Differential Hypothalamic Mechanisms Inciting Ovulation and Pseudopregnancy in the Rat

Abstract

By techniques of brain stimulation, neural mechanisms initiating pseudopregnancy in rats were distinguished from other neural mechanisms controlling ovulation. Although ovulatlon can regularly be induced during late diestrus or proestrus by unilateral "electrochemical" stimulation of the medial preoptic area under pento-barbital, pseudopregnancy proved to be infrequent sequel when such animals were observed for appropriate lengths of time. On the other hand, pseudopregnancy was regularly evoked by unilateral "electrical" stimulation of the tuberal region at sites and with electrical parameters not commonly yielding ovulation. "Electrochemical" stimulation in the present experiments was produced by a small direct current passed briefly through a stainless steel electrode. "Electrical" stimulation through either stainless steel or platinum electrodes was accomplished with trains of biphasic matched pairs of rectangular pulses. All electrodes were concentric bipolar units of 0.4 mm tubing with 0.13 mm cores, insulated to the tips, thus ensuring significant localization. Electrode positions were confirmed histologically. If an ovulation-inducing electrochemical stimulation of the preoptic region was administered in conjunction with electrical stimulation of the tuber, pseudopregnancy was initiated when corpora lutea formed that night; otherwise, if only the latter stimulus was given, corpus luteum formation and the start of pseudopregnancy were characteristically delayed an additional 24 hr, a striking parallel with delayed pseudopregnancy responses to cervical stimulation or infertile copulation. Sites of stimulation regularly leading to pseudopregnancy were in the medial hy-pothalamus, ranging from the anterior hypothalamic area and para-ventricular nuclei caudally into the premammillary complex and dorsally to include the dorsomedial nucleus. The medial preoptic areayieldedfew responses, and results of stimulating in and about the posterior nucleus and the mammillary peduncle were inconsistent. Electrode insertion into the basal tuberal region without current gave essentially negative results.