GONADOTROPIN-RELEASING HORMONES IN THE AFRICAN CATFISH - MOLECULAR-FORMS, LOCALIZATION, POTENCY AND RECEPTORS

Two gonadotropin releasing hormones (GnRHs) were identified in the Afr ican catfish: chicken GnRH-II (cGnRH-II) and catfish GnRH (cfGnRH). Im munological screening of HPLC fractions from pituitary extracts indica ted a third GnRH which co-eluted with lamprey GnRH-III. However, mass determination and amino acid sequencing identified this material as is otocin. This underlines the risk of ''identifying'' multiple forms of GnRH in tissue extracts on the basis of immunoreactivity in HPLC fract ions. In vivo and in vitro studies demonstrated that cGnRH-II is an ov er 100-fold more potent gonadotropin (GTH) secretagogue than cfGnRH. T his correlates with the respective receptor affinities. The presence o f both GnRHs in the pituitary gland suggests that they may modulate ea ch other's GTH release activity. Subthreshold or low doses of cGnRH-II partly inhibited cfGnRH-induced GTH II secretion. Conversely, combina tions of sub-threshold or low doses of cfGnRH with effective doses of cGnRH-II led to increases in GTH II levels similar to those induced by cGnRH-II alone. Combinations of submaximally effective dose of the 2 peptides resulted in additive effects. Hence, both GnRHs participate i n the regulation of GTH II release, and their relative concentrations may determine the overall effect. Immunocytochemistry, using anti-bodi es against the respective recombinant GnRH associated peptides (GAPs), as well as in situ hybridization showed that cfGnRH neurones are scat tered in the ventral forebrain and project into the pituitary gland, w hile cGnRH-II neurones are confined to the midbrain tegmentum and with out projections to the pituitary gland. Transfection experiments with GnRH receptor cDNA shows ligand activation characteristics similar to those of the native GnRH-R. Autoradiographic studies and hormone relea se studies indicate that GnRH-Rs in the African catfish pituitary glan d are restricted to the gonadotrophs.