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α2δ-1 (encoded by the Cacna2d1 gene) is a newly discovered NMDA receptor-interacting protein and is the therapeutic target of gabapentinoids (e.g., gabapentin and pregabalin) frequently used for treating patients with neuropathic pain. Nerve injury causes sustained α2δ-1 upregulation in the dorsal root ganglion (DRG), which promotes NMDA receptor synaptic trafficking and activation in the spinal dorsal horn, a hallmark of chronic neuropathic pain. However, little is known about how nerve injury initiates and maintains the high expression level of α2δ-1 to sustain chronic pain. Here, we show that nerve injury caused histone hyperacetylation and diminished enrichment of histone deacetylase-2 (HDAC2), but not HDAC3, at the Cacna2d1 promoter in the DRG. Strikingly, Hdac2 knockdown or conditional knockout in DRG neurons in male and female mice consistently induced long-lasting mechanical pain hypersensitivity, which was readily reversed by blocking NMDA receptors, inhibiting α2δ-1 with gabapentin or disrupting the α2δ-1-NMDA receptor interaction at the spinal cord level. Hdac2 deletion in DRG neurons increased histone acetylation levels at the Cacna2d1 promoter, upregulated α2δ-1 in the DRG, and potentiated α2δ-1-dependent NMDA receptor activity at primary afferent central terminals in the spinal dorsal horn. Correspondingly, Hdac2 knockdown-induced pain hypersensitivity was blunted in Cacna2d1 knockout mice. Thus, our findings reveal that HDAC2 functions as a pivotal transcriptional repressor of neuropathic pain via constitutively suppressing α2δ-1 expression and ensuing presynaptic NMDA receptor activity in the spinal cord. HDAC2 enrichment levels at the Cacna2d1 promoter in DRG neurons constitute a unique epigenetic mechanism that governs acute-to-chronic pain transition. SIGNIFICANCE STATEMENT Excess α2δ-1 proteins produced after nerve injury directly interact with glutamate NMDA receptors to potentiate synaptic NMDA receptor activity in the spinal cord, a prominent mechanism of nerve pain. Because α2δ-1 upregulation after nerve injury is long lasting, gabapentinoids relieve pain symptoms only temporarily. Our study demonstrates for the first time the unexpected role of intrinsic HDAC2 activity at the α2δ-1 gene promoter in limiting α2δ-1 gene transcription, NMDA receptor-dependent synaptic plasticity, and chronic pain development after nerve injury. These findings challenge the prevailing view about the role of general HDAC activity in promoting chronic pain. Restoring the repressive HDAC2 function and/or reducing histone acetylation at the α2δ-1 gene promoter in primary sensory neurons could lead to long-lasting relief of nerve pain., (Copyright © 2022 the authors.)

Background: Hyperalgesic priming (HP) is a model of the transition from acute to chronic pain. Electroacupuncture (EA) could inhibit pain development through the peripheral dorsal root ganglia; however, it is unclear whether it can mitigate the transition from acute to chronic pain by attenuating protein expression in the p38 MAPK (mitogen-activated protein kinase)/tumour necrosis factor alpha (TNF-α) pathway in the spinal dorsal horn., Aims: We aimed to determine whether EA could prevent the transition from acute to chronic pain by affecting the p38 MAPK/TNF-α pathway in the spinal dorsal horn in a rat model established using HP., Methods: We first randomly subdivided 30 male Sprague-Dawley (SD) rats into 5 groups ( n = 6 per group): control (N), sham HP (Sham-HP), HP, HP + SB203580p38 MAPK (HP+SB203580), and HP + Lenalidomide (CC-5013) (HP+Lenalidomide). We then randomly subdivided a further 30 male SD rats into 5 groups ( n = 6 per group): Sham-HP, HP, sham EA (Sham EA), EA (EA), and EA + U-46619 p38 MAPK agonist (EA+U-46619). We assessed the effects of EA on the mechanical paw withdrawal threshold and p38 MAPK/TNF-α expression in the spinal dorsal horn of rats subjected to chronic inflammatory pain., Results: Rats in the EA group had reduced p38 MAPK and TNF-α expression and had significantly reduced mechanical hyperalgesia compared with rats in the other groups., Conclusion: Our findings indicate that EA could increase the mechanical pain threshold in rats and inhibit the transition from acute pain to chronic pain. This mechanism could involve reduced p38 MAPK/TNF-α expression in the spinal dorsal horn.

The myeloid differentiation factor 88 (MyD88) adaptor mediates signaling by Toll-like receptors and some interleukins (ILs) in neural and non-neuronal cells. Recently, MyD88 protein was found to express in primary sensory neurons and be involved in the maintenance of persistent pain induced by complete Freund's adjuvant, chronic constriction injury and chemotherapy treatment in rodents. However, whether MyD88 in nociceptive neurons contributes to persistent pain induced by intraplantar injection of formalin remains elusive. Here, using conditional knockout (CKO) mice, we found that selective deletion of Myd88 in Nav1.8-expressing primary nociceptive neurons led to reduced pain response in the recovery phase of 1% formalin-induced mechanical pain and impaired the persistent thermal pain. Moreover, CKO mice exhibited reduced phase II pain response in 1%, but not 5%, formalin-induced acute inflammatory pain. Finally, nociceptor MyD88 deletion resulted in less neuronal c-Fos activation in spinal dorsal horns following 1% formalin stimulation. These data suggest that MyD88 in nociceptive neurons is not only involved in persistent mechanical pain but also promotes the transition from acute inflammatory pain to persistent thermal hyperalgesia induced by low-dose formalin stimulation., (Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.)

