Fos expression in luteinizing hormone-releasing hormone neurons of guinea pigs, with knife cuts separating the preoptic area and the hypothalamus, demonstrating luteinizing hormone surges.

LHRH neurons in guinea pigs, as in primates and other non-rodent species, are broadly distributed in the basal forebrain. In this study, knife cuts were made in the anterior hypothalamus, effectively separating more caudally positioned hypothalamic LHRH neurons from those in rostral preoptic areas. Guinea pigs with knife cuts displayed an LH surge in response to steroid administration. There was no significant difference in the number of LHRH neurons that expressed Fos in conjunction with an LH surge, although fewer total LHRH neurons were detected in the forebrain of knife-cut versus sham-cut animals. Knife-cut animals displayed a larger percentage of LHRH/Fos neurons in one region of the caudal hypothalamus than sham-cut animals. The area and perimeter of the LHRH reaction product within the cytoplasm of LHRH/Fos neurons were smaller than those of single-labeled LHRH neurons in sham-cut animals and in the caudal hypothalamus, but not the rostral preoptic area, of knife-cut animals. We conclude that caudal hypothalamic LHRH neurons separated from rostral preoptic regions are capable of sustaining an LH surge in guinea pigs. This finding is important, as LHRH neurons are present in the caudal hypothalamus, as well as in preoptic areas, of a large number of mammalian species, including humans.