Characterization of melatonin receptors in the rat area postrema: modulation of affinity with cations and guanine nucleotides.
We have localized and characterized the binding of the melatonin agonist, 2-[125I]iodomelatonin, in the rat area postrema (AP), by using quantitative autoradiography in vitro. At equilibrium conditions, Scatchard analysis revealed saturable high-affinity binding to a single class of sites (Kd 45.9 +/- (SE) 6.0 pM and Bmax 30.8 +/- 4.6 fmol/mg protein, n = 4 experiments with a total of 18 rats). Melatonin and 6-hydroxymelatonin were potent displacers of 2-[125I]iodomelatonin binding in the AP (IC50 20 and 500 pM, respectively) while N-acetylserotonin exhibited only a modest potency (IC50 25 nM). Micromolar concentrations of guanine nucleotides dose-dependently and specifically inhibited agonist binding at 22 degrees C. Saturation studies revealed that this was due to a decrease in binding affinity. Divalent cations (4 mM CaCl2 or 2 mM MgCl2) had no detectable effect on the affinity of the binding site, whereas physiological concentrations of NaCl significantly decreased the binding affinity. These results demonstrate specific high-affinity binding sites for 2-[125I]iodomelatonin in the rat AP and suggest coupling of these putative receptors to guanosine nucleotide binding regulatory protein(s).
Laitinen
J T
JT
Section on Pharmacology, National Institute of Mental Health, Bethesda, Md.
Flügge
G
G
Saavedra
J M
JM
D000818:Animals; D001345:Autoradiography; D001665:Binding Sites; D001667:Binding, Competitive; D002122:Calcium Chloride / Q000494:pharmacology*; D002413:Cations, Divalent; D002552:Cerebral Ventricles / Q000187:drug effects / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000502:physiology; D006150:Guanine Nucleotides / Q000494:pharmacology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D015636:Magnesium Chloride / Q000494:pharmacology*; D008297:Male; D008550:Melatonin / Q000031:analogs & derivatives / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter / Q000378:metabolism*; D012701:Serotonin / Q000031:analogs & derivatives / Q000378:metabolism; D013873:Thionucleotides / Q000494:pharmacology
D002413:Cations, Divalent; D006150:Guanine Nucleotides; D044094:Receptors, Melatonin; D017981:Receptors, Neurotransmitter; D013873:Thionucleotides; D015636:Magnesium Chloride; D012701:Serotonin; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; C044556:2-iodomelatonin; D019204:GTP-Binding Proteins; D008550:Melatonin; D002122:Calcium Chloride; C006389:N-acetylserotonin; C012365:6-hydroxymelatonin