THE HYPOTHALAMIC VENTROMEDIAL NUCLEI COUPLE ACTIVITY IN THE HYPOTHALAMO-PITUITARY-ADRENAL AXIS TO THE MORNING FED OR FASTED STATE

Function in the adrenocortical system is markedly altered by availabil ity of food. Basal activity is lowest and stress responsivity highest in the morning when nocturnal rats eat similar to 90% of their daily c alories during the dark. After an overnight fast, basal corticotrophin and corticosteroid levels are elevated, and responsivity to stressors is decreased. Central neural sites that control these changes are uni dentified. The hypothalamic ventromedial nuclei (VMN) appear to signal satiety; lesions result in increased food intake, obesity, and elevat ed basal insulin and corticosteroids. Thus, the VMN are good candidate s for calorically mediated control of adrenocortical system function i n satiated rats. We injected colchicine into the VMN to cause reversib le inhibition of activity (Avrith and Mogenson, 1978) and tested the e ffects on basal and stimulated function in the adrenocortical system. Colchicine-injected rats that fed ad libitum exhibited increased basal but reduced corticotrophin and corticosterone responses to restraint in the morning compared with controls. By contrast, after an overnight fast, control rats had increased basal adrenocortical hormones and de creased stress responses that did not differ from colchicine-injected rats. Colchicine was visualized within cells in the VMN for up to 5 d using fluorescein/colchicine, and the treatment did not cause increase d gliosis; moreover, the functional effects of the injections were rev ersed within 15 d. We conclude that (I) the VMN serve to couple activi ty in the adrenocortical system to energy intake and (2) discrete colc hicine injections provide a behaviorally and neuroendocrinologically u seful period of inhibition without causing permanent functional damage .