Cathodal weak direct current decreases epileptic excitability with reduced neuronal activity and enhanced delta oscillations

Seizures are manifestations of hyperexcitability in the brain. Non-invasive weak current stimulation, delivered through cathodal transcranial direct current stimulation (ctDCS), has emerged to treat refractory epilepsy and seizures, although the cellular-to-populational electrophysiological mechanisms remain unclear. Using the ctDCS in vivo model, we investigate how neural excitability is modulated through weak direct currents by analysing the local field potential (LFP) and extracellular unit spike recordings before, during and after ctDCS versus sham stimulation. In rats with kainic acid (KA)-induced acute hippocampal seizures, ctDCS reduced seizure excitability by decreasing the number and amplitude of epileptic spikes in LFP and enhancing delta (δ) power. We identified unit spikes of putative excitatory neurons in CA1 stratum pyramidale based on waveform sorting and validated via optogenetic inhibitions which increased aberrantly in seizure animals. Notably, cathodal stimulation significantly reduced these unit spikes, whereas anodal stimulation exhibited the opposite effect, showing polarity-specific and current strength-dependent responses. The reduced unit spikes after ctDCS coupled to δ oscillations with an increased coupling strength. These effects occurred during stimulation and lasted 90 min post-stimulation, accompanied by inhibitory short-term synaptic plasticity changes shown in paired-pulse stimulation after ctDCS. Consistently, neuronal activations measured by c-Fos significantly decreased after ctDCS, particularly in CaMKII⁺-excitatory neurons while increased in GAD⁺-inhibitory neurons. In conclusion, epileptic excitability was alleviated with cathodal weak direct current stimulation by diminishing excitatory neuronal activity and enhancing endogenous δ oscillations through strengthened coupling between unit spikes and δ waves, along with inhibitory plasticity changes, highlighting the potential implications to treat brain disorders characterized by hyperexcitability. KEY POINTS: Electric fields generated by transcranial weak electric current stimulation were measured at CA1, showing polarity-specific and current strength-dependent modulation of unit spike activity. Polyspike epileptiform discharges were observed in rats with kainic acid (KA)-induced hippocampal seizures. Cathodal transcranial direct current stimulation (ctDCS) reduced the number and amplitude of the epileptic spikes in local field potentials (LFPs) while increased δ oscillations. Neuronal unit spikes aberrantly increased in seizures and coupled with epileptiform discharges. ctDCS reduced excitatory neuronal firings at CA1 and strengthened the coupling between unit spikes and δ waves. Neuronal activations, measured by c-Fos, decreased in CaMKII⁺-excitatory neurons while increased in GAD⁺-inhibitory neurons after ctDCS. These effects on LFP and unit spikes lasted up to 90 min post-stimulation. Inhibitory short-term plasticity changes detected through paired-pulse stimulation underpin the enduring effects of ctDCS on seizures.