Functional characterization of eel dopamine D2 receptors and involvement in the direct inhibition of pituitary gonadotropins.

In various vertebrate species, dopamine (DA) exerts an inhibitory action on reproduction. In the European eel, DA plays a pivotal role in the inhibitory control of the gonadotrope function and the blockade of puberty. In vivo studies suggested that this effect is mediated by receptors pharmacologically related to the D2-family. In the European eel, two distinct D2 receptor (D2-R) paralogous genes have been identified (D2A-R and D2B-R) and both shown to be expressed in the pituitary. We investigated the potential role of each paralog in the control of gonadotrope function in this species. Eel recombinant D2A-R or D2B-R were expressed in HEK 293 cells, with a universal Gα subunit, and receptor activation was followed by inositol phosphate production. Recombinant D2-Rs exhibited a comparable affinity for DA, but differential affinities for mammalian D2-R agonists and antagonists, supporting subtle structure/activity differences. Further, using eel pituitary cell primary cultures, the expression by gonadotrope cells of both native eel D2-R paralogs was examined by in situ hybridisation of D2A-R or D2B-R transcripts, coupled to immunofluorescence of LHβ or FSHβ. LH and to a lesser extent, FSH cells expressed both D2-R transcripts, but with a clear predominance of D2B-R. Notably, D2B- R transcripts were detected on the majority of LH cells. Accordingly, using these cultures, we showed that DA potently inhibited basal and testosterone-stimulated LHβ expression and less potently basal and activin-stimulated FSHβ expression. We also tested some D2-R antagonists in order to select the most adequate one to be used in innovative protocols for induction of eel sexual maturation. We identified eticlopride as the most potent inhibitor of DA action on basal and stimulated LH expression in vitro. Our data suggest a differential functionalisation of the duplicated receptor genes and demonstrate that mainly D2B-R is involved in the dopaminergic inhibitory control of eel gonadotrope function. This article is protected by copyright. All rights reserved.