Crotalphine Modulates Microglia M1/M2 Phenotypes and Induces Spinal Analgesia Mediated by Opioid-Cannabinoid Systems

Abstract

Pain is a worldwide public health problem and its treatment is still a challenge since clinically available drugs do not completely reverse chronic painful states or induce undesirable effects. Crotalphine is a 14 amino acids synthetic peptide that induces a potent and long-lasting analgesic effect on acute and chronic pain models, peripherally mediated by the endogenous release of dynorphin A and the desensitization of the transient receptor potential ankyrin 1 (TRPA1) receptor. However, the effects of crotalphine on the central nervous system (CNS) and the signaling pathway have not been investigated. Thus, the central effect of crotalphine was evaluated on the partial sciatic nerve ligation (PSNL)-induced chronic neuropathic pain model. Crotalphine (100 µg/kg, p.o.)-induced analgesia on the 14th day after surgery lasting up to 24 h after administration. This effect was prevented by intrathecal administration of CB1 (AM251) or CB2 (AM630) cannabinoid receptor antagonists. Besides that, crotalphine-induced analgesia was reversed by CTOP, nor-BNI, and naltrindole, antagonists of mu, kappa, and delta-opioid receptors, respectively, and also by the specific antibodies for β-endorphin, dynorphin-A, and met-enkephalin. Likewise, the analgesic effect of crotalphine was blocked by the intrathecal administration of minocycline, an inhibitor of microglial activation and proliferation. Additionally, crotalphine decreased the PSNL-induced IL-6 release in the spinal cord. Importantly, in vitro, crotalphine inhibited LPS-induced CD86 expression and upregulated CD206 expression in BV-2 cells, demonstrating a polarization of microglial cells towards the M2 phenotype. These results demonstrated that crotalphine, besides activating opioid and cannabinoid analgesic systems, impairs central neuroinflammation, confirming the neuromodulatory mechanism involved in the crotalphine analgesic effect.

Microglia polarization in nociplastic pain: mechanisms and perspectives

“…In rat model of collagen-induced arthritis, administration of microglial inhibitor A-438079 (P2X7 antagonist) decreased the occurrence of mechanical allodynia, reduced IL-1β levels, inhibited microgliosis, in addition to the inhibition of spinal nociceptive withdrawal reflexes (Nieto et al 2016). In vitro, crotalphine downregulated CD86 expression and enhanced CD206 expression in LPS-treated BV-2 cells, shifting microglial polarization onto the anti-inflammatory M2 phenotype, confirming the neuromodulatory role contributed to crotalphine analgesic action (Lopes et al 2022). In the bone cancer pain mouse model, spinal cord microglia displayed augmented M1 activation and reduced M2 polarization, as well as up-regulated IL-1β and suppressed IL-10 expression throughout the development of bone cancer pain.…”