Pharmacological characterization of nicotinic receptor-mediated acetylcholine release in rat brain--an in vivo...

In vivo microdialysis was used to investigate nicotinic receptor-mediated acetylcholine release in the hippocampus, frontal cortex, and striatum of freely moving rats. Intraperitoneal administration of (-)-nicotine increased the release of acetylcholine in the hippocampus and frontal cortex but not in the striatum. (-)-Nicotine exhibited a bell-shaped dose-response relationship, and showed attenuation of response at the highest dose (5.0 mg/kg i.p.) in both the hippocampus and frontal cortex. In the hippocampus, (-)-nicotine (1.0 mg/kg i.p.)-induced increase of acetylcholine release was blocked by pretreatment with the centrally acting nicotinic receptor channel blocker, mecamylamine (1.0 mg/kg i.p.), but not by hexamethonium (5.0 mg/kg i.p.), suggesting that the effects of (-)-nicotine were mediated by the central nicotinic receptor. (S)-3-methyl-5-(1-methyl-2-pyrrolidinyl)isoxazole (ABT-418, 1.0 and 5.0 mg/kg i.p.), reported to be a selective agonist for alpha4beta2 nicotinic receptor subunits, also enhanced the release of acetylcholine in the hippocampus, while 3-(2,4-dimethoxybenzlidene)-anabaseine (GTS-21, 1.0 and 5.0 mg/kg i.p.), which has high affinity for the alpha7 nicotinic receptor subunit, was without effect. The natural alkaloids isolated from plants, (-)-cytisine and (-)-lobeline, had little effect on acetylcholine release from the hippocampus. A competitive antagonist for alpha4beta2 subunits of the nicotinic receptor, dihydro-beta-erythroidine, and a partial agonist for the beta2 subunit-containing nicotinic receptor, (-)-cytisine, inhibited (-)-nicotine-induced increase of acetylcholine release from the hippocampus, whereas a selective antagonist for the alpha7 subunit, methyllycaconitine, and a partial agonist for the alpha3 subunit-containing nicotinic receptor, (-)-lobeline, did not. These results indicate that there are certain differences among brain regions in the response of nicotinic receptor-mediated acetylcholine release and that (-)-nicotine-induced acetylcholine release in the rat hippocampus may be attributed to activation of the alpha4beta2 nicotinic receptor subunits.