STIMULATORY EFFECTS OF SEROTONIN ON MATURATIONAL GONADOTROPIN-RELEASE IN THE ATLANTIC CROAKER, MICROPOGONIAS-UNDULATUS

The effects of serotonin (5-HT) injection alone, and in combination with des Gly10 [d-Trp6]-luteinizing hormone-releasing hormone ethylamide (LHRHa), on plasma maturational gonadotropin (GtH) levels in the Atlantic croaker, Micropogonias undulatus, were examined. An injection of 5-HT (20 μg body wt) alone did not significantly elevate GtH levels, whereas simultaneous administration of LHRHa (20 ng/g body wt) and 5-HT elicited a greater elevation of plasma GtH levels than that induced by LHRHa alone. Pretreatment of 1-year-old fish with fluoxetine (10 μg/g), a 5-HT reuptake inhibitor, only slightly augmented the effect of 5-HT on GtH levels, whereas pretreatment with ketanserin (10 μg/g), a 5-HT receptor antagonist, completely inhibited the potentiating effect of 5-HT on the GtH response to LHRHa. Administration of LHRHa (20 μg/g) or 5-HT (20 μg/g) significantly elevated GtH levels in the 2- and 3-year-old croaker, but the combined treatment failed to increase GtH levels above those induced by LHRHa alone. However, with a lower dose of LHRHa (5 μg/g), the combination produced an additive effect. Serotonin (20 μg/ml media) alone, and in combination with LHRHa (10 ngiml media), significantly stimulated GtH release from the pituitaries of gonadally mature 2- and 3-year-old female croaker during an 18-hr incubation in vitro, but not from pituitaries of gonadally regressed fish. The combined treatment of LHRHa with 5-HT also significantly stimulated in vitro GtH release during 6-and 12-hr pituitary incubations, whereas these treatments alone were ineffective. The results demonstrate that a serotonergic stimulatory system for the control of GtH secretion is present in the Atlantic croaker, although preliminary results suggest that this system is only functional during certain stages of the reproductive cycle. © 1992.