Effect of plantar local anesthetic injection on dorsal horn neuron activity and pain behaviors caused by incision

Unveiling Targets for Treating Postoperative Pain: The Role of the TNF-α/p38 MAPK/NF-κB/Nav1.8 and Nav1.9 Pathways in the Mouse Model of Incisional Pain

Although the mouse model of incisional pain is broadly used, the mechanisms underlying plantar incision-induced nociception are not fully understood. This work investigates the role of Na_v1.8 and Na_v1.9 sodium channels in nociceptive sensitization following plantar incision in mice and the signaling pathway modulating these channels. A surgical incision was made in the plantar hind paw of male Swiss mice. Nociceptive thresholds were assessed by von Frey filaments. Gene expression of Na_v1.8, Na_v1.9, TNF-α, and COX-2 was evaluated by Real-Time PCR in dorsal root ganglia (DRG). Knockdown mice for Na_v1.8 and Na_v1.9 were produced by antisense oligodeoxynucleotides intrathecal treatments. Local levels of TNF-α and PGE₂ were immunoenzymatically determined. Incised mice exhibited hypernociception and upregulated expression of Na_v1.8 and Na_v1.9 in DRG. Antisense oligodeoxynucleotides reduced hypernociception and downregulated Na_v1.8 and Na_v1.9. TNF-α and COX-2/PGE₂ were upregulated in DRG and plantar skin. Inhibition of TNF-α and COX-2 reduced hypernociception, but only TNF-α inhibition downregulated Na_v1.8 and Na_v1.9. Antagonizing NF-κB and p38 mitogen-activated protein kinase (MAPK), but not ERK or JNK, reduced both hypernociception and hyperexpression of Na_v1.8 and Na_v1.9. This study proposes the contribution of the TNF-α/p38/NF-κB/Na_v1.8 and Na_v1.9 pathways to the pathophysiology of the mouse model of incisional pain.

Interaction Between Ropivacaine and a Self-Assembling Peptide: A Nanoformulation for Long-Acting Analgesia

Introduction

Methods

Results

Conclusion

Effect of pregabalin on nociceptive thresholds and immune responses in a mouse model of incisional pain

Background

It is known that some analgesics as well as pain can affect the immune system. The aim of this study was to investigate the analgesic effect and immunomodulation of pregabalin (PGB) in a mouse incisional pain model.

Methods

A postoperative pain model was induced by hind paw plantar incision in male BALB/c mice. Mice were randomly divided into four groups (n = 8) a saline-treated incision (incision), PGB-treated incision (PGB-incision), sham controls without incision or drug treatment (control), and a PGB-treated control (PGB-control). In the PGB treated groups, PGB was administered intraperitoneally (IP) 30 minutes before and 1 hour after the plantar incision. Changes of the mechanical nociceptive thresholds following incision were investigated. Mice were euthanized for spleen harvesting 12 hours after the plantar incision, and natural killer (NK) cytotoxicity to YAC 1 cells and lymphocyte proliferation responses to phytohemagglutinin were compared among these four groups.

Results

Mechanical nociceptive thresholds were decreased after plantar incision and IP PGB administration recovered these decreased mechanical nociceptive thresholds (P < 0.001). NK activity was increased by foot incision, but NK activity in the PGB-incision group was significantly lower than that in the Incision group (P < 0.001). Incisional pain increased splenic lymphocyte proliferation, but PGB did not alter this response.

Conclusions

Incisional pain alters cell immunity of the spleen in BALB/c mice. PGB showed antinocieptive effect on mouse incisional pain and attenuates the activation of NK cells in this painful condition. These results suggest that PGB treatment prevents increases in pain induced NK cell activity.

Challenges and opportunities in translational pain research - An opinion paper of the working group on translational pain research of the European pain federation (EFIC)

For decades, basic research on the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need. In this opinion paper bringing together pain researchers from very different disciplines, the opportunities and challenges of translational pain research are discussed. The many factors that may prevent the successful translation of bench observations into useful and effective clinical applications are reviewed, including interspecies differences, limited validity of currently available preclinical disease models of pain, and limitations of currently used methods to assess nociception and pain in non-human and human models of pain. Many paths are explored to address these issues, including the backward translation of observations made in patients and human volunteers into new disease models that are more clinically relevant, improved generalization by taking into account age and sex differences, and the integration of psychobiology into translational pain research. Finally, it is argued that preclinical and clinical stages of developing new treatments for pain can be improved by better preclinical models of pathological pain conditions alongside revised methods to assess treatment-induced effects on nociception in human and non-human animals. Significance: For decades, basic research of the underlying mechanisms of nociception has held promise to translate into efficacious treatments for patients with pain. Despite great improvement in the understanding of pain physiology and pathophysiology, translation to novel, effective treatments for acute and chronic pain has however been limited, and they remain an unmet medical need.

Does Threat Enlarge Nociceptive Reflex Receptive Fields?

Threat-induced pain modulation can increase survival by amplifying physiological and behavioral reactions toward danger. Threat can also modulate spinal nociception, suggesting engagement of endogenous top-down circuitry. A unique method to assess spinal nociception is via reflex receptive fields (RRF) associated with the nociceptive withdrawal reflex (NWR, a protective spinally-mediated reflex). The size of nociceptive RRFs can be modulated by top-down circuitry with the enlargement of RRFs related to increased spinal nociception. Threat has been previously shown to enhance pain and spinal nociception, but the relationship between threat and RRFs has not been investigated. The present study investigated this issue in 25 healthy individuals. RRFs were determined from NWRs measured by electromyography of the tibialis anterior following electrocutaneous stimulations. RRFs and pain were assessed during periods in which participants were under threat of unpredictable painful abdominal stimulations and when they were not under threat. Results indicated that threat periods led to significantly higher pain, larger nociceptive RRFs and NWR magnitudes. These findings imply that threat produces changes in protective reflexes related to spinal nociceptive sensitivity and increased pain perception. This is likely mediated by top-down circuitry that enhances dorsal horn nociceptive neurons by enlarging RRFs and amplifying ascending pain signals. PERSPECTIVE: This article presents the enlargement of RRF during periods of threat. The results from this study may help clarify the mechanism underlining emotional modulation of spinal nociception.

