The alterations in nerve growth factor concentration in plasma and synovial fluid before and after total knee arthroplasty

Total knee arthroplasty (TKA) is an effective procedure for pain relief; however, the emergence of postsurgical pain remains a concern. In this study, we investigated the production of nerve growth factor (NGF) and mediators that affect NGF production and their function in the synovial fluid and plasma after TKA. This study included 19 patients (20 knees) who had rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), and knee osteoarthritis (OA) who underwent TKA, categorized into OA and non-OA groups. The levels of NGF, inflammatory cytokines, and lipid mediators were analyzed before and after surgery. The intraoperative synovial fluid NGF concentration was more than seven times higher in the non-OA group than in the OA group. The intra-articular NGF levels increased significantly by more than threefold postoperatively in the OA group but not in the non-OA group. Moreover, the levels of inflammatory cytokines and lipid mediators were increased in the synovial fluid of both groups. The intra-articular cytokines or NGF concentrations positively correlated with postoperative pain. Targeted NGF control has the potential to alleviate postsurgical pain in TKA, especially in patients with OA, emphasizing the importance of understanding NGF dynamics under different knee conditions.

CXCL13/CXCR5 promote chronic postsurgical pain and astrocyte activation in rats by targeting NLRP3

Chronic postsurgical pain (CPSP) with high incidence negatively impacts the quality of life. X-C motif chemokine 13 (CXCL13) has been associated with postsurgery inflammation and exacerbates neuropathic pain in patients with CPSP. This study was aimed to illustrate the relationship between CXCL13 and nod-like receptor protein-3 (NLRP3), which is also involved in CPSP. A CPSP model was constructed by skin/muscle incision and retraction (SMIR) in right medial thigh, and the rats were divided into three groups: Sham, SMIR, and SMIR + anti-CXCL13 (intrathecally injected with anti-CXCL13 antibody). Then, the paw withdrawal threshold (PWT) score of rats was recorded. Primary rat astrocytes were isolated and treated with recombinant protein CXCL13 with or without NLRP3 inhibitor INF39. The expressions of CXCL13, CXCR5, IL-1β, IL-18, GFAP, NLRP3, and Caspase-1 p20 were detected by real-time quantitative reverse transcription PCR, western blot, ELISA, immunocytochemistry, and immunofluorescence analyses. The anti-CXCL13 antibody alleviated SMIR-induced decreased PWT and increased expression of GFAP, CXCL13, CXCR5, NLRP3, and Caspase-1 p20 in spinal cord tissues. The production of IL-1β, IL-18, and expression of CXCL13, CXCR5, GFAP, NLRP3, and Caspase-1 p20 were increased in recombinant protein CXCL13-treated primary rat astrocytes in a dose-dependent manner. Treatment with NLRP3 inhibitor INF39 inhibited the function of recombinant protein CXCL13 in primary rat astrocytes. The CXCL13/CXCR5 signaling could promote neuropathic pain, astrocytes activation, and NLRP3 inflammasome activation in CPSP model rats by targeting NLRP3. NLRP3 may be a potential target for the management of CPSP.

Microglial exosome miR-124-3p in hippocampus alleviates cognitive impairment induced by postoperative pain in elderly mice

Cognitive impairment induced by postoperative pain severely deteriorates the rehabilitation outcomes in elderly patients. The present study focused on the relationship between microglial exosome miR-124-3p in hippocampus and cognitive impairment induced by postoperative pain. Cognitive impairment model induced by postoperative pain was constructed by intramedullary nail fixation after tibial fracture. Morphine intraperitoneally was carried out for postoperative analgesia. Morris water maze tests were carried out to evaluate the cognitive impairment, while mRNA levels of neurotrophic factors (BDNF, NG) and neurodegenerative biomarker (VILIP-1) in hippocampus were tested by q-PCR. Transmission electron microscope was used to observe the axon degeneration in hippocampus. The levels of pro-inflammatory factors (TNF-α, IL-1β, IL-6), the levels of anti-inflammatory factors (Ym, Arg-1, IL-10) and microglia proliferation marker cyclin D1 in hippocampus were measured to evaluate microglia polarization. Bioinformatics analysis was conducted to identify key exosomes while BV-2 microglia overexpressing exosome miR-124-3p was constructed to observe microglia polarization in vitro experiments. Exogenous miR-124-3p-loaded exosomes were injected into hippocampus in vivo. Postoperative pain induced by intramedullary fixation after tibial fracture was confirmed by decreased mechanical and thermal pain thresholds. Postoperative pain induced cognitive impairment, promoted axon demyelination, decreased BDNF, NG and increased VILIP-1 expressions in hippocampus. Postoperative pain also increased pro-inflammatory factors, cyclin D1 and decreased anti-inflammatory factors in hippocampus. However, these changes were all reversed by morphine analgesia. Bioinformatics analysis identified the critical role of exosome miR-124-3p in cognitive impairment, which was confirmed to be down-regulated in hippocampus of postoperative pain mice. BV-2 microglia overexpressing exosome miR-124-3p showed decreased pro-inflammatory factors, cyclin D1 and increased anti-inflammatory factors. In vivo, stereotactic injection of exogenous miR-124-3p into hippocampus decreased pro-inflammatory factors, cyclin D1 and increased anti-inflammatory factors. The cognitive impairment, axon demyelination, decreased BDNF, NG and increased VILIP-1 expressions in hippocampus were all alleviated by exogenous exosome miR-124-3p. Microglial exosome miR-124-3p in hippocampus alleviates cognitive impairment induced by postoperative pain through microglia polarization in elderly mice.

SIRT2 overexpression decreases remifentanil-stimulated post-surgical hyperalgesia via microglia

Remifentanil (Remi)-induced hyperalgesia is a serious but common postoperative clinical problem. Sirtuin 2 (SIRT2) is essential in the pathogenetic mechanisms of several neurological disorders. However, whether SIRT2 contributes to the modulation of Remi-induced postsurgical hyperalgesia (POH) is unknown. Here, we investigated the regulatory potential of SIRT2 in Remi-stimulated POH. A rat Remi-stimulated POH model was built by infusing Remi in the surgical incision. Mechanical allodynia and thermal hyperalgesia were separately assessed by paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) measurements. SIRT2 and binding adaptor molecule 1 (Iba1) protein expressions and localization in spinal cord samples were detected by western blot and immunofluorescence. The results revealed SIRT2 downregulation in the spinal cord of rats with Remi-stimulated POH. Intrathecal administration of the overexpression plasmid harboring SIRT2 remarkably relieved mechanical allodynia, along with thermal hyperalgesia in the model animals. Iba1 amounts were increased upon intraoperative incision or Remi infusion, and this effect was more pronounced upon combining both treatments. Furthermore, SIRT2 overexpression suppressed microglia activation in the spinal cord of model animals, and starkly relieved incision- and/or Remi-associated pronociceptive processes as well as spinal microglia activation. SIRT2 elevation relieved Remi-associated POH, likely by suppressing spinal microglia activation. Thus, SIRT2 could be a potent target for treating neuropathic pain.

Role of magnesium-L-Threonate in alleviating skin/muscle incision and retraction induced mechanical allodynia and anxiodepressive-like behaviors in male rats

Chronic postsurgical pain (CPSP) and its emotional comorbidities poses health burden to patients who have received the surgical treatment. However, its underlying mechanism remains unclear. Emerging studies indicate that magnesium deficiency is associated with neurological diseases, and magnesium supplement confers protection under these disease conditions. In this study, we examined the role and mechanism of magnesium deficiency in the pathology of surgery-induced allodynia and negative emotion using a rat model of skin/muscle incision and retraction (SMIR) and investigated the therapeutic effects of magnesium supplementation by oral magnesium-L-Threonate (L-TAMS) in SMIR-injured rats. In the SMIR model, rats developed mechanical allodynia and anxiodepressive-like behaviors. Further, SMIR caused microglia and astrocyte activation and enhanced expression of pro-inflammatory cytokine (TNF-α, IL-1β and IL-6) in the anterior cingulate cortex (ACC). Importantly, magnesium ion (Mg2+) levels decreased in the serum and cerebrospinal fluid (CSF) of SMIR-injured rats, which exhibited high correlation with pain and emotion behavioral phenotypes in these rats. Repeated oral administration of L-TAMS increased serum and CSF levels of Mg2+ in SMIR-injured rats. Notably, L-TAMS administration reversed SMIR-induced mechanical allodynia and anxiodepressive-like behaviors but did not affect pain and emotional behaviors in sham rats. Moreover, L-TAMS administration suppressed SMIR-caused glial activation and proinflammatory cytokine expression in the ACC but had no such effect in sham rats. Together, our study demonstrates the contributing role of magnesium deficiency in the pathology of surgery-induced chronic pain and negative emotion. Moreover, we suggest that L-TAMS might be a novel approach to treat CPSP and its emotional comorbidities.

