2-[125i]iodomelatonin binding sites in hamster brain membranes: Pharmacological characteristics and regional distribution

M. J. Duncan, J. S. Takahashi, M. L. Dubocovich

Research output: Contribution to journalArticle

129 Scopus citations

Abstract

Studies in a variety of seasonally breeding mammals have shown that melatonin mediates photoperiodic effects on reproduction. Relatively little is known, however, about the site(s) or mechanisms of action of this hormone for inducing reproductive effects. Although binding sites for [3H] melatonin have been reported previously in bovine, rat, and hamster brain, the pharmacological selectivity of these sites was never demonstrated. In the present study, we have characterized binding sites for a new radioligand, 2-[125I]iodomelatonin, in brains from a photoperiodic species, the Syrian hamster. 2-[125I]Iodomelatonin labels a high affinity binding site in hamster brain membranes. Specific binding of 2-[125I]iodomelatonin is rapid, stable, saturable, and reversible. Saturation studies demonstrated that 2-[125I]iodomelatonin binds to a single class of sites with an affinity constant (KJ of 3.3 ± 0.5 nM and a total binding capacity (Bmax) of 110.2 ± 13.4 fmol/mg protein (n = 4). The Kj value determined from kinetic analysis (3.1 ± 0.9 nM; n = 5) was very similar to that obtained from saturation experiments. Competition experiments showed that the relative order of potency of a variety of indoles for inhibition of 2-[125I]iodomelatonin binding site to hamster brain membranes was as follows: 6-chloromelatonin > 2-iodomelatonin > N-acetylserotonin > 6-methoxymelatonin > melatonin > 6-hydroxymelatonin > 6,7-dichloro-2-methylmelatonin > 5-methoxytryptophol > 5-methoxytryptamine > 5- methoxy-iV, iV-dimethyltryptamine > AT-acetyltryptamine > serotonin > 5-methoxyindole (inactive). Compounds known to act at serotonergic, adrenergic, or dopaminergic receptors were either inactive or relatively ineffective as compared to melatonin. These results suggest that 2-[125I]iodomelatonin is a selective, high affinity probe for identifying melatonin receptor binding sites in rodent brain.

Original languageEnglish (US)
Pages (from-to)1825-1833
Number of pages9
JournalEndocrinology
Volume122
Issue number5
DOIs
StatePublished - May 1 1988

    Fingerprint

ASJC Scopus subject areas

  • Endocrinology

Cite this