“Multiphoton holographic photostimulation induces potassium-dependent spike silencing in label-free mouse cortex in vivo”

Multiphoton microscopy allows measurement of network activity as well as the manipulation of cell type specific or functionally identified neuronal subpopulations with optogenetic holographic stimulation. When neurons co-express an activity reporter (e.g. calcium or voltage-sensitive indicators) and an (excitatory or inhibitory) opsin, such “all optical” interrogation approaches in vivo allows to draw causal links between function of cell-type specific microcircuits and behaviour. However, the net effects of near-infrared stimulation on network activity per se remain to be adequately investigated in vivo. Here we show that multicell holographic photostimulation with near-infrared radiation with total powers to sample used in current literature halves the spike rate of the non-illuminated neurons in label-free mouse cortex in vivo. The effect is not mediated by GABA release, but depends on NIR-dependent gating of potassium channels as it is absent when neurons are intracellularly perfused with the broad potassium channel blocker cesium ions, and are possibly mediated by heating. The phenomenon may contribute to set an upper limit to holographic photostimulation efficacy, calls for the need to control the effects of holographic stimulation protocols per se in label free preparations, and might be of relevance to interpret the therapeutical effects on infrared stimulation in psychiatry and neurology. Highlights ### Competing Interest Statement The authors have declared no competing interest. * ### Abbreviations NIR : near-infrared radiation PSTH : peristimulus time histograms tpa : two-photon assisted