NETO2-GluK2 interaction contributes to postoperative pain hypersensitivity through inducing PKCγ activation and synaptic incorporation of AMPA receptor GluR1 subunits in rat dorsal horn

Important roles in the initiation and maintenance of postoperative pain are played by the functional control of kainate (KA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the rat dorsal horn (DH). However, the mechanisms underpinning the cross-talk between spinal KA and AMPA receptors in postoperative pain are poorly understood. We hypothesized that after the rat's plantar incision, the synaptic incorporation of AMPA receptor GluR1 subunits in the DH ipsilateral to the incision would increase due to the interaction between GluK2 and neuropilin tolloid-like 2 (NETO2). Our findings showed that incision stimuli caused severe pain responses, as measured by cumulative pain scores. GluK2-NETO2 but not GluK2-NETO1interaction was upregulated in ipsilateral dorsal horn neurons (DHNs) at 6 h post-incision. At 6 h post-incision, NETO2 small interfering ribonucleic acid (siRNA) intrathecal pretreatment increased mechanical withdrawal thresholds to von Freys and decreased ipsilateral paw cumulative pain scores. Further, PKCγactivation and synaptic abundance of GluK2 and GluR1 subunits in the ipsilateral DH were decreased by intrathecal pretreatment with NETO2 siRNA at 6 h post-incision. In conclusion, our findings imply that GluK2-NETO2 interaction could trigger PKCγactivation and the synaptic incorporation of AMPA receptor GluR1 subunits in rat DHs, which in turn led to the enhanced pain hypersensitivity after surgery. It sheds light on the interplay between KA and AMPA receptors in DHNs, which is thought to contribute to postoperative pain.

STING-IFN-I pathway relieves incision induced acute postoperative pain via inhibiting the neuroinflammation in dorsal root ganglion of rats

BACKGROUND

The purpose of this study was to study the effect of STING-IFN-I pathway on incision induced postoperative pain in rats and its possible mechanisms.

METHODS

The pain thresholds were evaluated by measuring the mechanical withdrawal threshold and the thermal withdrawal latency. The satellite glial cell and macrophage of DRG were analyzed. The expression of STING, IFN-a, P-P65, iNOS, TNF-α, IL-1β and IL-6 in DRG was evaluated.

RESULTS

The activation of STING-IFN-I pathway can reduce the mechanical hyperalgesia, thermal hyperalgesia, down-regulate the expression of P-P65, iNOS, TNF-α, IL-1β and IL-6, and inhibit the activation of satellite glial cell and macrophage in DRG.

CONCLUSIONS

The activation of STING-IFN-I pathway can alleviate incision induced acute postoperative pain by inhibiting the activation of satellite glial cell and macrophage, which reducing the corresponding neuroinflammation in DRG.

Sestrin2 prevents neonatal incision pain and re-incision enhanced hyperalgesia in adult rats

BACKGROUND

Improving the methods for recognizing pain is important for infants admitted to the neonatal intensive care unit. Sestrin2 is a novel stress-inducible protein with a neuroprotective role that functions as a molecular mediator of hormesis. Nevertheless, the role of sestrin2 in the pain process is still unclear. The following study examined the role of sestrin2 on mechanical hypersensitivity after pups incision, as well as enhanced pain hyperalgesia after adulthood re-incision in rats.

METHODS

The experiment was divided into two parts: (1) studying the effect of sestrin2 in the neonatal incision; (2) studying the priming effect in adulthood re-incision. An animal model was established in seven-day-old rat pups with a right hind paw incision. Pups were intrathecally administrated rh-sestrin2 (exogenous sestrin2). Paw withdrawal threshold testing was performed to assay mechanical allodynia; tissue was analyzed in ex vivo using Western blot and immunofluorescence. SB203580 was further used to inhibit microglial function and evaluate the sex-dependent effect in adulthood.

RESULTS

Sestrin2 expression increased transitorily in the spinal dorsal horn in pups after incision. Administration of rh-sestrin2 improved pups' mechanical hypersensitivity by regulating the AMPK/ERK pathway and alleviated re-incision-induced enhanced hyperalgesia in male and female adult rats. After administration of SB203580 in pups, the mechanical hyperalgesia following re-incision in adult rats was prevented in males but not females; however, the protective effect of SB203580 in males was counteracted by silencing sestrin2.

CONCLUSIONS

These data suggest that sestrin2 prevents neonatal incision pain and re-incision enhanced hyperalgesia in adult rats. Moreover, microglia inhibition affects enhanced hyperalgesia only in adult males, which may be regulated through the sestrin2 mechanism. To sum up, these sestrin2 data may be a potential common molecular target for treating re-incision hyperalgesia in different sexes.

Dehydrocorydaline alleviates sleep deprivation-induced persistent postoperative pain in adolescent mice through inhibiting microglial P2Y12 receptor expression in the spinal cord

During adolescence, a second period of central nervous system (CNS) plasticity that follows the fetal period, which involves sleep deprivation (SD), becomes apparent. SD during adolescence may result in abnormal development of neural circuits, causing imbalance in neuronal excitation and inhibition, which not only results in pain, but increases the chances of developing emotion disorders in adulthood, such as anxiety and depression. The quantity of surgeries during adolescence is also consistently on the rise, yet the impact and underlying mechanism of preoperative SD on postoperative pain remain unexplored. This study demonstrates that preoperative SD induces upregulation of the P2Y12 receptor, which is exclusively expressed on spinal microglia, and phosphorylation of its downstream signaling pathway p38Mitogen-activated protein/Nuclear transcription factor-κB (p38MAPK/NF-κB)in spinal microglia, thereby promoting microglia activation and microglial transformation into the proinflammatory M1 phenotype, resulting in increased expression of proinflammatory cytokines that exacerbate persisting postoperative incisional pain in adolescent mice. Both intrathecal minocycline (a microglia activation inhibitor) and MRS2395 (a P2Y12 receptor blocker) effectively suppressed microglial activation and proinflammatory cytokine expression. Interestingly, supplementation with dehydrocorydaline (DHC), an extract of Rhizoma Corydalis, inhibited the P2Y12/p38MAPK/NF-κB signaling pathway, microglia activation, and expression of pro-inflammatory cytokines in the model mice. Taken together, the results indicate that the P2Y12 receptor and microglial activation are important factors in persistent postoperative pain caused by preoperative SD in adolescent mice and that DHC has analgesic effects by acting on these targets.

Pretreatment of the ROS Inhibitor Phenyl-N-tert-butylnitrone Alleviates Sleep Deprivation-Induced Hyperalgesia by Suppressing Microglia Activation and NLRP3 Inflammasome Activity in the Spinal Dorsal Cord

Sleep deprivation, a common perioperative period health problem, causes ocular discomfort and affects postsurgical pain. However, the mechanism of sleep deprivation-induced increased pain sensitivity is elusive. This study aims to explore the role of ROS in sleep deprivation (SD)-induced hyperalgesia and the underlying mechanism. A 48-h continuous SD was performed prior to the hind paw incision pain modeling in mice. We measured ROS levels, microglial activation, DNA damage and protein levels of iNOS, NLRP3, p-P65 and P65 in mouse spinal dorsal cord. The involvement of ROS in SD-induced prolongation of postsurgical pain was further confirmed by intrathecal injection of ROS inhibitor, phenyl-N-tert-butylnitrone (PBN). Pretreatment of 48-h SD in mice significantly prolonged postsurgical pain recovery, manifesting as lowered paw withdrawal mechanical threshold and paw withdrawal thermal latency. It caused ROS increase and upregulation of iNOS on both Day 1 and 7 in mouse spinal dorsal cord. In addition, upregulation of NLRP3 and p-P65, microglial activation and DNA damage were observed in mice pretreated with 48-h SD prior to the incision. Notably, intrathecal injection of PBN significantly reversed the harmful effects of SD on postsurgical pain recovery, hyperalgesia, microglial activation and DNA damage via the NF-κB signaling pathway. Collectively, ROS increase is responsible for SD-induced hyperalgesia through activating microglial, triggering DNA damage and enhancing NLRP3 inflammasome activity in the spinal dorsal cord.

Preoperative Acute Sleep Deprivation Causes Postoperative Pain Hypersensitivity and Abnormal Cerebral Function

Preoperative sleep loss can amplify post-operative mechanical hyperalgesia. However, the underlying mechanisms are still largely unknown. In the current study, rats were randomly allocated to a control group and an acute sleep deprivation (ASD) group which experienced 6 h ASD before surgery. Then the variations in cerebral function and activity were investigated with multi-modal techniques, such as nuclear magnetic resonance, functional magnetic resonance imaging, c-Fos immunofluorescence, and electrophysiology. The results indicated that ASD induced hyperalgesia, and the metabolic kinetics were remarkably decreased in the striatum and midbrain. The functional connectivity (FC) between the nucleus accumbens (NAc, a subregion of the ventral striatum) and the ventrolateral periaqueductal gray (vLPAG) was significantly reduced, and the c-Fos expression in the NAc and the vLPAG was suppressed. Furthermore, the electrophysiological recordings demonstrated that both the neuronal activity in the NAc and the vLPAG, and the coherence of the NAc-vLPAG were suppressed in both resting and task states. This study showed that neuronal activity in the NAc and the vLPAG were weakened and the FC between the NAc and the vLPAG was also suppressed in rats with ASD-induced hyperalgesia. This study highlights the importance of preoperative sleep management for surgical patients.

