Cellular Neuroplasticity Mechanisms Mediating Pain Persistence

Transmission of noxious-stimulus-evoked inputs in the spinal and trigeminal systems is mediated primarily through excitatory glutamatergic synapses using alpha amino-3-hydroxy-5-methyl-4-isoxazole- propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors. Glutamatergic synapses exhibit multiple forms of short-lasting and long-lasting synaptic plasticity. Persistent enhancement of nociceptive transmission, known as "central sensitization", is a form of lasting plasticity that is similar mechanistically to long-term potentiation of glutamatergic transmission in other regions of the central nervous system. This potentiation of AMPA/kainate transmission is dependent upon the activity of NMDA receptors, which become enhanced following noxious peripheral stimulation as a result of several convergent mechanisms. Central sensitization is thus an expression of increased synaptic gain at glutamatergic synapses in central nociceptive- transmission neurons and thereby contributes importantly to pain hypersensitivity. In addition, recent evidence has revealed a new player in the mechanisms underlying pain hypersensitivity following nerve injury-microglia. Understanding of the roles of microglia may lead to new strategies for the diagnosis and management of neuropathic pain. Keywords: glutamate receptors, microglia, pain, spinal dorsal horn, synaptic plasticity, tyrosine kinases