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.

The origin, course and termination of two luteinizing hormone releasing hormone (LH-RH)-containing pathways were studied in the brain of rats by immunohistochemistry. In addition, the joint termination of LH-RH-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 three fascicles. Fibres of the median fascicle run in caudal direction on the dorsal and ventral surfaces of the optic chiasma. Fibres of the medial fascicle run along the lateral border of the SCH towards the retrochiasmatic area. Fibres of the lateral fascicle course laterally in close contact with the concave dorsomedial surface of the optic tract. These latter fibres gradually turn in medial direction and reach the lateral margin of the ME. The caudal LH-RH fibre tract was characterized only partially because the perikarya giving rise to these fibres were not identified. The fibres run rostrally among the ependymal cells of the mesencephalic aqueduct and form two fascicles. The fibres of the mammillary fascicle enter, and seem to terminate, in the medial mammillary nucleus, while the fibres of the tuberal fascicle course rostrally and join the fibres of the preoptico-infundibular tract. The surface areas of the brain along which LH-RH fibres terminate (retrochiasmatic area, inframammillary region, lateral margin of the ME) were found to display a special surface covering formed by tanycyte processes. The course of LH-RH fibre 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 LH-RH fibres and tanycyte processes, especially in areas where portal capillaries are absent, remains unclear.