Stereoselective L-[³H]quinuclidinyl benzilate-binding sites in nervous tissue of Aplysia californica: Evidence for muscarinic receptors

The muscarinic antagonist L-[³H]quinuclidinyl benzilate (L-[³H]QNB) binds with a high affinity (K(d) = 0.77 nm) to a single population of specific sites (B(max) = 47 fmol/mg of protein) in nervous tissue of the gastropod mullusc, 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.