LONG-TERM DESENSITIZATION OF NICOTINIC ACETYLCHOLINE-RECEPTORS IS REGULATED VIA PROTEIN-KINASE A-MEDIATED PHOSPHORYLATION

During prolonged application of transmitter, ligand-gated ion channels enter a nonconducting desensitized state. Studies on Torpedo electrop lax nicotinic acetylcholine (ACh) receptors have shown that entry into the desensitized state is accelerated by protein kinase A-dependent ( PKA) receptor phosphorylation. To examine the effects of phosphorylati on on desensitization of muscle-type ACh receptors, we expressed the f rog embryonic receptor type in Xenopus oocytes. Treatment of embryonic muscle ACh receptors with 8-Br cAMP had no measurable effect on the r ate of entry into a desensitized state, but it greatly accelerated the recovery from desensitization. Three complementary approaches to redu ce the levels of receptor phosphorylation provided additional evidence for a role of PKA-dependent phosphorylation in rescuing receptors fro m longterm desensitization. Inactivation of the endogenous PKA activit y by coexpression of an inhibitor protein, treatment of receptors with phosphatase, and removal of phosphorylation sites by site-specific su bunit mutation all resulted in slowed recovery. Our findings point to the existence of two distinct desensitized states: one requiring sever al seconds for full recovery and a second state from which recovery re quires minutes. Receptors lacking PKA phosphorylation sites exhibit a pronounced increase in the slowly recovering component of desensitizat ion, suggesting that receptor phosphorylation speeds overall recovery by reducing the entry into a deep desensitized state. This newly descr ibed effect of phosphorylation on ACh receptor function may serve as a n important modulator of postsynaptic receptor sensitivity.