Changes in the electrical conductivity of cerebral cortex during seizure activity

Abstract

The rabbit may respond to Metrazol-induced cortical convulsive activity with a long-lasting depression of the spontaneous electrocorticogram, a slowly developing and declining cortical negativity of considerable magnitude, and a marked drop in the electrical conductivity of the cortex. This reaction strikingly resembles spreading depression. However, since it is often elicited simultaneously over a considerable cortical area, it may lack one of the features of spreading depression, namely the slow propagation. The rabbit may also respond to convulsive activity with a moderate drop in cortical conductivity accompanied by cortical negativity of much smaller magnitude which is suggestive of an “abortive” spreading depression. Such a convulsion is followed by a short depression of the corticogram which may be ended by a new seizure, when sufficient Metrazol had been administered, or the recurrence of the nonconvulsive corticogram. The cat responded to Metrazol convulsions only with the second of the two reaction forms observed in the rabbit. A moderate drop in cortical conductivity was recorded during the seizure but no evidence for typical or even abortive spreading depressions was found in this species. Electrically induced convulsions caused similar but less pronounced drops in cortical conductivity in cats. Small decreases of conductivity were also recorded during indirect, transcallosal stimulation of the cortex in cats. The drop in conductivity observed during and after seizure activity may be indicative of a transport of electrolytes into neuronal elements, changing their ionic composition. This could account for a decrease in cortical excitability which arrests convulsive activity.