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, induced by a local injection of penicillin (PCN) into the posterior thalamus (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 motor cortex (MC). Using EEG recordings, 1) the distribution of focal cortical activity, and 2) the severity of the epileptiform activity (frequency of focal activity, occurrence and duration of generalized episodes) 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 sensory cortex during the last period of epileptiform activity; and (2) large potentials with a wave- or spike-wave-like shape that had their center of focal expression in the transition zone between the motor and sensory cortex. MC rats exhibited only a spike-like potential with or without short-lasting afterdischarges in the homotopic areas of the MC of both hemispheres. During periods with large potentials only, the number of generalized episodes was significantly reduced with respect to those periods with sharp potentials. When the epileptiform activity changed from large to sharp potentials, the interictal frequency increased significantly. It is postulated (a) and 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 interictal activity of the large potential type; and (b) that the wave of the large potentials is involved in an anticonvulsive mechanism that reduces the extent of ictal as well as interictal activity.