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

Role of phosphorylation and vanilloid ligand structure in ligand-dependent differential activations of transient receptor potential vanilloid 1

Vanilloid analogs, which can activate transient receptor potential vanilloid 1 (TRPV1), have been classified into two types based on susceptibility to forskolin (FSK). Treatment of cells expressing TRPV1 with FSK enhances TRPV1 responses to capsaicin-type ligands while diminishing the responses to eugenol-type ligands. In this study, we determined the effect of FSK on the activation of TRPV1 stimulated with vanilloid ligands, through the influx of Ca2+ in HEK293T cells expressing TRPV1. Our findings suggest that the effects of FSK can be attributed to the phosphorylation of TRPV1, as evidenced by using a protein kinase A inhibitor and TRPV1 mutants at potential phosphorylation sites. Furthermore, we examined the structure-activity relationship of 13 vanilloid analogs. Our results indicated that vanilloid compounds could be classified into three types, that is the previously reported two types and a novel type of 10-shogaol, by which TRPV1 activation was insusceptible to the FSK treatment.

Fluorescence labeling strategies for cell surface expression of TRPV1

Regulation of ion channel expression on the plasma membrane is a major determinant of neuronal excitability, and identifying the underlying mechanisms of this expression is critical to our understanding of neurons. Here, we present two orthogonal strategies to label extracellular sites of the ion channel TRPV1 that minimally perturb its function. We use the amber codon suppression technique to introduce a non-canonical amino acid (ncAA) with tetrazine click chemistry, compatible with a trans-cyclooctene coupled fluorescent dye. Additionally, by inserting the circularly permutated HaloTag (cpHaloTag) in an extracellular loop of TRPV1, we can incorporate a fluorescent dye of our choosing. Optimization of ncAA insertion sites was accomplished by screening residue positions between the S1 and S2 transmembrane domains with elevated missense variants in the human population. We identified T468 as a rapid labeling site (∼5 min) based on functional and biochemical assays in HEK293T/17 cells. Through adapting linker lengths and backbone placement of cpHaloTag on the extracellular side of TRPV1, we generated a fully functional channel construct, TRPV1exCellHalo, with intact wild-type gating properties. We used TRPV1exCellHalo in a single molecule experiment to track TRPV1 on the cell surface and validate studies that show decreased mobility of the channel upon activation. The application of these extracellular label TRPV1 (exCellTRPV1) constructs to track surface localization of the channel will shed significant light on the mechanisms regulating its expression and provide a general scheme to introduce similar modifications to other cell surface receptors.

TRPV1 channel in the pathophysiology of epilepsy and its potential as a molecular target for the development of new antiseizure drug candidates

Identification of transient receptor potential cation channel, subfamily V member 1 (TRPV1), also known as capsaicin receptor, in 1997 was a milestone achievement in the research on temperature sensation and pain signalling. Very soon after it became evident that TRPV1 is implicated in a wide array of physiological processes in different peripheral tissues, as well as in the central nervous system, and thereby could be involved in the pathophysiology of numerous diseases. Increasing evidence suggests that modulation of TRPV1 may also affect seizure susceptibility and epilepsy. This channel is localized in brain regions associated with seizures and epilepsy, and its overexpression was found both in animal models of seizures and in brain samples from epileptic patients. Moreover, modulation of TRPV1 on non-neuronal cells (microglia, astrocytes, and/or peripheral immune cells) may have an impact on the neuroinflammatory processes that play a role in epilepsy and epileptogenesis. In this paper, we provide a comprehensive and critical overview of currently available data on TRPV1 as a possible molecular target for epilepsy management, trying to identify research gaps and future directions. Overall, several converging lines of evidence implicate TRPV1 channel as a potentially attractive target in epilepsy research but more studies are needed to exploit the possible role of TRPV1 in seizures/epilepsy and to evaluate the value of TRPV1 ligands as candidates for new antiseizure drugs.

Divergent effects of olfactory receptors on transient receptor potential vanilloid 1 activation by capsaicin and eugenol

We analyzed the effects of olfactory receptors (ORs) on transient receptor potential vanilloid 1 (TRPV1) activation using HEK293T cells co-expressing TRPV1 and OR51E1. We demonstrate here that the effect of OR51E1 on TRPV1 activation varies depending on the two TRPV1 ligands: capsaicin and eugenol. Notably, both of these ligands are vanilloid analogs. OR51E1 enhanced the response of TRPV1 to capsaicin but diminished that to eugenol. OR51E2 also showed similar effects. Based on the susceptibility to the OR's modulatory effects, various TRPV1 ligands could be classified into capsaicin and eugenol types. Activation of OR51E1 enhanced cAMP production. In addition, forskolin exhibited almost identical effects as ORs on TRPV1 responses to both types of ligands. These results suggest that OR51E1-induced cAMP elevation leads to a modification of TRPV1, presumably phosphorylation of TRPV1, which amplifies the susceptibility of TRPV1 to the two types of ligands differently.

Fluorescence labeling strategies for the study of ion channel and receptor cell surface expression: A comprehensive toolkit for extracellular labeling of TRPV1

Regulation of ion channel expression on the plasma membrane is a major determinant of neuronal excitability, and identifying the underlying mechanisms of this expression is critical to our understanding of neurons. A critical aspect of measuring changes in ion channel expression is uniquely identifying ion channels located on the cell surface. To accomplish this goal we demonstrate two orthogonal strategies to label extracellular sites of the ion channel TRPV1 that minimally perturb the function of the channel: 1) We use the amber codon suppression technique to introduce a non-canonical amino acid (ncAA) with tetrazine click chemistry compatible with a trans-cyclooctene coupled fluorescent dye. 2) By inserting the circularly permutated HaloTag (cpHaloTag) in an extracellular loop of TRPV1, we incorporate a click-chemistry site for a chloroalkane-linked fluorescent dye of our choosing. Optimization of ncAA insertion sites was accomplished by screening residue positions between the S1 and S2 transmembrane domains with elevated missense variants in the human population, and we identified T468 as a rapid labeling site (~5 minutes) based on functional as well as biochemical assays in HEK293T/17 cells. After several rounds of adapting the linker lengths and backbone placement of cpHaloTag on the extracellular side of TRPV1, our efforts led to a channel construct that robustly expressed as a fully functional TRPV1exCellHalo fusion with intact wild-type gating properties. The TRPV1exCellHalo construct was used in a single molecule experiment to track TRPV1 on the cell surface and validate studies that show decreased mobility of the channel upon activation. The success of these extracellular label TRPV1 (exCellTRPV1) constructs as tools to track surface expression of the channel will shed significant light on the mechanisms regulating expression and provide a general scheme to introduce similar modifications to other cell surface receptors.