The effective prevention of postoperative cognitive dysfunction (POCD) needs to be explored, and the effect of preoperative pain on POCD remains unclear. We established a chronic pain model induced by chronic constriction injury (CCI) and models of acute pain and anxiety without pain in mice that were subsequently subjected to partial hepatectomy surgery. Morris water maze (MWM) tests were performed to evaluate the learning and memory abilities of the mice. ELISA was used to measure IL-1β, IL-6, and TNF-α in serum, and HPLC-MS was used to detect neurotransmitters in the prefrontal cortices and hippocampi of the mice. The results indicated that chronic pain induced by CCI might have significantly impaired the learning and memory abilities of mice, while acute pain and anxiety without pain only affected the memory abilities of mice. Perioperative acute pain increased the level of IL-1β in serum, and CCI might have increased the level of IL-6. CCI and acute pain increased dopamine (DA) levels in the cortex, similar to anxiety. Like anxiety, CCI increased 5-hydroxytryptamine (5-HT) levels in the prefrontal cortex and hippocampus. Acute pain led to a decrease in the acetylcholine (ACH) level in the hippocampus. Our results suggest that acute pain and CCI-induced chronic pain might aggravate postoperative cognitive dysfunction via neurotransmitters and by changing the levels of inflammatory factors such as IL-1β and IL-6.

Decreased dopaminergic activity and increased kappa opioid activity in the mesolimbic system underlie the negative emotional states related to chronic pain. However, it is not known whether these changes are just consequence of chronic pain or contribute to the sensorial changes associated with chronic pain. In this study, we asked whether the mesolimbic dopamine and kappa opioid systems contribute to the development and maintenance of chronic hyperalgesia, one of the most common sensorial changes related to chronic pain. The lesion of the dopaminergic cells of the ventral tegmental area prevented the transition from acute to chronic hyperalgesia when performed in pain-free rats, but did not affect the maintenance of chronic hyperalgesia, when performed in chronic pain in rats. As hyperalgesia becomes chronic, the dopamine levels in the nucleus accumbens decrease. The blockade of the kappa opioid receptors in the nucleus accumbens both prevented and reversed the development of chronic hyperalgesia, but did not affect its maintenance. Complementarily, the pharmacological activation of the kappa opioid receptors in the nucleus accumbens facilitated the transition from acute to chronic hyperalgesia. None of these interventions affected acute hyperalgesia. These findings suggest that the mesolimbic dopamine and kappa opioid systems specifically drive the pain chronification process, without affecting acute pain or the maintenance of chronic pain., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

This study aimed to evaluate whether the development and/or maintenance of chronic-latent muscle hyperalgesia is modulated by P2X3 receptors. We also evaluate the expression of P2X3 receptors and PKCε of dorsal root ganglions during these processes. A mouse model of chronic-latent muscle hyperalgesia, induced by carrageenan and evidenced by PGE 2 , was used. Mechanical muscle hyperalgesia was measured by Randall-Selitto analgesimeter. The involvement of P2X3 receptors was analyzed by using the selective P2X3 receptors antagonist A-317491 by intramuscular or intrathecal injections. Expression of P2X3 and PKCε in dorsal root ganglion (L4-S1) were evaluated by Western blotting. Intrathecal blockade of P2X3 receptors previously to carrageenan prevented the development and maintenance of acute and chronic-latent muscle hyperalgesia, while intramuscular blockade of P2X3 receptors previously to carrageenan only reduced the acute muscle hyperalgesia and had no effect on chronic-latent muscle hyperalgesia. Intrathecal, but not intramuscular, blockade of P2X3 receptors immediately before PGE 2 , in animals previously sensitized by carrageenan, reversed the chronic-latent muscle hyperalgesia. There was an increase in total and phosphorylated PKCε 48 h after the beginning of acute muscle hyperalgesia, and in P2X3 receptors at the period of chronic muscle hyperalgesia. P2X3 receptors expressed on spinal cord dorsal horn contribute to transition from acute to chronic muscle pain. We also suggest an interaction of PKCε and P2X3 receptors in this process. Therefore, we point out P2X3 receptors of the spinal cord dorsal horn as a pharmacological target to prevent the development or reverse the chronic muscle pain conditions.

The use of opioids for the relief of pain and headache disorders has been studied for years. Nowadays, particularly because of its ability to produce analgesia in various pain models, delta opioid receptor (DOPr) emerges as a promising target for the development of new pain therapies. Indeed, their potential to avoid the unwanted effects commonly observed with clinically used opioids acting at the mu opioid receptor (MOPr) suggests that DOPr agonists could be a therapeutic option. In this review, we discuss the use of opioids in the management of pain in addition to describing the evidence of the analgesic potency of DOPr agonists in animal models.

Background: Tissue injuries such as surgery and trauma are usually accompanied by simultaneous development of acute pain, which typically resolves along with tissue healing. However, in many cases, acute pain does not resolve despite proper tissue repair; rather, it transitions to chronic pain. In this study, we examined whether proliferator-activated receptor-gamma coactivator-1α (PGC-1α), a master regulator of mitochondria biogenesis, is implicated in pain chronification after burn injury in mice., Methods: We used PGC-1α and littermates PGC-1α mice of both sex. Burn injury was induced on these mice. Hindpaw mechanical withdrawal thresholds and thermal withdrawal latency were examined., Results: Hindpaw mechanical withdrawal thresholds and thermal withdrawal latencies were comparable at baseline between PGC-1α and PGC-1α mice. After burn injury, both PGC-1α and PGC-1α mice exhibited an initial dramatic decrease of withdrawal parameters at days 3 and 5 after injury. While PGC-1α mice fully recovered their withdrawal parameters to preinjury levels by days 11-14, PGC-1α mice failed to recover those parameters during the same time frame, regardless of sex. Moreover, we found that PGC-1α mice resolved tissue inflammation in a similar fashion to PGC-1α mice using a chemiluminescence-based reactive oxygen species imaging technique., Conclusions: Taken together, our data suggest that PGC-1α haploinsufficiency promotes pain chronification after burn injury.

