Constitutive endocytosis of GABA(A) receptors by an association with the adaptin AP2 complex modulates inhibitory synaptic currents in hippocampal neurons

Type A GABA receptors (GABA(A)) mediate the majority of fast synaptic inhib ition in the brain and are believed to be predominantly composed of alpha, beta, and gamma subunits. Although changes in cell surface GABA(A) receptor number have been postulated to be of importance in modulating inhibitory s ynaptic transmission, little is currently known on the mechanism used by ne urons to modify surface receptor levels at inhibitory synapses. To address this issue, we have studied the cell surface expression and maintenance of GABA(A) receptors. Here we show that constitutive internalization of GABA(A ) receptors in hippocampal neurons and recombinant receptors expressed in A 293 cells is mediated by clathrin- dependent endocytosis. Furthermore, we i dentify an interaction between the GABA(A) receptor beta and gamma subunits with the adaptin complex AP2, which is critical for the recruitment of int egral membrane proteins into clathrin-coated pits. GABA(A) receptors also c olocalize with AP2 in cultured hippocampal neurons. Finally, blocking clath rin-dependant endocytosis with a peptide that disrupts the association betw een amphiphysin and dynamin causes a large sustained increase in the amplit ude of miniature IPSCs in cultured hippocampal neurons. These results sugge st that GABA(A) receptors cycle between the synaptic membrane and intracell ular sites, and their association with AP2 followed by recruitment into cla thrin- coated pits represents an important mechanism in the postsynaptic mo dulation of inhibitory synaptic transmission.