GTP modulates [ 125I]iodomelatonin binding to a picomolar-affinity site in the Syrian hamster hypothalamus

Saturation binding experiments conducted with [ 125I]iodomelatonin at 0–4°C in the Syrian hamster hypothalamus, revealed a single nanomolar-affinity site which was not affected by GTP. In contrast, incubation at 30°C revealed two distinct binding sites with picomolar and nanomolar affinities, respectively. GTP caused a significant decrease in the affinity of only the picomolar site but did not alter its density: control: K d = 43 ± 6 pM, B max = 1.7 ± 0.3 fmol/mg protein; GTP (1 mM): K d = 250 ± 52, B max = 3.9 ± 2.6 fmol/mg protein. The foregoing indicates that the affinity of the putative melatonin receptor in the hamster hypothalamus is modulated by a regulatory G protein.