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

A Mouse Model of Postoperative Pain

Postoperative pain is highly debilitating and hinders recovery. Opioids are the main pain medication used for acute postoperative pain. Given the devastating opioid addiction and overdose epidemic across the US, non-opioid pain therapeutics are desperately needed. In order to develop novel, non-opioid therapies for the treatment of postoperative pain and identify the mechanisms underlying this pain, rodent models of incisional pain have been established. The protocol herein describes in detail how to create a mouse model of postoperative pain that was adapted from established protocols. This model of postoperative pain is frequently-used, highly reproducible, and results in peripheral and central nervous system alterations.

Effect of Size and Location of Nevi on Postoperative Pain and Emergence Agitation in Children Undergoing Nevi Excision

Congenital melanocytic nevi need surgical excisions. However, the effect of the size and location of the nevi on pain and emergence agitation have yet to be studied. The objective of this study was to evaluate (1) the ideal parameter of the nevus size and (2) the effects of the size and location of the nevus on pain and emergence agitation. This observational study enrolled 100 children scheduled for an excision of a nevus under sevoflurane anesthesia. The parameters of the nevus size included the long diameter, the area before resection, the area of resection, and the proportion (the area of resection/total body surface). The nevus locations included the trunk, face, scalp, and extremities. The proportion of the nevi was the most ideal parameter in evaluating the pain and emergence agitation. A large size showed a higher emergence agitation than a small size (median (range); 6 (0–20) in small groups vs. 12.5 (0–20) in large groups, p = 0.021). However, the pain was comparable. The nevus location did not influence pain or emergence agitation. In a multivariate regression analysis, a younger age and an extensive excision were associated with higher pain and emergence agitation. In conclusion, large nevi induced more severe emergence agitation. However, the nevus location did not affect the outcome. In addition, a younger age was associated with pain and emergence agitation. Clinicians need to consider the proportion of nevi when managing children undergoing a nevus excision.

Behavioral and Physiological Signs for Pain Assessment in Preterm and Term Neonates During a Nociception-Specific Response: A Systematic Review

BACKGROUND/GOAL

Assessment and treatment of neonatal pain is difficult because current scales are rarely validated against brain-based evidence. We sought to systematically evaluate published evidence to extract validation of the most promising markers of neonatal pain.

METHODS

We searched four databases using germane MeSH terms. We focused on assessments of pain and/or nociception that had at least two measures among behavioral, physiological, or cortical components in preterm and/or term neonates. We evaluated studies for quality of evidence and strength of recommendations using standardized tools.

RESULTS

Fifteen articles met our inclusion criteria. Among the behavioral components uncovered in this review, the withdrawal reflex and changes in facial expression are the most strongly associated with nociception-specific brain activity. These associations may be influenced by gestational age and change over time. Physiological signs, such heart rate and oxygen saturation, have little to no association with this type of response.

CONCLUSIONS

Current assessments of neonatal pain include behavioral components that are associated with nociceptive processing, but also other less valid components, while omitting newer measures based on neuroscientific research.

Mineralocorticoid Antagonist Improves Glucocorticoid Receptor Signaling and Dexamethasone Analgesia in an Animal Model of Low Back Pain

Low back pain, a leading cause of disability, is commonly treated by epidural steroid injections that target the anti-inflammatory glucocorticoid receptor (GR). However, their efficacy has been controversial. All currently used epidural steroids also activate the pro-inflammatory mineralocorticoid receptor (MR) with significant potency. Local inflammation of the dorsal root ganglia (DRG), a rat model of low back pain, was used. This model causes static and dynamic mechanical allodynia, cold allodynia and guarding behavior (a measure of spontaneous pain), and activates the MR, with pro-nociceptive effects. In this study, effects of local Dexamethasone (DEX; a glucocorticoid used in epidural injections), and eplerenone (EPL; a second generation, more selective MR antagonist) applied to the DRG at the time of inflammation were examined. Mechanical and spontaneous pain behaviors were more effectively reduced by the combination of DEX and EPL than by either alone. The combination of steroids was particularly more effective than DEX alone or the model alone (3-fold improvement for mechanical allodynia) at later times (day 14). Immunohistochemical analysis of the GR in the DRG showed that the receptor was expressed in neurons of all size classes, and in non-neuronal cells including satellite glia. The GR immunoreactivity was downregulated by DRG inflammation (48%) starting on day 1, consistent with the reduction of GR (57%) observed by Western blot, when compared to control animals. On day 14, the combination of DEX and EPL resulted in rescue of GR immunoreactivity that was not seen with DEX alone, and was more effective in reducing a marker for satellite glia activation/neuroinflammation. The results suggest that EPL may enhance the effectiveness of clinically used epidural steroid injections, in part by enhancing the availability of the GR. Thus, the glucocorticoid-mineralocorticoid interactions may limit the effectiveness of epidural steroids through the regulation of the GR in the DRG.

Isolated nociceptors reveal multiple specializations for generating irregular ongoing activity associated with ongoing pain

Ongoing pain has been linked to ongoing activity (OA) in human C-fiber nociceptors, but rodent models of pain-related OA have concentrated on allodynia rather than ongoing pain, and on OA generated in non-nociceptive Aβ fibers rather than C-fiber nociceptors. Little is known about how ongoing pain or nociceptor OA is generated. To define neurophysiological alterations underlying nociceptor OA, we have used isolated dorsal root ganglion neurons that continue to generate OA after removal from animals displaying ongoing pain. We subclassify OA as either spontaneous activity generated solely by alterations intrinsic to the active neuron or as extrinsically driven OA. Both types of OA were implicated previously in nociceptors in vivo and after isolation following spinal cord injury, which produces chronic ongoing pain. Using novel automated algorithms to analyze irregular changes in membrane potential, we have found, in a distinctive, nonaccommodating type of probable nociceptor, induction by spinal cord injury of 3 alterations that promote OA: (1) prolonged depolarization of resting membrane potential, (2) a hyperpolarizing shift in the voltage threshold for action potential generation, and (3) an increase in the incidence of large depolarizing spontaneous fluctuations (DSFs). Can DSFs also be enhanced acutely to promote OA in neurons from uninjured animals? A low dose of serotonin failed to change resting membrane potential but lowered action potential threshold. When combined with artificial depolarization to model inflammation, serotonin also strongly potentiated DSFs and OA. These findings reveal nociceptor specializations for generating OA that may promote ongoing pain in chronic and acute conditions.

Downregulation of neuroligin1 ameliorates postoperative pain through inhibiting neuroligin1/postsynaptic density 95-mediated synaptic targeting of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor GluA1 subunits in rat dorsal horns

Neuroligin1 is an important synaptic cell adhesion molecule that modulates the function of synapses through protein-protein interactions. Yet, it remains unclear whether the regulation of synaptic transmission in the spinal cord by neruoligin1 contributes to the development of postoperative pain. In a rat model of postoperative pain induced by plantar incision, we conducted Western blot study to examine changes in the expression of postsynaptic membrane of neuroligin1, postsynaptic density 95 (PSD-95), and α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor GluA1 and GluA2 subunits in the spinal cord dorsal horn after injury. The interaction between neuroligin1 and PSD-95 was further determined by using coimmunoprecipitation. Protein levels of neuroligin1 and GluA1, but not GluA2 and PSD-95, were significantly increased in the postsynaptic membrane of the ipsilateral dorsal horn at 3 h and 1 day after incision, as compared to that in control group (naïve). A greater amount of PSD-95 was coimmunoprecipitated with neuroligin1 at 3 h after incision than that in the control group. Intrathecal administration of small interfering RNAs (siRNAs) targeting neuroligin1 suppressed the expression of neuroligin1 in the spinal cord. Importantly, pretreatment with intrathecal neuroligin1 siRNA₂₄₉₇, but not scrambled siRNA or vehicle, prevented the upregulation of GluA1 expression at 3 h after incision, inhibited the enhanced neuroligin1/PSD-95 interaction, and attenuated postoperative pain. Together, current findings suggest that downregulation of spinal neuroligin1 expression may ameliorate postoperative pain through inhibiting neuroligin1/PSD-95 interaction and synaptic targeting of GluA1 subunit. Accordingly, spinal neuroligin1 may be a potential new target for postoperative pain treatment.

