Localization of luteinizing hormone-releasing hormone (LHRH) neurons that project to the median eminence.

The neuropeptide, luteinizing hormone-releasing hormone (LHRH), is released from nerve terminals in the median eminence and carried via the hypophysial portal system to the anterior pituitary, where it stimulates the release of gonadotropins. LHRH-containing neurons are located in many different regions of the rodent brain, including olfactory, septal, preoptic, and hypothalamic structures. Since those LHRH neurons that project to the median eminence form the final common pathway for the regulation of the pituitary/gonadal axis, we wished to determine which of these cell groups are afferent to this structure. A retrograde tracer, the lectin wheat germ agglutinin (WGA), was placed directly on the exposed surface of median eminence. Following survival times of 8-13 hr, brains were prepared for the dual immunocytochemical detection of WGA and LHRH. Approximately 50% of the LHRH neurons from the level of the septal nuclei caudalward were found to contain WGA immunoreactivity and therefore to project to the median eminence. The remaining single-labeled LHRH neurons were intermingled with the double-labeled cells. The 2 populations were not distinguishable from each other on either cytological or cytoarchitectonic criteria. Those LHRH neurons that were not retrogradely labeled following an injection of tracer into the median eminence are presumed to project to other regions of the central nervous system. We conclude that the LHRH neurons that are directly involved in the regulation of reproductive function are very heterogeneous, widely scattered in telencephalic and diencephalic regions.