Emerging importance of satellite glia in nervous system function and dysfunction

Satellite glial cells (SGCs) closely envelop cell bodies of neurons in sensory, sympathetic and parasympathetic ganglia. This unique organization is not found elsewhere in the nervous system. SGCs in sensory ganglia are activated by numerous types of nerve injury and inflammation. The activation includes upregulation of glial fibrillary acidic protein, stronger gap junction-mediated SGC-SGC and neuron-SGC coupling, increased sensitivity to ATP, downregulation of Kir4.1 potassium channels and increased cytokine synthesis and release. There is evidence that these changes in SGCs contribute to chronic pain by augmenting neuronal activity and that these changes are consistent in various rodent pain models and likely also in human pain. Therefore, understanding these changes and the resulting abnormal interactions of SGCs with sensory neurons could provide a mechanistic approach that might be exploited therapeutically in alleviation and prevention of pain. We describe how SGCs are altered in rodent models of four common types of pain: systemic inflammation (sickness behaviour), post-surgical pain, diabetic neuropathic pain and post-herpetic pain.

Truncal blocks and teenager postoperative pain perception after laparoscopic surgical procedures

Introduction:

The prevalence of moderate to severe pain is high in hospitalized teenage patients admitted to surgical services.

Objectives:

The aims of this study were to determine (1) the preoperative and postoperative factors influencing teenager postoperative pain perception; and (2) suffering, defined as the patient's anxiety, pain catastrophizing thoughts, and mood.

Methods:

Data were collected from medical records and from 2 medical interviews at the time of enrollment and postoperative day 1. Stepwise linear regression was conducted to assess variables that predicted teenagers' pain scores and suffering.

Results:

Two hundred two patients (mean age = 13.8 years, SD = 1.9), 56.4% females, scheduled for laparoscopic surgical procedures completed the study. The variables found to be significant predictors of pain response in teenagers were pain on the day of surgery (6.81, 95% confidence interval [CI] = 0.08–13.55, P = 0.05) and use of regional anesthesia (single-injection rectus sheath, transversus abdominis plane, and paravertebral nerve blocks) (−6.58, 95% CI = −12.87 to −0.30, P = 0.04). The use of regional anesthesia was found to predict mood responses (all patients: 2.60, 95% CI = 0.68–4.52, P = 0.01; girls: 3.45, 95% CI = 0.96–5.93, P = 0.01; 14–17-year-old teens: 2.77, 95% CI = 0.44–5.10, P = 0.02) and to negatively predict catastrophic thoughts among all patients as a group (−4.35, 95% CI = −7.51 to −1.19, P = 0.01) and among 14- to 17-year-old teens (−5.17, 95% CI = −9.44 to −0.90, P = 0.02).

Conclusion:

A comprehensive pain approach that includes truncal blocks may improve teenagers' postoperative pain control after laparoscopic surgeries.

Comparison of the peripheral antinociceptive effect of somatostatin with bupivacaine and morphine in the rodent postoperative pain model

BACKGROUND AND OBJECTIVES

Infiltration of surgical wound with local anaesthetics attenuate postoperative pain. However, side effects can also occur. Somatostatin (SST) and its analogues like octreotide reportedly reduce peripheral sensitisation. The current study evaluates peripherally mediated antinociceptive effect of SST in a rat model of postoperative pain. This was compared with bupivacaine and morphine under identical experimental conditions.

DESIGN

Randomised vehicle-controlled blind study.

SETTING

Pain research laboratory, All India Institute of Medical Sciences, New Delhi from February 2014 to July 2017.

EXPERIMENTAL SUBJECT

Rodent hind paw incision model.

INTERVENTIONS

Sprague-Dawley rats were subjected to incision and one of the following drugs administered into the open wound once by a micropipette: SST (10, 30 or 100 μg), bupivacaine (3, 10, 30, 50 or 100 μg) or morphine (100 μg). Antinociceptive effect of SST was further evaluated for its reversibility, site of action, effect on spinal c-fos expression and blood glucose level. The site of action of morphine was also investigated.

MAIN OUTCOME MEASURE

Nociception was estimated by nonevoked (guarding behaviour) and evoked (mechanical allodynia and thermal hyperalgesia) pain behaviours between 2 h and days 4 to 7.

RESULTS

Nociception was maximum 2 h after incision. SST (10 to 100 μg) significantly attenuated guarding behaviour between 2 h and day 2. A delayed inhibitory effect was observed on allodynia. Bupivacaine (10 to 100 μg doses) similarly decreased guarding score up to day 2 though evoked pain behaviours were relatively unaffected. In contrast, morphine produced a potent but transient inhibitory effect on guarding score at 2 h, which was mediated by both peripheral and central opioid receptors. The antinociceptive effect of SST was peripherally mediated by type 2 receptors and was associated with decreased c-fos staining. Blood glucose level was unaltered.

CONCLUSION

Guarding behaviour, which likely represents pain-at-rest following surgery, was attenuated by both bupivacaine and SST to comparable extents. This novel peripherally mediated antinociceptive effect of SST needs further evaluation.

Synergistic activation of ERK1/2 between A-fiber neurons and glial cells in the DRG contributes to pain hypersensitivity after tissue injury

Background

Intense nociceptive signaling arising from ongoing injury activates primary afferent nociceptive systems to generate peripheral sensitization. ERK1/2 phosphorylation in dorsal root ganglion can be used to visualize intracellular signal activity immediately after noxious stimulation. The aim of this study was to investigate spatiotemporal characteristics of ERK1/2 phosphorylation against tissue injury in the primary afferent neurons.

Methods

Plantar incisions were made in the hind paws of Sprague-Dawley rats (n =150). Levobupivacaine was injected into the plantar aspect of the paws and ankles, Mitogen-activated protein kinase kinase (MEK) inhibitor was injected into the paw, and carbenoxolone, dual inhibitor of the gap junction and pannexin channel, was intraperitoneally injected. Pain hypersensitivity was investigated by a behavioral study, while phosphorylated ERK1/2 was detected in dorsal root ganglion and hind paw using immunohistochemistry and Western blot.

Results

Phosphorylated ERK1/2 was induced in dorsal root ganglion (26.8 ± 2.9% at baseline, 65.6 ± 3.6% at 2 min, and 26.3 ± 3.4% at 2 h) after the incision. NF-200 positive A-fiber neurons and satellite glial cells were positive for phosphorylated ERK1/2. Injury-induced pain hypersensitivity was abolished by MEK inhibitor. Levobupivacaine treatment inhibited phosphorylated ERK1/2 induction, carbenoxolone treatment inhibited glial phosphorylated ERK1/2 at 2 min after the injury, and carbenoxolone inhibited pain hypersensitivity and neuronal phosphorylated ERK1/2 at 1 h after the injury.

