RESUMEN
Although teleost fish have higher levels of brain aromatase activity than any other vertebrate group, its function remains speculative, and no study has identified its cellular basis. A previous study determined aromatase activity in a vocal fish, the plainfin midshipman (Porichthys notatus), and found highest levels in the telencephalon and lower levels in the sonic hindbrain, which was dimorphic between and within (males) sexes. We have now localized aromatase-containing cells in the midshipman brain both by immunocytochemistry using teleost-specific aromatase antibodies and by in situ hybridization using midshipman-specific aromatase probes. Aromatase-immuno-reactivity and mRNA hybridization signal are consistent with relative levels of aromatase activity in different brain regions: concentrated in the dimorphic sonic motor nucleus, in a band just beneath the periaqueductal gray in the midbrain, in ventricular regions in the hypothalamus, and highest levels in the telencephalon especially in preoptic and ventricular areas. Surprisingly, double-label immunofluorescence does not show aromatase-immunoreactive colocalization in neurons, but instead in radial glia throughout the brain. This is the first study to identify aromatase expression mostly, if not entirely, in glial cells under normal rather than brain injury-dependent conditions. The abundance of aromatase in teleosts may represent an adaptation linked to continual neurogenesis that is known to occur throughout an individual's lifetime among fishes. The localization of aromatase within the intersexually and intrasexually dimorphic vocal-motor circuit further implies a function in the expression of alternative male reproductive phenotypes and, more generally, the development of natural, individual variation of specific brain nuclei.
Asunto(s)
Aromatasa/biosíntesis , Encéfalo/enzimología , Neuroglía/enzimología , ARN Mensajero/biosíntesis , Animales , Especificidad de Anticuerpos , Aromatasa/genética , Batrachoidiformes , Encéfalo/citología , Encéfalo/fisiología , Técnica del Anticuerpo Fluorescente , Hipotálamo/citología , Hipotálamo/enzimología , Inmunohistoquímica , Hibridación in Situ , Mesencéfalo/citología , Mesencéfalo/enzimología , Neuroglía/citología , Sustancia Gris Periacueductal/citología , Sustancia Gris Periacueductal/enzimología , Caracteres Sexuales , Telencéfalo/citología , Telencéfalo/enzimología , Vocalización Animal/fisiologíaRESUMEN
Gonadotropin-releasing hormone (GnRH) is a neuropeptide that occurs in multiple structural forms among vertebrate species. Bony fishes, amphibians, reptiles, birds, and mammals express different forms of GnRH in the forebrain and endocrine regions of the hypothalamus which regulate the release of reproductive 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, indicate that cGnRH-II is the most conserved form throughout vertebrate evolution. 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 radioimmunoassay to test for differential distributions of various GnRH forms in tissues from the terminal nerve (TN) ganglia, preoptic area, and midbrain 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 bodies are localized in the TN ganglia, the caudal ventral telencephalon, and the preoptic area. Axons of these cells project to regions of the hypothalamus and pituitary, diencephalic centers of sensory and behavioral integration, and the midbrain. A large, discrete, bilateral column of cGnRH-II-ir neurons in the midbrain tegmentum has sparse axonal projections to the hypothalamus and regions of the pituitary but numerous projections to sensory processing centers in the, midbrain and hindbrain. Immunocytochemical and chromatographic data are consistent with the presence of lGnRH-III and other GnRH 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 elasmobranch brain and supports the hypothesis of divergent function of GnRH variants related to gonadotropin control and neuromodulation of sensory function.