EXTRAPITUITARY GONADOTROPIN-RELEASING-HORMONE (GNRH) BINDING-SITES INGOLDFISH

In teleosts, as in other vertebrates, the secretion of pituitary gonad otropin (GTH) is mediated by the hypothalamic decapeptide, gonadotropi n-releasing hormone (GnRH). Recent findings in teleosts indicate that GnRH receptors are not restricted to the pituitary gonadotropes and ar e also associated with somatotropes as well as being present in a numb er of other tissues. In the present study, we provide novel informatio n on GnRH binding in a number of extrapituitary tissues in goldfish. H owever, we do not intend to provide full characterization of GnRH bind ing sites in various extrapituitary tissues in goldfish as this would clearly be outside the scope of this paper. In this study we examined GnRH binding in a number of extrapituitary tissues in goldfish and obs erved specific binding in ovary, testis, brain, liver and kidney. No s pecific GnRH binding was observed in muscle, skin, gut, gill and heart . In general, the present findings together with the results of other studies carried out in our laboratory demonstrate that mature goldfish ovary and testis contain two classes of GnRH binding sites, high affi nity/low capacity and low affinity/high capacity sites with binding ch aracteristics similar to those of the pituitary GnRH receptors. The br ain of goldfish was also found to contain two classes of GnRH binding sites, a super-high affinity/low capacity and a low affinity/high capa city sites. Furthermore, study of goldfish liver and kidney demonstrat ed the presence of a single class of GnRH binding sites with character istics different from those of pituitary, ovary, testis and brain. Ove rall, it is evident that goldfish contains a family of GnRH binding si tes which can be classified into four groups based on binding affiniti es: 1) A class of high affinity binding sites present in the pituitary , ovary and testis, 2) a class of super high affinity sites so far onl y detected in the brain, 3) a class of intermediate-affinity GnRH bind ing sites in the liver and kidney, and 4) a class of low affinity bind ing sites present in all the tissues containing specific GnRH binding sites except for liver and kidney.