THE LUTEINIZING-HORMONE RELEASING HORMONE-CONTAINING PATHWAYS AND THEIR CO-TERMINATION WITH TANYCYTE PROCESSES IN AND AROUND THE MEDIAN-EMINENCE AND IN THE PITUITARY-STALK OF THE RAT

Abstract

The origin, course and termination of 2 LHRH-containing pathways were studied in the brain of rats by immunohistochemistry. The joint termination of LHRH- containing nerve terminals and tanycyte processes over a rostro-caudally elongated belt surrounding the median eminence (ME) is described. The preoptico-infundibular tract originates from neurons confined to a pyramid-shaped area between the anterior commissure, the organum vasculosum laminae terminalis (OVLT) and the most caudal level of the suprachiasmatic nucleus (SCH). The axons of these neurons form 3 fascicles. Fibers of the median fascicle run in caudal direction on the dorsal and ventral surfaces of the optic chiasma. Fibers of the medial fascicle run along the lateral border of the SCH towards the retrochiasmatic area. Fibers of the lateral fascicle course laterally in close contact with the concave dorsomedial surface of the optic tract. These latter fibers gradually turn in medial direction and reach the lateral margin of the ME. The caudal LHRH fiber tract was characterized only partially because the perikarya giving rise to these fibers were not identified. The fibers run rostrally among the ependymal cells of the mesencephalic aqueduct and form 2 fascicles. The fibers of the mammillary fascicle enter, and seem to terminate, in the medial mammillary nucleus, while the fibers of the tuberal fascicle course rostrally and join the fibers of the preoptico-infundibular tract. The surface areas of the brain along with LHRH fibers terminate (retrochiasmatic area, inframammillary region, lateral margin of the ME) displayed a special surface covering formed by tanycyte processes. The course of LHRH fiber tracts described here is in good agreement with earlier experimental results on the localization of neural pathways controlling ovulation and the secretion of pituitary gonadotropic hormones. The significance of the joint termination of LHRH fibers and tanycyte processes, especially in areas where portal capillaries are absent, remains unclear.