Separate groups of dorsal horn neurons transmit spontaneous activity and mechanosensitivity one day after plantar incision

The mechanisms and management of persistent postsurgical pain

An estimated 10%-50% of patients undergoing a surgical intervention will develop persistent postsurgical pain (PPP) lasting more than 3 months despite adequate acute pain management and the availability of minimally invasive procedures. The link between early and late pain outcomes for surgical procedures remains unclear-some patients improve while others develop persistent pain. The elective nature of a surgical procedure offers a unique opportunity for prophylactic or early intervention to prevent the development of PPP and improve our understanding of its associated risk factors, such as pre-operative anxiety and the duration of severe acute postoperative pain. Current perioperative pain management strategies often include opioids, but long-term consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death. Pre-clinical models provide the opportunity to dissect mechanisms underpinning the transition from acute to chronic, or persistent, postsurgical pain. This review highlights putative mechanisms of PPP, including sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system and nociceptive signalling along the neuro-immune axis.

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.

The Protective Effects of Pre- and Post-Administration of Micronized Palmitoylethanolamide Formulation on Postoperative Pain in Rats

Background: Postoperative pain (PO) is a common form of acute pain. Inadequate PO treatment is an important health problem, as it leads to worse outcomes, such as chronic post-surgical pain. Therefore, it is necessary to acquire new knowledge on PO mechanisms to develop therapeutic options with greater efficacy than those available today and to lower the risk of adverse effects. For this reason, we evaluated the ability of micronized palmitoylethanolamide (PEA-m) to resolve the pain and inflammatory processes activated after incision of the hind paw in an animal model of PO. Methods: The animals were subjected to surgical paw incision and randomized into different groups. PEA-m was administered orally at 10 mg/kg at different time points before or after incision. Results: Our research demonstrated that the pre- and post-treatment with PEA-m reduced the activation of mast cells at the incision site and the expression of its algogenic mediator nerve growth factor (NGF) in the lumbar spinal cord. Furthermore, again at the spinal level, it was able to decrease the activation of phospho-extracellular signal-regulated kinases (p-ERK), ionized calcium binding adaptor molecule 1 (Iba1), glial fibrillary acidic protein (GFAP), and the expression of brain-derived neurotrophic factor (BDNF). PEA-m also reduced the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) spinal pathway, showing a protective effect in a rat model of PO. Conclusion: The results obtained reinforce the idea that PEA-m may be a potential treatment for the control of pain and inflammatory processes associated with PO. In addition, pre- and post-treatment with PEA-m is more effective than treatment alone after the surgery and this limits the time of taking the compound and the abuse of analgesics.

Oxycodone regulates incision-induced activation of neurotrophic factors and receptors in an acute post-surgery pain rat model

Background

Oxycodone, which is one of the most commonly used opiates in postoperative pain management, has a different affinity for μ-opioid receptors (MOR), κ-opioid receptors (KOR), and δ-opioid receptors (DOR). Accumulating research has suggested that neurotrophins (NTs) are involved in opioid analgesia. In the current exploratory study, we aimed to investigate the underlying mechanisms of the analgesic effects of oxycodone on post-surgery pain in rats and to determine whether neurotrophic factors and receptors were involved in these effects.

Methods

Mechanical and thermal sensitivity tests were used to evaluate the validity of the postoperative pain rat model and to determine the analgesic effect of oxycodone. Quantitative PCR and Western blot analysis were used to detect the changes in the expression of three types of opioid receptors and NTs and their high-affinity receptors in the spinal cord after surgery and oxycodone administration.

Results

Oxycodone showed an analgesic effect on plantar incision (PI)-induced hyperalgesia, especially thermal hyperalgesia. We detected an obvious increase in MOR expression levels but insignificant changes in KOR and DOR levels in the spinal cord after PI. Moreover, we found that oxycodone was able to reverse the increased expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tyrosine kinase receptor (TrK) A, and TrkB and the decreased expression of NT-3 and TrkC, after PI. Pretreatment with oxycodone also altered the expression of these mediators.

