Rationale:
In both in vivo and ex vivo experiments, neural populations have been observed to undergo progressive synchronization during seizure termination. It has been speculated that forced synchronization of neural populations can be used to thwart seizure progression or induce seizure termination. While electrical and cell-specific activation has been used to impact seizure trajectory in vivo, it is unclear what the contributions of specific neural populations are to the observed changes. Here, we present a strategy to investigate the impact of cell-type-specific secondary synchronization during seizures in freely moving, awake, chronically epileptic mice.
Methods:
Chronic epilepsy was induced in adult Thy1-ChR2 mice. In the hippocampus of these animals, an EEG recording setup (two cortical screws and two hippocampal wires) and an optical fiber capable of transmitting 473nm light were implanted. During experiments, both animal behavior (Video) and EEG were recorded. While the animals were awake, seizures were induced by optically exciting the Thy1-ChR2 neurons for five seconds. In some trials, the Thy1-ChR2 neurons were excited a second time (following a delay) to determine the effect of additional stimulation on seizure trajectory. Animal behavior during seizure was scored by applying the Racine scale and EEG features were analyzed for band power and area, among other measures.
Results:
In all chronically epileptic animals, seizures were reliably induced upon light stimulation. In trials where Thy1-ChR2 neurons were only activated once (for seizure induction), the average Racine scale was 3.14. Conversely, in trials when Thy1-ChR2 neurons were activated during the ongoing seizure, the average Racine scale was 3.66. We then compared EEG features towards the end of the seizure. Compared to single stimulation trials, when Thy1-ChR2 neurons were activated a second time, band power was elevated by 110%, line length was elevated by 50%, and area was elevated by 55%.
Conclusions:
This study showed that in awake, freely moving epileptic animals, a second synchronization of seizure-inducing excitatory neural population intensifies seizures. This suggests that not all neural synchronizations are beneficial for instantaneous seizure termination. Even if seizures could be shortened, the cost of the reduction may be immediate onset of large scale behavioral seizures. This result shows that additional investigation is necessary into the optimal neural populations to synchronize for seizure reduction and control.
Funding:
NIH P50HD105354
NIH NINDS 5-T32-NS-091006-07