Application of near infra-red laser light increases current-threshold in optic nerve consistent with increased Na+-dependent transport

Patients with optic neuritis or multiple sclerosis experience worsening symptoms and axonal impulse conduction failure with increased body temperature. Previously we have reported increases in current-threshold occur in optic nerve with localized warming, whose mechanism is only partly understood. Paired applications of near infra-red laser light to isolated optic nerve produced increases in threshold that were reduced on the second application, the response recovering with increasing application intervals, with a time-constant of 24 secs. 3 minute duration single applications of laser light gave rise to a rapid increase in threshold followed by a fade, where the time constant of the fade was between 40 and 50 seconds. Aftereffects were sometimes apparent following the light application, where the resting threshold was reduced. The increase in threshold with laser light illumination was partially blocked by 38.6 mM Li + in combination with 5 mM bumetanide, a manoeuvre increasing refractoriness and consistent with axonal depolarization. Assessing the effect of laser-light applications on the nerve input resistance ruled-out a previously suggested fall in myelin resistance as contributing to the changes in threshold. These data are consistent with an axonal membrane potential that partly relies on temperature-dependent electroneutral Na + influx, and where fade in the response to the laser is likely caused by a gradually diminishing Na + pump-induced hyperpolarization, in response to falling intracellular [Na + ].