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Melatonin Modulates Secretion of Growth Hormone and Prolactin by Trout Pituitary Glands and Cells in Culture

Laboratoire Arago (J.F., L.B., G.B.), Unité Mixte de Recherche 7628, Centre National de la Recherche Scientifique/Université P et M Curie, F-66651 Banyuls sur Mer, France; Université Jean Monnet-Laboratoire de Biologie Animale et Appliquée (D.F., J.A., M.B.), F-42023 Saint Etienne cedex 02, France; and National Institutes of Health, Laboratory of Developmental Neuroendocrinology (P.G.), Bethesda, Maryland 20892

Address all correspondence and requests for reprints to: Jack Falcón, Laboratoire Arago, Unité Mixte de Recherche 7628, Centre National de la Recherche Scientifique/University P et M Curie, BP 44, F-66651 Banyuls sur Mer, France. E-mail: falcon{at}obs-banyuls.fr.

In Teleost fish, development, growth, and reproduction are influenced by the daily and seasonal variations of photoperiod and temperature. Early in vivo studies indicated the pineal gland mediates the effects of these external factors, most probably through the rhythmic production of melatonin. The present investigation was aimed at determining whether melatonin acts directly on the pituitary to control GH and prolactin (PRL) secretion in rainbow trout. We show that 2-[¹²⁵I]-iodomelatonin, a melatonin analog, binds selectively to membrane preparations and tissue sections from trout pituitaries. The affinity was within the range of that found for the binding to brain microsomal preparations, but the number of binding sites was 20-fold less than in the brain. In culture, melatonin inhibited pituitary cAMP accumulation induced by forskolin, the adenyl cyclase stimulator. Forskolin also induced an increase in GH release, which was reduced in the presence of picomolar concentrations of melatonin. At higher concentrations, the effects of melatonin became stimulatory. In the absence of forskolin, melatonin induced a dose-dependent increase in GH release, and a dose-dependent decrease in PRL release. Melatonin effects were abolished upon addition of luzindole, a melatonin antagonist. Our results provide the first evidence that melatonin modulates GH and PRL secretion in Teleost fish pituitary. Melatonin effects on GH have never been reported in any vertebrate before. The effects result from a direct action of melatonin on pituitary cells. The complexity of the observed responses suggests several types of melatonin receptors might be involved.

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