Transient Receptor Potential Vanilloid 1 Signaling Is Independent on Protein Kinase A Phosphorylation of Ankyrin-Rich Membrane Spanning Protein

The sensory ion channel transient receptor potential vanilloid 1 (TRPV1) is mainly expressed in small to medium sized dorsal root ganglion neurons, which are involved in the transfer of acute noxious thermal and chemical stimuli. The Ankyrin-rich membrane spanning protein (ARMS) interaction with TRPV1 is modulated by protein kinase A (PKA) mediating sensitization. Here, we hypothesize that PKA phosphorylation sites of ARMS are crucial for the modulation of TRPV1 function, and that the phosphorylation of ARMS is facilitated by the A-kinase anchoring protein 79 (AKAP79). We used transfected HEK293 cells, immunoprecipitation, calcium flux, and patch clamp experiments to investigate potential PKA phosphorylation sites in ARMS and in ARMS-related peptides. Additionally, experiments were done to discriminate between PKA and protein kinase D (PKD) phosphorylation. We found different interaction ratios for TRPV1 and ARMS mutants lacking PKA phosphorylation sites. The degree of TRPV1 sensitization by ARMS mutants is independent on PKA phosphorylation. AKAP79 was also involved in the TRPV1/ARMS/PKA signaling complex. These data show that ARMS is a PKA substrate via AKAP79 in the TRPV1 signaling complex and that all four proteins interact physically, regulating TRPV1 sensitization in transfected HEK293 cells. To assess the physiological and/or therapeutic significance of these findings, similar investigations need to be performed in native neurons and/or in vivo.

Distribution and Assembly of TRP Ion Channels

In the last several decades, a large family of ion channels have been identified and studied intensively as cellular sensors for diverse physical and/or chemical stimuli. Named transient receptor potential (TRP) channels, they play critical roles in various aspects of cellular physiology. A large number of human hereditary diseases are found to be linked to TRP channel mutations, and their dysregulations lead to acute or chronical health problems. As TRP channels are named and categorized mostly based on sequence homology rather than functional similarities, they exhibit substantial functional diversity. Rapid advances in TRP channel study have been made in recent years and reported in a vast body of literature; a summary of the latest advancements becomes necessary. This chapter offers an overview of current understandings of TRP channel distribution and subunit assembly.

Opioid Receptors and Protonation-Coupled Binding of Opioid Drugs

Opioid receptors are G-protein-coupled receptors (GPCRs) part of cell signaling paths of direct interest to treat pain. Pain may associate with inflamed tissue characterized by acidic pH. The potentially low pH at tissue targeted by opioid drugs in pain management could impact drug binding to the opioid receptor, because opioid drugs typically have a protonated amino group that contributes to receptor binding, and the functioning of GPCRs may involve protonation change. In this review, we discuss the relationship between structure, function, and dynamics of opioid receptors from the perspective of the usefulness of computational studies to evaluate protonation-coupled opioid-receptor interactions.

Suppression of adenosine A2a receptors alleviates bladder overactivity and hyperalgesia in cyclophosphamide-induced cystitis by inhibiting TRPV1

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a type of chronic bladder inflammation characterized by increased voiding frequency, urgency and pelvic pain. The sensitization of bladder afferents is widely regarded as one of the pathophysiological changes in the development of IC/BPS. There is evidence that adenosine A_2a receptors are involved in regulating the sensitization of sensory afferents. However, the effect of adenosine A_2a receptors on cystitis remains unknown. In the present study, a rat model of chronic cystitis was established by intraperitoneal injection with cyclophosphamide (CYP). Cystometry and behavioral tests were performed to investigate bladder micturition function and nociceptive pain. The rats with chronic cystitis showed symptoms of bladder overactivity, characterized by an increase in bladder voiding frequency and voiding pressure. CYP treatment significantly increased the expression of the A_2a receptor in bladder afferent fibers and dorsal root ganglion (DRG) neurons. The A_2a receptor antagonist ZM241385 prevented bladder overactivity and hyperalgesia elicited by CYP-induced cystitis. In addition, the A_2a receptor and TRPV1 were coexpressed on DRG neurons. The TRPV1 antagonist capsazepine blocked bladder overactivity induced by the A_2a receptor agonist CGS21680. In contrast, ZM241385 significantly inhibited the capsaicin-induced increase in intracellular calcium concentration in DRG neurons. These results suggest that suppression of adenosine A_2a receptors in bladder afferents alleviates bladder overactivity and hyperalgesia elicited by CYP-induced cystitis in rats by inhibiting TRPV1, indicating that the adenosine A_2a receptor in bladder afferents is a potential therapeutic target for the treatment of IC/BPS.

TRPV1 promotes opioid analgesia during inflammation

Pain and inflammation are inherently linked responses to injury, infection, or chronic diseases. Given that acute inflammation in humans or mice enhances the analgesic properties of opioids, there is much interest in determining the inflammatory transducers that prime opioid receptor signaling in primary afferent nociceptors. Here, we found that activation of the transient receptor potential vanilloid type 1 (TRPV1) channel stimulated a mitogen-activated protein kinase (MAPK) signaling pathway that was accompanied by the shuttling of the scaffold protein β-arrestin2 to the nucleus. The nuclear translocation of β-arrestin2 in turn prevented its recruitment to the μ-opioid receptor (MOR), the subsequent internalization of agonist-bound MOR, and the suppression of MOR activity that occurs upon receptor desensitization. Using the complete Freund's adjuvant (CFA) inflammatory pain model to examine the role of TRPV1 in regulating endogenous opioid analgesia in mice, we found that naloxone methiodide (Nal-M), a peripherally restricted, nonselective, and competitive opioid receptor antagonist, slowed the recovery from CFA-induced hypersensitivity in wild-type, but not TRPV1-deficient, mice. Furthermore, we showed that inflammation prolonged morphine-induced antinociception in a mouse model of opioid receptor desensitization, a process that depended on TRPV1. Together, our data reveal a TRPV1-mediated signaling pathway that serves as an endogenous pain-resolution mechanism by promoting the nuclear translocation of β-arrestin2 to minimize MOR desensitization. This previously uncharacterized mechanism may underlie the peripheral opioid control of inflammatory pain. Dysregulation of the TRPV1-β-arrestin2 axis may thus contribute to the transition from acute to chronic pain.

