RECONSTRUCTIONS OF POPULATIONS OF LUTEINIZING-HORMONE-RELEASING HORMONE NEURONS IN YOUNG AND MIDDLE-AGED RATS REVEAL PROGRESSIVE INCREASES IN SUBGROUPS EXPRESSING FOS PROTEIN ON PROESTRUS AND AGE-RELATED DEFICITS

Fos expression has been used as a marker of activation of neuroendocri ne cells including LHRH neurons. In this study, Fos protein was locali zed within LHRH neurons in young and middle-aged rats to trace the tem poral and spatial pattern of LHRH neuronal activation associated with the preovulatory LH surge. Animals were killed during the late morning , afternoon, and evening of proestrus. Dual immunocytochemical protoco ls localized LHRH and LHRH/Fos neurons, and computer-assisted methods were used to reconstruct forebrain populations of single- and double-l abeled LHRH neurons. Although a significant increase in the number of LHRH/Fos neurons was noted by evening in both age groups, a greater in crease was observed in young (12% in morning, 28% in afternoon, and 62 % by evening) compared with aging females (5% in morning, 10% in after noon, and 40% by evening). Reconstructions of LHRH and LHRH/Fos neuron s revealed time- and age-dependent differences in Fos expression withi n LHRH neurons. In young females, LHRH/Fos neurons were restricted to central regions of the population of LHRH neurons on the morning of pr oestrus. By evening, Fos expression was also observed in more peripher al and caudal LHRH neurons. In middle-aged females, Fos expression was restricted to ventral subgroups of LHRH neurons on the afternoon of p roestrus. By evening, more LHRH neurons contained Fos protein, however , few were located in the dorsal aspect of the population. These data trace the progressive increase in activation of LHRH neurons during th e preovulatory LH surge in young females and reveal deficits in this p attern of activation by middle age.