The role of the nAChR subunits α5, β2, and β4 on synaptic transmission in the mouse superior cervical ganglion

Our previous immunoprecipitation analysis of nicotinic acetylcholine receptors (nAChRs) in the mouse superior cervical ganglion (SCG) revealed that approximately 55%, 24%, and 21% of receptors are comprised of alpha 3 beta 4, alpha 3 beta 4 alpha 5, and alpha 3 beta 4 beta 2 subunits, respectively. Moreover, mice lacking beta 4 subunits do not express alpha 5-containing receptors but still express a small number of alpha 3 beta 2 receptors. Here, we investigated how synaptic transmission is affected in the SCG of alpha 5 beta 4-KO and alpha 5 beta 2-KO mice. Using an ex vivo SCG preparation, we stimulated the preganglionic cervical sympathetic trunk and measured compound action potentials (CAPs) in the postganglionic internal carotid nerve. We found that CAP amplitude was unaffected in alpha 5 beta 4-KO and alpha 5 beta 2-KO ganglia, whereas the stimulation threshold for eliciting CAPs was significantly higher in alpha 5 beta 4-KO ganglia. Moreover, intracellular recordings in SCG neurons revealed no difference in EPSP amplitude. We also found that the ganglionic blocking agent hexamethonium was the most potent in alpha 5 beta 4-KO ganglia (IC50: 22.1 mu mol/L), followed by alpha 5 beta 2-KO (IC50: 126.7 mu mol/L) and WT ganglia (IC50: 389.2 mu mol/L). Based on these data, we estimated an IC50 of 568.6 mu mol/L for a receptor population consisting solely of alpha 3 beta 4 alpha 5 receptors; and we estimated that alpha 3 beta 4 alpha 5 receptors comprise 72% of nAChRs expressed in the mouse SCG. Similarly, by measuring the effects of hexamethonium on ACh-induced currents in cultured SCG neurons, we found that alpha 3 beta 4 alpha 5 receptors comprise 63% of nAChRs. Thus, in contrast to our results obtained using immunoprecipitation, these data indicate that the majority of receptors at the cell surface of SCG neurons consist of alpha 3 beta 4 alpha 5.