The mu opioid agonist morphine modulates potentiation of capsaicin-evoked TRPV1 responses through a cyclic AMP-dependent protein kinase A pathway

Opioid withdrawal increases transient receptor potential vanilloid 1 activity in a protein kinase A-dependent manner

Hyperalgesia is a cardinal symptom of opioid withdrawal. The transient receptor potential vanilloid 1 (TRPV1) is a ligand-gated ion channel expressed on sensory neurons responding to noxious heat, protons, and chemical stimuli such as capsaicin. TRPV1 can be inhibited via μ-opioid receptor (MOR)-mediated reduced activity of adenylyl cyclases (ACs) and decreased cyclic adenosine monophosphate (cAMP) levels. In contrast, opioid withdrawal following chronic activation of MOR uncovers AC superactivation and subsequent increases in cAMP and protein kinase A (PKA) activity. Here we investigated (1) whether an increase in cAMP during opioid withdrawal increases the activity of TRPV1 and (2) how opioid withdrawal modulates capsaicin-induced nocifensive behavior in rats. We applied whole-cell patch clamp, microfluorimetry, cAMP assays, radioligand binding, site-directed mutagenesis, and behavioral experiments. Opioid withdrawal significantly increased cAMP levels and capsaicin-induced TRPV1 activity in both transfected human embryonic kidney 293 cells and dissociated dorsal root ganglion (DRG) neurons. Inhibition of AC and PKA, as well as mutations of the PKA phosphorylation sites threonine 144 and serine 774, prevented the enhanced TRPV1 activity. Finally, capsaicin-induced nocifensive behavior was increased during opioid withdrawal in vivo. In summary, our results demonstrate an increased activity of TRPV1 in DRG neurons as a new mechanism contributing to opioid withdrawal-induced hyperalgesia.

HCN1 and HCN2 in Rat DRG neurons: levels in nociceptors and non-nociceptors, NT3-dependence and influence of CFA-induced skin inflammation on HCN2 and NT3 expression

I_h, which influences neuronal excitability, has recently been measured in vivo in sensory neuron subtypes in dorsal root ganglia (DRGs). However, expression levels of HCN (hyperpolarization-activated cyclic nucleotide-gated) channel proteins that underlie I_h were unknown. We therefore examined immunostaining of the most abundant isoforms in DRGs, HCN1 and HCN2 in these neuron subtypes. This immunostaining was cytoplasmic and membrane-associated (ring). Ring-staining for both isoforms was in neurofilament-rich A-fiber neurons, but not in small neurofilament-poor C-fiber neurons, although some C-neurons showed cytoplasmic HCN2 staining. We recorded intracellularly from DRG neurons in vivo, determined their sensory properties (nociceptive or low-threshold-mechanoreceptive, LTM) and conduction velocities (CVs). We then injected fluorescent dye enabling subsequent immunostaining. For each dye-injected neuron, ring- and cytoplasmic-immunointensities were determined relative to maximum ring-immunointensity. Both HCN1- and HCN2-ring-immunointensities were positively correlated with CV in both nociceptors and LTMs; they were high in Aβ-nociceptors and Aα/β-LTMs. High HCN1 and HCN2 levels in Aα/β-neurons may, via I_h, influence normal non-painful (e.g. touch and proprioceptive) sensations as well as nociception and pain. HCN2-, not HCN1-, ring-intensities were higher in muscle spindle afferents (MSAs) than in all other neurons. The previously reported very high I_h in MSAs may relate to their very high HCN2. In normal C-nociceptors, low HCN1 and HCN2 were consistent with their low/undetectable I_h. In some C-LTMs HCN2-intensities were higher than in C-nociceptors. Together, HCN1 and HCN2 expressions reflect previously reported I_h magnitudes and properties in neuronal subgroups, suggesting these isoforms underlie I_h in DRG neurons. Expression of both isoforms was NT3-dependent in cultured DRG neurons. HCN2-immunostaining in small neurons increased 1 day after cutaneous inflammation (CFA-induced) and recovered by 4 days. This could contribute to acute inflammatory pain. HCN2-immunostaining in large neurons decreased 4 days after CFA, when NT3 was decreased in the DRG. Thus HCN2-expression control differs between large and small neurons.