TRPV1 modulates morphine self-administration via activation of the CaMKII-CREB pathway in the nucleus accumbens

Opioid addiction is a growing problem for public health, and opioids have correspondingly become more heavily regulated over time. We have previously shown that TRPV1 plays a critical role in morphine addiction using a self-administration paradigm in rats, and the current study evaluates the effects of the TRPV1 signaling pathway on morphine self-administration (SA). We found that treatment with a selective TRPV1 antagonist, SB366791, significantly decreased the morphine SA-induced activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII), Akt and the cAMP response element binding protein (CREB) in the nucleus accumbens (NAc). In addition, phospho-PKA and phospho-PKC expression levels were significantly increased in the NAc of the morphine-SA groups, regardless of SB366791 treatment. Finally, local microinjection of SB366791 into the NAc significantly suppressed the maintenance of morphine SA. Taken together, our findings highlight that TRPV1 plays an important role in morphine addiction, likely via activation of the CaMKII-CREB pathway in the NAc.

Exploring Morphine-Triggered PKC-Targets and Their Interaction with Signaling Pathways Leading to Pain via TrkA

It is well accepted that treatment of chronic pain with morphine leads to μ opioid receptor (MOR) desensitization and the development of morphine tolerance. MOR activation by the selective peptide agonist, D-Ala2, N-MePhe4, Gly-ol]-enkephalin(DAMGO), leads to robust G protein receptor kinase activation, β-arrestin recruitment, and subsequent receptor endocytosis, which does not occur in an activation by morphine. However, MOR activation by morphine induces receptor desensitization, in a Protein kinase C (PKC) dependent manner. PKC inhibitors have been reported to decrease receptor desensitization, reduce opiate tolerance, and increase analgesia. However, the exact role of PKC in these processes is not clearly delineated. The difficulties in establishing a particular role for PKC have been, in part, due to the lack of reagents that allow the selective identification of PKC targets. Recently, we generated a conformation state-specific anti-PKC antibody that preferentially recognizes the active state of this kinase. Using this antibody to selectively isolate PKC substrates and a proteomics strategy to establish the identity of the proteins, we examined the effect of morphine treatment on the PKC targets. We found an enhanced interaction of a number of proteins with active PKC, in the presence of morphine. In this article, we discuss the role of these proteins in PKC-mediated MOR desensitization and analgesia. In addition, we posit a role for some of these proteins in mediating pain by TrKA activation, via the activation of transient receptor potential cation channel subfamily V member 1 (TRPV1). Finally, we discuss how these new PKC interacting proteins and pathways could be targeted for the treatment of pain.

Preferential selection of Arginine at the lipid-water-interface of TRPV1 during vertebrate evolution correlates with its snorkeling behaviour and cholesterol interaction

TRPV1 is a thermo-sensitive ion channel involved in neurosensory and other physiological functions. The trans-membrane helices of TRPV1 undergo quick and complex conformational changes governed by thermodynamic parameters and membrane components leading to channel opening. However, the molecular mechanisms underlying such events are poorly understood. Here we analysed the molecular evolution of TRPV1 at the lipid-water-interface region (LWI), typically defined as a layer of 6 Å thickness on each side of the membrane with less availability of free water. Amino acids demarcating the end of the trans-membrane helices are highly conserved. Residues present in the inner leaflet are more conserved and have been preferentially selected over others. Amino acids with snorkeling properties (Arginine and Tyrosine) undergo specific selection during the vertebrate evolution in a cholesterol-dependent and/or body temperature manner. Results suggest that H-bond formation between the OH- group of cholesterol and side chain of Arg557 or Arg575 at the inner leaflet is a critical parameter that can regulate channel functions. Different LWI mutants of TRPV1 have altered membrane localization and deficient colocalization with lipid raft markers. These findings may help to understand the lipid-protein interactions, and molecular basis of different neuronal functions. Such findings may have broad importance in the context of differential sensory responses, pathophysiologies, and application of pharmacological drugs such as anaesthetics acting on TRPVs.

G protein βγ subunits inhibit TRPM3 ion channels in sensory neurons

Transient receptor potential (TRP) ion channels in peripheral sensory neurons are functionally regulated by hydrolysis of the phosphoinositide PI(4,5)P₂ and changes in the level of protein kinase mediated phosphorylation following activation of various G protein coupled receptors. We now show that the activity of TRPM3 expressed in mouse dorsal root ganglion (DRG) neurons is inhibited by agonists of the G_i-coupled µ opioid, GABA-B and NPY receptors. These agonist effects are mediated by direct inhibition of TRPM3 by Gβγ subunits, rather than by a canonical cAMP mediated mechanism. The activity of TRPM3 in DRG neurons is also negatively modulated by tonic, constitutive GPCR activity as TRPM3 responses can be potentiated by GPCR inverse agonists. GPCR regulation of TRPM3 is also seen in vivo where G_i/o GPCRs agonists inhibited and inverse agonists potentiated TRPM3 mediated nociceptive behavioural responses.

DOI:http://dx.doi.org/10.7554/eLife.26138.001

TRPM3 belongs to a family of channel proteins that allow sodium and calcium ions to enter cells by forming pores in cell membranes. TRPM3 is found on the cell membranes of nerve cells; when ions flow into the nerves through the TRPM3 pores it triggers an electrical impulse. TRPM3 is responsible for helping us to detect heat, and mice without this protein find it difficult to sense painfully hot temperatures. Mice lacking TRPM3 also respond to other kinds of pain differently. Normally, a mouse with an injured paw becomes more sensitive to warm and hot temperatures, but this does not happen in mice that do not have TRPM3.

When activated, other proteins called G-protein coupled receptors (or GPCRs for short) can make some members of this family of channel proteins more or less likely to open their pore. This in turn increases or decreases the flow of ions through the pore, respectively. Yet it was not clear if GPCRs also affect TRPM3 channels on the membranes of nerve cells.

Quallo et al. have now discovered that “switching on” different GPCR proteins in sensory nerve cells from mice greatly reduces the flow of calcium ions though TRPM3 channels. The experiments made use of two pain-killing drugs, namely morphine and baclofen, and a molecule called neuropeptide Y to activate different GPCRs.

GPCRs interact with a group of small proteins called G-proteins that, when activated by the receptor, split into two subunits, known as the α subunit and the βγ subunit. Once detached these subunits are free to act as messengers and interact with other proteins in the cell membrane. Quallo et al. found that TRPM3 is one of a small group of proteins that interact with the βγ subunits of the G-protein, which can explain how “switching on” GPCRs reduces the activity of TRPM3. Two independent studies by Dembla, Behrendt et al. and Badheka, Yudin et al. also report similar findings.

