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Showing 1-2 of 2 resultsTwo-photon imaging and optogenetic stimulation rely on high illumination powers, particularly for state-of-the-art applications that target deeper structures, achieve faster measurements, or probe larger brain areas. However, little information is available on heating and resulting damage induced by high-power illumination in the brain. Here we used thermocouple probes and quantum dot nanothermometers to measure temperature changes induced by two-photon microscopy in the neocortex of awake and anaesthetized mice. We characterized heating as a function of wavelength, exposure time, and distance from the center of illumination. Although total power is highest near the surface of the brain, heating was most severe hundreds of microns below the focal plane, due to heat dissipation through the cranial window. Continuous illumination of a 1mm2 area produced a peak temperature increase of approximately 1.8°C/100mW. Continuous illumination with powers above 250 mW induced lasting damage, detected with immunohistochemistry against Iba1, GFAP, heat shock proteins, and activated Caspase-3. Higher powers were usable in experiments with limited duty ratios, suggesting an approach to mitigate damage in high-power microscopy experiments.
In CA1 pyramidal neurons, correlated inputs trigger dendritic plateau potentials that drive neuronal plasticity and firing rate modulation. Given the strong electrotonic coupling between soma and axon, the >25 mV depolarization associated with the plateau could propagate through the axon to influence action potential initiation, propagation, and neurotransmitter release. We examined this issue in brain slices, awake mice, and a computational model. Despite profoundly inactivating somatic and proximal axon Na(+) channels, plateaus evoked action potentials that recovered to full amplitude in the distal axon (>150 μm) and triggered neurotransmitter release similar to regular spiking. This effect was due to strong attenuation of plateau depolarizations by axonal K(+) channels, allowing full axon repolarization and Na(+) channel deinactivation. High-pass filtering of dendritic plateaus by axonal K(+) channels should thus enable accurate transmission of gain-modulated firing rates, allowing neuronal firing to be efficiently read out by downstream regions as a simple rate code.