Synaptically released glutamate reduces g-aminobutyric acid (GABA)ergic inhibition in the hippocampus via kainate receptors (disinhibitionyguinea pigs)

Exogenous application of agonists at the kainate subtype of glutamate receptors has been shown to depress evoked monosynaptic inhibition by g-aminobutyric acid (GABA)ergic interneurons in the hippocampus. This observation has led to the hypothesis that synaptic release of endogenous glutamate might have a disinhibitory effect on neuronal circuits, in addition to depolarizing neurons via postsynaptic a-amino-3-hydroxy-5-methyl-4-isoxazolepropi- onic acid (AMPA), kainate, and N-methyl-D-aspartic acid (NMDA) receptors. It is not known, however, if glutamate released from excitatory neurons has the same kainate re- ceptor-mediated effect on monosynaptic inhibitory transmis- sion as exogenous agonist application. Indeed, the recent demonstration that excitatory synaptic signals elicited in interneurons are partly mediated by kainate receptors sug- gests that these receptors may have a pro- rather than disinhibitory role. Here, we examine the effect of synaptically released glutamate on monosynaptic inhibitory signaling. In the presence of antagonists to AMPA and NMDA receptors, brief bursts of activity in glutamatergic afferent fibers reduce GABAergic transmission. This depression of inhibition is reversibly abolished by blocking kainate receptors. It persists when GABAB receptors are blocked and is enhanced by blocking metabotropic glutamate receptors, possibly ex- plained by presynaptic regulation of glutamate release from excitatory afferents by metabotropic autoreceptors. We con- clude that the net kainate receptor-mediated effect of synap- tically released glutamate is to reduce monosynaptic inhibi- tion. Since this form of disinhibition may contribute to seizure initiation, kainate receptors may constitute an important target for anticonvulsant drug development.