INHIBITION OF NICOTINE-EVOKED [H-3] DOPAMINE RELEASE BY PYRIDINO N-SUBSTITUTED NICOTINE ANALOGS - A NEW CLASS OF NICOTINIC ANTAGONIST

A series of N-substituted nicotine analogues were synthesized and eval uated for their ability to inhibit nicotine-evoked [H-3]dopamine ([H-3 ]DA) release from rat striatal slices and for displacement of [H-3]nic otine binding from rat striatal membranes. Of the eleven compounds exa mined, the most efficacious analogues in the [H-3]DA release assay wer e those that contained a pyridino N-alkyl substituent of three carbons or more in length. Structure-activity relationships indicate that pot ency in the [H-3]DA release assay increases with increase in alkyl cha in length. Introduction of an aromatic or unsaturated residue into the pyridino-N substituent also afforded compounds with significant antag onist activity. The most efficacious and potent compound in the series was S-N-octylnicotinium iodide (NONI). NONI had a potency approximate ly two-fold that of the classical nicotinic antagonists, mecamylamine (MEC) and dihydro-beta-erythroidine (DHBE). In addition, over the conc entration range examined, NONI completely blocked the effect of nicoti ne to evoke [H-3]DA release, whereas the classical antagonists inhibit ed but did not completely block nicotine's effect. Furthermore, NONI d id not possess agonist activity at concentrations which completely blo cked nicotine-evoked [H-3]DA release. Results from competition assays for [H-3]nicotine binding revealed that all the analogues were able to displace [H-3]nicotine binding with a lower affinity as compared to t he reference compound DHBE. A significant correlation between alkyl ch ain length and affinity for the [H-3]nicotine binding site was observe d. Moreover, there was a lack of correlation between displacement of [ H-3]nicotine binding and inhibition of nicotine-evoked [H-3]DA release , suggesting that different nicotinic receptor subtypes are responsibl e for modulation of DA release and [H-3]nicotine binding in striatum. The pKa values determined for S(-)nicotine and one of the active antag onists, S-N-allyl-nicotinium iodide (NANI), indicates that the quatern ary ammonium analogues exist predominantly in their unprotonated forms at physiological pH. Molecular modeling studies suggest that these an tagonists 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 informa tion on the antagonist pharmacophore of the nicotinic receptor subtype s responsible for modulation of DA release and for nicotine binding in brain. Thus, it is proposed that these antagonists bind to the recept or in their unprotonated forms and that the binding mode involves inte raction of the quaternary pyridinium-N atom with the anionic site of t he receptor. The unprotonated pyrrolidine-N atom serves as the hydroge n bond acceptor, which reverses the roles the nitrogens normally play in the binding of nicotine to the receptor. The N-alkyl substituent mo st likely binds to a site that extends beyond the normal agonist pharm acophore 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 nicoti ne-evoked [H-3]DA release from DA nerve terminals in brain and displac e [H-3]nicotine binding from brain membranes, providing new tools for unraveling the role of neuronal nicotinic receptors. (C) 1995 Wiley-Li ss, Inc.