Deep Tissue Incision Enhances Spinal Dorsal Horn Neuron Activity During Static Isometric Muscle Contraction in Rats

Translational correlates to pain with activities after deep tissue injury have been rarely studied. We hypothesized that deep tissue incision causes greater activation of nociception-transmitting neurons evoked by muscle contraction. In vivo neuronal activity was recorded in 203 dorsal horn neurons (DHNs) from 97 rats after sham, skin-only, or skin + deep muscle incision. We evaluated DHN responses to static, isometric muscle contractions induced by direct electrical stimulation of the muscle. The effect of pancuronium on DHN response to contractions was also examined. Approximately 50% of DHNs with receptive fields in the hindpaw were excited during muscle contraction. One-second .5- and 1.0-g muscle contractions produced greater DHN activity after skin + deep muscle incision (median [interquartile range], 32 [5-39] impulses, P = .021; and 36 [26-46] impulses, P = .006, respectively) than after sham (6 [0-21] and 15 [8-32] impulses, respectively). Neuromuscular blockade with pancuronium inhibited the muscle contractions and DHN activation during electrical stimulation, demonstrating contraction-induced activation. The greater response of spinal DHNs to static muscle contraction after skin + deep muscle incision may model and inform mechanisms of dynamic pain after surgery. PERSPECTIVE: Completion of various activities is an important milestone for recovery and hospital discharge after surgery. Skin + deep muscle incision caused greater activation of nociception-transmitting DHNs evoked by muscle contraction compared with skin-only incision. This result suggests an important contribution of deep muscle injury to activity-evoked hyperalgesia after surgery.

The Length of Surgical Skin Incision in Postoperative Inflammatory Reaction

BACKGROUND AND OBJECTIVES

Surgery provokes inflammatory and immune responses, so efforts have been made to reduce host response by using less invasive techniques. The purpose of this experimental study was to investigate the surgical stress induced by skin incision and the role of liver response in this process.

METHODS

Seventy male anesthetized Wistar rats were subjected to a midline incision confined strictly to the skin (dermis) of either 1 cm long (n = 20), 10 cm long (n = 20), or no incision (n = 20) or served as controls (n = 10). Skin trauma was left open for a 20-minutes period, and then was meticulously sutured. At 3 and 24 hours later, laparotomy was performed on half the rats of each group, for blood and liver sampling. In serum and liver homogenates, cytokine-induced neutrophil chemoattractant (CINC)1/interleukin (IL)-8 and tumor necrosis factor (TNF)-α levels were measured with enzyme-linked immunosorbent assays and nitric oxide (NO) using a Griess reaction.

RESULTS

Skin trauma was found to significantly (P < .01) increase all inflammatory mediators tested (CINC1/IL-8, TNF-α, NO) in serum of operated rats versus controls, the increase being proportionally dependent on the length of skin incision. In liver homogenates, CINC1/IL-8 was significantly (P < .01) increased in operated animals versus controls, similarly to serum levels. In contrast, liver TNF-α levels were inversely related to serum levels, and a significant (P < .01) decrease in TNF-α was observed in liver homogenates of operated animals compared with the controls, indicating that the increased TNF-α in blood reflects liver TNF-α secretion.

CONCLUSION

Our findings suggest that inflammatory and immune reactions induced by skin-only surgical trauma are closely correlated to the length of skin incision.

Role of Trpv1 and Trpv4 in surgical incision-induced tissue swelling and Fos-like immunoreactivity in the central nervous system of mice

Pain management remains a major concern regarding the treatment of postoperative patients. Transient receptor potential (TRP) channels are considered to be new therapeutic targets for pain control. We investigated whether the genes Trpv1 and Trpv4 are involved in hind paw swelling caused after surgical incision in mice or in incision-induced Fos-like immunoreactivity (Fos-LI) levels in the central nervous system. Mice were divided into four groups: wild-type (WT) control, WT incision, Trpv1 knockout (Trpv1^-/-) incision, and Trpv4 knockout (Trpv4^-/-) incision. Mice were anesthetized, and only those in the incision, and not control, groups received a surgical incision to their right plantar hind paw. Changes in paw diameter and in Fos-LI levels in the dorsal horn of the spinal cord, paraventricular nucleus of the hypothalamus (PVN), paraventricular nucleus of the thalamus, and central amygdala were evaluated 2 h after the incision. There was no significant difference in the paw diameter among groups. In contrast, in laminae I-II of the dorsal horn of the spinal cord and PVN, Fos-LI was significantly higher in all incision groups than in the WT control group. A significant increase in Fos-positive cells was also observed in the dorsal horn laminae III-IV in Trpv1^-/- and Trpv4^-/- incision groups compared with the WT incision group. Our results indicate that surgical incision activates the PVN and that Trpv1 and Trpv4 might be involved in neuronal activity in the dorsal horn laminae III-IV after surgical incision.

Increased Neuroligin 2 Levels in the Postsynaptic Membrane in Spinal Dorsal Horn may Contribute to Postoperative Pain

Neuroligin 2 is a synaptic cell adhesion molecule that is mainly located in inhibitory synapses and is crucial in the regulation of synapse function through protein-protein interactions. However, researchers have not clearly determined whether neuroligin 2 is involved in the development of postoperative pain. In the current study, Western blot, immunofluorescence staining and co-immunoprecipitation were used to examine the critical role of neuroligin 2 in postoperative pain hypersensitivity. A small interfering ribonucleic acid (siRNA)-targeting neuroligin 2 was used to inhibit neuroligin 2 expression. Our data found that plantar incision induced postoperative pain hypersensitivity, which was characterized by paw withdrawal threshold and cumulative pain score. The upregulation of neuroligin 2 and GluR1 expression in the postsynaptic membranes of ipsilateral spinal dorsal horn was observed at 3 h and 1 day after plantar incision. Additionally, at 3 h after plantar incision, the amount of PSD-95 that was co-immunoprecipitated with neuroligin 2 antibody was significantly increased in the ipsilateral dorsal horn, as compared to that of the control group. Intrathecal pretreatment of siRNA-targeting neuroligin 2 to reduce the neuroligin 2 expression in the spinal cord significantly inhibited the pain hypersensitivity and reduced the synaptic targeting of GluR1 in ipsilateral dorsal horns. Our study indicates that the incision-induced interaction between neuroligin 2 and PSD-95 and subsequent synaptic targeting of GluR1 in ipsilateral dorsal horns contribute to postoperative pain hypersensitivity.

