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 (PD) that are 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. 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. Significance StatementAbnormal 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 associated network. Epileptiform population discharges 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.

Probing cortical excitability under GABAergic modulation

“…The time of opto-stimulation necessary to induce a seizure -”time to seizure”-was used as an indicator of cortical resilience. As previously shown for principal cells in the CA1 subfield of the hippocampus 23,39 , optogenetic sustained stimulation of PC MEC->Hpp was sufficient to elicit limbic seizure in non-epileptic mice (n= 4 out of 4). During the train stimulations, LFP response in the right hippocampus showed spike-shaped discharges time-locked to the opto-pulses (Fig 3A and B).…”

“…injection (Dz, PTZ or NaCl), followed 3min later by 3) a paired pulse (PP) protocol lasting 45 min, 4) a second i.p. injection (NaCl, PTZ or NaCl, re-injecting PTZ due to its rapid elimination 23,35 , followed 3min later by 4) a single pulse (SP) protocol lasting 10min followed by 5) a seizure induction protocol.…”

Optogenetic activation of CA1 pyramidal neuronsin vivoinduces hypersynchronous and low voltage fast seizures

“…The seizure severity score was based on the modified Racine seizure behavioral scores The LFP recorded in the CA1 region showed a stereotypical evolution when a behavioral seizure was induced (Fig 1D). When the photostimulation began, each light pulse resulted in an immediate time-locked evoked potential without behavioral changes observed on video recording, this filed oscillation was defined as optogenetic population discharges (oPD) 26 and was observed in all stimulation epochs (Fig 1Da). As the photostimulation continued, the EEG morphology changed with multiple bursts unsynchronyzing to the light pulses (Fig 1Db) and continued with self-sustained, highamplitude, clustered spiking after the photostimulation ended (Fig 1Dc), typically associated with motionless behavioral stage (modified Racine scale 1-2).…”

Boosting L-type Ca2+channel activity in tuberous sclerosis mitigates excess glutamate receptors

“…Since BayK lowers GluA levels, we hypothesized that boosting L-VDCC activity will attenuate seizures. To do this, we recorded local field potentials using a chronic recording array implanted in hippocampal CA1 of freely behaving Tsc1 KO mice (9,40). Approximately three weeks after introducing AAV-Cre-GFP and implanting the electrodes, TS mice received a single i.p.…”

“…Approximately 3-month old Tsc1tm1Djk/J mice were anesthetized with isoflurane and placed in a stereotaxic device. As previously described, metal ground screws were secured to the cranium bilaterally and tungsten microwires were implanted in a satellite array in cortical, subcortical, and hippocampal locations (40). AAV-Cre-GFP or AAV-GFP was introduced into the hippocampus just prior to electrode implantation.…”