[3H]benzylpempidine, a new radioligand for probing a putative channel site on nicotinic cholinergic receptors.

A study was undertaken to assess the receptor binding characteristics of [3H]4-benzylpempidine to an allosteric site on calf brain membranes associated with nicotinic cholinergic receptors and to compare the binding affinity of novel arylpempidine analogs with their ability to antagonize the behavioral effects of nicotine in mice. Scatchard analysis of the binding yielded a K(d) of 20 nM and a B(max) of 330 fmols/mg membrane protein. [3H]4-benzylpempidine appears to be a more satisfactory ligand than [3H]mecamylamine, since it possessed a 50-fold greater affinity and its binding was far less sensitive to inorganic ions and Tris. Among the arylpempidine analogs 4-m-chlorobenzylidenepempidine and 4-benzylidenepempidine had the lowest K(i) values (1.4 nM and 5.0 nM, respectively) and were the most potent in antagonizing nicotine-induced seizures in mice. Although the K(i) values for pempidine and mecamylamine were 1-2 orders of magnitude greater than any of the arylpempidines, the dose required to antagonize nicotine-induced seizures in mice was comparable to the arylpempidines. One explanation for this apparent discrepancy in the correlation of binding affinity and nicotine antagonism is the lower brain penetration of arylpempidines compared to mecamylamine, following their systemic administration to mice.