Adenosine is a signaling molecule induced under stress such as energy insufficiency and ischemic/hypoxic conditions. Adenosine controls multiple physiological and pathological cellular and tissue function by activation of four G protein-coupled receptors (GPCR). Functional role of adenosine signaling in acute pain has been widely studied. However, the role of adenosine signaling in chronic pain is poorly understood. At acute levels, adenosine can be beneficial to anti-pain whereas a sustained elevation of adenosine can be detrimental to promote chronic pain. In recent years, extensive progress has been made to define the role of adenosine signaling in chronic pain and to dissect molecular new insight underlying the development of chronic pain. In this review, we summarize the differential role of adenosine signaling cascade in acute and chronic pain with a major focus on recent studies revealing adenosine ADORA2B receptor activation in the pathology of chronic pain. We further provide a therapeutic outlook of how multiple adenosine signaling components can be useful to treat chronic pain., (Copyright © 2019. Published by Elsevier B.V.)

Transient receptor melastatin 2 (TRPM2) is a nonselective Ca 2+ -permeable cation channel highly expressed in brain and other tissues. Studies showed that TRPM2 contributed to the induction of inflammatory cytokine and chemokine of immune cells, resulted in neuropathic pain. However, how TRPM2 regulates neuropathic pain is not clear. The sciatic nerve chronic constriction injury (CCI) rat model was used to induce chronic neuropathic pain. The RNA and protein level of TRPM2 was detected with real-time PCR and western blot. SiRNA targeting TRPM2 was used to knockdown the expression of TRPM2. Reactive oxygen species (ROS) levels were determined using H2DCFDA assay and NO production was analyzed by measuring the accumulated level of its stable metabolite (nitrite). We found that CCI significantly increased TRPM2 expression in dorsal root ganglion and spinal cord. Knockdown TRPM2 in early phase after CCI alleviated injury-induced neuropathic pain. Mechanistically, we demonstrated that TRPM2 knockdown drastically inhibited the iNOS expression and NO generation, with decreased ROS generation in CCI rat. TRPM2 participates in the transformation of acute pain to chronic pain during injury-induced neuropathic pain, which might serve as a potential therapeutic target for neuropathic pain., (© 2019 Wiley Periodicals, Inc.)

Systemic administration of the local anaesthetic lidocaine is antinociceptive in both acute and chronic pain states, especially in acute postoperative and chronic neuropathic pain. These effects cannot be explained by its voltage-gated sodium channel blocking properties alone, but the responsible mechanisms are still elusive. This narrative review focuses on available experimental evidence of the molecular mechanisms by which systemic lidocaine exerts its clinically documented analgesic effects. These include effects on the peripheral nervous system and CNS, where lidocaine acts via silencing ectopic discharges, suppression of inflammatory processes, and modulation of inhibitory and excitatory neurotransmission. We highlight promising objectives for future research to further unravel these antinociceptive mechanisms, which subsequently may facilitate the development of new analgesic strategies and therapies for acute and chronic pain., (Copyright © 2019 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.)

Background: Muscle energy techniques are applied to reduce pain and increase range of motion. These are applied to a variety of pathological conditions and on asymptomatic subjects. There is however limited knowledge on their effectiveness and which protocol may be the most beneficial., Objective: The aim of this review is to determine the efficacy of muscle energy techniques (MET) in symptomatic and asymptomatic subjects., Design: Systematic Review., Methods: A literature search was performed using the following database: Cochrane Library, MEDLINE, NLM Pubmed and ScienceDirect. Studies regarding MET in asymptomatic and symptomatic patients were considered for investigation. The main outcomes took into account range of motion, chronic and acute pain and trigger points. Two trained investigators independently screened eligible studies according to the eligibility criteria, extracted data and assessed risk of bias. Randomized control trials (RCT's) were analyzed for quality using the PEDro scale., Results: A total of 26 studies were considered eligible and included in the quantitative synthesis: 14 regarding symptomatic patients and 12 regarding asymptomatic subjects. Quality assessment of the studies through the PEDro scale observed a "moderate to high" quality of the included records., Conclusions: MET are an effective treatment for reducing chronic and acute pain of the lower back. MET are also effective in treating chronic neck pain and chronic lateral epicondylitis. MET can be applied to increase range of motion of a joint when a functional limitation is present. Other techniques seem to be more appropriate compared to MET for trigger points., Competing Interests: Competing interestsThe authors declare that they have no competing interests.

Pain and inflammation are inherently linked responses to injury, infection, or chronic diseases. Given that acute inflammation in humans or mice enhances the analgesic properties of opioids, there is much interest in determining the inflammatory transducers that prime opioid receptor signaling in primary afferent nociceptors. Here, we found that activation of the transient receptor potential vanilloid type 1 (TRPV1) channel stimulated a mitogen-activated protein kinase (MAPK) signaling pathway that was accompanied by the shuttling of the scaffold protein β-arrestin2 to the nucleus. The nuclear translocation of β-arrestin2 in turn prevented its recruitment to the μ-opioid receptor (MOR), the subsequent internalization of agonist-bound MOR, and the suppression of MOR activity that occurs upon receptor desensitization. Using the complete Freund's adjuvant (CFA) inflammatory pain model to examine the role of TRPV1 in regulating endogenous opioid analgesia in mice, we found that naloxone methiodide (Nal-M), a peripherally restricted, nonselective, and competitive opioid receptor antagonist, slowed the recovery from CFA-induced hypersensitivity in wild-type, but not TRPV1-deficient, mice. Furthermore, we showed that inflammation prolonged morphine-induced antinociception in a mouse model of opioid receptor desensitization, a process that depended on TRPV1. Together, our data reveal a TRPV1-mediated signaling pathway that serves as an endogenous pain-resolution mechanism by promoting the nuclear translocation of β-arrestin2 to minimize MOR desensitization. This previously uncharacterized mechanism may underlie the peripheral opioid control of inflammatory pain. Dysregulation of the TRPV1-β-arrestin2 axis may thus contribute to the transition from acute to chronic pain., (Copyright © 2019 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.)

