Characterization of the receptor for gonadotropin-releasing hormone in the pituitary of the African catfish,Clarias gariepinus

Abstract

Receptors for gonadotropin-releasing hormone (GnRH) were characterized using a radioligand prepared from a superactive analog of salmon GnRH (sGnRH), D-Arg(6)-Pro(9)-sGnRH-NEt (sGnRHa). Binding of(125)I-sGnRHa to catfish pituitary membrane fractions reached equilibrium after 2 h incubation at 4°C. Displacement experiments with several GnRH analogs as well as other peptides, demonstrated the specificity of(125)I-sGnRHa binding. Specific binding was enhanced in the presence of the cation chelator ethylene bis (oxyethylenenitrilo) tetra-acetic acid (EGTA), indicating an inhibitory effect of cations on GnRH-receptor binding. The binding of(125)I-sGnRHa to pituitary membranes was found to be saturable at radioligand concentrations of 5 nM and above. A Scatchard analysis of the saturation data suggested the presence of a single class of high-affinity binding sites (Ka=0.901±0.06×10(9)M(-1), Bmax=1678±150 fmol/mg protein). A comparative study on(125)I-sGnRHa binding to pituitary membrane fractions of male and female catfish, indicated that there were no differences in binding affinity and binding capacity between both sexes. The results demonstrate the presence of specific, saturable GnRH receptors in the African catfish pituitary.

Gonadotrophs but not somatotrophs carry gonadotrophin-releasing hormone receptors: receptor localisation, intracellular calcium, and gonadotrophin and GH release

“…A single class of high-affinity GnRH-binding sites was characterised in the pituitary of the African catfish (de Leeuw et al 1988). G nR H induces a rapid increase in the gonadotroph's cytosolic free calcium concentration ([C a2 +]j) in mammals (Stojilkovic & Catt 1992, Stojilkovic et al 1994.…”

Pituitary gonadotropin-releasing hormone (GnRH) receptor activity in goldfish and catfish: seasonal and gonadal effects

“…Development of a radioreceptor assay and characterization of GnRH receptors in the goldfish and catfish pituitary have made it possible to test this hypothesis; the goldfish pituitary contains two classes of GnRH binding sites, a high affinity/low capacity site and a low affinity/high capacity site (Habibi et al 1987a), whereas the catfish pituitary contains a single class of high affinity GnRH binding sites (De Leeuw et al 1988a). The present study provides information on the seasonal variations in the pituitary GnRH receptor binding and activity in the goldfish, and the effect of castration on pituitary GnRH receptor binding and activity in the catfish.…”

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The goldfish pituitary contains two classes of gonadotropin-releasing hormone (GnRH) binding sites, a high affinity/low capacity site and a low affinity/high capacity site (Habibi et al 1987a), whereas the catfish pituitary contains a single class of high affinity GnRH binding sites (De Leeuw et al 1988a). Seasonal variations in pituitary GnRH receptor binding parameters, and the effect of castration on pituitary GnRH receptor binding were investigated in goldfish and catfish, respectively.

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Gonadotropin-Releasing Hormone Receptor in the TeleostHaplochromis burtoni: Structure, Location, and Function1

“…To date, however, only tetraploid species [goldfish (6) and Xenopus (Troskie, B., N. Illing, and R. Millar, unpublished data)] have been shown to express more than one GnRH receptor. Evidence against the widespread existence of a second form is that radiolabeled GnRH apparently binds to only one high affinity class of sites in pituitary membranes in African catfish (20,42) and rats (43)(44)(45), as opposed to the two distinguishable binding affinities in goldfish (46). On the other hand, since both type I and type II GnRH-R sequences are expressed in perciform fish, it seems likely that two types of GnRH-R are expressed in individual perciform species, which appear to be diploid (47).…”