Excitation of Primary Afferent Neurons by Near-infrared Light in Vitro

Near-infrared light therapy is an emerging neurostimulation technology, but its cellular mechanism of action remains unresolved. Using standard intracellular recording techniques, we observed that 5-10 ms pulses of 1889 nm light depolarized the membrane potential for hundreds of milliseconds in more than 85% of dorsal root ganglion and nodose ganglion neurons tested. The laser-evoked depolarizations (LEDs) exhibited complex, multiphasic kinetics comprising fast and slow components. There was no discernable difference in the LEDs in intact ganglion neurons and in acutely isolated neurons. Thus, the LED sensor seems to reside within the neuronal membrane. The near-uniform distribution of responsive neurons increased membrane conductance, and the negative reversal potential value (-41+/-2.9 mV) suggests that LED is unrelated to the activation of heat-sensitive transient receptor potential cation channel subfamily V member 1 channels. The long duration of LEDs favors an involvement of second messengers.