The use of non-steroidal anti-inflammatory drugs (NSAIDs) for the treatment of inflammatory pain is limited by gastrointestinal complications. The rapid action of NSAIDs is associated with better pain relief. Previously, we demonstrated that fluoro-loxoprofen, a novel NSAID, has less ulcerogenic potential than other NSAIDs, attributable to its gastroprotective properties. The aim of this study was to investigate and compare the effects of fluoro-loxoprofen on inflammatory pain in rats with those of other NSAIDs. Oral administration of fluoro-loxoprofen, loxoprofen, and celecoxib resulted in equivalent analgesic action against yeast-induced inflammatory pain. The antinociceptive effect of fluoro-loxoprofen was maximized within 1 h after administration, which is less time than that observed for loxoprofen (2 h) and celecoxib (3 h). We confirmed that both fluoro-loxoprofen and loxoprofen suppressed the increases in prostaglandin E 2 in inflamed paws. In addition to yeast-induced pain, fluoro-loxoprofen produced a similar effect against adjuvant-induced inflammatory pain, with faster peak analgesic effects than those observed for loxoprofen and celecoxib. Taken together, these results suggest that the analgesic effect of fluoro-loxoprofen is equivalent to that of loxoprofen and celecoxib. Moreover, the analgesic effect of fluoro-loxoprofen against inflammatory pain was more rapid than that of other NSAIDs, and this may be associated with its rapid absorption property., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Sickle cell disease (SCD) is a genetic disorder associated with hemolytic anemia, end-organ damage, reduced survival, and pain. One of the unique features of SCD is recurrent and unpredictable episodes of acute pain due to vasoocclusive crisis requiring hospitalization. Additionally, patients with SCD often develop chronic persistent pain. Currently, sickle cell pain is treated with opioids, an approach limited by adverse effects. Because pain can start at infancy and continue throughout life, preventing the genesis of pain may be relatively better than treating the pain once it has been evoked. Therefore, we provide insights into the cellular and molecular mechanisms of sickle cell pain that contribute to the activation of the somatosensory system in the peripheral and central nervous systems. These mechanisms include mast cell activation and neurogenic inflammation, peripheral nociceptor sensitization, maladaptation of spinal signals, central sensitization, and modulation of neural circuits in the brain. In this review, we describe potential preventive/therapeutic targets and their targeting with novel pharmacologic and/or integrative approaches to ameliorate sickle cell pain.

Background and Purpose: Transient receptor potential (TRP) channels are a superfamily of non-selective cation permeable channels involved in peripheral sensory signalling. Animal studies have shown that several TRPs are important players in pain modulation. Among them, the TRP melastatin 8 (TRPM8) has elicited more interest for its controversial role in nociception. This channel, expressed by a subpopulation of sensory neurons in dorsal root ganglia (DRG) and trigeminal ganglia (TG), is activated by cold temperatures and cooling agents. In experimental neuropathic pain models, an up-regulation of this receptor in DRG and TG has been observed, suggesting a key role for TRPM8 in the development and maintenance of pain. Consistent with this hypothesis, TRPM8 knockout mice are less responsive to pain stimuli., Experimental Approach: In this study, the therapeutic potential and efficacy of two novel TRPM8 antagonists, DFL23693 and DFL23448, were tested., Key Results: Two potent and selective TRPM8 antagonists with distinct pharmacokinetic profiles, DFL23693 and DFL23448, have been fully characterized in vitro. In vivo studies in well-established models, namely, the wet-dog shaking test and changes in body temperature, confirmed their ability to block the TRPM8 channel. Finally, TRPM8 blockage resulted in a significant antinociceptive effect in formalin-induced orofacial pain and in chronic constriction injury-induced neuropathic pain, confirming an important role for this channel in pain perception., Conclusion and Implications: Our findings, in agreement with previous literature, encourage further studies for a better comprehension of the therapeutic potential of TRPM8 blockers as novel agents for pain management., (© 2018 The British Pharmacological Society.)

A growing body of evidence suggests that peptides may possess analgesic effects without tolerance development. The synthetic tetrapeptide Tyr-d-Arg-Phe-Gly-NH 2 was modified with the inclusion of a (d-Arg) 8 vector to prevent the action of endopeptidase and to increase the duration of the analgesic action of the tetrapeptide when administered orally. The aim of this study was to estimate the analgesic efficacy of the tetrapeptide with (d-Arg) 8 (tridecapeptide, TDP) in experimental models of acute and chronic pain. The analgesic effects of TDP were estimated using a model of acute visceral pain in mice (writhing test) and a model of chronic neuropathic pain (chronic constriction injury (CCI) of the sciatic nerve) in rats. The intravenous administration of morphine (0.32-1mg/kg) and TDP (0.32-1.8mg/kg) produced significant dose-related antinociceptive effects in the writhing test. The potency of TDP after i.g. administration was lower than that after i.v. administration but comparable with that of i.g. morphine. In the CCI model, TDP (0.1, 1 and 10mg/kg, i.g.) induced marked analgesia with repeated administration without any signs of tolerance. The single administration of TDP after morphine treatment (7days) produced a significant analgesic effect in morphine-tolerant rats, indicating the absence of cross-tolerance between these two drugs. The combined administration of TDP and morphine resulted in the reduction of analgesic tolerance to morphine. The absence of cross-tolerance to morphine and the ability to prevent morphine tolerance allows this compound to be a prospective candidate for chronic pain therapy. In order to find the target receptors for TDP, a docking study was performed. It was found that the molecule can bind to the NMDA receptor using electrostatic, hydrogen bonding and hydrophobic interactions., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Chemokines are chemotactic cytokines whose involvement in nociceptive processing is being increasingly recognized. Based on the previous description of the involvement of CC chemokine receptor type 1 (CCR1) in pathological pain, we have assessed the participation of CCR1 and its endogenous ligands CCL3 and CCL5 in hyperalgesia and allodynia in mice after acute inflammation with carrageenan and chronic inflammation with complete Freund's adjuvant (CFA). The subcutaneous administration of the CCR1 antagonist J113863 (3-30 mg/kg; 30 min. before) dose dependently inhibited carrageenan- and CFA-evoked thermal hyperalgesia and mechanical allodynia produced by CFA, but not by carrageenan. The maximal dose of J113863 did not modify the increase in paw thickness induced by carrageenan or CFA. An almost ten times augmentation of CCL3 levels was detected by ELISA assays in both carrageenan and CFA paws, but not in spinal cords of inflamed mice, whereas CCL5 concentrations remained unaltered. Accordingly, a marked increase of CCL3 mRNA expression was observed in inflamed paws, with CCL3 protein detected in neutrophils and macrophages by immunohistochemical experiments. The intraplantar administration of an anti-CCL3 antibody (0.3-3 μg) blocked thermal hyperalgesia in carrageenan- and CFA-inflamed mice as well as CFA-evoked mechanical allodynia. Our data suggest that the increased concentrations of CCL3 present in inflamed tissues can be involved in acute and chronic inflammatory hyperalgesia as well as in chronic mechanical allodynia, and that these hypernociceptive symptoms can be counteracted by its neutralization with an antibody or by the blockade of CCR1 receptors., (© 2015 Nordic Association for the Publication of BCPT (former Nordic Pharmacological Society).)

