The Effects of Helium-Neon Laser on Formalin-Induced Activity of Dorsal Horn Neurons in the Rat

Abstract

Extracellular recordings were made from single convergent neurons in the spinal cord dorsal horn of the halothane-anesthetized rat, and the effects of low power (8.5 mW; 632.8 nm) helium-neon laser irradiation were the neural responses to subcutaneous injection of formalin were examined. Formalin injection (5%, 100 μl) into the receptive fields of the neurons elicited an acute phasic and a prolonged tonic neural excitation, as has been reported previously. In the present study it was observed that prolonged (30 min) irradiation of the skin overlying the peripheral nerve with laser resulted in the attenuation of the formalin-induced discharge activities of the dorsal horn neurons. Relating the observed inhibitory effects of laser with clinically reported pain-relieving effects of laser irradiation, possible mechanisms for the inhibition of the dorsal horn neurons by laser are discussed.