CHARACTERIZATION OF MELATONIN BINDING-SITES IN CHICKEN AND HUMAN INTESTINES

The radioligand 2-[I-125]iodomelatonin was used to study melatonin binding sites in chicken and human intestines. In the chicken duodenum, 2-[I-125]iodomelatonin binding sites were enriched in the musculosa layer (B(max) congruent-to 1 fmol/mg protein) as compared to the mucosa/submucosa layer (B(max) congruent-to 0.2 fmol/mg protein). 2-[I-125]iodomelatonin bound with a K(d) of 68 +/- 18 pM (mean +/- S.E.M., n = 13) and was displaced by melatonin with a K(i) of 0.3 nM. The K(d) value for 2-[I-125]iodomelatonin was increased 2- to 4-fold by a GTP analog, suggesting that the binding sites might be coupled to a G-protein. The affinity order of nine melatonin analogs at the enteric binding sites was in agreement with the pharmacological profile of melatonin receptors described in other tissues. In the human jejunum, 2-[I-125]iodomelatonin binding could be observed in the mucosa/submucosa layer (K(d) = 150-200 pM, B(max) = 0.7 fmol/mg protein). The radioligand was efficiently displaced by melatonin (K(i) = 0.6 nM) but only marginally by N-acetyltryptamine (K(i) = 22 muM) and serotonin (K(i) = 14 muM).