Plasticity in inhibitory receptors, neurotransmission, and networks is an important mechanism for nociceptive signal amplification in the spinal dorsal horn. We studied potential changes in GABAergic pharmacology and its underlying mechanisms in hyperalgesic priming, a model of the transition from acute to chronic pain. We find that while GABAA agonists and positive allosteric modulators reduce mechanical hypersensitivity to an acute insult, they fail to do so during the maintenance phase of hyperalgesic priming. In contrast, GABAA antagonism promotes antinociception and a reduction in facial grimacing after the transition to a chronic pain state. During the maintenance phase of hyperalgesic priming, we observed increased neuroligin (nlgn) 2 expression in the spinal dorsal horn. This protein increase was associated with an increase in nlgn2A splice variant mRNA, which promotes inhibitory synaptogenesis. Disruption of nlgn2 function with the peptide inhibitor, neurolide 2, produced mechanical hypersensitivity in naive mice but reversed hyperalgesic priming in mice previously exposed to brain-derived neurotrophic factor. Neurolide 2 treatment also reverses the change in polarity in GABAergic pharmacology observed in the maintenance of hyperalgesic priming. We propose that increased nlgn2 expression is associated with hyperalgesic priming where it promotes dysregulation of inhibitory networks. Our observations reveal new mechanisms involved in the spinal maintenance of a pain plasticity and further suggest that disinhibitory mechanisms are central features of neuroplasticity in the spinal dorsal horn.

Complex regional pain syndrome type 1 (CRPS1) may be evoked by ischemia/reperfusion, eliciting acute and chronic pain that is difficult to treat. Despite this, the underlying mechanism of CRPS1 has not been fully elucidated. Therefore, the goal of this study is to evaluate the involvement of inflammation, oxidative stress, and the transient receptor potential ankyrin 1 (TRPA1) channel, a chemosensor of inflammation and oxidative substances, in an animal model of chronic post-ischemia pain (CPIP). Male Wistar rats were subjected to 3 h hind paw ischemia/reperfusion (CPIP model). Different parameters of nociception, inflammation, ischemia, and oxidative stress were evaluated at 1 (acute) and 14 (chronic) days after CPIP. The effect of a TRPA1 antagonist and the TRPA1 immunoreactivity were also observed after CPIP. In the CPIP acute phase, we observed mechanical and cold allodynia; increased levels of tumor necrosis factor-α (hind paw), ischemia-modified albumin (IMA) (serum), protein carbonyl (hind paw and spinal cord), lactate (serum), and 4-hydroxy-2-nonenal (4-HNE, hind paw and spinal cord); and higher myeloperoxidase (MPO) and N-acetyl-β-D-glucosaminidase (NAGase) activities (hind paw). In the CPIP chronic phase, we detected mechanical and cold allodynia and increased levels of IMA (serum), protein carbonyl (hind paw and spinal cord), and 4-HNE (hind paw and spinal cord). TRPA1 antagonism reduced mechanical and cold allodynia 1 and 14 days after CPIP, but no change in TRPA1 immunoreactivity was observed. Different mechanisms underlie acute (inflammation and oxidative stress) and chronic (oxidative stress) phases of CPIP. TRPA1 activation may be relevant for CRPS1/CPIP-induced acute and chronic pain.

Chronic postsurgical pain (CPSP) may affect up to 70% of patients after surgery. Glial and immune mediators have been implicated in the pathogenesis of chronic postsurgical pain. Our objective was to study cerebrospinal fluid (CSF) and serum concentrations of IL-1β, IL-6, IL-8, IL-10, IFNγ and TNFα over a 72-hour period in patients undergoing a thoracotomy and oesophagectomy. Despite adequate pain control, thoracotomy was still associated with significant central and peripheral inflammation. This must be taken into consideration in planning future strategies to prevent CPSP., (Copyright © 2015 Elsevier B.V. All rights reserved.)

Acute insults produce hyperalgesic priming, a neuroplastic change in nociceptors that markedly prolongs inflammatory mediator-induced hyperalgesia. After an acute initiating insult, there is a 72 h delay to the onset of priming, for which the underlying mechanism is unknown. We hypothesized that the delay is due to the time required for a signal to travel from the peripheral terminal to the cell body followed by a return signal to the peripheral terminal. We report that when an inducer of hyperalgesic priming (monocyte chemotactic protein 1) is administered at the spinal cord of Sprague Dawley rats, priming is detected at the peripheral terminal with a delay significantly shorter than when applied peripherally. Spinally induced priming is detected not only when prostaglandin E2 (PGE2) is presented to the peripheral nociceptor terminals, but also when it is presented intrathecally to the central terminals in the spinal cord. Furthermore, when an inducer of priming is administered in the paw, priming can be detected in spinal cord (as prolonged hyperalgesia induced by intrathecal PGE2), but only when the mechanical stimulus is presented to the paw on the side where the priming inducer was administered. Both spinally and peripherally induced priming is prevented by intrathecal oligodeoxynucleotide antisense to the nuclear transcription factor CREB mRNA. Finally, the inhibitor of protein translation reversed hyperalgesic priming only when injected at the site where PGE2 was administered, suggesting that the signal transmitted from the cell body to the peripheral terminal is not a newly translated protein, but possibly a newly expressed mRNA., (Copyright © 2015 the authors 0270-6474/15/350495-13$15.00/0.)

