Characterization of melatonin receptors in the rat suprachiasmatic nuclei: modulation of affinity with cations and guanine nucleotides.

We have characterized melatonin (MT) receptors in the rat suprachiasmatic nuclei by using quantitative autoradiography in vitro at equilibrium conditions for a picomolar affinity site. Binding of the MT agonist 2-[125I]iodomelatonin (30 pM to 6 nM) was saturable, of high affinity (Kd, 52.8 pM), of high specificity, and to a single class of sites (binding capacity, 16.5 fmol/mg protein). However, by shortening the washing time for bound and free ligand, we were able to demonstrate an additional low affinity form of this receptor (Kd, 761 pM; binding capacity, 72.2 fmol/mg protein). Micromolar concentrations of guanine nucleotides and millimolar concentrations of monovalent cations (Na+ and Li+) dose-dependently and specifically inhibited agonist binding at 22 C. Saturation studies revealed that this was due to a decrease in receptor affinity in both cases. Ca+2 promoted high affinity agonist binding, as omission of this cation decreased the affinity of the suprachiasmatic MT receptor. These results suggest coupling of the rat suprachiasmatic MT receptors to guanine nucleotide-binding regulatory protein(s). Moreover, our data demonstrate specific modulation of the affinity of these receptors with cations.