Association of inflammatory mediators with pain perception

(1)

Contents lists available atScienceDirect

Biomedicine & Pharmacotherapy

journal homepage:www.elsevier.com/locate/biopha

Association of in ﬂ ammatory mediators with pain perception

S. Ronchetti

^⁎

, G. Migliorati, D.V. Del ﬁ no

Department of Medicine, Section of Pharmacology, University of Perugia, P.le L. Severi 1, 06132, Perugia, Italy

A R T I C L E I N F O

Keywords:

NSAIDs Inﬂammation Pain Relief

A B S T R A C T

Treatment of pain has always been a major goal in the clinic, as it is related to several pathological conditions of inflammatory origin and surgical procedures, which are associated with inflammatory mediators. Understanding the molecular mechanisms underlying the association between inflammatory mediators and pain perception, from peripheral to central sensitization, can provide the basis for the development of new pharmacological treatments. Despite safety concerns, till date, the use of non-steroidal anti-inflammatory drugs (NSAIDs) has been shown to be efficacious, safe, and well tolerated by patients. Thus, choosing the appropriate administration route, developing new formulations and lowering the efficacious dose represent, currently, effective means of treating inflammation and relieving the pain, without inducing significant side effects.

1. Introduction

Inflammation is a process that has been well studied since ancient times. It is characterized by five typical signs:rubor (redness),calor (increased heat),tumor(swelling),dolor(pain), andfunctio laesa(loss of function). Thefirst four signs were described by Celsus in the 1^stcen- tury AD, while thefifth was added in 1871 by Virkow[1]. There has been considerable progress in our understanding of the cellular and molecular mechanisms of inflammation in the late 20th century. It has been revealed that inflammation is not just a sum of clinical signs, but a complex network of integrated signals between immune cells and injured tissues. Inflammation can be systemic (when it is caused by a trauma, surgery or severe infection) or local (when it is caused by an external injury). Pain is always associated with the region where the inflammation is localized. The detection of noxious stimuli, which is known as nociception, and the transmission of these stimuli to the brain lies at the basis of the pain. Primary afferent neurons can detect noxious chemical, thermal and mechanical stimuli, and the cell body of these neurons reside in the trigeminal and dorsal root ganglion (DRG). During the inflammatory process, the responses to noxious stimuli are enhanced (hyperalgesia) or pain is triggered by normal innocuous stimuli (allodynia). Sometimes, the pain becomes chronic if the inflammation is not promptly resolved. Moreover, it can persist even after the injury that causes the inflammation is healed. The hypersensitive state that underlies inflammatory pain is partially dependent on the plasticity of the nervous system, as a mechanism of adaptation to the nociceptive stimulus [2,3]. Such modifications include both post-translational changes and transcription-dependent changes, which all result in

modiﬁcation of the nociceptive pathway.

Although several molecular mechanisms underlying the inflammatory process have been revealed, some of them still remain to be elucidated. Once these mechanisms are elucidated, more molecules that are involved in the inflammatory process may be identified as targets of specific drugs, which could be used to resolve inflammation as well as relieve the associated pain.

A number of drugs are used to treat inﬂammatory pain, and there are several new drugs that are being developed. This review focuses on the molecular mechanisms that link inﬂammation to pain and on the treatment methods that are currently in use.

2. The nociceptive system

The nociceptive system contains neurons that are activated by noxious stimuli, such as mechanical, thermal and chemical stimuli. The primary nociceptive neurons are cutaneous nociceptors that are found at the terminals of Aδand C-fibres, which are thinly- and un-myelinated fibres, respectively. Peripheral afferent signals convey nociceptive stimuli to the grey matter of the dorsal horn in the spinal cord and if these stimuli persist, the C-fibres increase synaptic conduction in the dorsal root neurons, thus sensitizing the central nervous system. Some of these neurons project to the brain stem or to the thalamocortical system and this consequently leads to a conscious pain response. This central sensitization is essential for the development of hyperalgesia and allodynia and involves activation of the NMDA (N-methyl-D-aspartic acid) receptors by glutamate. On the other hand, the descending neural tract that inhibits transmission of the pain signal represents the

https://doi.org/10.1016/j.biopha.2017.12.001

Received 22 August 2017; Received in revised form 1 December 2017; Accepted 1 December 2017

⁎Corresponding author.

E-mail address:[email protected](S. Ronchetti).

T

(2)

antinociceptive system, and includes opioid peptides, serotonin, nor- epinephrine and dopamine. These neurotransmitters are released by intermediate neurons within the dorsal horn of the spinal cord, which in turn are activated by the opioid and GABAergic mechanism of the periacqueductal gray region[4,5].

3. Inﬂammation, pain and cytokines

The inﬂammatory pathway is typically triggered by a peripheral trauma or injury. This is accompanied by the release of arachidonic acid, which is subsequently converted to prostanoids by cyclo-oxige- nase enzymes (COXs). COX-2 is the predominant isoform of COX, but it is not an exclusive source of prostaglandins, which are constitutively expressed in the brain and induced in injured tissues by IL-1β, TNF-α and IL-6[6,7]. COX-1 is the major source of prostanoids that mediate physiological functions, but it may also be involved in pathological processes[8]. COX-2 transforms arachidonic acid into prostaglandin E2 (PGE2)[7]. PGE2 promotes local vasodilation and the activation and migration of neutrophils, macrophages and mast cells, and it can directly trigger nociceptorsvia the prostaglandin E (EP) receptors[9].

PGE2 is generally considered to be a sensitizing agent because it (i) enhances the sensitization of nociceptors by lowering the threshold of the tetrodoxin-resistant sodium channels, (ii) modulates the transient receptor potential vanilloid (TRPV) 1 channel for heat sensation, and (iii) sensitizes primary afferent neurons (peripheral sensitization) to bradykinin[10–14]. Bradykinin, which is released by mast cells and damaged tissues, binds to receptors located in all tissues, including the bradykinin 2 receptors that are constitutively expressed on neurons, such as the polymodal C-nociceptors[15]. As a consequence, the heat threshold is lowered, which causes the long-lasting pain associated with inflammation. In addition, the central production of PGE is believed to be the cause of fever. Studies on conditionalCOX-2-knockout mice have revealed importantfindings on the role of COX-2 in mediating pain. For example, Vardeh et al. demonstrated that when COX-2 is present at the site of inflammation, it plays a role in hypersensitivity to both mechanical and thermal pain after peripheral inflammation, while neural COX-2 contributes only to mechanical pain[16]. Since mechanical pain is the main symptom of postoperative pain and diseases such as arthritis, drugs that inhibit COX-2 can directly exert their effects on pain that has a mechanical origin. In the neuropatic pain, the up-regulation of COX2 and PGE2 is involved in the functions of EP-receptor bearing primary sensory neurons, thus contributing to the development of chronic pain,via de novosynthesis of pain mediators, such as IL-6 and BDNF[17]. BDNF contributes to the central sensitization of dorsal horn nociceptive neurons and it is also induced by inflammatory stimuli [18]. Furthermore, it has been recently demonstrated that the con- centration of BDNF is increased in the anterior cingulated cortex (ACC) during peripheral inflammation, and that this is sufficient to induce certain plastic changes. ACC is believed to be important in the antici- pation of a painful stimulus, so BDNF could be considered as a facil- itator of pain and as a contributor to the mechanisms underlying chronic pain[19].

Immune cells together with glia and neurons constitute an integrated network in which an immune response triggered by an inflammatory stimulus modulates the resulting pain. The interaction between immune cells and glial cells characterizes the neuroinflammation, in which release of inflammatory mediators has a critical role in the pathogenesis of chronic pain[20]. Immune cells are recruited at the site of inflammation, and their activation leads to the release of pro-inflammatory mediators such as cytokines and chemokines. Moreover, degranulation of resident mast cells is involved in triggering nociception, bothviadirect interaction with the peripheral nerve terminals through the cell adhesion molecule N-caderin, and in- directlyviathe release of histamine, bradykinin and other vasodilating mediators. In addition, mast cell degranulation leads to the rapid onset of nerve growth factor-induced hyperalgesia[21]. Interestingly, it was

very recently postulated that the cross-talk between mast cells and nerves may be bi-directional[22].

