Protease-activated receptors in neuropathic pain: an important mediator between neuron and glia

Chronic neuropathic pain is a refractory symptom in clinical practice due to nervous injury or inflammation, and affects millions of people all over the world. Although the neuronal functioning of pain pathways has been studied for many years, the induction and maintenance of this non-adaptive, pathological pain is still poorly understood. Recent evidence indicates that protease-activated receptors (PARs) participate in the initiation and maintenance of neuropathic pain and play a key role in mediating the interactions of nerve cells. Firstly, following nerve injury, alterations in neuron and neuron function induce an abnormal increase of some neurotransmitters and neuromodulators, such as substance P (SP), calcitonin gene-related peptide (CGRP), prostaglandins, kinins, and so on. Such abnormal factors can act on neuron reversely and then induce pain sensation directly, or activate glial cells (astrocytes and microglia) mediated by PARs, which trigger and accelerate the progress of neuropathic pain. Secondly, when the noxious factors invade, glial cells are activated as the first barrier of nervous system and secret many neuroinflammatory factors. These inflammatory factors have effects on PARs (especially PAR1 and PAR2) in the neurons around, and then aggravate the status of pain. Thirdly, in the progress of neuroinflammatory pain, microglia is activated first and initiates the status of pain, and then inflammatory factors and complements from microglia activate astrocytes and maintain or make the pain worse. All of these actions is protective to neurons first, but then turns to a kind of nociception and forms the feeling of pain under the continuous noxious stimuli. Conclusively, PARs may play an important role in the formation and maintenance of chronic pain through mediating the interactions among nerve cells, which may be a novel target in the study and control of neuropathic pain. This article focuses on recent developments of PARs in the progress of neuropathic pain, and provides a framework for addressing the major questions for the future.