Peripheral Nerve Block Facilitates Acute Inflammatory Responses Induced by Surgical Incision in Mice.

BACKGROUND AND OBJECTIVES: Anesthesia with peripheral nerve block (PNB) improves the early recovery profile of patients undergoing surgery, including the control of postoperative pain, opioid consumption, and the length of hospital stay. However, the influence of PNB on wound inflammation and the repair process has not been fully investigated. Therefore, we evaluated the effects of PNB on local inflammation of incised tissue in the acute phase of postoperative pain development. METHODS: Sciatic nerve block with 0.5% ropivacaine was performed before plantar incision in mice. Pain behavior, neutrophil infiltration, phagocytosis of apoptotic cells, and gene induction of inflammatory mediators were assessed for 7 days postoperatively. RESULTS: Sciatic nerve block with 0.5% ropivacaine treatment transiently increased the withdrawal threshold to mechanical stimuli and thermal latency for 2 hours after surgical incision, whereas no changes were observed from 3 hours after incision throughout the postoperative period. However, Gr-1 neutrophil infiltration and the number of CD68 macrophages engulfing TdT-mediated dUTP nick-end labeling apoptotic cells were significantly increased after incision. Tumor necrosis factor α and prostaglandin E2 were up-regulated at the incised sites. In addition, the expressions of lipoxygenase-15 and heme oxygenase-1, which resolve inflammation and promote wound healing after the acute inflammatory phase, were increased. CONCLUSIONS: Single PNB before incision promoted acute phase inflammation mediated by neutrophils and macrophages at the sites of incision, whereas postoperative pain was not altered. Peripheral nerve block might locally accelerate innate immune responses after surgical incision without altering the nociceptive profile.

Resolution of Inflammation by Resolvin D1 Is Essential for Peroxisome Proliferator-activated Receptor-γ-mediated Analgesia during Postincisional Pain Development in Type 2 Diabetes.

BACKGROUND: The wound healing process following acute inflammation after surgery is impaired in diabetes. Altered macrophage functions are linked to delayed tissue repair and pain development in diabetes. Although peroxisome proliferator-activated receptor (PPAR)-γ agonists are used to treat diabetes, their postoperative analgesic effects in diabetes have not been evaluated. METHODS: The PPARγ agonist rosiglitazone (rosi) was injected at the incision site of diabetic (db/db) mice with resolvin (Rv) D1, a lipid mediator involved in resolution of inflammation. Pain-related behavior, neutrophil infiltration, phagocytosis, and macrophage polarity were assessed for 7 days postoperatively. RESULTS: Rosiglitazone and RvD1 alleviated mechanical hyperalgesia in db/db (db) mice, whereas rosiglitazone alone did not alter mechanical thresholds on days 4 (db rosi + RvD1 vs. db rosi: 0.506 ± 0.106 vs. 0.068 ± 0.12) and 7 (0.529 ± 0.184 vs. 0.153 ± 0.183) after incision (n = 10 per group). In control m/m mice, the rosiglitazone-induced analgesic effects were reversed by knockdown with arachidonate 5-lipoxygenase small interfering RNA, but these were restored by addition of RvD1. In db/db mice treated with rosiglitazone and RvD1, local infiltration of neutrophils was markedly reduced, with an associated decrease in total TdT-mediated dUTP nick-end labeling cells. Acceleration of rosiglitazone-induced phenotype conversion of infiltrated macrophages from M1 to M2 was impaired in db/db mice, but it was effectively restored by RvD1 in db/db wounds. CONCLUSIONS: In diabetes, exogenous administration of RvD1 is essential for PPARγ-mediated analgesia during development of postincisional pain. Resolution of inflammation accelerated by RvD1 might promote PPARγ-mediated macrophage polarization to the M2 phenotype.

Peripheral administration of morphine attenuates postincisional pain by regulating macrophage polarization through COX-2-dependent pathway.

BACKGROUND: Macrophage infiltration to inflammatory sites promotes wound repair and may be involved in pain hypersensitivity after surgical incision. We recently reported that the development of hyperalgesia during chronic inflammation is regulated by macrophage polarity, often referred to as proinflammatory (M1) or anti-inflammatory (M2) macrophages. Although opioids such as morphine are known to alter the inflammatory milieu of incisional wounds through interactions with immunocytes, the macrophage-mediated effects of morphine on the development of postincisional pain have not been well investigated. In this study, we examined how morphine alters pain hypersensitivity through phenotypic shifts in local macrophages during the course of incision-induced inflammation. RESULTS: Local administration of morphine in the early phase, but not in the late phase alleviated mechanical hyperalgesia, and this effect was reversed by clodronate-induced peripheral depletion of local macrophages. At the morphine-injected incisional sites, the number of pro-inflammatory F4/80+iNOS+M1 macrophages was decreased during the course of pain development whereas increased infiltration of wound healing F4/80+CD206+M2 macrophages was observed during the early phase. Morphine increased the gene expression of endogenous opioid, proenkephalin, and decreased the pronociceptive cytokine, interleukin-1ß. Heme oxygenase (HO)-1 promotes the differentiation of macrophages to the M2 phenotype. An inhibitor of HO-1, tin protoporphyrin reversed morphine-induced analgesic effects and the changes in macrophage phenotype. However, local expression levels of HO-1 were not altered by morphine. Conversely, cyclooxygenase (COX)-2, primarily produced from peripheral macrophages in acute inflammation states, was up-regulated in the early phase at morphine-injected sites. In addition, the analgesic effects and a phenotype switching of infiltrated macrophages by morphine was reversed by local administration of a COX inhibitor, indomethacin. CONCLUSIONS: Local administration of morphine alleviated the development of postincisional pain, possibly by altering macrophage polarity at the incisional sites. A morphine-induced shift in macrophage phenotype may be mediated by a COX-2-dependent mechanism. Therefore, µ-opioid receptor signaling in macrophages may be a potential therapeutic target during the early phase of postincisional pain development.

