Evidence for a role of endocannabinoids, astrocytes and p38 phosphorylation in the resolution of postoperative pain

Additive antinociceptive action of intrathecal anandamide reuptake inhibitor and morphine in the management of post-incisional pain in rats

Spinal opioids have mixed efficacy and their adverse effects force treatment cessation of postoperative pain. Consequently, there is an ongoing search for new therapeutic strategies. Here, we evaluated the analgesic efficacy of intrathecal UCM707, an anandamide reuptake inhibitor, and morphine combination. Firstly, we assessed the effects of morphine (1, 5 and 10 μg), UCM707 (75 μg) and its combination in the hot plate. Then, morphine + UCM707 at sub-effective doses was evaluated in a rat post-incisional pain model. In addition, μ-, CB1r-, CB2r- and TRPV1-antagonists were pre-administered before the combination. Activation of μ-opioid and CB1r, and Cnr1, Cnr2, Oprm1 and TRPV1 expressions were evaluated in the lumbar sacra and periaqueductal grey by [35 S]-GTPγS binding autoradiography and qPCR studies. In the hot plate, morphine (1 μg) and UCM707 (75 μg) induced a more robust analgesic effect than each drug alone. Morphine plus UCM707 did not modify μ-opioid nor CB1 receptor function in the PAG or LS. Cnr1 and TRPV1 expression increased in the lumbar sacra (LS). Morphine plus UCM707 significantly reduced post-incisional pain at 1 and 4 days after surgery. Cnr1, Cnr2 and TRPV1 expressions increased in the LS. Blockade of μ-opioid receptor reduced combination effects on days 1 and 4. CB1r- and CB2r-antagonism reduced morphine + UCM707 effects on days 1 and 4, respectively. CB1r and TRPV1-antagonism improved their antinociceptive effects on day 4. These results revealed a synergistic/additive analgesic effect of UCM707 and morphine combination controlling postincisional pain. CB1r, CB2r and TRPV1 contribute differently as central sensitization occurs.

Structure-based identification of a G protein-biased allosteric modulator of cannabinoid receptor CB1

Cannabis sativa is known for its therapeutic benefit in various diseases including pain relief by targeting cannabinoid receptors. The primary component of cannabis, Δ9-tetrahydrocannabinol (THC), and other agonists engage the orthosteric site of CB1, activating both Gi and β-arrestin signaling pathways. The activation of diverse pathways could result in on-target side effects and cannabis addiction, which may hinder therapeutic potential. A significant challenge in pharmacology is the design of a ligand that can modulate specific signaling of CB1. By leveraging insights from the structure-function selectivity relationship (SFSR), we have identified Gi signaling-biased agonist-allosteric modulators (ago-BAMs). Further, two cryoelectron microscopy (cryo-EM) structures reveal the binding mode of ago-BAM at the extrahelical allosteric site of CB1. Combining mutagenesis and pharmacological studies, we elucidated the detailed mechanism of ago-BAM-mediated biased signaling. Notably, ago-BAM CB-05 demonstrated analgesic efficacy with fewer side effects, minimal drug toxicity and no cannabis addiction in mouse pain models. In summary, our finding not only suggests that ago-BAMs of CB1 provide a potential nonopioid strategy for pain management but also sheds light on BAM identification for GPCRs.

Cannabinoid Analgesia in Postoperative Pain Management: From Molecular Mechanisms to Clinical Reality

Postoperative pain (POP) is a challenging clinical phenomenon that affects the majority of surgical patients and demands effective management to mitigate adverse outcomes such as persistent pain. The primary goal of POP management is to alleviate suffering and facilitate a seamless return to normal function for the patient. Despite compelling evidence of its drawbacks, opioid analgesia remains the basis of POP treatment. Novel therapeutic approaches rely on multimodal analgesia, integrating different pharmacological strategies to optimize efficacy while minimizing adverse effects. The recognition of the imperative role of the endocannabinoid system in pain regulation has prompted the investigation of cannabinoid compounds as a new therapeutic avenue. Cannabinoids may serve as adjuvants, enhancing the analgesic effects of other drugs and potentially replacing or at least reducing the dependence on other long-term analgesics in pain management. This narrative review succinctly summarizes pertinent information on the molecular mechanisms, clinical therapeutic benefits, and considerations associated with the plausible use of various cannabinoid compounds in treating POP. According to the available evidence, cannabinoid compounds modulate specific molecular mechanisms intimately involved in POP. However, only two of the eleven clinical trials that evaluated the efficacy of different cannabinoid interventions showed positive results.

The New Orientation of Postoperative Analgesia: Remote Ischemic Preconditioning

Purpose of Review

Postoperative analgesia is currently a significant topic in anesthesiology. Currently, the predominant approach for achieving multimodal analgesia involves the utilization of pharmacotherapy and regional anesthesia procedures. The primary objectives of this approach are to mitigate postoperative pain, enhance patient satisfaction, and diminish overall opioid usage. Nevertheless, there is a scarcity of research on the use of remote ischemia preconditioning aimed at mitigating postoperative pain.

Recent Findings

Transient stoppage of blood flow to an organ has been found to elicit remote ischemia preconditioning (RIPC), which serves as a potent intrinsic mechanism for protecting numerous organs. In addition to its established role in protecting against reperfusion injury, RIPC has recently been identified as having potential benefits in the context of postoperative analgesia.

Summary

In addition to traditional perioperative analgesia, RIPC provides perioperative analgesia and organ protection.

Pain response to cannabidiol in induced acute nociceptive pain, allodynia, and hyperalgesia by using a model mimicking acute pain in healthy adults in a randomized trial (CANAB I)

ABSTRACT

Preclinical studies have demonstrated the analgesic potential of cannabidiol (CBD). Those suggesting an effect on pain-processing receptors have brought CBD back into focus. This study assessed the effect of CBD on acute pain, hyperalgesia, and allodynia compared with placebo. Twenty healthy volunteers were included in this randomized, placebo-controlled, double-blinded, crossover study assessing pain intensities (using numeric rating scale), secondary hyperalgesia (von Frey filament), and allodynia (dry cotton swab) in a well-established acute pain model with intradermal electrical stimulation. The authors compared the effect of 800-mg orally administered CBD on pain compared with placebo. They further examined the effect on hyperalgesia and allodynia. Cannabidiol whole blood levels were also measured. Pain ratings (mean ± SD) did not differ significantly after CBD application compared with placebo (5.2 ± 0.7 vs 5.3 ± 0.7, P-value 0.928), neither did the areas of hyperalgesia and allodynia differ significantly after CBD application compared with placebo (hyperalgesia 23.9 ± 19.2 cm2 vs 27.4 ± 17.0 cm2, P-value 0.597; allodynia 16.6 ± 13.1 cm2 vs 17.3 ± 14.1 cm2, P-value 0.884). The CBD whole blood level (median, first to third quartile) was 2.0 µg/L (1.5-5.1) 60 minutes and 5.0 µg/L (4.0-10.4) 130 minutes after CBD application. Although the oral application of 800-mg CBD failed to show a significant effect, it is important to focus future research on different dosing, routes of administration, and CBD as a part of multimodal treatment strategies before negating its effects on acute pain.

