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 degrees C in the Syrian hamster hypothalamus, revealed a single nanomolar-affinity site which was not affected by GTP. In contrast, incubation at 30 degrees 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: Kd = 43 +/- 6 pM, Bmax = 1.7 +/- 0.3 fmol/mg protein; GTP (1 mM): Kd = 250 +/- 52, Bmax = 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.