Citral (3,7-dimethyl-2,6-octadienal) is an open-chain monoterpenoid present in the essential oils of several medicinal plants. The aim of this work was to evaluate the effects of orally administered citral in experimental models of acute and chronic nociception, inflammation, and gastric ulcers caused by non-steroidal anti-inflammatory drugs (NSAIDs). Oral treatment with citral significantly inhibited the neurogenic and inflammatory pain responses induced by intra-plantar injection of formalin. Citral also had prophylactic and therapeutic anti-nociceptive effects against mechanical hyperalgesia in plantar incision surgery, chronic regional pain syndrome, and partial ligation of sciatic nerve models, without producing any significant motor dysfunction. In addition, citral markedly attenuated the pain response induced by intra-plantar injection of glutamate and phorbol 12-myristate 13-acetate (PMA, a protein kinase C activator), as well as by intrathecal (i.t.) injection of ionotropic and metabotropic glutamate receptor agonists (N-methyl-D-aspartic acid [NMDA] and 1-amino-1,3-dicarboxycyclopentane [trans-ACPD], respectively), substance P, and cytokine tumour necrosis factor-α. However, citral potentiated behaviours indicative of pain caused by i.t., but not intra-plantar, injection of a transient receptor potential vanilloid receptor type 1 (TRPV1) agonist. Finally, the anti-nociceptive action of citral was found to involve significant activation of the 5-HT2A serotonin receptor. The effect of citral was accompanied by a gastro-protective effect against NSAID-induced ulcers. Together, these results show the potential of citral as a new drug for the treatment of pain., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Background: Tissue acidosis is effective in causing chronic muscle pain. However, how muscle nociceptors contribute to the transition from acute to chronic pain is largely unknown., Results: Here we showed that a single intramuscular acid injection induced a priming effect on muscle nociceptors of mice. The primed muscle nociceptors were plastic and permitted the development of long-lasting chronic hyperalgesia induced by a second acid insult. The plastic changes of muscle nociceptors were modality-specific and required the activation of acid-sensing ion channel 3 (ASIC3) or transient receptor potential cation channel V1 (TRPV1). Activation of ASIC3 was associated with increased activity of tetrodotoxin (TTX)-sensitive voltage-gated sodium channels but not protein kinase Cϵ (PKCϵ) in isolectin B4 (IB4)-negative muscle nociceptors. In contrast, increased activity of TTX-resistant voltage-gated sodium channels with ASIC3 or TRPV1 activation in NaV1.8-positive muscle nociceptors was required for the development of chronic hyperalgesia. Accordingly, compared to wild type mice, NaV1.8-null mice showed briefer acid-induced hyperalgesia (5 days vs. >27 days)., Conclusion: ASIC3 activation may manifest a new type of nociceptor priming in IB4-negative muscle nociceptors. The activation of ASIC3 and TRPV1 as well as enhanced NaV1.8 activity are essential for the development of long-lasting hyperalgesia in acid-induced, chronic, widespread muscle pain.

Introduction: Population pharmacokinetic and pharmacodynamic (PK/PD) modeling is a critical component of drug development. Quantitative PK/PD models are used in drug development to improve both the design and interpretation of clinical trials across therapeutic areas., Areas Covered: In this review, the authors provide an overview of PK/PD modeling approaches and their applications in the management of acute and chronic pain as well as drug assessment. The advantages and limitations of these modeling approaches with regard to handling different end points of pain assessment in monotherapy and combination therapy are highlighted., Expert Opinion: New modeling approaches suitable for analgesics used in treatment of acute and chronic pain have started to emerge during the past few years. The application of the clinical utility index is limited but highly encouraged in pain drug assessment as it may inform the optimal window for treatment of pain to attain the best benefit:risk ratio. Owing to the restricted range of pain scores, beta regression models and coarsening models may be more appropriate modeling approaches for the bounded outcome data, rather than regular nonlinear/linear models that assume normal or lognormal error distribution. Additionally, modeling of exposure-response in flexible chronic pain studies remains challenging, and further investigations are needed.

Opioid receptor antagonists increase hyperalgesia in humans and animals, which indicates that endogenous activation of opioid receptors provides relief from acute pain; however, the mechanisms of long-term opioid inhibition of pathological pain have remained elusive. We found that tissue injury produced μ-opioid receptor (MOR) constitutive activity (MOR(CA)) that repressed spinal nociceptive signaling for months. Pharmacological blockade during the posthyperalgesia state with MOR inverse agonists reinstated central pain sensitization and precipitated hallmarks of opioid withdrawal (including adenosine 3',5'-monophosphate overshoot and hyperalgesia) that required N-methyl-D-aspartate receptor activation of adenylyl cyclase type 1. Thus, MOR(CA) initiates both analgesic signaling and a compensatory opponent process that generates endogenous opioid dependence. Tonic MOR(CA) suppression of withdrawal hyperalgesia may prevent the transition from acute to chronic pain.

