European Journal of Pharmacology (v.716, #1-3)

Pharmacology of pain by Richard Przewlocki (1).

Noradrenaline, through action on α1- and α2-adrenoceptors, is involved in intrinsic control of pain. Peripheral noradrenaline that is mainly released by the sympathetic nervous system has little influence on healthy tissues, whereas in injured or inflamed tissues it has varying effects, including aggravation of pain in neuropathy. The peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ion channel properties on primary afferent nociceptors, whereas an interaction with the immune system may contribute to peripheral antinociceptive effect of noradrenaline. In the spinal dorsal horn, noradrenaline released from descending pathways originating in the pontine A5–A7 cell groups attenuates pain by inhibitory action on α2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct α2-adrenergic action on spinal pain-relay neurons (postsynaptic inhibition), and by α1-adrenergic activation of inhibitory interneurons. Moreover, α2C-adrenoceptors on axon terminals of excitatory interneurons might contribute to spinal control of pain. At supraspinal levels, the effect of noradrenergic system on pain has varied depending on many factors such as the type of the adrenoceptor, pathophysiological condition, and the brain area. In general, the baseline pain sensitivity is only little influenced by the noradrenergic system, whereas in injured conditions the noradrenergic system contributes to feedback inhibition of pain. The central as well as the peripheral noradrenergic system is subject to various plastic changes following injury or inflammation that influence its antinociceptive efficiency. α2-Adrenoceptor agonists have proven effective in treating various pain conditions.
Keywords: Adrenergic receptors; α2-Adrenoceptor agonist; Descending noradrenergic pathways; Locus coeruleus; Nociceptive nerve fiber; Noradrenergic pain regulation; Spinal dorsal horn;

Multiple roles of serotonin in pain control mechanisms —Implications of 5-HT7 and other 5-HT receptor types by Florent Viguier; Benoît Michot; Michel Hamon; Sylvie Bourgoin (8-16).
Among monoamine neurotransmitters, serotonin (5-HT) is known to play complex modulatory roles in pain signaling mechanisms since the first reports, about forty years ago, on its essentially pro-nociceptive effects at the periphery and anti-nociceptive effects when injected directly at the spinal cord level. The discovery of multiple 5-HT receptor subtypes allowed possible explanations to this dual action at the periphery versus the central nervous system (CNS) since both excitatory and inhibitory effects can be exerted through 5-HT activation of different 5-HT receptors. However, it also appeared that activation of the same receptor subtype at CNS level might induce variable effects depending on the physiological or pathophysiological status of the animal administered with agonists. In particular, the marked neuroplastic changes induced by nerve lesion, which account for sensitization of pain signaling mechanisms, can contribute to dramatic changes in the effects of a given 5-HT receptor agonist in neuropathic rats versus intact healthy rats. This has notably been observed with 5-HT7 receptor agonists which exert a pronociceptive action in healthy rats but alleviate hyperalgesia consecutive to nerve lesion in neuropathic animals. Analysis of cellular mechanisms underlying such dual 5-HT actions mediated by a single receptor subtype indicates that the neuronal phenotype which expresses this receptor also plays a key role in determining which modulatory action 5-HT would finally exert on pain signaling mechanisms.
Keywords: Serotonin; Nociception; Neuropathic pain; 5-HT receptors; Hyperalgesia; GABA interneurons;

GABAA receptor modulation: Potential to deliver novel pain medicines? by Gordon Munro; Rikke R. Hansen; Naheed R. Mirza (17-23).
GABA (γ-aminobutyric acid) is abundantly expressed within the brain, and spinal cord pain circuits where it acts as the principal mediator of fast inhibitory neurotransmission. However, drugs that target GABAA receptor function such as the classical benzodiazepines have not been optimised to promote analgesia, are limited by side effects and are not routinely used for this purpose in humans. Compounds such as NS11394, L-838,417, HZ166 and TPA023 all bind to the same benzodiazepine site on the GABAA receptor to allosterically modulate receptor function and enhance the actions of GABA. By virtue of their ability to activate selected subtypes of GABAA receptors (principally those containing α2, α3 and α5 subunits) these compounds have been shown to possess excellent tolerability profiles in animals. Importantly, a number of these molecules also mediate profound analgesia in animal models of inflammatory and neuropathic pain. Other modulators such as neurosteroids bind to distinct sites on GABAA receptor α subunits, possess a unique pharmacology and are capable of targeting alternative GABAA receptor expressing populations. Moreover, neurosteroids also have pronounced analgesic actions in animal pain models. The continuing call for novel mechanism of action analgesics to target specific pathologies, especially in clinical neuropathic conditions, emphasizes the need to test modulators of GABAA receptor function in both human experimental pain models and pain patients.
Keywords: Allodynia; Benzodiazepine; Disinhibition; Hyperalgesia; Neuropathic; Neurosteroid;

