Basic research and clinical investigations of the neural basis of orofacial pain

Background Trigeminal nerve injury or orofacial inflammation causes severe pain in the orofacial regions innervated by uninjured nerves or uninflamed tissues as well as injured or inflamed tissues. Pathological orofacial pain associated with trigeminal nerve injury or inflammation is difficult to diagnose and treat. Highlight To develop appropriate treatments for patients with orofacial pathological pain, various animal models of trigeminal nerve injury or orofacial inflammation have been developed. Further, the possible mechanisms involving the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1-C2) have been studied. Conclusion (1) Neurotransmitters released from the somata of TG neurons are involved in peripheral sensitization. (2) Neurotransmitter release from TG neurons is decreased by botulinum toxin-type A administration, suggesting that this toxin suppressed neurotransmitter release and alleviated the neuropathic pain-related behavior. (3) Altered states of glial cells and nociceptive neurons, in the Vc and C1-C2 are involved in pathological orofacial pain associated with trigeminal nerve injury or orofacial inflammation. (4) The trigeminal sensory nuclear complex, especially the trigeminal spinal subnucleus oralis, is involved in normal and pathological orofacial pain conditions after peripheral nerve injury. (5) Neuroimaging analyses have suggested functional changes in the central and peripheral nervous systems in neuropathic pain conditions.