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Comparable surges of luteinizing hormone induced by preoptic or medial basal tuberal electrical stimulation in spontaneously persistent estrous or cyclic proestrous rats.

A previous report demonstrated in cyclic proestrous rats electrically stimulated in the medial preoptic area (MPOA) or the arcuate nucleus-median eminence (ARC/ME) region a close parallel in the progressive increase of serum LH with increasing stimulation time. The present report compares the LH surges induced in middle aged spontaneously persistent estrous (SPE) rats by electrical stimulation of the two regions. Electrical parameters were those previously used. In Series I, MPOA stimulation of SPE rats for 60 min or ARC/ME stimulation for 45 min produced serum LH concentrations 60 and 90 min after stimulation began that were essentially identical to those in proestrous rats similarly treated. In Series II, stimulations were prolonged to 120 min to determine whether LH would continue to rise with the longer stimulation. These rats were all chosen from one shipment and all were born on the same day. Sequential blood samples were taken before stimulation and 60, 90, 120, and 150 min after stimulation began. SPE rats were stimulated in either the MPOA or the ARC/ME and, for comparison, a few proestrous rats that had remained cyclic were stimulated in the MPOA. LH concentrations in these cyclic rats rose more abruptly than in the previous study to amounts significantly higher at both 60 and 90 min, but by 120 min the respective means were not significantly different. In the SPE rats, the 60-min LH levels resembled those in Series I. They continued to rise during the second hour, but only to average levels approximately half those attained in proestrous rats stimulated for 120 min. The comparative increments in MPOA-stimulated and ARC/ME-stimulated rats were similar, although all means in the latter were somewhat higher. The results agree with previous indications that stimulation of either the MPOA or the ARC/ME activates the rostral or caudal portions of a unitary preoptic-tuberal system. Although the tendency of SPE rats to release smaller amounts of LH may result from age-related lowered pituitary responsiveness to LHRH, only future study can determine whether a given degree of preoptic-tuberal stimulation releases equivalent amounts of LHRH in SPE and proestrous rats. Nevertheless, the absence of spontaneous LH surges in SPE points to a primary deficiency in control mechanisms impinging on the preoptic-tuberal system or in its responsiveness thereto.