Randomized control trial of topical clonidine for treatment of painful diabetic neuropathy

A length-dependent neuropathy with pain in the feet is a common complication of diabetes (painful diabetic neuropathy). It was hypothesized that pain may arise from sensitized-hyperactive cutaneous nociceptors, and that this abnormal signaling may be reduced by topical administration of the α(2)-adrenergic agonist, clonidine, to the painful area. This was a randomized, double-blind, placebo-controlled, parallel-group, multicenter trial. Nociceptor function was measured by determining the painfulness of 0.1% topical capsaicin applied to the pretibial area of each subject for 30minutes during screening. Subjects were then randomized to receive 0.1% topical clonidine gel (n=89) or placebo gel (n=90) applied 3 times a day to their feet for 12weeks. The difference in foot pain at week 12 in relation to baseline, rated on a 0-10 numerical pain rating scale (NPRS), was compared between groups. Baseline NPRS was imputed for missing data for subjects who terminated the study early. The subjects treated with clonidine showed a trend toward decreased foot pain compared to the placebo-treated group (the primary endpoint; P=0.07). In subjects who felt any level of pain to capsaicin, clonidine was superior to placebo (P<0.05). In subjects with a capsaicin pain rating ⩾2 (0-10, NPRS), the mean decrease in foot pain was 2.6 for active compared to 1.4 for placebo (P=0.01). Topical clonidine gel significantly reduces the level of foot pain in painful diabetic neuropathy subjects with functional (and possibly sensitized) nociceptors in the affected skin as revealed by testing with topical capsaicin. Screening for cutaneous nociceptor function may help distinguish candidates for topical therapy for neuropathic pain.

Agonist- and Ca2+-dependent desensitization of TRPV1 channel targets the receptor to lysosomes for degradation

TRPV1 receptor agonists such as the vanilloid capsaicin and the potent analog resiniferatoxin are well known potent analgesics. Depending on the vanilloid, dose, and administration site, nociceptor refractoriness may last from minutes up to months, suggesting the contribution of different cellular mechanisms ranging from channel receptor desensitization to Ca(2+) cytotoxicity of TRPV1-expressing neurons. The molecular mechanisms underlying agonist-induced TRPV1 desensitization and/or tachyphylaxis are still incompletely understood. Here, we report that prolonged exposure of TRPV1 to agonists induces rapid receptor endocytosis and lysosomal degradation in both sensory neurons and recombinant systems. Agonist-induced receptor internalization followed a clathrin- and dynamin-independent endocytic route, triggered by TRPV1 channel activation and Ca(2+) influx through the receptor. This process appears strongly modulated by PKA-dependent phosphorylation. Taken together, these findings indicate that TRPV1 agonists induce long-term receptor down-regulation by modulating the expression level of the channel through a mechanism that promotes receptor endocytosis and degradation and lend support to the notion that cAMP signaling sensitizes nociceptors through several mechanisms.

New strategies to develop novel pain therapies: addressing thermoreceptors from different points of view

One approach to develop successful pain therapies is the modulation of dysfunctional ion channels that contribute to the detection of thermal, mechanical and chemical painful stimuli. These ion channels, known as thermoTRPs, promote the sensitization and activation of primary sensory neurons known as nociceptors. Pharmacological blockade and genetic deletion of thermoTRP have validated these channels as therapeutic targets for pain intervention. Several thermoTRP modulators have progressed towards clinical development, although most failed because of the appearance of unpredicted side effects. Thus, there is yet a need to develop novel channel modulators with improved therapeutic index. Here, we review the current state-of-the art and illustrate new pharmacological paradigms based on TRPV1 that include: (i) the identification of activity-dependent modulators of this thermoTRP channel; (ii) the design of allosteric modulators that interfere with protein-protein interaction involved in the functional coupling of stimulus sensing and gate opening; and (iii) the development of compounds that abrogate the inflammation-mediated increase of receptor expression in the neuronal surface. These new sites of action represent novel strategies to modulate pathologically active TRPV1, while minimizing an effect on the TRPV1 subpopulation involved in physiological and protective roles, thus increasing their potential therapeutic use.

Modulation of urinary bladder innervation: TRPV1 and botulinum toxin A

The persisting interest around neurotoxins such as vanilloids and botulinum toxin (BoNT) derives from their marked effect on detrusor overactivity refractory to conventional antimuscarinic treatments. In addition, both are administered by intravesical route. This offers three potential advantages. First, intravesical therapy is an easy way to provide high concentrations of pharmacological agents in the bladder tissue without causing unsuitable levels in other organs. Second, drugs effective on the bladder, but inappropriate for systemic administration, can be safely used as it is the case of vanilloids and BoNT. Third, the effects of one single treatment might be extremely longlasting, contributing to render these therapies highly attractive to patients despite the fact that the reasons to the prolonged effect are still incompletely understood. Attractive as it may be, intravesical pharmacological therapy should still be considered as a second-line treatment in patients refractory to conventional oral antimuscarinic therapy or who do not tolerate its systemic side effects. However, the increasing off-label use of these neurotoxins justifies a reappraisal of their pharmacological properties.