Immunomodulatory Role of CB2 Receptors in Emotional and Cognitive Disorders

Emotional behavior, memory, and learning have been associated with alterations in the immune system in neuropsychiatric and neurodegenerative diseases. In recent years, several studies pointed out the involvement of the cannabinoid receptor 2 (CB2r) in the immune system and the regulation of inflammation. This receptor is widely distributed in different tissues and organs with higher expression in spleen and immune system cells. However, CB2r has also been detected in several brain areas and different brain cell types, such as neurons and glia. These findings suggest that CB2r may closely relate the immune system and the brain circuits regulating inflammation, mood, and cognitive functions. Therefore, we review the studies that may help elucidate the molecular bases of CB2r in regulating inflammation in different brain cells and its role in the pathophysiology of psychiatric and neurodegenerative disorders.

Electroacupuncture Alleviates Hyperalgesia by Regulating CB1 Receptor of Spinal Cord in Incisional Neck Pain Rats

Acupuncture therapy is effective in relieving postoperative pain of neck surgery, but its underlying mechanisms remain largely unknown. This study, in the incisional neck pain rat model, was designed to explore whether the endocannabinoid receptor 1 (CB1) in the cervical spinal cord is involved in the analgesic effect of electroacupuncture (EA) or not.The incisional neck pain model was established by making a longitudinal incision and applied EA treatment of Futu (LI18), Hegu-Neiguan (LI4-PC6), or Zusanli-Yanglingquan (ST36-GB34) for pain relief. The results showed that EA LI18 and EA LI4-PC6 effectively relieve allodynia caused by neck incision, which was obviously better than EA ST34-GB34 (P < 0.05). After EA, the expression levels of CB1 mRNA at 4h in the EALI18 group, and 24 and 48h in both EALI18 and EALI4-PC6 groups, and those of CB1 protein at 4, 24, and 48h in the EALI18 group, and the immunoactivity of CB1 in both EALI18 and EALI4-PC6 groups at 4h were significantly upregulated in contrast to those of the model group (P < 0.05). EA of either acupoint group had no effect on the expression of CB2 protein (P > 0.05). Moreover, the antinociceptive effect of EA was reversed by AM251 (CB1 antagonist). Immunofluorescence dual staining showed that CB1 expressed in astrocytes in the superficial layer (laminae I and II) of dorsal horns of the cervical spinal cord. Therefore, the findings of this study revealed that upregulation of CB1 expression in the cervical spinal cord contributes to the analgesic effect of EA in incisional neck pain rats. The CB1 receptor expresses on astrocytes.

Pharmacologic agents directed at the treatment of pain associated with maladaptive neuronal plasticity

INTRODUCTION

The definition of nociplastic pain in 2016 has changed the way maladaptive chronic pain is viewed in that it may emerge without neural lesions or neural disease. Many endogenous and pharmacologic substances are being investigated for their role in treating the pain associated with neuronal plasticity.

AREAS COVERED

The authors review promising pharmacologic agents for the treatment of pain associated with maladaptive neuronal plasticity. The authors then provide the reader with their expert opinion and provide their perspectives for the future.

EXPERT OPINION

An imbalance between the amplification of ascending pain signals and the poor activation of descending inhibitory signals may be at the root of many chronic pain syndromes. The inhibitory activity of noradrenaline reuptake may play a role in neuropathic and nociplastic analgesia. A better understanding of the brain's pain matrix, its signaling cascades, and the complex bidirectional communication between the immune system and the nervous system may help meet the urgent and unmet medical need for safe, effective chronic pain treatment, particularly for pain with a neuropathic and/or nociplastic component.

Interactions of Echinacea spp. Root Extracts and Alkylamides With the Endocannabinoid System and Peripheral Inflammatory Pain

Historical ethnobotanies of indigenous peoples of the North American prairies reveal treatment of many painful conditions by Echinacea spp. Recent evidence suggests a pharmacological basis for such use as the bioactivity of E. angustifolia and E. purpurea is mediated, in part, through activation of the endocannabinoid system (ECS). Whereas the cannabimimetic effects of individual echinacea products and alkylamides have been described, the activity of crude extracts has not been compared between cannabinoid (CB) receptors or across species or genotypes. Moreover, few studies have explored echinacea's engagement of the ECS for historic treatments or new therapeutic applications in peripheral inflammatory pain. We hypothesized that 1) the in vitro effects of root extracts on CB receptor internalization would vary with species and phytochemistry, and that echinacea root extracts would reduce inflammatory pain in vivo through activation of the ECS. Root extracts of different E. angustifolia and E. purpurea accessions were prepared, analyzed by HPLC-DAD to quantify caffeic acid derivatives and alkylamides (AKA), and tested for agonist and antagonist activities using receptor redistribution assays. Linear regression of activity relative to phytochemistry identified predictive compounds that were assessed individually in redistribution assays. Extracts were evaluated in the Hargreaves model of chronic inflammatory pain in rats with co-administration of selective CB1/2 antagonists to gauge involvement of the ECS. CB receptor agonist activity varied among accessions of both species with linear regression revealing a significant relationship between CB1 activity and AKA2 for E angustifolia, and AKA 9 + 10 for E purpurea. CB2 activity was positively related with AKA 9 + 10 and total AKAs in E. angustifolia. Four isolated AKA demonstrated agonist activity in the CB2, but not CB1, assay. In the inflammatory pain model, oral administration of either E angustifolia or E. purpurea root extract produced dose-dependent analgesic effects that were partially reversed by co-administration of CB receptor antagonists. This study demonstrates that in vitro effects of crude echinacea root extracts on CB receptors is predicted by phytochemistry. In vivo, echinacea has potential applications for peripheral inflammatory pain such as arthritis and burns, reflecting the traditional uses of Indigenous North Americans.

Chronic Pain in Dogs and Cats: Is There Place for Dietary Intervention with Micro-Palmitoylethanolamide?

The management of chronic pain is an integral challenge of small animal veterinary practitioners. Multiple pharmacological agents are usually employed to treat maladaptive pain including opiates, non-steroidal anti-inflammatory drugs, anticonvulsants, antidepressants, and others. In order to limit adverse effects and tolerance development, they are often combined with non-pharmacologic measures such as acupuncture and dietary interventions. Accumulating evidence suggests that non-neuronal cells such as mast cells and microglia play active roles in the pathogenesis of maladaptive pain. Accordingly, these cells are currently viewed as potential new targets for managing chronic pain. Palmitoylethanolamide is an endocannabinoid-like compound found in several food sources and considered a body's own analgesic. The receptor-dependent control of non-neuronal cells mediates the pain-relieving effect of palmitoylethanolamide. Accumulating evidence shows the anti-hyperalgesic effect of supplemented palmitoylethanolamide, especially in the micronized and co-micronized formulations (i.e., micro-palmitoylethanolamide), which allow for higher bioavailability. In the present paper, the role of non-neuronal cells in pain signaling is discussed and a large number of studies on the effect of palmitoylethanolamide in inflammatory and neuropathic chronic pain are reviewed. Overall, available evidence suggests that there is place for micro-palmitoylethanolamide in the dietary management of chronic pain in dogs and cats.