Botulinum toxin type A and gabapentin attenuate postoperative pain and NK1 receptor internalization in rats

Treatment of postoperative pain remains a challenge in clinic. Botulinum toxin type A (BoNT/A) and gabapentin regulate the release of neurotransmitters from primary afferent neurons, but their effects of on postoperative pain are not clear. In the current study, using pain behavioral tests, Western blot analysis, and immunocytochemistry, we examined whether BoNT/A, alone or in combination with intrathecal gabapentin, inhibited pain hypersensitivity and attenuated the increase in neurokinin 1 (NK1) receptor internalization in dorsal horn neurons after plantar incision. Our data showed that pretreatment of rats with an intraplantar (2 U) 24 h before plantar incision or intrathecal (0.5 U) injection of BoNT/A 48 h before plantar incision induced a prolonged (3-5 days) decrease in pain scores and mechanical hypersensitivity, as compared to those observed with saline pretreatment. Both intraplantar and intrathecal BoNT/A pretreatment reduced synaptosomal-associated protein 25 levels in the ipsilateral lumbar dorsal root ganglia and spinal cord dorsal horn, and attenuated the increase in NK1 receptor internalization in dorsal horn neurons. Intrathecal administration of a sub-effective dose of gabapentin (50 μg) with BoNT/A (0.5 U) induced greater inhibition of pain hypersensitivity and NK1 receptor internalization than BoNT/A alone. These findings suggest that pretreatment with BoNT/A, alone or in combination with intrathecal gabapentin, may present a promising multimodal analgesia regimen for postoperative pain treatment.

Transcriptional Changes in Dorsal Spinal Cord Persist after Surgical Incision Despite Preemptive Analgesia with Peripheral Resiniferatoxin

BACKGROUND

Peripheral nociceptors expressing the ion channel transient receptor potential cation channel, subfamily V, member 1, play an important role in mediating postoperative pain. Signaling from these nociceptors in the peri- and postoperative period can lead to plastic changes in the spinal cord and, when controlled, can yield analgesia. The transcriptomic changes in the dorsal spinal cord after surgery, and potential coupling to transient receptor potential cation channel, subfamily V, member 1-positive nociceptor signaling, remain poorly studied.

METHODS

Resiniferatoxin was injected subcutaneously into rat hind paw several minutes before surgical incision to inactivate transient receptor potential cation channel, subfamily V, member 1-positive nerve terminals. The effects of resiniferatoxin on postincisional measures of pain were assessed through postoperative day 10 (n = 51). Transcriptomic changes in the dorsal spinal cord, with and without peripheral transient receptor potential cation channel, subfamily V, member 1-positive nerve terminal inactivation, were assessed by RNA sequencing (n = 22).

RESULTS

Peripherally administered resiniferatoxin increased thermal withdrawal latency by at least twofold through postoperative day 4, increased mechanical withdrawal threshold by at least sevenfold through postoperative day 2, and decreased guarding score by 90% relative to vehicle control (P < 0.05). Surgical incision induced 70 genes in the dorsal horn, and these changes were specific to the ipsilateral dorsal horn. Gene induction with surgical incision persisted despite robust analgesia from resiniferatoxin pretreatment. Many of the genes induced were related to microglial activation, such as Cd11b and Iba1.

CONCLUSIONS

A single subcutaneous injection of resiniferatoxin before incision attenuated both evoked and nonevoked measures of postoperative pain. Surgical incision induced transcriptomic changes in the dorsal horn that persisted despite analgesia with resiniferatoxin, suggesting that postsurgical pain signals can be blocked without preventing transcription changes in the dorsal horn.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BACKGROUND

Chronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.

METHODS

Response properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.

RESULTS

Females have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.

CONCLUSIONS

Here, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

Peripheral Mechanisms of Ischemic Myalgia

Musculoskeletal pain due to ischemia is present in a variety of clinical conditions including peripheral vascular disease (PVD), sickle cell disease (SCD), complex regional pain syndrome (CRPS), and even fibromyalgia (FM). The clinical features associated with deep tissue ischemia are unique because although the subjective description of pain is common to other forms of myalgia, patients with ischemic muscle pain often respond poorly to conventional analgesic therapies. Moreover, these patients also display increased cardiovascular responses to muscle contraction, which often leads to exercise intolerance or exacerbation of underlying cardiovascular conditions. This suggests that the mechanisms of myalgia development and the role of altered cardiovascular function under conditions of ischemia may be distinct compared to other injuries/diseases of the muscles. It is widely accepted that group III and IV muscle afferents play an important role in the development of pain due to ischemia. These same muscle afferents also form the sensory component of the exercise pressor reflex (EPR), which is the increase in heart rate and blood pressure (BP) experienced after muscle contraction. Studies suggest that afferent sensitization after ischemia depends on interactions between purinergic (P2X and P2Y) receptors, transient receptor potential (TRP) channels, and acid sensing ion channels (ASICs) in individual populations of peripheral sensory neurons. Specific alterations in primary afferent function through these receptor mechanisms correlate with increased pain related behaviors and altered EPRs. Recent evidence suggests that factors within the muscles during ischemic conditions including upregulation of growth factors and cytokines, and microvascular changes may be linked to the overexpression of these different receptor molecules in the dorsal root ganglia (DRG) that in turn modulate pain and sympathetic reflexes. In this review article, we will discuss the peripheral mechanisms involved in the development of ischemic myalgia and the role that primary sensory neurons play in EPR modulation.

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.

High-fat diet increases pain behaviors in rats with or without obesity

Obesity is associated with increased risk for chronic pain. Basic mechanisms for this association are poorly understood. Using a milder version of a radicular pain model, local inflammation of the dorsal root ganglion (DRG), we observed marked increases in mechanical and cold allodynia in rats of both sexes that were maintained on a high-fat diet (HFD) for 6 weeks prior to DRG inflammation. Notably, this increase in pain-related behaviors occurred in both Long-Evans and Sprague-Dawley rats despite the fact that the 6-week HFD exposure induced obesity (e.g., increased insulin, leptin, weight, and percent body fat) in the Long-Evans, but not Sprague-Dawley, strains. This suggested that HFD, rather than obesity per se, increased pain behaviors. Increased pain behaviors were observed even after a much shorter (1 week) exposure to the HFD but the effect was smaller. HFD also increased behavioral responses and paw swelling to paw injection of complete Freund’s adjuvant, a model of peripheral inflammatory pain. No change was detected in plasma cytokine levels in HFD rats. However, increased macrophage infiltration of the DRG was observed in response to the HFD, absent any pain model. The results suggest that HFD can increase pain even when it does not cause obesity.

Effects of scalp nerve block on pain and emergence agitation after paediatric nevus surgery: a clinical trial

BACKGROUND

Pain is considered as being one cause of post-operative emergence agitation (EA) from sevoflurane anaesthesia. The purpose of this study was to investigate the pure effect of post-operative pain on EA after sevoflurane anaesthesia in preschool children undergoing excision of scalp nevi.

METHODS

Forty-four children, 1-7 years old, undergoing scalp nevus excision were enrolled. Patients were randomly assigned to two groups: the remifentanil group received single intravenous injection of short-acting synthetic opioid, remifentanil 1 μg/kg just before the scalp incision, and the block group received scalp nerve block with 0.25% ropivacaine after intubation. The end-tidal sevoflurane concentration was maintained around 1.5 vol% unless the mean arterial pressure is out of ±20% range of preoperative values during surgery in both groups. Watcha behaviour scale for EA and face, legs, activity, cry, consolability (FLACC) scale scores for pain were recorded post-operatively.

RESULTS

There was no difference in end-tidal sevoflurane concentration between the two groups during surgery and the emergence period. Agitation incidence and scores were not different between the two groups during the recovery period. FLACC scale was significantly lower in the block group than in the remifentanil group at post-anaesthesia care unit (PACU) arrival, at 10 and 20 min after PACU arrival, respectively.

CONCLUSION

The scalp nerve block decreased the early post-operative pain after paediatric nevus excision, but it did not decrease the incidence of EA with sevoflurane anaesthesia.

