Receptor protection studies comparing recombinant and native nicotinic receptors: Evidence for a subpopulation of mecamylamine-sensitive native α3β4* nicotinic receptors

Studies involving receptor protection have been used to define the functional involvement of specific receptor subtypes in tissues expressing multiple receptor subtypes. Previous functional studies from our laboratory demonstrate the feasibility of this approach when applied to neuronal tissues expressing multiple nicotinic acetylcholine receptors (nAChRs). In the current studies, the ability of a variety of nAChR agonists and antagonists to protect native and recombinant α3β4 nAChRs from alkylation were investigated using nAChR binding techniques. Alkylation of native α3β4* nAChRs from membrane preparations of bovine adrenal chromaffin cells resulted in a complete loss of specific [3H]epibatidine binding. This loss of binding to native nAChRs was preventable by pretreatment with the agonists, carbachol or nicotine. The partial agonist, cytisine, produced partial protection. Several nAChR antagonists were also tested for their ability to protect. Hexamethonium and decamethonium were without protective activity while mecamylamine and tubocurarine were partially effective. Addition protection studies were performed on recombinant α3β4 nAChRs. As with native α3β4* nAChRs, alkylation produced a complete loss of specific [3H]epibatidine binding to recombinant α3β4 nAChRs which was preventable by pretreatment with nicotine. However, unlike native α3β4* nAChRs, cytisine and mecamylamine, provide no protection for alkylation. These results highlight the differences between native α3β4* nAChRs and recombinant α3β4 nAChRs and support the use of protection assays to characterize native nAChR subpopulations.