Gonadotropin-releasing hormones in the African catfish: molecular forms, localization, potency and receptors |
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| Authors: | HJTh Goos PT Bosma J Bogerd CP Tensen CP Tensen KW Li KW Li MA Zandbergen RW Schulz |
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| Institution: | (1) Research Group for Comparative Endocrinology, University of Utrecht, P.O. Box 80.058, 3508 TB Utrecht, The Netherlands;(2) Present address: Leiden/Amsterdam Institute for Immunology, Department of Dermatology, Free University Hospital, De Boelelaan 1117, 1087 HV Amsterdam, The Netherlands;(3) Present address: Graduate School Neurosciences Amsterdam, Research Institute Neurosciences, Vrije Universiteit, Faculty of Biology, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands |
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| Abstract: | Two gonadotropin releasing hormones (GnRHs) were identified in the African catfish: chicken GnRH-II (cGnRH-II) and catfish GnRH (cfGnRH). Immunological screening of HPLC fractions from pituitary extracts indicated a third GnRH which co-eluted with lamprey GnRH-III. However, mass determination and amino acid sequencing identified this material as isotocin. This underlines the risk of  identifying multiple forms of GnRH in tissue extracts on the basis of immunoreactivity in HPLC fractions. In vivo and in vitro studies demonstrated that cGnRH-II is an over 100-fold more potent gonadotropin (GTH) secretagogue than cfGnRH. This correlates with the respective receptor affinities. The presence of both GnRHs in the pituitary gland suggests that they may modulate each other's GTH release activity. Sub-threshold or low doses of cGnRH-II partly inhibited cfGnRH-induced GTH II secretion. Conversely, combinations 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 in the regulation of GTH II release, and their relative concentrations may determine the overall effect. Immunocytochemistry, using anti-bodies against the respective recombinant GnRH associated peptides (GAPs), as well as in situ hybridization showed that cfGnRH neurones are scattered in the ventral forebrain and project into the pituitary gland, while cGnRH-II neurones are confined to the midbrain tegmentum and without 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 release studies indicate that GnRH-Rs in the African catfish pituitary gland are restricted to the gonadotrophs. |
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| Keywords: | gonadotropin releasing hormones African catfish potency GnRH receptors |
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