Modal Behavior of IGluR3 AMPA-Receptor-Channels in Cell Attached Recordings

Abstract

Ionotropic glutamate receptors (iGluR's) are ligand gated ion channels that mediate most of the fast excitatory neurotransmission in the CNS. Aberrant function of glutamate neurotransmission can lead to epilepsy and other neurodegenerative disorders. The extracellular ligand binding domain is a bilobal structure that binds an agonist and induces channel activation. Cell attached patch recordings were performed with both full and partial agonists on HEK 293 cells stably expressing homomeric GluR3-flip receptor channels. Single channel data were analyzed using QuB software to detect channel conductance states and to determine a simple model of agonist dependent channel activity and underlying modal behavior. Amplitude analysis uncovered three conductance states, 15 pS, 27 pS, and 40 pS, in the presence of the full agonist, glutamate, as well as the partial agonists, fluorowillardiine, chlorowillardiine and nitrowillardiine. Different modes of activation ranging from low to high open probability exist for this channel. The dwell times for the high mode are longer compared to the low mode. In the presence of the full agonist, glutamate, during a high mode of activation, the channel prefers to open to the intermediate and large conductance states. In the presence of the willardiine partial agonists, the channel opens more frequently to the smallest and intermediate conductance states. Kinetic modeling using maximum interval likelihood rate optimization revealed two time constants in each open state and at least three in the closed state for the full and partial agonists. These data suggests that the mechanisms of channel activation are similar for both full and partial agonists but the transition rates between states differ. Supported by NIH NS049223 and NS063518.