Arrest of seizure series induced by an intracortical injection of penicillin in the awake rat.

Experiments were performed to answer the question, whether series of generalized tonic-clonic seizures, induced in the awake rat by a local injection of Na-penicillin (PCN) solution into the motor cortex, terminates at the same critical concentration Ct of PCN within the focal area independently of the concentration C0 of PCN injected. Using the PCN diffusion coefficient D = 3.52 x 10(-4) mm2/s and the tortuosity factor lambda = 1.62, the concentration Ct at the onset of the last generalized seizure was calculated. The median duration of seizure series increased from 32 to 190 min, when the dose of injected PCN was raised from 32 to 1000 IU. At the onset of the last seizure, the median concentration Ct within the artificial focus ranged from 2.1 to 4.0 IU/0.5 microliter saline in rats treated with 32 to 125 IU PCN. After induction of convulsive behaviour with C0 = 250, 500, or 1000 IU PCN/0.5 microliter saline, however, Ct was at a higher level between 6.1 and 7.4 IU PCN/0.5 microliter saline. The difference between the cumulated data from the low-dose vs. the high-dose range was significant (p < 0.01). It is concluded that during long-lasting series of generalized seizures, the brain takes advantage of its plastic properties. By forming a counteracting mechanism, it protects itself from extreme epileptiform activity. This autoprotection may be due to the activation of neuronal networks which probably needs a certain frequency of seizures to become operatively.