Close juxtapositions between luteinizing hormone-releasing hormone-immunoreactive neurons and corticotropin-releasing factor-immunoreactive axons in the human diencephalon.

Gonadal functions are modulated by corticotropin-releasing factor (CRF) in the rat via direct suppression of LH-releasing hormone (LHRH) release. Although there is evidence of direct morphological contacts between the LHRH and CRF-immunoreactive (-IR) structures in the rat hypothalamus, little is known about the morphological base of CRF-influenced LHRH release in man. Thus, we studied the distribution of the CRF-IR and LHRH-IR systems in the human diencephalon and revealed putative CRF-LHRH juxtapositions using double label immunohistochemistry. LHRH-IR cells were present mainly in the infundibular region and the medial preoptic area. CRF-IR neuronal structures were observed in the periventricular area, paraventricular nucleus, infundibular region, and median eminence. CRF-LHRH juxtapositions were found mainly in the infundibulum and median eminence. Few juxtapositions were detected in the medial preoptic area. In these regions, black diaminobenzidine/silver-labeled CRF-IR fibers abutted fusiform brown diaminobenzidine-labeled LHRH neurons, usually forming multiple contacts. Examination of semithin sections of these close associations with the aid of oil immersion revealed no cleft between CRF-IR nerve terminals contacting LHRH-IR structures. These findings suggest that the juxtapositions between the LHRH-IR and CRF-IR neurons may be functional synapses forming the morphological substrate of the CRF-controlled LHRH secretion. Moreover, the wide distribution of CRF-IR elements suggests that CRF controls other diencephalic functions as well.