LHRH Pathways in Rat Brain: 'deafferentation' Spares a Sub-chiasmatic LHRH Projection to the Median Eminence

Localization of luteotrophic hormone-releasing hormone (LHRH) was examined by immunocytochemistry in untreated male rats and rats that received an anterior hypothalamic deafferentation. Gonadotrophic function was assessed by examining testicular weight and morphology. All of the antisera used in this study were able to reveal LHRH cell bodies and fibers. Cells from the medial preoptic area, (particularly the preoptic periventricular or median preoptic nuclei) and lateral anterior hypothalamus sent axons to the median eminence. The fiber tracts were loosely organized into (1) a tract that coursed through the organum vasculum of the lamina terminalis between the optic nerves and along the ventral surface of the optic chiasm; (2) a tract that coursed to the organum vasculum from the preoptic area and then traversed along the floor of the third ventricle; (3) a tract that coursed from the medial preoptic area to the median eminence along the lateral walls of the third ventricle; and (4) a tract that arose from the more caudal portions of the LHRH cell field in the lateral preoptic and lateral anterior hypothalamus, coursed along with the fibers of the medial forebrain bundle and turned medially at the caudal hypothalamus to enter the median eminence. An anterior cut which served most of the connections between the medial preoptic area and hypothalamus but did not penetrate through the optic chiasm served tract 3 and most of tracts 2 and 4, but spared the subchiasmatic projection (tract 1). The fibers that remained in the median eminence were sufficient to retain gonadotrophic function. This study provides an explanation for the variable effects of deafferentation and lesions of the anterior hypothalamus and preoptic area on gonadotrophic function.