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.

Characterization of cerebral cortical endocannabinoid levels in a rat inguinal surgery model using liquid chromatography-tandem mass spectrometry (LC-MS/MS)

BACKGROUND

The brain endocannabinoid system is believed to play significant roles in anti-nociception, fear response, anxiety, and stress. This study investigated the effects of rat inguinal surgery on the levels of endocannabinoids in the cerebral cortex.

AIM

The aim of this study was to investigate the effects of acute post-surgical pain on the levels of endocannabinoids in the cerebral cortex.

METHODS

Quantitation of endocannabinoids in the rat cerebral cortex was performed by liquid chromatography-tandem mass spectrometry.

RESULTS

There was no significant difference in the cerebral cortical levels of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) between the sham and surgery experimental groups. However, there were lateralized differences in the levels of these endocannabinoids between the right and left cerebral cortices irrespective of the two groups. The concentrations of AEA and 2-AG were significantly higher in the right cerebral cortex compared to the contralateral cerebral cortex.

CONCLUSION

Acute post-surgical pain did not induce significant alterations in the cerebral cortical levels of endocannabinoids in this study, but the phenomenon of lateralization of the cerebral cortical AEA and 2-AG levels was observed; this latter finding may be related to the role played by endocannabinoids in fear conditioning.

Propofol attenuates postoperative hyperalgesia via regulating spinal GluN2B-p38MAPK/EPAC1 pathway in an animal model of postoperative pain

BACKGROUND

Total intravenous anesthesia with propofol has been shown to reduce postoperative pain in some clinical studies, but knowledge of its underlying analgesic mechanism remains limited. In this study, we compared the analgesic effects of propofol versus isoflurane in an animal model of postoperative pain and evaluated its underlying molecular mechanisms.

METHODS

Plantar incision was made in the hind paws of rats under general anesthesia with 2.5% of inhalational isoflurane (isoflurane group) or intravenous infusion of propofol (1.5 mg kg^-1  min^-1 , propofol group). Mechanical allodynia was assessed by paw withdrawal threshold before and after incision. Spinal dorsal horns (L3-L5) were harvested 1 hr after incision to assess the level of phosphorylated GluN2B, p38MAPK, ERK, JNK, and EPAC using Western blot and immunofluorescence.

RESULTS

Mechanical allodynia induced by plantar incision peaked at 1 hr and lasted for 3 days after incision. It was significantly less in the propofol group compared with the isoflurane group in the first 2 hr following incision. The incision-induced increases in phosphorylated GluN2B, p38MAPK, and EPAC1 were significantly reduced in the propofol group. The number of spinal dorsal neurons co-expressed with EPAC1 and c-Fos after the incision was significantly lower in the propofol group.

CONCLUSION

Propofol reduced pain responses in an animal model of postoperative pain and suppressed the spinal GluN2B-p38MAPK/EPAC1 signaling pathway. Since the p38MAPK/EPAC pathway plays a critical role in the development of postoperative hyperalgesia, our results provide evidence-based behavioral, molecular, and cellular mechanisms for the analgesic effects of propofol when used for general anesthesia.

SIGNIFICANCE

These findings may provide a new mechanism for the postsurgical analgesic effect of propofol, which is particularly interesting during the subacute period after surgery as it is the critical period for the development of persistent postsurgical pain.

Electroacupuncture potentiates peripheral CB2 receptor-inhibited chronic pain in a mouse model of knee osteoarthritis

Purpose

Knee osteoarthritis (KOA) is a highly prevalent, chronic joint disorder, with chronic pain as its typical symptom. Although studies have shown that an activated peripheral CB2 receptor can reduce acute pain, whether the CB2 receptor is involved in electroacupuncture (EA) inhibiting chronic pain and the involved mechanism remains unclear. The aim of this study was to investigate whether EA may strengthen peripheral CB2 receptor-inhibited chronic pain in a mouse model of KOA.

Materials and methods:

KOA was induced by intra-articular injection of monosodium iodoacetate (MIA) into the left knee joint of mice. Thermal hyperalgesia was tested with the hot plate test, and mechanical allodynia was quantified using von Frey filaments. The expression of CB2 receptor and IL-1β were quantified by using immunofluorescence labeling.

Results

EA treatment at 2 Hz+1 mA significantly increased the expression of CB2 receptor in fibroblasts and decreased the expression of IL-1β in the menisci compared with that in the KOA group. However, EA had no effect on the expression of IL-1β in CB2^−/− mice. At 2 Hz+1 mA, EA significantly increased mechanical threshold, thermal latency, and weight borne after KOA modeling. However, knockout of the CB2 receptor blocked these effects of EA. After 2 Hz+1 mA treatment, EA significantly reduced the Osteoarthritis Research Society International (OARSI) score after KOA modeling. However, EA had no significant effect on the OARSI score in CB2^−/− mice.

Conclusion

EA reduced the expression of IL-1β by activating the CB2 receptor, thus inhibiting the chronic pain in the mouse model of KOA.

The role of the endocannabinoid system in the antihyperalgesic effect of Cedrus atlantica essential oil inhalation in a mouse model of postoperative pain

ETHNOPHARMACOLOGICAL RELEVANCE

Cedar is part of the phylum of conifers, and it's essential oil has been used for therapeutic purposes since ancient times. In our previous study, we have demonstrated that the inhalation of the Cedrus atlantica essential oil (CaEO) induces an antihyperalgesic effect in a model of postoperative pain. But the mechanism that underlies this effect is not yet fully known.

AIM OF THE STUDY

This study investigates the involvement of the endocannabinoid system in the antihyperalgesic effect produced by the inhalation of CaEO in a post operative pain model.

MATERIALS AND METHODS

Male Swiss mice (25-35±2g) were subjected to plantar incision. To assess the involvement of the endocannabinoid system, two different approaches were made: (1) by administering antagonists to the CB₁ and CB₂ receptors in different sites (intraperitoneal [i.p.], intraplantar [i.pl.] and intrathecal [i.t.]) and (2) by assessing the synergic effect of the inhalation of sub-effective doses of CaEO, Fatty acid hydrolase (FAAH) and Monoacylglycerol lipase (MAGL), and endocannabinoid degradation inhibitors (URB937 and JZL184, respectively).

RESULTS

The antihyperalgesic effect of CaEO inhalation was prevented by pretreatment with AM281 or AM630 given by i.p. and i.t., but not i.pl. Additionally, in mice pretreated with FAAH or the MAGL inhibitors, the antihyperalgesic effect of CaEO inhalation was significantly longer, which demonstrates the involvement of the endocannabinoid system in the antihyperalgesic effect of CaEO inhalation in a preclinical model of postoperative pain.

CONCLUSIONS

The present study shows that CaEO inhalation exerts an antihyperalgesic effect, possibly by the activation of the endocannabinoid system in a preclinical model of postoperative pain. It could be a new alternative to treat pain in a clinical environment.

