Study of Analgesic Effects of Laser Irradiation in Rats

<p>Low-level laser therapy (LLLT) has been employed to alleviate pain resulting from chronic non-infectious inflammation in muscles and joints. One of the challenges of LLLT is that a consistent understanding of mechanism of action remains elusive. The spinal dorsal horn, a key site where noxious signals are densely conveyed, offers a significant point of study for understanding the analgesic mechanism of LLLT. However, there’s a dearth of reports assessing LLLT’s effects on this region. In our study, the effect of laser irradiation on the sciatic nerve and the resultant firing of superficial neurons in the spinal dorsal horn—triggered by mechanical skin stimulation—was investigated using in vivo extracellular recording. To monitor the firing from individual neurons, electrodes were positioned in the superficial layer of the rat spinal dorsal horn. Mechanical stimulation, facilitated by von Frey filaments (vFF) ranging from 0.6 g to 26.0 g, was applied to the skin receptive field governed by the sciatic nerve, both pre and post laser exposure. The vFF-evoked neuronal firing in the superficial spinal dorsal horn was then recorded and analyzed. We examined two methods of laser application: direct irradiation onto the exposed sciatic nerve through a skin incision and percutaneous irradiation without making any incision. Our findings indicate that direct laser exposure to the sciatic nerve significantly inhibited the 26.0 g vFF-evoked firing frequency post-irradiation, with effects persisting for 3 hours. In an analysis 15 minutes post-irradiation, both 15.0 g and 26.0 g vFF-evoked firing frequencies were selectively inhibited. Sham irradiation showed no changes in firing frequency. A similar trend was observed with percutaneous laser irradiation. The firing frequency ratios between pre and 15 minutes post-irradiation remained consistent between the direct and percutaneous irradiation methods. When a photodiode sensor was implanted near the sciatic nerve, followed by percutaneous laser application, the power density reaching the nerve decreased to approximately 10% of the set irradiation parameters. Moreover, histopathological evaluations revealed no observable damage to the sciatic nerve due to direct laser exposure. Given that the 15.0 g and 26.0 g vFF are thought to correspond to noxious (painful) stimuli, our results suggest that laser irradiation selectively inhibits neuronal firing triggered by pain. This hints at the inhibition of neural activities in either or both Aδ and C fibers. Our findings suggest LLLT might not only be suitable for treating inflammation-induced pain (currently an insurable condition) but also for conditions where Aδ and C fibers play a pivotal role in the pathophysiological mechanism.</p>