Certain proinflammatory cytokines, such as TNF-α, IL-6 and IL-1β, play a direct role in the generation and maintenance of pain, facilitating central sensitization and hyperalgesia. Nociceptive neurons possess receptors for these cytokines on their surface, and neutralization of these cytokines may result in quick reduction of the pain before the attenuation of inflammation. These cytokines are released by activated macrophages, T lymphocytes and mast cells, both systemically and lo- cally at the site of inflammation. Several works have demonstrated that TNF-αis an initiator of neuropathic pain, and that intratecal injection of exogenous TNF-αhas a pro-nociceptive effect[23,24]. In addition, impairment of TNF-α signaling attenuates hypersensitivity in rodent models of neuropathy[25].

IL-6 is a cytokine that transfers peripheral immune signals to the CNS[26]. IL-6 has been shown to play an important role in the neuropathic pain following nerve injury, and the administration of anti-IL-6 antibody signiﬁcantly attenuates mechanical allodynia[27,28]. How- ever, it seems to play a dual role, since IL-6-deﬁcient mice show increased levels of cartilage loss in a spontaneous model of osteoarthritis [29]. Very recently, another cytokine, IL-17, released by specialized CD4+ T cells, was found in the spinal dorsal horn in a rat model of spinal nerve ligation, together with IL-1βand IL-6; these factors are believed to contribute to the development of neuropathic pain[30].

IL-1βis theﬁrst cytokine involved in peripheral nerve injury[31].

Its expression increases in a variety of autoimmune diseases in which pain is a main feature, and it is largely produced by glial cells and upregulated after peripheral nerve injury [23,32,33]. IL-1β has the ability to induce the expression of other pro-inﬂammatory factors that together contribute to the induction and maintenance of pain. The mechanism of its upregulation was recently discovered in the in- ﬂammasome, a multiprotein complex that, upon oligomerization, induces activation of pro-caspase1, initiating the processing of pro-IL-1β.

Diﬀerent inﬂammasomes are recruited in various diseases, ranging from neuropathic pain to infectious and autoimmune diseases, and they might be possible targets of pharmacological interventions for pain relief[34].

Interestingly, cytokines have been implicated in the cross-talk between the immune system and the brain: stress conditions induce the release of peripheral cytokines, which act on the endothelial cells of the blood-brain barrier, and activate microglia, which release pro-inflammatory cytokines[35]. Activation of microglia has been implicated in the regulation of mood and behaviour after prolonged exposure to psychological stress[36]. Psycological stress is a condition that causes activation of the neuroendocrine pathways towards the periphery that ultimately leads to the release of glucocorticoids by the hypothalamic- pituitary-adrenal (HPA) axis. The very well known modulation of the immune system by glucocorticoids includes changes in cellular trafficking, cytokine secretion, antibody production, and activation and proliferation of immune cells. An acute response to stress stimulates the immune system to react, while a prolonged one causes anti-inflammatory effects and immunosuppression. Therefore, glucocorticoids represent a fundamental link between the nervous system and the control of inflammation[37–39].

During an infection, as a result of activation of the immune response and release of pro-inflammatory cytokines, the body undergoes certain changes and exhibits sickness behavior. This sickness behavior is also observed in the early onset of major depression, which may indicate that these immunoinflammatory pathways are involved in the pathophysiology of major depression[40,41]. There is some interesting literature linking inflammation, pain and depression, but so far, anti-inflammatory drugs have not proven useful for the treatment of depression[42,43].

Bacterial infections activate the immune system, thus inducing in- ﬂammation and consequently triggering the nociceptive pathways. This is an indirect eﬀect that indicates the correlation of infection with pain.

(3)

Recently, bacteria were found to directly activate the nociceptors and thus lead to pain. The molecular elements involved in the bacterial process are formyl peptides and the pore-forming toxinα-hemolisin, which actviathe formyl peptide receptor 1 (FPR1) receptor and the pore assembly, respectively, and lead to ionic inﬂux. Moreover, mechanical and thermal hyperalgesia is directly dependent on bacterial load. In this context, both the triggered inﬂammation and the presence of bacteria activate the sensory neurons[44].

Thus, many factors participate in the complex network of pro-in- ﬂammatory mediators and receptors on nociceptive neurons.

4. Inflammatory mediators in peripheral and central pain Converse to physiological pain, inflammatory pain originates from tissue damage, and it initiates activation of the peripheral terminals of nociceptive nervefibers. The physiological threshold is lowered in the presence of a noxious stimulus, as such in the case of cutaneous nociceptors that are sensitized to thermal stimuli and nociceptors present in deep tissues, such as muscles and joints, which are sensitized to mechanical stimuli [45]. This sensitization, which is induced in a few minutes, encompasses a number of mediators that directly bind to receptors located at the end terminals of the peripheral fibers. One of these receptors is the well-known transient receptor vanilloid 1 (TRPV1), an ion channel activated by noxious heat, low pH and capsaicin, which is pivotal for inflammatory hyperalgesia[46]. It has been recently proposed that TRPV1 may regulate painviathe CNS, and not only the peripheral nervous system, in cases of chronic pain[47]. Also the transient receptor potential ankyrin 1 (TRPA1), which belongs to the same family TRP as TRPV1, is a central player in peripheral sensitization. Although natural ligands have not been identified, the hy- pothesis that bacterial or viral components, activated immune cells or products from damaged tissues release agonists for this receptor has been proposed[48]. The PGE2 receptors EP1, EP2 and EP4 contribute to inflammatory hyperalgesia, especially in peripheral sensitization.

Studies on single EP receptor-knockout mice provide new insights into the peculiar function of each receptor subtype. In a model of peripheral inﬂammation, EP2-knockout mice showed normal early peripheral hyperalgesia, but lacked the chronic hyperalgesic phase of spinal origin [49]. In EP1-null mice, this receptor was demonstrated to play a role in peripheral heat sensitization,viaactivation of TRPV1[50]. Recently, it was demonstrated that the expression and cell surface and axonal traﬃcking of TRPV1 are enhanced by PGE2, thus potentiating this receptor’s activity[51].

Proteases are degradative enzymes that can cleave protease-activated receptors (PARs), which are located on several tissues, including specific populations of neurons and astrocytes in the brain, spinal cord and associated ganglia. Proteases released by mast cells or neutrophils at the site of inflammation or from the coagulation cascade in injured tissues activate PARs by cleaving them at a specific site[52,53]. The PAR subtype PAR2 is co-expressed with TRPV1 in neurons of the DRG, which results in reduction of the temperature threshold of TRPV1 once it is activated and induces thermal hyperalgesia. Furthermore, mechanical hyperalgesia can be initiated by activation of PAR2, which in turn activates TRPV4. TRPVs are not the only receptors triggered by PARs, since other non-TRPV1 receptors might also be involved in hyperalgesia[54,55].

Other proteases, such as matrix metalloproteinases (MMPs), are associated with the inflammatory process. Some of them exert pro-inflammatory effects, while others are involved in the resolution of inflammation[56]. MMPs are released by a number of vascular and non- vascular cells, including endothelial cells,fibroblasts, monocytes, and macrophages and they increase vessel permeability. Importantly, MMP- 2 and MMP-9 have been implicated in the generation and maintenance of neuropathic pain, and their secretion by resident macrophages, Schwann cells, astrocytes and microglia plays an important role in the development of chronic pain hypersensitivity[57,58]. MMP-9 can also

stimulate the production of VEGF under pathologic conditions, thus favouring neovascularization and contributing to inﬂammation[59].