The cannabinoid system and pain

Chronic pain states are highly prevalent and yet poorly controlled by currently available analgesics, representing an enormous clinical, societal, and economic burden. Existing pain medications have significant limitations and adverse effects including tolerance, dependence, gastrointestinal dysfunction, cognitive impairment, and a narrow therapeutic window, making the search for novel analgesics ever more important. In this article, we review the role of an important endogenous pain control system, the endocannabinoid (EC) system, in the sensory, emotional, and cognitive aspects of pain. Herein, we briefly cover the discovery of the EC system and its role in pain processing pathways, before concentrating on three areas of current major interest in EC pain research; 1. Pharmacological enhancement of endocannabinoid activity (via blockade of EC metabolism or allosteric modulation of CB1receptors); 2. The EC System and stress-induced modulation of pain; and 3. The EC system & medial prefrontal cortex (mPFC) dysfunction in pain states. Whilst we focus predominantly on the preclinical data, we also include extensive discussion of recent clinical failures of endocannabinoid-related therapies, the future potential of these approaches, and important directions for future research on the EC system and pain. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Peripheral inflammation affects modulation of nociceptive synaptic transmission in the spinal cord induced by N-arachidonoylphosphatidylethanolamine

BACKGROUND AND PURPOSE

Endocannabinoids play an important role in modulating spinal nociceptive signalling, crucial for the development of pain. The cannabinoid CB₁ receptor and the TRPV1 cation channel are both activated by the endocannabinoid anandamide, a product of biosynthesis from the endogenous lipid precursor N-arachidonoylphosphatidylethanolamine (20:4-NAPE). Here, we report CB₁ receptor- and TRPV1-mediated effects of 20:4-NAPE on spinal synaptic transmission in control and inflammatory conditions.

EXPERIMENTAL APPROACH

Spontaneous (sEPSCs) and dorsal root stimulation-evoked (eEPSCs) excitatory postsynaptic currents from superficial dorsal horn neurons in rat spinal cord slices were assessed. Peripheral inflammation was induced by carrageenan. Anandamide concentration was assessed by mass spectrometry.

KEY RESULTS

Application of 20:4-NAPE increased anandamide concentration in vitro. 20:4-NAPE (20 μM) decreased sEPSCs frequency and eEPSCs amplitude in control and inflammatory conditions. The inhibitory effect of 20:4-NAPE was sensitive to CB₁ receptor antagonist PF514273 (0.2 μM) in both conditions, but to the TRPV1 antagonist SB366791 (10 μM) only after inflammation. After inflammation, 20:4-NAPE increased sEPSCs frequency in the presence of PF514273 and this increase was blocked by SB366791.

CONCLUSIONS AND IMPLICATIONS

While 20:4-NAPE treatment inhibited the excitatory synaptic transmission in both naive and inflammatory conditions, peripheral inflammation altered the underlying mechanisms. Our data indicate that 20:4-NAPE application induced mainly CB₁ receptor-mediated inhibitory effects in naive animals while TRPV1-mediated mechanisms were also involved after inflammation. Increasing anandamide levels for analgesic purposes by applying substrate for its local synthesis may be more effective than systemic anandamide application or inhibition of its degradation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Mitogen-activated protein kinase phosphatase-3 (MKP-3) in the surgical wound is necessary for the resolution of postoperative pain in mice

Mitogen-activated protein kinase (MAPK) phosphatase-3 (MKP-3) and its substrates (extracellular signal-regulated kinase [ERK] and p38) play an important role in pathophysiological mechanisms of acute postoperative and chronic neuropathic pain in the spinal cord. This study aimed to understand the role of MKP-3 and its target MAPKs at the site of surgical incision in nociceptive behavior. Wild-type (WT) and MKP-3 knockout (KO) mice underwent unilateral plantar hind paw incision. Mechanical allodynia was assessed by using von Frey filaments. Peripheral ERK-1/2 and p38 phosphorylation were measured by Western blot. Cell infiltration was determined using hematoxylin and eosin histological staining. Peripheral phosphorylated ERK-1/2 (p-ERK-1/2) inhibition was performed in MKP-3 KO mice. In WT mice, mechanical hypersensitivity was observed on postoperative day 1 (0.69±0.17 g baseline vs 0.13±0.08 g day 1), which resolved normally by postoperative day 12 (0.46±0.08 g, N=6). In MKP-3 KO mice, this hypersensitivity persisted at least 12 days after surgery (0.19±0.06 g; N=6). KO mice displayed higher numbers of infiltrating cells (51.4±6 cells/0.1 mm²) than WT mice (8.7±1.2 cells/0.1 mm²) on postoperative day 1 (vs 5–6 cells/0.1 mm² at baseline) that returned to baseline 12 days after surgery (10–12 cells/0.1 mm²). In WT mice, peripheral p-p38 and p-ERK-1/2 expression increased (5- and 3-fold, respectively) on postoperative days 1 and 5, and returned to basal levels 7–12 days after surgery (N=3 per group). Peripheral p-p38 levels in MKP-3 KO mice followed a similar expression pattern as WT mice. Peripheral p-ERK-1/2 levels in MKP-3 KO mice remained elevated 12 days after surgery (2.5-fold, N=3 per group). Administration of PD98059 (MEK inhibitor, N=8, vehicle N=9) reduced p-ERK-1/2 expression in the incised tissue and blocked hypersensitivity in MKP-3 KO mice (N=6). The findings of this study suggest that MKP-3 is pivotal for normal resolution of acute postoperative allodynia, through the regulation of peripheral p-ERK-1/2.

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.

Transmission pathways and mediators as the basis for clinical pharmacology of pain

INTRODUCTION

Mediators in pain transmission are the targets of a multitude of different analgesic pharmaceuticals. This review explores the most significant mediators of pain transmission as well as the pharmaceuticals that act on them. Areas covered: The review explores many of the key mediators of pain transmission. In doing so, this review uncovers important areas for further research. It also highlights agents with potential for producing novel analgesics, probes important interactions between pain transmission pathways that could contribute to synergistic analgesia, and emphasizes transmission factors that participate in transforming acute injury into chronic pain. Expert commentary: This review examines current pain research, particularly in the context of identifying novel analgesics, highlighting interactions between analgesic transmission pathways, and discussing factors that may contribute to the development of chronic pain after an acute injury.

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.

Endocannabinoids and acute pain after total knee arthroplasty

Osteoarthritis (OA) of the knee is a progressive disease that is associated with inflammation of the joints and lower extremity pain. Total knee arthroplasty (TKA) is a surgical procedure that aims to reduce pain and restore motor function in patients suffering from OA. The immediate postoperative period can be intensely painful leading to extended recovery times including persistent pain. The endocannabinoid system regulates nociception, and the activation of cannabinoid receptors produces antinociceptive effects in preclinical models of OA. To date, the influence of the endocannabinoid tone on pain and disability in OA patients and on acute postoperative pain in humans has not been explored. In this study, we provide the first comprehensive profile of endocannabinoids in serum, cerebrospinal fluid, and synovial fluid of patients with painful end-stage OA undergoing TKA and examine correlations between endocannabinoid levels, interleukin 6, functional disability, acute postoperative pain, and postoperative opioid use. Our results reveal that central (cerebrospinal fluid) and peripheral (synovial fluid) levels of the endocannabinoid 2-arachidonoyl glycerol were significantly elevated in patients who developed higher postoperative pain after TKA. In addition, synovial fluid 2-arachidonoyl glycerol levels were positively correlated with postoperative opioid use. Similarly, synovial fluid levels of the anti-inflammatory lipid palmitoylethanolamide correlated with functional disability in OA. Taken together, our results are the first to reveal associations between central and peripheral endocannabinoid levels and postoperative pain. This suggests that endocannabinoid metabolism may serve as a target for the development of novel analgesics both for systemic or local delivery into the joint.