There is currently a need to find more effective treatments for people suffering from long-term pain conditions and it has become clear that TRPM3 channels are involved in sensing both pain and temperature. These new findings show that drugs already used in the treatment of pain can dramatically change how TRPM3 works. These results might help scientists to find drugs that work in a similar way to dial down the activity of TRPM3 and to combat pain. Though first it will be important to confirm these new findings in human nerve cells.

DOI:http://dx.doi.org/10.7554/eLife.26138.002

Anti-nociceptive action of peripheral mu-opioid receptors by G-beta-gamma protein-mediated inhibition of TRPM3 channels

Opioids, agonists of µ-opioid receptors (µORs), are the strongest pain killers clinically available. Their action includes a strong central component, which also causes important adverse effects. However, µORs are also found on the peripheral endings of nociceptors and their activation there produces meaningful analgesia. The cellular mechanisms downstream of peripheral µORs are not well understood. Here, we show in neurons of murine dorsal root ganglia that pro-nociceptive TRPM3 channels, present in the peripheral parts of nociceptors, are strongly inhibited by µOR activation, much more than other TRP channels in the same compartment, like TRPV1 and TRPA1. Inhibition of TRPM3 channels occurs via a short signaling cascade involving Gβγ proteins, which form a complex with TRPM3. Accordingly, activation of peripheral µORs in vivo strongly attenuates TRPM3-dependent pain. Our data establish TRPM3 inhibition as important consequence of peripheral µOR activation indicating that pharmacologically antagonizing TRPM3 may be a useful analgesic strategy.

There are very few treatments available for people suffering from strong or long-lasting pain. Currently, substances called opioids – which include the well-known drug morphine – are the strongest painkillers. However, these drugs also cause harmful side effects, which makes them less useful.

Like all drugs, opioids mediate their effects by interacting with molecules in the body. In the case of opioids, these interacting molecules belong to a group of receptor proteins called G-protein coupled receptors (or GPCRs for short). These opioid receptors are widely distributed in the nerve cells and brain regions that detect and transmit pain signals. It was poorly understood how activation of opioid receptors reduces the activity of pain-sensing nerve cells, however several lines of evidence had suggested that a protein called TRPM3 might be involved.

TRPM3 is a channel protein that allows sodium and calcium ions to enter into nerve cells by forming pores in cell membranes, and mice that lack this protein are less sensitive to certain kinds of pain. Dembla, Behrendt et al. now show that activating opioid receptors on nerve cells from mice, with morphine and a similar substance, rapidly reduces the flow of calcium ions through TRPM3 channels. Further experiments confirmed that activating opioid receptors in a mouse’s paw also reduced the pain caused when TRPM3 proteins are activated.

GPCRs interact with a group of small proteins called G-proteins that, when activated by the receptor, split into two subunits. Based on studies with human kidney cells, Dembla, Behrendt et al. found the so-called G-beta-gamma subunit then carries the signal from the opioid receptor to TRPM3. Two independent studies by Quallo et al. and Badheka, Yudin et al. also report similar findings.

These new findings show that drugs already used in the treatment of pain can indirectly alter how TRPM3 works in a dramatic way. These results might help scientists to find drugs that work in a more direct way to dial down the activity of TRPM3 and to combat pain with fewer side effects. Though first it will be important to confirm these new findings in human nerve cells.

Modulation of neuroinflammation: Role and therapeutic potential of TRPV1 in the neuro-immune axis

Transient receptor potential vanilloid type 1 channel (TRPV1), as a ligand-gated non-selective cation channel, has recently been demonstrated to have wide expression in the neuro-immune axis, where its multiple functions occur through regulation of both neuronal and non-neuronal activities. Growing evidence has suggested that TRPV1 is functionally expressed in glial cells, especially in the microglia and astrocytes. Glial cells perform immunological functions in response to pathophysiological challenges through pro-inflammatory or anti-inflammatory cytokines and chemokines in which TRPV1 is involved. Sustaining inflammation might mediate a positive feedback loop of neuroinflammation and exacerbate neurological disorders. Accumulating evidence has suggested that TRPV1 is closely related to immune responses and might be recognized as a molecular switch in the neuroinflammation of a majority of seizures and neurodegenerative diseases. In this review, we evidenced that inflammation modulates the expression and activity of TRPV1 in the central nervous system (CNS) and TRPV1 exerts reciprocal actions over neuroinflammatory processes. Together, the literature supports the hypothesis that TRPV1 may represent potential therapeutic targets in the neuro-immune axis.

Ankyrin-rich membrane spanning protein as a novel modulator of transient receptor potential vanilloid 1-function in nociceptive neurons

BACKGROUND

The ion channel TRPV1 is mainly expressed in small diameter dorsal root ganglion (DRG) neurons, which are involved in the sensation of acute noxious thermal and chemical stimuli. Direct modifications of the channel by diverse signalling events have been intensively investigated, but little is known about the composition of modulating macromolecular TRPV1 signalling complexes. Here, we hypothesize that the novel adaptor protein ankyrin-rich membrane spanning protein/kinase D interacting substrate (ARMS) interacts with TRPV1 and modulates its function in rodent DRG neurons.

METHODS

We used immunohistochemistry, electrophysiology, microfluorimetry and immunoprecipitation experiments to investigate TRPV1 and ARMS interactions in DRG neurons and transfected cells.

RESULTS

We found that TRPV1 and ARMS are co-expressed in a subpopulation of DRG neurons. ARMS sensitizes TRPV1 towards capsaicin in transfected HEK 293 cells and in mouse DRG neurons in a PKA-dependent manner. Using a combination of functional imaging and immunocytochemistry, we show that the magnitude of the capsaicin response in DRG neurons depends not only on TRPV1 expression, but on the co-expression of ARMS alongside TRPV1.

CONCLUSION

These data indicate that ARMS is an important component of the signalling complex regulating the sensitivity of TRPV1.

SIGNIFICANCE

The study identifies ARMS as an important component of the signalling complex regulating the sensitivity of excitatory ion channels (TRPV1) in peripheral sensory neurons (DRG neurons) and transfected cells.

Sensitization of TRPV1 by protein kinase C in rats with mono-iodoacetate-induced joint pain

OBJECTIVE

To assess the functional changes of Transient receptor potential vanilloid 1 (TRPV1) receptor and to clarify its mechanism in a rat mono-iodoacetate (MIA)-induced joint pain model (MIA rats), which has joint degeneration with cartilage loss similar to osteoarthritis.

