Nicotinic-agonist stimulated (86)Rb(+) efflux and [(3)H]epibatidine binding of mice differing in beta2 genotype

Nicotinic acetylcholine receptor function and binding was measured in 12 brain regions from mice differing in β2 subunit expression. Function was measured by on-line detection of ⁸⁶Rb⁺ efflux stimulated under conditions that measure two pharmacologically distinct nicotinic responses: (1) stimulation with 10 μM nicotine, a response that is relatively sensitive to inhibition by the antagonist, dihydro-β-erythroidine (DHβE); and (2) stimulation with 10 μM epibatidine in the presence of 2 μM DHβE, a response that is relatively resistant to inhibition by DHβE. Deletion of the β2 subunit profoundly reduced both DHβE-sensitive and -resistant ⁸⁶Rb⁺ efflux in each brain region and essentially eliminated activity in regions such as cerebral cortex and thalamus. However, residual activity was observed in regions such as olfactory bulbs and inferior colliculus. [³H]Epibatidine binding was measured under conditions that allow estimation of both high- and low-affinity sites. High-affinity sites sensitive to inhibition by the nicotinic agonist, cytisine, were virtually eliminated in every region by the β2 null mutation. In contrast, only a subset of the high-affinity sites insensitive to inhibition by cytisine were eliminated in β2 null mutants, suggesting receptor heterogeniety. Similarly, low affinity [³H]epibatidine binding was heterogeneous in that a fraction of the sites required the β2 subunit. Many remaining sites were sensitive to inhibition by -bungarotoxin indicating that a subset of the low affinity [³H]epibatidine binding are of the 7* subtype. Distinct regional variation was observed among the 12 brain regions. These studies confirm important roles for β2-containing receptors in mediating pharmacologically distinct functions and as components of several identifiable binding sites.