Hind Paw Incision Research Articles

Change of p-CREB after surgical incisional pain in rat spinal cord

To explore the role of phosphorylation of cAMP response element binding protein (CREB) in the incision-induced pain hypersensitivity. A longitudinal incision was made in one plantar hind paw of isoflurane-anesthetized rats. Spinal cords were removed at various postoperative time after behavior test. Phosphorylation of CREB was determined by immunohistochemistry and double-labeling immunofluorescence. Morphine and gabapentin were intraperitoneally injected before the behavior test and were used to determine the interaction between phosphorylation of CREB and morphine and gabapentin. After the hind-paw incision, phosphorylation of CREB was enhanced in the ipsilateral lumbar spinal cord (P<0.05). The enhancement of p-CREB was mainly in the neurons in the dorsal horn of the spinal cord. All these were shown by double-labeling technique and p-CREB was mainly in the neurons. Intraperitoneal injection of morphine prevented the increased phosphorylation of CREB in the spinal cord and inhibited the mechanical allodynia induced by the incision (P>0.05). Gabapentin didn't inhibit the phosphorylation of CREB (P<0.05) but partly inhibited the mechanical allodynia. Incision induces the phosphorylation of CREB in the spinal cord, and the increase of p-CREB is mainly in the neurons. Phosphorylation of CREB in the spinal cord contributes to the pain hypersensitivity induced by surgical incision.

Contribution of the Chemokine (C-C Motif) Ligand 2 (CCL2) to Mechanical Hypersensitivity after Surgical Incision in Rats

Neural glial signaling in the spinal cord may underlie pain and sensitization after peripheral injury. The authors test the role of a glial activator, the chemokine (C-C motif) ligand 2 (CCL2), on mechanical hypersensitivity after plantar incision in a rat model of postoperative pain. Twenty-four hours after hind paw incision, rats were intrathecally administered an anti-CCL2 neutralizing antibody (3 and 10 microg) or control immunoglobulin G (10 microg). Mechanical hypersensitivity was assessed acutely and for several days after administration of anti-CCL2 antibody using von Frey filaments. Immunohistochemical analysis was conducted on spinal cord sections to examine the effects of treatment on measures of microglial activation, including levels of ionized calcium-binding adaptor molecule 1 and phosphorylated p38 mitogen-activated protein kinase. Neutralization of spinal CCL2 acutely reversed mechanical hypersensitivity within 30 min in a dose-dependent manner. A single administration also produced a sustained decrease in mechanical hypersensitivity 48 and 72 h after incision. Anti-CCL2 antibody reduced microglial activation as measured by the levels of ionized calcium-binding adaptor molecule 1 immunoreactivity and the number of microglia containing phosphorylated p38 mitogen-activated protein kinase 48 h after incision but not within 30 min of administration. These results provide evidence that CCL2 contributes to the maintenance of mechanical hypersensitivity after plantar incision and establish a role for neural glial signaling in postoperative pain. The long-term effects of anti-CCL2 treatment correlate with reduced microglial activation. Spinal blockade of CCL2 may serve as a useful therapy for the treatment of certain aspects of postoperative pain.

Surgical Incision Induces Anxiety-Like Behavior and Amygdala Sensitization: Effects of Morphine and Gabapentin

The role of affective dimension in the postoperative pain is still poorly understood. The present study investigated the development of anxiety-like behavior and amygdala sensitization in incisional pain. Using hind-paw incision model in rats, we showed that surgical incision induced the anxiety-like behavior as determined by elevated plus-maze and open-field tests. Intraperitoneal (IP) morphine administration reversed mechanical allodynia and anxiety-like behavior in a dose-dependent manner. Gabapentin also partially reduced incision-evoked mechanical allodynia and anxiety-like behavior in a dose-dependent manner. After incision, the expression of phosphorylated cAMP response elements (CRE-) binding protein (p-CREB) was transiently upregulated in the central and basolateral nuclei in the bilateral amygdala. The upregulation of p-CREB was inhibited by morphine and gabapentin. The present study suggested that surgical incision could induce anxiety and amygdala sensitization that can be inhibited by morphine and gabapentin. Thus treatment of surgery-induced affective disturbances by morphine and gabapentin may be a potential important adjunct therapy in the postoperative pain management.

