Hypothalamic LHRH and plasma LH levels after electrical stimulation of the deafferented medial basal hypothalamus.

The medial basal hypothalamus (MBH) of female rats was surgically isolated on the morning of proestrus and luteinizing hormone (LH) levels in the blood were determined before and after electrochemical stimulation (ECS) of the disconnected arcuate-median eminence (ARC-ME) region either on the same afternoon (Group 1) or on the 5th postoperative day (Group 2). Animals of Groups 3 and 4 were stimulated and sampled for LH on the 5th or 10th postoperative day, respectively, these rats having been primed with 5 micrograms estradiol injected daily throughout the experimental period. ECS resulted in a significant rise of plasma LH level in Group 1 and caused full ovulation as evaluated by the presence of ova found the next morning in the Fallopian tubes. Rats of Groups 2-4 failed to show any changes in plasma LH, and no ovulation was observed following ECS. Immunohistochemical examination of the brains revealed that axons staining for the luteinizing hormone-releasing hormone (LHRH) in the ARC-ME region remained at control levels 3 days after deafferentation (e.g. Group 1), whereas a marked decrease or complete absence of these structures was observed 8 or 13 days following surgical isolation of the MBH (e.g. Groups 2-4). These studies strongly support the view that no LHRH synthesizing perikarya are located within the MBH of the rat.