Contribution of the Sensitization of Supraspinal Nociceptive Transmission in Chronic Pain

Central sensitization in the spinal cord is well known to be involved in chronic pain. Recent investigations indicated that the protein expressions involving the synaptic plasticity are changed in several brain areas under a chronic pain condition. These changes in supraspinal neural function might cause the emotional and memory dysfunction. It is also possible that these changes are involved in the chronic pain. Indeed, since the improvement of spinal and peripheral sensitization showed limited relief in the neuropathic pain, the sensitization of supraspinal nociceptive transmission might be involved in the expression of chronic pain. We recently found that intra-thalamic treatment with excitatory neurotransmitter glutamate caused hyperalgesia, which is mediated by the stimulation of glutamate N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Moreover, intracerebroventricular treatment with gabapentin, a calcium channel alpha2delta-1 subunit blocker, attenuated the hyperalgesia in the nerve-injury model of mice. These results suggest that the sensitization of supraspinal nociceptive transmission is involved in neuropathic pain. It is also indicated that neuropathic pain is resulted from the activations of spinal glial cells. Likewise, the supraspinal glial activation was observed in the neuropathic pain. Therefore, the sensitization of supraspinal nociceptive transmission might be important for a chronic pain. In this review, we would like to discuss the possible involvement of the supraspinal sensitization in neuropathic pain and in its application for the curative treatment in chronic pain.