Differential modulation of β2 and β4 subunits of human neuronal nicotinic acetylcholine receptors by acidification

We have shown previously that acidification increases the affinity of agonists to rat alpha3beta4 nicotinic acetylcholine receptors ( nAChR) and accelerates both the activation and decay kinetics of agonist-induced currents recorded from human embryonic kidney 293 cells stably expressing the receptor (Abdrakhmanova et al., 2002b). Here, we report on experiments examining the effect of rapid acidification on four different subtypes (alpha3beta4alpha5, alpha4beta2, alpha3beta2, and alpha3beta2alpha5) of human neuronal nAChRs stably expressed in tsA201 cells using a piezoelectric device for rapid (< 5 ms) solution application. Application of ACh, at its EC50 concentration for each nAChR subtype, at pH values 7.4 and 6.0, showed that acidification, similarly to that reported for rat alpha 3 beta 4 acetylcholine receptors (AChRs), increased the amplitude and accelerated the activation and decay kinetics of the currents in human alpha 3 beta 4 alpha 5 AChRs by increasing their affinity to the agonist. In sharp contrast, acidification reduced the amplitude but accelerated the decay kinetics of the current in all human beta 2-containing nAChR subtypes (alpha 3 beta 2, alpha 3 beta 5 alpha 2, alpha 4 beta 2) examined in this study. Brief application of ACh at saturating concentration (1 mM) on alpha 3 beta 4 alpha 5 AChRs induced a rebound current upon rapid washout of the agonist at pH 7.4, but no rebound current was observed in alpha 3 beta 2 AChRs. Surprisingly, acidification, pH 6.0, applied only during the agonist pulse also accelerated the decay kinetics of the rebound current. Our data provide evidence for the specificity of proton-induced modulation of neuronal nAChRs based on their beta subunit composition. Furthermore, in alpha 3 beta 4 alpha 5 AChR, we find that protonation effects may persist, after washout of acidic solutions, consistent with proton-induced conformational changes of the receptor.