DISTRIBUTION OF CHICKEN .2. GONADOTROPIN-RELEASING-HORMONE IN MAMMALIAN BRAIN

Brains of nonmammalian vertebrates typically contain multiple forms of gonadotropin-releasing hormone (GnRH). Until recently, only the mamma lian form of GnRH (mGnRH) had been isolated in placental mammals. Bioc hemical and histological data show that both mGnRH and chicken-II GnRH (cGnRH-II) are present in a primitive placental mammal, the musk shre w (Suncus murinus). Similar to the case in nonmammalian species, in th e musk shrew, neurons that express cGnRH-II are located in a discrete cluster in the midbrain. We have used a combination of radioimmunoassa y and immunocytochemistry, analyzed at the light level and with electr on microscopy, to describe the distribution of cGnRH-II cell bodies an d fibers in the musk shrew brain. All cGnRH-II-immunoreactive (ir) neu rons reside in the midbrain, and this area contains the greatest conce ntration of cGnRH-II peptide in the brain. At the light and electron m icrographic levels, we have identified synaptic terminals containing d ense core vesicles that-are immunoreactive for cGnRH-II in the medial habenula. Radioimmunoassay reveals that this region contains the secon d greatest concentration of cGnRH-II in the brain. Widely scattered cG nRH-II-ir fibers are present throughout the forebrain, particularly in the medial septum, hypothalamus; and midbrain central gray. Scant cGn RH-II fibers are present in the median eminence, arcuate nucleus, and infundibular stem, and only low concentrations of the peptide are dete cted in these areas. Finally, intravenous administration of mGnRH is t en times more effective than cGnRH-II in promoting ovulation. Taken to gether, the results suggest that cGnRH-II is likely to act as a neurom odulator or neurotransmitter in mammals and that the medial habenula i s the major site of cGnRH-II action in mammalian brain. (C) 1995 Wiley -Liss, Inc.