Ultrafast (>400 Hz) network oscillations induced in thalamorecipient cortical layers by optogenetic activation of thalamocortical axons

Oscillations of extracellular voltage, reflecting synchronous rhythmic activity in large populations of neurons, are a ubiquitous feature in the mammalian brain and are thought to subserve critical, if not fully understood cognitive functions. Oscillations at different frequency bands are hallmarks of specific brain or behavioral states. At the higher end of the scale, ultrafast (400-600 Hz) oscillations in the somatosensory cortex, in response to peripheral stimulation, were observed in human and a handful of animal studies; however, their synaptic basis and functional significance remain largely unexplored. Here we report that brief optogenetic activation of thalamocortical axons ex-vivo elicited precisely reproducible, ~410 Hz local field potential wavelets ("ripplets") in middle layers of mouse somatosensory (barrel) cortex. Fast-spiking (FS) inhibitory interneurons were exquisitely synchronized with each other and fired spike bursts in anti-phase with ripplets, while excitatory neurons fired only 1-2 spikes per stimulus. Both subtypes received shared excitatory inputs at ripplet frequency, and bursts in layer 5 FS cells required intact connection with layer 4, suggesting that layer 4 excitatory cells were driving FS bursts in both layers. Ripplets may be a ubiquitous cortical response to exceptionally salient sensory stimuli, and could provide increased bandwidth for encoding and transmitting sensory information. Lastly, optogenetically-induced ripplets are a uniquely accessible model system for studying synaptic mechanisms of fast and ultrafast oscillations.