Role and mechanism of endothelin in the central nervous system in neuropathic pain

Abstract

Neuropathic pain, with an estimated 1.5% pervasiveness in the worldwide population is now considered as a distinct clinical entity and treatment is frequently inadequate. Endothelin-1 (ET-1) emerges to be a therapeutic target for the treatment of neuropathic pain in recent decade, which has been implicated in a remarkable variety of pain related processes. ET-1 and its two receptors, ETAR and ETBR, are found across multiple levels in pain pathway from peripheral nociceptors to the central high end of pain perception. Although studies have illustrated the players in the endothelin system, the mechanism remains obscure, especially its function in the central nervous system (CNS) in neuropathic pain. Therefore, work in this thesis was designed to determine 1) the functional significance and 2) the regulatory mechanism of central ET-1 axis in neuropathic pain of peripheral origin, 3) whether ET-1 axis underlies neuropathic pain of central origin and mechanisms of current clinical treatments of neuropathic pain, namely ketamine and voluntary exercise.

In the first study, sciatic nerve ligation (SNL)-induced neuropathic pain model was established in our unique transgenic mice which has selective over-expression of ET-1 in astrocyte. We found that persistent astrocytic over-expression of ET-1 exerted antiallodynic/ hyperalgesic effect via ETAR. However, opposite result was observed in nontransgenic mice that nociceptive behaviors were associated with up-regulated ET-1 and ETAR, suggesting a differential modification of ETAR under prolonged exposure to overexpressed astrocytic ET-1 in transgenic mice. We further showed that ET-1 modulates pain response by regulation of spinal excitatory amino acid transporter (EAAT2), which facilitates synaptic glutamate uptake.

In the second study, we demonstrated that, in SNL-induced neuropathic pain, spinal mRNA expressions of ET-1 and ETAR were enhanced accompanied by elevated expressions of Pax2 and NFAT5, two transcription factors that facilitate ET-1 transcription. Gene silencing of Pax2, but not NFAT5, reduced spinal ET-1 expression while inhibition of Pax2, NFAT5 and ETAR attenuated neuropathic pain. Additionally, inhibition of ETAR also reduced mRNA expressions of post-SNL Pax2 and NFAT5, suggesting a novel signaling transduction by Pax2/ET-1/ETAR/NFAT5. To the best of our knowledge, this is the first study reporting a new function of Pax2 and NFAT5, indicating their significant functional regulatory role in ET-1 axis-mediated neuropathic pain.

In the third study, we assessed the role of ET-1 axis in central neuropathic pain. In spinal cord injury (SCI)-induced neuropathic pain, up-regulation of ETAR and ETBR was associated with increased glutamate N-methyl-D-aspartate (NMDA) receptor and decreased EAAT2. Combination of multiday low dose ketamine and voluntary exercise reduced ET-1 receptors and subsequently corrected the dysregulation of glutamatergic transmission. In addition, joint treatment was superior in improving sensory and motor function than either ketamine or voluntary exercise alone.

It is hoped that studies described in this thesis have advanced the knowledge concerning the role of ET-1 in neuropathic pain and its importance as a potential target in combating neuropathic pain. Findings observed in our research may facilitate the development of novel and effective therapies for the treatment of neuropathic pain and related functional deficits, which is common in chronic pain patients.