Pharmacological identity of 2-[125I]iodomelatonin binding sites in chicken brain and sheep pars tuberalis.

2-[125I]Iodomelatonin was used to compare the binding and pharmacological characteristics of the melatonin receptor sites found in chicken brain and sheep pars tuberalis. Scatchard analysis and kinetic experiments showed that 2-[125I]iodomelatonin binds to a single class of site in both tissues with high affinity (Kd 20-34 pM). Competition experiments, using 21 analogues of melatonin, gave inhibition constants (Ki) for the two sites which were significantly correlated (r = 0.985, n = 21, P less than 0.0001). We conclude that the 2-[125I]iodomelatonin binding sites in sheep pars tuberalis and chicken brain have identical binding and pharmacological characteristics.