[3H]Resiniferatoxin autoradiography in the CNS of wild-type and TRPV1 null mice defines TRPV1 (VR-1) protein distribution

Cellular mechanisms of neuronal damage from hyperthermia

Hyperthermia can cause brain damage and also exacerbate the brain damage produced by stroke and amphetamines. The developing brain is especially sensitive to hyperthermia. The severity of, and mechanisms underlying, hyperthermia-induced neuronal death depend on both temperature and duration of exposure. Severe hyperthermia can produce necrotic neuronal death. For a window of less severe heat stresses, cultured neurons exhibit a delayed death with apoptotic characteristics including cytochrome c release and caspase activation. Little is known about mechanisms of hyperthermia-induced damage upstream of these late apoptotic effects. This chapter considers several possible upstream mechanisms, drawing on both in vivo and in vitro studies of the nervous system and other tissues. Hyperthermia-induced damage in some non-neuronal cells includes endoplasmic reticular stress due to denaturing of nascent polypeptide chains, as well as nuclear and cytoskeletal damage. Evidence is presented that hyperthermia produces mitochondrial damage, including depolarization, in cultured mammalian neurons.

Modulation of temperature-sensitive TRP channels

Animals sense temperature--either cold or hot--by the direct activation of temperature-sensitive members of the TRP family of ion channels, the thermo-TRPs. To date, six TRP channels--TRPV1-4, TRPM8 and TRPA1--have been reported to be directly activated by heat and to be involved in thermosensation. Temperature sensing can be modulated by phosphorylation of intracellular residues by protein kinases or by insertion of new channels into the cell membrane. In this review we provide a brief overview of the properties of thermo-TRPs, and we summarise signalling pathways involved in their regulation.

Hot receptors in the brain

Two major approaches have been employed for the development of novel drugs to treat chronic pain. The most traditional approach identifies molecules involved in pain as potential therapeutic targets and has focused mainly on the periphery and spinal cord. A more recent approach identifies molecules that are involved in long-term plasticity. Drugs developed through the latter approach are predicted to treat chronic, but not physiological or acute, pain. The TRPV1 (transient receptor potential vanilloid-1) receptor is involved in nociceptive processing, and is a candidate therapeutic target for pain. While most research on TRPV1 receptors has been conducted at the level of the spinal cord and peripheral structures, considerably less research has focused on supraspinal structures. This short paper summarizes progress made on TRPV1 receptors, and reviews research on the expression and function of TRPV1 receptors in supraspinal structures. We suggest that the TRPV1 receptor may be involved in pain processing in higher brain structures, such as the anterior cingulate cortex. In addition, some regions of the brain utilize the TRPV1 receptor for functions apparently unrelated to pain.

The Endocannabinoid System as a Target for the Treatment of Visceral Obesity and Metabolic Syndrome

The endogenous cannabinoid system is a novel, remarkably elaborate physiological signaling system, comprising the recently identified endogenous cannabinoid ligands, their corresponding selective receptors, and the machinery of proteins and enzymes that is involved in their biosynthesis, release, transport, and degradation. This system extends widely in both the central nervous system (CNS) and the periphery and exhibits a variety of actions implicated in vital functions (e.g., behavioral, antinociceptive, neuroprotective, immunosuppressive, cardiovascular, and metabolic). Particular interest has been focused on the apparent participation of endocannabinoids in metabolic homeostasis by modulating the activity of CNS circuits that control food intake and energy expenditure, the neuroendocrine response of the stress system, and the metabolic functions of crucial peripheral tissues, such as the adipose tissue, the gastrointestinal tract, the liver, and the skeletal muscles. These effects are predominantly CB(1) receptor mediated and, thus, selective antagonists of this receptor subtype are being vigorously investigated as potential therapeutic agents for the treatment of various metabolic derangements (e.g., obesity, insulin resistance, dyslipidemia, and metabolic syndrome). The first selective CB(1) receptor antagonist, rimonabant, has already successfully completed phase III clinical trials as adjunctive obesity treatment, with significant improvements in several associated metabolic and cardiovascular risk factors that led to the recent approval of its clinical use by the Food and Drug Administration.