The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 inhibits inflammatory pain-induced and incision-induced hypersensitivity in rat

This study characterized the contribution of metabotropic glutamate receptor 7 (mGlu7 receptor) activation to the development of inflammatory hyperalgesia and allodynia, using a novel, systemically active mGlu7 receptor allosteric agonist, N, N'-dibenzhydrylethane-1,2-diamine dihydrochloride (AMN082). The effects of AMN082 (0.1, 1 or 5 mg/kg, intraperitoneally; 5 or 50 nmol, intrathecally) or diclofenac (5 mg/kg, intraperitoneally) administered 30 min preprocedure or 3 h postprocedure on hindpaw withdrawal latency (in seconds) to thermal stimulation, and response threshold (in grams) to mechanical stimulation, were measured in adult rats (n = 6-8 per group) before and up to 24 h after intradermal injection of carrageenan into the hindpaw or hindpaw incision. Precarrageenan injection of 1 and 5 mg/kg AMN082, but not diclofenac inhibited thermal hyperalgesia, whereas postcarrageenan, both AMN082 and diclofenac attenuated thermal hyperalgesia and allodynia. In the paw incision model, presurgical and postsurgical administration of 1 and 5 mg/kg AMN082 inhibited thermal hyperalgesia, but not allodynia, whereas diclofenac was effective in attenuating both thermal hyperalgesia and allodynia but only when administered postsurgically. Intrathecal injection of AMN082 postcarrageenan and postsurgery also significantly attenuated thermal hyperalgesia. Enhancing endogenous mGlu7 receptor activity inhibits postinjury stimulus-evoked hypersensitivity and may be of therapeutic benefit for the treatment of inflammatory and incision-induced pain.

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

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

Spinal Microglial Expression and Mechanical Hypersensitivity in a Postoperative Pain Model: Comparison with a Neuropathic Pain Model

Postoperative pain control contributes to quality of life. Activation of spinal cord microglia after peripheral nerve injury contributes to mechanical hypersensitivity. The contribution of spinal cord microglia to hypersensitivity after surgery, however, is not well understood. Here, the authors evaluated whether inhibition of spinal microglia reduced postoperative mechanical hypersensitivity, and if so, whether the effect differed from that in a rat neuropathic pain model. Male Sprague-Dawley rats underwent either unilateral plantar hind paw incision (postoperative pain model) or L5 spinal nerve transection (neuropathic pain model), and the development of mechanical hypersensitivity was assessed using von Frey filaments. The microglial inhibitor minocycline was intraperitoneally administered daily for either 3 or 7 days. Spinal microglial activation was evaluated by OX42 immunohistochemistry. We also tested the effect of intrathecal administration of a p38 mitogen-activated protein kinase inhibitor, SB203580. In the postoperative pain model, minocycline did not suppress mechanical hypersensitivity, but did inhibit an increase in spinal OX42 expression. In contrast, in the neuropathic pain model, minocycline reduced mechanical hypersensitivity in a dose-related manner and inhibited spinal OX42 expression. SB203580 attenuated hypersensitivity in the neuropathic pain model, but not in the postoperative pain model. The results of the present study suggest that spinal OX42 expression has a more important role in the development of neuropathic pain than in postoperative pain, and that an increase in spinal OX42 expression does not contribute to postoperative mechanical hypersensitivity.