Muscle IL1β Drives Ischemic Myalgia via ASIC3-Mediated Sensory Neuron Sensitization

Musculoskeletal pain is a significantly common clinical complaint. Although it is known that muscles are quite sensitive to alterations in blood flow/oxygenation and a number of muscle pain disorders are based in problems of peripheral perfusion, the mechanisms by which ischemic-like conditions generate myalgia remain unclear. We found, using a multidisciplinary experimental approach, that ischemia and reperfusion injury (I/R) in male Swiss Webster mice altered ongoing and evoked pain-related behaviors in addition to activity levels through enhanced muscle interleukin-1 beta (IL1β)/IL1 receptor signaling to group III/IV muscle afferents. Peripheral sensitization depended on acid-sensing ion channels (ASICs) because treatment of sensory afferents in vitro with IL1β-upregulated ASIC3 in single cells, and nerve-specific knock-down of ASIC3 recapitulated the results of inhibiting the enhanced IL1β/IL1r1 signaling after I/R, which was also found to regulate afferent sensitization and pain-related behaviors. This suggests that targeting muscle IL1β signaling may be a potential analgesic therapy for ischemic myalgia.

SIGNIFICANCE STATEMENT

Here, we have described a novel pathway whereby increased inflammation within the muscle tissue during ischemia/reperfusion injury sensitizes group III and IV muscle afferents via upregulation of acid-sensing ion channel 3 (ASIC3), leading not only to alterations in mechanical and chemical responsiveness in individual afferents, but also to pain-related behavioral changes. Furthermore, these I/R-induced changes can be prevented using an afferent-specific siRNA knock-down strategy targeting either ASIC3 or the upstream mediator of its expression, interleukin 1 receptor 1. Therefore, this knowledge may contribute to the development of alternative therapeutics for muscle pain and may be especially relevant to pain caused by issues of peripheral circulation, which is commonly observed in disorders such as complex regional pain syndrome, sickle cell anemia, or fibromyalgia.

Impact of different analgesic depths and abdominal trauma of different severities on stress and recovery of rats undergoing total intravenous anesthesia

A number of animal models have been developed to examine the pathophysiological consequences of surgical procedures, but anesthetic methods, monitoring, and management measures in these models are very different from those used in humans. This study was designed to create a rat model of abdominal surgery using anesthetic methods and perioperative treatment similar to those used in the clinic and to investigate the effects of different injury severities and depths of anesthesia and analgesia on surgical stress and postoperative recovery. Abdominal skin/muscle incision was compared with exploratory laparotomy in rats under propofol intravenous anesthesia, accompanied by perioperative measures such as oxygen inhalation, fluid infusion, warmth, blood gas analysis, and infection prevention. Stress indices (mean arterial pressure, heart rate, blood glucose, and plasma corticosterone) were monitored during anesthesia and surgery, and recovery indicators (body weight, food consumption, and pain) were measured after surgery. In addition, animals undergoing laparotomy were subjected to low and high dosages of propofol and sufentanil, in order to examine the relationship between anesthetic and analgesic depth and stress on recovery. Exploratory laparotomy induced a greater stress response and caused slower postoperative recovery as measured than somatic injury. High-dose sufentanil downregulated plasma corticosterone and improved postoperative recovery more effectively than high-dose propofol (P<0.05). Taken together, a rat model of abdominal surgery using anesthetic methods and perioperative treatment similar to those used in the clinic was successfully developed. It showed a positive correlation between severity of surgical trauma and stress response and postoperative recovery and a significant role of adequate analgesia in reducing surgical stress and improving postoperative recovery.

Comparative antinociceptive effect of arachidonylcyclopropylamide, a cannabinoid 1 receptor agonist & lignocaine, a local anaesthetic agent, following direct intrawound administration in rats

Background & objectives:

Treatment of inflammatory pain with opioids is accompanied by unpleasant and, at times, life-threatening side effects. Cannabis produces antinociception as well as psychotropic effects. It was hypothesized that peripheral cannabinoid receptors outside the central nervous system could be selectively activated for relief of pain. This study was undertaken to measure the antinociceptive effect of type 1 cannabinoid receptor (CB1r) agonist arachidonylcyclopropylamide (ACPA) in a rat model of inflammatory pain after intrawound administration and the effects were compared with lignocaine.

Methods:

Wounds were produced under controlled conditions by an incision in the right hind paw in rats. ACPA (10, 30 or 100 μg/10 μl) was administered directly into the wound. Antinociception was evaluated by guarding, allodynia and thermal hyperalgesia. This was compared to lignocaine (30 μg/10 μl). Reversal of ACPA (30 μg)-mediated antinociceptive effect was attempted by intrawound AM251 (100 μg), a CB1r antagonist. Antinociception was also evaluated after contralateral administration of ACPA (30 μg). Primary afferent nociceptive input to the spinal cord was investigated by c-Fos expression after ACPA treatment (100 μg).

Results:

ACPA, but not lignocaine, inhibited guarding behaviour, which was locally mediated. Conversely, lignocaine, but not ACPA, inhibited thermal hyperalgesia and mechanical allodynia. ACPA-mediated inhibitory effect was reversible and dose dependent. It was associated with a decreased c-Fos expression. Locomotor activity was unaffected following ACPA (100 μg) treatment.

Interpretation & conclusions:

Lignocaine attenuated evoked pain behaviour whereas ACPA decreased guarding score. This difference was likely due to blockade of sodium ion channels and the activation of peripheral CB1r, respectively. Central side effects were absent after ACPA treatment. Further studies need to be done to assess the effect of ACPA treatment in clinical conditions.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

Active Nerve Regeneration with Failed Target Reinnervation Drives Persistent Neuropathic Pain

Peripheral nerves can regenerate and, when injured, may cause neuropathic pain. We propose that the active regeneration process plays a pivotal role in the maintenance of neuropathic pain. In one commonly used rodent neuropathic pain model, pronounced pain behaviors follow ligation and cutting of the L5 spinal nerve. We found that the injured nerve regenerates into the sciatic nerve and functionally reinnervates target tissues: the regenerated nerve conducts electrical signals, mechanical responses, and tracers between the leg/hindpaw and axotomized sensory ganglion. The regenerating nerve is the primary source of abnormal spontaneous activity detected in vivo. Disrupting the regeneration inhibited pain. First, semaphorin 3A, an inhibitory axonal guidance molecule, reduced functional regeneration, spontaneous activity, and pain behaviors when applied to the injury site in vivo. Second, knockdown of the upregulated growth-associated protein 43 (GAP43) with siRNA injected into the axotomized sensory ganglion reduced pain behaviors. We next examined the spared nerve injury model, in which pain behaviors are essentially permanent. The regeneration resulted in tangled GAP43-positive neuromas at the nerve injury site without target reinnervation. Perfusing the nerve stump with semaphorin 3A, but not removing the tangled fibers, prevented or reversed pain behaviors. This effect far outlasted the semaphorin 3A perfusion. Hence, in this model the long-lasting chronic pain may reflect the anatomical inability of regenerating nerves to successfully reinnervate target tissues, resulting in an ongoing futile regeneration process. We propose that specifically targeting the regeneration process may provide effective long-lasting pain relief in patients when functional reinnervation becomes impossible.

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.

