Activity Clamp Provides Insights into Paradoxical Effects of the Anti-Seizure Drug Carbamazepine

Abstract

A major challenge in experimental epilepsy research is to reconcile the effects of anti-epileptic drugs (AEDs) on individual neurons with their network-level actions. Highlighting this difficulty, it is unclear why carbamazepine (CBZ), a frontline AED with a known molecular mechanism, has been reported to increase epileptiform activity in several clinical and experimental studies. We confirmed in an in vitro mouse model (in both sexes) that the frequency of interictal bursts increased after CBZ perfusion. To address the underlying mechanisms, we developed a method, activity clamp, to distinguish the response of individual neurons from network-level actions of CBZ. We first recorded barrages of synaptic conductances from neurons during epileptiform activity and then replayed them in pharmacologically isolated neurons under control conditions and in the presence of CBZ. CBZ consistently decreased the reliability of the second action potential in each burst of activity. Conventional current-clamp recordings using excitatory ramp or square-step current injections failed to reveal this effect. Network modeling showed that a CBZ-induced decrease of neuron recruitment during epileptic bursts can lead to an increase in burst frequency at the network level by reducing the refractoriness of excitatory transmission. By combining activity clamp with computer simulations, the present study provides a potential explanation for the paradoxical effects of CBZ on epileptiform activity.SIGNIFICANCE STATEMENT The effects of anti-epileptic drugs on individual neurons are difficult to separate from their network-level actions. Although carbamazepine (CBZ) has a known anti-epileptic mechanism, paradoxically, it has also been reported to increase epileptiform activity in clinical and experimental studies. To investigate this paradox during realistic neuronal epileptiform activity, we developed a method, activity clamp, to distinguish the effects of CBZ on individual neurons from network-level actions. We demonstrate that CBZ consistently decreases the reliability of the second action potential in each burst of epileptiform activity. Network modeling shows that this effect on individual neuronal responses could explain the paradoxical effect of CBZ at the network level.

dCas9-Based Scn1a Gene Activation Restores Inhibitory Interneuron Excitability and Attenuates Seizures in Dravet Syndrome Mice

“…Recently, we developed a new electrophysiological approach ('activity clamp') to analyze how a neuron in a given epileptic network responds to antiepileptic drugs 41 . Here, we modified this method to adapt it for primary neuronal cultures in order to compare interneurons transduced with either the Ctrl-dCas9A or Scn1a-dCas9A system.…”

“…The sampling frequencies in voltage and in current clamp configurations were set at 20 kHz to perfectly overlap the conductance traces with the software voltage reading. To analyze the dynamic clamp traces an automatic Matlab script was used41 to detect events and calculate APs parameters. An event was selected as AP if its peak crossed 0mV and if its dv/dt was >20.…”

In vivo CRISPRa decreases seizures and rescues cognitive deficits in a rodent model of epilepsy

“…Activity clamp. The template simulating the barrage of synaptic conductances during epileptiform bursts was previously described (Morris et al, 2017). Dynamic clamp software (Signal 6.0, Cambridge Electronic Design, Cambridge, UK) and a Power3 1401 (CED) were used to inject both excitatory and inhibitory conductance templates simultaneously in a neuron recorded in current clamp configuration (iteration frequency 15 kHz).…”

“…However, a significant decrease in half width was seen when all the APs during the current step protocol were pooled (Figure 2 F). Finally, we applied activity clamp, a method to assess neuronal excitability during epileptiform barrages of excitation (Morris et al, 2017). Neurons expressing Kcna1-dCas9A fired less than neurons expressing Ctrl-dCas9A when exposed to the same simulated synaptic input.…”

Ictal Inhibition: Sync Globally, Slack Locally

“…In the worst cases, antiepileptic drugs can have paradoxical effects, worsening seizures. 1 Experimental studies aimed at dissecting the interplay between excitation and inhibition suggest that transiently dysfunctional inhibition, in which GABA synapses temporarily become ineffective or even excitatory, may be responsible for the paradoxical drug effects. For example, in human and animal studies, interneurons fire earlier and/or harder than principal cells during the onset of epileptiform activity, 2,3 before failing by either entering depolarization block 4,5 or causing postsynaptic chloride to accumulate such that the GABAA reversal potential becomes depolarized.…”