Abstract
BackgroundActivated astrocytes play important roles in chronic post-surgical pain (CPSP). Recent studies have shown reactive astrocytes are classified into A1 and A2 phenotypes, but their precise roles in CPSP remain unknown. In this study, we investigated the roles of spinal cord A1 and A2 astrocytes and related mechanisms in CPSP.MethodsWe used a skin/muscle incision and retraction (SMIR) model to establish a rat CPSP model. Microglia, CXCR7, and the phosphoinositide 3-kinase/Akt (PI3K/Akt) signaling pathways were regulated by intrathecal injections of minocycline (a non-specific microglial inhibitor), AMD3100 (a CXCR7 agonist), and LY294002 (a specific PI3K inhibitor), respectively. Mechanical allodynia was detected with von Frey filaments. The changes in microglia, A1 astrocytes, A2 astrocytes, CXCR7, and PI3K/Akt signaling pathways were examined by enzyme-linked immunosorbent assay (ELISA), western blot, and immunofluorescence.ResultsMicroglia were found to be activated, with an increase in interleukin-1 alpha (IL-1α), tumor necrosis factor alpha (TNFα), and complement component 1q (C1q) in the spinal cord at an early stage after SMIR. On day 14 after SMIR, spinal cord astrocytes were also activated; these were mainly of the A1 phenotype and less of the A2 phenotype. Intrathecal injection of minocycline relieved SMIR-induced mechanical allodynia and reverted the ratio of A1/A2 reactive astrocytes. The expression of CXCR7 and PI3K/Akt signaling was decreased after SMIR, while they were increased after treatment with minocycline. Furthermore, intrathecal injection of AMD3100 also relieved SMIR-induced mechanical allodynia, reverted the ratio of A1/A2 reactive astrocytes, and activated the PI3K/Akt signaling pathway, similar to the effects produced by minocycline. However, intrathecal injection of AMD3100 did not increase the analgesic effect of minocycline. Last, LY294002 inhibited the analgesic effect and A1/A2 transformation induced by minocycline and AMD3100 after SMIR.ConclusionOur results indicated that microglia induce the transformation of astrocytes to the A1 phenotype in the spinal cord via downregulation of the CXCR7/PI3K/Akt signaling pathway during CPSP. Reverting A1 reactive astrocytes to A2 may represent a new strategy for preventing CPSP.
Highlights
Chronic postsurgical pain (CPSP) refers to pain in and around the surgical area that lasts longer than 2 months, excluding pain due to disease recurrence, inflammation, and other factors [1, 2]
Microglia are activated in the spinal cord in the early stages of skin/muscle incision and retraction (SMIR) Behavioral testing showed that mechanical paw withdrawal threshold (PWT) was decreased in the ipsilateral hindpaw from days 1 to 21 in the SMIR group, compared with baseline
The results of the enzyme-linked immunosorbent assay (ELISA) showed that spinal cord Interleukin-1 alpha (IL-1α), TNFα, and component 1q (C1q) were significantly higher in the SMIR group than in the sham group (Fig. 2c–e, IL-1α: F(2, 6) = 510, P < 0.0001; TNFα: F(2, 6) = 536.2, P < 0.0001; C1q: F(2, 6) = 41.76, P = 0.0003)
Summary
Chronic postsurgical pain (CPSP) refers to pain in and around the surgical area that lasts longer than 2 months, excluding pain due to disease recurrence, inflammation, and other factors [1, 2]. CPSP seriously affects post-operative recovery and the quality of life of the patient [3,4,5]. The mechanism underlying CPSP is complex and remains to be elucidated. Previous studies have shown that CPSP is a neuropathic pain caused mainly by surgical damage to peripheral nerves [6]. Flatters demonstrated that skin/ muscle incision and retraction (SMIR)-induced CPSP lasted at least 22 days without nerve damage [3]. Activated astrocytes play important roles in chronic post-surgical pain (CPSP). Recent studies have shown reactive astrocytes are classified into A1 and A2 phenotypes, but their precise roles in CPSP remain unknown. We investigated the roles of spinal cord A1 and A2 astrocytes and related mechanisms in CPSP
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