Spike-wave rhythms in cat cortex induced by parenteral penicillin. II. Cellular features.

Epileptiform potentials, consisting of spontaneous, generalized bursts frequently assuming a 3/sec spike-wave form and tonic clonic electrographic seizures were produced in 32 lightly anesthetized cats by parenteral injections of penicillin. The activity of 83 identified pyramidal tract cells and 207 cortical non-pyramidal tract cells was correlated with the surface EEG. The majority of both cell types generated depolarizations and action potentials with the EEG spike. Hyperpolarizations, during which cells were inhibited, followed the depolarizations. The depolarizations responded to injected current as if they were generated by excitatory synapses; and hyperpolarizations to injected current and chloride ions as if generated by proximal inhibitory synapses. Attempts to identify a class of neurons firing during the surface-negative wave (presumed inhibitory interneurons) were unsuccessful. Forty-two units were recorded during tonic-clonic seizures. Intracellular records disclosed tonic oscillations of membrane potential, phased bursting with "depolarization shifts", abortive action potentials and post-ictal hyperpolarizations. Cell somata often depolarized to the point of inactivation, but axons continued to fire at high rates. These results emphasize the role of EPSP-IPSP sequences in the generation of spike-wave rhythms.