Conclusion

Based on the results, possible underlying mechanisms for the antinociceptive properties of oxycodone in acute postoperative pain include the activation of MOR downstream signaling and the regulation of NTs and receptor expression through attenuation of glial activation and fortification of antinociceptive mediators in the spinal cord. This study may provide new insights into the molecular mechanisms underlying the analgesic action of oxycodone.

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.

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.

Disruption of Spinal Noradrenergic Activation Delays Recovery of Acute Incision-Induced Hypersensitivity and Increases Spinal Glial Activation in the Rat

Results of clinical studies suggest that descending inhibitory controls from the brainstem are important for speeding recovery from pain after surgery. We examined the effects of destroying spinally projecting noradrenergic neurons via intrathecally administered antibody to dopamine β-hydroxylase conjugated to saporin (DβH-saporin) on recovery in an acute incisional pain model. Mechanical and thermal paw withdrawal thresholds and nonevoked spontaneous guarding scores were tested for several weeks postoperatively and analyzed using mixed effects growth curve modeling. DβH-saporin treatment resulted in a significant prolongation in the duration of mechanical and to a lesser degree thermal hypersensitivity in the ipsilateral paw of incised rats but did not increase the duration of spontaneous guarding. DβH-saporin treatment was also associated with increased microglial and astrocyte activation in the ipsilateral spinal cord 21 days after incision compared with immunoglobulin G-saporin treated controls. Chronic intrathecal administration of the α2 adrenergic receptor antagonist atipamezole (50-200 μg/d) produced similar effects. These data suggest that spinally projecting noradrenergic pathways and spinal α2 adrenergic receptor activation are important for speeding recovery from hypersensitivity after surgical incision possibly by reducing spinal glial activation. Interventions that augment the noradrenergic system might be important to speed recovery from pain after surgery.

PERSPECTIVE

Endogenous descending spinal noradrenergic activation promotes resolution of incision-induced hypersensitivity and inhibits spinal microglial and astrocyte activation in part through α2 adrenergic receptors.

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.

Mast cell stabilization promotes antinociceptive effects in a mouse model of postoperative pain

BACKGROUND

Nerve injury and consequent inflammatory responses produced by surgical incision result in a complicated pain status which still affects half of all surgical patients. Therefore, it is essential for anesthesiologists to identify the mechanisms of postoperative pain. Mast cells are resident cells of connective tissue and the mucosa that participate in the immune response. Degranulation of mast cells is involved in the development of postoperative pain and can be induced by surgical incision. The aim of this study was to investigate whether stabilization of mast cells causes an antinociceptive effect in a mouse model of postoperative pain.

METHODS

Postoperative pain was induced by making an incision in the hind paw of BALB/c mice. The mast cell membrane stabilizer cromoglycate (200 μg/20 μL) was injected before incision of the paw, and postoperative pain responses were measured by assessing guarding behavior, withdrawal threshold to mechanical stimuli, and latency of heat pain behavior 1, 2, and 7 days after the incision.

RESULTS

The incision produced guarding pain, mechanical allodynia, and heat hypersensitivity. Cromoglycate decreased the guarding pain score (day 1) and the withdrawal threshold to mechanical stimuli (days 1, 2, and 7). However, the withdrawal latency to heat was not affected by cromoglycate treatment.

CONCLUSION

Cromoglycate significantly attenuated the pain response expressed as guarding pain and mechanical allodynia in a mouse model of postoperative pain. Thus, mast cell activation is likely a mechanism of postoperative pain and is an interesting target for the development of new therapies.

Intense focused ultrasound preferentially stimulates subcutaneous and focal neuropathic tissue: preliminary results

OBJECTIVE

Potential peripheral sources of pain from subcutaneous tissue can require invasive evocative tests for their localization and assessment. Here, we describe studies whose ultimate goal is development of a noninvasive evocative test for subcutaneous, painful tissue.