METHODS

Sensitization of TRPV1 in MIA rats was assessed by transient spontaneous pain behavior induced by capsaicin injection in knee joints and electrophysiological changes of dorsal root ganglion (DRG) neurons innervating knee joints in response to capsaicin. Mechanisms of TRPV1 sensitization were analyzed by a newly developed sandwich enzyme-linked immunosorbent assay that detects phosphorylated TRPV1, followed by functional and expression analyses of protein kinase C (PKC) in vivo and in vitro, which involves TRPV1 phosphorylation.

RESULTS

Pain-related behavior induced by intra-articular injection of capsaicin was significantly increased in MIA rats compared with sham rats. In addition, capsaicin sensitivity, evaluated by capsaicin-induced inward currents, was significantly increased in DRG neurons of MIA rats. Protein levels of TRPV1 remained unchanged, but phosphorylated TRPV1 at Ser800 increased in DRG neurons of MIA rats. Phosphorylated-PKCɛ (p-PKCɛ) increased and co-localized with TRPV1 in DRG neurons of MIA rats. Capsaicin-induced pain-related behavior in MIA rats was inhibited by intra-articular pretreatment of the PKC inhibitor bisindolylmaleimide I. In addition, intra-articular injection of the PKC activator phorbol 12-myristate 13-acetate increased capsaicin-induced pain-related behavior in normal rats.

CONCLUSION

TRPV1 was sensitized at the knee joint and at DRG neurons of MIA rats through PKC activation. Thus, TRPV1 sensitization might be involved in chronic pain caused by osteoarthritis.

Capsaicin, Nociception and Pain

Capsaicin, the pungent ingredient of the hot chili pepper, is known to act on the transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1). TRPV1 is involved in somatic and visceral peripheral inflammation, in the modulation of nociceptive inputs to spinal cord and brain stem centers, as well as the integration of diverse painful stimuli. In this review, we first describe the chemical and pharmacological properties of capsaicin and its derivatives in relation to their analgesic properties. We then consider the biochemical and functional characteristics of TRPV1, focusing on its distribution and biological effects within the somatosensory and viscerosensory nociceptive systems. Finally, we discuss the use of capsaicin as an agonist of TRPV1 to model acute inflammation in slices and other ex vivo preparations.

Recent advances in therapeutic strategies that focus on the regulation of ion channel expression

A number of different ion channel types are involved in cell signaling networks, and homeostatic regulatory mechanisms contribute to the control of ion channel expression. Profiling of global gene expression using microarray technology has recently provided novel insights into the molecular mechanisms underlying the homeostatic and pathological control of ion channel expression. It has demonstrated that the dysregulation of ion channel expression is associated with the pathogenesis of neural, cardiovascular, and immune diseases as well as cancers. In addition to the transcriptional, translational, and post-translational regulation of ion channels, potentially important evidence on the mechanisms controlling ion channel expression has recently been accumulated. The regulation of alternative pre-mRNA splicing is therefore a novel therapeutic strategy for the treatment of dominant-negative splicing disorders. Epigenetic modification plays a key role in various pathological conditions through the regulation of pluripotency genes. Inhibitors of pre-mRNA splicing and histone deacetyalase/methyltransferase have potential as potent therapeutic drugs for cancers and autoimmune and inflammatory diseases. Moreover, membrane-anchoring proteins, lysosomal and proteasomal degradation-related molecules, auxiliary subunits, and pharmacological agents alter the protein folding, membrane trafficking, and post-translational modifications of ion channels, and are linked to expression-defect channelopathies. In this review, we focused on recent insights into the transcriptional, spliceosomal, epigenetic, and proteasomal regulation of ion channel expression: Ca(2+) channels (TRPC/TRPV/TRPM/TRPA/Orai), K(+) channels (voltage-gated, KV/Ca(2+)-activated, KCa/two-pore domain, K2P/inward-rectifier, Kir), and Ca(2+)-activated Cl(-) channels (TMEM16A/TMEM16B). Furthermore, this review highlights expression of these ion channels in expression-defect channelopathies.

Receptome: Interactions between three pain-related receptors or the "Triumvirate" of cannabinoid, opioid and TRPV1 receptors

A growing amount of data demonstrates the interactions between cannabinoid, opioid and the transient receptor potential (TRP) vanilloid type 1 (TRPV1) receptors. These interactions can be bidirectional, inhibitory or excitatory, acute or chronic in their nature, and arise both at the molecular level (structurally and functionally) and in physiological processes, such as pain modulation or perception. The interactions of these three pain-related receptors may also reserve important and new therapeutic applications for the treatment of chronic pain or inflammation. In this review, we summarize the main findings on the interactions between the cannabinoid, opioid and the TRPV1 receptor regarding to pain modulation.

Opioids and TRPV1 in the peripheral control of neuropathic pain--Defining a target site in the injured nerve

Targeting peripheral neuropathic pain at its origin may prevent the development of hypersensitivity. Recently we showed this can be mediated by opioid receptors at the injured nerve trunk. Here, we searched for the most relevant peripheral site to block transient receptor potential vanilloid 1 (TRPV1), and investigated analgesic interactions between TRPV1 and opioids in neuropathy. In a chronic constriction injury (CCI) of the sciatic nerve in mice, we assessed the effects of μ-, δ- and κ-opioid receptor agonists and TRPV1 antagonist (SB366791) injected at the CCI site or into the injured nerve-innervated paw on spontaneous paw lifting, heat and mechanical sensitivity. We also examined TRPV1 expression in total membrane and plasma membrane fractions from nerves and paws. We found that opioids and SB366791 co-injected in per se nonanalgesic doses at the CCI site or into the paw diminished heat and mechanical sensitivity. SB366791 alone dose-dependently alleviated heat and mechanical sensitivity. TRPV1 blockade in the paw was more effective than at the CCI site. None of the treatments diminished spontaneous paw lifting. TRPV1 expression analysis suggests that the levels of functional TRPV1 do not critically determine the TRPV1 antagonist-mediated analgesia. Together, the identification of the primary action site in damaged nerves is crucial for effective pain control. Contrary to opioids, the TRPV1 blockade in the injured nerve peripheral terminals, rather than at the nerve trunk, appears promising against heat pain. Opioid/TRPV1 antagonist combinations at both locations partially reduced neuropathy-triggered heat and mechanical pain.