Molecular assays for characterization of alternatively spliced isoforms of the u opioid receptor (MOR)

Mu-opioid receptor (MOR) belongs to a family of heptahelical G-protein-coupled receptors (GPCRs). Studies in humans and rodents demonstrated that the OPRM1 gene coding for MOR undergoes extensive alternative splicing afforded by the genetic complexity of OPRM1. Evidence from rodent studies also demonstrates an important role of these alternatively spliced forms in mediating opiate analgesia via their differential signaling properties. MOR signaling is predominantly G(ia) coupled. Release of the alpha subunit from G-protein complex results in the inhibition of adenylyl cyclase/cAMP pathway, whereas release of the betagamma subunits activates G-protein-activated inwardly rectifying potassium channels and inhibits voltage-dependent calcium channels. These molecular events result in the suppression of cellular activities that diminish pain sensations. Recently, a new isoform of OPRM1, MOR3, has been identified that shows an increase in the production of nitric oxide (NO) upon stimulation with morphine. Hence, there is a need to describe molecular techniques that enable the functional characterization of MOR isoforms. In this review, we describe the methodologies used to assay key mediators of MOR activation including cellular assays for cAMP, free Ca(2+), and NO, all of which have been implicated in the pharmacological effects of MOR agonists.

TRPV1: on the road to pain relief

Historically, drug research targeted to pain treatment has focused on trying to prevent the propagation of action potentials in the periphery from reaching the brain rather than pinpointing the molecular basis underlying the initial detection of the nociceptive stimulus: the receptor itself. This has now changed, given that many receptors of nociceptive stimuli have been identified and/or cloned. Transient Receptor Potential (TRP) channels have been implicated in several physiological processes such as mechanical, chemical and thermal stimuli detection. Ten years after the cloning of TRPV1, compelling data has been gathered on the role of this channel in inflammatory and neuropathic states. TRPV1 activation in nociceptive neurons, where it is normally expressed, triggers the release of neuropeptides and transmitters resulting in the generation of action potentials that will be sent to higher CNS areas where they will often be perceived as pain. Its activation also will evoke the peripheral release of pro-inflammatory compounds that may sensitize other neurons to physical, thermal or chemical stimuli. For these reasons as well as because its continuous activation causes analgesia, TRPV1 has become a viable drug target for clinical use in the management of pain. This review will provide a general picture of the physiological and pathophysiological roles of the TRPV1 channel and of its structural, pharmacological and biophysical properties. Finally, it will provide the reader with an overall view of the status of the discovery of potential therapeutic agents for the management of chronic and neuropathic pain.

Morphine peripheral analgesia depends on activation of the PI3Kgamma/AKT/nNOS/NO/KATP signaling pathway

Morphine is one of the most prescribed and effective drugs used for the treatment of acute and chronic pain conditions. In addition to its central effects, morphine can also produce peripheral analgesia. However, the mechanisms underlying this peripheral action of morphine have not yet been fully elucidated. Here, we show that the peripheral antinociceptive effect of morphine is lost in neuronal nitric-oxide synthase null mice and that morphine induces the production of nitric oxide in primary nociceptive neurons. The activation of the nitric-oxide pathway by morphine was dependent on an initial stimulation of PI3Kgamma/AKT protein kinase B (AKT) and culminated in increased activation of K(ATP) channels. In the latter, this intracellular signaling pathway might cause a hyperpolarization of nociceptive neurons, and it is fundamental for the direct blockade of inflammatory pain by morphine. This understanding offers new targets for analgesic drug development.

Sustained morphine treatment augments capsaicin-evoked calcitonin gene-related peptide release from primary sensory neurons in a protein kinase A- and Raf-1-dependent manner

Studies have shown that long-term (5alpha,6alpha)-7,8-didehydro-4,5-epoxy-17-methylmorphinan-3,6-diol (morphine) treatment increases the sensitivity to painful heat stimuli (thermal hyperalgesia). The cellular adaptations contributing to sustained morphine-mediated pain sensitization are not fully understood. It was shown previously (J Neurosci 22:6747-6755, 2002) that sustained morphine exposure augments pain neurotransmitter [such as calcitonin gene-related peptide (CGRP)] release in the dorsal horn of the spinal cord in response to the heat-sensing transient receptor potential vanilloid 1 receptor agonist 8-methyl-N-vanillyl-6-nonenamide (capsaicin). In the present study, we demonstrate that sustained morphine-mediated augmentation of CGRP release from isolated primary sensory dorsal root ganglion neurons is dependent on protein kinase A and Raf-1 kinase. Our data indicate that, in addition to neural system adaptations, sustained opioid agonist treatment also produces intracellular compensatory adaptations in primary sensory neurons, leading to augmentation of evoked pain neurotransmitter release from these cells.