Conclusion

ERK1/2 phosphorylation in A-fiber neurons and satellite glial cells immediately after injury contributes to the generation of pain hypersensitivity. Signal communication between neurons and satellite glial cells expands the duration of neuronal ERK1/2 phosphorylation and pain hypersensitivity at 1 h after tissue injury.

Ropivacaine preperitoneal wound infusion for pain relief and prevention of incisional hyperalgesia after laparoscopic colorectal surgery: a randomized, triple-arm, double-blind controlled evaluation vs intravenous lidocaine infusion, the CATCH study

AIM

The abdominal incision for specimen extraction could trigger postoperative pain after laparoscopic colorectal resections (LCRs). Continuous wound infusion (CWI) of ropivacaine may be a valuable option for postoperative analgesia. This study was undertaken to evaluate the potential benefits of ropivacaine CWI on pain relief, metabolic stress reaction, prevention of wound hyperalgesia and residual incisional pain after LCR. A subgroup with intravenous lidocaine infusion (IVL) was added to discriminate between the peripheral and systemic effects of local anaesthetic infusions.

METHOD

Patients were randomly allocated to three subgroups: CWI (0.2% ropivacaine 10 ml/h for 48 h); IVL (lidocaine 1.5% at 4 ml/h for 48 h); control group.

RESULTS

In all, 95 patients were randomized (86 patients analysed). Postoperative pain intensity did not differ significantly between groups. Within the first 24 h after surgery, morphine requirement was significantly lower in the CWI group compared with the IVL group, but there was no significant difference compared with the control group (P = 0.02 and P = 0.15, respectively). The area of hyperalgesia did not differ significantly between subgroups, nor did the hyperalgesia ratio which was 1.2 cm (0.0-6.7) vs 1.9 cm (0.4-4.0) vs 2.0 cm (0.5-7.0) in the CWI, IVL and control groups respectively (P = 0.35). The number of patients reporting residual incisional pain after 3 months (3/26 vs 4/23 vs 4/23 in the CWI, IVL and control groups respectively) did not differ significantly between the groups, nor did their metabolic stress reactions.

CONCLUSION

Ropivacaine CWI at the site of the abdominal incision did not provide any significant benefit either on analgesia or on the prevention of wound hyperalgesia after LCR.

BDNF contributes to the neonatal incision-induced facilitation of spinal long-term potentiation and the exacerbation of incisional pain in adult rats

Neonatal surgical injury exacerbates spinal microglial reactivity, modifies spinal synaptic function, leading to exaggerated pain hypersensitivity after adult repeated incision. Whether and how the alteration in microglial reactivity and synaptic plasticity are functionally related remain unclear. Previously, we and others have documented that spinal brain-derived neurotrophic factor (BDNF), secreted from microglia, contributes to long-term potentiation (LTP) in adult rodents with neuropathic pain. Here, we demonstrated that the mRNA and protein expression of spinal BDNF are significantly upregulated in adult rats subjected to neonatal incision and adult repeated incision (nIN-IN). Neonatal incision facilitates spinal LTP induced by BDNF or high frequency electrical stimulation after adult incision, including a decreased induction threshold and an increased magnitude of LTP. Coincidently, inhibition of spinal BDNF abrogates the LTP facilitation, alleviates the mechanical allodynia and thermal hyperalgesia in nIN-IN rats. By contrast, spinal application of exogenous BDNF in the adult rats with a single neonatal incision mimics the LTP facilitation and pain hypersensitivity, which have been found in nIN-IN rats. Exogenous BDNF-induced exacerbation of pain hypersensitivity could be blocked by BDNF inhibitor. In addition, blockade of microglial reactivity by intrathecal application of minocycline attenuates the elevation of BDNF and the LTP facilitation, and also, alleviates pain hypersensitivity in nIN-IN rats. In conclusion, spinal BDNF, at least partly derived from microglia, contributes to the neonatal incision-induced facilitation of spinal LTP and to the exacerbation of incisional pain in adult rats. Thus, spinal BDNF may combine the changes of microglial reactivity and synaptic plasticity in nIN-IN rats.

Hydrogen Peroxide Induces Muscle Nociception via Transient Receptor Potential Ankyrin 1 Receptors

BACKGROUND

H2O2 has a variety of actions in skin wounds but has been rarely studied in deep muscle tissue. Based on response to the transient receptor potential ankyrin 1 antagonists after plantar incision, we hypothesized that H2O2 exerts nociceptive effects via the transient receptor potential ankyrin 1 in muscle.

METHODS

Nociceptive behaviors in rats (n = 269) and mice (n = 16) were evaluated after various concentrations and volumes of H2O2 were injected into the gastrocnemius muscle or subcutaneous tissue. The effects of H2O2 on in vivo spinal dorsal horn neuronal activity and lumbar dorsal root ganglia neurons in vitro were evaluated from 26 rats and 6 mice.

RESULTS

Intramuscular (mean ± SD: 1,436 ± 513 s) but not subcutaneous (40 ± 58 s) injection of H2O2 (100 mM, 0.6 ml) increased nociceptive time. Conditioned place aversion was evident after intramuscular (-143 ± 81 s) but not subcutaneous (-2 ± 111 s) injection of H2O2. These H2O2-induced behaviors were blocked by transient receptor potential ankyrin 1 antagonists. Intramuscular injection of H2O2 caused sustained in vivo activity of dorsal horn neurons, and H2O2 activated a subset of dorsal root ganglia neurons in vitro. Capsaicin nerve block decreased guarding after plantar incision and reduced nociceptive time after intramuscular H2O2. Nociceptive time after intramuscular H2O2 in transient receptor potential ankyrin 1 knockout mice was shorter (173 ± 156 s) compared with wild-type mice (931 ± 629 s).

CONCLUSIONS

The greater response of muscle tissue to H2O2 may help explain why incision that includes deep muscle but not skin incision alone produces spontaneous activity in nociceptive pathways.

