Two-photon imaging induces brain heating and calcium microdomain hyper-activity in cortical astrocytes

Abstract

Unraveling how neural networks process and represent sensory information and how this cellular dynamics instructs behavioral output is a main goal in current neuroscience. Two-photon activation of optogenetic actuators and fluorescence calcium (Ca2+) imaging with genetically encoded Ca2+ indicators allow, respectively, the all-optical stimulation and read-out of activity from genetically identified cell populations. However, these techniques expose the brain to high near-infrared light doses raising the concern of light-induced adverse effects on the biological phenomena being studied. Combing Ca2+ imaging of GCaMP6f-expressing cortical astrocytes as a sensitive readout for photodamage and an unbiased machine-based event detection, we demonstrate the subtle build-up of aberrant microdomain Ca2+ signals in fine astroglial processes. Illumination conditions routinely being used in biological two-photon microscopy (920-nm excitation, 100-fs regime, ten mW average power) increased the frequency of microdomain Ca2+ events, but left their amplitude, area and duration rather unchanged. This increase in local Ca2+ activity was followed by Ca2+ transients in the otherwise silent soma. Ca2+ hyperactivity occurred without overt morphological damage. Surprisingly, at the same average power, continuous-wave 920-nm illumination was as damaging as fs pulses, indicating a linear, heating-mediated (rather than a highly non-linear) damage mechanism. In an astrocyte-specific IP3-receptor knock-out mouse (IP3R2-KO), Near-infrared light-induced Ca2+ microdomains signals persisted in the small processes, underpinning their resemblance to physiological IP3R2-independent Ca2+ signals, while somatic activity was abolished. Contrary to what has generally been believed in the field, shorter pulses and lower average power are advantageous to alleviate photodamage and allow for longer useful recording windows.