Purinergic mechanisms and pain—An update by Geoffrey Burnstock (24-40).
There is a brief summary of the background literature about purinergic signalling. The review then considers purinergic mechanosensory transduction involved in visceral, cutaneous and musculoskeletal nociception and on the roles played by P2X3, P2X2/3, P2X4, P2X7 and P2Y12 receptors in neuropathic and inflammatory pain. Current developments of compounds for the therapeutic treatment of both visceral and neuropathic pain are discussed.
Keywords: ATP; Bladder; Gut; Inflammatory; Mechanosensory transduction; Neuropathic;

The exploitation of preparations of Cannabis sativa to combat pain seems to date back to time immemorial, although their psychotropic effects, which are at the bases of their recreational use and limit their therapeutic use, are at least as ancient. Indeed, it has always been different to tease apart the unwanted central effects from the therapeutic benefits of Δ9-tetrahydrocannabinol (THC), the main psychotropic component of cannabis. The discovery of the cannabinoid receptors and of their endogenous ligands, the endocannabinoids, which, unlike THC, play a pro-homeostatic function in a tissue- and time-selective manner, offered the opportunity to develop new analgesics from synthetic inhibitors of endocannabinoid inactivation. The advantages of this approach over direct activation of cannabinoid receptors as a therapeutic strategy against neuropathic and inflammatory pain are discussed here along with its potential complications. These latter have been such that clinical success has been achieved so far more rapidly with naturally occurring THC or endocannabinoid structural analogues acting at a plethora of cannabinoid-related and -unrelated molecular targets, than with selective inhibitors of endocannabinoid enzymatic hydrolysis, thus leading to revisit the potential usefulness of “multi-target” versus “magic bullet” compounds as new analgesics.
Keywords: Endocannabinoid system; CB1 receptors; TRPV1 receptors; Inflammatory pain; Neuropathic pain;

Neurotensin (NT) is a tridecapeptide, which – since its discovery in 1973 – has been demonstrated to be involved in the control of various physiological activities in both the central nervous system and in the periphery. Its biological effects are mediated by four receptor types. Exogenously administered NT exerts different behavioral effects, including antinociception. Structure–activity relationship studies performed in recent years resulted in development of several peptidomimetic receptor agonists and non-peptidic receptor antagonists that are useful tools for studies of NT mechanisms in tissue and on cellular level. This may result in design of new generation of analgesics based on neurotensin. NT antinociceptive effects are distinct from opioid analgesia. This creates opportunity of development of hybride analgesics that may simultaneously activate both opioid and NT antinociceptive pathways.Display Omitted
Keywords: Neurotensin; Neurotensin receptor; Pain; Antinociception;

