Localization of immunoreactive lamprey gonadotropin-releasing hormone in the rat brain

A highly specific antiserum against lamprey gonadotropin-releasing hormone (GnRH) was used to localize 1-GnRH in areas of the rat brain associated wit h reproductive function. Immunoreactive 1-GnRH-like neurons were observed i n the ventromedial preoptic area (POA), the region of the diagonal band of Broca and the organum vasculosum lamina terminalis, with fiber projections to the rostral wall of the third ventricle and the organum vasculosum lamin a terminalis. Another population of 1-GnRH-like neurons was localized in th e dorsomedial and lateral POA, with nerve fibers projecting caudally and ve ntrally to terminate in the external layer of the median eminence. Other fi bers apparently projected caudally and circumventrically to terminate aroun d the cerebral aqueduct in the mid-brain central gray. By using a highly sp ecific antiserum directed against mammalian luteinizing hormone-releasing h ormone (m-LHRH), the localization of the LHRH neuronal system was compared to that of the 1-GnRH system. There were no LHRH neurons in the dorsomedial or the lateral region of the POA that contained the 1-GnRH neurons. As exp ected, there was a large population of LHRH neurons in the ventromedial POA associated with the diagonal band of Broca and organum vasculosum lamina t erminalis: in both of these regions, there were many more LHRH neurons than 1-GnRH neurons and the LHRH neurons extended more dorsally and laterally t han the 1-GnRH neurons. The LHRH neurons seemed to project to the median em inence in the same areas as those that were innervated by the 1-GnRH neuron s. Absorption studies indicated that 1-GnRH cell bodies were eliminated by adding 1 mu g of either 1-GnRH-I or 1-GnRH-III, but not m-LHRH to the antis erum before use. Fibers were largely eliminated by the addition of 1 mu g 1 -GnRH-III to the antiserum. No chicken GnRH-II neurons or nerve fibers coul d be visualized by immunostaining. Because the antiserum recognized GnRH-I and GnRH-III equally, we have visualized an 1-GnRH system in rat brain. The results are consistent: with the presence of either one or both of these p eptides within the rat hypothalamus. Because 1-GnRH-I has only weak nonsele ctive gonadotropin-releasing activity, whereas 1-GnRH-III is a highly selec tive releaser of, follicle-stimulating, hormone, and because 1-GnRH neurons are located in areas known to control follicle-stimulating hormone release selectively, our results support the hypothesis that 1-GnRH-III, or a clos ely related peptide, may be mammalian follicle-stimulating hormone-releasin g factor. (C) 1999 Elsevier Science Inc. All rights reserved.