The existence of 125I-labelled iodomelatonin-binding sites in chicken ovaries and testes was investigated. The specific binding of 125I-labelled iodomelatonin to chicken ovarian and testicular tissue satisfies all the criteria for a binding site. It was rapid, stable, saturable, reversible, specific and of high affinity. Equilibrium studies showed that total and non-specific binding increased over a range of 5–150 pmol 125I-labelled iodomelatonin/1 tested, with specific binding reaching saturation towards the middle range of radioligand concentrations. Scatchard analyses indicated a dissociation constant (Kd) of 36·5 ± 5·3 pmol/l (means ± s.e.m.) in the membrane preparations of chicken testes at the middle point in the period of light and a maximum number of binding sites (Bmax) of 0·93 ±0·40 fmol/mg protein (n = 6). In membrane preparations of chicken ovaries, the Kd was 102·2 ± 27·3 pmol/l and the Bmax was 2·77± 0·38 fmol/mg protein (n= 6). Equilibrium and kinetic dissociation constants in the picomolar range indicate high-affinity and physiologically relevant 125I-labelled iodomelatonin-binding sites. Competitive inhibition studies determined the following order of relative potency for inhibition of 125I-labelled iodomelatonin-binding to chicken gonadal membranes: 6-chloromelatonin > melatonin > N-acetylserotonin > > > 5-hydroxytryptamine, tryptamine, 5-methoxytryptophol, l-acetylindole-3-acetic acid, 5-hydroxyindole-3-acetic acid and l-tryptophan. The presence of 125I-labelled iodomelatonin-binding sites suggests a direct pinealgonadal connection in the chicken.
Journal of Endocrinology (1992) 133, 5–11
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