Inhibition of nicotine‐evoked [3H] dopamine release by pyridino N‐substituted nicotine analogues: A new class of nicotinic antagonist

A series of N-substituted nicotine analogues were synthesized and evaluated for their ability to inhibit nicotine-evoked [ 3 H]dopamine ([ 3 H]DA) release from rat striatal slices and for displacement of [ 3 H]nicotine binding from rat striatal membranes. Of the eleven compounds examined, the most efficacious analogues in the [ 3 H]DA release assay were those that contained a pyridino N-alkyl substituent of three carbons or more in length. Structure-activity relationships indicate that potency in the [ 3 H]DA release assay increases with increase in alkyl chain length. Introduction of an aromatic or unsaturated residue into the pyridino-N substituent also afforded compounds with significant antagonist activity. The most efficacious and potent compound in the series was S-N-octylnicotinium iodide (NONI). NONI had a potency approximately two-fold that of the classical nicotinic antagonists, mecamylamine (MEC) and dihydro-β-erythroidine (DHBE). In addition, over the concentration range examined, NONI completely blocked the effect of nicotine to evoke [ 3 H]DA release, whereas the classical antagonists inhibited but did not completely block nicotine's effect. Furthermore, NONI did not possess agonist activity at concentrations which completely blocked nicotine-evoked [ 3 H]DA release. Results from competition assays for [ 3 H]nicotine binding revealed that all the analogues were able to displace [ 3 H]nicotine binding with a lower affinity as compared to the reference compound DHBE. A significant correlation between alkyl chain length and affinity for the [ 3 H]nicotine binding site was observed. Moreover, there was a lack of correlation between displacement of [ 3 H]nicotine binding and inhibition of nicotine-evoked [ 3 H]DA release, suggesting that different nicotinic receptor subtypes are responsible for modulation of DA release and [ 3 H]nicotine binding in striatum. The pKa values determined for S(-)nicotine and one of the active antagonists, S-N-allyl-nicotinium iodide (NANI), indicates that the quaternary ammonium analogues exist predominantly in their unprotonated forms at physiological pH. Molecular modeling studies suggest that these antagonists may interact with the nicotinic receptor in a novel binding mode which is different from the mode of interaction of nicotine with this receptor. This structure-activity data may provide useful information on the antagonist pharmacophore of the nicotinic receptor subtypes responsible for modulation of DA release and for nicotine binding in brain. Thus, it is proposed that these antagonists bind to the receptor in their unprotonated forms and that the binding mode involves interaction of the quaternary pyridinium-N atom with the anionic site of the receptor. The unprotonated pyrrolidine-N atom serves as the hydrogen bond acceptor, which reverses the roles the nitrogens normally play in the binding of nicotine to the receptor. The N-alkyl substituent most likely binds to a site that extends beyond the normal agonist pharmacophore volume, which may prevent the receptor protein from achieving its open-channel quaternary form. In conclusion, this study describes a new class of efficacious nicotinic antagonists which inhibit nicotine-evoked [ 3 H]DA release from DA nerve terminals in brain and displace [ 3 H]nicotine binding from brain membranes, providing new tools for unraveling the role of neuronal nicotinic receptors.