Extracellular free calcium and potassium during paroxysmal activity in the cerebral cortex of the cat

Extracellular calcium and potassium activities (a_Ca and a_K) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline a_Ca was 1.2-1.5 mM/1, baseline a_k 2.7-3.2 mM/1. Transient decreases in a_Ca and simultaneous increases in a_K were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in a_Ca (by up to 0.7 mM/1) were found during seizure activity. A fall in a_Ca preceded the onset of paroxysmal discharges and the rise in a_K after injection of pentylene tetrazol. The decrease in a_Ca led also the rise in a_K during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca⁺⁺ dependent mechanisms participate in the generation of epileptic activity.