High-frequency, high-intensity electrical stimulation selectively activates human fast-spiking interneurons

Electrical brain stimulation has been used to treat epilepsy with therapeutic success, but without complete understanding of its cellular mechanisms of seizure suppression. Using acute human brain slices obtained from pharmaco-resistant epilepsy patients, we delivered high-frequency, high-intensity extracellular electrical stimulation while recording from neocortical layer 2/3 pyramidal neurons (PNs), fast-spiking interneurons (FSINs), and non-fast-spiking interneurons (nFSINs). Electrical stimulation led to an increasingly selective activation of FSINs at higher stimulation frequencies, which consistently fired with high fidelity throughout prolonged stimulation periods while both PNs and nFSINs were suppressed rapidly. In addition, stimulation caused a long-term rebalancing of synaptic weights through selective and differential depression of synaptic inputs, resulting in an approximately twofold increase in the normalized excitatory-to-inhibitory postsynaptic conductance at FSINs while not affecting PNs, thereby providing support for FSIN activation toward increased inhibitory tone and network stabilization. These stimulation-induced effects were accompanied by only minimal changes in neuronal intrinsic excitability.