Transient receptor potential vanilloid subtype 1 mediates microglial cell death in vivo and in vitro via Ca2+-mediated mitochondrial damage and cytochrome c release

The present study examined the expression of transient receptor potential vanilloid subtype 1 (TRPV1) in microglia, and its association with microglial cell death. In vitro cell cultures, RT-PCR, Western blot analysis, and immunocytochemical staining experiments revealed that rat microglia and a human microglia cell line (HMO6) showed TRPV1 expression. Furthermore, exposure of these cells to TRPV1 agonists, capsaicin (CAP) and resiniferatoxin (RTX), triggered cell death. This effect was ameliorated by the TRPV1 antagonists, capsazepine and iodo-resiniferatoxin (I-RTX), suggesting that TRPV1 is directly involved. Further examinations revealed that TRPV1-induced toxicity was accompanied by increases in intracellular Ca(2+), and mitochondrial damage; these effects were inhibited by capsazepine, I-RTX, and the intracellular Ca(2+) chelator BAPTA-AM. Treatment of cells with CAP or RTX led to increased mitochondrial cytochrome c release and enhanced immunoreactivity to cleaved caspase-3. In contrast, the caspase-3 inhibitor z-DEVD-fmk protected microglia from CAP- or RTX-induced toxicity. In vivo, we also found that intranigral injection of CAP or 12-hydroperoxyeicosatetraenoic acid, an endogenous agonist of TRPV1, into the rat brain produced microglial damage via TRPV1 in the substantia nigra, as visualized by immunocytochemistry. To our knowledge, this study is the first to demonstrate that microglia express TRPV1, and that activation of this receptor may contribute to microglial damage via Ca(2+) signaling and mitochondrial disruption.

TRPV1 receptors in the CNS play a key role in broad-spectrum analgesia of TRPV1 antagonists

Vanilloid receptor type 1 (TRPV1) is a ligand-gated nonselective cation channel that is considered to be an important integrator of various pain stimuli such as endogenous lipids, capsaicin, heat, and low pH. In addition to expression in primary afferents, TRPV1 is also expressed in the CNS. To test the hypothesis that the CNS plays a differential role in the effect of TRPV1 antagonists in various types of pain, the analgesic effects of two TRPV1 antagonists with similar in vitro potency but different CNS penetration were compared in vivo. Oral administration of either A-784168 (1-[3-(trifluoromethyl)pyridin-2-yl]-N-[4-(trifluoromethylsulfonyl)phenyl]-1,2,3,6-tetrahydropyridine-4-carboxamide) (good CNS penetration) or A-795614 (N-1H-indazol-4-yl-N'-[(1R)-5-piperidin-1-yl-2,3-dihydro-1H-inden-1-yl]urea) (poor CNS penetration) blocked capsaicin-induced acute pain with the same potency. In complete Freund's adjuvant (CFA)-induced chronic inflammatory pain, oral administration of either compound blocked thermal hyperalgesia with similar potency. Furthermore, intraplantar or intrathecal administration of A-784168 blocked CFA-induced thermal hyperalgesia, suggesting that both peripheral and CNS TRPV1 receptors may play a role in inflammatory thermal hyperalgesia. The effects of the two TRPV1 antagonists were further assessed in models presumably mediated by central sensitization, including CFA- and capsaicin-induced mechanical allodynia and osteoarthritic pain. In these models, the potency of the two compounds was similar after intrathecal administration. However, when administered orally, A-784168, with good CNS penetration, was much more potent than A-795614. Together, these results demonstrate that TRPV1 receptors in the CNS play an important role in pain mediated by central sensitization. In addition, these results demonstrate that significant CNS penetration is necessary for a TRPV1 antagonist to produce broad-spectrum analgesia.

The TRPV1 receptor and nociception

The capsaicin receptor TRPV1 is an emerging target for the treatment of pain with a unique expression profile in peripheral nociceptors and the ability to show polymodal activation, TRPV1 is an important integrator of responses to inflammatory mediators. Sensitization of TRPV1 during chronic pain is believed to contribute to the transduction of noxious signaling for normally innocuous stimuli and consequently the search for novel TRPV1 therapeutics is intense. The current understanding of the physiological role the receptor, as well as the potential therapeutic utility and emerging liabilities of TRPV1 modulators are discussed.

