Desensitized states prolong GABA_A channel responses to brief agonist pulses

We studied the role of desensitization at inhibitory synapses by comparing nonequilibrium GABA_A channel gating with inhibitory postsynaptic currents (IPSCs). Currents activated by brief pulses of 1-10 mM GABA to outside-out patches from cultured hippocampal neurons mimicked GABA-mediated IPSCs. Although the average open time of single GABA_A channels following brief pulses was less than 10 ms, channels entered long (τ = 38-69 ms) closed states and subsequently reopened. Movement through these states resulted in paired-pulse desensitization. The time required for deactivation after removal of agonist also increased in proportion to the extent of desensitization. These results suggest that visits to desensitized states buffer the channel in bound conformations and underlie the expression of long-lasting components of the IPSC. Reopening after GABA_A receptor desensitization may thus enhance inhibitory synaptic transmission by prolonging the response to a brief synaptic GABA transient.