Regulation by guanine nucleotides and cations of melatonin binding sites in the goldfish brain.

Effects of nucleotides and cations on 2-[125I]iodomelatonin binding sites in the goldfish brain were examined. Nucleotides (10(-6)-10(-3) M) dose-dependently inhibited the specific binding with the following order of potency: guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) > GTP = GDP > GMP = ATP > cyclic GMP. Cyclic AMP was ineffective. The treatment of membranes with GTP gamma S induced rapid dissociation of 2-[125I]iodomelatonin from membranes when added at the steady state, increased the Kd and decreased the Bmax values as revealed by saturation analysis, and increased the IC50 value of melatonin to inhibit the specific binding. The treatment decreased the specific binding to membrane preparations obtained from six brain regions as well. Inorganic salts (5-200 mM) dose-dependently inhibited the specific binding with the following order of potency: CaCl2 > MgCl2 > LiCl > NaCl > choline chloride > KCl, except for 5 mM MgCl2, which enhanced the specific binding. Saturation experiments demonstrated that 75 mM CaCl2, 100 mM MgCl2 and 200 mM NaCl increased the Kd and decreased the Bmax while 5 mM MgCl2 increased the Bmax value. These results imply that G protein and physiological concentrations of cations are involved in the regulation of melatonin binding sites in the goldfish brain.