Postsurgical Latent Pain Sensitization Is Driven by Descending Serotonergic Facilitation and Masked by µ-Opioid Receptor Constitutive Activity in the Rostral Ventromedial Medulla

Following tissue injury, latent sensitization (LS) of nociceptive signaling can persist indefinitely, kept in remission by compensatory µ-opioid receptor constitutive activity (MORCA) in the dorsal horn of the spinal cord. To demonstrate LS, we conducted plantar incision in mice and then waited 3-4 weeks for hypersensitivity to resolve. At this time (remission), systemic administration of the opioid receptor antagonist/inverse agonist naltrexone reinstated mechanical and heat hypersensitivity. We first tested the hypothesis that LS extends to serotonergic neurons in the rostral ventral medulla (RVM) that convey pronociceptive input to the spinal cord. We report that in male and female mice, hypersensitivity was accompanied by increased Fos expression in serotonergic neurons of the RVM, abolished on chemogenetic inhibition of RVM 5-HT neurons, and blocked by intrathecal injection of the 5-HT3R antagonist ondansetron; the 5-HT2AR antagonist MDL-11 939 had no effect. Second, to test for MORCA, we microinjected the MOR inverse agonist d-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) and/or neutral opioid receptor antagonist 6β-naltrexol. Intra-RVM CTAP produced mechanical hypersensitivity at both hindpaws; 6β-naltrexol had no effect by itself, but blocked CTAP-induced hypersensitivity. This indicates that MORCA, rather than an opioid ligand-dependent mechanism, maintains LS in remission. We conclude that incision establishes LS in descending RVM 5-HT neurons that drives pronociceptive 5-HT3R signaling in the dorsal horn, and this LS is tonically opposed by MORCA in the RVM. The 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic postsurgical pain.SIGNIFICANCE STATEMENT Surgery leads to latent pain sensitization and a compensatory state of endogenous pain control that is maintained long after tissue healing. Here, we show that either chemogenetic inhibition of serotonergic neuron activity in the RVM or pharmacological inhibition of 5-HT3 receptor signaling at the spinal cord blocks behavioral signs of postsurgical latent sensitization. We conclude that MORCA in the RVM opposes descending serotonergic facilitation of LS and that the 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic postsurgical pain.

P2Y1 purinergic receptor inhibition attenuated remifentanil-induced postoperative hyperalgesia via decreasing NMDA receptor phosphorylation in dorsal root ganglion

BACKGROUND

Remifentanil-induced postoperative hyperalgesia is an intractable side effect of the clinical use of remifentanil, the mechanism of which remains obscure, especially in the peripheral nervous system. N-methyl-D-aspartate receptor (NMDAR) phosphorylation in dorsal root ganglion (DRG) plays a pronociceptive role in neuropathic pain. The contribution of the P2Y1 purinergic receptor (P2Y1R) in DRG to pain hypersensitivity derived from various origins and P2Y1R upregulation-induced NMDAR activation in neurons have also been uncovered. This study aimed to investigate whether P2Y1R participates in nociceptive processing in the DRG and spinal cord in remifentanil-induced postoperative hyperalgesia.

METHODS

Rats with remifentanil-induced postoperative hyperalgesia were intrathecally injected with NMDAR antagonist MK801 or P2Y1R antagonist MRS2179 at 10 min prior to remifentanil infusion. Mechanical allodynia, heat hyperalgesia, and cold hyperalgesia were measured at -24 h, 2 h, 6 h, 24 h, and 48 h following remifentanil infusion. The P2Y1R expression and NMDAR expression and phosphorylation in DRG ipsilateral to the incision were detected by Western blot and immunofluorescence.

RESULTS

Incision and remifentanil induced mechanical allodynia, heat hyperalgesia, and cold hyperalgesia accompanied by upregulated P2Y1R expression, increased NMDAR subunit NR1 expression and phosphorylation at Ser896, and NR2B expression and phosphorylation at Tyr1472 in DRG. Inhibition of NMDAR phosphorylation by MK801 effectively attenuated remifentanil-induced postoperative hyperalgesia. Furthermore, P2Y1R blockade by MRS2179 not only lessened remifentanil-evoked postoperative hypersensitivity to mechanical, heat, and cold stimuli, but also suppressed the increases in NR1 and NR2B expression and phosphorylation in DRG induced by incision and remifentanil.

CONCLUSION

The process by which P2Y1R mediates NMDAR expression and phosphorylation represents a mechanism of remifentanil-induced postoperative hyperalgesia in the DRG and/or spinal cord.

In Vivo Calcium Imaging Visualizes Incision-Induced Primary Afferent Sensitization and Its Amelioration by Capsaicin Pretreatment

Previous studies have shown that infiltration of capsaicin into the surgical site can prevent incision-induced spontaneous pain like behaviors and heat hyperalgesia. In the present study, we aimed to monitor primary sensory neuron Ca2+ activity in the intact dorsal root ganglia (DRG) using Pirt-GCaMP3 male and female mice pretreated with capsaicin or vehicle before the plantar incision. Intraplantar injection of capsaicin (0.05%) significantly attenuated spontaneous pain, mechanical, and heat hypersensitivity after plantar incision. The Ca2+ response in in vivo DRG and in in situ spinal cord was significantly enhanced in the ipsilateral side compared with contralateral side or naive control. Primary sensory nerve fiber length was significantly decreased in the incision skin area in capsaicin-pretreated animals detected by immunohistochemistry and placental alkaline phosphatase (PLAP) staining. Thus, capsaicin pretreatment attenuates incisional pain by suppressing Ca2+ response because of degeneration of primary sensory nerve fibers in the skin.SIGNIFICANCE STATEMENT Postoperative surgery pain is a major health and economic problem worldwide with ∼235 million major surgical procedures annually. Approximately 50% of these patients report uncontrolled or poorly controlled postoperative pain. However, mechanistic studies of postoperative surgery pain in primary sensory neurons have been limited to in vitro models or small numbers of neurons. Using an innovative, distinctive, and interdisciplinary in vivo populational dorsal root ganglia (DRG) imaging (>1800 neurons/DRG) approach, we revealed increased DRG neuronal Ca2+ activity from postoperative pain mouse model. This indicates widespread DRG primary sensory neuron plasticity. Increased neuronal Ca2+ activity occurs among various sizes of neurons but mostly in small-diameter and medium-diameter nociceptors. Capsaicin pretreatment as a therapeutic option significantly attenuates Ca2+ activity and postoperative pain.

The different mechanisms of peripheral and central TLR4 on chronic postsurgical pain in rats

Chronic postsurgical pain (CPSP) is a common complication after surgery; however, the underlying mechanisms of CPSP are poorly understood. As one of the most important inflammatory pathways, the Toll-like receptor 4/nuclear factor-kappa B (TLR4/NF-κB) signaling pathway plays an important role in chronic pain. However, the precise role of the TLR4/NF-κB signaling pathway in CPSP remains unclear. In the present study, we established a rat model of CPSP induced by skin/muscle incision and retraction (SMIR) and verified the effects and mechanisms of central and peripheral TLR4 and NF-κB on hyperalgesia in SMIR rats. The results showed that TLR4 expression was increased in both the spinal dorsal horn and dorsal root ganglia (DRGs) of SMIR rats. However, the TLR4 expression pattern in the spinal cord was different from that in DRGs. In the spinal cord, TLR4 was expressed in both neurons and microglia, whereas it was expressed in neurons but not in satellite glial cells in DRGs. Further results demonstrate that the central and peripheral TLR4/NF-κB signaling pathway is involved in the SMIR-induced CPSP by different mechanisms. In the peripheral nervous system, we revealed that the TLR4/NF-κB signaling pathway induced upregulation of voltage-gated sodium channel 1.7 (Nav1.7) in DRGs, triggering peripheral hyperalgesia in SMIR-induced CPSP. In the central nervous system, the TLR4/NF-κB signaling pathway participated in SMIR-induced CPSP by activating microglia in the spinal cord. Ultimately, our findings demonstrated that activation of the peripheral and central TLR4/NF-κB signaling pathway involved in the development of SMIR-induced CPSP.

Preemptive Analgesic Effect of Intrathecal Applications of Neuroactive Steroids in a Rodent Model of Post-Surgical Pain: Evidence for the Role of T-Type Calcium Channels

Preemptive management of post-incisional pain remains challenging. Here, we examined the role of preemptive use of neuroactive steroids with activity on low-voltage activated T-type Ca²⁺ channels (T-channels) and γ-aminobutyric acid A (GABA_A) receptors in the development and maintenance of post-incisional pain. We use neuroactive steroids with distinct effects on GABA_A receptors and/or T-channels: Alphaxalone (combined GABAergic agent and T-channel inhibitor), ECN (T-channel inhibitor), CDNC24 (GABAergic agent), and compared them with an established analgesic, morphine (an opioid agonist without known effect on either T-channels or GABA_A receptors). Adult female rats sustained the skin and muscle incision on the plantar surface of the right paw. We injected the agents of choice intrathecally either before or after the development of post-incisional pain. The pain development was monitored by studying mechanical hypersensitivity. Alphaxalone and ECN, but not morphine, are effective in alleviating mechanical hyperalgesia when administered preemptively whereas morphine provides dose-dependent pain relief only when administered once the pain had developed. CDNC24 on the other hand did not offer any analgesic benefit. Neuroactive steroids that inhibit T-currents—Alphaxalone and ECN—unlike morphine, are effective preemptive analgesics that may offer a promising therapeutic approach to the treatment of post-incisional pain, especially mechanical hypersensitivity.

Ketamine relieves depression-like behaviors induced by chronic postsurgical pain in rats through anti-inflammatory, anti-oxidant effects and regulating BDNF expression

RATIONALE

Clinically, chronic postsurgical pain (CPSP) is very common. Many CPSP patients may experience depression. Thus far, little is known about the mechanism of the comorbidity of CPSP and depression. Ketamine has been confirmed to possess analgesic and rapid antidepressant effects, but it is unclear whether ketamine can relieve the comorbidity of CPSP and depression.

OBJECTIVES

The present study evaluated the effects of ketamine in rats with the comorbidity of CPSP and depression.

METHODS

We induced CPSP in rats by thoracotomy and screened for rats with or without depression-like phenotype by hierarchical cluster analysis based on the results of depression-related behavioral experiments. Subsequently, rats were intraperitoneally injected with ketamine (20 mg/kg) and were evaluated by mechanical withdrawal threshold, cold hyperalgesia test, sucrose preference test, forced swimming test, and open field test. The inflammatory-related cytokines (IL-1, IL-6, TNF-α, nuclear factor-kappaB), oxidative stress parameters (superoxide dismutase, malondialdehyde, glutathione, catalase), and brain-derived neurotrophic factor (BDNF) in rat hippocampus were detected.