Thus, proteases represent pivotal players in inﬂammation-induced pain[60].

Glutamate is an important excitatory neurotransmitter in the CNS that was found to be involved in acute and chronic pain. Conditional knockout mice that lack vesicular glutamate transporters (VGLUT1–VGLUT3) show a decrease in nociception that is caused by a reduction in theﬁring of the superﬁcial dorsal horn neurons[61,62].

Moreover, PGE2 amplifies synaptic transmission by stimulating the release of glutamate, and inhibition of COX-2 by celecoxib prevents glutamate release from nerve terminals, thus linking inflammation to glutamate and pain[63–65]. Recently, glutamate was found to play a modulatory role in the DRG following peripheral inflammatory mechanical hyperalgesia. Glutamate release activates N-methyl-^D-aspartate receptors (NMDARs) in satellite glial cells, but the mechanism by which glutamate-activated satellite cells influence neuronal excitability remains to be elucidated[66]. NMDARs are very extensively studied receptors in the context of pain. They play an important role in both inflammation and nerve injury-induced central sensitization, and have been proposed to be pharmacological targets for the treatment of neuropathic pain[67–69]. Interestingly, it has been recently demonstrated that two distinct receptors of glutamate, metabotropic mGlutRI and mGlutR5, which are located in different region of the dorsal horn of the spinal cord, induce opposing effects, with mGlutRI playing a pro- nociceptive role and mGlutR5 exerting an antinociceptive effect[70].

These newﬁndings may have important therapeutic implications for the pharmacological treatment of inﬂammatory pain.

Another interesting recently published work demonstrates that central GABAAreceptors play a role in mediating mechanical allodynia induced by large myelinated Aβfibers following inflammation. This finding indicates that normally, the central GABAAreceptor-mediated anti-nociceptive effect is the result of inhibitory action, while the central GABAA receptor-mediated pro-nociceptive effect is the result of excitatory action following inflammation [71]. Generally, the loss of inhibition mediated via GABA receptors contributes to spinal excitability, which is necessary for the delivery of a supraspinal pain signal after injury. In fact, intrathecal administration of GABA or GABAA

agonists has been found to attenuate nociceptive behaviours in rodent models of pain; however, there is still no clinical evidence for the direct pain-relieving eﬀects of benzodiazepines in neuropathic pain[72–74].

Endogenous opiods are important players in the modulation of pain.

Inﬂammation increases the expression of opioids in the cells of the immune system and in the early stages of inﬂammation. At the peripheral terminals of sensory neurons, opioids released by immune cells activate opioid receptors, by passing through the damaged perineural sheath. This interaction between immune cells and peripheral sensory neurons is probably necessary for the delivery of opioid peptides in close proximity to opioid receptors, which results in pain relief.

Peripheral opioids have also been postulated to play an anti-in- ﬂammatory roleviainhibition of NA, substance P and TNF-αsecretion from neuronal cells. However, exogenous opioids, the most powerful painkillers known, have been found to paradoxically prolong neuropatic pain and induce nociceptive sensitization, thereby favouring the transition from acute to chronic pain. In fact, short-term treatment with morphine can prolong allodyniaviaNOD-like receptor 3 inﬂammasome activation in the spinal cord microglia, and its expression is increased by TLR4-signaling[75–77]. Therefore, in-depth knowledge about the molecular mechanisms of opioids will ensure the best use of these drugs in the management of pain.

In addition to the above mentioned mediators, many other neuro- peptides are involved in neurogenic inﬂammation. For example, substance P is expressed throughout the nervous and immune systems and can induce cytokine release by various cell types, thereby playing a critical role in modulating immune responses at peripheral sites such as the gastrointestinal and respiratory tracts during inﬂammation [78].

(4)

Substance P has also been shown to augment inﬂammation in the CNS and stimulate the production of inﬂammatory cytokines by monocytes/

macrophages, T lymphocytes and mast cells, as well as the release of inﬂammatory mediator-containing granules from neutrophils, mast cells and eosinophils [79,80]. Together with neurokinin A (NKA), neurokinin B (NKB), and calcitonin gene-related peptide (CGRP), Sub- stance P contributes to neurogenic inﬂammation and activates mast cells, microglia and astrocytes[81].

Finally, systemic inflammation can modify the integrity of the blood-brain barrier (BBB). BBB permeability can be altered by nociceptive stimuli as demonstrated by a number of pre-clinical studies in healthy animals. In one such study, injection ofλ-carrageenan into the rat hind paw, a peripheral inflammatory stimulus, was found to disrupt the disulphide-bonded occludin oligomeric assemblies in the BBB, thus compromising its permeability[82]. In the rodent model of systemic inflammation induced by LPS injection, disruption of the BBB was found to be caused by prostanoids and nitric oxide (NO), modifications of tight junctions, or degradation of glycocalyx, a complex structure lining the apical endothelium. In addition, leucocyte trafficking and expression of the pro-inflammatory mediators TNF-α and IL-1β, induced by peripheral inflammation, also contribute to the permeabili- zation of the BBB[83–85]. The changes in BBB function during pain and systemic inflammation have clinical relevance, and understanding the underlying mechanisms is important for the development of new treatments for CNS diseases.

Overall, many mediators of the inflammatory cascade directly or indirectly induce a nociceptive response, but specific pharmacological targets still need to be identified (Fig. 1).

5. Gender-based diﬀerences in pain

Differences in pain perception between males and females have been known since long, but it is not entirely clear as to what contributes to such differences. Women are more sensitive to painful stimuli than men and are often not properly treated. The prevalence of pain of chronic or inflammatory origin is higher in women, including neck and knee pain, migraine, rheumatoid arthritis and irritable bowel syndrome [86]. However, although many studies have tried to understand the mechanisms underlying the more acute sensitivity to pain in females, pre-clinical research mostly includes male animals and is therefore limited in this regard. Beyond the role of estrogen and its modulation of the nociceptive response, which is thought to be controversial, differences in the modulation of the immune system in the two sexes should be considered[87]. According to Rosen et al., differences in the modulation of the immune system between males and females could account for differences in pain. Women exhibit an enhanced inflammatory response to tissue damage that is more pronounced than

the response in men, and this can result in the high production of pro- inflammatory cytokines with consequent pain. In addition, even cir- culating estrogens can increase the release of pro-inflammatory cytokines, thus resulting in pain. A very interesting finding in this field involves the role of TLR4 in the mediation of chronic pain in mice:

injection of lipopolysaccharide in the spinal cords of male mice induces allodynia, but this is not observed in female mice. This effect is observed only in the spinal cord and not in other tissues[88]. In addition, at the molecular level, p38 signaling was found to be involved in neuropathic pain, which is differentially regulated in females and males. Intrathecal injection of p38 inhibitors reduced mechanical allodynia, a symptom of neuropathic pain, only in male mice, but not in females, and this difference was found to be confined to the spinal cord in a mouse model of chronic constriction injury[89,90]. Proteins of the inflammatory pathway may be differentially regulated as well. A study on COX-1- and COX-2-knockout mice of both sexes found that COX-1- knockout females show a higher degree of thermal hyperalgesia than males, whereas COX-2-knockout females exhibit a decrease in thermal hyperalgesia compared with males. Therefore, qualitative rather than quantitative differences are very likely to account for these differences between the sexes. Thesefindings indicate that the effect of COX inhibition with specific drugs may be sex dependent[91]. Another important sex-based difference lies in the effectiveness of opioid treatment in chronic pain. Several studies have demonstrated that morphine is more effective in males than in females, while some others have demonstrated a lower degree of analgesia in males than in females.

Nonetheless, women experience the side effects of opioids to a greater extent than men do. This could be probably be explained by the tight link between the opioid system and immune cells. Morphine can bind to TLR4 on cells of the immune system, specifically glia cells, thereby triggering cytokine release and excitation of neurons, and causing a reduction in the analgesic effect of morphine. This binding of morphine to TLR4 on glia cells attenuates the responses in females more than in males[92].

