Kainate receptors in hippocampal CA3 subregion: evidence for a role in regulating neurotransmitter release

The hippocampal CA3 subregion of the rat is characteristically enriched in kainate receptors. At the synaptic level, the subcellular localization of these receptors is still a matter of debate. The CA3 pyramidal cells are particularly sensitive to excitotoxicity induced by kainate, which is in agreement with the high levels of kainate receptors in the stratum lucidum of the hippocampal CA3 subregion. Immunocytochemical studies, using antibodies against kainate receptor subunits, clearly demonstrated the presence of postsynaptic kainate receptors. However, it was not possible at the time to identify the activity of postsynaptic kainate receptors as mediators of the synaptic transmission. There are also reports showing the labeling of unmyelinated axons and nerve terminals with antibodies against kainate receptor subunits. The evidence for the presence of presynaptic kainate receptors in the hippocampus is further substantiated by the demonstration that stimulation of kainate receptors in synaptosomes isolated from the rat hippocampal CA3 subregion increases the intracellular free Ca2+ concentration ([Ca2+]i) coupled to the release of glutamate. These results support the model proposed by Coyle (1983), in which the excitotoxicity induced by kainate involves the activation of presynaptic kainate receptors, causing the release of glutamate. According to this model, the neurotoxic effect of kainate in the rat hippocampal CA3 subregion involves a direct effect on presynaptic kainate receptors and an indirect effect on postsynaptic glutamate receptors due to the enhanced release of glutamate.