The synaptic activation of kainate receptors

L-Glutamate, the principal excitatory neurotransmitter in the vertebrate central nervous system, acts on three classes of ionotropic glutamate receptors, named after the agonists AMPA, NMDA and kainate. AMPA receptors are known to mediate fast synaptic responses and NMDA receptors to mediate slow synaptic responses at most excitatory synapses in the brain. Kainate receptors are formed from a separate set of genes (GluR5-7, KA-1 and KA-2) and are widely distributed throughout the brain. They are implicated in epileptogenesis and cell death. However, the physiological functions of kainate receptors are not known. The development of 2,3-benzodiazepine antagonists that are selective for AMPA receptors enables kainate receptors to be specifically activated by exogenous ligands, such as kainate. Here we demonstrate that high-frequency stimulation of mossy fibres in rat hippocampal slices, in the presence of the highly selective AMPA receptor antagonist GYKI 53655 plus NMDA- and GABA-receptor antagonists, activates an inward current in CA3 neurons that has a pharmacology typical of kainate receptors. The finding that kainate receptors can be activated synaptically adds to the diversity of information transfer at glutamatergic synapses.