title: Stereoselective L-[3H]quinuclidinyl benzilate-binding sites in nervous tissue of Aplysia californica: evidence for muscarinic receptors.
abstract: The muscarinic antagonist L-[3H]quinuclidinyl benzilate (L-[3H]QNB) binds with a high affinity (Kd = 0.77 nM) to a single population of specific sites (Bmax = 47 fmol/mg of protein) in nervous tissue of the gastropod mollusc, Aplysia. The specific L-[3H]QNB binding is displaced stereoselectively by the enantiomers of benzetimide, dexetimide, and levetimide. The pharmacologically active enantiomer, dexetimide, is more potent than levetimide as an inhibitor of L-[3H]QNB binding. Moreover, the muscarinic cholinergic ligands, scopolamine, atropine, oxotremorine, and pilocarpine are effective inhibitors of the specific L-[3H]QNB binding, whereas nicotinic receptor antagonists, decamethonium and d-tubocurarine, are considerably less effective. These pharmacological characteristics of the L-[3H]QNB-binding site provide evidence for classical muscarinic receptors in Aplysia nervous tissue. The physiological relevance of the dexetimide-displaceable L-[3H]QNB-binding site was supported by the demonstration of the sensitivity of the specific binding to thermal denaturation. Specific binding of L-[3H]QNB was also detected in nervous tissue of another marine gastropod, Pleurobranchaea californica. The characteristics of the Aplysia L-[3H]QNB-binding site are in accordance with studies of numerous vertebrate and invertebrate tissues indicating that the muscarinic cholinergic receptor site has been highly conserved through evolution.
mesh_terms: D000818:Animals; D001048:Aplysia / Q000378:metabolism*; D001665:Binding Sites; D001667:Binding, Competitive; D009417:Nerve Tissue / Q000378:metabolism*; D011812:Quinuclidines / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000378:metabolism*; D013237:Stereoisomerism; D014316:Tritium
chemical_list: D011812:Quinuclidines; D011976:Receptors, Muscarinic; D014316:Tritium; D011813:Quinuclidinyl Benzilate