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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Bursting activity in myelinated sensory neurons plays a key role in pain behavior induced by localized inflammation of the rat sensory ganglion

Abnormal spontaneous activity of sensory neurons is observed in many different preclinical pain models, but its basis is not well understood. In this study mechanical and cold hypersensitivity were induced in rats after inflammation of the L5 dorsal root ganglion (DRG), initiated by local application of the immune stimulator zymosan in incomplete Freund's adjuvant. Mechanical hypersensitivity was evident by day 1 and maintained for 2 months. The model also showed reduction of rearing behavior in a novel environment. Microelectrode recordings made in isolated whole DRG on day 3 after inflammation showed a marked increase of spontaneous activity, predominantly with a bursting pattern. The incidence was especially high (44%) in Aαβ cells. Spontaneous activity and subthreshold membrane potential oscillations were completely blocked by tetrodotoxin (500 nM) and by riluzole (10 μM), a blocker of persistent sodium currents. In vivo, local perfusion of the inflamed DRG for the first 7 days with riluzole gave long-lasting, dose-dependent reduction in mechanical pain behaviors. Riluzole perfusion did not affect mechanical sensitivity in normal animals. Unmyelinated C cells had a very low incidence of spontaneous activity and were much less affected by riluzole in vitro. Taken together these results suggest that high-frequency and/or bursting spontaneous activity in Aαβ sensory neurons may play important roles in initiating pain behaviors resulting from inflammatory irritation of the DRG.

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.

The complement component C5a receptor mediates pain and inflammation in a postsurgical pain model

The complement system is an important part of innate immunity. Complement activation generates a set of effector molecules with diverse biological functions. C5a is a crucial terminal component of the complement cascade. Several reports suggest that C5a can support nociceptive sensitization and inflammation in various models, including models of incisional pain. However, information concerning the differential effects of C5a on specific modalities of nociception, the role of C5a in supporting neutrophil infiltration, secondary nociceptive mediator generation, and the location of the relevant populations of C5a receptors supporting incisional sensitization are needed. In these studies we utilized C5a receptor-null mice (C5aR(-/-)) and matched controls to study nociceptive changes after hind paw incision. Heat hyperalgesia and mechanical allodynia were measured for 4 days after incision. We also followed hind paw edema, wound area neutrophil infiltration using the myeloperoxidase assay, and interleukin-1β and nerve growth factor levels using both enzyme-linked immunosorbent assay and immunohistochemical techniques. The main findings were: (1) Heat vs mechanical nociceptive sensitization after incision were differentially reduced in C5aR(-/-) mice, with thermal sensitization affected throughout the postincisional period but mechanical sensitization affected only at later time points; (2) Edema developed after incision in wild-type mice but only slightly and transiently in C5aR(-/-) mice, and (3) Deletion of C5aR blocked interleukin-1β and nerve growth factor production near the wound site. These findings demonstrate that the complement system component C5a is a novel biomarker and mediator associated with postsurgical nociceptive processing. C5aR may provide a novel target for the control of pain and inflammation after surgery.

Acid-sensing ion channels in postoperative pain

Iatrogenic pain consecutive to a large number of surgical procedures has become a growing health concern. The etiology and pathophysiology of postoperative pain are still poorly understood, but hydrogen ions appear to be important in this process. We have investigated the role of peripheral acid-sensing ion channels (ASICs), which form depolarizing channels activated by extracellular protons, in a rat model of postoperative pain (i.e., hindpaw skin/muscle incision). We report high levels of ASIC-type currents (∼ 77%) in sensory neurons innervating the hindpaw muscles, with a prevalence of ASIC3-like currents. The ASIC3 protein is largely expressed in lumbar DRG neurons innervating the plantar muscle, and its mRNA and protein levels are increased by plantar incision 24 h after surgery. Pharmacological inhibition of ASIC3 channels with the specific toxin APETx2 or in vivo knockdown of ASIC3 subunit by small interfering RNA led to a significant reduction of postoperative spontaneous, thermal, and postural pain behaviors (spontaneous flinching, heat hyperalgesia, and weight bearing). ASIC3 appears to have an important role in deep tissue but also affects prolonged pain evoked by skin incision alone. The specific homomeric ASIC1a blocker PcTx1 has no effect on spontaneous flinching, when applied peripherally. Together, these data demonstrate a significant role for peripheral ASIC3-containing channels in postoperative pain.

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.