Capsaicin-evoked iCGRP release from human dental pulp: a model system for the study of peripheral neuropeptide secretion in normal healthy tissue

The mechanisms underlying trigeminal pain conditions are incompletely understood. In vitro animal studies have elucidated various targets for pharmacological intervention; however, a lack of clinical models that allow evaluation of viable innervated human tissue has impeded successful translation of many preclinical findings into clinical therapeutics. Therefore, we developed and characterized an in vitro method that evaluates the responsiveness of isolated human nociceptors by measuring basal and stimulated release of neuropeptides from collected dental pulp biopsies. Informed consent was obtained from patients presenting for extraction of normal wisdom teeth. Patients were anesthetized using nerve block injection, teeth were extracted and bisected, and pulp was removed and superfused in vitro. Basal and capsaicin-evoked peripheral release of immunoreactive calcitonin gene-related peptide (iCGRP) was analyzed by enzyme immunoassay. The presence of nociceptive markers within neurons of the dental pulp was characterized using confocal microscopy. Capsaicin increased the release of iCGRP from dental pulp biopsies in a concentration-dependent manner. Stimulated release was dependent on extracellular calcium, reversed by a TRPV1 receptor antagonist, and desensitized acutely (tachyphylaxis) and pharmacologically by pretreatment with capsaicin. Superfusion with phorbol 12-myristate 13-acetate (PMA) increased basal and stimulated release, whereas PGE2 augmented only basal release. Compared with vehicle treatment, pretreatment with PGE2 induced competence for DAMGO to inhibit capsaicin-stimulated iCGRP release, similar to observations in animal models where inflammatory mediators induce competence for opioid inhibition. These results indicate that the release of iCGRP from human dental pulp provides a novel tool to determine the effects of pharmacological compounds on human nociceptor sensitivity.

Functional Interaction Between TRPV1 and μ-Opioid Receptors in the Descending Antinociceptive Pathway Activates Glutamate Transmission and Induces Analgesia

The transient receptor potential vanilloid-1 (TRPV1) receptor is involved in peripheral and spinal nociceptive processing and is a therapeutic target for pain. We have shown previously that TRPV1 in the ventrolateral periaqueductal gray (VL-PAG) tonically contributes to brain stem descending antinociception by stimulating glutamate release into the rostral ventromedial medulla and off neuron activity. Because both opioid and vanilloid systems integrate and transduce pain sensation in these pathways, we studied the potential interaction between TRPV1 and μ-opioid receptors in the VL-PAG–rostral ventromedial medulla (RVM) system. We found that the TRPV1 agonist, capsaicin, and the μ-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin, when coadministered into the ventrolateral-PAG at doses nonanalgesic per se, produce 1) antinociception in tests of thermal nociception; 2) stimulation of glutamate release into the RVM; and 3) inhibition of on neuron activity in the RVM. These effects were all antagonized by the TRPV1 and opioid receptor antagonists 5′-iodo-resiniferatoxin and naloxone, respectively, thus suggesting the existence of a TRPV1–μ-opioid interaction in the VL-PAG–RVM system. By using double immunofluorescence techniques, we found that TRPV1 and μ-opioid receptors are coexpressed in several neurons of the VL-PAG. These findings suggest that μ-receptor activation not only acts on inhibitory neurons to disinhibit PAG output neurons but also interacts with TRPV1 activation at increasing glutamate release into the RVM, possibly by acting directly on PAG output neurons projecting to the RVM.

The pharmacological challenge to tame the transient receptor potential vanilloid-1 (TRPV1) nocisensor

The transient receptor potential vanilloid-1 (TRPV1) cation channel is a receptor that is activated by heat (>42 degrees C), acidosis (pH<6) and a variety of chemicals among which capsaicin is the best known. With these properties, TRPV1 has emerged as a polymodal nocisensor of nociceptive afferent neurones, although some non-neuronal cells and neurones in the brain also express TRPV1. The activity of TRPV1 is controlled by a multitude of regulatory mechanisms that either cause sensitization or desensitization of the channel. As many proalgesic pathways converge on TRPV1 and this nocisensor is upregulated and sensitized by inflammation and injury, TRPV1 is thought to be a central transducer of hyperalgesia and a prime target for the pharmacological control of pain. As a consequence, TRPV1 agonists causing defunctionalization of sensory neurones and a large number of TRPV1 blockers have been developed, some of which are in clinical trials. A major drawback of many TRPV1 antagonists is their potential to cause hyperthermia, and their long-term use may carry further risks because TRPV1 has important physiological functions in the peripheral and central nervous system. The challenge, therefore, is to pharmacologically differentiate between the physiological and pathological implications of TRPV1. There are several possibilities to focus therapy specifically on those TRPV1 channels that contribute to disease processes. These approaches include (i) site-specific TRPV1 antagonists, (ii) modality-specific TRPV1 antagonists, (iii) uncompetitive TRPV1 (open channel) blockers, (iv) drugs interfering with TRPV1 sensitization, (v) drugs interfering with intracellular trafficking of TRPV1 and (vi) TRPV1 agonists for local administration.