A membrane-delimited N-myristoylated CRMP2 peptide aptamer inhibits CaV2.2 trafficking and reverses inflammatory and postoperative pain behaviors

Targeting proteins within the N-type voltage-gated calcium channel (CaV2.2) complex has proven to be an effective strategy for developing novel pain therapeutics. We describe a novel peptide aptamer derived from the collapsin response mediator protein 2 (CRMP2), a CaV2.2-regulatory protein. Addition of a 14-carbon myristate group to the peptide (myr-tat-CBD3) tethered it to the membrane of primary sensory neurons near surface CaV2.2. Pull-down studies demonstrated that myr-tat-CBD3 peptide interfered with the CRMP2-CaV2.2 interaction. Quantitative confocal immunofluorescence revealed a pronounced reduction of CaV2.2 trafficking after myr-tat-CBD3 treatment and increased efficiency in disrupting CRMP2-CaV2.2 colocalization compared with peptide tat-CBD3. Consequently, myr-tat-CBD3 inhibited depolarization-induced calcium influx in sensory neurons. Voltage clamp electrophysiology experiments revealed a reduction of Ca, but not Na, currents in sensory neurons after myr-tat-CBD3 exposure. Current clamp electrophysiology experiments demonstrated a reduction in excitability of small-diameter dorsal root ganglion neurons after exposure to myr-tat-CBD3. Myr-tat-CBD3 was effective in significantly attenuating carrageenan-induced thermal hypersensitivity and reversing thermal hypersensitivity induced by a surgical incision of the plantar surface of the rat hind paw, a model of postoperative pain. These effects are compared with those of tat-CBD3-the nonmyristoylated tat-conjugated CRMP2 peptide as well as scrambled versions of CBD3 and CBD3-lacking control peptides. Our results demonstrate that the myristoyl tag enhances intracellular delivery and local concentration of the CRMP2 peptide aptamer near membrane-delimited calcium channels resulting in pronounced interference with the calcium channel complex, superior suppression of calcium influx, and better antinociceptive potential.

Update on the role of spinal cord TRPV1 receptors in pain modulation

The structure, expression and function of the transient receptor potential vanilloid 1 (TRPV1) receptor were intensively studied since the cloning in 1997 and TRPV1 receptors are now considered to act as transducers and molecular integrators of nociceptive stimuli in the periphery. In contrast, spinal TRPV1 receptors were studied less extensively and their role in pain modulation is still not fully understood. This short review is a follow up on our previous summary in this area (Spicarova and Palecek 2008). The aim was to review preferentially the most recent findings concerning the role of the spinal TRPV1 receptors, published within the last five years. The update is given on the expression and function of the spinal TRPV1 receptors, their activation by endogenous agonists, interaction between the endocannabinoid and endovanillod system and possible role of the spinal TRPV1 receptors in pathological pain states. There is now mounting evidence that TRPV1 receptors may be an important element in modulation of nociceptive information at the spinal cord level and represent an interesting target for analgesic therapy.

Endogenous analgesia, dependence, and latent pain sensitization

Endogenous activation of µ-opioid receptors (MORs) provides relief from acute pain. Recent studies have established that tissue inflammation produces latent pain sensitization (LS) that is masked by spinal MOR signaling for months, even after complete recovery from injury and re-establishment of normal pain thresholds. Disruption with MOR inverse agonists reinstates pain and precipitates cellular, somatic, and aversive signs of physical withdrawal; this phenomenon requires N-methyl-D-aspartate receptor-mediated activation of calcium-sensitive adenylyl cyclase type 1 (AC1). In this review, we present a new conceptual model of the transition from acute to chronic pain, based on the delicate balance between LS and endogenous analgesia that develops after painful tissue injury. First, injury activates pain pathways. Second, the spinal cord establishes MOR constitutive activity (MORCA) as it attempts to control pain. Third, over time, the body becomes dependent on MORCA, which paradoxically sensitizes pain pathways. Stress or injury escalates opposing inhibitory and excitatory influences on nociceptive processing as a pathological consequence of increased endogenous opioid tone. Pain begets MORCA begets pain vulnerability in a vicious cycle. The final result is a silent insidious state characterized by the escalation of two opposing excitatory and inhibitory influences on pain transmission: LS mediated by AC1 (which maintains the accelerator) and pain inhibition mediated by MORCA (which maintains the brake). This raises the prospect that opposing homeostatic interactions between MORCA analgesia and latent NMDAR-AC1-mediated pain sensitization creates a lasting vulnerability to develop chronic pain. Thus, chronic pain syndromes may result from a failure in constitutive signaling of spinal MORs and a loss of endogenous analgesic control. An overarching long-term therapeutic goal of future research is to alleviate chronic pain by either (a) facilitating endogenous opioid analgesia, thus restricting LS within a state of remission, or (b) extinguishing LS altogether.

Mitogen-activated protein kinase (MAPK) phosphatase-3 (MKP-3) displays a p-JNK-MAPK substrate preference in astrocytes in vitro

Mitogen-activated protein kinases (MAPKs) play critical roles in the central nervous system immune responses through glial function, which are regulated with relative selectivity (or preference) by MAPK phosphatases (MKP). Phosphorylated extracellular signal-regulated protein kinase (p-ERK) is preferentially dephosphorylated by MKP-3, which display little activity over p-p38 and p-c-Jun NH2-terminal kinases (p-JNK). It has been proposed that these substrate preferences may vary depending on tissue or functional cellular processes. Since astrocytes display a prominent activity of JNK>ERK under stressed or reactive phenotype, we hypothesize that MKP-3 possess a similar or differential substrate preference in astrocytes for JNK and ERK (ERK=JNK or JNK>ERK). We generated transient expression of MKP-3 by transfecting a specific cDNA in primary rat neonatal brain cortex astrocytes. Cells were stimulated with lipopolysaccharide (LPS), and MAPKs and downstream pro-inflammatory products were measured by Western blot and ELISA analyses. MKP-3 expression in primary astrocytes reduced LPS-induced p-ERK and p-p38 by ∼50%, and p-JNK by ∼75%, and moderately reduced nitrite oxide (NO), while completely blocked Interleukin (IL)-6 and tumor necrosis factor alpha (TNFα). We confirmed MKP-3 specific activity by developing a BV-2 microglia cell line stably overexpressing MKP-3 and using a specific siRNA against MKP-3. Our data demonstrate MKP-3 has differential substrate preference in astrocytes compared to other cells types, since it preferentially dephosphorylated p-JNK over p-ERK. Our results indicate also that astrocytic immune functions can be modulated by MKP-3 induction, a strategy that could be beneficial in neurological conditions in which astrocytes play a pathophysiological role, i.e. persistent pain.

