REDUCED PROPORTION OF LUTEINIZING-HORMONE (LH)-RELEASING HORMONE NEURONS EXPRESS FOS PROTEIN DURING THE PREOVULATORY OR STEROID-INDUCED LH SURGE IN MIDDLE-AGED RATS

Results of previous studies suggest that altered patterns of LHRH neur osecretion contribute to attenuated LH surges and the eventual cessati on of ovulation in aging female rats. The present study compared evide nce of LHRH neuronal activation in conjunction with the preovulatory a nd steroid-induced LH surge in young and middle-aged animals to determ ine whether age-related alterations could be detected. Double immunocy tochemical protocols were used to colocalize LHRH and the protein prod uct of the proto-oncogene c-fos, which increases within the nucleus of LHRH neurons in association with spontaneous or induced LH surges. Th e mean proportion of LHRH neurons containing immunoreactive Fos was hi gher in the brains of young compared to middle-aged females in associa tion with both the preovulatory (P < 0.01) and the steroid-induced LH surge (p < 0.001). The time course of activation of LHRH neurons was d elayed in the brains of aging females, and the proportion of double-la beled LHRH neurons remained elevated longer in the brains of young com pared to middle-aged steroid-treated females. Moreover, regional diffe rences in LHRH neuronal activation were observed both within and betwe en age groups. The data presented suggest that reduced LHRH neuronal a ctivation may contribute to the attenuation and eventual loss of preov ulatory LH surges in middle-aged female rats.