Intracellular signaling in primary sensory neurons and persistent pain

During evolution, living organisms develop a specialized apparatus called nociceptors to sense their environment and avoid hazardous situations. Intense stimulation of high threshold C- and Adelta-fibers of nociceptive primary sensory neurons will elicit pain, which is acute and protective under normal conditions. A further evolution of the early pain system results in the development of nociceptor sensitization under injury or disease conditions, leading to enhanced pain states. This sensitization in the peripheral nervous system is also called peripheral sensitization, as compared to its counterpart, central sensitization. Inflammatory mediators such as proinflammatory cytokines (TNF-alpha, IL-1beta), PGE(2), bradykinin, and NGF increase the sensitivity and excitability of nociceptors by enhancing the activity of pronociceptive receptors and ion channels (e.g., TRPV1 and Na(v)1.8). We will review the evidence demonstrating that activation of multiple intracellular signal pathways such as MAPK pathways in primary sensory neurons results in the induction and maintenance of peripheral sensitization and produces persistent pain. Targeting the critical signaling pathways in the periphery will tackle pain at the source.

Capsaicin inhibits the in vitro binding of peptides selective for mu- and kappa-opioid, and nociceptin-receptors

Capsaicin inhibited the equilibrium specific binding of endogenous opioid-like peptide ligands such as endomorphin-1, nociceptin, and dynorphin((1-17)) in rat brain membrane preparations. We studied the in vitro effect of capsaicin (1-10 microM) on homologous and heterologous competitive binding of opioid ligands, using unlabeled synthetic peptides and the following tritiated compounds: [(3)H]endomorphin-1, [(3)H]endomorphin-2, [(3)H]nociceptin((1-17)) and [(3)H]dynorphin((1-17)). Capsaicin-dependent inhibition was also observed in [(35)S]GTPgammaS stimulation assays in the presence of certain opioid peptides. The inhibition of opioid binding was further investigated using other synthetic and natural mu-opioid ligands such as [D-Ala(2),(NMe)Phe(4),Gly(5)-ol]enkephalin (DAMGO), morphine and naloxone. The decrease in opioid ligand affinity upon capsaicin treatments was most apparent with endomorphin-1, followed by nociceptin and dynorphin. The binding of other investigated opioids were not affected in the presence of capsaicin. In [(3)H]endomorphin-1 binding assays, capsazepine antagonized the inhibitory effect of capsaicin in rat brain membranes suggesting the involvement of TRPV1 receptors. In Chinese hamster ovary (CHO) cells stably expressing mu-opioid receptors, but lacking vanilloid receptors, the inhibition by capsaicin on the binding of [(3)H]endomorphin-1 was not present. It is concluded that the inhibitory effect of capsaicin on the receptor binding affinity of endogenous opioid peptides in brain membrane preparations seems not to be a direct effect, it is rather a negative feedback interaction with opioid receptors.

Rapid, opioid-sensitive mechanisms involved in transient receptor potential vanilloid 1 sensitization

TRPV1 is a nociceptive, Ca2+-selective ion channel involved in the development of several painful conditions. Sensitization of TRPV1 responses by cAMP-dependent PKA crucially contributes to the development of inflammatory hyperalgesia. However, the pathways involved in potentiation of TRPV1 responses by cAMP-dependent PKA remain largely unknown. Using HEK cells stably expressing TRPV1 and the mu opioid receptor, we demonstrated that treatment with the adenylate cyclase activator forskolin significantly increased the multimeric TRPV1 species. Pretreatment with the mu opioid receptor agonist morphine reversed this increased TRPV1 multimerization. FRET analysis revealed that treatment with forskolin did not cause multimerization of pre-existing TRPV1 monomers on the plasma membrane and that intracellular pools of TRPV1 exist mostly as monomers in this model. This suggests that increased TRPV1 multimerization occurred from an intracellular store of inactive TRPV1 monomers. Treatment with forskolin also caused an increase in TRPV1 expression on the plasma membrane not resulting from increased TRPV1 expression, and this rapid TRPV1 translocation was inhibited by treatment with morphine. Thus, potentiation of TRPV1 responses by cAMP-dependent PKA involves plasma membrane insertion of functional TRPV1 multimers formed from an intracellular store of inactive TRPV1 monomers. This potentiation occurs rapidly and can be dynamically modulated by activation of the mu opioid receptor under conditions where cAMP levels are raised, such as with inflammation. Increased translocation and multimerization of TRPV1 channels provide a cellular mechanism for fine-tuning of nociceptive responses that allow for rapid modulation of TRPV1 responses independent of transcriptional changes.

Capsaicin (TRPV1 Agonist) therapy for pain relief: farewell or revival?

OBJECTIVE

In this review, we explain our current understanding of the molecular basis for pain relief by capsaicin and other transient receptor potential vanilloid subfamily, member 1 (TRPV1) agonists. We summarize disease-related changes in TRPV1 expression and its implications for therapy and potential adverse effects. Last, we provide an overview of the current clinical uses of topical and injectable TRPV1 agonist preparations in both oncologic and nononcologic populations.

