Structural determinants of fast desensitization and desensitization-deactivation coupling in GABAa receptors

Fast IPSCs in the brain are predominantly caused by presynaptic release of GABA that activates GABA(A) receptor (GABA(A)R) channels. The IPSCs are shaped by the gating and desensitization properties of postsynaptic GABA(A)Rs. Specifically, fast desensitization has been suggested to decrease IPSC amplitude and to increase IPSC duration by slowing deactivation; however, the mechanisms underlying desensitization, deactivation, and their coupling are poorly understood. Consistent with this suggestion, alpha1beta3gamma2L GABA(A)Rs desensitize with a prominent fast phase and deactivate slowly, whereas alpha1beta3delta GABA(A)Rs desensitize without a fast phase and deactivate rapidly. Using the concentration-jump technique applied to excised patches, we studied GABA(A)Rs containing chimeras or exchange mutants between delta and gamma2L subunits to gain insight into the structural bases for fast desensitization and its coupling to deactivation. We demonstrated that the N terminus and two adjacent residues (V233, Y234) in the first transmembrane domain (TM1) of the delta subunit were both required to abolish fast desensitization. Additionally, these residues in TM1 of the gamma2L subunit (Y235, F236) were critical for desensitized states to prolong deactivation after removal of GABA, because mutations resulted in accelerated deactivation despite unaltered desensitization time course. Interestingly, control of desensitization and deactivation was independent of the identity (gamma2L or delta subunit sequence) of TM2, indicating that structures related to the putative channel gate may play a less direct role in desensitization than previously suggested.