Even though there are not enough pre-clinical studies on sex-based differences in pain, given the preferential use of male rodents, the treatment of pain should be re-considered in view of the differences in pain between sexes, not in terms of efficacy of NSAIDs or other drugs but rather choosing the most appropriate doses and treatment regimen depending on the sex.

6. Epigenetic regulators of inﬂammation and pain

Environmental factors can impact synaptic plasticity as well as the inﬂammatory process, thereby inﬂuencing the development of persistent painviaDNA-sequence independent mechanisms. Many diseases such as diabetes and cancer, which are involved in epigenetic

Fig. 1.Mediators of inﬂammatory pain. Several receptors, either directly or indirectly, induce a nociceptive response. For a detailed description of each receptor, please refer to the text (paragraph #4).

PAR2: protease-activator receptor 2; TRPV 1: transient receptor potential cation channel subfamily V member 1; TRPV 4: transient receptor potential cation channel subfamily V member 4; EP: prostaglandin receptor; NMDAR: N-methyl-D-aspartate receptor; GABAA: gamma-aminobutyric acid A receptor. TLR4: toll-like receptor 4.

(5)

regulation, have been found to be associated with chronic pain[93,94].

Although the exact mechanisms through which all these changes occur are still not clear, there is evidence that histone deacetylase inhibitors (HDACs) are able to attenuate inflammatory diseases, including arthritis, hepatitis and colitis, and that their effects are partially mediated by cytokine suppression. In addition, modification of DNA methylation is another mechanism that influences the expression of pronociceptive genes: for example, the methyl-binding protein MeCP2 can promote the upregulation of inflammatory genes during inflammatory pain. A growing body of evidence has demonstrated a close relationship between epigenetics and pain, and is unravelling the mechanisms of underlying the regulation of the nociceptive pathways by environmental factors. All of them can have an impact on the development of pain as well as on the transition from acute to persistent pain[95,96]. Future studies will pave the way for the development of new drugs that target epigenetic regulators.

7. Use of non-steroidal anti-inﬂammatory drugs in the treatment of pain associated with inﬂammation

Non-steroidal anti-inflammatory drugs (NSAIDs) are largely used in the treatment of pain and inflammation associated with a number of diseases and other traumatic conditions. Both chronic and acute pain can be effectively treated with NSAIDs. However, there are always safety concerns associated with the use of these drugs, such as clinically relevant gastrointestinal, renal and cardiovascular damage. Therefore, the route of administration, dose and treatment duration should be decided based on each specific case and type of drug.

Da Costa and colleagues conducted an in-depth analysis of trials from the literature on the use of NSAIDs in the treatment of osteoarthritis in elderly people: they showed that the most effective NSAID is diclofenac (150 mg/day), which is useful for both reducing pain and enhancing function, and is superior to ibrupofen, naproxen and celecoxib. The longest treatment duration in their study was 3 months. A COX-2-selective inhibitor, etoricoxib (60 mg/day), was proven to be as effective as diclofenac, but it is commercially available only in a few countries[97]. US and EU guidelines for osteoarthritis management, however, recommend the use of topical rather than oral NSAIDs, especially in the elderly [98]. Accordingly, a review by Greig on the prodrug type loxoprofen concluded that although the incidence of adverse effects did not differ between topical and oral therapy, the risk-to- benefit ratio is in favor of topical administration in long-term treatment [99].

NSAIDs have been proven to be useful in the treatment of postoperative pain triggered by inflammatory mediators such as bradikinin, PGE2 and cytokines, which are released after tissue damage. Even though the opioids are very effective drugs in the relief of this type of pain, which can become chronic if not properly treated, they are unable to block the inflammatory component. In addition, opiate-related adverse effects usually affect patient satisfaction. In this context, NSAIDs can be considered to be as effective as opioids, and their use in association when necessary allows for reduction of the opioid dosage. Gupta and Bah concluded in their study that NSAIDs can be administered to all patients who require postoperative acute pain relief, but patients who have undergone colorectal surgery should be treated with caution [100]. The work of Fujita and colleagues supports the use of NSAIDs in the treatment of postoperative pain. They have demonstrated, in a rat model of postoperative pain, that COX inhibition with the NSAIDs piroxicam and ketorolac at an early stage after the incision surgery is important for the suppression of allodynia that develops at the same time as inflammation [101]. Therefore, early treatment or even pre- treatment with NSAIDs before surgery helps reduce the pain associated with COX activation. In addition, although NSAIDs inhibit the expression of TXA2, which is involved in platelet aggregation, there is no evidence of significant perioperative bleeding[102].

One of the most important adverse eﬀects of NSAIDs is

cardiovascular (CV) risk, which is associated with vascular COX-2 inhibition and the consequent reduction in the synthesis of prostanoids [103,104]. The extent of CV risk depends on multiple factors, such as COX-2 selectivity, the dose responsivity, the plasma half-life, and the interaction with ASA. As an example, naproxen has demonstrated lower risk compared with other nonselective and COX-2 selective NSAIDs because of an antiplatelet eﬀect similar to ASA, and, due to its relatively long half-life of 14 h, induces a persistent inhibition of thromboxane production. On the other hand, diclofenac has generally been associated with the highest CV risk among nonselective NSAIDs and, due to its much shorter half-life, has more transient antiplatelet eﬀects. Treat- ment with celecoxib, COX-2 inhibitor, implies an increased risk of CV events, especially at higher doses but also when used twice-daily[105].

Many clinical studies have demonstrated that low doses of NSAIDs in short-term treatment can effectively reduce the CV risk. The EMA and FDA recommend the use of the lowest effective dose for the shortest time for the relief of the symptoms[106]. Recently, some pharmaceu- tical attempts have been made to determine the proper formulations for efficaciously delivering the drug without increasing the risk of CV.

Among the FDA-approved formulations, one uses the SoluMatrix tech- nology, with the capsules containing submicroscopic particles that improve the pharmacokinetics of diclofenac, reducing the dosage by up to 20%, which leads to a significant reduction of the pain 48 h after administration[107]. Another uses beta-cyclodextrins in the form of a complex with diclofenac for topical subcutaneous self-administration in pre-filled syringes, which allows the dosage to be decreased to as low as 25 mg. The efficacy and safety profiles of this new formulation were demonstrated in a number of studies for the treatment of pain following orthopedic surgery or neuropathic pain[108–111]. All these new formulations have been well tolerated, and no serious adverse effects have been reported so far. This means that increasing the efficiency of delivery and decreasing the dosage can be as safe as efficacious, and thus ensure the reduction of inflammation and the associated pain.

Among the pre-clinical studies conducted so far, one treatment that looks promising is the formulation of diclofenac sodium-loaded alginate microspheres-chitosan hydrogel that is intra-articularly administered for the treatment of arthritis and osteoarthritis. This intra-articular in- jectable treatment targets morbid joints to achieve drug enrichment and minimizes the probability of drug exposure to normal tissue[112].

Another promising study uses gellan gum, a bacterial exo-poly- saccharide, for topical delivery of diclofenac. This method seems to be an alcohol-free alternative to the current commercially available topical diclofenac formulations[113].

8. Conclusions

Several studies have attempted to unravel the molecular mechanisms underlying the role of the inflammatory mediators that trigger nociception, and, in the near future, it is expected that new pharmacological treatments will be developed for the simultaneous treatment of inflammation and pain. Nevertheless, so far, the most useful available drugs for these purposes are NSAIDs, the use of which is sometimes restricted by their adverse effects, depending on the type of NSAID, dose and treatment regimen. In order to circumvent this problem, choosing the appropriate administration route, developing new formulations and lowering the efficacious dose are rather effective means of managing inflammation and, therefore, interfering with the process of pain sensitization, without inducing significant side effects.

Authors contribution

S.R. wrote the manuscript, G.M. and D.V.D. substantially revised the manuscript adding important criticisms.