Rationale: Treatment with a carbon monoxide-releasing molecule (tricarbonyldichlororuthenium(II) dimer, CORM-2) or a classical heme oxygenase 1 inducer (cobalt protoporphyrin IX, CoPP) has potent anti-inflammatory effects, but the role played by these treatments in the antinociceptive effects of morphine during acute and chronic pain was not evaluated., Objectives: In wild type (WT), neuronal (NOS1-KO), or inducible (NOS2-KO) nitric oxide synthases knockout mice, we evaluated the effects of CORM-2 and CoPP treatments in the antinociceptive actions of morphine and their interaction with nitric oxide during acute, visceral, and chronic inflammatory or neuropathic pain., Methods: Acute and visceral pain was assessed through formalin and acid acetic writhing tests. Chronic inflammatory pain induced by the intra-articular administration of complete Freund's adjuvant and neuropathic pain by partial ligation of sciatic nerve were evaluated by measuring allodynia and hyperalgesia using the von Frey filaments, plantar, or cold plate tests., Results: While nitric oxide, synthetized by NOS1 and/or NOS2, increased the local antinociceptive effects of morphine during acute and chronic pain, it decreased the inhibitory effects of morphine after visceral pain. Moreover, while CORM-2 or CoPP treatments did not alter or reduced the antinociceptive effects of morphine during acute and visceral pain, both treatments improved the local antiallodynic and antihyperalgesic effects of morphine after chronic inflammatory or neuropathic pain in WT, but not in KO mice., Conclusions: CORM-2 and CoPP treatments improved the local antinociceptive effects of morphine during chronic inflammatory and neuropathic pain by interaction with nitric oxide synthetized by NOS1 and NOS2 isoforms.

We have previously shown that activation of protein kinase Cε (PKCε) in male rats induces a chronic, long-lasting change in nociceptors such that a subsequent exposure to proinflammatory mediators produces markedly prolonged mechanical hyperalgesia. This neuroplastic change, hyperalgesic priming, is dependent on activation of cytoplasmic polyadenylation element-binding protein (CPEB), downstream of PKCε, and consequent translation of mRNAs in the peripheral terminal of the nociceptor. Since α calmodulin-dependent protein kinase II (αCaMKII), a molecule implicated in neuroplasticity, is a target of CPEB and can also affect CPEB function, we investigated its role in the transition from acute to chronic pain. Priming induced by direct activation of PKCε can be prevented by inhibition of αCaMKII. In addition, direct activation of αCaMKII induces priming, which was not prevented by pretreatment with PKCε antisense, suggesting that αCaMKII is downstream of PKCε in the induction of priming. Activation of ryanodine receptors (RyRs), which can lead to activation of αCaMKII, also induced priming, in a calcium- and αCaMKII-dependent manner. Similarly, inhibition of the RyR and a calcium buffer prevented induction of priming by PKCε. Unlike activation of PKCε, ryanodine and αCaMKII induced priming in female as well as male rats. Our results demonstrate a contribution of αCaMKII to induction of hyperalgesic priming, a phenomenon implicated in the transition from acute to chronic pain.

Object: Disc-related disorders such as herniation and chronic degenerative disc disease (DDD) are often accompanied by acute or chronic pain. Different mediators have been identified in the development of radicular pain and DDD. Previous studies have not analyzed individual cytokine profiles discriminating between acute sciatic and chronic painful conditions, nor have they distinguished between different anatomical locations within the disc. The aim of this study was to elucidate the protein biochemical mechanisms in DDD., Methods: The authors determined expression levels of matrix metalloproteinase-3, transforming growth factor-β (TGF-β), tumor necrosis factor-α, interleukin-1α, and pro-substance P using enzyme-linked immunosorbent assay and Western blot analyses in patients suffering from DDD (n = 7), acute back pain due to herniated discs with radiculopathy (n = 7), and a control group (n = 7). Disc tissue samples from the anulus fibrosus (AF) and nucleus pulposus (NP) were analyzed. Statistical analysis was performed using nonparametric tests., Results: A distinct distribution of cytokines was found in different anatomical regions of intervertebral discs in patients with DDD and herniated NP. Increased TGF-β levels were predominantly found in DDD. Matrix metalloproteinase-3 was increased in acute herniated disc material. Increased levels of substance P were found in patients suffering from DDD but not in patients with disc herniation. The data showed significantly higher levels of proinflammatory cytokines in the AF and NP of patients with DDD, and the expression levels in the AF were even higher than in the NP, suggesting that the inflammatory response initiates from the AF., Conclusions: These results highlight the complex mechanisms involved during disc degeneration and the need to distinguish between acute and chronic processes as well as different anatomical regions, namely the AF and NP. They also highlight potential problems in disc nucleus replacement therapies because the results suggest a biochemical link between AF and NP cytokine expression.

Glucomannan (GM) is a polysaccharide obtained from Heterodermia obscurata lichens. The present study was conducted to elucidate the antinociceptive effect of GM in behavioural models of acute and chronic pain in mice. GM reduced mechanical allodynia and the levels of interleukin 1-β (IL-1β) in spinal cord and nerve in the partial sciatic nerve ligation (PSNL) model. Systemic treatment with GM inhibited the nociception induced by intraplantar injection of glutamate and by intrathecal injection of N-methyl-d-aspartic acid (NMDA), (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), tumour necrosis factor α (TNF-α) and IL-1β. Taken together, our data demonstrate that GM has significant antinociceptive effect in acute and chronic pain, suggesting a potential interest in the development of new clinically relevant drugs for the management of pain., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Background: The role of extracellular signal-regulated kinases (ERKs) in nociception has been explored in the last years. While in spinal cord their activation is frequently correlated with pain or acute noxious stimuli, supraspinally, this association is not so evident and remains unclear. This study aims to evaluate ERK1/2 activation in the spinal cord and brainstem nuclei upon neuropathy and/or an additional mechanical stimulus., Methods: Acute noxious mechanical stimulation was applied in the left hindpaw of anaesthetized SHAM-operated and chronic constriction injured (CCI, neuropathic pain model) rats. Other SHAM or CCI rats did not receive any stimulus. Immunohistochemistry against the phosphorylated isoforms of ERK1/2 (pERK1/2) was performed in lumbar spinal cord and brainstem sections to assess ERK1/2 activation., Results: In the spinal cord, stimulation promoted an increase in pERK1/2 expression in the superficial dorsal horn of SHAM rats. No significant effects were caused by CCI alone. At supraspinal level, changes in ERK1/2 activation induced by CCI were observed in A5, locus coeruleus (LC), raphe obscurus (ROb), raphe magnus, dorsal raphe (DRN), lateral reticular and paragigantocellularis nucleus. CCI increased pERK1/2 expression in all these nuclei, with exception of LC, where a significant decrease was verified. Mechanical noxious stimulation of CCI rats decreased pERK1/2 expression in ROb and DRN, but no further changes were detected in either SHAM- or CCI-stimulated animals., Conclusion: ERK1/2 are differentially activated in the spinal cord and in selected brainstem nuclei implicated in nociception, in response to an acute noxious stimulus and/or to a neuropathic pain condition., (© 2012 European Federation of International Association for the Study of Pain Chapters.)

