Function in the adrenocortical system is markedly altered by availability of food. Basal activity is lowest and stress responsivity highest in the morning when nocturnal rats eat approximately 90% of their daily calories 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 unidentified. The hypothalamic ventromedial nuclei (VMN) appear to signal satiety; lesions result in increased food intake,
obesity, and elevated basal
insulin and
corticosteroids. Thus, the VMN are good candidates for calorically mediated control of adrenocortical system function in satiated rats. We injected
colchicine into the VMN to cause reversible inhibition of activity (Avrith and Mogenson, 1978) and tested the effects 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 decreased 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 increased
gliosis; moreover, the functional effects of the
injections were reversed within 15 d. We conclude that (1) the VMN serve to couple activity in the adrenocortical system to energy intake and (2) discrete
colchicine injections provide a behaviorally and neuroendocrinologically useful period of inhibition without causing permanent functional damage.