Abstract

Nicotinic agonist-stimulated efflux of ⁸⁶Rb⁺from mouse brain synaptosomes was monitored continuously by on-line radioactivity detection. The concentration-effect curve following a 5-s stimulation with acetylcholine was biphasic (EC₅₀ = 7.2 and 550 μM). α-Bungarotoxin (100 nM) did not inhibit the response, but dihydro-β-erythroidine (DHβE) blocked both phases with differing potency (average IC₅₀ = .22 and 8.9 μM for responses activated by low and high acetylcholine concentrations, respectively). Differential sensitivity DHβE inhibition was used to measure stimulation of ⁸⁶Rb⁺ efflux by 17 nicotinic agonists, which differed markedly in potency and efficacy. All agonists were more potent at the DHβE-sensitive site. Both components were inhibited by the six antagonists tested. Methyllycaconitine and DHβE were more potent for the DHβE-sensitive component, whereas hexamethonium was more potent at the DHβE-resistant component. Both DHβE-sensitive and DHβE-resistant responses were reduced more than 95% in β2-null mutant mice, establishing the requirement for the β2 subunit for both components. Both components were widely, but not identically, distributed throughout the brain. The DHβE-sensitive component appears to be identical with agonist-stimulated⁸⁶Rb⁺ efflux described previously and is likely to be mediated by α4β2 receptors. The DHβE-resistant component is a novel, active, and widely distributed response mediated by nicotinic receptor(s) that also require the β2 subunit.