Rescue of HSP70 in Spinal Neurons Alleviates Opioids-Induced Hyperalgesia via the Suppression of Endoplasmic Reticulum Stress in Rodents.

Tong-Tong Lin,Chun-Yi Jiang,Fan Hu,Liang Hu,Wen-Tao Liu,Xing Yang,Chao-Yu Wang,Yuan Han,Xue-Feng Wu,Jie Qu

doi:10.3389/fcell.2020.00269

Abstract

A major unresolved issue in treating pain is the paradoxical hyperalgesia produced by the gold-standard analgesic morphine and other opioids. Endoplasmic reticulum (ER) stress has been shown to contribute to neuropathic or inflammatory pain, but its roles in opioids-induced hyperalgesia (OIH) are elusive. Here, we provide the first direct evidence that ER stress is a significant driver of OIH. GRP78, the ER stress marker, is markedly upregulated in neurons in the spinal cord after chronic morphine treatment. At the same time, morphine induces the activation of three arms of unfolded protein response (UPR): inositol-requiring enzyme 1α/X-box binding protein 1 (IRE1α/XBP1), protein kinase RNA-like ER kinase/eukaryotic initiation factor 2 subunit alpha (PERK/eIF2α), and activating transcription factor 6 (ATF6). Notably, we found that inhibition on either IRE1α/XBP1 or ATF6, but not on PERK/eIF2α could attenuate the development of OIH. Consequently, ER stress induced by morphine enhances PKA-mediated phosphorylation of NMDA receptor subunit 1(NR1) and leads to OIH. We further showed that heat shock protein 70 (HSP70), a molecular chaperone involved in protein folding in ER, is heavily released from spinal neurons after morphine treatment upon the control of KATP channel. Glibenclamide, a classic KATP channel blocker that inhibits the efflux of HSP70 from cytoplasm to extracellular environment, or HSP70 overexpression in neurons, could markedly suppress morphine-induced ER stress and hyperalgesia. Taken together, our findings uncover the induction process and the central role of ER stress in the development of OIH and support a novel strategy for anti-OIH treatment.

Highlights

Opioids, such as morphine, are indispensable in the treatment of severe pain
Immunoblot results indicated that the levels of activating transcription factor 6 (ATF6), IRE1α, p-eIF2, XBP1s, and Caspase-12 were increased in the spinal cord of mice under the treatment of morphine (Figure 1B)
The principal findings are: (1) during the development of opioids-induced hyperalgesia (OIH), Endoplasmic reticulum (ER) is robustly induced by morphine in the neurons in the spinal cord, with activation of all the three unfolded protein response (UPR) pathways, including IRE1α/XBP1s, ATF6, and protein kinase-like ER kinase (PERK)/eIF2α

Summary

Introduction

Opioids, such as morphine, are indispensable in the treatment of severe pain. Use of these drugs is plagued by two major unresolved problems: hyperalgesia and tolerance. Numerous studies have been devoted to understanding the mechanisms underlying opioids-induced hyperalgesia (OIH), including the central glutaminergic system (Mao, 2002; Mao et al, 2002; Gupta et al, 2011), spinal dynorphins (Gardell et al, 2002), descending facilitation (Barbaro et al, 1986; Heinricher et al, 1992; Morgan et al, 1992), decreased reuptake and enhanced nociceptive response, and genetic influences (Mao et al, 2002; King et al, 2005; Liang et al, 2006). N-methyl-D-aspartate (NMDA) receptor, which mediates the nociceptive effect of glutamate and plays pivotal roles in the development of hyperalgesia, may be activated by opioids. The NR2 subunit can be phosphorylated by Ca2+/calmodulin (CaM)−dependent protein kinase II (CaMKII) and PKC (MacDonald et al, 2001; Robison et al, 2005)

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Conclusion