METHOD

Search of MEDLINE and other databases.

RESULTS

The capsaicin receptor TRPV1 is a polymodal nociceptor exhibiting a dynamic threshold of activation that could be lowered under inflammatory conditions. Consistent with this model, TRPV1 knock-out mice are devoid of post-inflammatory thermal hyperalgesia. TRPV1 desensitization of primary sensory neurons is a powerful approach to relieve symptoms of nociceptive behavior in animal models of chronic pain. However, over-the-counter capsaicin creams have shown moderate to poor analgesic efficacy. This is in part related to low dose, poor skin absorption, and compliance factors. Recently developed site-specific capsaicin therapy with high-dose patches and injectable preparations seem to be safe and reportedly provide long-lasting analgesia with rapid onset.

CONCLUSIONS

We argue that TRPV1 agonists and antagonists are not mutually exclusive but rather complimentary pharmacologic approaches for pain relief and we predict a "revival" for capsaicin and other TRPV1 agonists in the clinical management of pain associated with inflammation, metabolic imbalances (eg, diabetes), infections (HIV), and cancer, despite the current focus of the pharmaceutical industry on TRPV1 antagonists.

Phospholipase C and protein kinase A mediate bradykinin sensitization of TRPA1: a molecular mechanism of inflammatory pain

Bradykinin is an inflammatory mediator that plays a pivotal role in pain and hyperalgesia in inflamed tissues by exciting and/or sensitizing nociceptors. TRPA1 is an important component of the transduction machinery through which environmental irritants and endogenous proalgesic agents depolarize nociceptors to elicit inflammatory pain. Here, using electrophysiological, immunocytochemical and behavioural analyses, we showed a functional interaction of these two inflammation-related molecules in both heterologous expressing systems and primary sensory neurons. We found that bradykinin increased the TRPA1 currents evoked by allyl isothiocyanate (AITC) or cinnamaldehyde in HEK293 cells expressing TRPA1 and bradykinin receptor 2 (B2R). This potentiation was inhibited by phospholipase C (PLC) inhibitor or protein kinase A (PKA) inhibitor, and mimicked by PLC or PKA activator. The functional interaction between B2R and TRPA1, as well as the modulation mechanism, was also observed in rat dorsal root ganglia neurons. In an occlusion experiment, the PLC activator could enhance AITC-induced TRPA1 current further even in saturated PKA-mediated potentiation, indicating the additive potentiating effects of the PLC and PKA pathways. These data for the first time indicate that a cAMP-PKA signalling is involved in the downstream from B2R in dorsal root ganglia neurons in addition to PLC. Finally, subcutaneous pre-injection of a sub-inflammatory dose of bradykinin into rat hind paw enhanced AITC-induced pain behaviours, which was consistent with the observations in vitro. Collectively, these results represent a novel mechanism through which bradykinin released in response to tissue inflammation might trigger the sensation of pain by TRPA1 activation.

Targeting of opioid-producing leukocytes for pain control

It is accepted that inflammatory mediators released from leukocytes contribute to the generation of pain. However, it is less well known that immune cells also produce mediators that can effectively counteract pain. These include anti-inflammatory cytokines and opioid peptides. This article concentrates on recent evidence that interactions between leukocyte-derived opioid peptides and their receptors on peripheral sensory neurons can result in potent, clinically relevant inhibition of pathological pain. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G-protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, all these changes lead to an enhanced peripheral analgesic efficacy of opioids. The major source of local endogenous opioid ligands (beta-endorphin, enkephalins, endomorphins and dynorphin) are leukocytes. These cells contain and upregulate signal-sequence encoding mRNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor-triggered release of Ca(2+) from endoplasmic reticulum. Once secreted opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of excitatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness or addiction. Future aims include the selective targeting of opioid-containing leukocytes to sites of painful injury and the augmentation of opioid peptide and receptor synthesis.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

GPR35 is a functional receptor in rat dorsal root ganglion neurons

GPR35, previously an orphan G-protein coupled receptor, is a receptor for kynurenic acid. Here we examine the distribution of GPR35 in the rat dorsal root ganglion (DRG) and the effects of its selective activation. GPR35 was expressed predominantly by small- to medium-diameter neurons of the DRG. Many of these same neurons also expressed the transient receptor potential vanilloid 1 channel, a nociceptive neuronal marker. The GPR35 agonists kynurenic acid and zaprinast inhibited forskolin-stimulated cAMP production by cultured rat DRG neurons. Inhibition required G(i/o) proteins as the effect was completely abolished by pretreatment with pertussis toxin. This is the first study to report the expression and function of GPR35 in rat nociceptive DRG neurons. We propose that GPR35 modulates nociception and that continued study of this receptor will provide additional insight into the role of kynurenic acid in pain perception.

