A high-affinity presynaptic kainate-type glutamate receptor facilitates glutamate exocytosis from cerebral cortex nerve terminals (synaptosomes)

Abstract

Ionotropic glutamate receptor agonists, kainate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and domoate, all facilitated 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes). The non-selective, non- N-methyl- d-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked kainate facilitation of glutamate release. AMPA responses were non-desensitizing and insensitive to the AMPA receptor desensitization inhibitor, cyclothiazide. The AMPA receptor antagonist GYKI 52466 failed to block ionotropic glutamate receptor-mediated facilitation, but the ionotropic glutamate receptor 6 kainate receptor subunit antagonist NS-102 was a potent blocker. Furthermore, kainate and AMPA responses were not additive. Taken together, our results indicate that, in the cerebral cortex, both kainate and AMPA may be facilitating glutamate release through the activation of a high-affinity kainate receptor containing glutamate receptor 6 kainate subunits. Kainate enhanced 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential, indicating that a ligand-gated ion channel that conducts cations may underlie the mechanism by which kainate mediates facilitation of glutamate release. While the facilitatory effect of kainate on glutamate release is consistent with a classical ionotropic action of ionotropic glutamate receptors, our observation that kainate inhibits GABA release suggests that alternative presynaptic mechanisms may operate in cerebrocortical nerve terminals to mediate the ionotropic glutamate receptor modulation of glutamate and GABA release. We conclude that high-affinity kainate-type glutamate autoreceptors represent a positive feed-forward system for potentiating the release of glutamate from cerebrocortical nerve terminals.