(6)

Funding

This research did not receive any speciﬁc grant from funding agencies in the public, commercial, or not-for-proﬁt sectors.

Conﬂict of interest

S.R. has received honoraria from IBSA Farmaceutici Italia srl. G.M.

and D.V.D. declare no conﬂict of interest.

References

[1] A. Scott, K.M. Khan, J.L. Cook, V. Duronio, What is“inﬂammation”? Are we ready to move beyond Celsus? Br. J. Sports Med. 38 (3) (2004) 248–249.

[2] I.E. Demir, K.H. Schafer, E. Tieftrunk, H. Friess, G.O. Ceyhan, Neural plasticity in the gastrointestinal tract: chronic inﬂammation, neurotrophic signals, and hypersensitivity, Acta Neuropathol. 125 (4) (2013) 491–509.

[3] A. Latremoliere, C.J. Woolf, Central sensitization: a generator of pain hypersensitivity by central neural plasticity, J. Pain 10 (9) (2009) 895–926.

[4] R.D. Treede, Gain control mechanisms in the nociceptive system, Pain 157 (6) (2016) 1199–1204.

[5] B.A. Vogt, R.W. Sikes, The medial pain system, cingulate cortex, and parallel processing of nociceptive information, Prog. Brain Res. 122 (2000) 223–235.

[6] Y. Geng, F.J. Blanco, M. Cornelisson, M. Lotz, Regulation of cyclooxygenase-2 expression in normal human articular chondrocytes, J. Immunol. 155 (2) (1995) 796–801.

[7] M. Burian, G. Geisslinger, COX-dependent mechanisms involved in the antinociceptive action of NSAIDs at central and peripheral sites, Pharmacol. Ther. 107 (2) (2005) 139–154.

[8] M.G. Perrone, A. Scilimati, L. Simone, P. Vitale, Selective COX-1 inhibition: a therapeutic target to be reconsidered, Curr. Med. Chem. 17 (32) (2010) 3769–3805.

[9] T. Aoki, S. Narumiya, Prostaglandins and chronic inﬂammation, Trends Pharmacol. Sci. 33 (6) (2012) 304–311.

[10] S. England, S. Bevan, R.J. Docherty, PGE2 modulates the tetrodotoxin-resistant sodium current in neonatal rat dorsal root ganglion neurones via the cyclic AMP- protein kinase A cascade, J. Physiol. 495 (Pt 2) (1996) 429–440.

[11] T. Kumazawa, K. Mizumura, H. Koda, Involvement of EP3 subtype of prostaglandin E receptors in PGE2-induced enhancement of the bradykinin response of nociceptors, Brain Res. 632 (1–2) (1993) 321–324.

[12] J.B. Daher, C.R. Tonussi, A spinal mechanism for the peripheral anti-inﬂammatory action of indomethacin, Brain Res. 962 (1–2) (2003) 207–212.

[13] R.R. Ji, Peripheral and central mechanisms of inﬂammatory pain, with emphasis on MAP kinases, Curr. Drug Targets Inﬂamm. Allergy 3 (3) (2004) 299–303.

[14] B. St-Jacques, W. Ma, Peripheral prostaglandin E2 prolongs the sensitization of nociceptive dorsal root ganglion neurons possibly by facilitating the synthesis and anterograde axonal traﬃcking of EP4 receptors, Exp. Neurol. 261 (2014) 354–366.

[15] G. Petho, P.W. Reeh, Sensory and signaling mechanisms of bradykinin, eicosa- noids, platelet-activating factor, and nitric oxide in peripheral nociceptors, Physiol. Rev. 92 (4) (2012) 1699–1775.

[16] D. Vardeh, D. Wang, M. Costigan, M. Lazarus, C.B. Saper, C.J. Woolf, G.A. Fitzgerald, T.A. Samad, COX2 in CNS neural cells mediates mechanical in- ﬂammatory pain hypersensitivity in mice, J. Clin. Invest. 119 (2) (2009) 287–294.

[17] W. Ma, B. St-Jacques, P.C. Duarte, Targeting pain mediators induced by injured nerve-derived COX2 and PGE2 to treat neuropathic pain, Expert Opin. Ther.

Targets 16 (6) (2012) 527–540.

[18] F. Calabrese, A.C. Rossetti, G. Racagni, P. Gass, M.A. Riva, R. Molteni, Brain-derived neurotrophic factor: a bridge between inﬂammation and neuroplasticity, Front. Cell. Neurosci. 8 (2014) 430.

[19] K. Thibault, W.K. Lin, A. Rancillac, M. Fan, T. Snollaerts, V. Sordoillet, M. Hamon, G.M. Smith, Z. Lenkei, S. Pezet, BDNF-dependent plasticity induced by peripheral inﬂammation in the primary sensory and the cingulate cortex triggers cold allodynia and reveals a major role for endogenous BDNF as a tuner of the aﬀective aspect of pain, J. Neurosci. 34 (44) (2014) 14739–14751.

[20] R.R. Ji, Z.Z. Xu, Y.J. Gao, Emerging targets in neuroinﬂammation-driven chronic pain, Nat. Rev. Drug Discov. 13 (7) (2014) 533–548.

[21] G.R. Lewin, A. Rueﬀ, L.M. Mendell, Peripheral and central mechanisms of NGF- induced hyperalgesia, Eur. J. Neurosci. 6 (12) (1994) 1903–1912.

[22] S.A. van Diest, O.I. Stanisor, G.E. Boeckxstaens, W.J. de Jonge, R.M. van den Wijngaard, Relevance of mast cell-nerve interactions in intestinal nociception, Biochim. Biophys. Acta 1822 (1) (2012) 74–84.

[23] A.K. Clark, E.A. Old, M. Malcangio, Neuropathic pain and cytokines: current perspectives, J. Pain Res. 6 (2013) 803–814.

[24] D. Gruber-Schoﬀnegger, R. Drdla-Schutting, C. Honigsperger,

G. Wunderbaldinger, M. Gassner, J. Sandkuhler, Induction of thermal hyperalgesia and synaptic long-term potentiation in the spinal cord lamina I by TNF-alpha and IL-1beta is mediated by glial cells, J. Neurosci. 33 (15) (2013) 6540–6551.

[25] C.I. Svensson, M. Schafers, T.L. Jones, H. Powell, L.S. Sorkin, Spinal blockade of TNF blocks spinal nerve ligation-induced increases in spinal P-p38, Neurosci. Lett.

379 (3) (2005) 209–213.

[26] L. Vallieres, S. Rivest, Regulation of the genes encoding interleukin-6, its receptor, and gp130 in the rat brain in response to the immune activator lipopolysaccharide

and the proinﬂammatory cytokine interleukin-1beta, J. Neurochem. 69 (4) (1997) 1668–1683.

[27] M.S. Ramer, P.G. Murphy, P.M. Richardson, M.A. Bisby, Spinal nerve lesion-induced mechanoallodynia and adrenergic sprouting in sensory ganglia are atte- nuated in interleukin-6 knockout mice, Pain 78 (2) (1998) 115–121.

[28] J.L. Arruda, S. Sweitzer, M.D. Rutkowski, J.A. DeLeo, Intrathecal anti-IL-6 antibody and IgG attenuates peripheral nerve injury-induced mechanical allodynia in the rat: possible immune modulation in neuropathic pain, Brain Res. 879 (1–2) (2000) 216–225.

[29] A.S. de Hooge, F.A. van de Loo, M.B. Bennink, O.J. Arntz, P. de Hooge, W.B. van den, Berg, Male IL-6 gene knock out mice developed more advanced osteoarthritis upon aging, Osteoarthr. Cartil. 13 (1) (2005) 66–73.

[30] C. Sun, J. Zhang, L. Chen, T. Liu, G. Xu, C. Li, W. Yuan, H. Xu, Z. Su, IL-17 con- tributed to the neuropathic pain following peripheral nerve injury by promoting astrocyte proliferation and secretion of proinﬂammatory cytokines, Mol. Med.