Transcutaneous electric acupoint stimulation reduces intra-operative remifentanil consumption and alleviates postoperative side-effects in patients undergoing sinusotomy: a prospective, randomized, placebo-controlled trial

BACKGROUND

Although opioids are widely used as analgesics in general anaesthesia, they have unpleasant side-effects and can delay postoperative recovery. Acupuncture and related techniques are effective for acute and chronic pain, and reduces some side-effects. We assessed the effect of transcutaneous electric acupoint stimulation (TEAS) on intra-operative remifentanil consumption and the incidences of anaesthesia-related side-effects.

METHODS

Sixty patients undergoing sinusotomy were randomly assigned to TEAS or control group. TEAS consisted of 30 min of stimulation (6-9 mA, 2/10 Hz) on the Hegu (LI4), Neiguan (PC6), and Zusanli (ST36) before anaesthesia. The patients in the control group had the electrodes applied, but received no stimulation. Bispectral index was used to monitor the depth of anaesthesia. Perioperative haemodynamics were recorded, and peripheral blood samples were collected to measure the levels of mediators of surgical stress. The primary end point was intraoperative remifentanil consumption and the secondary endpoints were recovery quality and anaesthesia-related side-effects.

RESULTS

Patients in the TEAS group required 39% less remifentanil during surgery than controls [0.0907 (SD 0.026) μg kg(-1) min(-1) vs 0.051 (0.018) μg kg(-1) min(-1)]. There were no differences in intra-operative haemodynamics or surgical stress between groups. However, the time to extubation and recall in the control group was 16.8 (6.8) min and 23.0 (5.0) min, respectively, significantly longer than that in the TEAS group (P<0.01). TEAS also decreased the incidence of dizziness and pruritus within the first 24 h after surgery (P<0.01).

CONCLUSION

The use of TEAS significantly reduced intra-operative remifentanil consumption and alleviated postoperative side-effects in patients undergoing sinusotomy.

CLINICAL TRIAL REGISTRATION

The trial was registered at clinicaltrials.gov (NCT01700855).

Spinal mitogen-activated protein kinase phosphatase-3 (MKP-3) is necessary for the normal resolution of mechanical allodynia in a mouse model of acute postoperative pain

The mechanisms that drive the normal resolution of acute postoperative pain are not completely understood. We hypothesize a pivotal role of a major spinal mitogen-activated protein kinase (MAPKs) regulator, MAPK phosphatase (MKP)-3, in the resolution of postoperative pain. We used wild-type and MKP-3 knock-out (KO) mice, a paw incision model of acute postoperative pain, and behavioral and molecular biology experiments. We observed persistent mechanical allodynia in mice lacking MKP-3 (postoperative day 21), concurrently with persistent phosphorylation of spinal p38 and extracellular signal-regulated kinases (ERK)-1/2 on postoperative day 12, while both MAPK phosphorylation and allodynia resolved on postoperative day 7 in wild-type mice. Spinal p-ERK was expressed mainly in neurons and microglia, while spinal p-p38 was expressed mostly in microglia in MKP-3 KO mice, and their selective pharmacological inhibition reduced the persistent allodynia observed in these mice. Our findings strongly suggest that dysregulation of MKP-3 prevents spontaneous resolution of acute postoperative pain and drives its transition to persistent pain via persistent neuronal and microglial MAPK phosphorylation in the spinal cord.

Endocannabinoid system and pain: an introduction

The endocannabinoid (EC) system consists of two main receptors: cannabinoid type 1 receptor cannabinoid receptors are found in both the central nervous system (CNS) and periphery, whereas the cannabinoid type 2 receptor cannabinoid receptor is found principally in the immune system and to a lesser extent in the CNS. The EC family consists of two classes of well characterised ligands; the N-acyl ethanolamines, such as N-arachidonoyl ethanolamide or anandamide (AEA), and the monoacylglycerols, such as 2-arachidonoyl glycerol. The various synthetic and catabolic pathways for these enzymes have been (with the exception of AEA synthesis) elucidated. To date, much work has examined the role of EC in nociceptive processing and the potential of targeting the EC system to produce analgesia. Cannabinoid receptors and ligands are found at almost every level of the pain pathway from peripheral sites, such as peripheral nerves and immune cells, to central integration sites such as the spinal cord, and higher brain regions such as the periaqueductal grey and the rostral ventrolateral medulla associated with descending control of pain. EC have been shown to induce analgesia in preclinical models of acute nociception and chronic pain states. The purpose of this review is to critically evaluate the evidence for the role of EC in the pain pathway and the therapeutic potential of EC to produce analgesia. We also review the present clinical work conducted with EC, and examine whether targeting the EC system might offer a novel target for analgesics, and also potentially disease-modifying interventions for pathophysiological pain states.

Synergistic activation of lipopolysaccharide-stimulated glial cells by propofol

Despite the extensive use of propofol in general anesthetic procedures, the effects of propofol on glial cell were not completely understood. In lipopolysaccharide (LPS)-stimulated rat primary astrocytes and BV2 microglial cell lines, co-treatment of propofol synergistically induced inflammatory activation as evidenced by the increased production of NO, ROS and expression of iNOS, MMP-9 and several cytokines. Propofol augmented the activation of JNK and p38 MAPKs induced by LPS and the synergistic activation of glial cells by propofol was prevented by pretreatment of JNK and p38 inhibitors. When we treated BV2 cell culture supernatants treated with LPS plus propofol on cultured rat primary neuron, it induced a significant neuronal cell death. The results suggest that the repeated use of propofol in immunologically challenged situation may induce glial activation in brain.

Ankle joint mobilization affects postoperative pain through peripheral and central adenosine A1 receptors

BACKGROUND

Physical therapists frequently use joint mobilization therapy techniques to treat people with musculoskeletal dysfunction and pain. Several studies suggest that endogenous adenosine may act in an analgesic fashion in various pain states.

OBJECTIVE

The purpose of this study was to investigate the contribution of the adenosinergic system on the antihyperalgesic effect of ankle joint mobilization (AJM).

DESIGN

This was a experimental study.

METHODS

To test the hypothesis that the adrenosinergic system is involved in the antihyperalgesic effect of AJM, mice (25-35 g) submitted to plantar incision surgery were used as a model of acute postoperative pain. The mice were subjected to AJM for 9 minutes. Withdrawal frequency to mechanical stimuli was assessed 24 hours after plantar incision surgery and 30 minutes after AJM, adenosine, clonidine, or morphine treatments. The adenosinergic system was assessed by systemic (intraperitoneal), central (intrathecal), and peripheral (intraplantar) administration of caffeine. The participation of the A1 receptor was investigated using a selective adenosine A1 receptor subtype antagonist. In addition, previous data on the involvement of the serotonergic and noradrenergic systems in the antihyperalgesic effect of AJM were confirmed.

RESULTS

Ankle joint mobilization decreased mechanical hyperalgesia, and this effect was reversed by pretreatment of the animals with caffeine given by intraperitoneal, intraplantar, and intrathecal routes. In addition, intraplanar and intrathecal administrations of 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, a selective adenosine A1 subtype receptor antagonist) or systemic administration of yohimbine or ρ-chlorophenylalanine methyl ester hydrochloride (PCPA) blocked the antihyperalgesia induced by AJM.