Centhaquin attenuates hyperalgesia and non-evoked guarding in a rat model of postoperative pain primarily through α2B-adrenoceptors

Centhaquin has been shown to produce antinociception in the mouse hot plate and tail flick assays through the opioid, the α2A and α2B adrenoceptors. Present study was conducted to determine the effects of centhaquin in a rat model of postoperative pain. Involvement of opioid, and adrenergic receptors was assessed by pretreating rats with antagonists at the opioid (naloxone), α2-(atipamezole) or α2B-(imiloxan) adrenergic receptors. Postoperative pain was induced by hind paw plantar incision in male Sprague Dawley rats. Antihyperalgesic effects were determined by measurement of paw withdrawal latencies and withdrawal force, using dynamic von Frey filaments; attenuation of non-evoked guarding was measured by assigning pain scores to spontaneous behaviors. Rotarod test was used to determine motor impairment. Animals received saline, centhaquin or antagonist plus centhaquin. Centhaquin produced dose-dependent antihyperalgesic effect and attenuation of non-evoked guarding behavior, versus saline treated rats (P<0.05). Naloxone partially blocked while atipamezole and imiloxan significantly reversed centhaquin's antihyperalgesic effects (P<0.05). Attenuation of non-evoked guarding behavior was also blocked, but was not statistically significant. Imiloxan produced a greater block compared to atipamezole while naloxone had no significant effect. Rotarod testing indicated that centhaquin did not cause motor impairment. This is the first report demonstrating centhaquin antinociception in the rat postoperative pain model. Opioid, α2 adrenergic, and particularly α2B adrenergic receptors are involved in mediating antihyperalgesia while attenuation of nonevoked guarding is mediated by α2B/α2 adrenergic receptors. Centhaquin could be an effective non-sedating alternative in treating postoperative pain in ambulatory surgeries.

Role of neurokinin type 1 receptor in nociception at the periphery and the spinal level in the rat

OBJECTIVES

Noxious stimuli activate small to medium-sized dorsal root ganglion (DRG) neurons. Intense noxious stimuli result in the release of substance P (SP) from the central terminals of these neurons. It binds to the neurokinin type 1 receptor (NK1r) and sensitises the dorsal horn neurons. SP is also released from the peripheral terminals leading to neurogenic inflammation. However, their individual contribution at spinal and peripheral levels to postincisional nociception has not been delineated as yet.

METHODS

Sprague-Dawley rats were administered different doses (3-100 μg) of an NK1r antagonist (L760735) by intrathecal (i.t.) route before hind paw incision. On the basis of its antinociceptive effect on guarding behaviour, the 30 μg dose was selected for further study. In different sets of animals, this was administered i.t. (postemptive) and intrawound (i.w.). Finally, in another group, drug (30 μg) was administered through both i.t and i.w. routes. The antinociceptive effect was assessed and compared. Expression of SP was examined in the spinal cord. Intrawound concentration of SP and inflammatory mediators was also evaluated.

RESULTS

Postemptive i.t. administration significantly attenuated guarding and allodynia. Guarding was alone decreased after i.w. drug treatment. Combined drug administration further attenuated all nociceptive parameters, more so after postemptive treatment. Expression of SP in the spinal cord decreased post incision but increased in the paw tissue. Inflammatory mediators like the nerve growth factor also increased after incision.

CONCLUSION

In conclusion, SP acting through the NK1r appears to be an important mediator of nociception, more so at the spinal level. These findings could have clinical relevance.

Endogenous opioid activity in the anterior cingulate cortex is required for relief of pain

Pain is aversive, and its relief elicits reward mediated by dopaminergic signaling in the nucleus accumbens (NAc), a part of the mesolimbic reward motivation pathway. How the reward pathway is engaged by pain-relieving treatments is not known. Endogenous opioid signaling in the anterior cingulate cortex (ACC), an area encoding pain aversiveness, contributes to pain modulation. We examined whether endogenous ACC opioid neurotransmission is required for relief of pain and subsequent downstream activation of NAc dopamine signaling. Conditioned place preference (CPP) and in vivo microdialysis were used to assess negative reinforcement and NAc dopaminergic transmission. In rats with postsurgical or neuropathic pain, blockade of opioid signaling in the rostral ACC (rACC) inhibited CPP and NAc dopamine release resulting from non-opioid pain-relieving treatments, including peripheral nerve block or spinal clonidine, an α2-adrenergic agonist. Conversely, pharmacological activation of rACC opioid receptors of injured, but not pain-free, animals was sufficient to stimulate dopamine release in the NAc and produce CPP. In neuropathic, but not sham-operated, rats, systemic doses of morphine that did not affect withdrawal thresholds elicited CPP and NAc dopamine release, effects that were prevented by blockade of ACC opioid receptors. The data provide a neural explanation for the preferential effects of opioids on pain affect and demonstrate that engagement of NAc dopaminergic transmission by non-opioid pain-relieving treatments depends on upstream ACC opioid circuits. Endogenous opioid signaling in the ACC appears to be both necessary and sufficient for relief of pain aversiveness.

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.

Calcitonin gene-related peptide is involved in inflammatory pain but not in postoperative pain

BACKGROUND

The aim of this study was to clarify the roles of calcitonin gene-related peptide (CGRP) in postoperative pain and inflammatory pain.

METHODS

αCGRP knockout mice that the authors have developed and wild-type mice were used. Pain behaviors were assessed after incision and complete Freund's adjuvant (CFA) injection. Changes in CGRP and c-Fos expression in the dorsal horn were also examined.

RESULTS

Guarding pain scores in αCGRP knockout mice were lower than those in wild-type mice at 24 h (3.8 ± 1.6 vs. 6.8 ± 1.5, P = 0.044) and 48 h (1.8 ± 1.7 vs. 6.0 ± 1.5, P = 0.001) after CFA injection (n = 8 to 9). Withdrawal latencies to heat stimulation in αCGRP knockout mice were higher than those in wild-type mice at 24 to 72 h after CFA injection (4.9 ± 1.0 vs. 3.4 ± 0.8 at 24 h, P = 0.04; 5.1 ± 0.3 vs. 3.2 ± 0.9 at 48 h, P = 0.047; and 5.4 ± 1.6 vs. 3.5 ± 0.5 s at 72 h, P = 0.045) (n = 11 to 13), but withdrawal thresholds to mechanical stimulation were comparable. CGRP expression was increased at 24 h after CFA injection in wild-type mice, and the c-Fos-positive profile was increased at 4 h after CFA injection (ipsilateral vs. contralateral: 12.3 ± 4.6 vs. 1.3 ± 1.9, P < 0.0001) and maintained at 24 h (10.0 ± 4.1 vs. 0.8 ± 1.3, P < 0.0001) (n = 4 to 6).

CONCLUSION

These results suggest that contribution of the αCGRP system depends on the modality of pain and the stage of inflammation.

Down-regulation of Stargazin inhibits the enhanced surface delivery of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor GluR1 subunit in rat dorsal horn and ameliorates postoperative pain

BACKGROUND

Stargazin is the first transmembrane protein known to regulate synaptic targeting of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors. However, it is unclear whether regulation of the surface delivery of spinal AMPA receptor subunits by stargazin contributes to postoperative pain development.

METHODS

Western blot analysis was used to examine changes in the surface delivery of AMPA receptor subunits, GluR1 and GluR2, in rat dorsal horn. The interaction between stargazin and GluR1 and GluR2 was examined by coimmunoprecipitation. Expression of stargazin was suppressed by intrathecal administration of small interfering RNA311.

