Desensitization of Kainate Receptors by Kainate, Glutamate and Diastereomers of 4-Methylglutamate

Abstract

The potencies of kainate, glutamate and diastereomers of 4-methylglutamate were determined for activation and steady-state desensitization of GluR6 and dorsal root ganglion-type kainate receptors using whole-cell voltage clamp. In HEK293 cells expressing GluR6, all four diastereomers induced desensitizing inward currents at relatively high concentrations (>50 μM), however, the 2S,4R diastereomer (2S,4R-4MG; SYM 2081) was approximately 100-fold more potent than the other three. The ec 50 for receptor activation by 2S,4R-4MG (1.0 μM) was similar to that for kainic acid (1.8 μM), but 2S,4R-4MG was significantly more potent than kainate, glutamate or the other diastereomers of 4-methylglutamate at producing steady-state desensitization of GluR6 receptors. ic 50 s for desensitization quantified using a fixed concentration of kainate as a test agonist were 7.6, 31 and 667 nM for 2S,4R-4MG, kainate and glutamate, respectively. In addition, 2S,4R-4MG fully desensitized native kainate receptors (of the GluR5 subtype) in dorsal root ganglion neurons with an ic 50 of 11 nM, compared to 3.4 μM for glutamate. For GluR6, recovery from desensitization displayed a similar time course for kainate and glutamate (τ = 3–4 s) but was roughly 20-fold slower for 2S,4R-4MG, which suggests that the rate of recovery is not entirely dependent on the affinity of ligand for the desensitized receptor. Following exposure to concanavalin A, application of kainate, glutamate and 2S,4R-4MG evoked very similar maximal currents that showed little or no desensitization. Lectin pretreatment produced a leftward shift in the concentration-response relationship for 2S,4R-4MG with an 11-fold reduction in the ec 50; however, no significant change in the ec 50 for kainate was observed. The characteristic of 2S,4R-4MG to potently and completely desensitize both recombinant GluR6 receptors and native receptors on dorsal root ganglion neurons suggests that this compound will be useful to study selective blockade of these receptors in the nervous system. © 1997 Elsevier Science Ltd.