1. Kainate receptors with pharmacological properties similar to those of the GluR5 subunit have been shown to modulate inhibitory synaptic transmission in the CA1 region of the hippocampus. The kinetic properties of currents gated by GluR5 receptors have not been examined in detail. Here we describe several biophysical features of recombinant GluR5 receptors expressed in HEK293 cells. 2. We found that homomeric GluR5 receptors can exhibit striking inter-cell variability in channel kinetics in response to the agonists kainate and glutamate. Desensitization rates in response to kainate varied between individual cells by nearly 1000-fold (range, 1.5 ms to 1.5 s), while glutamate desensitization rates differed by 9-fold (range, 1.0 to 9.0 ms). 3. The time course of recovery from desensitization in response to glutamate also showed intercell variation. The majority of glutamate currents in GluR5-expressing cells recovered from desensitization with two widely separated exponential components: 50 ± 10 ms and 5.1 ± 1.0 s (contributing 37.6% and 62.4% of the sum of the exponential fits, respectively). In contrast, currents with the fastest desensitization kinetics had a recovery time course of 4.8 ± 0.3 s. 4. Kainate receptors in murine dorsal root ganglion neurons are likely to be composed of homomeric GluR5 subunits. These receptor currents recovered from glutamate desensitization with a biexponential time course of 36 ± 4 ms and 4.7 ± 0.7 s. 5. These results suggest that aspects of GluR5 kainate receptor function are modulated by intracellular mechanism(s). At synapses such mechanisms could regulate the frequency-response relationship of synaptic kainate receptors by altering their rate of entry into and recovery from desensitization.
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