Abstracts

Rationale: Current pre-clinical methods for characterizing the efficacy of anti-epileptic drugs and/or surgical interventions rely on the ability to prevent, delay, or lessen the severity of artificially induced seizures, either as a result of the administration of pro-convulsant drugs or electric shock. These measures exhibit a large degree of baseline variability, requiring large numbers of replicates to achieve the statistical power required to demonstrate efficacy. We have developed a sensitive new method to detect small shifts in network excitability (e.g. seizure susceptibility) using optogenetically induced population discharge thresholds (oPDT), in mice, as a surrogate for seizure. Methods: By combining optogenetic stimulation of the hippocampus with chronic multi-site recording in peri-hippocampal structures of awake behaving animals, we can induce and detect abnormal network wide population discharges that mirror spontaneous interictal spikes in terms of waveform shape and latency. By varying the intensity of light we can compare the magnitude of the optogenetically mediated current to the probability of population discharge. This probability curve is well fit by a Boltzmann curve, allowing calculation of a V50. Results: We demonstrate that the V50 is a sensitive and reliable metric of network excitability. Manipulations known to increase excitability such as sub-convulsive doses (20 mg/kg) of pentylenetetrazol (PTZ ), or chronic EtOH withdrawal, produce a leftward shift in the curve compared to baseline. Anti-epileptic drugs, in combination with pro-convulsive manipulations, produce a rightward shift to back to baseline. oPDT baselines are remarkably stable over time, allowing for within-subject experimental design with multiple pharmacological manipulations, reducing the total number of animals needed. Furthermore we have fully characterized the close relationship between the oPDT and optogenetically induced seizure thresholds, thus demonstrating that the oPDT is a useful surrogate. Conclusions: The oPDT is the first example of a sensitive measure of subconvulsive network excitability, with broad applicability to a number of areas of investigation. Funding: Support: R01AA016852 and the Tab Williams Family Fund