Pharmacological characterization of a nicotinic autoreceptor in rat hippocampal synaptosomes

The modulation of ³H]ACh release by nicotinic compounds was studied in superfused rat hippocampal synaptosomes loaded with ³H]choline. (−)-Nicotine (0.1–10 μM) evoked a dose-dependent increase in ³H]ACh release; higher concentrations were less effective. Nicotine-evoked release was Ca²⁺-dependent, and blocked by the nicotinic antagonists dihydro-β-erythroidine, mecamylamine, and pempidine. The α7-selective antagonist methyllycaconitine did not inhibit nicotine-evoked release when tested at 1 μM, although at 10 μM some attenuation of the response was observed. Six agonists tested were equally efficacious in stimulating ³H]ACh release, as judged by the maximum responses, and gave the following EC₅₀ values: (±)-epibatidine 0.12 μM; (+)-anatoxin-a 0.14 μM; (−)-nicotine 0.99 μM; (−)-cytisine 1.06 μM; ABT-418 2.6 μM; isoarecolone 43 μM. 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. Special issue dedicated to Dr. Herman Bachelard.