Nociceptive scores and endorphin-containing cells reduced by Low Level Laser Therapy (LLLT) in inflamed paws of Wistar rats

Abstract

Objective: This study aimed to investigate how local pain relief is mediated by laser therapy and how dose affects the relationship. Methods: Inflammation was induced in the hind-paws of Wistar rats. Two groups of rats received 780-nm laser therapy (Spectra-Medics Pty Ltd.) at one of two doses (2.5 and 1 J/cm2). One group acted as a control. Scores of nociceptive threshold were recorded using paw pressure and paw thermal threshold measures. Results: A dose of 1 J/cm2 had no statistically significant effect on antinociceptive responses. A dose of 2.5 J/cm2 demonstrated a statistically significant effect on paw pressure threshold ( p < 0.029) compared to controls. There was no difference in paw thermal threshold responses and paw volumes at either dose. Immunohistochemistry in control animals demonstrated normal beta-endorphin containing lymphocytes in control inflamed paws but no beta-endorphin containing lymphocytes in rats that received laser at 2.5 J/cm2. Conclusion: The results confirm previous findings that the effect of laser therapy is dose-related. The mechanism of effect may occur via a differentiated pressure-sensitive neural pathway rather than a thermal-sensitive neural pathway. The significance of the immunohistochemistry findings remains unknown.