Increased sensitivity of group III and group IV afferents from incised muscle in vitro

Understanding deep muscle pain is of increasing importance for evaluating clinical pathologic pain states. Previously, a central role of deep muscle tissue in the development of ongoing pain behavior after incision was demonstrated. The underlying mechanisms, however, remain unclear. Using a new in vitro plantar flexor digitorum brevis (FDB) muscle-nerve preparation, we investigated properties of mechanosensitive group III and IV afferents innervating incised and unincised muscle, and explored potential mediators of afferent excitation after incision. Afferents of uninjured muscle had a low incidence (14.3%) of ongoing activity. A high proportion (65.8%) of afferents responded to heat and a minority, 20.8%, were activated by pH 6.0 lactic acid. Incision increased the prevalence of afferents with ongoing activity to 54.7%. A greater proportion of group III and IV afferents responded to pH 6.0 lactic acid after incision compared to control (55.4% vs. 20.8%). Sensitization of afferents to heat and mechanical stimulation was prominent in group IV afferents after incision; both heat (38.0 vs. 40.5°C in control) and mechanical response threshold (median: 5.0 vs. 22.0 mN in control) were decreased. The finding hat incision increased ongoing activity of muscle afferents is consistent with our previous in vivo studies and supports the idea that deep muscle tissue has a prominent role in the genesis of ongoing pain after incision. The enhanced chemosensitivity of muscle afferents to lactic acid after incision suggests an increased response to an ischemic mediator that may contribute to pain and hyperalgesia caused by surgery in deep tissues.

Chronic spontaneous activity generated in the somata of primary nociceptors is associated with pain-related behavior after spinal cord injury

Mechanisms underlying chronic pain that develops after spinal cord injury (SCI) are incompletely understood. Most research on SCI pain mechanisms has focused on neuronal alterations within pain pathways at spinal and supraspinal levels associated with inflammation and glial activation. These events might also impact central processes of primary sensory neurons, triggering in nociceptors a hyperexcitable state and spontaneous activity (SA) that drive behavioral hypersensitivity and pain. SCI can sensitize peripheral fibers of nociceptors and promote peripheral SA, but whether these effects are driven by extrinsic alterations in surrounding tissue or are intrinsic to the nociceptor, and whether similar SA occurs in nociceptors in vivo are unknown. We show that small DRG neurons from rats (Rattus norvegicus) receiving thoracic spinal injury 3 d to 8 months earlier and recorded 1 d after dissociation exhibit an elevated incidence of SA coupled with soma hyperexcitability compared with untreated and sham-treated groups. SA incidence was greatest in lumbar DRG neurons (57%) and least in cervical neurons (28%), and failed to decline over 8 months. Many sampled SA neurons were capsaicin sensitive and/or bound the nociceptive marker, isolectin B4. This intrinsic SA state was correlated with increased behavioral responsiveness to mechanical and thermal stimulation of sites below and above the injury level. Recordings from C- and Aδ-fibers revealed SCI-induced SA generated in or near the somata of the neurons in vivo. SCI promotes the entry of primary nociceptors into a chronic hyperexcitable-SA state that may provide a useful therapeutic target in some forms of persistent pain.

Spinal cord protein interacting with C kinase 1 is required for the maintenance of complete Freund's adjuvant-induced inflammatory pain but not for incision-induced post-operative pain

Protein interacting with C kinase 1 (PICK1) is a PDZ-containing protein that binds to AMPA receptor (AMPAR) GluR2 subunit and protein kinase Cα (PKCα) in the central neurons. It functions as a targeting and transport protein, presents the activated form of PKCα to synaptic GluR2, and participates in synaptic AMPAR trafficking in the nervous system. Thus, PICK1 might be involved in many physiological and pathological processes triggered via the activation of AMPARs. We report herein that PICK1 knockout mice display impaired mechanical and thermal pain hypersensitivities during complete Freund's adjuvant (CFA)-induced inflammatory pain maintenance. Acute transient knockdown of spinal cord PICK1 through intrathecal injection of PICK1 antisense oligodeoxynucleotide had a similar effect. In contrast, knockout and knockdown of spinal cord PICK1 did not affect incision-induced guarding pain behaviors or mechanical or thermal pain hypersensitivities. We also found that PICK1 is highly expressed in dorsal horn, where it interacts with GluR2 and PKCα. Injection of CFA into a hind paw, but not a hind paw incision, increased PKCα-mediated GluR2 phosphorylation at Ser880 and GluR2 internalization in dorsal horn. These increases were absent when spinal cord PICK1 was deficient. Given that dorsal horn PKCα-mediated GluR2 phosphorylation at Ser880 and GluR2 internalization contribute to the maintenance of CFA-induced inflammatory pain, our findings suggest that spinal PICK1 may participate in the maintenance of persistent inflammatory pain, but not in incision-induced post-operative pain, through promoting PKCα-mediated GluR2 phosphorylation and internalization in dorsal horn neurons.

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.