Targeted therapy to prevent the progression from acute to chronic pain in cancer patients remains elusive. We developed three novel cancer models in mice that together recapitulate the anatomical, temporal, and functional characteristics of acute and chronic head and neck cancer pain in humans. Using pharmacologic and genetic approaches in these novel cancer models, we identified the interaction between protease-activated receptor 2 (PAR2) and serine proteases to be of central importance. We show that serine proteases such as trypsin induce acute cancer pain in a PAR2-dependent manner. Chronic cancer pain is associated with elevated serine proteases in the cancer microenvironment and PAR2 upregulation in peripheral nerves. Serine protease inhibition greatly reduces the severity of persistent cancer pain in wild-type mice, but most strikingly, the development of chronic cancer pain is prevented in PAR2-deficient mice. Our results demonstrate a direct role for PAR2 in acute cancer pain and suggest that PAR2 upregulation may favor the development and maintenance of chronic cancer pain. Targeting the PAR2-serine protease interaction is a promising approach to the treatment of acute cancer pain and prevention of chronic cancer pain.

Tibia fracture in rats followed by cast immobilization leads to nociceptive, trophic, vascular and bone-related changes similar to those seen in Complex Regional Pain Syndrome (CRPS). Substance P (SP) mediated neurogenic inflammation may be responsible for some of the signs of CRPS in humans. We therefore hypothesized that SP acting through the SP receptor (NK1) leads to the CRPS-like changes found in the rat model. In the present study, we intradermally injected rats with SP and monitored hindpaw mechanical allodynia, temperature, and thickness as well as tissue levels of tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β), interleukin 6 (IL-6), and nerve growth factor-β (NGF) for 72 h. Anti-NGF antibody was utilized to block the effects of SP-induced NGF up-regulation. Fracture rats treated with the selective NK1 receptor antagonist LY303870 prior to cast removal were assessed for BrdU, a DNA synthesis marker, incorporation in skin cells to examine cellular proliferation. Bone microarchitecture was measured using micro computed tomography (μCT). We observed that: (1) SP intraplantar injection induced mechanical allodynia, warmth and edema as well as the expression of nociceptive mediators in the hindpaw skin of normal rats, (2) LY303870 administered intraperitoneally after fracture attenuated allodynia, hindpaw unweighting, warmth, and edema, as well as cytokine and NGF expression, (3) LY303870 blocked fracture-induced epidermal thickening and BrdU incorporation after fracture, (4) anti-NGF antibody blocked SP-induced allodynia but not warmth or edema, and (5) LY303870 had no effect on bone microarchitecture. Collectively our data indicate that SP acting through NK1 receptors supports the nociceptive and vascular components of CRPS, but not the bone-related changes.

Glutamate is an essential transmitter in pain pathways. However, its broad usage in the central and peripheral nervous system prevents us from designing efficient glutamate-based pain therapies without causing harmful side effects. The discovery of vesicular glutamate transporters (VGLUT1-3) has been a crucial step in describing specific glutamatergic neuronal subpopulations and glutamate-dependent pain pathways. To assess the role of VGLUT2-mediated glutamatergic contribution to pain transmission from the entire primary sensory population, we crossed our Vglut2(f/f) line with the Ht-Pa-Cre line. Such Vglut2-deficient mice showed significantly decreased, but not completely absent, acute nociceptive responses. The animals were less prone to develop an inflammatory-related state of pain and were, in the partial sciatic nerve ligation chronic pain model, much less hypersensitive to mechanical stimuli and did not develop cold allodynia or heat hyperalgesia. To take advantage of this neuropathic pain-resistant model, we analyzed Vglut2-dependent transcriptional changes in the dorsal spinal cord after nerve injury, which revealed several novel candidate target genes potentially relevant for the development of neuropathic pain therapeutics. Taken together, we conclude that VGLUT2 is a major mediator of nociception in primary afferents, implying that glutamate is the key somatosensory neurotransmitter., (Copyright © 2012 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved.)

In acute and chronic pain, the objective of pharmacokinetic-pharmacodynamic (PKPD) modeling is the development and application of mathematical models to describe and/or predict the time course of the pharmacokinetics (PK) and pharmacodynamics (PD) of analgesic agents and link PK to PD. Performing population PKPD modeling using nonlinear mixed effects modeling allows, apart from the estimation of fixed effects (the PK and PD model estimates), the quantification of random effects as within- and between-subject variability. Effect-compartment models and mechanism-based biophase distribution models that incorporate drug-association and -dissociation kinetics are applied in PKPD modeling of pain treatment. Mechanism-based models enable the quantification of the rate-limiting factors in drug effect owing to drug distribution versus receptor kinetics (since receptor kinetics are nonlinear they are discernable from the linear effect-compartment kinetics). It is a helpful technique in understanding the complex behavior of specific analgesics, such as buprenorphine, but also morphine and its active metabolite morphine-6-glucuronide, especially with respect to the reversal of opioid-induced side effects, most importantly life-threatening respiratory depression. One approach in chronic pain studies is the application of mixture models. Mixture models do not necessarily need to take PK data into account and allow the objective differentiation of measured responses to analgesics into specific response subgroups, and as such, may play an important role in analyzing Phase I and II analgesia studies. Appropriate application of PKPD modeling leads to the improvement of current therapeutics with respect to dose design and outcome, understanding the interaction of analgesics within complex chronic pain disease processes and may play an important role in drug development. In the current article, novel observations using the aforementioned techniques on opioids, NSAIDs, epidural analgesia, ketamine and GABA-ergic drugs in acute and chronic pain are discussed.