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 female rats at two different time points associated with the LH surge on proestrus. More immunopositive LHRH neurons were detected in the brains of young but not middle-aged females killed on the evening of proestrus (1830-2030 h) compared with those killed earlier in the afternoon (1300-1430 h). To facilitate the investigation of potential regional differences within the population, sections through the basal forebrain were divided into four areas based upon their rostral-caudal position. Whereas no change in detectability was observed in the most rostral subset of LHRH neurons, more LHRH perikarya were observed at the later compared to the earlier time point in the other three areas surveyed in the brains of young females. Only the increase observed in sections through the preoptic area, however, reached statistical significance. Computer assisted three-dimensional reconstruction of all LHRH neurons in sections through the preoptic area revealed a marked increase in a subgroup of LHRH neurons in the dorsomedial aspect of this region of the population. Changes in detectability of LHRH neurons in association with the LH surge suggest that these neurons may be involved in regulating preovulatory LH release. Examination of additional time points could reveal changes in other subgroups of LHRH neurons as well as significant changes in detectability of LHRH neurons in aging brains.