Participation of cortical recurrent inhibition in the genesis of spike and wave discharges in feline generalized penicillin epilepsy.

Cortical recurrent inhibition (RI) evoked in pericruciate cortex by antidromic stimulation of the cerebral peduncle (CP) was studied in normal cats and in cats exhibiting the signs of feline generalized penicillin epilepsy (FGPE) following the i.m. injection of penicillin. Two measures of RI evoked by antidromic CP stimulation were used: (i) the averaged focal potential in the pericruciate gyrus; and (ii) the duration of the suppression or diminution of extracellularly recorded action potential (ap) discharge of antidromically activated pericruciate neurons measured in peristimulus time histograms (PSTHs). After i.m. injection of 350,000 IU/kg of penicillin RI remained preserved as long as only generalized spike and wave (SW) discharges appeared in the EEG, although in 5/17 neurons a modest to moderate reduction in the duration of RI occurred once SW discharges had appeared in the EEG. This inconstant reduction was probably not caused by a direct anti-inhibitory action of penicillin, but is a consequence of the increased number of ap discharges curtailing RI. At the small concentrations of penicillin existing in brain in FGPE its anti-inhibitory action evident with larger concentrations cannot be demonstrated. When focal or generalized tonic-clonic (T-C) seizures occurred, RI was reduced in slightly more than half of the instances for a few minutes before the onset of these seizures. This suggests that the transition from SW discharge to T-C seizure may be caused by a breakdown of RI.