Absence of spinal response to extracorporeal shock waves on the endogenous opioid systems in the rat

Extracorporeal shock wave therapy (ESWT) seems to be a new therapeutic strategy for chronic pain due to tendopathies. Neurophysiological mechanisms of action for pain relief following ESWT are still unknown. The aim of this study was to investigate if the analgesic effect of ESWT is caused by modulation of the endogenous spinal opioid system. Rats were treated with two different energy flux densities (0.04 and 0.11mJ/mm(2)) and immunohistochemical analysis of met-enkephalin (MRGL) and dynorphin (Dyn) was performed at 4 or 72 h after ESWT. ESWT had no modulatory influence on the expression of the spinal opioid systems. Different energy doses or repetitive treatment did not alter MRGL or Dyn immunoreactivity in the spinal cord. Furthermore, a delayed effect of ESWT at 72 h after treatment was not detectable. We conclude from these findings that the analgesic effects of ESWT treatment are not supported by endogenous opioids.