RESULTS

In the hippocampus of rats with the comorbidity of CPSP and depression, IL-1, IL-6, TNF-α, nuclear factor-kappaB, and malondialdehyde were significantly increased, while superoxide dismutase, glutathione, catalase, and BDNF were significantly decreased. Ketamine relieved depression but did not attenuate hyperalgesia in CPSP rats. Additionally, ketamine reduced proinflammatory cytokines, inhibited oxidative stress, and elevated BDNF levels in rat hippocampus.

CONCLUSIONS

Ketamine can rapidly relieve CPSP-induced depression in rats, which may be related to the reduction of proinflammatory cytokines, regulating oxidative stress and increasing BDNF in the hippocampus.

High Estrogen Level Modifies Postoperative Hyperalgesia via GPR30 and MMP-9 in Dorsal Root Ganglia Neurons

The cycling of sex hormones is one of the factors affecting pain in females, and the mechanisms are not fully understood. G-protein coupled estrogen receptor 30 (GPR30) is the estrogen receptor known to be involved in mechanical hyperalgesia. Studies have demonstrated that matrix metalloproteinase-9 (MMP-9) is a critical component in peripheral/central nervous system hypersensitivity and neuroinflammation, both of which participate in hyperalgesia. Here, ovariectomized rats were treated with low or high dose estrogen replacement, and then plantar incisions were made. Subsequently, mechanical allodynia was evaluated by determining the paw withdrawal mechanical threshold before and after the incision. In rats with incisions, high estrogen levels induced postoperative hyperalgesia and upregulation of GPR30 and MMP-9 in dorsal root ganglia (DRGs). MMP-9 was expressed primarily in DRG neurons co-expressing GPR30, and led to the activation of IL-1β. After intrathecal injection of the GPR30 agonist G1, female rats with low estrogen and plantar incisions continued to exhibit significant hyperalgesia until 48 h post-incision. In high estrogen level rats with plantar incisions, intrathecal injection of GPR30 antagonist G15 significantly attenuated postoperative hyperalgesia. Intraperitoneal injection of N-acetyl-cysteine, a source of cysteine that prevents the oxidation of cysteine residues on MMP-9, significantly relieved high estrogen-induced postoperative hyperalgesia via suppression of MMP-9 and IL-1β activation in DRGs. These results demonstrate that high estrogen level in rats with incisions elicit GPR30 and MMP-9 upregulation in DRGs and subsequently activate IL-1β, leading to induced postoperative hyperalgesia.

Dezocine attenuates the remifentanil-induced postoperative hyperalgesia by inhibition of phosphorylation of CaMKⅡα

Remifentanil, ultra-short-acting μ-opioid receptor agonist, has the greatest advantage in analgesia but could increase postoperative pain scores and induces postoperative hyperalgesia. Dezocine is a mixed opioid receptor partial agonist/antagonist and has been used for postoperative hyperalgesia management in clinical patients,but the potential molecular mechanism is still unclear. Ca²⁺/calmodulin-dependent protein kinase Ⅱ(CaMKⅡ) has been reported involved in remifentanil-induced hyperalgesia (RIH) in previous studies, but the relationship between CaMKⅡ and dezocine in RIH is still unclear. To investigate the mechanism of dezocine in RIH, we used a remifentanil induced postoperative hyperalgesia (RIPH) in incisional pain model of mouse. We subcutaneously infused remifentanil (40 μg/kg) to induce postoperative hyperalgesia. Dezocine (1.5 mg/kg, 3.0 mg/kg, and 6.0 mg/kg) was infused subcutaneously with remifentanil using the apparatus pump for 30 min. Paw withdrawal thermal latency (PWTL) and paw withdrawal mechanical threshold (PWMT) were used to assess thermal hyperalgesia and mechanical allodynia. Western blotting analysis and immunohistochemistry analysis were used to assess the expression of phosphorylated CaMKⅡα (p-CaMKⅡα) in somatosensory cortex, hippocampus and spinal cord. Subcutaneous infusion of remifentanil enhanced postoperative pain induced by surgical incision and increased PWTL and PWMT. Dezocine dose-dependently decreased the PWTL and PWMT in RIPH model. Correlating with behavioral effects, dezocine inhibited remifentanil-induced up-regulation of p-CaMKⅡα expression in somatosensory cortex, hippocampus and spinal cord. Dezocine could attenuate RIPH by suppressing p-CaMKⅡα.

NOD-like receptor protein 3 inflammasome drives postoperative mechanical pain in a sex-dependent manner

Postoperative pain management continues to be suboptimal because of the lack of effective nonopioid therapies and absence of understanding of sex-driven differences. Here, we asked how the NLRP3 inflammasome contributes to postoperative pain. Inflammasomes are mediators of the innate immune system that are responsible for activation and secretion of IL-1β upon stimulation by specific molecular signals. Peripheral IL-1β is known to contribute to the mechanical sensitization induced by surgical incision. However, it is not known which inflammasome mediates the IL-1β release after surgical incision. Among the 9 known inflammasomes, the NLRP3 inflammasome is ideally positioned to drive postoperative pain through IL-1β production because NLRP3 can be activated by factors that are released by incision. Here, we show that male mice that lack NLRP3 (NLRP3) recover from surgery-induced behavioral and neuronal mechanical sensitization faster and display less surgical site inflammation than mice expressing NLRP3 (wild-type). By contrast, female NLRP3 mice exhibit minimal attenuation of the postoperative mechanical hypersensitivity and no change in postoperative inflammation compared with wild-type controls. Sensory neuron-specific deletion of NLRP3 revealed that in males, NLRP3 expressed in non-neuronal cells and potentially sensory neurons drives postoperative pain. However, in females, only the NLRP3 that may be expressed in sensory neurons contributes to postoperative pain where the non-neuronal cell contribution is NLRP3 independent. This is the first evidence of a key role for NLRP3 in postoperative pain and reveals immune-mediated sex differences in postoperative pain.

Ultrasound therapy reduces persistent post-thoracotomy tactile allodynia and spinal substance P expression in rats

BACKGROUND

Therapeutic ultrasound (TU) alleviates nerve injury-associated pain, while the molecular mechanisms are less clear. This is an investigator-initiated experimental study to evaluate the mechanisms and effects of ultrasound on prolonged post-thoracotomy pain in a rodent model.

METHODS

The rats were randomly separated into four groups (n=8 per group): sham-operation (sham; group 1), thoracotomy and rib retraction (TRR; group 2), and TRR procedure followed by TU (TRR+TU-3; group 3) or TU with the ultrasound power turned off (TRR+TU-0; group 4). TU was delivered daily, beginning on postoperative day 11 (POD 11) for the next 2 weeks. Mechanical sensitivity, subcutaneous tissue temperature, and spinal substance P and interleukin-1 beta (IL-1β) were evaluated on PODs 11 and 23.

RESULTS

Group 3, which received ultrasound treatment (3 MHz; 1.0 W/cm²) for 5 min each day, demonstrated higher mechanical withdrawal thresholds when compared with the group without ultrasound intervention (group 2) or sham ultrasound (group 4). Ultrasound treatment also inhibited the upregulation of spinal substance P and IL-1β measured from spinal cord dorsal horns extract and increased subcutaneous temperature.

CONCLUSIONS

The results of this study suggest an increase in mechanical withdrawal thresholds and subcutaneous temperature, as well as a downregulation of spinal substance P and IL-1β, in the group which received ultrasound treatment. The regulation of spinal substance P and IL-1β may mediate potential effects of this non-invasive treatment.

The role of P2X7R/ERK signaling in dorsal root ganglia satellite glial cells in the development of chronic postsurgical pain induced by skin/muscle incision and retraction (SMIR)

The mechanisms of chronic postsurgical pain remain to be elucidated. We reported here that skin/muscle incision and retraction (SMIR), a rat model of postsurgical pain, phosphorylated the extracellular regulated protein kinases (ERK) signaling components c-Raf, MEK (ERK kinase) and ERK1/2 in lumbar 3 dorsal root ganglion (L3 DRG) in rats. Intrathecal injection of ERK specific inhibitor SCH772984 suppressed the mechanical allodynia induced by SMIR. Furthermore, SMIR upregulated tumor necrosis factor alpha (TNFα) in L3 DRG, which could be inhibited by SCH772984. Intrathecal injection of TNF antagonist Etanercept could also inhibit the mechanical allodynia and the increased ERK phosphorylation in L3 DRG induced by SMIR. In addition, immunofluorescent data showed that P2X7R was located exclusively in GFAP labeled satellite glial cells and was highly colocalized with p-ERK1/2 following SMIR. Pretreatment with P2X7R antagonist Brilliant Blue G (BBG) could also block the mechanical allodynia, inhibited the phosphorylation of c-Raf, MEK, ERK1/2, and decrease the expression of TNF-α. Finally, intrathecal injection of BzATP produced mechanical allodynia and induced ERK phosphorylation in satellite glial cells in L3 DRG. Thus, P2X7R activation in satellite glial cells in L3 DRG, leading to a positive feedback between ERK pathway activation and TNF-α production, is suggested to be involved in the induction of chronic postsurgical pain following SMIR.

