[3H]acetylcholine and [3H](-)nicotine label the same recognition site in rat brain

High affinity binding sites for [3H]acetylcholine and [3H](-)nicotine in rat brain were compared with respect to key characteristics, any one of which should distinguish them if they are different. The density of binding sites for each ligand varied approximately 4-fold in five areas of rat forebrain, but in each of these areas and in human cerebral cortex as well, the densities of [3H]acetylcholine- and [3H](-)nicotine-binding sites were indistinguishable. The affinity of [3H](-)nicotine was higher than that of [3H]acetylcholine, but nicotinic cholinergic drugs competed for the sites labeled by the two ligands with similar affinities; and in each case, the site labeled displayed marked stereoselectivity for the enantiomers of nicotine. The binding of [3H]acetylcholine and [3H](-)nicotine was decreased to the same extent by preincubation of tissues with dithiothreitol, and the binding was restored by subsequent treatment with 5,5'-dithiobis-2-nitrobenzoic acid, indicating that a disulfide bond is required at or near the binding site for each ligand. Treatment of rats with nicotine for 10 days increased the density of binding sites for both ligands, and treatment with the cholinesterase inhibitor soman for 9 days decreased the density of binding sites for both ligands. Taken together, these results indicate that [3H]acetylcholine and [3H](-)nicotine bind to the same nicotinic cholinergic recognition site in rat brain.