RESULTS

Membrane-bound GluR1, but not GluR2, in ipsilateral dorsal horn was increased at 3 h (1.49 ± 0.15-fold of β-tubulin, mean ± SEM) and 1 day (1.03 ± 0.25) after incision, as compared with that in control rats (naive, 0.63 ± 0.23, P < 0.05, n = 6 per group). The amount of GluR1 coimmunoprecipitated with stargazin was greater at 3 h after incision (1.48 ± 0.31-fold of input) than that in control animals (0.45 ± 0.24, P < 0.05, n = 6 per group). Importantly, the increase in membrane GluR1 at 3 h after incision was normalized to near control level (0.72 ± 0.20-fold of β-tubulin) by pretreatment with intrathecal stargazin small interfering RNA311 (0.87 ± 0.09), but not scrambled small interfering RNA (1.48 ± 0.24) or vehicle (1.25 ± 0.13, P < 0.05, n = 6 per group). Stargazin small interfering RNA311 pretreatment prevented the increase in stargazin-GluR1 interaction and decreased postoperative pain after incision.

CONCLUSIONS

This study suggests a critical role of stargazin-mediated surface delivery of GluR1 subunit in the development of postoperative pain. A better therapeutic strategy for postoperative pain may involve selectively down-regulating spinal stargazin to inhibit synaptic targeting of GluR1 subunit.

Sensitization of group III and IV muscle afferents in the mouse after ischemia and reperfusion injury

Ischemic myalgia is a unique type of muscle pain in the patient population. The role that discrete muscle afferent subpopulations play in the generation of pain during ischemic events, however, has yet to be determined. Using 2 brachial artery occlusion models to compare prolonged ischemia or transient ischemia with reperfusion of the muscles, we found that both injuries caused behavioral decrements in grip strength, as well as increased spontaneous pain behaviors. Using our ex vivo forepaw muscles, median and ulnar nerves, dorsal root ganglion, and spinal cord recording preparation, we found after both prolonged and transient ischemia that there was a significant increase in the number of afferents that responded to both noxious and non-noxious chemical (lactate, adenosine triphosphate, varying pH) stimulation of the muscles compared to uninjured controls. However, we found an increase in firing to heat stimuli specifically in muscle afferents during prolonged ischemia, but a distinct increase in afferent firing to non-noxious chemicals and decreased mechanical thresholds after transient ischemia. The unique changes in afferent function observed also corresponded with distinct patterns of gene expression in the dorsal root ganglia. Thus, the development of ischemic myalgia may be generated by unique afferent-based mechanisms during prolonged and transient ischemia.

PERSPECTIVE

This study analyzed the response properties of thinly myelinated group III and unmyelinated group IV muscle afferents during prolonged and transient ischemia in addition to pain behaviors and alterations in DRG gene expression in the mouse. Results suggest that mechanisms of pain generation during prolonged ischemia may be different from ischemia/reperfusion.

Effect of painless diabetic neuropathy on pressure pain hypersensitivity (hyperalgesia) after acute foot trauma

Introduction and objective

Acute injury transiently lowers local mechanical pain thresholds at a limb. To elucidate the impact of painless (diabetic) neuropathy on this post-traumatic hyperalgesia, pressure pain perception thresholds after a skeletal foot trauma were studied in consecutive persons without and with neuropathy (i.e. history of foot ulcer or Charcot arthropathy).

Design and methods

A case–control study was done on 25 unselected clinical routine patients with acute unilateral foot trauma (cases: elective bone surgery; controls: sprain, toe fracture). Cases were 12 patients (11 diabetic subjects) with severe painless neuropathy and chronic foot pathology. Controls were 13 non-neuropathic persons. Over 1 week after the trauma, cutaneous pressure pain perception threshold (CPPPT) and deep pressure pain perception threshold (DPPPT) were measured repeatedly, adjacent to the injury and at the opposite foot (pinprick stimulators, Algometer II^®).

Results

In the control group, post-traumatic DPPPT (but not CPPPT) at the injured foot was reduced by about 15–25%. In the case group, pre- and post-operative CPPPT and DPPPT were supranormal. Although DPPPT fell post-operatively by about 15–20%, it remained always higher than the post-traumatic DPPPT in the control group: over musculus abductor hallucis 615 kPa (kilopascal) versus 422 kPa, and over metatarsophalangeal joint 518 kPa versus 375 kPa (medians; case vs. control group); CPPPT did not decrease post-operatively.

Conclusion

Physiological nociception and post-traumatic hyperalgesia to pressure are diminished at the foot with severe painless (diabetic) neuropathy. A degree of post-traumatic hypersensitivity required to ‘pull away’ from any one, even innocuous, mechanical impact in order to avoid additional damage is, therefore, lacking.

Novel use of perineural pregabalin infusion for analgesia in a rat neuropathic pain model

BACKGROUND

The anticonvulsant drugs pregabalin and gabapentin are often used systemically to treat some forms of chronic neuropathic pain. However, many patients report side effects serious enough to cause discontinuation of the drug. Here we present evidence that pregabalin may block neuropathic pain when applied to the site of nerve injury in a rat neuropathic pain model.

METHODS

Forty male Sprague Dawley rats were randomized into 4 groups: sciatic nerve crush injury with perineural pregabalin treatment (treatment), crush injury with perineural saline treatment (saline control), crush injury with subcutaneous pregabalin treatment (systemic drug control), and sham surgery (sham surgery control). Animals received either continuous infusions of 1% pregabalin for 7 days (treatment and systemic control) or saline (saline control) and were tested for pain behaviors using incapacitance meter, guarding scores, and radiant heat withdrawal latency (Hargreaves method). Nerves were studied using histology and immunohistochemistry for α(2)δ-1 receptors thought to mediate the central analgesic action of pregabalin.

RESULTS

Treatment rats had significantly better guarding scores than systemic drug controls or saline controls (P < 0.0001) and had significantly better incapacitance scores than systemic drug controls and saline controls (P ≤ 0.001). Hargreaves method data showed hypoalgesia in all injured animals with no difference among injured groups (P = 0.80). Qualitatively, immunohistochemistry likely showed equivalent expression of the α(2)δ-1 calcium channel at the injured nerve site in all nerve-injured animals.

CONCLUSIONS

Perineural pregabalin administration produced superior analgesia compared with that of systemic pregabalin in this neuropathic pain model. Perineural pregabalin treatment may provide a useful alternative to systemic pregabalin treatment for neuropathic pain.

Preclinical assessment of pain: improving models in discovery research

To date, animal models have not sufficiently "filtered" targets for new analgesics, increasing the failure rate and cost of drug development. Preclinical assessment of "pain" has historically relied on measures of evoked behavioral responses to sensory stimuli in animals. Such measures can often be observed in decerebrated animals and therefore may not sufficiently capture affective and motivational aspects of pain, potentially diminishing translation from preclinical studies to the clinical setting. Further, evidence indicates that there are important mechanistic differences between evoked behavioral responses of hypersensitivity and ongoing pain, limiting evaluation of mechanisms that could mediate aspects of clinically relevant pain. The mechanisms underlying ongoing pain in preclinical models are currently being explored and may serve to inform decisions towards the transition from drug discovery to drug development for a given target.

Differential suppression of intracranial self-stimulation, food-maintained operant responding, and open field activity by paw incision and spinal nerve ligation in rats

BACKGROUND

Detection of ongoing spontaneous pain behaviors in laboratory animals remains a research challenge. Most preclinical pain studies measure elicited behavioral responses to an external noxious stimulus; however, ongoing spontaneous pain in humans and animals may be unrelated to hypersensitivity, and likely diminishes many behaviors, particularly motivated behaviors, that we hypothesize will decrease after induction of acute and chronic pain.