Rep. 15 (1) (2017) 89–96.

[31] S.H. Ferreira, B.B. Lorenzetti, A.F. Bristow, S. Poole, Interleukin-1 beta as a potent hyperalgesic agent antagonized by a tripeptide analogue, Nature 334 (6184) (1988) 698–700.

[32] J.A. DeLeo, R.W. Colburn, A.J. Rickman, Cytokine and growth factor im- munohistochemical spinal proﬁles in two animal models of mononeuropathy, Brain Res. 759 (1) (1997) 50–57.

[33] K. Ren, R. Torres, Role of interleukin-1beta during pain and inﬂammation, Brain Res. Rev. 60 (1) (2009) 57–64.

[34] H. Zhang, F. Li, W.W. Li, C. Stary, J.D. Clark, S. Xu, X. Xiong, The inﬂammasome as a target for pain therapy, Br. J. Anaesth. 117 (6) (2016) 693–707.

[35] E. Haroon, C.L. Raison, A.H. Miller, Psychoneuroimmunology meets neuropsychopharmacology: translational implications of the impact of inﬂammation on behavior, Neuropsychopharmacology 37 (1) (2012) 137–162.

[36] E.S. Wohleb, D.B. McKim, J.F. Sheridan, J.P. Godbout, Monocyte trafficking to the brain with stress and inflammation: a novel axis of immune-to-brain commu- nication that influences mood and behavior, Front. Neurosci. 8 (2014) 447.

[37] D. Cruz-Topete, J.A. Cidlowski, One hormone, two actions: anti- and pro-in- ﬂammatory eﬀects of glucocorticoids, Neuroimmunomodulation 22 (1–2) (2015) 20–32.

[38] D.A. Padgett, R. Glaser, How stress inﬂuences the immune response, Trends Immunol. 24 (8) (2003) 444–448.

[39] N.C. Nicolaides, E. Kyratzi, A. Lamprokostopoulou, G.P. Chrousos, E. Charmandari, Stress, the stress system and the role of glucocorticoids, Neuroimmunomodulation 22 (1–2) (2015) 6–19.

[40] M. Maes, J. Lambrechts, E. Bosmans, J. Jacobs, E. Suy, C. Vandervorst, C. de Jonckheere, B. Minner, J. Raus, Evidence for a systemic immune activation during depression: results of leukocyte enumeration byﬂow cytometry in conjunction with monoclonal antibody staining, Psychol. Med. 22 (1) (1992) 45–53.

[41] B.E. Leonard, Pain, depression and inﬂammation: are interconnected causative factors involved? Mod. Trends Pharmacopsychiatry 30 (2015) 22–35.

[42] M. Berk, O. Dean, H. Drexhage, J.J. McNeil, S. Moylan, A. O'Neil, C.G. Davey, L. Sanna, M. Maes, Aspirin: a review of its neurobiological properties and therapeutic potential for mental illness, BMC Med. 11 (2013) 74.

[43] M. Maes, Targeting cyclooxygenase-2 in depression is not a viable therapeutic approach and may even aggravate the pathophysiology underpinning depression, Metab. Brain Dis. 27 (4) (2012) 405–413.

[44] I.M. Chiu, B.A. Heesters, N. Ghasemlou, C.A. Von Hehn, F. Zhao, J. Tran, B. Wainger, A. Strominger, S. Muralidharan, A.R. Horswill, J. Bubeck Wardenburg, S.W. Hwang, M.C. Carroll, C.J. Woolf, Bacteria activate sensory neurons that modulate pain and inﬂammation, Nature 501 (7465) (2013) 52–57.

[45] H.O. Handwerker, Advancing our understanding of the mechanisms and mediators underlying pain and inﬂammation, Clin. Drug Invest. 27 (Suppl. 1) (2007) 1–6.

[46] S.I. Choi, J.Y. Lim, S. Yoo, H. Kim, S.W. Hwang, Emerging role of spinal cord TRPV1 in pain exacerbation, Neural Plast. 2016 (2016) 5954890.

[47] M. Silva, D. Martins, A. Charrua, F. Piscitelli, I. Tavares, C. Morgado, V.Di Marzo, Endovanilloid control of pain modulation by the rostroventromedial medulla in an animal model of diabetic neuropathy, Neuropharmacology 107 (2016) 49–57.

[48] A. Koivisto, H. Chapman, N. Jalava, T. Korjamo, M. Saarnilehto, K. Lindstedt, A. Pertovaara, TRPA1: a transducer and ampliﬁer of pain and inﬂammation, Basic Clin. Pharmacol. Toxicol. 114 (1) (2014) 50–55.

[49] H. Reinold, S. Ahmadi, U.B. Depner, B. Layh, C. Heindl, M. Hamza, A. Pahl, K. Brune, S. Narumiya, U. Muller, H.U. Zeilhofer, Spinal inﬂammatory hyperalgesia is mediated by prostaglandin E receptors of the EP2 subtype, J. Clin. Invest.

115 (3) (2005) 673–679.

[50] T. Moriyama, T. Higashi, K. Togashi, T. Iida, E. Segi, Y. Sugimoto, T. Tominaga, S. Narumiya, M. Tominaga, Sensitization of TRPV1 by EP1 and IP reveals peripheral nociceptive mechanism of prostaglandins, Mol. Pain 1 (2005) 3.

[51] W. Ma, B. St-Jacques, U. Rudakou, Y.N. Kim, Stimulating TRPV1 externalization and synthesis in dorsal root ganglion neurons contributes to PGE2 potentiation of TRPV1 activity and nociceptor sensitization, Eur. J. Pain 4 (2016).

[52] A. Grant, S. Amadesi, N.W. Bunnett, Protease-activated receptors: mechanisms by which proteases sensitize TRPV channels to induce neurogenic inﬂammation and pain, in: W.B. Liedtke, S. Heller (Eds.), TRP Ion Channel Function in Sensory Transduction and Cellular Signaling Cascades, CRC Press/Taylor & Francis, Boca Raton (FL), 2007.

[53] F. Gieseler, H. Ungefroren, U. Settmacher, M.D. Hollenberg, R. Kaufmann, Proteinase-activated receptors (PARs)–focus on receptor-receptor-interactions and their physiological and pathophysiological impact, Cell Commun. Signal. CCS 11 (2013) 86.

[54] K.A. Alier, J.A. Endicott, P.L. Stemkowski, N. Cenac, L. Cellars, K. Chapman,

(7)

P. Andrade-Gordon, N. Vergnolle, P.A. Smith, Intrathecal administration of proteinase-activated receptor-2 agonists produces hyperalgesia by exciting the cell bodies of primary sensory neurons, J. Pharmacol. Exp. Ther. 324 (1) (2008) 224–233.

[55] S. Amadesi, J. Nie, N. Vergnolle, G.S. Cottrell, E.F. Grady, M. Trevisani, C. Manni, P. Geppetti, J.A. McRoberts, H. Ennes, J.B. Davis, E.A. Mayer, N.W. Bunnett, Protease-activated receptor 2 sensitizes the capsaicin receptor transient receptor potential vanilloid receptor 1 to induce hyperalgesia, J. Neurosci. 24 (18) (2004) 4300–4312.

[56] B. Fingleton, Matrix metalloproteinases as regulators of inﬂammatory processes, Biochim. Biophys. Acta 1864 (11 Pt A) (2017) 2036–2042.

[57] S.E. Lakhan, M. Avramut, Matrix metalloproteinases in neuropathic pain and migraine: friends, enemies, and therapeutic targets, Pain Res. Treat. 2012 (2012) 952906.

[58] F.A. White, H. Jung, R.J. Miller, Chemokines and the pathophysiology of neuropathic pain, Proc. Natl. Acad. Sci. U. S. A. 104 (51) (2007) 20151–20158.

