Abstracts

Rationale: Sixty percent of epilepsies are focal, among which 40% are resistant to anti epileptic drugs and only 30% of the latter are suitable for resective surgery. Indeed, when eloquents parts of the brain like the motor cortex are involved, resective surgery cannot be performed unless unacceptable functional deficit. In this specific case of epilepsy involving the motor cortex, deep brain stimulation of the basal ganglia (BG) has been proposed as an alternative treatment. The first step to understand the implication of BG in the control of epileptic activity is to determine the influence of neocortical paroxystics spikes on the input structures of the BG (subthalamic nucleus (STN), putamen and caudate nucleus). Here, we aimed to characterize the electrophysiological properties of the BG activity during focal, acute, motor seizure, induced in two monkeys (macacus fascicularis).Methods: The animals were equipped with a chronic recording chamber and epidural screws for EEG recording. We induced on demand focal motor seizure by injecting penicillin (7-40 µg) in the motor cortex. We recorded before (baseline) and after penicillin injection (inter ictal and ictal periods) the extracellular multi-sites activities of STN, caudate and putamen. Our set up allowed us to record simultaneously the BG extracellular activities with 4 electrodes concomitant to epidural EEG activity. We studied the firing rate (FR), the firing pattern (FP) based on the analysis of the interval interspike histogram and the autocorrelogram. Oscillatory activities were assessed based on the study of the power spectra of spikes train. In addition, we looked at the synchronization between BG extracellular and spikes activities and between pairs of BG neurons using the stimulus histogram and the cross correlogram. Results: A total number of 79, 41 and 92 neurons were recorded respectively in STN, caudate and putamen. During seizure, the FR was significantly increased compared to baseline and compared to interictal period in STN (27.46±20.65 vs 14.57±12.75; p<0.01 and 16.93±13.67; p<0.01, respectively) and in putamen (18.80±16.08 vs 6.17±6.26; p<0.0001 and 11.19±13.53; p<0.05, respectively). In caudate, the ictal FR did not differ compared to baseline and to interictal period (5.07±3.48 vs 6.31±3.66; p>0.5 and 6.77±5.11; p>0.1, respectively). The analysis of FP showed that 50 % of cells in STN, 24% in putamen and only 13% in caudate changed their FP from an irregular pattern in inter ictal period to a burst pattern during the seizure. During ictal period 30%, 57% and 3% of pairs of STN, putamen and caudate neurons became highly correlated. In addition 65%, 62% and 23% of cells in STN, putamen and caudate showed a strong synchronisation with cortical activity.Conclusions: We conclude that the modification of the FR and FP during seizure associate with the increase of oscillatory activity and cell synchronisation for STN and putamen point to a robust reorganization of BG circuitry during focal motor seizure. However, our data suggest that the caudate is not involved in this network.