TARP‐associated AMPA receptors display an increased maximum channel conductance and multiple kinetically distinct open states

Abstract

Fast excitatory synaptic transmission in the CNS is mediated mainly by AMPA-type glutamate receptors (AMPARs), whose biophysical properties are dramatically modulated by the presence of transmembrane AMPAR regulatory proteins (TARPs). To help construct a kinetic model that will realistically describe native AMPAR/TARP function, we have examined the single-channel properties of homomeric GluA1 AMPARs in combination with the TARPs, γ-2, γ-4 and γ-5. In a saturating concentration of agonist, each of these AMPAR/TARP combinations gave rise to single-channel currents with multiple conductance levels that appeared intrinsic to the receptor-channel complex, and showed long-lived subconductance states. The open time and burst length distributions of the receptor complexes displayed multiple dwell-time components. In the case of γ-2- and γ-4-associated receptors, these distributions included a long-lived component lasting tens of milliseconds that was absent from both GluA1 alone and γ-5-associated receptors. The open time distributions for each conductance level required two dwell-time components, indicating that at each conductance level the channel occupies a minimum of two kinetically distinct open states. We have explored how these data place novel constraints on possible kinetic models of TARP-associated AMPARs that may be used to define AMPAR-mediated synaptic transmission.