LIMITATIONS

The results are limited to animal models and cannot be generalized to acute pain in humans.

CONCLUSIONS

This study demonstrated the involvement of the adenosinergic system in the antihyperalgesic effect of AJM in a rodent model of pain and provides a possible mechanism basis for AJM-induced relief of acute pain.

Monoacylglycerol lipase - a target for drug development?

The endocannabinoid (eCB) system is involved in processes as diverse as control of appetite, perception of pain and the limitation of cancer cell growth and invasion. The enzymes responsible for eCB breakdown are attractive pharmacological targets, and fatty acid amide hydrolase inhibitors, which potentiate the levels of the eCB anandamide, are now undergoing pharmaceutical development. 'Drugable' selective inhibitors of monoacylglycerol lipase, a key enzyme regulating the levels of the other main eCB, 2-arachidonoylglycerol, were however not identified until very recently. Their availability has resulted in a large expansion of our knowledge concerning the pharmacological consequences of monoacylglycerol lipase inhibition and hence the role(s) played by the enzyme in the body. In this review, the pharmacology of monoacylglycerol lipase will be discussed, together with an analysis of the therapeutic potential of monoacylglycerol lipase inhibitors as analgesics and anticancer agents.

Spinal cannabinoid receptor type 2 agonist reduces mechanical allodynia and induces mitogen-activated protein kinase phosphatases in a rat model of neuropathic pain

Peripheral nerve injury generally results in spinal neuronal and glial plastic changes associated with chronic behavioral hypersensitivity. Spinal mitogen-activated protein kinases (MAPKs), eg, p38 or extracellular signal-regulated kinases (ERKs), are instrumental in the development of chronic allodynia in rodents, and new p38 inhibitors have shown potential in acute and neuropathic pain patients. We have previously shown that the cannabinoid type 2 receptor agonist JWH015 inhibits ERK activity by inducing MAPK phosphatase (MKP)-1 and MKP-3 (the major regulators of MAPKs) in vitro in microglial cells. Therefore, we decided to investigate the role of these phosphatases in the mechanisms of action of JWH015 in vivo using the rat L5 nerve transection model of neuropathic pain. We observed that peripheral nerve injury reduced spinal MKP-1/3 expression and activity and that intrathecal JWH015 reduced established L5 nerve injury-induced allodynia, enhanced spinal MKP-1/3 expression and activity, and reduced the phosphorylated form of p38 and ERK-1/2. Triptolide, a pharmacological blocker of MKP-1 and MKP-3 expression, inhibited JWH015's effects, suggesting that JWH015 exerts its antinociceptive effects by modulating MKP-1 and MKP-3. JWH015-induced antinociception and MKP-1 and MKP-3 expression were inhibited by the cannabinoid type 2 receptor antagonist AM630. Our data suggest that MKP-1 and MKP-3 are potential targets for novel analgesic drugs.

PERSPECTIVE

MAPKs are pivotal in the development of chronic allodynia in rodent models of neuropathic pain. A cannabinoid type 2 receptor agonist, JWH015, reduced neuropathic allodynia in rats by reducing MAPK phosphorylation and inducing spinal MAPK phosphatases 1 and 3, the major regulators of MAPKs.

Spinal interleukin-10 therapy to treat peripheral neuropathic pain

INTRODUCTION

  Current research indicates that chronic peripheral neuropathic pain includes a role for glia and the actions of proinflammatory factors. This review briefly discusses the glial and cytokine responses that occur following peripheral nerve damage in support of utilizing anti-inflammatory cytokine interleukin-10 (IL-10) therapy to suppress chronic peripheral neuropathic pain. SPINAL NONVIRAL INTERLEUKIN-10 GENE THERAPY:  IL-10 is one of the most powerful endogenous counter-regulators of proinflammatory cytokine function that acts in the nervous system. Subarachnoid (intrathecal) spinal injection of the gene encoding IL-10 delivered by nonviral vectors has several advantages over virally mediated gene transfer methods and leads to profound pain relief in several animal models. NONVIRAL GENE DELIVERY:  Lastly, data are reviewed that nonviral deoxyribonucleic acid (DNA) encapsulated by a biologically safe copolymer, poly(lactic-co-glycolic) acid (PLGA), thought to protect DNA, leads to significantly improved therapeutic gene transfer in animal models, which additionally and significantly extends pain relief.

CONCLUSIONS

  The impact of these early studies exploring anti-inflammatory genes emphasizes the exceptional therapeutic potential of new biocompatible intrathecal nonviral gene delivery approaches such as PLGA microparticles. Ultimately, ongoing expression of therapeutic genes is a viable option to treat chronic neuropathic pain in the clinic.

Mitogen activated protein kinase phosphatase-1 prevents the development of tactile sensitivity in a rodent model of neuropathic pain

Background

Neuropathic pain due to nerve injury is one of the most difficult types of pain to treat. Following peripheral nerve injury, neuronal and glial plastic changes contribute to central sensitization and perpetuation of mechanical hypersensitivity in rodents. The mitogen activated protein kinase (MAPK) family is pivotal in this spinal cord plasticity. MAPK phosphatases (MKPs) limit inflammatory processes by dephosphorylating MAPKs. For example, MKP-1 preferentially dephosphorylates p-p38. Since spinal p-p38 is pivotal for the development of chronic hypersensitivity in rodent models of pain, and p-p38 inhibitors have shown clinical potential in acute and chronic pain patients, we hypothesize that induction of spinal MKP-1 will prevent the development of peripheral nerve-injury-induced hypersensitivity and p-p38 overexpression.

Results

We cloned rat spinal cord MKP-1 and optimize MKP-1 cDNA in vitro using transfections to BV-2 cells. We observed that in vitro overexpression of MKP-1 blocked lipopolysaccharide-induced phosphorylation of p38 (and other MAPKs) as well as release of pro-algesic effectors (i.e., cytokines, chemokines, nitric oxide). Using this cDNA MKP-1 and a non-viral, in vivo nanoparticle transfection approach, we found that spinal cord overexpression of MKP-1 prevented development of peripheral nerve-injury-induced tactile hypersensitivity and reduced pro-inflammatory cytokines and chemokines and the phosphorylated form of p38.

Conclusions

Our results indicate that MKP-1, the natural regulator of p-p38, mediates resolution of the spinal cord pro-inflammatory milieu induced by peripheral nerve injury, resulting in prevention of chronic mechanical hypersensitivity. We propose that MKP-1 is a potential therapeutic target for pain treatment or prevention.

Shifts in cell-type expression accompany a diminishing role of spinal p38-mapkinase activation over time during prolonged postoperative pain

BACKGROUND

Surgery often causes prolonged postoperative pain, the mechanisms of which are unknown. The authors investigated the role of p38, a pain-associated mitogen-activated protein kinase, in induction and maintenance of such pain.

