Modulation of pituitary dopamine D-1 or D-2 receptors and secretion of follicle stimulating hormone and luteinizing hormone during the annual reproductive cycle of female rainbow trout

The two gonadotrophins follicle stimulating hormone (FSH) and luteinizing h ormone (LH) have distinct temporal expression and release profiles in fish, but little is known regarding their neuroendocrine control, especially for FSH. The present experiments were performed on previtellogenic, mature and preovulatory female trout. The catecholamine synthesis inhibitor, alpha -m ethyl-p-tyrosine, increased plasma LH and FSH concentrations of mature fish . The dopamine agonist apomorphine decreased and the dopamine antagonist do mperidone increased plasma LH concentration of preovulatory fish and delaye d ovulation, but did not modify plasma FSH concentration. The dopamine D-2 agonist bromocryptine inhibited LH release in cultured gonadotrophs from ma ture and preovulatory fish, but not from previtellogenic fish. Bromocryptin e also Significantly inhibited basal and salmon gonadotrophin releasing-hor mone (sGnRH)-induced FSH release from cultured gonadotrophs of mature fish, but not of preovulatory fish, and increased FSH release from gonadotrophs of previtellogenic fish. The dopamine D-1 agonist SKF 38393 had no observed effect on the release of FSH and LH, at any reproductive stage studied. Th e D-1 agonist SKF 38393, the D-2 agonist bromocriptine and sGnRH had no obs erved effects on cell contents of FSH and LH. Taken together, these data su ggest that, at the level of the pituitary, dopamine inhibits LH release as vitellogenesis proceeds, via activation of dopamine D-2 receptors. We demon strate for the first time in fish a control of FSH release (a dopamine cont rol), especially in mature fish which have low circulating concentrations o f FSH.