[Postoperative pain therapy in Germany. Status quo]

A great deal of progress has been made in the field of postoperative pain therapy in the last 20 years. Beginning from clinical trials on the effectiveness of individual procedures, such as epidural anesthesia and patient-controlled analgesia, a wide range of healthcare services research as well as basic research with human and animal experiments has been established. Whereas health services research in the 1980s and 1990s focused more on the implementation of acute pain services, outcome-oriented research approaches are nowadays the center of attention. Acute pain registries and pain certification projects initiated in Germany have to be mentioned particularly in this respect. Basic research papers from recent years increasingly address specific aspects of acute postoperative pain and have provided translational approaches that are applied around the world for studying neurobiological mechanisms of postoperative pain. At the same time, interdisciplinary cooperation in research projects has led to a better understanding of complex correlations regarding predictors and mechanisms (including psychosocial aspects) of acute and in recent times also chronic pain after surgery. In parallel, evidence-based medicine has found its way into acute pain medicine in Germany. In 2007, clinical acute pain therapy in Germany was enhanced by S3 level guidelines for the first time; however, the implementation is still incomplete. In future, questions concerning mechanism-based therapy of acute pain need to be equally in the center of attention of research, such as prevention of persisting pain after surgery and acute pain of different origins.

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.

Conditioned place preference and spontaneous dorsal horn neuron activity in chronic constriction injury model in rats

Patients with neuropathic pain commonly present with spontaneous pain, in addition to allodynia and hyperalgesia. Although evoked responses in neuropathic pain models are well characterized, determining the presence of spontaneous pain is more challenging. We determined whether the chronic constriction injury (CCI) model could be used to measure effects of treatment of spontaneous pain, by evaluating dorsal horn neuron (DHN) spontaneous activity and spontaneous pain-related behaviors. We measured conditioned place preference (CPP) to analgesia produced by sciatic nerve block with bupivacaine in rats with established CCI. We undertook another CPP experiment using hind paw incision. We also examined DHN spontaneous activity in CCI rats. Although CCI produced nocifensive responses to mechanical stimuli, CPP to analgesic nerve block was not evident 14 days after injury: Compared with baseline (314 ± 65 seconds), CCI rats did not show a preference for the bupivacaine-paired chamber after conditioning (330 ± 102 seconds). However, sciatic nerve block after hind paw incision produced CPP on postoperative day 1, serving as a positive control. The proportion of spontaneously active DHNs (33%) was not significantly increased in CCI rats compared with the sham (21%). The median rate of spontaneous activity in the CCI group (12.6 impulses per second) was not different from the sham group (9.2 impulses per second). Also, there was no change in DHN spontaneous activity after sciatic nerve block with bupivacaine. Our findings suggest that CCI as a neuropathic pain model should not be used to measure effects of treatment of spontaneous pain driven by the peripheral input.

100-Hz Electroacupuncture but not 2-Hz Electroacupuncture is Preemptive Against Postincision Pain in Rats

Preemptive analgesia involves introducing an analgesic before noxious stimulation. Electroacupuncture (EA) activates descending mechanisms that modulate nociceptive inputs into the spinal dorsal horn. This study evaluated whether preoperative EA is more effective than postoperative EA in reducing incision pain in rats. The nociceptive threshold to mechanical stimulation was utilized to examine the effects of an intraperitoneal injection of saline (0.1 mL/kg) or naloxone (1 mg/kg) on antinociception induced by a 20-minute period of 2-Hz or 100-Hz EA applied to the Zusanli (ST36) and Sanyinjiao (SP6) acupoints before surgical incision, or 10 minutes after or 100 minutes after surgical incision of the hind paw. The extent of mechanical hyperalgesia after the incision was significantly attenuated by the application of 100-Hz EA preoperatively, but not by its application at 10 minutes or 100 minutes postoperatively. By contrast, 2-Hz EA was effective against postoperative hyperalgesia when applied 10 minutes or 100 minutes after surgery but not when it was applied preoperatively. Only the effect of 2-Hz EA applied 10 minutes after surgery was sensitive to naloxone. The present study showed for the first time that 100-Hz EA, but not 2-Hz EA, exerts a nonopioidergic preemptive effect against postincision pain in rats.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

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.

Local, Controlled Delivery of Local Anesthetics In Vivo from Polymer - Xerogel Composites

PURPOSE

Polymer-xerogel composite materials have been introduced to better optimize local anesthetics release kinetics for the pain management. In a previous study, it was shown that by adjusting various compositional and nano-structural properties of both inorganic xerogels and polymers, zero-order release kinetics over 7 days can be achieved in vitro. In this study, in vitro release properties are confirmed in vivo using a model that tests for actual functionality of the released local anesthetics.

METHODS

Composite materials made with tyrosine-polyethylene glycol(PEG)-derived poly(ether carbonate) copolymers and silica-based sol-gel (xerogel) were synthesized. The in vivo release from the composite controlled release materials was demonstrated by local anesthetics delivery in a rat incisional pain model.

RESULTS

The tactile allodynia resulting from incision was significantly attenuated in rats receiving drug-containing composites compared with the control and sham groups for the duration during which natural healing had not yet taken place. The concentration of drug (bupivacaine) in blood is dose dependent and maintained stable up to 120 h post-surgery, the longest time point measured.

CONCLUSIONS

These in vivo studies show that polymer-xerogel composite materials with controlled release properties represent a promising class of controlled release materials for pain management.

Activation of spinal phosphatidylinositol 3-kinase/protein kinase B mediates pain behavior induced by plantar incision in mice

The etiology of postoperative pain may be different from antigen-induced inflammatory pain and neuropathic pain. However, central neural plasticity plays a key role in incision pain. It is also known that phosphatidylinositol 3-kinase (PI3K) and protein kinase B/Akt (PKB/Akt) are widely expressed in laminae I-IV of the spinal horn and play a critical role in spinal central sensitization. In the present study, we explored the role of PI3K and Akt in incision pain behaviors. Plantar incision induced a time-dependent activation of spinal PI3K-p110γ and Akt, while activated Akt and PI3K-p110γ were localized in spinal neurons or microglias, but not in astrocytes. Pre-treatment with PI3K inhibitors, wortmannin or LY294002 prevented the activation of Akt brought on by plantar incision in a dose-dependent manner. In addition, inhibition of spinal PI3K signaling pathway prevented pain behaviors (dose-dependent) and spinal Fos protein expression caused by plantar incision. These data demonstrated that PI3K signaling mediated pain behaviors caused by plantar incision in mice.

