Effect of Lesion Size in the Ventromedial Hypothalamus on Growth Hormone and Insulin Levels in Weanling Rats

Electrolytic lesions of three different sizes were placed in and around the ventromedial hypothalamic nuclei (VMN) in weanling rats. After three weeks, during which food intake was measured, the rats were killed and the following parameters were measured: plasma growth hormone (GH), insulin and glucose, carcass fat, water and lean body mass, pituitary weight and GH content, and ponderal and linear growth. Progressive decreases in pituitary weight, pituitary and plasma GH, linear growth, and carcass water and lean body mass were produced by increasing lesion size. Concomitant increases occurred in plasma insulin and carcass fat. Ponderal growth and plasma glucose were unaffected by any of the lesions and none of the experimental groups exhibited hyperphagia. The data indicate that the VMN are a control site for GH and insulin secretion. The lack of correlation between parameters of growth and those of obesity suggests that the two hormonal control mechanisms are subserved by independent neuronal assemblies.