Induction of suppressor of cytokine signaling 3 via HSF-1-HSP70-TLR4 axis attenuates neuroinflammation and ameliorates postoperative pain

Postoperative pain is a common form of acute pain that, if not managed effectively, can become chronic pain. Evidence has shown that glia, especially microglia, mediate neuroinflammation, which plays a vital role in pain sensitization. Moreover, toll-like receptor 4 (TLR4), the tumor necrosis factor receptor (TNF-R), the interleukin-1 receptor (IL-1R), and the interleukin-6 receptor (IL-6R) have been considered key components in central pain sensitization and neuroinflammation. Therefore, we hypothesized that activation of the body's endogenous "immune brakes" will inhibit these receptors and achieve inflammation tolerance as well as relieve postoperative pain. After searching for potential candidates to serve as this immune brake, we identified and focused on the suppressor of cytokine signaling 3 (SOCS3) gene. To regulate SOCS3 expression, we used paeoniflorin to induce heat shock protein 70 (HSP70)/TLR4 signaling. We found that paeoniflorin significantly induced SOCS3 expression both in vitro and in vivo and promoted the efflux of HSP70 from the cytoplasm to the extracellular environment. Furthermore, paeoniflorin markedly attenuated incision-induced mechanical allodynia, and this effect was abolished by small interfering RNAs targeting SOCS3. These findings demonstrated an effective and safe strategy to alleviate postoperative pain.

Local Application of Ultrasound Attenuates Neuropathic Allodynia and Proinflammatory Cytokines in Rats After Thoracotomy

BACKGROUND AND OBJECTIVES

We aimed to investigate the effect of therapeutic ultrasound (TU) on pain sensitivity and the concentration inflammatory cytokines in a thoracotomy rat model.

METHODS

Rats were distributed randomly into 4 groups: (1) sham operated, (2) thoracotomy and rib retraction (TRR), (3) TRR rats that received TU (TRR + TU-1), and (4) TRR rats that received TU with the ultrasound turned off (TRR + TU-0). Ultrasound was set at 1-MHz frequency (1.0-W/cm intensity and 100% duty cycle for 5 minutes), began on postoperative day (POD) 10, and then continued once per day, 5 days a week for 3 weeks.

RESULTS

The TRR and TRR + TU-0 rats encountered tactile hypersensitivity from PODs 10 to 28. Mechanical withdrawal thresholds were increased (all P < 0.05) following 5 days of TU, but thresholds remained significantly lower than baseline values. Therapeutic ultrasound increased the subcutaneous, but not body temperature. All groups receiving TRR demonstrated an increase in concentration of interleukin 1β and tumor necrosis factor α (TNF-α) on POD 14; however, the rise in TNF-α concentration was less in the TU-treated group than in the others. The decrease in concentration was greatest in the TRR + TU-1 group and similar between the TRR and TRR + TU-0 groups.

CONCLUSIONS

Mechanical allodynia was partially resolved with TU. Tissue temperature increased with ultrasound, while TU restricted the up-regulation of interleukin 1β and TNF-α around the injured intercostal nerve.

Preoperative transcranial direct current stimulation: Exploration of a novel strategy to enhance neuroplasticity before surgery to control postoperative pain. A randomized sham-controlled study

BACKGROUND

An imbalance in the excitatory/inhibitory systems in the pain networks may explain the persistent chronic pain after hallux valgus surgery. Thus, to contra-regulate this dysfunction, the use of transcranial direct current stimulation (tDCS) becomes attractive.

OBJECTIVE

We tested the hypothesis that two preoperative active(a)-tDCS sessions compared with sham(s)-tDCS could improve the postoperative pain [as indexed by Visual Analogue Scale (VAS) at rest and during walking (primary outcomes)]. To assess their effect on the change in the Numerical Pain Scale (NPS0-10) during Conditioned Pain Modulation (CPM-task), disability related to pain (DRP) and analgesic consumption (secondary outcomes). Also, we assessed if the brain derived neurotrophic factor (BDNF) in the cerebral spinal fluid (CSF) after tDCS could predict the intervention's effect on the DRP.

METHODS

It is a prospective, double blind, sham-controlled, randomized single center, 40 women (18-70 years-old) who had undergone hallux valgus surgery were randomized to receive two sessions (20 minutes each) of anodal a-tDCS or s-tDCS on the primary motor cortex at night and in the morning before the surgery. To assess the DRP was used the Brazilian Profile of Chronic Pain: Screen (B-PCP:S).

RESULTS

A-tDCS group showed lower scores on VAS at rest and during walking (P<0.001). At rest, the difference between groups was 2.13cm (95%CI = 1.59 to 2.68) while during walking was 1.67cm (95%CI = 1.05 to 2.28). A-tDCS, when compared to s-tDCS reduced analgesic doses in 73.25% (P<0.001), produced a greater reduction in B-PCP:S (mean difference of 9.41 points, 95%CI = 0.63 to 18.21) and higher function of descending pain modulatory system (DPMS) during CPM-task.

CONCLUSION

A-tDCS improves postoperative pain, the DRP and the function of DPMS. Also, the CSF BDNF after a-tDCS predicted the improvement in the DRP. In overall, these findings suggest that a-tDCS effects may be mediated by top-down regulatory mechanisms associated with the inhibitory cortical control.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02360462.

Analgesic effects and pharmacologic mechanisms of the Gelsemium alkaloid koumine on a rat model of postoperative pain

Postoperative pain (POP) of various durations is a common complication of surgical procedures. POP is caused by nerve damage and inflammatory responses that are difficult to treat. The neuroinflammation-glia-steroid network is known to be important in POP. It has been reported that the Gelsemium alkaloid koumine possesses analgesic, anti-inflammatory and neurosteroid modulating activities. This study was undertaken to test the analgesic effects of koumine against POP and explore the underlying pharmacologic mechanisms. Our results showed that microglia and astroglia were activated in the spinal dorsal horn post-incision, along with an increase of proinflammatory cytokines (interleukin 1β, interleukin 6, and tumor necrosis factor α). Both subcutaneous and intrathecal (i.t.) koumine treatment after incision significantly prevented mechanical allodynia and thermal hyperalgesia, inhibited microglial and astroglial activation, and suppressed expression of proinflammatory cytokines. Moreover, the analgesic effects of koumine were antagonized by i.t. administration of translocator protein (18 kDa) (TSPO) antagonist PK11195 and GABAA receptor antagonist bicuculline. Together, koumine prevented mechanical allodynia and thermal hyperalgesia caused by POP. The pharmacologic mechanism of koumine-mediated analgesia might involve inhibition of spinal neuroinflammation and activation of TSPO. These data suggested that koumine might be a potential pharmacotherapy for the management of POP.

Muscle Reactive Oxygen Species (ROS) Contribute to Post-Incisional Guarding via the TRPA1 Receptor

BACKGROUND

Deep tissues and their afferents have unique responses to various stimuli and respond to injury distinctively. However, the types of receptors and endogenous ligands that have a key role in pain after deep tissue incision are unknown. TRPA1 has been shown to mediate pain-related responses in inflammation- and nerve injury-induced pain models. We hypothesized that TRPA1 has an important role in pain behaviors after deep tissue incision.

METHODS

The effect of various doses of intraperitoneal (i.p.) TRPA1 antagonist, HC-030031, on pain behaviors after skin + deep tissue incision of the rat hind paw was measured. In vivo reactive oxygen species (ROS)-imaging and hydrogen peroxide (H2O2) levels after incision were also evaluated. Separate groups of rats were examined for H2O2-evoked pain-related behaviors after injections into the deep tissue or the subcutaneous tissue.

RESULTS

Guarding pain behavior after skin + deep tissue incision was decreased by i.p. HC-030031. However, HC-030031 did not affect mechanical or heat responses after incision. Treatment either before or after incision was effective against incision-induced guarding behavior. ROS increased after skin + deep tissue incision in both the incised muscle and the skin. Tissue H2O2 also increased in both skin and muscle after incision. H2O2 injection produced pain behaviors when injected into muscle but not after subcutaneous injection.

CONCLUSIONS

This study demonstrates that TRPA1 antagonist HC-030031 reduced spontaneous guarding pain behavior after skin + deep tissue incision. These data indicate that TRPA1 receptors on nociceptors are active in incised fascia and muscle but this is not evident in incised skin. Even though endogenous TRPA1 agonists like ROS and H2O2 were increased in both incised skin and muscle, those in skin do not contribute to nociceptive behaviors. This study suggests that endogenous TRPA1 ligands and the TRPA1 receptor are important targets for acute pain from deep tissue injury.

Activation of glial glutamate transporter via MAPK p38 prevents enhanced and long-lasting non-evoked resting pain after surgical incision in rats

Pain after surgery has recently become a major issue not only due to lack of treatment success in the acute phase; even more alarming is the large number of patients developing prolonged pain after surgery. Because spinal glutamate as well as spinal glia plays a major role in acute incisional pain, we investigated the role of the spinal glial glutamate transporters (GT), GLAST, GLT-1, for acute and prolonged pain and hyperalgesia caused by an incision. Spinal administration of the GT-inhibitor DL-TBOA increased non-evoked pain but not evoked pain behavior (hyperalgesia) up to 2 weeks after incision. In accordance, spinal GLAST (and to a lesser degree GLT-1) were upregulated after incision for several days. Long-term incision induced GT upregulation was prevented by long-lasting p38-inhibitor administration but not by long-lasting ERK1/2-inhibition after incision. In accordance, daily treatment with the p38-inhibitor (but not the ERK1/2 inhibitor) prolonged non-evoked but not evoked pain behavior after incision. In electrophysiological experiments, spontaneous activity of high threshold (HT) (but not wide dynamic range (WDR)) neurons known to transmit incision induced non-evoked pain was increased after prolonged treatment with the p38-inhibitor. In conclusion, our findings indicate a new spinal pathway by which non-evoked pain behavior after incision is modulated. The pathway is modality (non-evoked pain) and neuron (HT) specific and disturbance contributes to prolonged long-term pain after surgical incision. This may have therapeutic implications for the treatment of acute and - even more relevant - for prevention of chronic pain after surgery in patients.

