Synaptic activation of presynaptic kainate receptors on hippocampal mossy fiber synapses

Kainate receptors (KARs) are a poorly understood family of ionotropic glutamate receptors. A role for these receptors in the presynaptic control of transmitter release has been proposed but remains controversial. Here, KAR agonists are shown to enhance fiber excitability, and a number of experiments show that this is a direct effect of KARs on the presynaptic fibers. In addition, KAR activation inhibits evoked transmitter release from mossy fiber synapses. Synaptic release of glutamate from either neighboring mossy fiber synapses or associational/commisural (A/C) synapses results in the activation of these presynaptic ionotropic KARs. These results, along with previous studies, indicate that KARs, through the endogenous release of glutamate, mediate excitatory postsynaptic potentials (EPSPs), alter presynaptic excitability, and modulate transmitter release.