Role of substance P signaling in enhanced nociceptive sensitization and local cytokine production after incision

Substance P (SP) signaling facilitates nociceptive sensitization in various inflammatory and chronic pain models and we postulated that SP signaling might also contribute to the development of post-incisional hyperalgesia. These studies used mice with a deletion of the pre-protachykinin A gene ( ppt-A −/−) which codes for SP to determine the role of SP signaling in post-incisional pain and in the increased cytokine and nerve growth factor (NGF) expression observed in the incised skin. SP deficient ppt-A −/− mice displayed reduced mechanical allodynia and heat hyperalgesia compared to the wild-type ( wt) mice at all post-incision time points, despite similar baseline values ( p < 0.001). Furthermore, the NK-1 receptor antagonist LY303870 attenuated mechanical allodynia produced by incision in the wt mice ( p < 0.001). Incision also up-regulated IL-6, TNF-α and KC levels but not IL-1β after 2 h in the wt mice skin. However, ppt-A −/− mice had more skin NGF levels 2 h post-incision. Subcutaneous hind paw SP injection produced acute and transient elevations of IL-1β, IL-6, and KC but modest elevations in TNF-α levels in the wt mice. Systemic LY303870 reversed the SP-induced elevations of these cytokines. Hind paw injection of IL-6 and NGF dose dependently produced less mechanical allodynia in the ppt-A −/− compared to wt mice. Additionally, SP produced mechanical allodynia in a dose-dependent fashion in wt mice. Therefore, SP supports nociceptive sensitization after hind paw incision and potentially participates directly in modulating the intensity of inflammatory response in peri-incisional tissue.

Long-Term Pain Vulnerability After Surgery in Rats: Prevention by Nefopam, an Analgesic with Antihyperalgesic Properties

Tissue damage associated with surgery often produces peripheral and central sensitization that may outlast the stimuli, leading to exaggerated postoperative pain. Paradoxically, the use of opioid analgesia, which is essential for surgical pain management may induce pain sensitization leading to enhanced postoperative pain and an increased risk of developing chronic pain. We studied whether a surgical incision in the rat hindpaw may favor the development of long-term pain vulnerability by estimating hyperalgesia induced by an inflammatory stimulation of the unlesioned contralateral hindpaw 3 wk later. We also evaluated the ability of nefopam, an analgesic drug commonly used in postoperative pain management, to prevent not only exaggerated postoperative pain but also long-term pain vulnerability. The efficacy of morphine was assessed 1 day after surgical incision. On Day 0, a surgical plantar incision was performed in one hindpaw of rats treated or untreated with fentanyl (4 x 100 microg/kg, one injection every 15 min). Nefopam (10 mg/kg) or saline was subcutaneously injected 30 min before injury. Three weeks later, once pain measures had returned to basal values, a subsequent nociceptive stimulus, specifically intraplantar carrageenan injection, was performed to evaluate pain sensitivity in incision- and fentanyl-experienced rats. Pain was measured by the paw-pressure vocalization test and the weight bearing test. Surgical incision in rats induced latent and long-term pain hypersensitivity, which was manifested by exaggerated hyperalgesia on carrageenan injection. Administering fentanyl in association with the surgical incision induced exaggerated postoperative pain. When injected before incision, nefopam reduced the exaggerated postoperative pain induced by perioperative fentanyl treatment and prevented the development of long-term pain hypersensitivity. Preoperative nefopam administration also improved morphine analgesic efficacy in the context of fentanyl-induced postoperative hyperalgesia. Given preemptively, nefopam may be effective at improving postoperative pain management and at reducing the risk of developing postoperative chronic pain, because the drug has both analgesic and antihyperalgesic properties.

POSTOPERATIVE ANALGESIA INDUCED BY INTRATHECAL NEOSTIGMINE OR BETHANECHOL IN RATS

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

Electrophysiologic Characteristics of Large Neurons in Dorsal Root Ganglia during Development and after Hind Paw Incision in the Rat

Withdrawal thresholds in the paw are lower in younger animals, and incision further reduces these thresholds. The authors hypothesized that these differences result in part from changes in intrinsic electrophysiologic properties of large neurons. Using isolated whole dorsal root ganglion, current clamping was performed to determine the electrophysiologic properties of large neurons before and after incision in animals aged 1 and 4 weeks. Mechanical withdrawal thresholds were used to follow paw sensitivity. After paw incision, withdrawal thresholds decreased to a similar degree at both ages, but returned to control threshold at 72 h only in the 1-week-old animals. The resting membrane potential was less negative and the rheobase and the resistance of the membrane were lower at baseline in the 1-week-old animals (P < 0.05). After incision, the membrane potential became more depolarized and the rheobase was less in both ages. These changes remained 72 h after the incision in both ages. These findings suggest that lower mechanical thresholds in the younger animals may be partially attributed to the intrinsic electrophysiologic properties of the larger-diameter afferent neurons. The lack of resolution of the electrophysiologic changes in the young despite the resolution of the withdrawal response suggests that continued input from large fibers into the central nervous system may occur at this age despite the apparent resolution of behavioral changes. Further studies are needed to determine the etiology of these differences, their impact in the central nervous system, and whether theses changes can be prevented.

