Cellular mechanisms of neocortical secondary epileptogenesis.

Firing activity, membrane parameters and postsynaptic responses were studied by recording intracellularly from different types of neurons during the development of a secondary neocortical epileptiform focus (mirror focus, Mf) contralateral to the site of an aminopyridine-induced focus (primary focus, Pf) in anesthetized rats. Three different stages in the development of secondary epileptogenesis were observed. (i) in the Pf stage epileptiform discharges appeared only in the ECoG recorded from the Pf, but neurons in the Mf showed reduced firing activity; (ii) in the Pf + Mf stage, synchronous ictal epileptiform activity occurred in the Pf and Mf. Changes in the balance between inhibition and excitation, appearance of novel electrophysiological phenomena (e.g. antidromic like action potentials, PDS (paroxysmal depolarization shift) potentials, rebound bursts), enhanced intrinsic bursting, and a transition from regular spiking to bursting were observed at the cellular level; (iii) in the Pf/Mf stage in 10% of the animals, the surface epileptic discharges were in synchrony with cellular activity in the Mf but were temporally independent of Pf activity, suggesting that during secondary epileptogenesis the Pf and the Mf can have underlying epileptogenic mechanisms which are different in origin.