Thermal constraints on in vivo optogenetic manipulations

A key assumption of optogenetics is that light only affects opsin-expressing neurons. However, illumination invariably heats tissue, and many physiological processes are temperature-sensitive. Commonly-used illumination protocols increased temperature by 0.2–2°C and suppressed spiking in multiple brain regions. In striatum, light delivery activated an inwardly-rectifying potassium conductance and biased rotational behavior. Thus, careful consideration of light delivery parameters is required, as even modest intracranial heating can confound interpretation of optogenetic experiments.