Transient infrared laser exposure modulates calcium activity in cortical dendritic spines.

Significance: Infrared neural stimulation (INS) is an optical neuromodulation technique that elicits neural activity through photothermal temperature gradients. Photothermal gradients have been shown to alter neuronal excitability, but the effects on the dendritic spine (DS) structure and function have not been characterized. Aim: We aim to determine whether INS induces calcium signaling in DS and whether this signaling alters filamentous actin (F-actin). Approach: We used confocal imaging of cortical neurons expressing a genetically encoded calcium indicator (GCaMP)8f-Syn1 and mCardinal-LifeAct during laser exposure. Fluorescence signals were measured within regions of interest covering the DS. The resulting fluorescence measurements were baseline-normalized, and transient spiking activity in DS was quantified. Results: Transient thermal gradients induce persistent calcium activity in a fraction of DS, independent of pulse duration but dependent on the absolute temperature and number of laser exposures. There is no relationship between the calcium activity and F-actin. Calcium signaling in DS was not mediated by ionotropic glutamate receptors but depended on extracellular entry. Conclusions: These results indicate that transient thermal gradients elicit calcium activity in DS, but the induced activity does not alter F-actin, suggesting that transient thermal gradients may be a useful tool for studying calcium dynamics in DS without triggering traditional structural plasticity pathways.