Epileptic activity can induce both long-lasting potentiation and long-lasting depression

Epileptic seizures are associated with massive changes of intracellular ion concentrations which could cause persistent changes in efficacy of afferent excitation. Such alterations were investigated in the CA1 area of hippocampal slices using the acute high potassium model of epilepsy which allows rapid wash of the epileptogenic solution. Field potentials elicited by stimulation of Schaffer collaterals were recorded in stratum pyramidale and stratum radiatum. The experiments revealed that long-lasting potentiation as well as long-lasting depression of field potentials could result from superfusion with high potassium. These changes were NMDA-dependent, but did not depend on the discharge type (interictal vs. ictal) of the slice. It is concluded that epileptic activity cannot only lead to an enhancement of responses to afferent excitation, but also to a long-lasting depression. This depression could represent a self-protective mechanism of the brain which may also be involved in post-ictal depression of cerebral activity.