[59] M. Hollborn, C. Stathopoulos, A. Steﬀen, P. Wiedemann, L. Kohen, A. Bringmann, Positive feedback regulation between MMP-9 and VEGF in human RPE cells, Invest. Ophthalmol. Vis. Sci. 48 (9) (2007) 4360–4367.

[60] F.A. Russell, J.J. McDougall, Proteinase activated receptor (PAR) involvement in mediating arthritis pain and inﬂammation, Inﬂamm. Res. 58 (3) (2009) 119–126.

[61] Y. Liu, O. Abdel Samad, L. Zhang, B. Duan, Q. Tong, C. Lopes, R.R. Ji, B.B. Lowell, Q. Ma, VGLUT2-dependent glutamate release from nociceptors is required to sense pain and suppress itch, Neuron 68 (3) (2010) 543–556.

[62] G. Scherrer, S.A. Low, X. Wang, J. Zhang, H. Yamanaka, R. Urban, C. Solorzano, B. Harper, T.S. Hnasko, R.H. Edwards, A.I. Basbaum, VGLUT2 expression in primary aﬀerent neurons is essential for normal acute pain and injury-induced heat hypersensitivity, Proc. Natl. Acad. Sci. U. S. A. 107 (51) (2010) 22296–22301.

[63] I. Nishihara, T. Minami, Y. Watanabe, S. Ito, O. Hayaishi, Prostaglandin E2 stimulates glutamate release from synaptosomes of rat spinal cord, Neurosci. Lett.

196 (1–2) (1995) 57–60.

[64] T.Y. Lin, C.W. Lu, C.C. Wang, S.K. Huang, S.J. Wang, Cyclooxygenase 2 inhibitor celecoxib inhibits glutamate release by attenuating the PGE2/EP2 pathway in rat cerebral cortex endings, J. Pharmacol. Exp. Ther. 351 (1) (2014) 134–145.

[65] R.M. Langford, V. Mehta, Selective cyclooxygenase inhibition: its role in pain and anaesthesia, Biomed. Pharmacother. 60 (7) (2006) 323–328.

[66] L.F. Ferrari, C.M. Lotufo, D. Araldi, M.A. Rodrigues, L.P. Macedo, S.H. Ferreira, C.A. Parada, Inﬂammatory sensitization of nociceptors depends on activation of NMDA receptors in DRG satellite cells, Proc. Natl. Acad. Sci. U. S. A. 111 (51) (2014) 18363–18368.

[67] D. Bleakman, A. Alt, E.S. Nisenbaum, Glutamate receptors and pain, Semin. Cell Dev. Biol. 17 (5) (2006) 592–604.

[68] M. Niesters, A. Dahan, Pharmacokinetic and pharmacodynamic considerations for NMDA receptor antagonists in the treatment of chronic neuropathic pain, Expert Opin. Drug Metab. Toxicol. 8 (11) (2012) 1409–1417.

[69] S. Collins, M.J. Sigtermans, A. Dahan, W.W. Zuurmond, R.S. Perez, NMDA receptor antagonists for the treatment of neuropathic pain, Pain Med. 11 (11) (2010) 1726–1742.

[70] H. Radwani, O. Roca-Lapirot, F. Aby, M.J. Lopez-Gonzalez, R. Benazzouz, M. Errami, A. Favereaux, M. Landry, P. Fossat, [EXPRESS] Group I metabotropic glutamate receptor plasticity after peripheral inﬂammation alters nociceptive transmission in the dorsal of the spinal cord in adult rats, Mol. Pain 13 (2017) 1744806917737934.

[71] M.J. Kim, Y.H. Park, K.Y. Yang, J.S. Ju, Y.C. Bae, S.K. Han, D.K. Ahn, Participation of central GABAA receptors in the trigeminal processing of mechanical allodynia in rats, Korean J. Physiol. Pharmacol. 21 (1) (2017) 65–74.

[72] D.M. Dirig, T.L. Yaksh, Intrathecal baclofen and muscimol, but not midazolam, are antinociceptive using the rat-formalin model, J. Pharmacol. Exp. Ther. 275 (1) (1995) 219–227.

[73] M.J. Eaton, M.A. Martinez, S. Karmally, A single intrathecal injection of GABA permanently reverses neuropathic pain after nerve injury, Brain Res. 835 (2) (1999) 334–339.

[74] G. Munro, P.K. Ahring, N.R. Mirza, Developing analgesics by enhancing spinal inhibition after injury: GABAA receptor subtypes as novel targets, Trends Pharmacol. Sci. 30 (9) (2009) 453–459.

[75] P.M. Grace, K.A. Strand, E.L. Galer, D.J. Urban, X. Wang, M.V. Baratta, T.J. Fabisiak, N.D. Anderson, K. Cheng, L.I. Greene, D. Berkelhammer, Y. Zhang, A.L. Ellis, H.H. Yin, S. Campeau, K.C. Rice, B.L. Roth, S.F. Maier, L.R. Watkins, Morphine paradoxically prolongs neuropathic pain in rats by amplifying spinal NLRP3 inﬂammasome activation, Proc. Natl. Acad. Sci. U. S. A. 113 (24) (2016) E3441–E3450.

[76] E. Latz, T.S. Xiao, A. Stutz, Activation and regulation of the inﬂammasomes, Nat.

Rev. Immunol. 13 (6) (2013) 397–411.

[77] A. Ellis, P.M. Grace, J. Wieseler, J. Favret, K. Springer, B. Skarda, M. Ayala, M.R. Hutchinson, S. Falci, K.C. Rice, S.F. Maier, L.R. Watkins, Morphine ampliﬁes mechanical allodynia via TLR4 in a rat model of spinal cord injury, Brain Behav.

Immun. 58 (2016) 348–356.

[78] T.M. O'Connor, J. O'Connell, D.I. O'Brien, T. Goode, C.P. Bredin, F. Shanahan, The role of substance P in inﬂammatory disease, J. Cell Physiol. 201 (2) (2004) 167–180.

[79] V.S. Chauhan, D.G. Sterka Jr., D.L. Gray, K.L. Bost, I. Marriott, Neurogenic exacerbation of microglial and astrocyte responses to Neisseria meningitidis and Borrelia burgdorferi, J. Immunol. 180 (12) (2008) 8241–8249.

[80] M.B. Johnson, A.D. Young, I. Marriott, The therapeutic potential of targeting substance P/NK-1R interactions in inﬂammatory CNS disorders, Front. Cell.

Neurosci. 10 (2016) 296.

[81] F. Corrigan, K.A. Mander, A.V. Leonard, R. Vink, Neurogenic inﬂammation after traumatic brain injury and its potentiation of classical inﬂammation, J.

Neuroinﬂamm. 13 (1) (2016) 264.

[82] G. McCaﬀrey, M.J. Seelbach, W.D. Staatz, N. Nametz, C. Quigley, C.R. Campos, T.A. Brooks, T.P. Davis, Occludin oligomeric assembly at tight junctions of the blood-brain barrier is disrupted by peripheral inﬂammatory hyperalgesia, J.

Neurochem. 106 (6) (2008) 2395–2409.

[83] W.A. Banks, A.M. Gray, M.A. Erickson, T.S. Salameh, M. Damodarasamy, N. Sheibani, J.S. Meabon, E.E. Wing, Y. Morofuji, D.G. Cook, M.J. Reed, Lipopolysaccharide-induced blood-brain barrier disruption: roles of cyclooxygenase, oxidative stress, neuroinﬂammation, and elements of the neurovascular unit, J. Neuroinﬂamm. 12 (2015) 223.

[84] A. Varatharaj, I. Galea, The blood-brain barrier in systemic inﬂammation, Brain Behav. Immun. 60 (2017) 1–12.

