Distribution of gonadotropin-releasing hormone immunoreactivity in the brain ofIchthyophis beddomei (amphibia: gymnophiona)

Abstract

From a comparative viewpoint, we have investigated the presence and neuroanatomical distribution of gonadotropin-releasing hormone (GnRH)-immunoreactive material in the brain of a gymnophione amphibian, Ichthyophis beddomei. Immunocytochemical analysis of the adult brain and terminal nerves in both sexes shows the presence of neurons and fibers containing mammalian GnRH (mGnRH)- and chicken GnRH-II (cGnRH-II)-like peptides. With respect to GnRH-immunoreactive material, there are two distinct neuronal systems in the brain: one containing mGnRH, which is located in the forebrain and terminal nerve, and the other containing cGnRH-II, which is restricted to the midbrain tegmentum. Basically, this distribution pattern parallels that of many species of anurans and a urodele. Whereas the presence of cGnRH-II-immunoreactive fibers in the dorsal pallium of L. beddomei is a feature in common with a urodele amphibian, the total absence of cGnRH-II-like material in the median eminence is unique to this species. It is suggested here that the distribution profile of GnRH-like material within the brain and terminal nerve of I. beddomei represents a primitive pattern.

A DiI-tracing study of the neural connections of the pineal organ in two elasmobranchs ( Scyliorhinus canicula and Raja montagui ) suggests a pineal projection to the midbrain GnRH-immunoreactive nucleus

“…Although sGnRHir fibers have been observed in the dogfish median eminence, they do not form any special accumulation in this neurohemal region. This result conforms with the lack of immunostaining of the preoptic-hypothalamic GnRH neurons with this antiserum and suggests that the sGnRHir system reported here is not primarily hypophysiotrophic, in line with observations on the midbrain GnRHir system of other vertebrates (Mikami et al 1988;Amano et al 1991;Zandbergen et al 1995;Pinelli et al 1997).…”

“…While searching for neural circuits that can mediate pineal responses, we have focused our attention on possible relationships of the pineal organ with an enigmatic midbrain gonadotropin-immunoreactive (GnRHir) nucleus previously described in three elasmobranchs (Wright and Demski 1991) and many other vertebrate species (chondrosteans: Leprêtre et al 1993;teleosts: Münz et al 1981;Borg et al 1982;Kah et al 1986;Batten et al 1990;Amano et al 1991;Zandbergen et al 1995;Parhar et al 1998a;White et al 1994;White and Fernald 1998;Yamamoto et al 1998;amphibians: D'Aniello et al 1995;Iela et al 1996;Pinelli et al 1997;birds: Mikami et al 1988;primitive mammals: Rissman et al 1995). The midbrain GnRHir nucleus generally contains a different gonadotropin molecule than the other central GnRHir neurons, and in some species, the midbrain GnRHir nucleus does not project to the hypophysis (Mikami et al 1988;Amano et al 1991;Zandbergen et al 1995;Pinelli et al 1997).…”

“…The midbrain GnRHir nucleus generally contains a different gonadotropin molecule than the other central GnRHir neurons, and in some species, the midbrain GnRHir nucleus does not project to the hypophysis (Mikami et al 1988;Amano et al 1991;Zandbergen et al 1995;Pinelli et al 1997). The regulation of the midbrain GnRHir nucleus is not understood because, unlike the preoptic-hypothalamic GnRH populations, it does not appear to be influenced by sexual or thyroid hormones (Parhar et al 1998b(Parhar et al , 2000.…”

Current research in amphibians: Studies integrating endocrinology, behavior, and neurobiology

“…Similar work continues to be done on courtship behavior in two of the three major amphibian groups, the urodeles (salamanders and newts) and the anurans (frogs and toads). The third group, the apodans, is largely uninvestigated outside of neuroanatomical studies (Pinelli et al, 1997;Hilscher-Conklin et al, 1998;Ebersole and Boyd, 2000). The urodeles and anurans share some basic behavioral characteristics, such as males producing advertisement signals and females using them for mate choice, and the presence of amplexus or stereotyped clasping behavior in which males tightly grasp females as females and males release gametes for external fertilization.…”

Identification and neuroanatomical distribution of immunoreactivity for mammalian gonadotropin-releasing hormone (mGnRH) in the brain and neural hypophyseal lobe of the toad Bufo arenarum

“…However, the possibility of these cells containing small amounts of GnRH undetectable by immunocytochemical techniques cannot be excluded. In other amphibian species, ir-GnRH neurons have been detected in the terminal nerve (D'Aniello et al 1994;Iella et al 1996;Pinelli et al 1997). It is worth noting that ir-GnRH neurons have been identified in metamorphic larvae in this structure (Somoza et al 1996).…”