Upregulation of Dorsal Horn Microglial Cyclooxygenase-1 and Neuronal Cyclooxygenase-2 After Thoracic Deep Muscle Incisions in the Rat

Plantar hindpaw incision produces hyperalgesia, transient upregulation of cyclooxygenase-2 (COX-2) and prolonged upregulation of cyclooxygenase-1 (COX-1) in rat lumbar spinal cord. Our hypothesis in this study was that a deep thoracic incision causes COX-1 and COX-2 upregulation in the dorsal horn coincident with pain-related behavior, and that specific cell types contribute to this increase in COX expression. A left lateral thoracic skin incision was made in anesthetized rats, and superficial and deep muscles were incised. Postoperative pain-related behavior was quantified by recording exploratory rearing. Four and 24 h postsurgery, COX-1 and COX-2 immunohistochemistry, with co-labeling for cell type, were performed on the spinal cord. Deep thoracic muscle incision produced a 42% decrease in rearing compared to sham skin-incision controls at 4 h postsurgery (P = 0.001). There was an increase in both COX-1 and COX-2 immunoreactivity in the thoracic dorsal horn at 4 h postsurgery on the ipsilateral side of surgery animals compared to the ipsilateral side of control animals, contralateral side of surgery animals or contralateral side of control animals. No surgery-induced differences were seen at the lumbar level. At 24 h postsurgery, there was no longer a decrease in rearing, and no surgery-induced differences in COX-1 or COX-2 were seen at any level. At 4 h postsurgery, 96% of COX-1 immunoreactive cells co-localized with microglia and 98% of COX-2 immunoreactive cells co-localized with neurons. A unilateral deep thoracic wound produces pain-related behavior and, at the same time, ipsilateral upregulation of microglial COX-1 and neuronal COX-2 in the thoracic dorsal horn.

Chronic Morphine Administration Enhances Nociceptive Sensitivity and Local Cytokine Production after Incision

Background -The chronic use of opioids prior to surgery leads to lowered pain thresholds and exaggerated pain levels after these procedures. Several mechanisms have been proposed to explain this heightened sensitivity commonly termed opioid-induced hyperalgesia (OIH). Most of these proposed mechanisms involve plastic events in the central or peripheral nervous systems. Alterations in the abundance of peripheral mediators of nociception have not previously been explored.Results -In these experiments mice were treated with saline (control) or ascending daily doses of morphine to generate a state of OIH followed by hind paw incision. In other experiments morphine treatment was initiated at the time of incision. Both mechanical allodynia and peri-incisional skin cytokine levels were measured. Myeloperoxidase (MPO) assays were used to determine neutrophil activity near the wounds. The cytokine production inhibitor pentoxifylline was used to determine the functional significance of the excess cytokines in previously morphine treated animals. Mice treated chronically treated with morphine prior to incision were found to have enhanced skin levels of IL-1β, IL-6, G-CSF, KC and TNFα after incision at one or more time points compared to saline pretreated controls. The time courses of individual cytokines followed different patterns. There was no discernable effect of chronic morphine treatment on wound area neutrophil infiltration. Pentoxifylline reduced cytokine levels and reversed the excess mechanical sensitization caused by chronic morphine administration prior to incision. Morphine treatment initiated at the time of incision did not lead to a generalized enhancement of cytokine production or nociceptive sensitization in excess of the levels observed after incision alone.Conclusion -The enhanced level of nociceptive sensitization seen after incision in animals chronically exposed to morphine is associated with elevated levels of several cytokines previously reported to be relevant to this incisional pain model. The cytokines may be functional in supporting nociceptive sensitization because pentoxifylline reverses both peri-incisional skin cytokine levels and OIH. Opioid administration beginning at the time of incision does not seem to have the same cytokine enhancing effect. Approaches to postoperative pain control involving a reduction of cytokines may be an effective way to control excessive pain in patients chronically using opioids prior to surgical procedures.

