An antagonist to GnRH in the control of reproduction in teleost fish: dopaminergic inhibition. Ancestral origin and differential conservation within vertebrates ? [Un contre-pouvoir au contrôle de la reproduction par la GnRH chez les poissons téléostéens: l'inhibition dopaminergique. Rôle ancestral et conservation différentielle chez les Vertébrés.]
Résumé
In mammals, the neurohormonal control of the pituitary gonadotropes is provided by the gonadoliberin GnRH. Several studies on teleost fish indicate that a single positive control by GnRH is not a general rule among vertebrates. Peter and colleagues presented the first evidence of an inhibitory neurohormonal factor, "GRIF" (gonadotropin-release inhibiting factor). They induced a preovulatory LH surge by injuring particular brain areas in the goldfish. Subsequent in vivo and in vitro studies identified dopamine as GRIF, and neuroanatomical investigations have demonstrated that dopaminergic neurones in the anterior preoptic area projecting to the pituitary represent the anatomical substrate for GRIF activity. An inhibitory role of dopamine on the control of LH and ovulation/spermiation has been evidenced in many adult teleosts, including its implications for aquaculture. However, dopamine does not play an inhibitory role in all adult teleosts. As regards the early stages of gametogenesis and especially the control of puberty, a role for dopamine has been suggested or rejected depending on species. The European eel has a unique life cycle with a long prepubertal stage, which has made it a useful model to demonstrate the key-role of dopamine in the control of puberty. Data from tetrapods suggest that the role of dopamine as a GRIF is not restricted to the teleosts, but that it may have an ancient evolutionary origin, and has been differentially conserved throughout vertebrate evolution.