Duration of action of a topical anaesthetic formulation for pain management of mulesing in sheep

OBJECTIVE

To investigate the effect of topical anaesthesia on 'mothering up' of lambs after mulesing and marking, and for pain alleviation over a 24-h period.

DESIGN

Two separate trials were performed on Merino lambs undergoing the mules procedure for flystrike prevention, to assess the efficacy of immediate postoperative topical anaesthetic wound dressing containing lignocaine (hydrochloride) 40.6 g/L, bupivacaine (hydrochloride) 4.5 g/L, adrenaline (tartrate) 24.8 mg/L and cetrimide 5.0 g/L in a gel base (Bayer Animal Health, Gordon, NSW, Australia).

METHODS

In both trials, lambs were assigned to one of three treatment regimens: control, mules procedure with topical anaesthetic (0.5 mL/kg) and mules procedure without topical anaesthetic treatment. Parameters measured included body weight, assessment of skin and wound sensitivity to light touch and pain stimulation, behavioural responses and time to mother up and to feed.

RESULTS

In both trials there was rapid (1 min) and prolonged (up to 24 h) wound analgesia as shown by lower scores for light touch (P<0.001) and pain responses (P<0.001), with absent or significantly diminished primary and secondary hyperalgesia (P≤0.05) and significant reduction in pain-related behaviours (P<0.001) in treated versus untreated lambs.

CONCLUSION

Significant pain alleviation and improved recovery can be achieved in lambs for at least 24 h after mulesing through the use of topical anaesthesia. It is suggested that the haemostatic action of adrenalin, together with inhibition of the inflammatory cascade and the barrier effect of the gel within the product, may explain the prolonged anaesthesia up to 24 h observed in the present study. These results suggest that topical anaesthesia has the capacity to dramatically improve the welfare of lambs undergoing mulesing.

Glial cell activation in the spinal cord and dorsal root ganglia induced by surgery in mice

In rodents, surgery and/or remifentanil induce postoperative pain hypersensitivity together with glial cell activation. The same stimulus also produces long-lasting adaptative changes resulting in latent pain sensitization, substantiated after naloxone administration. Glial contribution to postoperative latent sensitization is unknown. In the incisional pain model in mice, surgery was performed under sevoflurane+remifentanil anesthesia and 21 days later, 1 mg/kg of (-) or (+) naloxone was administered subcutaneously. Mechanical thresholds (Von Frey) and glial activation were repeatedly assessed from 30 min to 21 days. We used ionized calcium binding adaptor molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) to identify glial cells in the spinal cord and dorsal root ganglia by immunohistochemistry. Postoperative hypersensitivity was present up to 10 days, but the administration of (-) but not (+) naloxone at 21 days, induced again hyperalgesia. A transient microglia/macrophage and astrocyte activation was present between 30 min and 2 days postoperatively, while increased immunoreactivity in satellite glial cells lasted 21 days. At this time point, (-) naloxone, but not (+) naloxone, increased GFAP in satellite glial cells; conversely, both naloxone steroisomers similarly increased GFAP in the spinal cord. The report shows for the first time that surgery induces long-lasting morphological changes in astrocytes and satellite cells, involving opioid and toll-like receptors, that could contribute to the development of latent pain sensitization in mice.

Combination of nerve blockade and intravenous alfentanil is better than single treatment in relieving postoperative pain

BACKGROUND/PURPOSE

Multimodal analgesia can improve perioperative analgesia but knowledge of combination protocols is still incomplete. This study was designed to evaluate whether the combination of sciatic nerve blockade (SNB) and intravenous alfentanil (IVA) is more effective than either single treatment in relieving postoperative pain in rats.

METHODS

In a plantar incision model, withdrawal thresholds were evaluated by von Frey test before incision as baselines and for 7 days after incision. The animals were randomly allocated into various groups to receive SNB with 1% or 2% lidocaine, IVA of 50 or 150 μg/kg, or combined treatments (SNB 1% + 50 μg/kg IVA or SNB 2% + 150 μg/kg IVA) before incision. The results were compared with those of sham procedures--i.e., injections of peri-sciatic or intravenous saline, or a combination of both.

RESULTS

Plantar incision caused postoperative allodynia for 3 days. SNB with 2% lidocaine reduced allodynia at 1 hour, 3 hours, day 1, and day 2, but not at postoperative 5 hours or days 3-7, whereas 150 μg/kg IVA produced short analgesia for only 3 hours after surgery. Neither low-dose SNB nor low-dose IVA had a significant effect. When high-dose SNB and high-dose IVA were combined, a strong antiallodynic effect was shown in an additive manner. No synergism was evidently displayed by the combination.

CONCLUSION

Our results indicated that in an incisional pain model, multimodal analgesia is superior to single or no pretreatment; however, the combination of multimodal analgesic treatments should be individually discerned depending on nociceptive types and analgesic mechanisms.

The pathophysiology of acute pain: animal models

PURPOSE OF REVIEW

Trauma, surgery, and burns are three common clinical scenarios that are associated with significant acute pain. This review describes the pathophysiology of acute pain utilizing three preclinical models: surgery, burn, and fracture.

RECENT FINDINGS

In general, there is greater interest directed toward peripheral mediators of acute pain. Studies indicate that treatment against nerve growth factor, interleukins, and ischemic-like mediators may provide valuable avenues for treatment of acute pain. By targeting the periphery, analgesic therapies may have reduced side-effects.

SUMMARY

Peripheral mediators of acute pain can vary depending upon the type of injury. Treatment aimed toward those mediators specific to the injury may improve acute pain management in the future. It will be important to translate these findings into clinical trials in the future.

Radiotelemetric and symptomatic evaluation of pain in the rat after laparotomy: long-term benefits of perioperative ropivacaine care

Effective relief of acute and long-term postoperative pain is of utmost importance to patients undergoing surgery. Here, we worked on a controlled procedure of abdominal surgery in the rat inducing persistent postoperative pain symptoms for up to 10 days and tested the efficacy of perioperative care with the local anesthetic ropivacaine. Laparotomy was likewise used to implant radiotelemetric probes by which electrocardiogram, body temperature, and locomotor activity were recorded in freely moving animals. We showed that postoperative pain symptoms (mechanical allodynia) measured in periphery of the scar were associated over time with persistent tachycardia, elevated heart rate variability, and loss of mobility. Furthermore, a single subcutaneous infiltration of the local anesthetic ropivacaine in the periphery of the abdominal incision was sufficient to prevent the appearance of allodynia and the associated cardiac and motor signs of pain, monitored by radiotelemetry. These beneficial effects were observed when the infiltration was performed in the perioperative period, but not later. This study on freely moving animals exhibiting long-lasting postoperative pain symptoms and altered autonomic/motor function illustrates well the importance of the timing of preemptive analgesia care with long-acting local anesthetics. Moreover, it emphasizes the utility of monitoring heart rate variability to quantify spontaneous expression of long-lasting postoperative pain.