Lack of evidence for functional TRPV1 vanilloid receptors in rat hippocampal nerve terminals

Although TRPV(1) vanilloid receptors (TRPV(1)Rs) have been assumed to be present in the brain, their role is not well-defined. Here, we tested the widely used TRPV(1)R agonists (E)-capsaicin (0.1-100 microM) and resiniferatoxin (RTX, 0.1 microM) on resting and K(+)-evoked Ca(2+) entry and radiolabelled GABA release in rat hippocampal nerve terminals. (E)-capsaicin and RTX failed to evoke Ca(2+) transients or to trigger [(3)H]GABA outflow. Both (E)-capsaicin (EC(50), 40.4 microM) and its enantiomer (Z)-capsaicin (EC(50), 22.9 microM), which is inactive at the TRPV(1)R, inhibited the K(+)-evoked Ca(2+) entry, and to similar extent, the Ca(2+)-dependent K(+)-evoked [(3)H]GABA release. The TRPV(1)R enhancer/partial agonist 2-aminoethoxydiphenyl borate (1-300 microM) induced rapid Ca(2+) entry. None of the above-mentioned findings proved to be sensitive to the TRPV(1)R antagonists iodoresiniferatoxin (I-RTX; 3 microM) and SB366791 (3 microM). The CB(1) cannabinoid receptor antagonist AM251 (EC(50), 1.1 microM) and I-RTX (EC(50), 4.6 microM) also diminished the K(+)-evoked Ca(2+) entry per se. We observed competitive antagonism between I-RTX and AM251, indicating that the two molecules may act at the same site. In conclusion, there is a need to examine the discrepancy between ex vivo and in vitro data to understand the neurochemical and physiological functions of brain TRPV(1)Rs.

Modulation of human TRPV1 receptor activity by extracellular protons and host cell expression system

The transient receptor potential vanilloid 1 (TRPV1) receptor is a ligand-gated cation channel that can be activated by capsaicin, heat, protons and cytosolic lipids. We compared activation of recombinant human TRPV1 receptors stably expressed in human 293 cells, derived from kidney embryonic cells, and in human 1321N1 cells, derived from brain astrocytes. Cellular influx of calcium was measured in response to acid, endovanilloids (N-arachidonoyl-dopamine, N-oleoyl-dopamine and anandamide), capsaicin and other traditional vanilloid agonists under normal (pH 7.4) and acidic (pH 6.7 and 6.0) assay conditions. The host cell expression system altered the agonist profile of endogenous TRPV1 receptor agonists without affecting the pharmacological profile of either exogenous TRPV1 receptor agonists or antagonists. Our data signify that the host cell expression system plays a modulatory role in TRPV1 receptor activity, and suggests that activation of native human TRPV1 receptors in vivo will be dependent on cell-specific regulatory factors/pathways.

Endocannabinoids activate transient receptor potential vanilloid 1 receptors to reduce hyperdopaminergia-related hyperactivity: therapeutic implications

BACKGROUND

Knockout (KO) mice invalidated for the dopamine transporter (DAT) constitute a powerful animal model of neurobiological alterations associated with hyperdopaminergia relevant to schizophrenia and attention-deficit/hyperactivity disorder (ADHD).

METHODS

Because of continuously increasing evidence for a neuromodulatory role of endocannabinoids in dopamine-related pathophysiological responses, we assessed endocannabinoid signaling in DAT KO mice and evaluated the ability of endocannabinoid ligands to normalize behavioral deficits, namely spontaneous hyperlocomotion in these mice.

RESULTS

In DAT KO mice, we found markedly reduced anandamide levels, specifically in striatum, the dopamine nerve terminal region. Furthermore, three distinct indirect endocannabinoid agonists, the selective anandamide reuptake inhibitors AM404 and VDM11 and the fatty acid amidohydrolase inhibitor AA5HT, attenuated spontaneous hyperlocomotion in DAT KO mice. The hypolocomotor effects of AM404, VDM11, and AA5HT were significantly attenuated by co-administration of the transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine but not the selective cannabinoid type 1 (CB1)receptor antagonist AM251. Interestingly, TRPV1 binding was increased in the striatum of DAT KO mice, while CB1 receptor binding was unaffected.

CONCLUSIONS

These data indicate a dysregulated striatal endocannabinoid neurotransmission associated with hyperdopaminergic state. Restoring endocannabinoid homeostasis in active synapses might constitute an alternative therapeutic strategy for disorders associated with hyperdopaminergia. In this process, TRPV1 receptors seem to play a key role and represent a novel promising pharmacological target.

