Abstract
Understanding light intensity and temperature increase is of considerable importance in designing or performing in vivo optogenetic experiments. Our study describes the optimal light power at target depth in the rodent brain that would maximize activation of light-gated ion channels while minimizing temperature increase. Monte Carlo (MC) simulations of light delivery were used to provide a guideline for suitable light power at a target depth. In addition, MC simulations with the Pennes bio-heat model using data obtained from measurements with a temperature-measuring cannula having 12.3 mV/°C of thermoelectric sensitivity enabled us to predict tissue heating of 0.116 °C/mW on average at target depth of 563 μm and specifically, a maximum mean plateau temperature increase of 0.25 °C/mW at 100 μm depth for 473 nm light. Our study will help to improve the design and performance of optogenetic experiments while avoiding potential over- and under-illumination.
Highlights
Optogenetics, which combines optics with genetic engineering, permits cell-type-specific probing of neural circuits with millisecond precision [1]
The differences between the fiber diameters become reduced as the light propagates deeper. This means that both the numerical aperture (NA) and the fiber diameter are not prominent factors in practical in vivo optogenetic applications, and the fiber output power has the greatest effect on the ATA and optimized fiber-to-target distances (OFD)
Optimizing light intensity and target distance for in vivo optogenetic experiments is critical for minimizing unwanted heating of tissues yet is often overlooked when designing experiments
Summary
Optogenetics, which combines optics with genetic engineering, permits cell-type-specific probing of neural circuits with millisecond precision [1]. During in vivo optogenetic experiments, light is typically delivered into the brain via an optical fiber inserted with stereotactic guidance, and intensity measured in mW/mm is the recommended notation for reporting light requirement because of its inherent simplicity in measuring total light power [2, 3]. The experimental requirement is for the intensity to be sufficiently powerful enough to achieve neural activation. To compensate for scattering loss, a 100-fold or higher intensity is used in vivo to illuminate neural cells located further away from the light source [2]. Christie et al (2013) warned of false opto-fMRI (combinational study of fMRI with optogenetic tools) responses due to tissue heating despite the absence of optogenetic activation [5]
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