2-[125I]iodo-5-methoxycarbonylamino-N-acetyltryptamine: a selective radioligand for the characterization of melatonin ML2 binding sites.
We now describe the preparation and characterization of a novel radioligand, 2-[125I]iodo-5-methoxy-carbonylamino-N-acetyltryptamine (2-[125I]MCA-NAT), with high affinity and pharmacological selectivity for melatonin ML2 receptor sites. 2-[125I]MCA-NAT was prepared by introducing an [125I]iodine molecule on carbon 2 of 5-MCA-NAT (5-methoxycarbonylamino N-acetyltryptamine), a selective melatonin ML2 receptor ligand. The specific binding (88%) of 2-[125I]MCA-NAT (50 pM) to whole washed hamster brain membranes showed rapid kinetics of association/dissociation, and was of high affinity and saturable (Kd value = 116 +/- 14 pM; Bmax value = 15.5 +/- 1.8 fmol/mg protein, n = 3). 2-[125I]MCA-NAT showed no affinity for melatonin ML1 receptors of chicken retina. Competition curves of various melatonin analogues for 2-[125I]MCA-NAT binding to hamster brain, testes and kidney were monophasic and showed a pharmacological order of affinities (Ki values for brain, nM) identical to that of the ML2 sites [2-iodomelatonin (0.77) > 6-chloro-2-methyl-melatonin (2.56) > 6-chloromelatonin (6.8) > prazosin (21.7) > or = N-acetylserotonin (23.3 nM) > or = 5-MCA-NAT (29.5) > or = melatonin (83.9) > luzindole (1687) > serotonin (2120)]. Affinity constants for competition of melatonin analogues on [125I]MCA-NAT binding to hamster brain, testes, and kidney correlated significantly [r = 0.962, P < 0.001, n = 9; r = 0.982, P < 0.0001, n = 13; r = 0.975, P < 0.0001, n = 9, respectively) with the affinities determined on 2-[125I]iodomelatonin binding to ML2 sites (hamster brain) but not to ML1 sites (chicken retina, r = 0.33, P > 0.05, n = 16). In conclusion, 2-[125I]MCA-NAT is a specific radioligand for the identification and characterization of ML2 binding sites in brain and peripheral tissues.
Molinari
E J
EJ
Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago 60611-3008, USA.
North
P C
PC
Dubocovich
M L
ML
D000818:Animals; D001921:Brain / Q000378:metabolism; D002645:Chickens; D006224:Cricetinae; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007457:Iodine Radioisotopes; D007700:Kinetics; D008024:Ligands; D008566:Membranes / Q000378:metabolism; D008647:Mesocricetus; D051379:Mice; D008809:Mice, Inbred C3H; D017111:Phodopus; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D011956:Receptors, Cell Surface / Q000187:drug effects*; D018160:Receptors, Cytoplasmic and Nuclear / Q000187:drug effects*; D044094:Receptors, Melatonin; D012160:Retina / Q000378:metabolism; D014363:Tryptamines / Q000138:chemical synthesis / Q000493:pharmacokinetics / Q000494:pharmacology*
C103347:2-iodo-5-methoxycarbonylamino-N-acetyltryptamine; D007457:Iodine Radioisotopes; D008024:Ligands; D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D044094:Receptors, Melatonin; D014363:Tryptamines