Effects of guanethidine sympathectomy on ventromedial hypothalamic obesity

Since ventromedial hypothalamic (VMH) lesions apparently produce chronic increases in parasympathetic activity and decreases in sympathetic responses, two experiments were performed to determine whether decreases in sympathetic tone alone are sufficient either to produce elements of the VMH syndrome or to potentiate the expression of the syndrome in animals with lesions. In experiment 1, rats that had been injected with guanethidine sulfate for 5 wk (40 mg X kg-1 X day-1) to produce a permanent sympathectomy (SympX) were maintained on a high-fat diet and subsequently subjected to VMH lesions. In experiment 2, adrenal-demedullated animals were treated with guanethidine for 6 wk and then subsequently underwent VMH lesions. SympX in the studies (81 and 85% depletion of superior cervical ganglia neurons, respectively) did duplicate the effects of VMH lesions on salivary gland weights. In neither experiment, however, did SympX alone or in combination with demedullation mimic the effects of VMH lesions on food intake, body weight, or body fat. Determinations of free fatty acids (FFAs) in the experiments confirmed that VMH lesions elevate basal FFA levels, but they also indicated that effective VMH lesions need not impair the FFA mobilization to 2-deoxyglucose challenges as some basomedial hypothalamic lesions do. In addition, experiment 2, which employed a prolonged period of high-fat feeding, revealed that SympX plus adrenal demedullation could potentiate (a 13% increase) the effects of VMH lesions on body weight. The results taken together do not support the conclusion that a preponderance of the VMH syndrome can be accounted for by the type of reduction in sympathetic tone produced by guanethidine.