Characterization of Melatonin Binding Sites in the Pars Tuberalis of the European Hamster

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Abstract
Melatonin binding sites in the pars tuberalis of the European hamster (Cricetus cricetus) have been characterized using the radioligand 2-[(125) I]iodomelatonin. Specific 2-[(125) I]iodomelatonin binding was assessed using radioreceptor studies of pars tuberalis membrane preparations. Saturation studies revealed a single, high affinity site (K(d) 39.8 (± 7.6 SEM) pM and B(max) 4.1 (± 0.5 SEM) fmol/mg protein, n=4). Kinetic experiments showed the 2-[(125) I]iodomelatonin binding to be rapid, saturable and reversible. The K(d) calculated from the dissociation and association rate constants was 19.4 pM. The order of potency of different indoles for inhibition of 2-[(125) I]iodomelatonin binding was 6-chloromelatonin > melatonin > 6-hydroxymelatonin > N-acetylserotonin > 5-methoxytryptophol > serotonin > 5-methoxytryptamine. GTP caused a dose-dependent inhibition of the 2-[(125) I]iodomelatonin binding. A saturation study showed that GTP reduced the number of binding sites by a third without altering their affinity. These results imply the presence of a G-protein-coupled melatonin receptor in the pars tuberalis of sexually active European hamsters.