Brain Derived Neurotrophic Factor (BDNF) Contributes to the Pain Hypersensitivity Following Surgical Incision in the Rats

BackgroundThe pathogenic role of brain derived neurotrophic factor (BDNF) in the incisional pain is poorly understood. The present study explores the role of the BDNF in the incision-induced pain hypersensitivity.MethodsA longitudinal incision was made in one plantar hind paw of isoflurane-anesthetized rats. Dorsal root ganglias (DRG) and spinal cords were removed at various postoperative times (1–72 h). Expression pattern of BDNF was determined by immunohistochemistry and double-labeling immunofluorescence. Lidocaine-induced blockade of sciatic nerve function was used to determine the importance of afferent nerve activity on BDNF expression in the DRG and spinal cord after incision. BDNF antibody was administered intrathecally (IT) or intraperitoneal (IP) to modulate the spinal BDNF or peripheral BDNF after incision.ResultsAfter hind-paw incision, the BDNF was upregulated in the ipsilateral lumbar DRG and spinal cord whereas thoracic BDNF remained unchanged in response to incision. The upregulated BDNF was mainly expressed in the large-sized neurons in DRG and the neurons and the primary nerve terminals in the spinal cord. Sciatic nerve blockade prevented the increase of BDNF in the DRG and spinal cord. IT injection of BDNF antibody greatly inhibited the mechanical allodynia induced by incision whereas IP administration had only marginal effect.ConclusionThe present study showed that incision induced the segmental upregulation of BDNF in the DRG and spinal cord through somatic afferent nerve transmission, and the upregulated BDNF contributed to the pain hypersensitivity induced by surgical incision.

Hind Paw Incision in the Rat Produces Long-Lasting Colon Hypersensitivity

Visceral injury has been shown to alter somatic sensitivity, but little is known about the effect of somatic insult on the viscera. In the present study, we examined (1) the effect of colon inflammation on somatic sensitivity and (2) the affect of hind paw incision on colon sensitivity. After intracolonic administration of trinitrobenzene sulfonic acid (TNBS) or zymosan, visceromotor responses to colorectal distension were increased to post-treatment day 8. Mechanical withdrawal thresholds in the hind paw were decreased in TNBS- and in zymosan-treated rats until post-intracolonic treatment day 2. There was no change in hind paw heat withdrawal latency in either group. Plantar incision of the hind paw resulted in a decrease in both hind paw mechanical withdrawal threshold and heat withdrawal latency and significantly increased the visceromotor response to colorectal distension from postincision days 1 to 8. The colon hypersensitivity was of longer duration than hyperalgesia at the site of hind paw incision. These results support the hypothesis that somatic injury and visceral inflammation can alter central processing of visceral and somatic inputs, respectively. Surgical procedures are common and typically associated with hyperalgesia at and around the site of incision. This report establishes in a model of postsurgical pain and hyperalgesia that a long-lasting visceral hypersensitivity may also accompany postsurgical hyperalgesia.

Oral administration of Ginkgo biloba extract, EGb‐761 inhibits thermal hyperalgesia in rodent models of inflammatory and post‐surgical pain

