The regulatory action of estrogen and vasoactive intestinal peptide on prolactin secretion in sea bream (Sparus aurata, L.)

The effect of estradiol-17b (E2) implants on the in vitro secretion of prolactin (PRL) and its modulation by vasoactive intestinal peptide (VIP) in a marine teleost, sea bream (Sparus aurata L.), was determined. Experiments were conducted during winter and spring. During winter, fish (n¼130, body weight 50–70 g) were randomly divided into 2 groups; control and E2 treated (10 mg/kg, wet weight). Fish were sacrificed after 7 days treatment and in vitro pituitary cultures in Ringer bicarbonate supplemented with increasing doses (0–200 nM) of VIP were carried out for 18 h. Culture medium was analysed by PAGE and secreted PRL quantified by densitometry. Fish treated with E2 secreted significantly more PRL ðP < 0:05Þ in vitro than control fish. In E2 primed fish VIP caused a dose-dependent inhibition of PRL secretion in vitro. VIP had no detectable effect on the secretion of PRL from control pituitaries. Treatment with E2 had a different effect during spring; PRL secretion was significantly decreased ðP < 0:01Þ compared with the control fish. Anatomical evidence of abundant VIPimmunoreactive nerve fibres in neurohypophysial (NH) tissue penetrating the rostral pars distalis provide further evidence supporting an action for VIPin the regulation of PRL cells. In conclusion, the responsiveness of PRL in the pituitary gland varied with season. Moreover, in the sea bream VIP appears to modulate PRL secretion from E2 primed pituitary glands.

The present work was funded by the Fundac ~ao para a Ci^encia e a Tecnologia, Portugal (PraxisXXI 2/2.1/ BIA/211/94). J.F. and L.B. were funded by FCT Grants BPD/22033 and 4/5.3/PRODEP/2000, respectively. The authors thank the anonymous reviewers for their helpful critique.