Sodium-dependent high-affinity binding of [3H]hemicholinium-3 in the rat brain: a potentially selective marker for presynaptic cholinergic sites

This report describes the membrane binding properties of 3H]hemicholinium-3 (3H]HC-3), a selective inhibitor of sodium-dependent high-affinity choline uptake (SDHACU) in cholinergic nerve terminals. Under the described assay conditions, 3H]HC-3 bind with a saturable population of high-affinity (apparent Kd = 1.9 nM) CNS membrane sites having the regional distribution: striatum much greater than hippocampus greater than cerebral cortex greater than cerebellum. High-affinity 3H]HC-3 binding is entirely dependent upon the presence of sodium chloride (EC50 = 35-50 mM) and is markedly reduced when other salts of sodium or monovalent ions are substituted. 3H]HC-3 binding is inhibited by choline (Ki = 6 microM) and acetylcholine (Ki = 35 microM) but markedly less sensitive to other cholinergic agents and metabolic inhibitors. In light of the similar ionic dependencies, regional distributions and pharmacological specificities of 3H]HC-3 binding and SDHACU, closely associated sites may be involved in both processes.