title: An 125I-labeled binding probe for the muscarinic cholinergic receptor.
abstract: We describe the synthesis and characterization of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), a binding probe for the muscarinic cholinergic receptor of high specific radioactivity. The binding isotherm of this 125I-labeled compound to a rat synaptic plasma membrane-enriched fraction consists of two components: a 'specific' component which is saturable and closely fits hyperbolic binding to a single class of sites as evaluated by Scatchard analysis (Kd = 1.5 to 3 nM), and a linear component which may be measured directly by preincubating membranes in 0.1 microM 3-quinuclidinyl-benzilate, the most potent muscarinic antagonist known. The specific binding of [3-3H]3-quinuclidinyl-benzilate and [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) to rat brain subcellular fractions is parallel in myelin, synaptic plasma membrane and mitochondrial fractions with a 3--4-fold enrichment observed in synaptic plasma membrane over crude mitochondrial fractions. The concentrations of muscarinic antagonists required to block [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate) binding parallel that reported for tritiated binding probes and are consistent with physiological measurements of their dissociation constants. Because of the high specific radioactivity of [125I]3-quinuclidinyl-(3-iodo-4-hydroxy-benzilate), this iodinated binding probe should prove useful in assaying low levels of muscarinic receptor in tissue culture and other biological sources.
mesh_terms: D000818:Animals; D002540:Cerebral Cortex / Q000378:metabolism; D002855:Chromatography, Thin Layer; D007457:Iodine Radioisotopes / Q000378:metabolism*; D008928:Mitochondria / Q000378:metabolism; D010276:Parasympatholytics / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology; D011812:Quinuclidines / Q000378:metabolism; D051381:Rats; D011950:Receptors, Cholinergic / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism; D013570:Synaptic Membranes / Q000378:metabolism; D014316:Tritium / Q000378:metabolism
chemical_list: D007457:Iodine Radioisotopes; D010276:Parasympatholytics; D010277:Parasympathomimetics; D011812:Quinuclidines; D011950:Receptors, Cholinergic; D011976:Receptors, Muscarinic; D014316:Tritium