METHODS

In this study, 201 male rats were subjected to paw incision (INC), L5/L6 spinal nerve ligation (SNL), or INC in SNL rats, and the effects on paw withdrawal threshold (PWT) were assessed. For comparison, the behavioral-decreasing effects on nonevoked measures, including lever pressing for rewarding electrical stimulation of the ventral tegmental area intracranial self-stimulation (VTA ICSS) or food reinforcement (FR), and open field activity (OFA), were also assessed in these same rats.

RESULTS

INC decreased PWT for 4 days, decreased VTA ICSS for 2 days, and FR for 1 day but did not alter OFA. SNL decreased PWT similarly to INC but did not decrease VTA ICSS or FR; SNL did however decrease OFA. INC in SNL rats reduced PWT, VTA ICSS, and FR similarly to INC alone and did not decrease OFA compared with SNL alone.

CONCLUSIONS

The acute effects of INC on decreasing lever pressing for VTA ICSS and FR (1-2 days after incision) correspond to the timeframe in which ongoing spontaneous pain is expected to occur after INC. Therefore, these decreases are likely mediated by ongoing spontaneous pain, which may be unrelated to mechanical hypersensitivity that persists for up to 4 days after INC. PWT is decreased similarly by SNL, yet operant behavior (lever pressing for VTA ICSS and FR) was not decreased by SNL. SNL, but not INC, decreased rearing behavior but not total distance traveled during OFA. This further indicates that the presence and the extent of hypersensitivity are not predictive of many behavioral changes in rats thought to be mediated by the presence of ongoing pain. Surprisingly, the behavioral effects of INC are not exacerbated in SNL rats. These data support the growing belief that acute pain models produce short-lived spontaneous pain behaviors that are often less pronounced or absent in neuropathic pain models, and highlight the need for assessment of both evoked and nonevoked pain behaviors in developing future therapies for acute and chronic pain.

Direct intrawound administration of dimethylsulphoxide relieves acute pain in rats

Wounds associated with injuries such as burns can produce moderate to severe pain. Besides causing distress to the patient, unrelieved pain could delay healing owing to stress-related problems. Thus, pain needs to be treated as early as possible after injury. It was hypothesised that local treatment of wounds with appropriate analgesic drugs could attenuate pain. HOE 140, a bradykinin receptor antagonist, reduced acute inflammatory pain in rats after intrawound administration. In this study, the analgesic effect of dimethylsulphoxide (DMSO) was investigated in a similar hind-paw incision model in rats. An extremely small quantity (10 µl) of 100% DMSO was administered into the incision site just before closure of the wound. It persistently attenuated guarding behaviour in rats over a period of 3 days without affecting thermal hyperalgesia or allodynia. Accumulated evidence indicates that guarding is equivalent to pain at rest in humans. The possible mechanisms of the analgesic effect could be inhibition of C group of peripheral nerve fibres or even free radical scavenging. Healing of the wound was found to be normal at the end of the study period. In conclusion, DMSO could be useful in the treatment of acute pain resulting from tissue injuries such as burns.

Extended duration local anesthetic agent in a rat paw model

Encapsulated local anesthetics extend postoperative analgesic effect following site-directed nerve injection; potentially reducing postoperative complications. Our study aim was to investigate efficacy of our improved extended duration formulation - 15% bupivacaine in poly(DL-lactic acid co castor oil) 3:7 synthesized by ring opening polymerization. In vitro, around 70% of bupivacaine was released from the p(DLLA-CO) 3:7 after 10 days. A single injection of the optimal formulation of 15% bupivacaine-polymer or plain (0.5%) bupivacaine (control), was injected via a 22G needle beside the sciatic nerve of Sprague-Dawley rats under anesthesia; followed (in some animals) by a 1cm longitudinal incision through the skin and fascia of the paw area. Behavioral tests for sensory and motor block assessment were done using Hargreave's hot plate score, von Frey filaments and rearing count. The 15% bupivacaine formulation significantly prolonged sensory block duration up to at least 48 h. Following surgery, motor block was observed for 48 h following administration of bupivacaine-polymer formulation and rearing was reduced (returning to baseline after 48 h). No significant differences in mechanical nociceptive response were observed. The optimized bupivacaine-polymer formulation prolonged duration of local anesthesia effect in our animal model up to at least 48 h.

The challenge of evaluating pain and a pre-incisional local anesthetic block

Background. Our objective was to test the effectiveness of a local anesthetic line block administered before surgery in reducing postoperative pain scores in dogs undergoing ovariohysterectomy (OVHX).

Methods. This study is a prospective, randomized, blinded, clinical trial involving 59 healthy female dogs. An algometric pressure-measuring device was used to determine nociceptive threshold, and compared to three subjective pain scales. Group L/B received a line block of lidocaine (4 mg/kg) and bupivacaine (1 mg/kg) subcutaneously in the area of the incision site and saline subcutaneously as premedication; group L/BM (positive control) received a similar block and morphine (0.5 mg/kg) subcutaneously for premedication; and group SS (negative control) received a saline line block and saline premedication. Criteria for rescue analgesia were defined before the study. Dogs were assessed prior to surgery, at extubation (time 0) and at 2, 4, 6, 8 and 24 h post-recovery. The data were analyzed with one-way ANOVA, and a Split Plot Repeated Measures ANOVA with one grouping factor and one repeat factor (time). P < 0.05 was considered statistically significant.

Results. Approximately 33% of dogs required rescue analgesia at some point during the study, with no significant difference between groups. There was no significant difference between treatment groups with any assessment method.

Conclusions. As there were no statistically significant differences between positive and negative controls, the outcome of this technique cannot be proven.

Effect of deep tissue incision on pH responses of afferent fibers and dorsal root ganglia innervating muscle

BACKGROUND

Understanding the mechanisms underlying deep tissue pain in the postoperative period is critical to improve therapies. Using the in vitro plantar flexor digitorum brevis muscle-nerve preparation and patch clamp recordings from cultured dorsal root ganglia neurons innervating incised and unincised muscle, the authors investigated responses to various pH changes.

METHODS

Incision including the plantar flexor digitorum brevis muscle or sham operation was made in the rat hind paw. On postoperative day 1, in vitro single-fiber recording was undertaken. On the basis of previous studies, the authors recorded from at least 40 fibers per group. Also DiI-labeled dorsal root ganglia innervating muscle from rats undergoing incision and a sham operation were cultured and tested for acid responses, using whole cell patch clamp recordings.

RESULTS

The prevalence of responsive group IV afferents to lactic acid pH 6.5 in the incision group (15 of 67; 22.3%) was greater than that in the control group (2 of 35; 5.7%; P=0.022). In dorsal root ganglia neurons innervating muscle, incision increased mean current amplitudes of acid-evoked currents; the acid-sensing ion channel blocker, amiloride 300 μM, inhibited more than 75% of the acid-evoked current, whereas, the transient receptor vanilloid receptor 1 blocker (AMG9810 1 μM) did not cause significant inhibition.

CONCLUSION

The authors' experiments demonstrated that incision increases the responses of flexor digitorum brevis muscle afferent fibers to weak acid solutions, and increased acid-evoked currents in dorsal root ganglia innervating muscle. The authors' data suggest that up-regulation of acid-sensing ion channels might underlie this increased chemosensitivity caused by surgery.

