EXPRESSION OF EPILEPTIFORM ACTIVITY-INDUCED BY A PENICILLIN FOCUS WITHIN THE POSTERIOR THALAMUS IN THE AWAKE RAT

Investigations were performed to study the epileptiform activity, indu ced by a local injection of penicillin (PCN) into the posterior thalam us (pTh) of the awake rat, and to compare it with the epilepsy induced in the same animals 2 weeks later by an injection of PCN into the mot or cortex (MC). Using EEG recordings, 1) the distribution of focal cor tical activity, and 2) the severity of the epileptiform activity (freq uency of focal activity, occurrence and duration of generalized episod es) were analyzed. The focal activity of pTh rats was characterized by two types of potentials: (a) sharp potentials with a spike-like shape that developed during the first hour after PCN injection only in the visual cortex, but in the transition area between the motor and sensor y cortex during the last period of epileptiform activity; and (2) larg e potentials with a wave- or spike-wave-like shape that had their cent er of focal expression in the transition zone between the motor and se nsory cortex. MC rats exhibited only a spike-like potential with or wi thout short-lasting afterdischarges in the homotopic areas of the MC o f both hemispheres. During periods with large potentials only, the num ber of generalized episodes was significantly reduced with respect to those periods with sharp potentials. When the epileptiform activity ch anged from large to sharp potentials, the interictal frequency increas ed significantly. It is postulated (a) that a pTh focus activates the lateral and/or the reticular thalamic areas, which, due to their high intrinsic potential for synchronization, cause a self-sustained interi ctal activity of the large potential type; and (b) that the wave of th e large potentials is involved in an anticonvulsive mechanism that red uces the extent of ictal as well as interictal activity.