is in the suprachiasmatic nuclei (SCN), the others, food-e ntrained, are in unknown sites. To determine how the hypothalamic vent romedial nuclei (VMN) and feeding affect rhythms, we compared nocturna lly active rats fed either ad libitum or for 2 hr/d during light [rest ricted feeding (RF)] and either with or without colchicine-induced dis ruption of VMN. We measured rhythms in temperature, locomotor activity , feeding, drinking, corticosterone, and the numbers of cells expressi ng c-Fos in light/dark in hypothalamic nuclei, the suprachiasmatic nuc lei, and two major SCN targets, the subparaventricular zone (sPVNz) an d paraventricular thalamus (pvTHAL). c-Fos cells were always light > d ark in SCN, whereas the VMN and sPVNz lacked light/dark differences ex cept after RF and RI-plus VMN disruption, respectively. Controls fed a d libitum had high-amplitude rhythms and, generally, c-Fos cells dark > light. In RF controls, a c-Fos pattern dark > light occurred in VMN; generally, c-Fos cell numbers increased elsewhere maintaining dark > light. By contrast, levels of corticosterone peaked before food. In ra ts fed ad libitum, VMN with colchicine markedly reduced rhythm amplitu des, not phase. c-Fos patterns were abolished except in pvTHAL and SCN . In RF, VMN disruption blocked corticosterone and light/dark c-Fos pa tterns in all nuclei but produced a pattern in the sPVNz like SCN. We conclude that VMN amplify rhythmic output from the SCN, and the RF-ind uced rhythm in VMN enhances c-Fos activity driven by the SCN. The VMN may contain a food-entrained oscillator, and the sPVNz may integrate o utput from several oscillators.