Studies in vitro suggest that the standardised extract of Ginkgo biloba, EGb-761 has anti-inflammatory properties and modulatory effects on key pain-related molecules. This study investigated the analgesic and anti-inflammatory effects of EGb-761 on carrageenan-induced inflammatory and hindpaw incisional pain. Adult male Wistar rats (n=6-10/group; 250-420 g) were injected intradermally with carrageenan into the left hindpaw or anaesthetised with isoflurane (2%) and a longitudinal 1 cm incision was made through the skin, fascia and plantaris muscle of the hindpaw. EGb-761 (3, 10, 30, 100 or 300 mg kg(-1)), diclofenac (5 mg kg(-1)) or drug-vehicle was administered 3 h post-carrageenan/post-surgery. Hindpaw withdrawal latency (in seconds) to thermal stimulation, response threshold (in grams) to mechanical stimulation and paw volume were measured. Carrageenan induced significant mechanical allodynia, thermal hyperalgesia and paw oedema at 6 h post-carrageenan, while paw incision surgery induced significant mechanical allodynia and thermal hyperalgesia at 6 and 24 h post-surgery. Administration of EGb-761 dose-dependently inhibited thermal hyperalgesia and was equally effective as diclofenac (5 mg kg(-1)) in both the carrageenan and hindpaw incision model. EGb-761 had no effect on carrageenan- or incision-induced mechanical allodynia or paw oedema. Diclofenac significantly reduced mechanical allodynia in both models and carrageenan-induced paw oedema. EGb-761 dose-dependently alleviates acute inflammatory and surgically induced thermal hyperalgesia and is comparable to diclofenac, a commonly prescribed non-steroidal anti-inflammatory drug. This indicates that EGb-761 has analgesic potential in acute inflammatory pain.

Modulation of persistent nociceptive inputs in the anterior pretectal nucleus of the rat

The anterior pretectal nucleus (APtN) participates in nociceptive and antinociceptive mechanisms. Drugs were injected into the ventral APtN to evaluate how intrinsic mechanisms interact in the nucleus during persistent allodynia produced by a surgical incision in a rat hind paw. Naloxone (1 and 10 ng/0.08 μl), methysergide (0.037 and 3.7 ng/0.08 μl) or atropine (0.1 and 10 ng/0.08 μl) increased the allodynia. The effect of methysergide was intensified by naloxone or atropine, the effect of atropine was intensified by naloxone or methysergide, but the effect of naloxone was not changed by methysergide or atropine. DAMGO (1.5 μg/0.08 μl), oxotremorine (5 μg/0.08 μl) or serotonin (5 μg/0.08 μl) reduced the allodynia. The effect of DAMGO was less intense in methysergide-treated rats but was not changed in atropine-treated rats, the effect of serotonin was not changed by naloxone or atropine, and the effect of oxotremorine was not changed by naloxone or methysergide. Baclofen (150 ng/0.08 μl) increased, whereas phaclofen (300 ng/0.1 μl) reduced the allodynia. Bicuculline (50 ng/0.08 μl) increased the incision pain, while muscimol (50 ng/0.08 μl) did not change it. Phaclofen was inhibited by methysergide but was unchanged by atropine. The effect of DAMGO was reduced by phaclofen (100 ng/0.1 μl). We interpret these results as indicative that noxious inputs utilize cholinergic and serotonergic pathways in the vAPtN for the activation of descending pain control mechanisms, the serotonergic pathway being under the control of GABAergic neurons which, in turn, are modulated negatively by opioid nerve terminals.

Spinal Glial Activation Contributes to Postoperative Mechanical Hypersensitivity in the Rat

The activation of spinal cord microglia and astrocytes after peripheral nerve injury or inflammation contributes to behavioral hypersensitivity. The contribution of spinal cord glia to mechanical hypersensitivity after hind paw incision has not been investigated previously. Male Sprague-Dawley rats underwent a unilateral plantar hind paw incision, and the development of mechanical hypersensitivity was assessed by using von Frey filaments. The activation of spinal cord microglia and astrocytes was measured 1, 2, 3, and 5 days after hind paw incision by using immunohistochemistry. The glial activation inhibitor, fluorocitrate, was administered intrathecally 24 hours after hind paw incision to determine glial involvement in mechanical hypersensitivity. Hind paw incision induced an activation of spinal astrocytes ipsilateral to incision within 24 hours. Both microglia and astrocytes reached a maximum activation 3 days after hind paw incision. Fluorocitrate produced a dose-dependent reduction in mechanical hypersensitivity when administered 24 hours after hind paw incision. Spinal cord glial activation contributes to the mechanical hypersensitivity that develops after hind paw incision. Perspective Hind paw incision produces mechanical hypersensitivity that can be alleviated with the inhibition of spinal cord glia. Our results suggest that the activation of spinal cord astrocytes within 24 hours of incision contributes to mechanical hypersensitivity. Therefore, spinal cord astrocytes might represent a novel target for the treatment of postoperative pain.