Targeting TRP channels for pain relief by Jill-Desiree Brederson; Philip R. Kym; Arpad Szallasi (61-76).
Preclinical research has recently uncovered new molecular mechanisms underlying the generation and transduction of pain, many of which represent opportunities for pharmacological intervention. Manipulating temperature-sensitive Transient Receptor Potential (TRP) channels (so-called “thermoTRPs”) on nociceptive neurons is a particularly attractive strategy in that it targets the beginning of the pain pathway. In the focus of current drug development efforts are the heat-sensitive TRPV1, warm-activated TRPV3, cold-responsive TRPA1, and cool-activated TRPM8 channels. TRPV1 desensitization by topical agonists (e.g. high concentration capsaicin creams and patches) has been in clinical use for decades to alleviate chronic painful conditions like diabetic neuropathy. Currently, site-specific resiniferatoxin (an ultrapotent capsaicin analogue) injections are being evaluated as “molecular scalpels” to achieve permanent analgesia in cancer patients with chronic, intractable pain. In the past few years a number of potent, small molecule TRPV1, TRPV3 and TRPA1 antagonists have been advanced into clinical trials for the treatment of inflammatory, neuropathic and visceral pain. TRPM8 antagonists are following closely behind for cold allodynia. Early TRPV1 antagonists in the clinic, however, showed worrisome adverse effects including hyperthermia and impaired noxious heat sensation. These adverse effects placed the patients at risk for scalding injury and prompted their withdrawal from the clinical trials. Second generation TRPV1 antagonists that do not cause core body temperature elevation have been reported, although the therapeutic utility of this class of compounds is not yet known. This review discusses the promise and challenges of developing TRP channel antagonists as a new generation of pain therapeutics.
Keywords: ThermoTRP channels; TRPV1; TRPV3; TRPM8; TRPA1; Resiniferatoxin; Capsaicin;

Sigma 1 receptor: A new therapeutic target for pain by Daniel Zamanillo; Luz Romero; Manuel Merlos; José Miguel Vela (78-93).
Sigma 1 receptor (σ1 receptor) is a unique ligand-regulated molecular chaperone located mainly in the endoplasmic reticulum and the plasma membrane. σ1 receptor is activated under stress or pathological conditions and interacts with several neurotransmitter receptors and ion channels to modulate their function. The effects reported preclinically with σ1 receptor ligands are consistent with a role for σ1 receptor in central sensitization and pain hypersensitivity and suggest a potential therapeutic use of σ1 receptor antagonists for the management of neuropathic pain as monotherapy. Moreover, data support their use in opioid adjuvant therapy: combination of σ1 receptor antagonists and opioids results in potentiation of opioid analgesia, without significant increases in opioid-related unwanted effects. Results from clinical trials using selective σ1 receptor antagonists in several pain conditions are eagerly awaited to ascertain the potential of σ1 receptor modulation in pain therapy.
Keywords: Sigma 1 receptor; Pain; Analgesia; Opioid; Neuropathic; Sensitization;

The plasticity of the association between mu-opioid receptor and glutamate ionotropic receptor N in opioid analgesic tolerance and neuropathic pain by Pilar Sánchez-Blázquez; Maria Rodríguez-Muñoz; Esther Berrocoso; Javier Garzón (94-105).
Multiple groups have reported the functional cross-regulation between mu-opioid (MOP) receptor and glutamate ionotropic receptor N (GluN), and the post-synaptic association of these receptors has been implicated in the transmission and modulation of nociceptive signals. Opioids, such as morphine, disrupt the MOP receptor–GluN receptor complex to stimulate the activity of GluN receptors via protein kinase C (PKC)/Src. This increased GluN receptor activity opposes MOP receptor signalling, and via neural nitric oxide synthase (nNOS) and calcium and calmodulin regulated kinase II (CaMKII) induces the phosphorylation and uncoupling of the opioid receptor, which results in the development of morphine analgesic tolerance. Both experimental in vivo activation of GluN receptors and neuropathic pain separate the MOP receptor–GluN receptor complex via protein kinase A (PKA) and reduce the analgesic capacity of morphine. The histidine triad nucleotide-binding protein 1 (HINT1) associates with the MOP receptor C-terminus and connects the activities of MOP receptor and GluN receptor. In HINT1(−/−) mice, morphine promotes enhanced analgesia and produces tolerance that is not related to GluN receptor activity. In these mice, the GluN receptor agonist N-methyl-d-aspartate acid (NMDA) does not antagonise the analgesic effects of morphine. Treatments that rescue morphine from analgesic tolerance, such as GluN receptor antagonism or PKC, nNOS and CaMKII inhibitors, all induce MOP receptor–GluN receptor re-association and reduce GluN receptor/CaMKII activity. In mice treated with NMDA or suffering from neuropathic pain (induced by chronic constriction injury, CCI), GluN receptor antagonists, PKA inhibitors or certain antidepressants also diminish CaMKII activity and restore the MOP receptor–GluN receptor association. Thus, the HINT1 protein stabilises the association between MOP receptor and GluN receptor, necessary for the analgesic efficacy of morphine, and this coupling is reduced following the activation of GluN receptors, similar to what is observed in neuropathic pain.
Keywords: Mu-opioid receptor; Glutamate ionotropic receptor N; Neuropathic pain; Analgesic tolerance; Allodynia; Analgesia;