Inhalation of Cedrus atlantica essential oil alleviates pain behavior through activation of descending pain modulation pathways in a mouse model of postoperative pain

ETHNOPHARMACOLOGICAL RELEVANCE

Cedrus atlantica essential oil (CaEO) presents analgesic and anti-inflammatory sedative properties. However, it remains unknown whether CaEO alleviates acute postoperative pain.

MATERIALS AND METHODS

Here, we investigated the effect of CaEO on postoperative pain and its mechanisms related to the descending pain control in Swiss males mice induced by a plantar incision surgery (PIS) in the hindpaw.

RESULTS

Inhalation of CaEO (5', 30' or 60') markedly reduced mechanical hypersensitivity. This effect was prevented by pre-treatment with naloxone or p-chlorophenylalanine methyl ester (PCPA, 100mg/kg, i.p.)-induced depletion of serotonin. In addition, p-alpha-methyl-para-tyrosin (AMPT, 100mg/kg, i.p.)-induced depletion of norepinephrine, intraperitoneal injection of the α2-adrenergic receptor antagonist yohimbine (0.15 mg/kg, i.p.) or haloperidol (1mg/kg, i.p.) an antagonist of dopaminergic (D1 and D2) receptors prevented the effect of CaEO on hypersensitivity.

CONCLUSIONS

These findings suggest that CaEO alleviates postoperative pain by activating the descending pain modulation pathways on the opioidergic, serotonergic, noradrenergic (α2-adrenergic) and dopaminergic (dopamine D1 and D2 receptors) systems.

A membrane-delimited N-myristoylated CRMP2 peptide aptamer inhibits CaV2.2 trafficking and reverses inflammatory and postoperative pain behaviors

Targeting proteins within the N-type voltage-gated calcium channel (CaV2.2) complex has proven to be an effective strategy for developing novel pain therapeutics. We describe a novel peptide aptamer derived from the collapsin response mediator protein 2 (CRMP2), a CaV2.2-regulatory protein. Addition of a 14-carbon myristate group to the peptide (myr-tat-CBD3) tethered it to the membrane of primary sensory neurons near surface CaV2.2. Pull-down studies demonstrated that myr-tat-CBD3 peptide interfered with the CRMP2-CaV2.2 interaction. Quantitative confocal immunofluorescence revealed a pronounced reduction of CaV2.2 trafficking after myr-tat-CBD3 treatment and increased efficiency in disrupting CRMP2-CaV2.2 colocalization compared with peptide tat-CBD3. Consequently, myr-tat-CBD3 inhibited depolarization-induced calcium influx in sensory neurons. Voltage clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after myr-tat-CBD3 exposure. Current clamp electrophysiology experiments demonstrated a reduction in excitability of small-diameter dorsal root ganglion neurons after exposure to myr-tat-CBD3. Myr-tat-CBD3 was effective in significantly attenuating carrageenan-induced thermal hypersensitivity and reversing thermal hypersensitivity induced by a surgical incision of the plantar surface of the rat hind paw, a model of postoperative pain. These effects are compared with those of tat-CBD3-the nonmyristoylated tat-conjugated CRMP2 peptide as well as scrambled versions of CBD3 and CBD3-lacking control peptides. Our results demonstrate that the myristoyl tag enhances intracellular delivery and local concentration of the CRMP2 peptide aptamer near membrane-delimited calcium channels resulting in pronounced interference with the calcium channel complex, superior suppression of calcium influx, and better antinociceptive potential.

[M1-OPIOID RECEPTOR AND CATECHOL-O-METILTRANSFERASE GENES POLYMORPHISM EFFECTS ON PERIOPERATIVE PSYCHOLOGICAL CONDITION OF THE PATIENTS AND THE EFFECTIVENESS OF POSTOPERATIVE ANALGESIA WITH OPIOIDS]

OBJECTIVE

To evaluate the influence of µl-opioid receptor (OPRM1) 118A>G and catechol-O-methyltransferase (COMT) 1947G>A gene single nucleotide polymorphisms (SNP) combinations on postoperative opioid analgesia (POA) efficacy.

MATERIAL AND METHODS

In 100 consecutive patients scheduled for major urologic surgeries the SNPs of OPRM1 118A>G and COMT 1947G>A (real-time PCR), preoperative anxiety and depression level (HADS scale), POA efficacy ("Pain Out" questionnaire) have been assessed.

RESULTS

Preoperative anxiety and depression, additional analgesia requirements, severity of nausea, drowsiness and incidence of vomiting were higher in OPRM1 118G allele carriers. Maximum pain intensity and pain-related restriction of mobility on first postoperative day were lower in homozygous carriers of COMT 1947A. The best response to POA in view of preoperative anxiety, analgesia efficacy and severity of nausea has been revealed in carriers of combination COMT-OPRM1 AA-AA.

CONCLUSIONS

SNPs of OPRM1 118A>G and COMT 1947G>A affect the preoperative patients psychological status and POA efficacy.

[Expression of connexin 43 in spinal cord dorsal horn of rats with acute incisional pain]

OBJECTIVE

To examine the effect of acute incisional pain on the expression of connexin 43 in rat spinal cord dorsal horn.

METHODS

Eighty rats were assigned into control group without any treatment and incisional pain group with incision surgery. For paw incisions, a 1-cm longitudinal incision was made through the skin and fascia of the plantar aspect of the right hind paw. After surgery, the 50% paw withdrawal threshold (PWT) was assessed in response to a tactile stimulus with calibrated von Frey monofilaments at 1, 2, 4 and 24 h, respectively. The spinal cord dorsal horn of rats was isolated at 1, 2, and 4 h after the surgery to assess the expression of connexin 43 using Western blotting and immunofluorescence assay.

RESULTS

The 50% PWT of the rats was significantly decreased after the incision surgery, and this decrement was the most obvious at 2 and 4 h. Western blotting and immunofluorescence assay showed that the expression of connexin 43 in the spinal cord dorsal horn was significantly increased in rats receiving the surgery especially at 2 and 4 h after the surgery.

CONCLUSION

Incision surgery induces an significant increase in connexin 43 expression in rat spinal cord dorsal horn, suggestting an potential role of connexin43 in postoperative incisional pain.

Role of somatostatin and somatostatin receptor type 2 in postincisional nociception in rats

Somatostatin (SST) and the somatostatin receptor type 2 (sstr2) are expressed in the superficial part (Laminae I-III) of the dorsal horn of the spinal cord. Since the neurons in these laminae also receive nociceptive sensation from the periphery, it was hypothesized that both SST and sstr2 could be involved in the modulation of nociceptive transmission. To the best of knowledge, there are no studies on the involvement of SST and sstr2 in hind paw incision model in rats, which mimics postoperative pain in humans. Sprague-Dawley rats were subjected to hind paw incision under isoflurane anaesthesia and the resulting mechanical allodynia and thermal hyperalgesia were evaluated for 5 days. In another set of animals, the spinal cord was isolated at specified time intervals after incision and examined for SST and sstr2 expression using immunohistochemistry and immunoblotting procedures. Finally, nociceptive parameters were again evaluated in incised rats, which had received SST (400 µg/kg i.p. three times per day). Blood glucose level and locomotor activity were determined after SST treatment. Both allodynia and hyperalgesia were highest immediately after incision. Spinal SST expression increased at 2 h. A further increase was noted on day 3. Expression of sstr2 increased initially but decreased at day 1. These changes could be due to exocytosis of SST and internalization of the ligand-receptor complex. SST injection significantly attenuated mechanical allodynia but not thermal hyperalgesia. Significant change in blood glucose level or locomotor activity was absent. SST appears to contribute to postincisional pain. This finding could be of clinical relevance.

[Genetic variation may influence the development of persistent postsurgical pain]

Persistent postsurgical pain is a major clinical problem. It is not fully understood why some patients develop persistent postsurgical pain while others do not. The genetic profile might play an important role in this development. In this article, we summarize the existing studies examining the effects of single nucleotide polymorphisms on the development of persistent postsurgical pain. The evidence in this area is sparse and the results are heterogeneous. Further studies are needed before firm conclusions can be made.

[Effects of epidural anesthesia on stress-induced immune supression during major corrective spine surgery]

STUDY DESIGN

A prospective, randomized study wias performed to compare two anesthetic methods.

OBJECTIVE

To evaluate the effect of epidural analgesia on postoperative pain, endocrine- metabolic and inflammatory stress response and cellular inmmune responses during major corrective spine surgetry.

METHODS

The study included 350 patients aged 15 to 65 who were randomly allocated to two equal groups. Group I (n=205) had continuous epidural analgesia (E4) and sevoflurane anesthesia during surgety and continuous epidural analgesia with ropivacaine and fentanil after surgery; Group 2 (n= 145) had general anesthesia with sevoflurane and fentanil and systemic administration of opioids after surgery. Patient pain, PONV syndrome, mobility, and satisfaction were measured after surgery along with plasma levels ofcortisol, ghmcose, interleukins IL-1β, IL-6, and IL-10 during and after surgemy C-reactive protein (CRP), and cell-surface receptor expression of immune cells (cluster of differentiation) HLA-DR+/CD3-, HLA-DR+/CD3+, HILA-DR, CD3, CD4, CD8, CD16, CD19 CD16/56+, and CD16/56+/CD3+) were measured perioperatively.