DESIGN

We used a rat model of a focal and subcutaneous neuroma to test the hypothesis that intense focused ultrasound can differentiate focal and subcutaneous neuropathic tissue from control tissue. To do so, we first applied intense focused ultrasound (2 MHz, with individual pulses of 0.1 second in duration) to the rat's neuroma while the rat was under light anesthesia. We started with low values of intensity, which we increased until intense focused ultrasound stimulation caused the rat to reliably flick its paw. We then applied that same intense focused ultrasound protocol to control tissue away from the neuroma and assayed for the rat's response to that stimulation.

RESULTS

Intense focused ultrasound of sufficient strength (I(SATA) of 600 +/- 160 W/cm(2) ) applied to the neuroma caused the rat to flick its paw, while the same intense focused ultrasound applied millimeters to a centimeter away failed to induce a paw flick.

CONCLUSION

Successful stimulation of the neuroma by intense focused ultrasound required colocalization of the neuroma and intense focused ultrasound supporting our hypothesis.

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.

Enduring prevention and transient reduction of postoperative pain by intrathecal resolvin D1

Postoperative pain slows surgical recovery, impacting the return of normal function for weeks, months, or longer. Here we report the antihyperalgesic actions of a new compound, resolvin D1 (RvD1), known to reduce inflammation and to suppress pain after peripheral nerve injury, on the acute pain occurring after paw incision and the prolonged pain after skin-muscle retraction. Injection of RvD1 (20-40ng) into the L5-L6 intrathecal space 30minutes before surgery reduces the postincisional primary mechanical hypersensitivity, lowering the peak change by approximately 70% (with 40ng) and reducing the area under the curve (AUC) for the entire 10-day postincisional course by approximately 60%. Intrathecal injection of RvD1 on postoperative day (POD) 1 reduces the hyperalgesia to the same level as that from preoperative injection within a few hours, an effect that persists for the remaining PODs. Tactile allodynia and hyperalgesia following the skin/muscle incision retraction procedure, measured at the maximum values 12 to 14days, is totally prevented by intrathecal RvD1 (40ng) given at POD 2. However, delaying the injection until POD 9 or POD 17 results in RvD1 causing only transient and incomplete reversal of hyperalgesia, lasting for <1day. These findings demonstrate the potent, effective reduction of postoperative pain by intrathecal RvD1 given before or shortly after surgery. The much more limited effect of this compound on retraction-induced pain, when given 1 to 2weeks later, suggests that the receptors or pathways for resolvins are more important in the early than the later stages of postoperative pain. Single intrathecal injections of resolvin D1 in rats before or 1 to 2days after surgery strongly reduce postoperative pain for several weeks.

Transient inflammation-induced ongoing pain is driven by TRPV1 sensitive afferents

Background

Neuropathic pain is a chronic disease resulting from dysfunction within the "pain matrix". The basolateral amygdala (BLA) can modulate cortical functions and interactions between this structure and the medial prefrontal cortex (mPFC) are important for integrating emotionally salient information. In this study, we have investigated the involvement of the transient receptor potential vanilloid type 1 (TRPV1) and the catabolic enzyme fatty acid amide hydrolase (FAAH) in the morphofunctional changes occurring in the pre-limbic/infra-limbic (PL/IL) cortex in neuropathic rats.

Results

The effect of N-arachidonoyl-serotonin (AA-5-HT), a hybrid FAAH inhibitor and TPRV1 channel antagonist, was tested on nociceptive behaviour associated with neuropathic pain as well as on some phenotypic changes occurring on PL/IL cortex pyramidal neurons. Those neurons were identified as belonging to the BLA-mPFC pathway by electrical stimulation of the BLA followed by hind-paw pressoceptive stimulus application. Changes in their spontaneous and evoked activity were studied in sham or spared nerve injury (SNI) rats before or after repeated treatment with AA-5-HT. Consistently with the SNI-induced changes in PL/IL cortex neurons which underwent profound phenotypic reorganization, suggesting a profound imbalance between excitatory and inhibitory responses in the mPFC neurons, we found an increase in extracellular glutamate levels, as well as the up-regulation of FAAH and TRPV1 in the PL/IL cortex of SNI rats. Daily treatment with AA-5-HT restored cortical neuronal activity, normalizing the electrophysiological changes associated with the peripheral injury of the sciatic nerve. Finally, a single acute intra-PL/IL cortex microinjection of AA-5-HT transiently decreased allodynia more effectively than URB597 or I-RTX, a selective FAAH inhibitor or a TRPV1 blocker, respectively.

