Journal of Neuroscience, Vol 5, 3184-3188, Copyright © 1985 by Society for Neuroscience
Stereoselective L-[3H]quinuclidinyl benzilate-binding sites in nervous tissue of Aplysia californica: evidence for muscarinic receptors
TF Murray, GJ Mpitsos, JF Siebenaller and DL Barker
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.