Functional kainate-selective glutamate receptors in cultured hippocampal neurons

Glutamate mediates fast synaptic transmission at the majority of excitatory synapses throughout the central nervous system by interacting with different types of receptor channels. Cloning of glutamate receptors has provided evidence for the existence of several structurally related subunit families, each composed of several members. It has been proposed that KA1 and KA2 and GluR-5, GluR-6, and GluR-7 families represent subunit classes of high-affinity kainate receptors and that in vivo different kainate receptor subtypes might be constructed from these subunits in heteromeric assembly. However, despite some indications from autoradiographic studies and binding data in brain membranes, no functional pure kainate receptors have so far been detected in brain cells. We have found that early after culturing, a high percentage of rat hippocampal neurons express functional, kainate-selective glutamate receptors. These kainate receptors show pronounced desensitization with fast onset and very slow recovery and are also activated by quisqualate and domoate, but not by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate. Our results provide evidence for the existence of functional glutamate receptors of the kainate type in nerve cells, which are likely to be native homomeric GluR-6 receptors.