[85] J.J. Lochhead, P.T. Ronaldson, T.P. Davis, Hypoxic stress and inﬂammatory pain disrupt blood-brain barrier tight junctions: implications for drug delivery to the central nervous system, AAPS J. 19 (4) (2017) 910–920.

[86] J.S. Mogil, Sex diﬀerences in pain and pain inhibition: multiple explanations of a controversial phenomenon, Nat. Rev. Neurosci. 13 (12) (2012) 859–866.

[87] A. Amandusson, A. Blomqvist, Estrogenic inﬂuences in pain processing, Front.

Neuroendocrinol. 34 (4) (2013) 329–349.

[88] R.E. Sorge, M.L. LaCroix-Fralish, A.H. Tuttle, S.G. Sotocinal, J.S. Austin, J. Ritchie, M.L. Chanda, A.C. Graham, L. Topham, S. Beggs, M.W. Salter, J.S. Mogil, Spinal cord Toll-like receptor 4 mediates inﬂammatory and neuropathic hypersensitivity in male but not female mice, J. Neurosci. 31 (43) (2011) 15450–15454.

[89] R.E. Sorge, J.C. Mapplebeck, S. Rosen, S. Beggs, S. Taves, J.K. Alexander, L.J. Martin, J.S. Austin, S.G. Sotocinal, D. Chen, M. Yang, X.Q. Shi, H. Huang, N.J. Pillon, P.J. Bilan, Y. Tu, A. Klip, R.R. Ji, J. Zhang, M.W. Salter, J.S. Mogil, Diﬀerent immune cells mediate mechanical pain hypersensitivity in male and female mice, Nat. Neurosci. 18 (8) (2015) 1081–1083.

[90] S. Taves, T. Berta, D.L. Liu, S. Gan, G. Chen, Y.H. Kim, T. Van de Ven, S. Laufer, R.R. Ji, Spinal inhibition of p38 MAP kinase reduces inﬂammatory and neuropathic pain in male but not female mice: sex-dependent microglial signaling in the spinal cord, Brain Behav. Immun. 55 (2016) 70–81.

[91] N.L. Chillingworth, S.G. Morham, L.F. Donaldson, Sex differences in inflammation and inflammatory pain in cyclooxygenase-deficient mice, Am. J. Physiol. Regul.

Integr. Comp. Physiol. 291 (2) (2006) R327–R334.

[92] H.H. Doyle, A.Z. Murphy, Sex diﬀerences in innate immunity and its impact on opioid pharmacology, J. Neurosci. Res. 95 (1–2) (2017) 487–499.

[93] P. Blancafort, J. Jin, S. Frye, Writing and rewriting the epigenetic code of cancer cells: from engineered proteins to small molecules, Mol. Pharmacol. 83 (3) (2013) 563–576.

[94] L.M. Villeneuve, R. Natarajan, The role of epigenetics in the pathology of diabetic complications, Am. J. Physiol. Ren. Physiol. 299 (1) (2010) F14–F25.

[95] F. Denk, S.B. McMahon, Chronic pain: emerging evidence for the involvement of epigenetics, Neuron 73 (3) (2012) 435–444.

[96] G. Bai, K. Ren, R. Dubner, Epigenetic regulation of persistent pain, Transl. Res. 165 (1) (2015) 177–199.

[97] B.R. da Costa, S. Reichenbach, N. Keller, L. Nartey, S. Wandel, P. Juni, S. Trelle, Eﬀectiveness of non-steroidal anti-inﬂammatory drugs for the treatment of pain in knee and hip osteoarthritis: a network meta-analysis, Lancet 387 (10033) (2016) 2093–2105.

[98] O. Bruyere, C. Cooper, J.P. Pelletier, E. Maheu, F. Rannou, J. Branco, M. Luisa Brandi, J.A. Kanis, R.D. Altman, M.C. Hochberg, J. Martel-Pelletier, J.Y. Reginster, A consensus statement on the European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO) algorithm for the management of knee Osteoarthritis-From evidence-based medicine to the real-life setting, Semin. Arthr.

Rheum. 45 (4 Suppl) (2016) S3–S11.

[99] S.L. Greig, K.P. Garnock-Jones, Loxoprofen: a review in pain and inﬂammation, Clin. Drug Invest. 36 (9) (2016) 771–781.

[100] A. Gupta, M. Bah, NSAIDs in the treatment of postoperative pain, Curr. Pain Headache Rep. 20 (11) (2016) 62.

[101] I. Fujita, T. Okumura, A. Sakakibara, Y. Kita, Involvement of inﬂammation in severe post-operative pain demonstrated by pre-surgical and post-surgical treatment with piroxicam and ketorolac, J. Pharm. Pharmacol. 64 (5) (2012) 747–755.

[102] R.R. Nir, H. Nahman-Averbuch, R. Moont, E. Sprecher, D. Yarnitsky, Preoperative preemptive drug administration for acute postoperative pain: a systematic review and meta-analysis, Eur. J. Pain 20 (7) (2016) 1025–1043.

[103] W.J. Elliott, Do the blood pressure effects of nonsteroidal antiinflammatory drugs influence cardiovascular morbidity and mortality? Curr. Hypertens. Rep. 12 (4) (2010) 258–266.

[104] C.C. Chan, C.M. Reid, T.J. Aw, D. Liew, S.J. Haas, H. Krum, Do COX-2 inhibitors raise blood pressure more than nonselective NSAIDs and placebo? An updated meta-analysis, J. Hypertens 27 (12) (2009) 2332–2341.

[105] N. Pawlosky, Cardiovascular risk: are all NSAIDs alike? Can. Pharm. J. 146 (2) (2013) 80–83.

[106] F. Franceschi, L. Saviano, C. Petruzziello, M. Gabrielli, L. Santarelli, L. Capaldi, M.Di Leo, A. Migneco, E. Gilardi, G. Merra, V. Ojetti, Safety and eﬃcacy of low doses of diclofenac on acute pain in the emergency setting, Eur. Rev. Med.

Pharmacol. Sci. 20 (20) (2016) 4401–4408.

[107] R. Altman, M. Hochberg, A. Gibofsky, M. Jaros, C. Young, Eﬃcacy and safety of low-dose SoluMatrix meloxicam in the treatment of osteoarthritis pain: a 12-week, phase 3 study, Curr. Med. Res. Opin. 31 (12) (2015) 2331–2343.

[108] T. Dietrich, R. Leeson, B. Gugliotta, B. Petersen, Eﬃcacy and safety of low dose subcutaneous diclofenac in the management of acute pain: a randomized double- blind trial, Pain Pract. 14 (4) (2014) 315–323.

(8)

[109] E. Chiarello, S. Bernasconi, B. Gugliotta, S. Giannini, Subcutaneous injection of diclofenac for the treatment of pain following minor orthopedic surgery (DIRECT study): a randomized trial, Pain Pract. 15 (1) (2015) 31–39.

[110] H.A. Blair, G.L. Plosker, Diclofenac sodium injection (akis((R)), dicloin ((R))): a review of its use in the management of pain, Clin. Drug Invest. 35 (6) (2015) 397–404.

[111] C. Scavone, A.C. Bonagura, S. Fiorentino, D. Cimmaruta, R. Cenami, M. Torella, T. Fossati, F. Rossi, Eﬃcacy and safety proﬁle of diclofenac/cyclodextrin and

progesterone/cyclodextrin formulations: a review of the literature data, Drugs R&D 16 (2) (2016) 129–140.

[112] X. Qi, X. Qin, R. Yang, J. Qin, W. Li, K. Luan, Z. Wu, L. Song, Intra-articular administration of chitosan thermosensitive in situ hydrogels combined with diclofenac sodium-loaded alginate microspheres, J. Pharm. Sci. 105 (1) (2016) 122–130.

[113] M.H. Mahdi, B.R. Conway, T. Mills, A.M. Smith, Gellan gumﬂuid gels for topical administration of diclofenac, Int. J. Pharm. 515 (1–2) (2016) 535–542.