Placebo-induced changes in spinal cord pain processing.

Pain is an essential sensory modality, signaling injury or threat of injury. Pain perception depends on both biological and psychological factors. However, it is not known whether psychological factors modify spinal mechanisms or if its effect is limited to cortical processing. Here, we use a placebo analgesic model to show that psychological factors affect human spinal nociceptive processes. Mechanical hyperalgesia (hypersensitivity) after an injury is attributable to sensitized sensory neurons in the spinal cord. After a 5 min, 46 degrees C heating of the skin, subjects developed areas of mechanical hyperalgesia. This area was smaller in a placebo condition compared with a baseline condition. This result suggests that placebo analgesia affects the spinal cord as well as supra-spinal pain mechanisms in humans and provides strong supporting evidence that placebo analgesia is not simply altered reporting behavior. Central sensitization is thought to mediate the exaggerated pain after innocuous sensory stimulation in several clinical pain conditions that follow trauma and nervous-system injury. These new data indicate that expectation about pain and analgesia is an important component of the cognitive control of central sensitization.