THE NUMBER AND DISTRIBUTION OF DETECTABLE LUTEINIZING-HORMONE (LH)-RELEASING HORMONE CELL-BODIES CHANGES IN ASSOCIATION WITH THE PREOVULATORY LH SURGE IN THE BRAINS OF YOUNG BUT NOT MIDDLE-AGED FEMALE RATS

The population of LHRH neurons was examined in young and middle-aged f emale rats at two different time points associated with the LH surge o n proestrus. More immunopositive,LHRH neurons were detected in the bra ins of young but not middle-aged females killed on the evening of proe strus (1830-2030 h) compared with those killed earlier in the afternoo n (1300-1430 h). To facilitate the investigation of potential regional differences within the population, sections through the basal forebra in were divided into four areas based upon their rostral-caudal positi on. Whereas no change in detectability was observed in the most rostra l subset of LHRH neurons, more LHRH perikarya were observed at the lat er compared to the earlier time point in the other three areas surveye d in the brains of young females. Only the increase observed in sectio ns through the preoptic area, however, reached statistical significanc e. Computer assisted three-dimensional reconstruction of all LHRH neur ons in sections through the preoptic area revealed a marked increase i n a subgroup of LHRH neurons in the dorsomedial aspect of this region of the population. Changes in detectability of LHRH neurons in associa tion with the LH surge suggest that these neurons may be involved in r egulating preovulatory LH release. Examination of additional time poin ts could reveal changes in other subgroups of LHRH neurons as well as significant changes in detectability of LHRH neurons in aging brains.