Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Comparative Study
, 91 (4), 1336-46

The Relationship Between Agonist Potency and AMPA Receptor Kinetics

Affiliations
Comparative Study

The Relationship Between Agonist Potency and AMPA Receptor Kinetics

Wei Zhang et al. Biophys J.

Abstract

AMPA-type glutamate receptors are tetrameric ion channels that mediate fast excitatory synaptic transmission in the mammalian brain. When agonists occupy the binding domain of individual receptor subunits, this domain closes, triggering rearrangements that couple agonist binding to channel opening. Here we compare the kinetic behavior of GluR2 channels activated by four different ligands, glutamate, AMPA, quisqualate, and 2-Me-Tet-AMPA, full agonists that vary in potency by up to two orders of magnitude. After reduction of desensitization with cyclothiazide, deactivation decays were strongly agonist dependent. The time constants of decay increased with potency, and slow components in the multiexponential decays became more prominent. The desensitization decays of agonist-activated currents also contained multiple exponential components, but they were similar for the four agonists. The time course of recovery from desensitization produced by each agonist was described by two sigmoid components, and the speed of recovery varied substantially. Recovery was fastest for glutamate and slowest for 2-Me-Tet-AMPA, and the amplitude of the slow component of recovery increased with agonist potency. The multiple kinetic components appear to arise from closed-state transitions that precede channel gating. Stargazin increases the slow kinetic components, and they likely contribute to the biexponential decay of excitatory postsynaptic currents.

Figures

FIGURE 1
FIGURE 1
Deactivation shows agonist-dependent components that do not depend on receptor occupancy. (a) Mean concentration–response data for steady-state currents (in 100 μM CTZ). The smooth curves are Hill-type fits. Current amplitudes were normalized to currents evoked in the same patch by near-saturating concentrations. Error bars show mean ± SE values, which in most cases were less than half the symbol size. (b) Currents evoked in individual patches by 2-ms applications (arrowheads) of 10 mM glutamate, 0.5 mM AMPA, 0.2 mM quisqualate, and 0.1 mM 2-Me-Tet-AMPA (in CTZ). The decays of the currents were fitted with multiexponential functions (solid curves; individual components shown as dashed lines). (c) For each of the four agonists, weighted mean time constants are plotted against the EC50 ratio (glutamate EC50/agonist EC50). (d) Currents evoked by different concentrations of 2-Me-Tet-AMPA (left). In the panels to the right, the individual currents were scaled to have the same peak amplitude, and the slow components obtained from the biexponential fits to the decays are shown as solid curves.
FIGURE 2
FIGURE 2
Glutamate-induced desensitization shows multiple kinetic components that do not depend on receptor occupancy. (a) Current evoked by 100-ms applications of 10 mM glutamate. Left and right panels show one- and two-exponential fits (solid curves) to the decays. The residuals are also shown (res; obtained by subtracting the data from the fit). Insets are the same currents and fits on an expanded timescale (dashed lines show individual components). (b) Currents evoked by sustained applications of two different concentrations of glutamate (left). In the panels to the right, the individual currents were scaled to have the same peak amplitude. The individual components obtained from the biexponential fits to the decays are shown as dashed curves.
FIGURE 3
FIGURE 3
Desensitization decays are similar for each agonist. Currents evoked by 100-ms applications of (a) 10 mM glutamate, (b) 0.5 mM AMPA, (c) 0.2 mM quisqualate, and (d) 0.1 mM 2-Me-Tet-AMPA. The decays of the currents were fitted with biexponential functions (solid curves; dashed lines show individual components).
FIGURE 4
FIGURE 4
Possible kinetic mechanisms. In model 1, one cleft closure step is followed by gating transitions to three different open states. In models 2 and 3, gating transitions are preceded by three cleft-closure steps that occur in parallel or sequentially. Only a single subunit is shown.
FIGURE 5
FIGURE 5
Deactivation depends on the duration of exposure to agonist and is faster for T686S mutant channels. (a) Decays of currents at the end of 2-ms (dotted traces) and 500-ms (solid traces) agonist applications (in CTZ). (b) Plot of weighted mean time constants of deactivation versus application time (log scale) for glutamate and 2-Me-Tet-AMPA. The curves are single-exponential fits. (c) Currents at the end of 50-ms applications of 0.2 mM quisqualate for GluR2-WT (solid trace) and GluR2-T686S (dotted curve). (d) The time constants of deactivation were faster and the relative amplitude of the slow component was smaller for the T686S mutant (mean ± SE, four to six patches).
FIGURE 6
FIGURE 6
Recovery from desensitization shows fast and slow components. Left panels are examples of the results from two-pulse protocols to measure recovery from desensitization with 10 mM glutamate, 0.5 mM AMPA, 0.2 mM quisqualate, and 0.1 mM 2-Me-Tet-AMPA. Other panels show mean (± SE) data from six to eight patches. The data were fitted with two Hodgkin-Huxley–type components. In b and d, the right panels show the early phase of recovery.
FIGURE 7
FIGURE 7
Stargazin increases the slow component of deactivation. (a and b) Currents evoked by 2-ms applications of 10 mM glutamate (no CTZ) in patches containing GluR4 (a) or GluR4 and stargazin (b). The slow components from biexponential fits to the decays are shown (solid curves). Peak currents are off-scale. (c) Currents evoked by 2-ms applications of 10 mM glutamate (in CTZ) in patches containing GluR4 (dotted trace) and GluR4/stargazin (solid trace) channels. (d) Mean time constants for the fast and slow deactivation components (τ1, τ2: left and middle panels) and the mean relative amplitude of the slow component (right) without and with coexpression of stargazin in the absence and presence of CTZ. Stargazin increased both deactivation time constants and the relative amplitude of the slow deactivation component seen with and without CTZ (mean ± SE, data from four and six patches).

Similar articles

See all similar articles

Cited by 30 articles

See all "Cited by" articles

Publication types

MeSH terms

Substances

Feedback