RESULTS

In group 1, there were significantly less pain, less nausea, earlier mobility, and higher satisfaction than those in group 2. Group I has also demonstrated significantly less plasma levels of glucose, cortisol, CRP, IL-lβ, IL-6, IL-10 at various stages. The ratio of CD4/CD8 (p=0.001) and B cells (p=0.01) have increased by postoperative day 3 in group 1. NK-cells (CD16/56+) have decreased significantly by day 3 after surgery (p=0.001) compared to the group 2. T-lymphocytes, (CD3) have decreased in all patients, but they were significantly lower in patients receiving opioids, compared wiith EA.

CONCLUSIONS

Polerfulr afferent stimulation in major corrective spine surgery accompanied by immunosuppression for at least a wieek after surgery. EA reduces the surgical stress response, prevents postoperative lymphocyte apoptosis and thus, increases stress and infectious resistance.

Effect of the cytochrome P450 2D6*10 genotype on the pharmacokinetics of tramadol in post-operative patients

The cytochrome P450 2D6 (CYP2D6) is the most highly polymorphic isoenzyme of the cytochrome P-450-system, which affects the metabolism of one-fourth of all prescription drugs. Tramadol, a narcotic-like pain reliever used to treat moderate to severe pain, is primarily metabolized by CYP2D6. The CYP2D6*10 allele is the most common allele in the Chinese population. Therefore, we investigated the effects of CYP2D6*10 on tramadol pharmacokinetics in 45 post-operative patients who had undergone gastrointestinal tract surgery. Tramadol was administered to the patients after the operation, and the plasma concentrations of tramadol and O-desmethyltramadol were subsequently evaluated at 12 time points. Pharmacokinetic analyses were performed using non-compartmental methods. The area under the curve (AUC), plasma clearance (CL), elimination half-life (T1/2), mean residence time (MRT), peak concentration, and peak time of tramadol and O-desmethyltramadol were calculated. CYP2D6*10 was genotyped by polymerase chain reaction-restriction fragment length polymorphism. The frequency of CYP2D6*10 alleles was 51% in the 45 patients. The patients were divided into three groups according to their CYP2D6*10 genotype: wild-type, heterozygous, and homozygous mutant. Pharmacokinetic parameters were compared among the three groups. The analyses showed that T1/2, MRT, and AUC of tramadol were larger, and CL was lower in homozygous mutant patients compared to the wild-type group (P< 0.05). These results show that the CYP2D6*10 genetic polymorphism has a significant impact on the pharmacokinetics of tramadol in Chinese post-operative patients.

[Acute pain syndrome and levels of peripheral blood saturation after various thoracic surgical approaches]

The traumatic effect of surgical approaches was determined by the intensity and duration of pain syndrome and by the degree of lung ventilation disturbances after thoracic surgery. An acute pain syndrome was considered by visual analog pain scale and a blood saturation level for the first 5 days after operation. There were 3 groups, each group consisted of 31 patients. All patients were after thoracotomies, thoracoscopies, rethoracoscopies. Maximal intensity of pain appeared to be after thoracotomies and its degree has been reducing since the first till fifth day (from 8.1 +/- 1.7 to 4.2 +/- 0.9 points). The pain syndrome was reliably less after thoracoscopy (from 5.9 +/- 1.6 to 3.5 +/- 1.4 points). Minimal pain was noted after revideothoracoscopies with the dynamics from 4.0 +/- 2.4 to 2.7 +/- 1.2 points. The rate of blood saturation was more reduced after thoracotomy for the first two days till 92.9 +/- 4.6% and the saturation level became equal on the third day in all groups. Obtained data objectively confirmed the considerably less injury in the case of endoscopic thoracic approaches in comparison with open intervention.

[Dexmedetomidine use for postoperative adrenergic analgesia and sedation in abdominal surgery]

Comparative study of postoperative analgesia and sedation with trimeperidine and dexmedetomidine and their effects on haemodynamics and vegetative nervous system was performed. Assessment of analgesia and sedation during vagotonia (first part of the study) and hypokinetic type of haemodynamics (second part of the study) was carried out with visual analogue scale (VAS) and Richmond scale. Results of the study showed that dexmedetomidine is more effective and safer than trimeperidine for analgesia and sedation in patients with spontaneous breathing after abdominal surgery. Dexmedetomidine use allows keeping optimal type of haemodynamics and vegetative nervous system parameters on first day of postoperative period.

Platelet aggregation in rhesus macaques (Macaca mulatta) in response to short-term meloxicam administration

NSAID administration is often chosen as a method of minimizing pain and discomfort for nonhuman primates. Of concern when using NSAID is the potential for decreased platelet aggregation due to the inhibition of cyclooxygenases 1 and 2. In both dogs and humans, the use of NSAID that are selective for cyclooxygenase 2, like meloxicam, minimizes the inhibition of platelet aggregation in comparison to nonselective NSAID, like aspirin, that inhibit both isoforms of cyclooxygenase. In this study, we measured platelet aggregation in rhesus macaques (n = 6) by using the impedance method on a multiple-electrode aggregometer at baseline, at 1 and 4 d after initiating treatment with aspirin or meloxicam, and after a washout period. There was no statistical difference between aggregation at baseline and after 1 or 4 d of meloxicam treatment, but platelet aggregation decreased after both 1 and 4 d of aspirin therapy. Our data suggest that clinically significant postoperative hemorrhage is unlikely in rhesus macaques briefly treated with meloxicam.

Effects of multimodal analgesia with LowDose buprenorphine and meloxicam on fecal glucocorticoid metabolites after surgery in New Zealand white rabbits (Oryctolagus cuniculus)

Despite the increasing use of rabbits as companion animals and models for biomedical research, rabbits have not been extensively studied to identify an efficacious postsurgical analgesic that does not cause systemic complications. The synergy of NSAID and systemic opioids is well-documented, and their combined use reduces the amount of either drug required for adequate analgesia. We measured fecal corticosterone metabolites (FCM) in rabbits after a minimally invasive vascular cut-down procedure. Rabbits received buprenorphine (0.03 mg/kg SC every 12 h for 3 d), meloxicam (0.2 mg/kg SC every 24 h for 3 d), buprenorphine-meloxicam (0.01 mg/kg-0.1 mg/kg SC every 24 h for 3 d), or a single dose of 0.5% bupivacaine (0.5 mL) infused locally at the incision site. By day 3 after surgery, buprenorphine, meloxicam, and bupivacaine groups showed elevated FCM levels, which continued to rise until day 7 and then gradually returned to baseline by day 28. In the buprenorphine-meloxicam group, FCM was relatively unchanged until day 3, when treatment was discontinued, and then began to rise. Rabbits in the buprenorphine-meloxicam group gained more weight over the 28-d study than did those in the other 3 treatment groups. This study shows that in rabbits low-dose buprenorphine administered with meloxicam effectively mitigates the FCM response that develops after surgery without the adverse effects associated with higher doses.

Ketorolac reduces spinal astrocytic activation and PAR1 expression associated with attenuation of pain after facet joint injury

Chronic neck pain affects up to 70% of persons, with the facet joint being the most common source. Intra-articular injection of the non-steroidal anti-inflammatory drug ketorolac reduces post-operative joint-mediated pain; however, the mechanism of its attenuation of facet-mediated pain has not been evaluated. Protease-activated receptor-1 (PAR1) has differential roles in pain maintenance depending on the type and location of painful injury. This study investigated if the timing of intra-articular ketorolac injection after painful cervical facet injury affects behavioral hypersensitivity by modulating spinal astrocyte activation and/or PAR1 expression. Rats underwent a painful joint distraction and received an injection of ketorolac either immediately or 1 day later. Separate control groups included injured rats with a vehicle injection at day 1 and sham operated rats. Forepaw mechanical allodynia was measured for 7 days, and spinal cord tissue was immunolabeled for glial fibrillary acidic protein (GFAP) and PAR1 expression in the dorsal horn on day 7. Ketorolac administered on day 1 after injury significantly reduced allodynia (p=0.0006) to sham levels, whereas injection immediately after the injury had no effect compared with vehicle. Spinal astrocytic activation followed behavioral responses and was significantly decreased (p=0.009) only for ketorolac given at day 1. Spinal PAR1 (p=0.0025) and astrocytic PAR1 (p=0.012) were significantly increased after injury. Paralleling behavioral data, astrocytic PAR1 was returned to levels in sham only when ketorolac was administered on day 1. Yet, spinal PAR1 was significantly reduced (p<0.0001) by ketorolac independent of timing. Spinal astrocyte expression of PAR1 appears to be associated with the maintenance of facet-mediated pain.

Developmental fluoxetine exposure normalizes the long-term effects of maternal stress on post-operative pain in Sprague-Dawley rat offspring

Early life events can significantly alter the development of the nociceptive circuit. In fact, clinical work has shown that maternal adversity, in the form of depression, and concomitant selective serotonin reuptake inhibitor (SSRI) treatment influence nociception in infants. The combined effects of maternal adversity and SSRI exposure on offspring nociception may be due to their effects on the developing hypothalamic-pituitary-adrenal (HPA) system. Therefore, the present study investigated long-term effects of maternal adversity and/or SSRI medication use on nociception of adult Sprague-Dawley rat offspring, taking into account involvement of the HPA system. Dams were subject to stress during gestation and were treated with fluoxetine (2×/5 mg/kg/day) prior to parturition and throughout lactation. Four groups of adult male offspring were used: 1. Control+Vehicle, 2. Control+Fluoxetine, 3. Prenatal Stress+Vehicle, 4. Prenatal Stress+Fluoxetine. Results show that post-operative pain, measured as hypersensitivity to mechanical stimuli after hind paw incision, was decreased in adult offspring subject to prenatal stress alone and increased in offspring developmentally exposed to fluoxetine alone. Moreover, post-operative pain was normalized in prenatally stressed offspring exposed to fluoxetine. This was paralleled by a decrease in corticosteroid binding globulin (CBG) levels in prenatally stressed offspring and a normalization of serum CBG levels in prenatally stressed offspring developmentally exposed to fluoxetine. Thus, developmental fluoxetine exposure normalizes the long-term effects of maternal adversity on post-operative pain in offspring and these effects may be due, in part, to the involvement of the HPA system.

