The site of inhibition, by melatonin, of GnRH-dependent testosterone secretion was investigated in adult rat Leydig cells cultured in vitro. The various effects downstream of the binding of GnRH to its own receptor were isolated and mimicked by specific drugs. Testosterone secretion was then evaluated after 3 h stimulation with GnRH, thapsigargin (1 microM), phorbol-12-myristate-13-acetate (100 nM), arachidonic acid (20 microM), and ionomycin (1 microM) in the presence or absence of melatonin (215 nM). The effect of melatonin on the GnRH-induced changes in cytoplasmic calcium concentration ([Ca(2+)](i)) was also studied, using Fura-2 as fluorescent Ca(2+) indicator. Melatonin attenuated the increase in [Ca(2+)](i) and inhibited the testosterone secretion induced by GnRH, but not that induced by ionomycin. Both ionomycin and thapsigargin potentiated GnRH-induced testosterone secretion; however, ionomycin, but not thapsigargin, partially prevented the inhibitory effect of melatonin on cells stimulated with GnRH. The effect of melatonin was probably dependent on the binding of melatonin to its Gi-protein-coupled receptor, as the inhibitory effect on GnRH-induced secretion was supressed in cells pretreated with pertussis toxin in a concentration of 180 ng/ml for 20 h. Assay of 17-hydroxy-progesterone showed that, irrespective of the treatment, cells cultured with melatonin secreted greater amounts than controls. We conclude that melatonin reduces GnRH-induced testosterone secretion by 1) decreasing [Ca(2+)](i), through impairment of the GnRH-dependent release of Ca(2+) from intracellular stores and 2) blocking 17-20 desmolase enzymatic activity, an effect that occurs irrespective of changes in [Ca(2+)](i).
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S Valenti, S Thellung, T Florio, M Giusti, G Schettini, and G Giordano
D. N. Foster, D. Galehouse, T. Giordano, B. Min, I. C. Lamb, D. A. Porter, K. J. Intehar, and W. L. Bacon
Recombinant cDNA clones that encode the α subunit of the chicken pituitary glycoprotein hormones were isolated from a pituitary library. The longer of the two cDNA clones that were sequenced was 754bp in length. It contained 81 nucleotides of the 5′-untranslated region (UTR), an open-reading frame of 360bp that encoded a 24 amino acid leader polypeptide sequence as well as the 96 amino acid mature α subunit, and 268 nucleotides of the 3′-UTR, followed by a 45 bp poly(A) tract. There was 69–79% homology between the nucleotide sequence of the coding region for the chicken and mammalian α-subunit cDNAs. Northern blot analysis revealed that the steady-state levels of an approximately 800 bp α-subunit specific transcript increased quantitatively when dispersed chicken pituitary glands were treated in culture with chicken gonadotrophin-releasing hormone-I.