Analgesia and the neural substrate of reward

All the known strong analgesics have high abuse potential and, conversely, powerful rewards such as self-stimulation and drugs of abuse have analgesic properties. This coincidence of effects suggests that analgesia and reward may be related processes. Two types of analgesia have been described. One type is antinociceptive in that the analgesic reduces the intensity of the sensory input reaching the CNS from nociceptors. Another type of analgesia, sometimes called dissociative analgesia and exemplified by the patient reporting "It hurts but the pain does not bother me," has been described by clinicians. Thus strong analgesics can relieve the suffering without necessarily reducing the sensory intensity of pain. If analgesia and abuse potency are related it should be expected that the neural substrates of analgesia and abuse liability have considerable overlap. This paper discusses evidence that the analgesic effects of morphine and amphetamine in the formalin test are synergistic, and depend on dopaminergic neurotransmission involving the dopamine cells of the ventral tegmental area.