Hypothetical Mechanisms for the Cellular and Neurophysiologic Basis of Secondary Epileptogenesis: Proposed Role of Synaptic Reorganization

Abstract

This review article evaluates the hypothetical cellular mechanisms responsible for chronic lesion-induced epilepsy. Emphasis is given to particular clinical characteristics of secondary epileptogenesis: (a) a temporal latency, (b) the involvement of distant but related sites, and (c) irreversibility. Although loss of GABAergic inhibitory interneurons or increased excitatory input to these interneurons may contribute to epileptogenesis, several studies have provided evidence that inhibition is not depressed in epileptogenic regions and may actually be enhanced. Axonal sprouting, synaptic reorganization, and formation of new recurrent excitatory circuits have been proposed to account for the increased seizure susceptibility of temporal lobe epilepsy. Recent data support the hypothesis that local inhibitory circuits mask the multisynaptic excitatory interactions that are associated with mossy fiber sprouting in the dentate gyrus and that physiological mechanisms that reduce inhibition or increase excitability unmask the new recurrent excitatory circuits responsible for seizures. A hypothesis based on axonal sprouting and synaptic reorganization can account for the essential clinical characteristics of secondary epileptogenesis and may have widespread applicability to the general phenomenon of lesion-induced epilepsy.