Conclusion

These data suggest a possible involvement of endovanilloids in the cortical plastic changes associated with peripheral nerve injury and indicate that therapies able to normalize endovanilloid transmission may prove useful in ameliorating the symptoms and central sequelae associated with neuropathic pain.

Transient inflammation-induced ongoing pain is driven by TRPV1 sensitive afferents

BACKGROUND

Tissue injury elicits both hypersensitivity to evoked stimuli and ongoing, stimulus-independent pain. We previously demonstrated that pain relief elicits reward in nerve-injured rats. This approach was used to evaluate the temporal and mechanistic features of inflammation-induced ongoing pain.

RESULTS

Intraplantar Complete Freund's Adjuvant (CFA) produced thermal hyperalgesia and guarding behavior that was reliably observed within 24 hrs and maintained, albeit diminished, 4 days post-administration. Spinal clonidine produced robust conditioned place preference (CPP) in CFA treated rats 1 day, but not 4 days following CFA administration. However, spinal clonidine blocked CFA-induced thermal hyperalgesia at both post-CFA days 1 and 4, indicating different time-courses of ongoing and evoked pain. Peripheral nerve block by lidocaine administration into the popliteal fossa 1 day following intraplantar CFA produced a robust preference for the lidocaine paired chamber, indicating that injury-induced ongoing pain is driven by afferent fibers innervating the site of injury. Pretreatment with resiniferatoxin (RTX), an ultrapotent capsaicin analogue known to produce long-lasting desensitization of TRPV1 positive afferents, fully blocked CFA-induced thermal hypersensitivity and abolished the CPP elicited by administration of popliteal fossa lidocaine 24 hrs post-CFA. In addition, RTX pretreatment blocked guarding behavior observed 1 day following intraplantar CFA. In contrast, administration of the selective TRPV1 receptor antagonist, AMG9810, at a dose that reversed CFA-induced thermal hyperalgesia failed to reduce CFA-induced ongoing pain or guarding behavior.

CONCLUSIONS

These data demonstrate that inflammation induces both ongoing pain and evoked hypersensitivity that can be differentiated on the basis of time course. Ongoing pain (a) is transient, (b) driven by peripheral input resulting from the injury, (c) dependent on TRPV1 positive fibers and (d) not blocked by TRPV1 receptor antagonism. Mechanisms underlying excitation of these afferent fibers in the early post-injury period will offer insights for development of novel pain relieving strategies in the early post-traumatic period.

Skin incision induces expression of axonal regeneration-related genes in adult rat spinal sensory neurons

Skin incision and nerve injury both induce painful conditions. Incisional and postsurgical pain is believed to arise primarily from inflammation of tissue and the subsequent sensitization of peripheral and central neurons. The role of axonal regeneration-related processes in development of pain has only been considered when there has been injury to the peripheral nerve itself, even though tissue damage likely induces injury of resident axons. We sought to determine if skin incision would affect expression of regeneration-related genes such as activating transcription factor 3 (ATF3) in dorsal root ganglion (DRG) neurons. ATF3 is absent from DRG neurons of the normal adult rodent, but is induced by injury of peripheral nerves and modulates the regenerative capacity of axons. Image analysis of immunolabeled DRG sections revealed that skin incision led to an increase in the number of DRG neurons expressing ATF3. RT-PCR indicated that other regeneration-associated genes (galanin, GAP-43, Gadd45a) were also increased, further suggesting an injury-like response in DRG neurons. Our finding that injury of skin can induce expression of neuronal injury/regeneration-associated genes may impact how clinical postsurgical pain is investigated and treated.

PERSPECTIVE

Tissue injury, even without direct nerve injury, may induce a state of enhanced growth capacity in sensory neurons. Axonal regeneration-associated processes should be considered alongside nerve signal conduction and inflammatory/sensitization processes as possible mechanisms contributing to pain, particularly the transition from acute to chronic pain.

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.