Importance of glial activation in neuropathic pain by Joanna Mika; Magdalena Zychowska; Katarzyna Popiolek-Barczyk; Ewelina Rojewska; Barbara Przewlocka (106-119).
Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.
Keywords: Neuropathic pain; Astroglia; Microglia; Opioids; Glial inhibitors;

Astrocytes—Multitaskers in chronic pain by Rikke Rie Hansen; Marzia Malcangio (120-128).
Treatment of chronic pain remains a clinical challenge and sufficient pharmacological management is difficult to achieve without concurrent adverse drug effects. Recently the concept of chronic pain as a solely neuron-mediated phenomenon has evolved and it is now appreciated that also glial cells are of critical importance in pain generation and modulation. Astrocytes are macroglial cells that have close structural relationships with neurons; they contact neuronal somata and dendrites and enwrap synapses, where small astrocytic processes have been shown to be highly motile. This organization allows astrocytes to directly influence and coordinate neurons located within their structural domains. Moreover, astrocytes form astroglial networks and calcium wave propagations can spread through neighbouring astrocytes. ATP, which is released from astrocytes in response to elevated intracellular calcium concentrations, can contribute to the central mechanisms in chronic pain via purinergic receptors. In this review we highlight the structural organization and the functionalities of astrocytes that allow them to undertake critical roles in pain processing and we stress the possibility that astrocytes contribute to chronic pain not via a single pathway, but by undertaking various roles depending on the pain condition.
Keywords: Astrocyte; Pain; c-Jun N-terminal kinase; ATP;

Large panel of gene-based techniques is used for many years specifically in the pain research field. From the first identification (cloning) of some “mythic” genes, such as those encoding opioid or capsaicin receptors allowing then the creation of first-generation knockout mice, to the today conditional (time, tissue, cell-type and even pathology-dependent) and regulatable modulation of a gene function, these approaches largely contributed to fundamental leaps forward in our understanding of the function of some proteins and of their interest as possible druggable targets. Perhaps one of the most remarkable evolution in the last years is the passage of these approaches from the bench to the patient; whether it concerns the identification of genes involved in inherited pain insensibility/susceptibility, the search for genetic markers of pain types, the individual pharmacogenomics or even the first gene therapy trials. From many possible variants of gene-grounded techniques used in pain research we focus here on gene knockouts and some recent developments, on viral vectors-based gene transfer and on transgenic models for the tracing of pain pathways. Through these selected examples we attempted to emphasize the immense potential of these approaches and their already well-recognized contribution in both the basic and clinical pain research.
Keywords: Gene expression modification; Gene transfer; Viral vector; Primary sensory neuron; Spinal cord; Nerve pathways tracing;

Involvement of the opioid and cannabinoid systems in pain control: New insights from knockout studies by Xavier Nadal; Carmen La Porta; S. Andreea Bura; Rafael Maldonado (142-157).
The endogenous opioid and cannabinoid systems are involved in the physiological inhibitory control of pain and are of particular interest for the development of therapeutic approaches for pain management. The involvement of these endogenous systems in pain control has been studied from decades by the use of compounds with different affinities for each cannabinoid and opioid receptor or for the different enzymes involved in endocannabinoid and endogenous opioid metabolism. However, the selectivity of these pharmacological tools in vivo has represented an important limitation for these studies. The generation of genetically modified mice with selective mutations in specific components of the endocannabinoid and endogenous opioid system has provided important advances in the identification of the specific contribution of each component of these endogenous systems in the perception of noxious stimuli and the development of pathological pain states. Different lines of constitutive and conditional knockout mice deficient in specific cannabinoid and opioid receptors, specific precursors of the endogenous opioid peptides and the main enzymes involved in endocannabinoid and endogenous opioid degradation are now available. These knockout mice have also been used to evaluate the contribution of each component of the endocannabinoid and opioid system in the antinociceptive effects of cannabinoid and opioid agonists, including those currently used to treat pain in humans. This review summarizes the main advances provided in the last 15 years by the use of these genetic tools in the knowledge of the physiological control of pain and the pharmacology of cannabinoid and opioid compounds for pain management.
Keywords: Opioid receptor; Cannabinoid receptor; Fatty-acid amide hydrolase; Monoacylglycerol lipase; Neprilysin; Tolerance;

