The synaptic nature of the paroxysmal depolarizing shift in hippocampal neurons

It is well known that there are two types of neuronal burst discharges: endogenous and network driven. Frequently, the two types are similar in appearance. But careful analysis has shown them to be quite different mechanistically and to share few common properties. Endogenous bursts are a common feature of certain hippocampal neurons. Application of a convulsant agent to a hippocampal slice, however, results in the appearance of interictal discharges whose intracellular correlate is the paroxysmal depolarizing shift (PDS). In contrast to the normal endogenous burst, the PDS is a network-driven burst. It has been proposed that the underlying depolarization of the PDS constitutes a large, compound, excitatory synaptic potential. In support of this hypothesis, recent voltage clamp experiments have demonstrated that a large synaptic conductance underlies the PDS. In this report we review those experiments and others that have established the synaptic nature of epileptiform activity in the hippocampus.