Mechanisms involved in potentiation of transient receptor potential vanilloid 1 responses by ethanol

The transient receptor potential vanilloid 1 or TRPV1 is a calcium-permeable ion channel that is activated by capsaicin, the active component of hot chilli peppers, and is involved in the development of inflammatory and neuropathic hyperalgesias. Ethanol can sensitise TRPV1-mediated responses, but the pathways contributing to the potentiation of TRPV1 by ethanol have not been clearly defined. Since the mu opioid receptor (MOP) agonist morphine can inhibit TRPV1 responses potentiated by cAMP-dependent protein kinase A (PKA), and ethanol-mediated modulation of other ion channels involves activation of PKA, we aimed to assess the contribution of MOP-sensitive pathways to the potentiation of TRPV1-mediated capsaicin responses by ethanol. Calcium responses elicited by the TRPV1 agonist capsaicin were potentiated by treatment with ethanol, but morphine was not able to inhibit ethanol-sensitised capsaicin responses. Indeed, cAMP-dependent PKA did not appear to contribute to potentiation of TRPV1 responses by ethanol, as the PKA inhibitor Rp-cAMPS did not inhibit ethanol-potentiated capsaicin responses. Similarly, treatment with specific PKC and PI3K inhibitors did not affect capsaicin responses in the presence of ethanol. However, treatment with wortmannin at concentrations reported to cause PIP2 depletion limited the ability of ethanol to sensitise TRPV1-mediated capsaicin responses. Among other plausible mechanisms, such as non-specific inhibition of kinases including mTOR, DNA-PK, MLCK, MAPK and polo-like kinases, this suggests that ethanol may affect the PIP2-TRPV1 interaction. This was confirmed by inhibition of ethanol-potentiation by the PLC inhibitor U73122. The results presented here suggest that morphine may be of limited use in inhibiting nociceptive TRPV1 responses that have been sensitised by exposure to ethanol.

The vanilloid receptor TRPV1: 10 years from channel cloning to antagonist proof-of-concept

The clinical use of TRPV1 (transient receptor potential vanilloid subfamily, member 1; also known as VR1) antagonists is based on the concept that endogenous agonists acting on TRPV1 might provide a major contribution to certain pain conditions. Indeed, a number of small-molecule TRPV1 antagonists are already undergoing Phase I/II clinical trials for the indications of chronic inflammatory pain and migraine. Moreover, animal models suggest a therapeutic value for TRPV1 antagonists in the treatment of other types of pain, including pain from cancer. We argue that TRPV1 antagonists alone or in conjunction with other analgesics will improve the quality of life of people with migraine, chronic intractable pain secondary to cancer, AIDS or diabetes. Moreover, emerging data indicate that TRPV1 antagonists could also be useful in treating disorders other than pain, such as urinary urge incontinence, chronic cough and irritable bowel syndrome. The lack of effective drugs for treating many of these conditions highlights the need for further investigation into the therapeutic potential of TRPV1 antagonists.

Inflammatory mediators modulating the transient receptor potential vanilloid 1 receptor: therapeutic targets to treat inflammatory and neuropathic pain

The transient receptor potential vanilloid 1 receptor (TRPV1) plays an important role in inflammatory heat hyperalgesia. TRPV1 is a non-selective cation channel gated by noxious heat, protons and capsaicin, thus being regarded as a polymodal molecular integrator in nociception. Abundant evidence has demonstrated that TRPV1 is also modulated by numerous inflammatory mediators, including growth factors, neurotransmitters, peptides or small proteins, lipids, chemokines and cytokines. By activating multiple protein kinases to increase the phosphorylation of TRPV1, pronociceptive inflammatory mediators sensitise the TRPV1 response to noxious heat, protons and capsaicin, thus augmenting thermal hyperalgesia. In contrast, by inhibiting protein kinases or other mechanisms, antinociceptive inflammatory mediators suppress the response of TRPV1 to these stimuli, thus damping thermal hyperalgesia. The positive modulation of TRPV1 by inflammatory mediators may constitute a novel mechanism underlying sustained inflammatory or neuropathic pain. Blocking pronociceptive inflammatory mediator-exerted sensitising effects or boosting antinociceptive inflammatory mediator-induced suppressing effects on TRPV1 should be considered as sources of novel potential therapies to more effectively treat chronic pain conditions.

