Some effects of ventromedial hypothalamic lesions, adrenalectomies, and corticosterone replacement therapy on energy expenditure in rats

Abstract

The purpose of the present study was to test the hypothesis that decreases in energy-expenditure reported following VMH lesions in rats were a function of decreases in adrenal function. Five groups of rats were given acquisition training in Wahman activity-wheels for 21 consecutive one-hour per day sessions and then were subjected to either VMH lesions, adrenalectomies, the respective sham-operations, or were subjected to neither sham-operation. At the time of surgery two fat pads were removed from each animal and the rates of free fatty acid mobilization were determined. Following surgery each subject was again given access to an activity-wheel for a one-hour test session every other day for a total of 24 measurement days. During the postsurgical test phase the subjects were maintained on a sequence of drug/no drug conditions which consisted of: 1. no injection; 2. injections of the vehicle substance; or 3. injections of corticosterone in the vehicle substance. Under the no drug conditions there were significant decreases in the running levels of the VMH-lesioned and adrenalectomized subjects although the VMH-lesioned animals were the more severely depressed. Corticosterone replacement therapy was effective in reinstating the activity levels of the adrenalectomized subjects but produced no increment in those of the VMH subjects. Free fatty acid mobilization rates taken at the time of sacrifice under the influence of corticosterone or the vehicle substance reflected similar results. The hypothesis that the reductions in activity following destruction of the VMH or adrenalectomy are due to disruption of the same mechanism was not supported.