Pharmacological characterization of a nicotinic autoreceptor in rat hippocampal synaptosomes

The modulation of [H-3]ACh release by nicotinic compounds was studied in superfused rat hippocampal synaptosomes loaded with [H-3]choline. (-)-Nicotine (0.1-10 mu M) evoked a dose-dependent increase in [H-3]ACh release; higher concentrations were less effective. Nicotine-evoked release was Ca2+-dependent, and blocked by the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, and pempidine. The alpha 7-selective antagonist methyllycaconitine did not inhibit nicotine-evoked release when tested at 1 mu M, although at 10 mu M some attenuation of the response was observed. Six agonists tested were equally efficacious in stimulating [H-3]ACh release, as judged by the maximum responses, and gave the following EC(50) values: (+/-)-epibatidine 0.12 mu M; (+)-anatoxin-a 0.14 mu M; (-)-nicotine 0.99 mu M; (-)-cytisine 1.06 mu M; ABT-418 2.6 mu M; isoarecolone 43 mu M Each agonist generated a ''bell-shaped'' dose response curve, suggesting desensitisation at higher concentrations. This is supported by analysis of repetitive stimulation with (-)-nicotine and (-)-cytisine: S2/S1 ratios declined sharply with increasing concentration, whereas subsequent KCl-evoked release remained constant. These results are discussed in terms of possible nicotinic receptor subtypes that might be present on hippocampal nerve terminals.