EXPRESSION OF FOS WITHIN LUTEINIZING-HORMONE-RELEASING HORMONE NEURONS, IN RELATION TO THE STEROID-INDUCED LUTEINIZING-HORMONE SURGE IN GUINEA-PIGS

The induction of Fos protein was examined within LHRH neurons of guine a pigs; the aim was to delineate relationships between subgroups of LH RH neurons during an LH surge in a laboratory rodent in which the dist ribution of LHRH neurons and the presence of a true luteal phase in th e reproductive cycle resemble those in primates. Approximately one thi rd of the forebrain population of LHRH neurons was examined in ovariec tomized steroid-treated guinea pigs killed either before or during a s teroid-induced LH surge, LHRH/Fos double-labeled neurons were more abu ndant in surging compared to presurge guinea pigs (p = 0.008) and were most abundant within the preoptic area and anterior hypothalamus. Non etheless, double-labeled LHRH/Fos neurons were observed throughout the remainder of the population of LHRH neurons in surging guinea pigs. A relative loss of LHRH reaction product was detected by image analysis in the LHRH terminals in the median eminence of surging guinea pigs, consistent with augmented LHRH release. Thus, there appears to be a co ordinated increase in Fos expression in subgroups of LHRH neurons, mor e pronounced in rostral, as compared to caudal, regions in guinea pigs killed after the peak of the steroid-induced LH surge.