PERSPECTIVE

Speeding the recovery time after surgery using perioperative ropivacaine care is of significant clinical relevance because it might limit the risk of chronic pain and postoperative complications. In humans, chronobiological analysis of heart rate variability could also help quantify spontaneous pain expression with minimal emotional bias.

Topical anaesthesia alleviates short-term pain of castration and tail docking in lambs

OBJECTIVE

To investigate the effect of a topical anaesthetic formulation on pain alleviation, wound healing and systemic levels of local anaesthetic actives in lambs undergoing castration and tail docking.

DESIGN

Three placebo-controlled and/or randomised experiments were conducted using three groups of Merino lambs (n = 62, 68 and 19) undergoing routine castration and tail docking.

PROCEDURE

Surgical castration, with either surgical or hot-iron tail docking, was performed with and without the application of topical anaesthetic (Tri-Solfen) or placebo. The effects of this procedure were compared with those of rubber ring castration and tail docking, and of the handled but unmarked controls. Wound pain was assessed using calibrated Von-Frey monofilaments over a 4-h period, pain-related behaviour was assessed over 5 h, wound healing was assessed at 14 and 28 days, and the plasma levels of lignocaine and bupivacaine were determined.

RESULTS

Rapid and up to 4 h primary hyperalgesia developed following surgical castration and tail docking in the untreated and placebo-treated lambs. It was absent in the castration wounds, and significantly reduced in the tail-docking wounds, of the treated lambs. Hot-iron docking was associated with mild and transient secondary hyperalgesia, which was abolished by the topical anaesthesia. There was a significant reduction in pain-related behaviours in treated lambs, which were not significantly different in their behaviour to the sham-operation handled controls. Plasma lignocaine and bupivacaine levels were below the toxic thresholds in all tested lambs.

CONCLUSION

Topical anaesthesia alleviates wound pain and significantly reduces pain-related behaviours in lambs undergoing surgical castration plus surgical or hot-iron tail docking, without a negative effect on wound healing or a risk of systemic toxicity.

Guarding pain and spontaneous activity of nociceptors after skin versus skin plus deep tissue incision

BACKGROUND

Guarding pain after rat plantar incision is similar to pain at rest in postoperative patients. Spontaneous activity (SA) in nociceptive pathways quite likely transmits such ongoing pain. This study examined the extent of tissue injury by incision on pain behaviors and nociceptor SA.

METHODS

Rat pain behaviors were measured after a sham procedure, skin incision, or skin plus deep tissue incision. Separate groups of rats underwent in vivo single-fiber recording 1 day after a sham procedure, skin, or skin plus deep tissue incision or 7 days after skin plus deep tissue incision.

RESULTS

Compared with the control procedure, skin incision induced moderate guarding on the day of incision only, whereas skin plus deep tissue incision caused guarding for 5 days. Mechanical and heat hyperalgesia were similar in both incised groups, except that mechanical hyperalgesia lasted longer after skin plus deep tissue incision. On Postoperative Day 1, skin incision (18.2%) produced a similar prevalence of SA in nociceptors as in controls (13.0%), whereas skin plus deep tissue incision generated a greater prevalence of SA (61.0%); SA rate also tended to be greater (6.1 vs. 10.0 imp/s) after skin plus deep tissue incision. Seven days after skin plus deep tissue incision, the SA prevalence was similar (13.6%) as in controls.

CONCLUSIONS

These data demonstrated that incised deep tissue rather than skin had a central role in the genesis of guarding behavior and nociceptor SA. Understanding the responses of deep tissue to incision and the mechanisms for deep tissue pain will improve postoperative pain management.

Somatic paravertebral block decreases opioid requirements in children undergoing appendectomy

PURPOSE

Somatic paravertebral block (SPVB) appears to provide effective and prolonged nerve block in children; however, study of its use in this population is limited. We compared SPVB with no block in children undergoing appendectomy.

METHODS

Thirty-six children aged 3-16 yr undergoing open appendectomy were involved in this prospective randomized controlled study. Anesthesia was induced with propofol and maintained with isoflurane in N(2)O/oxygen. All subjects received fentanyl, acetaminophen and ketorolac during anesthesia. Group I (SPVB) subjects received a right SPVB at T(11), T(12), and L(1) using 0.2% ropivacaine 0.25 mL.kg(-1) with epinephrine 1:200,000 preoperatively. Group II (Control) had only bandaids applied to skin. Both groups were given morphine 0.05 mg.kg(-1) iv every 2 hr if pain scores reached 5/10 on a visual analogue scale. Acetaminophen was administered postoperatively every 6 hr to both groups. Time to first dose of morphine, total dose of morphine in 24 hr, and any adverse effects up to 24 hr after surgery were recorded.

RESULTS

Group I (SPVB) subjects required significantly less morphine than Group II (Control) patients (0.12 +/- 0.07 vs 0.34 +/- 0.15 mg.kg(-1), respectively; P < 0.001), and time to their first dose was significantly longer (7.1 +/- 4.4 vs 2.5 +/- 1.6 hr, respectively; P < 0.001). Incidence of vomiting was 11% with Group I and 27% with Group II (P = 0.21). No other adverse effects were observed in either group.

CONCLUSIONS

In children undergoing appendectomy, SPVB provides better pain relief than no block and reduces opioid requirements. Side effects were not statistically different between groups.

Ketoprofen produces modality-specific inhibition of pain behaviors in rats after plantar incision

BACKGROUND

Postoperative pain remains a significant problem despite optimal treatment with current drugs. Nonsteroidal antiinflammatory drugs reduce inflammation and provide analgesia but are associated with adverse side effects.

