Gonadotropin-releasing hormone neurons in the preoptic-hypothalamic region of the rat contain lamprey gonadotropin-releasing hormone III, mammalian luteinizing hormone-releasing hormone, or both peptides.

This study utilized a newly developed antiserum, specific for lamprey gonadotropin-releasing hormone III (l-GnRH-III), to determine the following: in which regions of the rat hypothalamus the neuronal perikarya producing l-GnRH-III are localized; and whether this peptide, known to selectively induce follicle-stimulating hormone release, is coexpressed in neurons containing mammalian luteinizing hormone-releasing hormone (m-LHRH). Double-label immunocytochemistry was performed by using an l-GnRH-III polyclonal antiserum and an LHRH monoclonal antiserum. Immunopositive neurons for l-GnRH-III, m-LHRH, or neurons coexpressing both peptides were detected within the organum vasculosum lamina terminalis (OVLT) region of the preoptic area (POA). Caudal to the OVLT, l-GnRH-III-positive neurons were also observed dorso-medially, above the third ventricle in the medial POA. The m-LHRH neurons were not observed in this area. The lateral POA region contained neurons positive for both peptides along with single-labeled neurons for each peptide. Importantly, neurons that expressed l-GnRH-III, m-LHRH, or both peptides were also detected in the ventral regions of the rostral hypothalamus, dorsolateral to the borders of the supraoptic nuclei. In both of these latter areas, neurons containing l-GnRH-III were slightly dorsal to neurons containing only m-LHRH. The l-GnRH-III perikarya and fibers were eliminated by absorption of the primary antiserum with l-GnRH-III, but not by l-GnRH-I, chicken-GnRH-II, or m-LHRH. These results indicate that, unlike other isoforms of GnRH found in the mammalian brain, l-GnRH-III neurons not only are observed in regions that control follicle-stimulating hormone release but also are colocalized with m-LHRH neurons in areas primarily controlling LH release. These findings suggest an interrelationship between these two peptides in the control of gonadotropin secretion.