Infrared laser-induced fast thermal gradient affects the excitability of primary hippocampal neurons

Infrared laser (IRL) exposure can induce a rapid temperature change (fast thermal gradient or FTG) that is able to stimulate or inhibit neurons and, thereby, modify neurological functions. Despite extensive research into this effect, the fundamental mechanism(s) underlying how FTG causes neurological stimulation or inhibition remains unclear. While it is hypothesized that IRL-induced FTG acts directly on the neuronal plasma membrane (PM), it is uncertain if the neurological effects observed in previous studies are mostly derived from presynaptic effects (i.e., modifications in action potential (AP) firing) or also from postsynaptic effects (i.e., alteration of the synaptic responses of the excitatory and inhibitory neuronal receptors). In the present study, we present an analysis of FTG-mediated changes in neuronal PM, AP firing rate, and miniature postsynaptic excitatory and inhibitory currents (mEPSCs and mIPSCs). Our results suggest FTG induces changes in both presynaptic and postsynaptic neurophysiological mechanisms. Specifically, we found that, after IRL pulse (IRLP)-induced FTG exposure, the amplitudes of APs are reduced, but the rate of APs are increased. In contrast, the quantities of both mEPSCs and mIPSCs are reduced, but the peak-to-peak frequency and peak amplitudes are increased. The results outlined in this study demonstrate the impact of FTG on neurons and neuronal network. This information is critical for understanding the complexity of the effects of FTG on neurological functions and for demonstrating how post-synaptic mechanisms might play a crucial role in neurological excitation or inhibition seen following IRL pulse exposure.