FACILITATION OF THE LORDOSIS REFLEX OF FEMALE RATS FROM THE VENTROMEDIAL NUCLEUS OF THE HYPOTHALAMUS

Abstract

Effects of electrical stimulation of hypothalamic ventromedial nucleus (v.m.n.) on the lordosis reflex of female rats were examined in ovariectomized and estrogen-primed animals with chronically implanted electrodes. Lordosis triggered either by manual cutaneous stimulation or by male mounting, was facilitated by electrical stimulation of the v.m.n. A gradual increase in lordosis performance followed a relatively long period of stimulation. The optimal frequency of stimulation was between 10 and 30 Hz. Threshold for effective facilitation was, on the average, 12.5 .mu.A. Stimulation tended to induce larger facilitation when applied to the lateral side of v.m.n. Pre-treatment with estrogen was necessary to obtain facilitation by v.m.n. stimulation. The threshold dosage of estrogen was 2.5 .mu.g/animal. Stimulation was effective in adrenalectomized rats, in dexamethasone-primed animals, and in rats pre-treated with exogenous progesterone. Adrenal progesterone release is probably not required for the v.m.n. facilitation of lordosis. Medial preoptic stimulation with the same parameters suppressed the lordosis reflex. The v.m.n. participates in the control of lordosis by a facilitatory output. The delay before facilitation implies that the v.m.n. is not in the direct reflex-arc for the execution of lordosis. A summation or interaction process with an unusually long time course is involved.