METHODS

We tested low doses (0.5-5 mg/kg) of parenteral ketoprofen against pain-related behaviors after plantar incision in rats. To further evaluate the potential sites of action of ketoprofen in our model, a novel, sustained-release microparticle formulation of ketoprofen was placed into the wound, and tested for its effects on pain behaviors. Intrathecal ketoprofen (150 microg) was also studied. Plasma samples were assayed for drug concentrations.

RESULTS

We found that low doses of parenterally administered ketoprofen produced a modality-specific effect on pain behaviors; guarding after incision was decreased, whereas no inhibition of exaggerated responses to heat or mechanical stimuli was evident. Very low doses, 0.5 mg/kg, could produce inhibition of guarding. The locally applied sustained-release ketoprofen-eluting microparticles and intrathecally administered ketoprofen also produced a modality-specific effect on pain behaviors after incision, inhibiting only guarding. Plasma levels of ketoprofen after parenteral or local administration were in the range of therapeutic blood levels in postoperative patients.

CONCLUSIONS

This study demonstrates that ketoprofen is an effective analgesic for nonevoked guarding in rats after plantar incision. There was no effect on mechanical or heat responses, which highlights the importance of multiple-modality testing of pain behaviors for drug evaluation. We found efficacy at doses used clinically in postoperative patients.

Hindpaw incision in early life increases the hyperalgesic response to repeat surgical injury: critical period and dependence on initial afferent activity

Pain in early life can enhance the response to subsequent injury, but effects are influenced by both the nature and timing of neonatal injury. Using plantar hindpaw incision, we investigated how postnatal age influences the response to repeat surgical injury two weeks later. The degree and time course of behavioural changes in mechanical withdrawal threshold were measured, and injury-related hyperalgesia was further quantified by flexion reflex electromyographic responses to suprathreshold mechanical stimuli 24 h following incision. Plantar hindpaw incision produces acute mechanical hyperalgesia in neonatal and adult rats, but incision in neonatal pups has an additional effect on the response to subsequent injury. With initial incision at postnatal day (P) 3 or 6, the degree of hyperalgesia following repeat incision 2 weeks later was greater than in animals having a single incision at the same age. At older ages (initial incision at P10, P21 or P40) responses did not differ in repeat and single incision groups. To test the role of primary afferent activity, levobupivacaine sciatic block was performed prior to P6 plantar incision, and controls received saline or subcutaneous levobupivacaine. Repeat peri-operative, but not a single pre-operative sciatic block, prevented the enhanced response to repeat incision two weeks later. Our results show that the first postnatal week represents a critical period when incision increases hyperalgesia following repeat surgery two weeks later, and effects are initiated by peripheral afferent activity. This has potential therapeutic implications for the type and duration of peri-operative analgesia used for neonatal surgery.

Comparison of skin incision vs. skin plus deep tissue incision on ongoing pain and spontaneous activity in dorsal horn neurons

Surgery injures both skin and deep tissue causing pain at rest and evoked pain with activities. In this study, we examined the extent of injury by incision and dorsal horn neuron (DHN) spontaneous activity (SA) in rats that underwent a sham operation, skin incision or skin plus deep tissue incision. Pain behaviors were measured 1 day later followed by DHN recordings in the same rats. On postoperative day (POD) 1, guarding pain, assessed with an abbreviated pain score, was increased in the skin plus deep tissue incision group (7.0+/-0.7 vs. 0.1+/-0.6 in control, P<0.001), but not in the skin incision group (1.8+/-1.0); yet, mechanical and heat hyperalgesia were similar in both incised groups. In the rats that underwent skin plus deep tissue incision, more DHNs expressed SA (78.1% vs. 35.7% in control, P<0.01) and SA rate also tended to be greater (13.8+/-2.9 vs. 5.6+/-2.0 imp/s). Bupivacaine infiltration into the incision decreased SA in both skin incision and skin plus deep tissue incision (POD1) groups to the same level as in the sham-operated rats. In a separate group of rats that underwent skin plus deep tissue incision, guarding pain was not present (0.1+/-0.6) on POD7 and the percentage and rate of DHN SA were the same as in the sham control. These data demonstrate that incised deep tissue rather than skin is critical for the development of guarding pain and increased SA of DHNs. Skin incision alone is sufficient for primary mechanical and heat hyperalgesia.

Postoperative analgesia induced by intrathecal neostigmine or bethanechol in rats

1. Cholinergic agonists and acetylcholinesterase inhibitors, such as neostigmine, produce a muscarinic receptor-mediated antinociception in several animal species that depends on activation of spinal cholinergic neurons. However, neostigmine causes antinociception in sheep only in the early, and not late, postoperative period. 2. In the present study, a model of postoperative pain was used to determine the antinociceptive effects of bethanechol (a muscarinic agonist) and neostigmine administered intrathecally 2, 24 or 48 h after a plantar incision in a rat hind paw. Changes in the threshold to punctate mechanical stimuli were evaluated using an automated electronic von Frey apparatus. 3. Mechanical hyperalgesia was obtained following plantar incision, the effect being stronger during the immediate (2 h) than the late post-surgical period. Bethanechol (15-90 microg/5 microL) or neostigmine (1-3 microg/5 microL) reduced incision-induced mechanical hyperalgesia, the effects of both drugs being more intense during the immediate (2 h) than the late post-surgical period. 4. The ED(50) for bethanechol injected at 2, 24 and 48 h was 5.6, 51.9 and 82.5 microg/5 microL, respectively. The corresponding ED(50) for neostigmine was 1.62, 3.02 and 3.8 microg/5 microL, respectively. 5. The decline in the antinociceptive potency of neostigmine with postoperative time is interpreted as resulting from a reduction in pain-induced activation of acetylcholine-releasing descending pathways. However, the similar behaviour of bethanechol in the same model points to an additional mechanism involving intrinsic changes in spinal muscarinic receptors.

Impact of topical anaesthesia on pain alleviation and wound healing in lambs after mulesing

OBJECTIVE

To investigate the impact of using the topical anaesthetic preparation Tri-Solfen on pain alleviation and wound healing in lambs undergoing mulesing.

DESIGN

Three separate trials, placebo controlled and/or randomised, were carried out over a 5 month period on three mobs of between 60 and 263 merino lambs undergoing routine mulesing.

PROCEDURE

Wound pain was assessed using 10 and 75 g calibrated Von-Frey monofilaments to determine sensitivity to light touch and pain stimulation over a 4 to 8 h period. Pain-related behaviour was documented by trained, blinded observers using a numerical rating scale. Wound healing rates were determined using scaled digital photography and image analysis software to calculate contraction in wound surface area 2 and 4 weeks after mulesing.

