“…A similar response has also been demonstrated with MWM of the elbow joint 35 . In a number of follow-up studies, which used methods to discern involvement of endogenous opioids, such as naloxone blockade and tolerance 34,35,43,69,70 , the hypoalgesic effect was found to be non-opioid in nature. Subsequently, an animal study of knee manipulation for capsaicin-induced foot hyperalgesia reported that the substrates of the hypoalgesic effect were nonopioid in nature, involving both serotonergic-and noradrenergic-mediated descending pain inhibition pathways 71 .…”