Peroxisome proliferator-activated receptor-gamma agonist rosiglitazone attenuates inflammatory pain through the induction of heme oxygenase-1 in macrophages.

Macrophage infiltration to inflammatory sites promotes tissue repair and may be involved in pain hypersensitivity. Peroxisome proliferator-activated receptor (PPAR)γ signaling is known to regulate polarity of macrophages, which are often referred to as proinflammatory (M1) and antiinflammatory (M2) macrophages. We recently showed that the PPARγ agonist rosiglitazone ameliorated the development of postincisional hyperalgesia by increasing the influx of M2 macrophages to inflamed sites. It has been suggested that heme oxygenase (HO)-1, upregulated by PPARγ signaling, promotes differentiation of macrophages to M2 phenotype. In this study, we investigated how rosiglitazone alters pain hypersensitivity by a PPARγHO-1-dependent mechanism during the course of inflammation induced by complete Freund's adjuvant. Local administration of rosiglitazone alleviated mechanical hyperalgesia, with increased gene induction of HO-1. Phenotype switching of infiltrated macrophages to M2 by rosiglitazone was reversed by an HO-1 inhibitor, tin protoporphyrin, at the inflamed sites. Direct stimulation of peritoneal macrophages with rosiglitazone also increased HO-1 induction in the presence of lipopolysaccharide/interferon-γ. Moreover, rosiglitazone increased gene induction of endogenous opioid proenkephalin, both at inflamed sites and in isolated macrophages. Administration of naloxone blocked the analgesic effects of rosiglitazone. We speculate that rosiglitazone alleviated the development of inflammatory pain, possibly through regulating the M1/M2 balance at the inflamed site by a PPARγ/HO-1-dependent mechanism. PPARγ signaling in macrophages may be a potential therapeutic target for the treatment of acute pain development.

Peroxisome proliferator-activated receptor-gamma agonist rosiglitazone attenuates postincisional pain by regulating macrophage polarization.

Acute inflammation triggered by macrophage infiltration to injured tissue promotes wound repair and may induce pain hypersensitivity. Peroxisome proliferator-activated receptor γ (PPAR)γ signaling is known to regulate heterogeneity of macrophages, which are often referred to as classically activated (M1) and alternatively activated (M2) macrophages. M1 macrophages have considerable antimicrobial activity and produce a wide variety of proinflammatory cytokines. In contrast, M2 macrophages are involved in anti-inflammatory and homeostatic functions linked to wound healing and tissue repair. Although it has been suggested that PPARγ agonists attenuate pain hypersensitivity, the molecular mechanism of macrophage-mediated effects of PPARγ signaling on pain development has not been explored. In this study, we investigated the link between the phenotype switching of macrophage polarization induced by PPARγ signaling and the development of acute pain hypersensitivity. Local administration of rosiglitazone significantly ameliorated hypersensitivity to heat and mechanical stimuli, and paw swelling. Consistent with the down-regulation of nuclear factor κB (NFκB) phosphorylation by rosiglitazone at the incisional sites, the number of F4/80(+)iNOS(+) M1 macrophages was decreased whereas numbers of F4/80(+)CD206(+) M2 macrophages were increased in rosiglitazone-treated incisional sites 24 h after the procedure. In addition, gene induction of anti-inflammatory M2-macrophage-associated markers such as arginase1, FIZZ1 and interleukin (IL)-10 were significantly increased, whereas M1-macrophage-related molecules such as integrin αX, IL-1ß, MIP2α and leptin were decreased at rosiglitazone-treated incisional sites. Moreover, transplantation of rosiglitazone-treated peritoneal macrophages into the incisional sites significantly attenuated hyperalgesia. We speculate that local administration of rosiglitazone significantly alleviated the development of postincisional pain, possibly through regulating macrophage polarity at the inflamed site. PPARγ signaling in macrophages may be a potential therapeutic target for the treatment of acute pain development.