Topography and associations of luteinizing hormone-releasing hormone and neuropeptide Y-immunoreactive neuronal systems in the human diencephalon

Neuropeptide Y (NPY) potentiates the effect of luteinizing hormone-releasin g hormone (LHRH) on luteinizing hormone secretion in several species, inclu ding human. In addition to the pituitary sites, the interactions of the NPY and LHRH systems may involve diencephalic loci. However, the morphologic b asis of this putative communication has not yet been elucidated in the huma n brain. To discover interaction sites, the distribution and connections of LHRH and NPY-immunoreactive (IR) neuronal elements in the human hypothalam us were investigated by means of light microscopic single- and double-label immunocytochemistry. NPY-IR perikarya and fibers were found to be widely d istributed in the ventral diencephalon, with high densities in the preoptic oseptal, periventricular, and tuberal regions. Small neuronal cell groups w ere infiltrated with a dense network of varicose NPY-IR fibers in the later al preoptic area. The LHRH-IR perikarya were located mainly in the preoptic oseptal region, diagonal band of Broca, lamina terminalis, and periventricu lar and infundibular nuclei. A few LHRH-IR neurons and fibers were scattere d in the mamillary region. The overlap between the NPY and LHRH systems was apparent in the periventricular, paraventricular, and infundibular nuclei. Double-labeling immunohistochemistry showed NPY-IR axon varicosities in co ntact with LHRH-IR perikarya and main dendrites. The putative innervation o f LHRH neurons by NPY-IR fibers was also seen in 1-mum-thick plastic sectio ns and with confocal laser scanning microscope, thus further supporting the functional impact of NPY-IR terminals on LHRH-IR neurons. The present find ings suggest that the hypophysiotropic LHRH-synthesizing neurons may be inn ervated by intrahypothalamic NPY-IR fibers. Confirmation by ultrastructural analysis would demonstrate that the LHRH system in the human hypothalamus is regulated by NPY, as has been demonstrated in nonhuman species. J. Comp. Neurol. 427:593-603, 2000. (C) 2000 Wiley-Liss, Inc.