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 sensitive, quantitative estimate of network excitability in freely behaving mice using a novel optogenetic intensity-response procedure. Synchronous activation of deep sublayer CA1 pyramidal cells produces abnormal network-wide epileptiform population discharges (PDs) that are nearly indistinguishable from spontaneously-occurring interictal spikes (IISs). By systematically varying light intensity, and therefore the magnitude of the optogenetically-mediated current, we generated intensityresponse curves using the probability of PD as the dependent variable. 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 (LEV; 40 mk/kg), in combination with PTZ, produced a rightward shift. Optogenetically-induced PD threshold (oPDT) baselines were stable over time, suggesting the metric is appropriate for within-subject experimental designs with multiple pharmacological manipulations.Abnormal excitability is associated with a number of neurologic 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 associated network. Epileptiform population discharges (PDs) are network-wide events that represent a distinct transition from normal to abnormal functional modes. We developed a new method that uses light intensity-response curves to precisely determine the threshold for this transition as a surrogate measure of network excitability.

Alcohol withdrawal produces changes in excitability, population discharge probability, and seizure threshold

“…Video was used to score the severity of evoked seizure using a modified Racine scale (Racine, 1972). The modified Racine scale is as follows; stage 1: behavioral arrest, stage 2: head nodding, stage 3: forelimb clonus, stage 4: rearing bilateral clonus, stage 5: falling hindlimb clonus, and stage 6: erratic running, jumping (Klorig et al, 2019).…”

“…Here, we apply a newly developed paradigm, the optogenetically induced population discharge threshold (oPDT) (Klorig et al, 2019), in the context of chronic alcohol exposure and withdrawal using the chronic intermittent EtOH (CIE) exposure paradigm (Becker & Hale, 1993). We demonstrate that oPDT testing permits assessment of the mechanisms of AWS along several dimensions, including increases in spontaneous IS (Veatch & Gonzalez, 1996), increases in seizure severity (Becker & Hale, 1993), and reduction in seizure threshold to a given stimulus (Kokka et al, 1993).…”

A Step-by-Step Protocol for Optogenetic Kindling

“…Indeed, a recent study shows that optogenetic kindling works well in the hippocampus (Klorig et al, 2019). Using optokindling, seizures can be elicited by avoiding gross brain damage and targeting other cell populations of interest such as interneurons.…”

Optogenetic activation of nonhuman primate cortical and subcortical brain circuits highlights detection capabilities of MEG source imaging

“…We anticipate that experimenter control over discrete brain regions of interest coupled to whole brain recording and detection of downstream responders will provide a platform for improved investigation of brain function in both normal and pathological conditions. Subthreshold modulation could be used to investigate population activity that gives rise to neural oscillations 49,50 and suprathreshold synchronous modulation could be used to mimic epileptiform discharges in the study of epilepsy 50 .…”

“…We have developed a MEG-compatible optogenetic preparation in vervet monkeys that allows simultaneous optical stimulation and MEG recordings, along with associated local field potential (LFP) recordings from deep and superficial structures. Stimulation and LFP recordings were achieved with an optrode, which is a combined optical fiber/recording electrode 20,50 . Optogenetic techniques are well suited for use with MEG/MSI in that they are magnetically silent and allow for precise control over neural population potentials through the use and activation of virally-expressed, light sensitive ion channels 17 .…”