Opioid Receptors

Opioids are the oldest and most potent drugs for the treatment of severe pain. Their clinical application is undisputed in acute (e.g., postoperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny. This article reviews mechanisms underlying opioid analgesia and other opioid actions. It discusses the structure, function, and plasticity of opioid receptors; the central and peripheral sites of analgesic actions and side effects; endogenous and exogenous opioid receptor ligands; and conventional and novel opioid compounds. Challenging clinical situations, such as the tension between chronic pain and addiction, are also illustrated.

The mechanism of μ-opioid receptor (MOR)-TRPV1 crosstalk in TRPV1 activation involves morphine anti-nociception, tolerance and dependence

Initiated by the activation of various nociceptors, pain is a reaction to specific stimulus modalities. The μ-opioid receptor (MOR) agonists, including morphine, remain the most potent analgesics to treat patients with moderate to severe pain. However, the utility of MOR agonists is limited by the adverse effects associated with the use of these drugs, including analgesic tolerance and physical dependence. A strong connection has been suggested between the expression of the transient receptor potential vanilloid type 1 (TRPV1) ion channel and the development of inflammatory hyperalgesia. TRPV1 is important for thermal nociception induction, and is mainly expressed on sensory neurons. Recent reports suggest that opioid or TRPV1 receptor agonist exposure has contrasting consequences for anti-nociception, tolerance and dependence. Chronic morphine exposure modulates TRPV1 activation and induces the anti-nociception effects of morphine. The regulation of many downstream targets of TRPV1 plays a critical role in this process, including calcitonin gene-related peptide (CGRP) and substance P (SP). Additional factors also include capsaicin treatment blocking the anti-nociception effects of morphine in rats, as well as opioid modulation of TRPV1 responses through the cAMP-dependent PKA pathway and MAPK signaling pathways. Here, we review new insights concerning the mechanism underlying MOR-TRPV1 crosstalk and signaling pathways and discuss the potential mechanisms of morphine-induced anti-nociception, tolerance and dependence associated with the TRPV1 signaling pathway and highlight how understanding these mechanisms might help find therapeutic targets for the treatment of morphine induced antinociception, tolerance and dependence.

A synergistic effect of simultaneous TRPA1 and TRPV1 activations on vagal pulmonary C-fiber afferents

Transient receptor potential ankyrin type 1 (TRPA1) and vanilloid type 1 (TRPV1) receptors are coexpressed in vagal pulmonary C-fiber sensory nerves. Because both these receptors are sensitive to a number of endogenous inflammatory mediators, it is conceivable that they can be activated simultaneously during airway inflammation. This study aimed to determine whether there is an interaction between these two polymodal transducers upon simultaneous activation, and how it modulates the activity of vagal pulmonary C-fiber sensory nerves. In anesthetized, spontaneously breathing rats, the reflex-mediated apneic response to intravenous injection of a combined dose of allyl isothiocyanate (AITC, a TRPA1 activator) and capsaicin (Cap, a TRPV1 activator) was ∼202% greater than the mathematical sum of the responses to AITC and Cap when they were administered individually. Similar results were also observed in anesthetized mice. In addition, the synergistic effect was clearly demonstrated when the afferent activity of single vagal pulmonary C-fiber afferents were recorded in anesthetized, artificially ventilated rats; C-fiber responses to AITC, Cap and AITC + Cap (in combination) were 0.6 ± 0.1, 0.8 ± 0.1, and 4.8 ± 0.6 impulses/s (n = 24), respectively. This synergism was absent when either AITC or Cap was replaced by other chemical activators of pulmonary C-fiber afferents. The pronounced potentiating effect was further demonstrated in isolated vagal pulmonary sensory neurons using the Ca(2+) imaging technique. In summary, this study showed a distinct positive interaction between TRPA1 and TRPV1 when they were activated simultaneously in pulmonary C-fiber sensory nerves.

Trafficking of ThermoTRP Channels

ThermoTRP channels (thermoTRPs) define a subfamily of the transient receptor potential (TRP) channels that are activated by changes in the environmental temperature, from noxious cold to injurious heat. Acting as integrators of several stimuli and signalling pathways, dysfunction of these channels contributes to several pathological states. The surface expression of thermoTRPs is controlled by both, the constitutive and regulated vesicular trafficking. Modulation of receptor surface density during pathological processes is nowadays considered as an interesting therapeutic approach for management of diseases, such as chronic pain, in which an increased trafficking is associated with the pathological state. This review will focus on the recent advances trafficking of the thermoTRP channels, TRPV1, TRPV2, TRPV4, TRPM3, TRPM8 and TRPA1, into/from the plasma membrane. Particularly, regulated membrane insertion of thermoTRPs channels contributes to a fine tuning of final channel activity, and indeed, it has resulted in the development of novel therapeutic approaches with successful clinical results such as disruption of SNARE-dependent exocytosis by botulinum toxin or botulinomimetic peptides.

Morphine Reduces Expression of TRPV1 Receptors in the Amygdala but not in the Hippocampus of Male Rats

BACKGROUND

Chronic use of opioids usually results in physical dependence. The underlying mechanisms for this dependence are still being evaluated. Transient receptor potential vanilloid type 1 (TRPV1) are important receptors of pain perception. Their role during opioid dependence has not been studied well. The aim of this study was to evaluate the effect of morphine-dependence on the expression of TRPV1 receptors in the amygdala and CA1 region of the hippocampus.

METHODS

This study used four groups of rats. Two groups of rats (morphine and morphine+naloxone) received morphine based on the following protocol: 10 mg/kg (twice daily, 3 days) followed by 20, 30, 40 and 50 mg/kg (twice daily), respectively, for 4 consecutive days. Another group received vehicle (1 ml/kg) instead of morphine given using the same schedule. The morphine+naloxone group of rats additionally received naloxone (5 mg/kg) at the end of the protocol. The control group rats received no injections or intervention. The amygdala and CA1 regions of the morphine, saline-treated and intact animals were isolated and prepared for real-time PCR analysis.

RESULTS

Administration of naloxone induced withdrawal signs in morphine-treated animals. The results showed a significant decrease in TRPV1 gene expression in the amygdala (P<0.05) but not the CA1 region of morphine dependent rats.

CONCLUSION

TRPV1 receptors may be involved in morphine-induced dependence.