Transient receptor potential cation channels in disease

The transient receptor potential (TRP) superfamily consists of a large number of cation channels that are mostly permeable to both monovalent and divalent cations. The 28 mammalian TRP channels can be subdivided into six main subfamilies: the TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPP (polycystin), TRPML (mucolipin), and the TRPA (ankyrin) groups. TRP channels are expressed in almost every tissue and cell type and play an important role in the regulation of various cell functions. Currently, significant scientific effort is being devoted to understanding the physiology of TRP channels and their relationship to human diseases. At this point, only a few channelopathies in which defects in TRP genes are the direct cause of cellular dysfunction have been identified. In addition, mapping of TRP genes to susceptible chromosome regions (e.g., translocations, breakpoint intervals, increased frequency of polymorphisms) has been considered suggestive of the involvement of these channels in hereditary diseases. Moreover, strong indications of the involvement of TRP channels in several diseases come from correlations between levels of channel expression and disease symptoms. Finally, TRP channels are involved in some systemic diseases due to their role as targets for irritants, inflammation products, and xenobiotic toxins. The analysis of transgenic models allows further extrapolations of TRP channel deficiency to human physiology and disease. In this review, we provide an overview of the impact of TRP channels on the pathogenesis of several diseases and identify several TRPs for which a causal pathogenic role might be anticipated.

Mu-opioid receptor activation modulates transient receptor potential vanilloid 1 (TRPV1) currents in sensory neurons in a model of inflammatory pain

Current therapy for inflammatory pain includes the peripheral application of opioid receptor agonists. Activation of opioid receptors modulates voltage-gated ion channels, but it is unclear whether opioids can also influence ligand-gated ion channels [e.g., the transient receptor potential vanilloid type 1 (TRPV1)]. TRPV1 channels are involved in the development of thermal hypersensitivity associated with tissue inflammation. In this study, we investigated mu-opioid receptor and TRPV1 expression in primary afferent neurons in the dorsal root ganglion (DRG) in complete Freund's adjuvant (CFA)-induced paw inflammation. In addition, the present study examined whether the activity of TRPV1 in DRG neurons can be inhibited by mu-opioid receptor (mu-receptor) ligands and whether this inhibition is increased after CFA inflammation. Immunohistochemistry demonstrated colocalization of TRPV1 and mu-receptors in DRG neurons. CFA-induced inflammation increased significantly the number of TRPV1- and mu-receptor-positive DRG neurons, as well as TRPV1 binding sites. In whole-cell patch clamp studies, opioids significantly decreased capsaicin-induced TRPV1 currents in a naloxone- and pertussis toxinsensitive manner. The inhibitory effect of morphine on TRPV1 was abolished by forskolin and 8-bromo-cAMP. During inflammation, an increase in TRPV1 is apparently rivaled by an increase of mu-receptors. However, in single dissociated DRG neurons, the inhibitory effects of morphine are not different between animals with and without CFA inflammation. In in vivo experiments, we found that locally applied morphine reduced capsaicin-induced thermal allodynia. In summary, our results indicate that mu-receptor activation can inhibit the activity of TRPV1 via G(i/o) proteins and the cAMP pathway. These observations demonstrate an important new mechanism underlying the analgesic efficacy of peripherally acting mu-receptor ligands in inflammatory pain.

Effects of inhibition of fatty acid amide hydrolase vs. the anandamide membrane transporter on TRPV1-mediated calcium responses in adult DRG neurons; the role of CB1receptors

The aim of the present study was to investigate the relationship between TRPV1 stimulation and endocannabinoid-driven CB(1) receptor-mediated inhibition of activity in adult rat dorsal root ganglion (DRG) neurons, a model of primary afferent nociceptors. Calcium-imaging studies were performed to compare the effects of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 microm) vs. the anandamide (AEA) uptake inhibitor UCM707 (1 microm) on capsaicin (100 nm) and N-arachidonoyl dopamine (NADA; 1 microm)-evoked changes in intracellular calcium [Ca(2+)](i) in DRG neurons. The ability of the CB(1) receptor antagonist AM251 (1 microm) to modulate the effects of URB597 and UCM707 was also determined. Suprafusion of NADA and capsaicin evoked robust increases in [Ca(2+)](i) in DRG neurons (89 +/- 4% and 132 +/- 6% of the depolarizing KCl response, respectively). Co-incubation with URB597 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (39 +/- 3% and 79 +/- 4% of KCl response, respectively). Similarly, co-incubation with UCM707 significantly attenuated both NADA and capsaicin-evoked increases in [Ca(2+)](i) (59 +/- 7% and 72 +/- 4% of KCl response, respectively). The CB(1) receptor antagonist AM251 significantly attenuated the effects of URB597 on NADA-evoked increases in [Ca(2+)](i) but not the effects of URB597 on capsaicin-evoked increases in [Ca(2+)](i). By contrast, AM251 significantly attenuated the inhibitory effects of UCM707 on both NADA and capsaicin-evoked increases in [Ca(2+)](i.) These data suggest that transport of both NADA and capsaicin into DRG neurons and the subsequent activation of TRPV1 is partly governed by FAAH-dependent mechanisms as well as via the putative AEA membrane transporter.