Near-Infrared Photobiomodulation of the Peripheral Nerve Inhibits the Neuronal Firing in a Rat Spinal Dorsal Horn Evoked by Mechanical Stimulation

Abstract

Photobiomodulation has analgesic effects via inhibition of nerve activity, but few reports have examined the effects on the spinal dorsal horn, the entry point for nociceptive information in the central nervous system. In this study, we evaluated the effects of laser irradiation of peripheral nerve axons, which are conduction pathways for nociceptive stimuli, on the neuronal firing in lamina II of the spinal dorsal horn of a rat evoked by mechanical stimulation with von Frey filaments (vFF). In order to record neuronal firing, electrodes were inserted into lamina II of the exposed rat spinal dorsal horn. The exposed sciatic nerve axons were irradiated with an 808 nm laser. The 26.0 g vFF-evoked firing frequency was inhibited from 5 min after laser irradiation and persisted for 3 h. Sham irradiation did not alter the firing frequency. Laser irradiation selectively inhibited 15.0 and 26.0 g vFF-evoked firing, which corresponded to nociceptive stimuli. Histopathological evaluation revealed no damage to the sciatic nerve due to laser irradiation. These results indicate that neuronal firing is inhibited in lamina II of the spinal dorsal horn, suggesting that laser irradiation inhibits Aδ and/or C fibers that conduct nociceptive stimuli.

Relationship between Laser Intensity at the Peripheral Nerve and Inhibitory Effect of Percutaneous Photobiomodulation on Neuronal Firing in a Rat Spinal Dorsal Horn

“…We attempted to compare the effects of direct laser irradiation on the sciatic nerve using the same experimental setup in the past [23] with the effects of percutaneous irradiation in this study. The vFF-evoked firing frequency of the neurons varied widely, and it was difficult to compare the firing frequency values.…”

“…This study suggested that laser irradiation inhibited the activity of Aδ and/or C fibers [23] because Aδ and C fibers innervate lamina II [30]. However, because the sciatic nerve was exposed and the laser was applied directly [23], it is unknown whether percutaneous irradiation, used in clinical practice, also inhibits neuronal firing in the lamina II of the spinal dorsal horn.…”

“…Although the mechanism is not fully understood [2][3][4][5]21,22], previous studies using electrophysiological techniques have shown that PBM affects the activity of paintransmitting nerves [23][24][25][26][27][28][29]. These include a few studies in which neural activity was recorded in the spinal dorsal horn, the entry point for pain signals in the central nervous system [30], but these studies do not mention the Rexed laminae of the spinal dorsal horn [28,29].…”

“…These include a few studies in which neural activity was recorded in the spinal dorsal horn, the entry point for pain signals in the central nervous system [30], but these studies do not mention the Rexed laminae of the spinal dorsal horn [28,29]. We previously reported that the 808 nm laser irradiation of the sciatic nerve, exposed through a skin incision in rats, inhibited neuronal firing in the lamina II of the spinal dorsal horn evoked by mechanical stimulation, corresponding to the noxious stimulus of the peripheral cutaneous receptive field [23]. This study suggested that laser irradiation inhibited the activity of Aδ and/or C fibers [23] because Aδ and C fibers innervate lamina II [30].…”

“…We previously reported that the 808 nm laser irradiation of the sciatic nerve, exposed through a skin incision in rats, inhibited neuronal firing in the lamina II of the spinal dorsal horn evoked by mechanical stimulation, corresponding to the noxious stimulus of the peripheral cutaneous receptive field [23]. This study suggested that laser irradiation inhibited the activity of Aδ and/or C fibers [23] because Aδ and C fibers innervate lamina II [30]. However, because the sciatic nerve was exposed and the laser was applied directly [23], it is unknown whether percutaneous irradiation, used in clinical practice, also inhibits neuronal firing in the lamina II of the spinal dorsal horn.…”