Optogenetically-Induced Population Discharge Threshold as a Sensitive Measure of Network Excitability

Abstract

Network excitability is governed by synaptic efficacy, intrinsic excitability, and the circuitry in which these factors are expressed. The complex interplay between these factors determines how circuits function, and at the extreme, their susceptibility to seizure. We have developed a novel optogenetic intensity response procedure that provides a sensitive, quantitative estimate of network excitability. By combining optogenetic stimulation of the hippocampus with chronic multi-site recordings in peri-hippocampal structures of awake behaving mice, we induced abnormal network-wide epileptiform population discharges (PD) that were nearly indistinguishable from spontaneously occurring interictal spikes. By systematically varying light intensity, and therefore the magnitude of the optogenetically-mediated current, we generated intensity-response curves using the probability of PD as the dependent variable. This probability curve was well fit by a Boltzmann function, from which we calculated the intensity that produces a half-maximal probability of discharge (I50). This novel metric, the I50, is correlated with the optogenetic after-discharge threshold (oADT) in the same mice. Manipulations known to increase excitability, such as sub-convulsive doses (20 mg/kg) of the chemoconvulsant pentylenetetrazol (PTZ), produced a leftward shift in the curve compared to baseline. The anti-epileptic drug levetiracetam (40 mk/kg), in combination with PTZ, produced a rightward shift. Optogenetically-induced population discharge threshold (oPDT) baselines were stable over time, suggesting the metric is appropriate for within-subject experimental designs with multiple pharmacological manipulations. The oPDT is a sensitive measure of subconvulsive network excitability, with broad applicability to a number of areas of investigation. Significance StatementNetwork excitability is carefully regulated by homeostatic mechanisms in the brain in order to maintain optimal functional conditions. Abnormal excitability is associated with a number of neurological disorders including epilepsy. Excitability can be measured in single cells in vitro, but it is difficult to extrapolate from these values to the functional impact on the network as a whole. Epileptiform discharges are network wide events that represent a distinct transition from normal to abnormal functional modes. We developed a new technique that uses light intensity-response curves to precisely determine the threshold for this transition as a surrogate measure of network excitability and seizure susceptibility.