Fig. 3.

The decay of GluR1 and GluR4 currents varies little with subunit occupancy. a, Hypothetical kinetic model for AMPA receptor gating. Each subunit of the tetrameric channel contains a binding site for glutamate. There are therefore five discrete closed states, which differ in the number of subunits occupied by glutamate (0–4, states R0 toR4). Channels with two, three, or four glutamates bound can open to discrete subconductance levels (O2,O3, O4) where unitary conductance increases with receptor occupancy. Channels with one, two, three, or four subunits occupied by glutamate can desensitize (statesD1 to D4). b,c, Currents (dotted lines) evoked by sustained applications of the indicated concentrations of glutamate in outside-out patches from cells transfected with the cDNAs encoding GluR1 (b) or GluR4 (c). The decays of the currents have been fitted with single exponential functions (smooth curves) that gave time constants of 1.92–2.18 msec for GluR1 and 2.78–3.27 msec for GluR4. The insets show the currents superimposed after they were normalized to their peak amplitudes. d, Plot of the decay time constants for GluR1 (circles) and GluR4 (squares) channels obtained from single exponential fits to the decay of currents evoked by sustained applications of a range of glutamate concentrations (mean ± SEM values from 6 and 7 patches, respectively). e,f, Currents (dotted lines) through GluR1 (e) and GluR4 (f) channels evoked by sustained applications of 5 mm glutamate before (control) and after equilibrating individual patches in solutions containing 100–500 nmNBQX. The decays of the currents have been fitted with single exponential functions (smooth curves) that gave time constants of 1.88–2.21 msec for GluR1 and 2.71–3.35 msec for GluR4.g, Plot of the decay time constants for GluR1 (circles) and GluR4 (squares) channels as a function of NBQX concentration (mean ± SEM values from 4 and 5 patches, respectively).