Wound hypoxia in deep tissue after incision in rats

Our previous studies using rat models of incisional pain have shown that tissue lactate levels increase and pH decreases for several days after incision, suggesting the presence of an ischemic-like condition. The purpose of this study was to evaluate the time course and the extent of tissue hypoxia that develops in incised muscle and skin. We directly measured oxygen tension at several time points after incisions of the gastrocnemius muscle, the paraspinal skin, and the plantar hindpaw in anesthetized rats using an oxygen-sensitive microelectrode. In vivo hypoxia of the incised tissues was also evaluated immunohistochemically using a hypoxia marker, pimonidazole hydrochloride. To minimize intersubject variability, unincised contralateral tissues were used as a control. Tissue oxygen tension was decreased in both skeletal muscle and skin compared with control, for several days after incision. When measured directly, oxygen tension decreased immediately and remained low for several days after incisions. Pimonidazole immunostaining revealed hypoxic areas in incised muscle and skin for several days. By postoperative day 10, tissue oxygen tension recovered to that of control tissue. These results support the evidence that a hypoxic condition is present in deep tissue after incisions and that an ischemic-like mechanism may contribute to postoperative pain.

Inhibition by local bupivacaine-releasing microspheres of acute postoperative pain from hairy skin incision

BACKGROUND

Acute postoperative pain causes physiological deficits and slows recovery. Reduction of such pain by local anesthetics that are delivered for several days postoperatively is a desirable clinical objective, which is approached by a new formulation and applied in animal studies reported here.

METHODS

We subcutaneously injected a new formulation of poly-lactic-co-glycolic acid polymer microspheres, which provides steady drug release for 96+ hours into rats at the dorsal region 2 hours before surgery. A single 1.2-cm-long skin incision was followed by blunt dissection of skin away from the underlying fascia, and closed by 2 sutures, followed by 14 days of testing. Microspheres containing 5, 10, 20, and 40 mg bupivacaine were injected locally 2 hours before surgery; bupivacaine-free microspheres were the vehicle control, and bupivacaine HCl solution (0.5%), the positive control. Mechanical sensitivity was determined by the frequency of local muscle contractions to repeated pokes with nylon monofilaments (von Frey hairs) exerting 4 and 15 g forces, testing, respectively, allodynia and hyperalgesia, and by pinprick.

RESULTS

Injection of bupivacaine microspheres (40 mg drug) into intact skin reduced responses to 15 g von Frey hairs for 6 hours and to pinprick for 36 hours. Respective reductions from bupivacaine HCl lasted for 3 and 2 hours. Skin incision and dissection alone caused mechanical allodynia and hyperalgesia for 14 days. Microspheres containing 20 or 40 mg bupivacaine suppressed postoperative hypersensitivity for up to 3 days, reduced integrated allodynia (area under curve of response versus time) over postoperative days 1 to 5 by 51% ± 20% (mean ± SE) and 78% ± 12%, and reduced integrated hyperalgesia by 55% ± 13% and 64% ± 11%, for the respective doses. Five and ten milligrams bupivacaine in microspheres and the 0.5% bupivacaine solution were ineffective in reducing postoperative hypersensitivity, as were 40 mg bupivacaine microspheres injected contralateral to the incision.

CONCLUSIONS

Significant suppression of postoperative pain by the slow-release bupivacaine preparation outlasts its anesthetic action on intact skin. These findings demonstrate preventive analgesia and indicate the importance of acute processes in the development of chronic postoperative pain.

Evaluation of reward from pain relief

The human experience of pain is multidimensional and comprises sensory, affective, and cognitive dimensions. Preclinical assessment of pain has been largely focused on the sensory features that contribute to nociception. The affective (aversive) qualities of pain are clinically significant but have received relatively less mechanistic investigation in preclinical models. Recently, operant behaviors such as conditioned place preference, avoidance, escape from noxious stimulus, and analgesic drug self-administration have been used in rodents to evaluate affective aspects of pain. An important advance of such operant behaviors is that these approaches may allow the detection and mechanistic investigation of spontaneous neuropathic or ongoing inflammatory/nociceptive (i.e., nonevoked) pain that is otherwise difficult to assess in nonverbal animals. Operant measures may allow the identification of mechanisms that contribute differentially to reflexive hypersensitivity or to pain affect and may inform the decision to progress novel mechanisms to clinical trials for pain therapy. Additionally, operant behaviors may allow investigation of the poorly understood mechanisms and neural circuits underlying motivational aspects of pain and the reward of pain relief.

TRPV1, but not TRPA1, in primary sensory neurons contributes to cutaneous incision-mediated hypersensitivity

Background

Mechanisms underlying postoperative pain remain poorly understood. In rodents, skin-only incisions induce mechanical and heat hypersensitivity similar to levels observed with skin plus deep incisions. Therefore, cutaneous injury might drive the majority of postoperative pain. TRPA1 and TRPV1 channels are known to mediate inflammatory and nerve injury pain, making them key targets for pain therapeutics. These channels are also expressed extensively in cutaneous nerve fibers. Therefore, we investigated whether TRPA1 and TRPV1 contribute to mechanical and heat hypersensitivity following skin-only surgical incision.

Results

Behavioral responses to mechanical and heat stimulation were compared between skin-incised and uninjured, sham control groups. Elevated mechanical responsiveness occurred 1 day post skin-incision regardless of genetic ablation or pharmacological inhibition of TRPA1. To determine whether functional changes in TRPA1 occur at the level of sensory neuron somata, we evaluated cytoplasmic calcium changes in sensory neurons isolated from ipsilateral lumbar 3–5 DRGs of skin-only incised and sham wild type (WT) mice during stimulation with the TRPA1 agonist cinnamaldehyde. There were no changes in the percentage of neurons responding to cinnamaldehyde or in their response amplitudes. Likewise, the subpopulation of DRG somata retrogradely labeled specifically from the incised region of the plantar hind paw showed no functional up-regulation of TRPA1 after skin-only incision. Next, we conducted behavior tests for heat sensitivity and found that heat hypersensitivity peaked at day 1 post skin-only incision. Skin incision-induced heat hypersensitivity was significantly decreased in TRPV1-deficient mice. In addition, we conducted calcium imaging with the TRPV1 agonist capsaicin. DRG neurons from WT mice exhibited sensitization to TRPV1 activation, as more neurons (66%) from skin-incised mice responded to capsaicin compared to controls (46%), and the sensitization occurred specifically in isolectin B4 (IB4)-positive neurons where 80% of incised neurons responded to capsaicin compared to just 44% of controls.

Conclusions

Our data suggest that enhanced TRPA1 function does not mediate the mechanical hypersensitivity that follows skin-only surgical incision. However, the heat hypersensitivity is dependent on TRPV1, and functional up-regulation of TRPV1 in IB4-binding DRG neurons may mediate the heat hypersensitivity after skin incision injury.

Activation of p38 mitogen-activated protein kinase in the dorsal root ganglion contributes to pain hypersensitivity after plantar incision

BACKGROUND

The phosphorylation of p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) promotes primary afferent sensitization. The role of p38MAPK signaling in the DRG in the pathogenesis of plantar incision hyperalgesia has not been investigated.

RESULTS

Levels of phosphorylated p38MAPK (p-p38MAPK) obviously increased in the DRG after plantar incision. Unmyelinated and myelinated DRG neurons that express p-p38MAPK contained small to medium cell bodies, suggesting that p-p38MAPK expression is induced in neurons with C- and Aδ-fibers. The p-p38MAPK inhibitors FR167653 or SB203580 inhibited incision-induced mechanical hypersensitivity and spontaneous pain behavior. The systemic administration of tumor necrosis factor-α (TNF-α) inhibitor prevented subsequent incision-induced activation of p38MAPK in the DRG and alleviated mechanical hypersensitivity after the incision.

CONCLUSIONS

p38MAPK signaling in the DRG plays a crucial role in the development of primary afferent sensitization and pain behavior caused by plantar incision.