Pain is a private experience that involves both sensory and emotional components. Animal studies of pain can only be inferred by their responses, and therefore the measurement of reflexive responses dominates the pain literature for nearly a century. It has been argued that although reflexive responses are important to unveil the sensory nature of pain in organisms, pain affect is equally important but largely ignored in pain studies primarily due to the lack of validated animal models. One strategy to begin to understand pain affect is to use conditioning principles to indirectly reveal the affective condition of pain. This review critically analyzed several procedures that are thought to measure affective learning of pain. The procedures regarding the current knowledge, the applications, and their advantages and disadvantages in pain research are discussed. It is proposed that these procedures should be combined with traditional reflex-based pain measurements in future studies of pain, which could greatly benefit both the understanding of neural underpinnings of pain and preclinical assessment of novel analgesics.
Keywords: Pavlovian conditioning; Operant conditioning; Animal model; Pain; Pain affect;

Peripheral mechanisms of burn injury-associated pain by Helen Laycock; Joao Valente; Carsten Bantel; Istvan Nagy (169-178).
It is estimated that burn injury affects about 1 in 3000 people annually world-wide. Burn injury induces severe pain, which is difficult to control in the great majority of cases making burn injury-associated pain a serious clinical challenge. In order to meet this clinical need, novel targets should be identified. Like pain developing in other peripheral pathologies, burn injury-associated pain is also initiated and maintained by signalling between the injured tissues and primary sensory fibres that supply those tissues. This signalling is underlain by the formation and accumulation of a mixture of agents at the site of the injury, some of which could be targets for intervention. However, at present the composition of this “burn injury tissue fluid” is incompletely established. Here, we summarise our current understanding of the composition of this burn injury tissue fluid and explore how already known agents in that tissue fluid may activate nociceptors to initiate and maintain pain associated with burn injury.
Keywords: Burn; Inflammatory; Nociception; Nociceptor; Primary sensory neuron;

Opioids, sensory systems and chronic pain by Christoph Stein (179-187).
Opioids are the oldest and most potent drugs for the treatment of severe pain. Their clinical application is undisputed in acute pain (e.g. associated with trauma or surgery) but their long-term use in chronic pain has met increasing scrutiny. Therefore, this article will review sensory mechanisms related to opioid analgesia and side effects with a special emphasis on chronic pain. Central and peripheral sites of analgesic actions and side effects, as well as conventional and novel opioid compounds will be discussed. Since pain is a complex bio-psycho-social phenomenon, non-pharmacological considerations important for the understanding of opioid analgesic efficacy are also included. Finally, examples of challenging clinical situations such as the perioperative management of patients receiving long-term opioid treatment are illustrated.
Keywords: Morphine; Inflammation; Arthritis; Dorsal root ganglion; Peripheral; Chonic pain; Non-malignant pain; Cancer pain; Surgery; Postoperative pain; Dependence; Addiction; Tolerance; Clinical applications;

Genes, molecules and patients—Emerging topics to guide clinical pain research by Shafaq Sikandar; Ryan Patel; Sital Patel; Sanam Sikander; David L.H. Bennett; Anthony H. Dickenson (188-202).
This review selectively explores some areas of pain research that, until recently, have been poorly understood. We have chosen four topics that relate to clinical pain and we discuss the underlying mechanisms and related pathophysiologies contributing to these pain states. A key issue in pain medicine involves crucial events and mediators that contribute to normal and abnormal pain signaling, but remain unseen without genetic, biomarker or imaging analysis. Here we consider how the altered genetic make-up of familial pains reveals the human importance of channels discovered by preclinical research, followed by the contribution of receptors as stimulus transducers in cold sensing and cold pain. Finally we review recent data on the neuro-immune interactions in chronic pain and the potential targets for treatment in cancer-induced bone pain.