Characterization of quisqualate receptor desensitization in cultured postnatal rat hippocampal neurons

The quisqualate class of glutamate receptors is thought to play an important role in excitatory synaptic transmission, synaptic plasticity, and neuronal death. Since desensitization is a prominent feature of the responses mediated by this class of receptors, we have characterized the rapidly desensitizing quisqualate response in cultured postnatal rat hippocampal neurons using the whole-cell patch-clamp technique. Quisqualate and its structural analogs elicit a peak current that rapidly decays to a steady-state level. In contrast, currents induced by kainate, NMDA, and their structural analogs exhibit either no decay or a much slower decay. The biophysical and pharmacological properties of the peak and steady-state quisqualate currents indicate that both are mediated by an ionotropic quisqualate receptor. Quisqualate currents desensitized monoexponentially by ~70% with a time constant near 80 msec. Both the rate and percentage of desensitization showed slight voltage dependence and were concentration dependent, reaching maximal values at saturation. Additionally, the overlap of the dose-response curves for activation of the steady-state current and desensitization of the peak current by a conditioning dose suggests that the two processes are related. Furthermore, desensitizing quisqualate currents were observed when Ca²⁺, Mg²⁺, Na⁺, K⁺, and Cl^- were removed from the extracellular solution or their concentrations greatly reduced. These results suggest that the decline in the response is not caused by a simple open channel block mechanism. Despite the lack of desensitization by kainate, our observations are consistent with the hypothesis that quisqualate and kainate act at a single receptor-channel complex. Kainate and quisqualate appeared to interact competitively when applied simultaneously and noncompetitively when quisqualate was applied first. In addition, saturating doses of quisqualate and kainate gave steady-state currents of equal amplitude in neurons treated with the lectin WGA, an inhibitor of quisqualate receptor desensitization.