The discovery of leptin has sparked a rapidly growing number of publications concerning the role of leptin in the regulation of body adiposity, feeding, and reproductive system in mammals. To date, there have been no reports on the presence of leptin-related peptide, and functional studies on the role of leptin remain limited in fishes. We investigated the effect of mouse recombinant leptin on basal and sea bream (sb) GnRH-induced LH release from dispersed pituitary cells obtained from male European sea bass (Dicentrarchus labrax) at different stages of sexual development. The potential interaction of leptin with the porcine neuropeptide Y (pNPY), known to play a dual role in feeding and reproduction in vertebrates, was also investigated. High doses of leptin (10(-8)-10(-6) M) and/or pNPY (0.1 and 1 nM) had different effects on LH release at various stages of sexual development. Porcine NPY alone was weakly effective on basal LH release, but it enhanced LH release induced by leptin (10(-6) M) in late prepuberty but not in early postpuberty. Additive or inhibitory effects of leptin were observed on sbGnRH-induced LH release depending on sbGnRH dose and stage of sexual development. The direct action of leptin on LH release at the pituitary level in sea bass suggests that leptin is a regulator of the reproductive system in fishes.