Two pharmacologically distinct components of nicotinic receptor-mediated rubidium efflux in mouse brain require the beta 2 subunit

Nicotinic agonist-stimulated efflux of Rb-86(+) from mouse brain synaptosom es was monitored continuously by on-line radioactivity detection. The conce ntration-effect curve following a 5-s stimulation with acetylcholine was bi phasic (EC50 = 7.2 and 550 mu M). alpha-Bungarotoxin (100 nM) did not inhib it the response, but dihydro-beta-erythroidine (DH beta E) blocked both pha ses with differing potency (average IC50 =.22 and 8.9 mu M for responses ac tivated by low and high acetylcholine concentrations, respectively). Differ ential sensitivity DH beta E inhibition was used to measure stimulation of Rb-86(+) efflux by 17 nicotinic agonists, which differed markedly in potenc y and efficacy. All agonists were more potent at the DH beta E-sensitive si te. Both components were inhibited by the six antagonists tested. Methyllyc aconitine and DH beta E were more potent for the DH beta E-sensitive compon ent, whereas hexamethonium was more potent at the DH beta E-resistant compo nent, Both DH beta E-sensitive and DH beta E-resistant responses were reduc ed more than 95% in beta 2-null mutant mice, establishing the requirement f or the beta 2 subunit for both components, Both components were widely, but not identically, distributed throughout the brain. The DH beta E-sensitive component appears to be identical with agonist-stimulated Rb-86(+) efflux described previously and is likely to be mediated by alpha 4 beta 2 recepto rs. The DH beta E-resistant component is a novel, active, and widely distri buted response mediated by nicotinic receptor(s) that also require the beta 2 subunit.