Limbic epilepsy induced by tetanus toxin: a longitudinal electroencephalographic study

An epileptiform syndrome was induced in rats by injection of tetanus toxin (approximately 10 mouse LD50) unilaterally into the hippocampus. Continuous EEG records were obtained from implanted hippocampal electrodes for periods of 4-7 weeks in 14 rats. In a pilot study of two of these rats, three more recording electrodes were placed in other brain areas. Six of the rats (including the latter two) were simultaneously filmed using time-lapse videorecording, and the relationships between EEG events and overt motor fits were assessed using a split-screen video monitoring system. Characteristic peaks and troughs in the numbers of overt fits occurring each day were noted in all the rats that were filmed, and less marked peaks occurred in the numbers of hippocampal seizure discharges. At the start of the syndrome, seizure discharges occurred without accompanying fits; then overt fits occurred with some of the discharges; later the animals stopped having fits but seizure discharges continued to occur alone for several weeks. Fits only occurred with longer seizure discharges (more than approximately 30 s), but not all longer seizures were associated with fits. Whether or not a hippocampal seizure discharge leads to a motor fit appears to depend not on the nature of the electrical activity in the hippocampus but probably on the properties of areas, such as the cingulate gyrus, to which the seizure activity may spread. Large epileptiform spikes occurred throughout the syndrome, and their frequency was often increased for some minutes after a seizure discharge. While there was an overall correlation between the number of fits and the number of seizure discharges occurring during the entire syndrome, within relatively brief periods this relation was not consistent. The occurrence of motor fits was often associated with a decrease in seizure discharge frequency. In the four rats with bilateral electrodes, some independent EEG activity was observed in the uninjected hippocampus.