Transient epileptiform experience affects the duration of penicillin-induced convulsive activity in the awake rat

1. Experiments were performed to investigate whether the sensitivity of the cerebral cortex to produce convulsive activity was modified by a transient epileptic focus. For this reason, 30 awake rats received two intracerebral penicillin (PCN) injections (125 IU/0.5 microliter solvent), the first one into the motor or visual cortex, the second one two weeks later into the motor cortex. Convulsive activity was recorded by means of EEG. 2. After the first PCN injection, interictal activity lasted 143.4 +/- 19.3, S.E. min (n = 17) in the motor cortex group, but 426 min (n = 13) in the visual group (p < 0.05). The median frequency of interictal potentials (np/min) was significantly lower in the motor cortex group than in the visual cortex group (18.4 and 25.5 np/min, respectively, p < 0.05). The period between the onset of the first and last seizure was 110.6 +/- 10.2 min for the motor cortex group, and 119.8 +/- 31.2 min for the visual cortex group. The seizures lasted longer and were less frequent in the motor than in the visual group (length: 12.5 +/- 1.1 vs 7.5 +/- 2.8 s; number: 43 +/- 11 vs 131 +/- 40, respectively, 0.05 < p < 0.1). 3. After the second PCN injection which was performed in all rats into the motor cortex, the duration but not the frequency of the interictal activity depended on the site of the first injection. The duration after this treatment was 96.8 +/- 43.3, S.E. min in the visual and 173.2 +/- 30.5 min in the motor cortex group (p < 0.05), while the frequency values of both groups were at a similar level (20.5 +/- 2.1 vs 18.9 +/- 1.7 np/min, respectively). Also the period with generalized activity after the second PCN injection was shorter in the visual than in the motor cortex group (43.9 +/- 32.3 vs 114.3 +/- 31.8 min, respectively, p < 0.05), and the number of seizures slightly lower (8 +/- 210 vs 54 +/- 30, respectively, p < 0.1). The length of the seizures was rather equal 11.6 +/- 2.5 vs 12.0 +/- 3.5 s, respectively, p > 0.1). 4. It is postulated that a transient epileptiform focus can induce, in a site-specific manner, long-lasting anticonvulsant effects. Therefore, epileptogenic experience may not only sensitize the brain, but also desensitize it with respect to the induction and duration of epileptiform activity.