N-n-alkylpyridinium analogs, a novel class of nicotinic receptor antagonists: Selective inhibition of nicotine-evoked [³H]dopamine overflow from superfused rat striatal slices

Structural simplification of N-n-alkylnicotinium analogs, antagonists at neuronal nicotinic acetylcholine receptors (nAChRs), was achieved by removal of the N-methylpyrrolidino moiety affording N-n-alkylpyridinium analogs with carbon chain lengths of C1 to C20. N-n-Alkylpyridinium analog inhibition of [³H]nicotine and [³H]methyllycaconitine binding to rat brain membranes assessed interaction with α4β2* and α7* nAChRs, respectively, whereas inhibition of nicotine-evoked ³H overflow from [³H]dopamine ([ ³H]DA)-preloaded rat striatal slices assessed antagonist action at nAChR subtypes mediating nicotine-evoked DA release. No inhibition of [ ³H]methyllycaconitine binding was observed, although N-n-alkylpyridinium analogs had low affinity for [³H]nicotine binding sites, i.e., 1 to 3 orders of magnitude lower than that of the respective N-n-alkylnicotinium analogs. These results indicate that the N-methylpyrrolidino moiety in the N-n-alkylnicotinium analogs is a structural requirement for potent inhibition of α4β2* nAChRs. Importantly, N-n-alkylpyridinium analogs with n-alkyl chains < C10 did not inhibit nicotine-evoked [³H]DA overflow, whereas analogs with n-alkyl chains ranging from C10 to C20 potently and completely inhibited nicotine-evoked [ ³H]DA overflow (IC₅₀ = 0.12-0.49 μM), with the exceptions of N-n-pentadecylpyridinium bromide (C15) and N-n-eicosylpyridinium bromide (C20), which exhibited maximal inhibition of ∼50%. The mechanism of inhibition of a representative analog of this structural series, N-n-dodecylpyridinium iodide, was determined by Schild analysis. Linear Schild regression with slope not different from unity indicated competitive antagonism at nAChRs mediating nicotine-evoked [³H]DA overflow and a K _B value of 0.17 μM. Thus, the simplified N-n-alkylpyridinium analogs are potent, selective, and competitive antagonists of nAChRs mediating nicotine-evoked [³H]DA overflow, indicating that the N-methylpyrrolidino moiety is not a structural requirement for interaction with nAChR subtypes mediating nicotine-evoked DA release.