Influence of CYP2D6 activity on pre-emptive analgesia by the N-methyl-D-aspartate antagonist dextromethorphan in a randomized controlled trial of acute pain

BACKGROUND

There is some evidence that dextromethorphan (DM) is effective as a pre-emptive analgesic agent.  DM is mainly metabolized to dextrorphan (DOR) by CYP2D6 whose activity can be inhibited by pharmacologic intervention.

OBJECTIVES

To investigate the efficacy of DM as a pre-emptive analgesic agent and describe the population pharmacokinetics in the presence of normal and poor CYP2D6 metabolism in acute post-operative pain.

STUDY DESIGN

Double blind, randomized, placebo-controlled trial

SETTING

Post-surgical analgesic consumption after knee ligament surgery, a setting of acute pain.

METHODS

Forty patients were randomized to a single oral dose of 50 mg quinidine or placebo, administered 12 hours before 50 mg DM. Patients were genotyped for the major CYP2D6 and ABCB1 variants and phenotyped for CYP2D6 using urine DM/DOR metabolic ratios and blood samples for population pharmacokinetic modeling.

RESULTS

Quinidine was effective in inhibiting CYP2D6 activity, with 2-fold reduction of DM to DOR biotransformation clearance, prolonged DM half-life, and increased DM systemic availability. Patients in the quinidine group required significantly less often NSAIDs than patients in the placebo group (35.3% vs. 75.0%, P = 0.022). The odds ratio for NSAID consumption in the placebo vs. quinidine group was 5.5 (95% confidence interval (CI) 1.3 - 22.7) at 48 hours after surgery.

LIMITATIONS

While this study shows an impact of DM on pre-emptive analgesia and is mechanistically interesting, the findings need to be confirmed in larger trials.

CONCLUSION

CYP2D6 inhibition by quinidine influenced the pre-emptive analgesic effectiveness of DM confirming that CYP2D6 phenotypic switch increases the neuromodulatory effect of oral dextromethorphan.

Gene expression in skin, muscle, and dorsal root ganglion after plantar incision in the rat

BACKGROUND

Treating postoperative pain remains a significant challenge for perioperative medicine. Recent studies have shown that nerve growth factor is up-regulated and contributes to incisional pain. To date, few studies have examined expression of other neurotrophin-related mediators that may contribute to the development and/or maintenance of incisional pain.

METHODS

Male Sprague-Dawley rats underwent a plantar incision, and pain behaviors were examined (n = 6). In a separate group of rats, expression of neurotrophic factors were studied. At various times after incision (n = 4) or sham surgery (n = 4), the skin, muscle, and dorsal root ganglia were harvested and total RNA isolated. Real-time reverse transcription polymerase chain reaction was performed and the fold change in gene expression was analyzed using significance analysis of microarrays.

RESULTS

Several genes were changed (P < 0.05) as early as 1 h after incision. Expression of artemin and nerve growth factor were increased in both incised skin and muscle. Brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-5 were all down-regulated in the skin but up-regulated in the muscle 48 h after incision. Few genes changed in the dorsal root ganglion. Most changes in expression occurred in the first 48 h after incision, a timeframe when pain behavior was the greatest.

CONCLUSION

Surgical incision is associated with pain-related gene expression changes in skin, muscle, and, to a lesser extent, dorsal root ganglion. The gene expression profile provides clues as to mediators that are involved in peripheral sensitization and pain transmission after surgical incision and also suggest mechanisms for resolution of postoperative pain when more persistent pain syndromes like neuropathic pain continue.

Blockade of metabotropic glutamate receptor 5 activation inhibits mechanical hypersensitivity following abdominal surgery

This study used the metabotropic glutamate 5 (mGlu5) receptor subtype-selective antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) to characterise the contribution of mGlu5 receptor activity to pain and hypersensitivity in an animal model of post-surgical pain. Adult female Wistar rats (200-250g) were anaesthetised with isoflurane (2%) and underwent a midline laparotomy with gentle manipulation of the viscera, and the effects of pre- (30min) or post- (5h) operative treatment with MPEP (1, 3 or 10mgkg(-1); i.p.) or drug-vehicle on hindpaw withdrawal latency (in seconds) to thermal stimulation (Hargreave's Test) and response threshold (in grams) to mechanical stimulation (using a dynamic plantar aesthesiometer) were measured. Animals that underwent surgery displayed significant hypersensitivity to mechanical stimulation of the hindpaws. Hypersensitivity was maximum at 6h post-surgery (44.5+/-2.4% decrease; p<0.01 vs. anaesthesia only controls) and persisted for 48h. Surgery had no effect on thermal withdrawal latency. Both pre-operative and post-operative administration of 10mgkg(-1)MPEP blocked mechanical hypersensitivity induced by surgery (p<0.01 vs. vehicle treatment). MPEP had no effect on acute nociceptive thresholds in naïve animals. These data suggest that activity at mGlu5 receptors contributes to development of pain and hypersensitivity following surgery.

[Effects of electroacupuncture at "Futu" (LI 18), etc. on expression of spinal 5-HT 1 AR mRNA, 5-HT 2 AR mRNA and protein in rats with neck incision pain]

OBJECTIVE

To observe the effect of electroacupuncture (EA) at bilateral "Futu" (LI 18), etc. on the expression of 5-HT 1 A receptor (R) mRNA, 5-HT2 AR mRNA and protein in the spinal cord of rats with neck incision pain, so as to explore its underlying mechanism in relieving incision pain.

METHODS

A total of 48 Wistar rats were randomly divided into control, model (incision pain), Futu (LI 18), Hegu (LI 4)-Neiguan (PC 6, LI 4-PC 6), Zusanli (ST 36)-Yanglingquan (GB 34) cervical cord (ST 36-GB 34 C) and lumbar cord (ST 36-GB 34 L) groups. A 1.5 cm longitudinal incision was made in the middle of the neck under Isoflurane inhalation anesthesia. Pain threshold (PT) was measured using radiant heat. EA (1-2 mA, 2 Hz/15 Hz) was applied to bilateral LI 18, LI 4-PC 6, ST 36-GB 34 for 30 min. The expression of 5-HT 1 AR mRNA, 5-HT 2 AR mRNA and protein in the cervical spinal cord (C1-C4) tissue, and 5-HT 1 AR mRNA and 5-HT 2 AR mRNA in the lumbar cord (L1- L3) were detected with Real-time PCR and Western blot analysis, respectively.

RESULTS

After the operation, the thermal PT was shortened obviously In comparison with pre-operation (P < 0.05). Compared with post-operation, the PT values were increased markedly in the LI 18 group and LI 4-PC 6 group (P < 0.05), rather than in the ST 36-GB 34 group (P > 0.05). The expression levels of 5-HT 1 AR mRNA, 5-HT 2 AR mRNA and 5-HT 2 AR protein in the cervical cord of the model group were increased significantly compared with those of the control group (P < 0.05). In comparison with the model group, the expression level of 5-HT 1 AR mRNA was down-regulated considerably in the LI 18 and LI 4-PC 6 groups (P < 0.05), and those of spinal 5-HT 2 AR mRNA and protein were up-regulated significantly in the LI 18 and LI 4-PC 6 groups (P < 0.05). No significant changes were found in the expression of 5-HT 1 AR mRNA and 5-HT 2 AR mRNA in the ST 36-GB 34 C group in comparison with the model group (P > 0.05). The expression levels of 5-HT 1 AR mRNA and 5-HT 2 AR mRNA in the ST 36-GB 34 L group were significantly lower than those of the ST 36-GB 34 C group (P < 0.05).

CONCLUSION

EA of LI 18 and LI 4-PC 6 can significantly suppress pain reaction of neck incision in the rat, which is closely associated with its effects in down-regulating the expression of 5-HT 1 AR mRNA and in up-regulating 5-HT 2 AR mRNA and 5-HT 2 AR protein in the cervical spinal cord.

Evaluation of leukemia inhibitory factor (LIF) in a rat model of postoperative pain

Postoperative pain remains a significant problem despite optimal treatment with current pharmaceutical agents. In an effort to provide better postoperative pain control, there is a need to understand the factors that contribute to the development of pain after surgery. Leukemia inhibitory factor (LIF) is a pleiotropic cytokine released from tissues after injury. We hypothesized that LIF expression in skin, muscle, and dorsal root ganglion (DRG) would increase after plantar incision. The mRNA and protein expression of LIF and LIF receptor (LIF-R) were measured after plantar incision in the rat. Pain behaviors, immunohistochemistry, and C-fiber heat responses to LIF were also studied. LIF expression increased after incision in skin and muscle, and LIF-R was present in large and small DRG neurons. LIF administration to the hindpaw increased pain behaviors, a process that was reversed by anti-LIF. However, LIF and anti-LIF treatment at the time of incision did not augment or ameliorate pain behaviors. LIF treatment activated the second messenger system, JAK-STAT3, in cultured DRG neurons, but failed to alter spontaneous activity or heat responses in C-fiber nociceptors. In conclusion, LIF is not a target for postoperative analgesia; LIF may be important for skin and muscle repair and regeneration after incision.

PERSPECTIVE

This article highlights an incision pain model for the study of factors involved in nociception. The study demonstrates that LIF in is an unlikely target for novel early postoperative analgesics.