Carbon Monoxide-Releasing Molecule-2 Inhibits Connexin 43-Hemichannel Activity in Spinal Cord Astrocytes to Attenuate Neuropathic Pain

Abstract

Carbon monoxide-releasing molecule (CORM-2) acts as a carbon monoxide (CO) deliverer in a more controlled manner without altering carboxyhemoglobin level and exerts potential function in inhibiting inflammation and/or acute nociception. However, the regulatory mechanism of CORM-2 on spinal nerve ligation (SNL)-induced neuropathic pain is not currently clear. Our study aims to investigate the role of CORM-2 in neuropathic pain and the underlying mechanism. We found that spinal cord astrocytes were dramatically activated on day 7 after SNL. L-α-aminoadipate (L-α-AA), an astroglial toxin, reversed SNL-induced astrocyte activation at sub-toxic dose. Intrathecal administration of CO donor CORM-2 induced antiallodynic and antihyperalgesic effects in neuropathic animals induced by SNL and suppressed SNL-induced spontaneous excitatory postsynaptic current (EPSC) frequency in lamina II neurons of spinal cord slices. CORM-2 administration markedly inhibited SNL-induced connexin 43 (Cx43) expression, hemichannel function, and gap junction function on spinal astrocyte membranes. Moreover, exogenous CORM-2 could attenuate HO-1 expression, while overexpressed heme oxygenase-1 (HO-1) increased intracellular CO production, attenuated Cx43 expression, hemichannel function, and gap junction function on spinal astrocyte membranes. Additionally, Cx43 over-expression markedly reduced CORM-2-induced mechanical threshold and thermal hyperalgesia and elevated CORM-2-induced spontaneous EPSC frequency. In conclusion, CORM-2 attenuated SNL-induced neuropathic pain via suppressing Cx43-hemichannel function, which may contribute to understanding of the pathology of neuropathic pain.

Connexin Hemichannels in Astrocytes: Role in CNS Disorders

“…The expression changes of Cx43 following pain is still inconclusive (Table 2). Multiple studies have shown that Cx43 is upregulated in astrocytes following nerve ligation and spinal cord injury, and that inhibition of Cx43 can attenuate pain hypersensitivity (Wu et al, 2011; Chen et al, 2012, 2014, 2018; Shen et al, 2014; Neumann et al, 2015; Robinson and Dougherty, 2015; Choi et al, 2016; Hang et al, 2016; Kaji et al, 2016; Mao et al, 2017a,b; Wang and Sun, 2017; Yang et al, 2018). On the contrary, few studies showed that a decrease of Cx43 following nerve injury could contribute to pain hypersensitivity (Xu et al, 2014; Morioka et al, 2015; Zhang et al, 2016).…”

Role of Connexins in Chronic Pain and Their Potential as Therapeutic Targets for Next-Generation Analgesics

“…Cx43 expression appears to be upregulated within the spinal cord grey matter following a spinal cord injury, oxaliplatin and bortezomib-induced peripheral neuropathy. [37][38][39][40][41][42][43]…”

“…50) The NLRP3 inflamasome complex was activated following a CCI in mice. 37) Although a direct link between increased astrocytic Cx43 and activation of NLRP3 inflammasome was not demonstrated, they speculated that nerve injury evoked increased Cx43 hemichannel activity, resulting in increased ATP release, which in turn, activated NLRP3 inflammasome. 37) Other yet-to-be-identified substances that fit though the hemichannel could also activate other enzymes and lead to nociceptive hypersensitivity.…”

“…44,61) Intrathecal injection of Peptide5, which is a short peptide mimetic of the extracellular EL1 loop of the Cx43 channel, ameliorated mechanical hypersensitivity in mice with a CCI. 37) Suppression of Cx43 expression with RNA interference, in addition to blocking Cx functioning, could be a powerful approach to reducing chronic pain. 32) However, previous studies have also shown that downregulation of spinal Cx43 expression leads to nociceptive hypersensitivity, so upregulation, to basal levels, would be called for in some pain states.…”

Connexin43 Hemichannels in Satellite Glial Cells, Can They Influence Sensory Neuron Activity?

“…For example, Cx43 increased expression has been associated with the maintenance of the late-phase of neuropathic pain in mice (Chen et al, 2012 , 2014 ). Accordingly, the use of Gap26 -a Cx43 hemichannel blocker- attenuated the pain hypersensitivity in a mice cancer pain model (Li et al, 2017 ) and CORM-2—a CO donor and inhibitor of Cx43 hemichannels (León-Paravic et al, 2014 ) also decreased the levels of neuropathic pain in mice (Wang and Sun, 2017 ). Currently, the exact mechanism by which Cx43 hemichannels and/or GJCs present in spinal astrocytes are involved in pain-like behavior remains unknown.…”