RESULTS

There was rapid (3 min) and prolonged (up to 8 h) wound analgesia as shown by pain response scores (P < or = 0.01), with absent or significantly diminished primary and secondary hyperalgesia (P < or = 0.01) and significant reduction in pain-related behaviour (P < 0.001) in treated versus untreated lambs. In addition there was improved wound healing in the treated lambs (P < or = 0.05).

CONCLUSION

Tri-Solfen effects rapid and prolonged wound analgesia, reduction in pain-related behaviour and improved wound healing in lambs undergoing routine mulesing, providing effective alleviation of pain associated with routine mulesing in sheep.

Postoperative pain--clinical implications of basic research

Postoperative incisional pain is a unique and common form of acute pain. Although ample evidence indicates that an efficeous postoperative pain treatment reduces patient morbidity and patient outcome, recent studies demonstrate that about 50-70% of patients experience moderate to severe pain after surgery indicating that postoperative pain remains poorly treated. Perhaps important reasons for this quandary are distinct mechanisms of incisional nociception compared to other pain conditions limiting our regimen to drugs designed for other clinical pain problems. Another reason might be the lack of an in depth knowledge about the pathophysiology and neuropharmacology of postoperative pain. Basic research offers important insights in the mechanisms of postsurgical incisional pain and the translation of experimental results into clinical practice will have important implications on the improvement of new multimodal treatment regimens based postoperative pain mechanisms. In the present review, recent developments in experimental postsurgical incisional pain research will be described and their possible relevance for clinical practice discussed.

Spinale Glutamatrezeptorantagonisten: Differenzierung von primärer und sekundärer mechanischer Hyperalgesie nach operativer Schnittinzision im Tierexperiment

Secondary mechanical hyperalgesia has been demonstrated in postoperative patients indicating that central sensitization occurs after surgery. However, the underlying mechanisms are unknown. Here, we studied the role of spinal AMPA/kainate receptors for pain behaviors indicating secondary hyperalgesia caused by gastrocnemius incision in the rat. These were reduced by NBQX, a selective antagonist of AMPA/kainate receptors. However, administration of NMDA receptor antagonists caused no or only a modest decrease in behaviors for secondary hyperalgesia but produced associated motor deficits and supraspinal side effects. We further determined that only secondary mechanical hyperalgesia was reversed by JSTX, a selective antagonist of calcium-permeable AMPA receptor; primary mechanical hyperalgesia and guarding behavior were unchanged. These findings indicate that JSTX influenced a spinal amplification process that leads to secondary hyperalgesia but does not contribute to primary hyperalgesia and guarding after incision. This amplification process likely requires Ca(2) influx through spinal AMPA/KA (but not NMDA) receptors. Behaviors for secondary mechanical hyperalgesia after incision can be inhibited without affecting primary mechanical hyperalgesia and guarding. Mechanisms for central sensitization causing secondary hyperalgesia in postoperative patients may therefore be separated from spontaneous pain and hyperalgesia that arises adjacent to the area of the incision.

Effects of systemic administration of lidocaine and QX-314 on hyperexcitability of spinal dorsal horn neurons after incision in the rat

Although systemic lidocaine has been shown to suppress postoperative pain in a clinical setting, the mechanisms of action of lidocaine have not been elucidated. The present study was therefore designed to determine the relative contribution of central and peripheral sites to the action of lidocaine on incision-induced hyperexcitation of spinal dorsal horn (SDH) neurons in the rat. Receptive field (RF) areas, spontaneous activities, and responses of single wide-dynamic-range (WDR) neurons of the SDH to nonnoxious and noxious stimuli were recorded before and after longitudinal incisions of 1cm through the skin, fascia, and muscle had been made in the center of their RFs of the hindquarters. Significant increases in spontaneous activities, RF sizes, and responses of WDR neurons to both nonnoxious and noxious stimuli were observed at 30 min after the incision (P<0.001). Systemic administration of lidocaine (1 mg/kg bolus plus 0.5 mg/kg/h and 2 mg/kg bolus plus 1 mg/kg/h) and QX-314 (1mg/kg bolus plus 0.5 mg/kg/h and 2 mg/kg bolus plus 1 mg/kg/h) significantly but temporarily suppressed and reversed the increases in spontaneous activity, responses to nonnoxious, and noxious stimuli and RF sizes (P<0.01). Systemic administration of the same doses of lidocaine and QX-314 did not affect responses of WDR neurons to nonnoxious or noxious stimuli or their RF sizes in sham-operated animals in which an incision had not been made. The results suggest that systemic administration of lidocaine has suppressive effects on postoperative pain mainly through peripheral sites of action.

Heat hyperalgesia after incision requires TRPV1 and is distinct from pure inflammatory pain

Postoperative pain significantly impacts patient recovery. However, postoperative pain management remains suboptimal, perhaps because treatment strategies are based mainly on studies using inflammatory pain models. We used a recently developed mouse model of incisional pain to investigate peripheral and spinal mechanisms contributing to heat hyperalgesia after incision. Behavioral experiments involving TRPV1 KO mice demonstrate that, as previously observed in inflammatory models, TRPV1 is necessary for heat (but not mechanical) hyperalgesia after incision. However, in WT mice, neither the proportion of TRPV1 immunoreactive neurons in the DRG nor the intensity of TRPV1 staining in the sciatic nerve was different from that in controls up to 4 days after incision. This result was corroborated by immunoblot analysis of sciatic nerve in rats subjected to an incision, and is distinct from that following inflammation of the rat hind paw, a situation in which TRPV1 expression levels in sciatic nerve increases. In the absence of heat exposure, spinal c-Fos staining was similar between incised TRPV1 KO and WT mice. However, differences in c-Fos staining between heat exposed TRPV1 KO and WT mice after incision suggest that the incision-mediated enhancement of heat-evoked signaling to the spinal cord involves a TRPV1-dependent mechanism. Finally, heat hyperalgesia after incision was reversed by antagonism of spinal non-NMDA receptors, unlike inflammatory hyperalgesia, which is mediated via NMDA receptors . Thus, TRPV1 is important for the generation of thermal hyperalgesia after incision. Our observations suggest that all experimental pain models may not be equally appropriate to guide the development of postoperative pain therapies.