Activation of mu opioid receptors sensitizes transient receptor potential vanilloid type 1 (TRPV1) via β-arrestin-2-mediated cross-talk

The transient receptor potential family V1 channel (TRPV1) is activated by multiple stimuli, including capsaicin, acid, endovanilloids, and heat (>42C). Post-translational modifications to TRPV1 result in dynamic changes to the sensitivity of receptor activation. We have previously demonstrated that β-arrestin2 actively participates in a scaffolding mechanism to inhibit TRPV1 phosphorylation, thereby reducing TRPV1 sensitivity. In this study, we evaluated the effect of β-arrestin2 sequestration by G-protein coupled receptors (GPCRs) on thermal and chemical activation of TRPV1. Here we report that activation of mu opioid receptor by either morphine or DAMGO results in β-arrestin2 recruitment to mu opioid receptor in sensory neurons, while activation by herkinorin does not. Furthermore, treatment of sensory neurons with morphine or DAMGO stimulates β-arrestin2 dissociation from TRPV1 and increased sensitivity of the receptor. Conversely, herkinorin treatment has no effect on TRPV1 sensitivity. Additional behavioral studies indicate that GPCR-driven β-arrestin2 sequestration plays an important peripheral role in the development of thermal sensitivity. Taken together, the reported data identify a novel cross-talk mechanism between GPCRs and TRPV1 that may contribute to multiple clinical conditions.

Modulation of transient receptor vanilloid 1 activity by transient receptor potential ankyrin 1

Transient receptor potential vanilloid 1 (TRPV1) is a nonselective ligand-gated cation channel responding to noxious heat, protons, and chemicals such as capsaicin. TRPV1 is expressed in sensory neurons and plays a critical role in pain associated with tissue injury, inflammation, and nerve lesions. Transient receptor potential ankyrin 1 (TRPA1) is coexpressed with TRPV1. It is activated by compounds that cause a burning sensation (e.g., mustard oil) and, indirectly, by components of the inflammatory milieu eliciting nociceptor excitation and pain hypersensitivity. Previous studies indicate an interaction of TRPV1 and TRPA1 signaling pathways. Here we sought to examine the molecular mechanisms underlying such interactions in nociceptive neurons. We first excluded physical interactions of both channels using radioligand binding studies. By microfluorimetry, electrophysiological experiments, cAMP measurements, and site-directed mutagenesis we found a sensitization of TRPV1 after TRPA1 stimulation with mustard oil in a calcium and cAMP/protein kinase A (PKA)-dependent manner. TRPA1 stimulation enhanced TRPV1 phosphorylation via the putative PKA phosphorylation site serine 116. We also detected calcium-sensitive increased TRPV1 activity after TRPA1 activation in dorsal root ganglion neurons. The inhibition of TRPA1 by HC-030031 (1,2,3,6-tetrahydro-1,3-dimethyl-N-[4-(1-methylethyl)phenyl]-2,6-dioxo-7H-purine-7-acetamide, 2-(1,3-dimethyl-2,6-dioxo-1,2,3,6-tetrahydro-7H-purin-7-yl)-N-(4-isopropylphenyl)acetamide) after its initial stimulation (and the calcium-insensitive TRPA1 mutant D477A) still showed increased capsaicin-induced TRPV1 activity. This excludes a calcium-induced additive TRPA1 current after TRPV1 stimulation. Our study shows sensitization of TRPV1 via activation of TRPA1, which involves adenylyl cyclase, increased cAMP, subsequent translocation and activation of PKA, and phosphorylation of TRPV1 at PKA phosphorylation residues. This suggests that cross-sensitization of TRP channels contributes to enhanced pain sensitivity in inflamed tissues.

Opioids for the treatment of arthritis pain

INTRODUCTION

Centrally acting opioids are well established in the treatment of acute, surgical and cancer pain. However, their use in chronic noncancer pain (CNCP) is controversial because of side effects such as tolerance, somnolence, respiratory depression, confusion, constipation and addiction. Chronic arthritis and other musculoskeletal diseases are among the leading causes of CNCP.

AREAS COVERED

This manuscript will discuss the role of conventional opioids in chronic arthritis. In addition, future developments and strategies exploiting peripheral effects of opioids on pain and inflammation will be outlined.

EXPERT OPINION

Aims in drug development include the design of peripherally restricted opioid agonists, selective targeting of endogenous opioids to sites of painful injury and the augmentation of peripheral ligand and receptor synthesis, for example, by gene therapy. Although a large number of peripherally acting opioid compounds have been developed, clinical Phase III studies have not been published so far. Another strategy is to augment the effects of endogenously released opioid peptides by the inhibition of their degrading enzymes. Technology-oriented research is needed to find novel ways of peripheral restriction of opioids. Such analgesics would be desirable for their lack of central side effects and of adverse effects typical of nonsteroidal anti-inflammatory drugs (gastrointestinal ulcers, bleeding, myocardial infarction and stroke).

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.

TRPV1: a stress response protein in the central nervous system

The transient receptor potential (TRP) family comprises a diverse group of cation channels that regulate a variety of intracellular signaling pathways. The TRPV1 (vanilloid 1) channel is best known for its role in nociception and sensory transmission. First studied in the dorsal root ganglia as the receptor for capsaicin, TRPV1 is now recognized to have a broader distribution and function within the central nervous system (CNS). Because it can be activated by a range of potentially noxious stimuli, TRPV1's polymodal nature and ability to interact with other receptor pathways make it a candidate for a stress response protein. As a result, TRPV1 is emerging as a key mediator of CNS function through modulation of both glial and neuronal activity. Growing evidence has suggested that TRPV1 can mediate a variety of pathways from glial reactivity and cytokine release to synaptic transmission and plasticity. This review highlights the increasing importance of TRPV1 as a regulator of CNS function in response to stress.

Development and optimization of FLIPR high throughput calcium assays for ion channels and GPCRs

Ca(2+) permeable ion channels and GPCRs linked to Ca(2+) release are important drug targets, with modulation of Ca(2+) signaling increasingly recognized as a valid therapeutic strategy in a range of diseases. The FLIPR is a high throughput imaging plate reader that has contributed substantially to drug discovery efforts and pharmacological characterization of receptors and ion channels coupled to Ca(2+). Now in its fourth generation, the FLIPR(TETRA) is an industry standard for high throughput Ca(2+) assays. With an increasing number of excitation LED banks and emission filter sets available; FLIPR Ca(2+) assays are becoming more versatile. This chapter describes general methods for establishing robust FLIPR Ca(2+) assays, incorporating practical aspects as well as suggestions for assay optimization, to guide the reader in the development and optimization of high throughput FLIPR assays for ion channels and GPCRs.

