Targeting spinal TRAF6 expression attenuates chronic visceral pain in adult rats with neonatal colonic inflammation

The YTHDF1-TRAF6 pathway regulates the neuroinflammatory response and contributes to morphine tolerance and hyperalgesia in the periaqueductal gray

Long-term use of opioids such as morphine has negative side effects, such as morphine analgesic tolerance and morphine-induced hyperalgesia (MIH). These side effects limit the clinical use and analgesic efficacy of morphine. Elucidation of the mechanisms and identification of feasible and effective methods or treatment targets to solve this clinical phenomenon are important. Here, we discovered that YTHDF1 and TNF receptor-associated factor 6 (TRAF6) are crucial for morphine analgesic tolerance and MIH. The m6A reader YTHDF1 positively regulated the translation of TRAF6 mRNA, and chronic morphine treatments enhanced the m6A modification of TRAF6 mRNA. TRAF6 protein expression was drastically reduced by YTHDF1 knockdown, although TRAF6 mRNA levels were unaffected. By reducing inflammatory markers such as IL-1β, IL-6, TNF-α and NF-κB, targeted reduction of YTHDF1 or suppression of TRAF6 activity in ventrolateral periaqueductal gray (vlPAG) slows the development of morphine analgesic tolerance and MIH. Our findings provide new insights into the mechanism of morphine analgesic tolerance and MIH indicating that YTHDF1 regulates inflammatory factors such as IL-1β, IL-6, TNF-α and NF-κB by enhancing TRAF6 protein expression.

Astrocytes in Chronic Pain: Cellular and Molecular Mechanisms

Chronic pain is challenging to treat due to the limited therapeutic options and adverse side-effects of therapies. Astrocytes are the most abundant glial cells in the central nervous system and play important roles in different pathological conditions, including chronic pain. Astrocytes regulate nociceptive synaptic transmission and network function via neuron–glia and glia–glia interactions to exaggerate pain signals under chronic pain conditions. It is also becoming clear that astrocytes play active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Therefore, this review presents our current understanding of the roles of astrocytes in chronic pain, how they regulate nociceptive responses, and their cellular and molecular mechanisms of action.

Glial IL-33 signaling through an ST2-to-CXCL12 pathway in the spinal cord contributes to morphine-induced hyperalgesia and tolerance

[Figure: see text].

FMRP acts as a key messenger for visceral pain modulation

Visceral pain is a common clinical symptom, which is caused by mechanical stretch, spasm, ischemia and inflammation. Fragile X syndrome (FXS) with lack of fragile X mental retardation protein (FMRP) protein is an inherited disorder that is characterized by moderate or severe intellectual and developmental disabilities. Previous studies reported that FXS patients have self-injurious behavior, which may be associated with deficits in nociceptive sensitization. However, the role of FMRP in visceral pain is still unclear. In this study, the FMR1 knock out (KO) mice and SH-SY5Y cell line were employed to demonstrate the role of FMRP in the regulation of visceral pain. The data showed that FMR1 KO mice were insensitive to zymosan treatment. Recording in the anterior cingulate cortex (ACC), a structure involved in pain process, showed less presynaptic glutamate release and postsynaptic responses in the FMR1 KO mice as compared to the wild type (WT) mice after zymosan injection. Zymosan treatment caused enhancements of adenylyl cyclase 1 (AC1), a pain-related enzyme, and NMDA GluN2B receptor in the ACC. However, these up-regulations were attenuated in the ACC of FMR1 KO mice. Last, we found that zymosan treatment led to increase of FMRP levels in the ACC. These results were further confirmed in SH-SY5Y cells in vitro. Our findings demonstrate that FMRP is required for NMDA GluN2B and AC1 upregulation, and GluN2B/AC1/FMRP forms a positive feedback loop to modulate visceral pain.

TRAF6 Contributes to CFA-Induced Spinal Microglial Activation and Chronic Inflammatory Pain in Mice

Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been reported to be expressed in spinal astrocytes and is involved in neuropathic pain. In this study, we investigated the role and mechanism of TRAF6 in complete Freund's adjuvant (CFA)-evoked chronic inflammatory hypersensitivity and the effect of docosahexaenoic acid (DHA) on TRAF6 expression and inflammatory pain. We found that TRAF6 was dominantly increased in microglia at the spinal level after intraplantar injection of CFA. Intrathecal TRAF6 siRNA alleviated CFA-triggered allodynia and reversed the upregulation of IBA-1 (microglia marker). In addition, intrathecal administration of DHA inhibited CFA-induced upregulation of TRAF6 and IBA-1 in the spinal cord and attenuated CFA-evoked mechanical allodynia. Furthermore, DHA prevented lipopolysaccharide (LPS)-caused increase of TRAF6 and IBA-1 in both BV2 cell line and primary cultured microglia. Finally, intrathecal DHA reduced LPS-induced upregulation of spinal TRAF6 and IBA-1, and alleviated LPS-induced mechanical allodynia. Our findings indicate that TRAF6 contributes to pain hypersensitivity via regulating microglial activation in the spinal dorsal horn. Direct inhibition of TRAF6 by siRNA or indirect inhibition by DHA may have therapeutic effects on chronic inflammatory pain.