Differential distribution of gonadotropin-releasing hormone-immunoreactiveneurons in the stingray brain: Functional and evolutionary considerations

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that occurs in mult iple structural forms among vertebrate species. Bony fishes, amphibians, re ptiles, birds, and mammals express different forms of GnRH in the forebrain and endocrine regions of the hypothalamus which regulate the release of re productive gonadotropins from the pituitary. In contrast, previous studies on bony fishes and tetrapods have localized the chicken GnRH-II (cGnRH-II) nucleus in the midbrain tegmentum and, combined with cladistic analyses, in dicate that cGnRH-II is the most conserved form throughout vertebrate evolu tion. However, in elasmobranch fishes, the neuroanatomical distribution of cGnRH-II and dogfish GnRH (dfGnRH) cells and their relative projections in the brain are unknown We used high-performance liquid chromatography and ra dioimmunoassay to test for differential distributions of various GnRK forms in tissues from the terminal nerve (TN) ganglia, preoptic area, and midbra in of the Atlantic stingray, Dasyatis sabina. These experiments identified major peaks that coelute with cGnRH-II and dfGnRH, minor peaks that coelute with lamprey GnRH-III (lGnRH-III), and unknown forms. Immunocytochemistry experiments on brain sections show that dfGnRH-immunoreactive (-ir) cell bo dies are localized in the TN ganglia, the caudal ventral telencephalon, and the preoptic area. Axons of these cells project to regions of the hypothal amus and pituitary, diencephalic centers of sensory and behavioral integrat ion, and the midbrain. A large, discrete, bilateral column of cGnRH-II-ir n eurons in the midbrain tegmentum has sparse axonal projections to the hypot halamus and regions of the pituitary but numerous projections to sensory pr ocessing centers in the midbrain and hindbrain. Immunocytochemical and chro matographic data are consistent with the presence of lGnRH-III and other Gn RH forms in the TN that differ from dfGnRH and cGnRH-II. This is the first study that shows differential distribution of cGnRH-II and dfGnRH in the el asmobranch brain and supports the hypothesis of different function of GnRH variants related to gonadotropin control and neuromodulation of sensory fun ction. (C) 2000 Academic Press.