Differential interactions of gentamicin with mouse junctional and extrajunctional ACh receptors expressed in Xenopus...

The nicotinic acetylcholine receptors (AChRs) from Torpedo electric organ and mouse muscles when expressed in Xenopus oocytes desensitize with different time courses. Initially, the role of cAMP-dependent phosphorylation on the gamma subunits in the different desensitization rates was investigated by expressing normal and mutant AChRs in the oocytes cultured in the presence of gentamicin. Mutant Torpedo AChRs lacking the potential cAMP-dependent phosphorylation sites in the gamma subunit appear to desensitize like normal Torpedo AChRs. Similarly, mutant mouse extrajunctional AChRs containing a newly created phosphorylation site in the gamma subunit appeared to desensitize like normal mouse AChRs, which lack the potential cAMP-dependent phosphorylation site in the gamma subunit. These results suggest that different rates of desensitization between the Torpedo and muscle extrajunctional AChRs are not attributable to differential cAMP-dependent phosphorylation of these AChRs. Subsequently, to determine whether gentamicin used in culturing oocytes differentially interacts with muscle junctional and extrajunctional AChRs, we analyzed rates of current decay following different gentamicin treatments. Both chronic and acute treatment with gentamicin profoundly accelerated the decay of whole-cell currents mediated by both types of AChR. The effect of prolonged gentamicin treatment on junctional AChRs was long lasting when compared to treatment on extrajunctional AChRs. Although the two types of AChR still desensitize differently in the absence of gentamicin, these results suggest that the characteristic desensitization of junctional and extrajunctional AChRs observed previously is largely due to differential interactions of gentamicin with the two types of AChR.