METHODS

Male rats were subjected to the skin-muscle incision retraction procedure at the saphenous region; the procedure causes ~4 weeks of secondary tactile hyperalgesia in the ipsilateral plantar region, indicating central sensitization. The spinal cord was sectioned from L3 and L4 + L5 vertebral segments and stained for activated p38 (P-p38) at postoperative day 3 (POD 3), just as secondary hyperalgesia develops; at PODs 10-12, the time of maximum hyperalgesia; and at POD 35, after the resolution of hyperalgesia. Some sections were costained for microglia, astrocytes, and neurons. Intrathecal injections of a P-p38 inhibitor were performed at POD 2 or POD 9, and subsequent changes in pain were monitored.

RESULTS

Skin-muscle incision retraction increased the numbers of dorsal horn P-p38 positive cells in L3 by ~3-fold and in L4 + L5 by ~7-fold from POD 3 to PODs 11-12. This increase was accompanied by a shift from microglia to neurons, resulting in a ~20-fold increase in P-p38-positive neurons in L4-L5 over this time. No P-p38 was detected in astrocytes. A P-p38 inhibitor given at POD 2 prevented development of secondary hypersensitivity, but when given at POD 9 the same dose gave weak relief of pain for less than 3 h.

CONCLUSIONS

Spinal P-p38 mitogen-activated protein kinase, activated after incision retraction, is important for the induction of prolonged pain, but despite increased pain near the time of maximum pain, its functional importance for the maintenance of pain is not great.

Prospects for cannabinoid therapies in basal ganglia disorders

Cannabinoids are promising medicines to slow down disease progression in neurodegenerative disorders including Parkinson's disease (PD) and Huntington's disease (HD), two of the most important disorders affecting the basal ganglia. Two pharmacological profiles have been proposed for cannabinoids being effective in these disorders. On the one hand, cannabinoids like Δ(9) -tetrahydrocannabinol or cannabidiol protect nigral or striatal neurons in experimental models of both disorders, in which oxidative injury is a prominent cytotoxic mechanism. This effect could be exerted, at least in part, through mechanisms independent of CB(1) and CB(2) receptors and involving the control of endogenous antioxidant defences. On the other hand, the activation of CB(2) receptors leads to a slower progression of neurodegeneration in both disorders. This effect would be exerted by limiting the toxicity of microglial cells for neurons and, in particular, by reducing the generation of proinflammatory factors. It is important to mention that CB(2) receptors have been identified in the healthy brain, mainly in glial elements and, to a lesser extent, in certain subpopulations of neurons, and that they are dramatically up-regulated in response to damaging stimuli, which supports the idea that the cannabinoid system behaves as an endogenous neuroprotective system. This CB(2) receptor up-regulation has been found in many neurodegenerative disorders including HD and PD, which supports the beneficial effects found for CB(2) receptor agonists in both disorders. In conclusion, the evidence reported so far supports that those cannabinoids having antioxidant properties and/or capability to activate CB(2) receptors may represent promising therapeutic agents in HD and PD, thus deserving a prompt clinical evaluation.

The Central Role of Glia in Pathological Pain and the Potential of Targeting the Cannabinoid 2 Receptor for Pain Relief

Under normal conditions, acute pain processing consists of well-characterized neuronal signaling events. When dysfunctional pain signaling occurs, pathological pain ensues. Glial activation and their released factors participate in the mediation of pathological pain. The use of cannabinoid compounds for pain relief is currently an area of great interest for both basic scientists and physicians. These compounds, bind mainly either the cannabinoid receptor subtype 1 (CB(1)R) or cannabinoid receptor subtype 2 (CB(2)R) and are able to modulate pain. Although cannabinoids were initially only thought to modulate pain via neuronal mechanisms within the central nervous system, strong evidence now supports that CB(2)R cannabinoid compounds are capable of modulating glia, (e.g. astrocytes and microglia) for pain relief. However, the mechanisms underlying cannabinoid receptor-mediated pain relief remain largely unknown. An emerging body of evidence supports that CB(2)R agonist compounds may prove to be powerful novel therapeutic candidates for the treatment of chronic pain.

Antinociceptive effects induced through the stimulation of spinal cannabinoid type 2 receptors in chronically inflamed mice

The stimulation of spinal cannabinoid type 2 (CB(2)) receptors is a suitable strategy for the alleviation of experimental pain symptoms. Several reports have described the up-regulation of spinal cannabinoid CB(2) receptors in neuropathic settings together with the analgesic effects derived from their activation. Besides, we have recently reported in two murine bone cancer models that the intrathecal administration of cannabinoid CB(2) receptor agonists completely abolishes hyperalgesia and allodynia, whereas spinal cannabinoid CB(2) receptor expression remains unaltered. The present experiments were designed to measure the expression of spinal cannabinoid CB(2) receptors as well as the analgesic efficacy derived from their stimulation in mice chronically inflamed by the intraplantar injection of complete Freund's adjuvant 1 week before. Both spinal cannabinoid CB(2) receptors mRNA measured by real-time PCR and cannabinoid CB(2) receptor protein levels measured by western blot remained unaltered in inflamed mice. Besides, the intrathecal (i.t.) administration of the cannabinoid CB(2) receptor agonists AM1241, (R,S)-3-(2-Iodo-5-nitrobenzoyl)-1-(1-methyl-2-piperidinylmethyl)-1H-indole, (0.03-1 μg) and JWH 133, (6aR,10aR)-3-(1,1-Dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]pyran, (3-30 μg) dose-dependently blocked inflammatory thermal hyperalgesia and mechanical allodynia. The analgesic effects induced by both agonists were counteracted by the coadministration of the selective cannabinoid CB(2) receptor antagonist SR144528, 5-(4-chloro-3-methylphenyl)-1-[(4-methylphenyl)methyl]-N-[(1S,2S,4R)-1,3,3-trimethylbicyclo[2.2.1]hept-2-yl]-1H-pyrazole-3-carboxamide, (5 μg) but not by the cannabinoid CB(1) receptor antagonist AM251, N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide, (10 μg). The effects induced by AM1241 were also inhibited by the coadministration of the opioid receptor antagonist, naloxone (1 μg). These results demonstrate that effective analgesia can be achieved in chronic inflammatory settings through the stimulation of spinal cannabinoid CB(2) receptors even if this receptor population is not up-regulated.

An update on analgesics

Recent introduction of new analgesics into the clinic is best described as a slow process with activity classified into two main areas: improving analgesic efficacy/potency and reducing side-effect profile. This review article describes some of the recent advances with an emphasis on use in the acute setting. In this respect, opioids continue to be the mainstay (but not the only) analgesic and there have been important improvements in their clinical effect profile. For example, tapentadol has been introduced as a mixed opioid and norepinephrine uptake inhibitor which, unlike tramadol, does not require metabolic activation and does not suffer from isomer-dependent pharmacodynamics. Opioid antagonists have received much attention recently either used alone, methylnaltrexone (s.c) or alvimopan (p.o), or in combination, Targinact (oxycodone/naloxone), and appear to be effective in reducing opioid side-effects such as those in the gastrointestinal tract. Other agents where there has been recent development include the use of gabapentin, methylxanthines, and local anaesthetics. An interesting area of translation of basic research is in the inhibition of breakdown of endogenous opioids with opiorphin, targeting of the endocannabinoid system, and the use of ampakines to obtund opioid-induced side-effects. It is clear that there is still much work to be done, but the need for highly efficacious analgesics with good side-effect profile remains.