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The effects of dopamine on the secretion of two sturgeon gonadotropins (stGTH I and stGTH II) in sexually mature male white sturgeon (Acipenser transmontanus) were evaluated. In Experiment I, sturgeon were given intraperitoneal injections of physiological saline (PS), dopamine (100 mg kg-'), the gonadotropin releasing hormone analog D-Ala 6 -des-Gly 10 -GnRH ethylamide (GnRHA) (10 jig/kg-'), and a combination of GnRHa and dopamine. Fish receiving only GnRHa had significantly higher (p < 0.05) concentrations of plasma stGTH I and stGTH II compared to fish receiving PS, dopamine, or a combination of GnRHa and dopamine. Two hours following its administration, dopamine was effective in decreasing plasma concentrations of both stGTHs that were previously elevated by GnRHa. Dopamine or PS administered by themselves did not alter plasma concentrations of either stGTH. In Experiment II, sturgeon injected intraperitoneally with a combination of GnRHa and pimozide had significantly higher (p < 0.05) concentrations of plasma stGTH I and stGTH II compared to males receiving GnRHa or pimozide alone. While this effect of GnRHAa + pimozide was observed in the spring, no such potentiation was seen in these fish during the summer (Experiment III). These results represent the first evidence of dopaminergic inhibition of GnRH-induced pituitary gonadotropin secretion in Chondrostean fish.

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