Lactate Concentrations in Incisions Indicate Ischemic-like Conditions May Contribute to Postoperative Pain

The substances in wounds that cause incisional pain and hyperalgesia after surgery are poorly understood. We have developed and characterized rat models for incision-induced pain behaviors and measured increased tissue hydrogen ion concentration. Because lactate may facilitate nociceptor responses to low pH and contribute to ischemic pain mechanisms, we measured tissue lactate after incision of the plantar region of the hindpaw, gastrocnemius muscle, and paraspinal region in halothane anesthetized rats using in vivo microdialysis. Incisions were performed at 1 site (plantar, gastrocnemius, or paraspinal incision) in each rat. The corresponding contralateral side was used as the control. In anesthetized rats, a microdialysis fiber was passed into the incision and the control side. L-Lactate was measured using the lactate oxidase method. Tissue concentration was determined from postoperative day 0 to postoperative day 14 using the no net flux method. Lactate was increased on the day of hindpaw incision to 3.6 ± 1.6 mmol/L compared with control (2.1 ± .6 mmol/L) and remained increased through 7 days. In the gastrocnemius muscle, lactate was increased the day after incision (4.2 ± 1.2 mmol/L vs 1.7 ± .5 mmol/L) until postoperative day 7. On the day of the paraspinal incision, lactate was 3.4 ± 1.1 mmol/L on the operated side and 2.2 ± .6 mmol/L in the control side. Lactate remained increased through postoperative day 8 at the paraspinal incision. These experiments demonstrate that incision of the plantar hindpaw, the gastrocnemius muscle, and the paraspinal region increased tissue lactate concentration. The wound environment contains increased lactate at the same time that pH is decreased; lactate could potentially facilitate nociceptor activation by low pH and contribute to pain after surgery. Perspective This study demonstrates that lactate is increased in wounds when pain behaviors and acid are increased. Lactate and low pH are present in incisions and indicate an ischemic pain mechanism that may contribute to postsurgical pain.

(634): Epidural Ngx 424, an ampa-kainate receptor antagonist, produces analgesia in rats after plantar incision

Excitatory amino acid receptors are important for sensory transmission in the spinal cord. We studied the analgesic effects of epidural administration of NGX424, a competitive non-methyl-D-aspartate (nonNMDA) ionotropic excitatory amino acid (EAA) receptor antagonist, in normal rats and in rats after hindpaw incision. After institutional approval, rats were acclimated to testing for withdrawal to radiant heat applied to the hindpaw. Then using halothane anesthesia, PE10 catheters were inserted into the lumbar epidural space using a small lumbar incision.

Glial Activation and Segmental Upregulation of Interleukin-1β (IL-1β) in the Rat Spinal Cord after Surgical Incision

The present study investigated the expression patterns of glial cells and interleukin-1beta (IL-1beta) in the rat spinal cord after a surgical incision, which is closely related with clinical postoperative pain. Microglia and astrocytes became activated in the spinal cord following incision. Real-time polymerase chain reaction (PCR) and immunohistochemisty showed that IL-1beta mRNA and protein level in the spinal cord was transiently upregulated after surgical incision. The increased IL-1beta-immunoreactivity (IR) was mainly localized in neurons but not the activated microglia or astrocytes. Although obvious increase in IL-1beta-IR could be observed in the lumbar segments of the spinal cord ipsilateral to a hind paw incision, significant upregulation of IL-1beta was not detected in the lumbar segments following thoracic incision. The present study indicated that surgical incision could induce glial activation and segmental upregulation of IL-1beta in the spinal cord. The activated glial cells and upregulated IL-1beta, in turn, may be involved in the incision-induced pain hypersensitivity.