Heteromeric heat-sensitive transient receptor potential channels exhibit distinct temperature and chemical response

TRPV1 and TRPV3 are two heat-sensitive ion channels activated at distinct temperature ranges perceived by human as hot and warm, respectively. Compounds eliciting human sensations of heat or warmth can also potently activate these channels. In rodents, TRPV3 is expressed predominantly in skin keratinocytes, whereas in humans TRPV1 and TRPV3 are co-expressed in sensory neurons of dorsal root ganglia and trigeminal ganglion and are known to form heteromeric channels with distinct single channel conductances as well as sensitivities to TRPV1 activator capsaicin and inhibitor capsazepine. However, how heteromeric TRPV1/TRPV3 channels respond to heat and other stimuli remains unknown. In this study, we examined the behavior of heteromeric TRPV1/TRPV3 channels activated by heat, capsaicin, and voltage. Our results demonstrate that the heteromeric channels exhibit distinct temperature sensitivity, activation threshold, and heat-induced sensitization. Changes in gating properties apparently originate from interactions between TRPV1 and TRPV3 subunits. Our results suggest that heteromeric TRPV1/TRPV3 channels are unique heat sensors that may contribute to the fine-tuning of sensitivity to sensory inputs.

Modulation of peripheral sensory neurons by the immune system: implications for pain therapy

The concept that the immune system can communicate with peripheral sensory neurons to modulate pain is based mostly on documented interactions between opioid ligands and receptors. Such findings may have broad implications for the development of safer pain medication. Innovative strategies take into account that analgesics should be particularly active in pathological states rather than producing a general suppression of the central nervous system, as with conventional morphine- or cannabinoid-like drugs. Inflammation of peripheral tissue leads to increased functionality of opioid receptors on peripheral sensory neurons and to local production of endogenous opioid peptides. In addition, endocannabinoids were detected in leukocytes, but their role in pain modulation has yet to be addressed. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing immune cells to sites of painful injury, and the augmentation of peripheral ligand and receptor synthesis (e.g., by gene therapy). Similar approaches may be pursued for cannabinoids. The ultimate goal is to avoid detrimental side effects of currently available analgesics such as respiratory depression, cognitive impairment, addiction, gastrointestinal bleeding, and thromboembolic complications.

Acidic NAADP-releasable Ca(2+) compartments in the megakaryoblastic cell line MEG01

BACKGROUND

A novel family of intracellular Ca(2+)-release channels termed two-pore channels (TPCs) has been presented as the receptors of NAADP (nicotinic acid adenine dinucleotide phosphate), the most potent Ca(2+) mobilizing intracellular messenger. TPCs have been shown to be exclusively localized to the endolysosomal system mediating NAADP-evoked Ca(2+) release from the acidic compartments.

OBJECTIVES

The present study is aimed to investigate NAADP-mediated Ca(2+) release from intracellular stores in the megakaryoblastic cell line MEG01.

METHODS

Changes in cytosolic and intraluminal free Ca(2+) concentrations were registered by fluorimetry using fura-2 and fura-ff, respectively; TPC expression was detected by PCR.

RESULTS

Treatment of MEG01 cells with the H(+)/K(+) ionophore nigericin or the V-type H(+)-ATPase selective inhibitor bafilomycin A1 revealed the presence of acidic Ca(2+) stores in these cells, sensitive to the SERCA inhibitor 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ). NAADP releases Ca(2+) from acidic lysosomal-like Ca(2+) stores in MEG01 cells probably mediated by the activation of TPC1 and TPC2 as demonstrated by TPC1 and TPC2 expression silencing and overexpression. Ca(2+) efflux from the acidic lysosomal-like Ca(2+) stores or the endoplasmic reticulum (ER) results in ryanodine-sensitive activation of Ca(2+)-induced Ca(2+) release (CICR) from the complementary Ca(2+) compartment.

CONCLUSION

Our results show for the first time NAADP-evoked Ca(2+) release from acidic compartments through the activation of TPC1 and TPC2, and CICR, in a megakaryoblastic cell line.

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.

Regulation of TRP signalling by ion channel translocation between cell compartments

The TRP (transient receptor potential) family of ion channels is a heterogeneous family of calcium permeable cation channels that is subdivided into seven subfamilies: TRPC ("Canonical"), TRPV ("Vanilloid"), TRPM ("Melastatin"), TRPA ("Ankyrin"), TRPN ("NOMPC"), TRPP ("Polycystin"), and TRPML ("Mucolipin"). TRP-mediated ion currents across the cell membrane are determined by the single channel conductance, by the fraction of activated channels, and by the total amount of TRP channels present at the plasma membrane. In many cases, the amount of TRP channels at the plasma membrane is altered in response to physiological stimuli by translocation of channels to and from the plasma membrane. Regulated translocation has been described for channels of the TRPC, TRPV, TRPM, and TRPA family and is achieved by vesicular transport of these channels along cellular exocytosis and endocytosis pathways. This review summarizes the stimuli and signalling cascades involved in the translocation of TRP channels and highlights interactions of TRP channels with proteins of the endocytosis and exocytosis machineries.

Heteromerization of TRP channel subunits: extending functional diversity

Transient receptor potential (TRP) channels are widely found throughout the animal kingdom. By serving as cellular sensors for a wide spectrum of physical and chemical stimuli, they play crucial physiological roles ranging from sensory transduction to cell cycle modulation. TRP channels are tetrameric protein complexes. While most TRP subunits can form functional homomeric channels, heteromerization of TRP channel subunits of either the same subfamily or different subfamilies has been widely observed. Heteromeric TRP channels exhibit many novel properties compared to their homomeric counterparts, indicating that co-assembly of TRP channel subunits has an important contribution to the diversity of TRP channel functions.

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.

A G-protein-coupled neuropeptide Y-like receptor suppresses behavioral and sensory response to multiple stressful stimuli in Drosophila

Recent studies suggest that human neuropeptide Y (NPY) plays a prominent role in management of stress response and emotion, and higher NPY levels observed in combat-exposed veterans may help coping with posttraumatic stress. Neuropeptide F (NPF), the counterpart of NPY in Drosophila melanogaster, also displays parallel activities, including promotion of resilience to diverse stressors and prevention of uncontrolled aggressive behavior. However, it remains unclear how NPY family peptides modulate physical and emotional responses to various stressors. Here we show that NPFR1, a G-protein-coupled NPF receptor, exerts an inhibitory effect on larval aversion to diverse stressful stimuli mediated by different subtypes of fly and mammalian transient receptor potential (TRP) family channels. Imaging analysis in larval sensory neurons and cultured human cells showed that NPFR1 attenuates Ca(2+) influx mediated by fly TRPA and rat TRPV1 channels. Our findings suggest that suppression of TRP channel-mediated neural excitation by the conserved NPF/NPFR1 system may be a major mechanism for attaining its broad anti-stress function.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 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 neurologic 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).