The effects of activation of kainate receptors on tonic and phasic gabaergic inhibition in interneurons in field CA1 of guinea pig hippocampus slices

Kainate receptor agonists are powerful convulsants and excitotoxins. Until recently, there have been several contradictory views as to the roles of these receptors in the CNS. We report here experiments showing that application of kainate led to concentration-dependent increases in evoked GABAergic inhibitory postsynaptic currents (phasic currents) in interneurons in field CA1 of guinea pig hippocampus slices. This evidently occurred as a result of a decrease in the action potential generation threshold in inhibitory axons and an increase in the number of endings responding at a given stimulus strength. Increases in phasic inhibitory postsynaptic currents were accompanied by increases in the tonic GABAergic current (the constant component of GABAergic conduction). Increases in the tonic current occurred because of increases in the discharge frequency of interneurons, leading to action-potential-dependent GABA release and, as a result, increases in the extracellular concentration of endogenous agonist. The high level of extracellular GABA after addition of kainate led to desensitization of synaptic GABAergic receptors, while the tonic conductivity led to shunting of synaptic currents. Thus, while 1 microM kainate increased inhibitory postsynaptic currents, this was preceded by a transient depression. The different dynamics of the effects of kainate on phasic and tonic inhibitory GABAergic currents in hippocampal interneurons and the decrease in inhibition of glutamatergic pyramidal cells which may result from these changes may explain the epileptogenic properties of kainate.