Immunohistochemical localization of cannabinoid type 1 and vanilloid transient receptor potential vanilloid type 1 receptors in the mouse brain

Cannabinoid type 1 receptors and transient receptor potential vanilloid type 1 channels have been proposed to act as metabotropic and ionotropic receptors, respectively, for two classes of endogenous polyunsaturated fatty acid amides, the acylethanolamides and the acyldopamides. Furthermore, we and others have shown that functional crosstalk occurs between these two receptors when they are expressed in the same cell. Although demonstrated in sensory neurons of the dorsal root ganglia, spinal cord and myenteric neurons, co-expression of cannabinoid type 1 and transient receptor potential vanilloid type 1 has not yet been studied in the brain. In the present study, we addressed this issue by using commercially available specific antibodies whose specificity was confirmed by data obtained with brains from cannabinoid type 1(-/-) and transient receptor potential vanilloid type 1(-/-) mice. Double cannabinoid type 1/transient receptor potential vanilloid type 1 immunofluorescence and single cannabinoid type 1 or transient receptor potential vanilloid type 1 avidin-biotin complex immunohistochemistry techniques were performed and both methods used point to the same results. Cannabinoid type 1/transient receptor potential vanilloid type 1 expression was observed in the hippocampus, basal ganglia, thalamus, hypothalamus, cerebral peduncle, pontine nuclei, periaqueductal gray matter, cerebellar cortex and dentate cerebellar nucleus. In particular, in the hippocampus, cannabinoid type 1/transient receptor potential vanilloid type 1 expression was detected on cell bodies of many pyramidal neurons throughout the CA1-CA3 subfields and in the molecular layer of dentate gyrus. In the cerebellar cortex, expression of cannabinoid type 1/transient receptor potential vanilloid type 1 receptors was found surrounding soma and axons of the vast majority of Purkinje cell bodies, whose cytoplasm was found unstained for both receptors. Cannabinoid type 1 and transient receptor potential vanilloid type 1 immunoreactivity was also detected in: a) the globus pallidus and substantia nigra, in which some intensely transient receptor potential vanilloid type 1 immunopositive cell bodies were found in dense and fine cannabinoid type 1/transient receptor potential vanilloid type 1 positive and cannabinoid type 1 positive nerve fiber meshworks, respectively; b) the cytoplasm of thalamic and hypothalamic neurons; and c) some neurons of the ventral periaqueductal gray. These data support the hypothesis of a functional relationship between the two receptor types in the CNS.

Anandamide as an intracellular messenger regulating ion channel activity

The endocannabinoid anandamide (N-arachidonoylethanolamine) was proposed to be an extracellular retrograde messenger, which regulates excitability of neurons by cannabinoid CB1 receptor-dependent inhibition of neurotransmitter release. Recent findings indicate that the neuromodulatory actions of anandamide might be more complex. Anandamide has been shown to directly modulate various ion channels, such as alpha7-nicotinic acetylcholine receptors, T-type Ca2+ channels, voltage-gated and background K+-channels and Transient Receptor Potential Vanilloid type 1 (TRPV1) channels. The binding site of anandamide at some of these ion channels appears to be intracellular or at the bilayer interface. This rises the intriguing possibility that anandamide, prior to its release into the synaptic cleft, may regulate ion homeostasis and excitability of neurons as an intracellular modulator of ion channels independent of its action at cannabinoid CB1 receptors. This possibility might extend the concept of anandamide as an endocannabinoid retrograde messenger and may have profound implications for its role in neurotransmission and neuronal function. Here, we will review the evidence for this hypothesis.

The vanilloid receptor as a putative target of diverse chemicals in multiple chemical sensitivity

The vanilloid receptor (TRPV1 or VR1), widely distributed in the central and peripheral nervous system, is activated by a broad range of chemicals similar to those implicated in Multiple Chemical Sensitivity (MCS) Syndrome. The vanilloid receptor is reportedly hyperresponsive in MCS and can increase nitric oxide levels and stimulate N-methyl-D-aspartate (NMDA) receptor activity, both of which are important features in the previously proposed central role of nitric oxide and NMDA receptors in MCS. Vanilloid receptor activity is markedly altered by multiple mechanisms, possibly providing an explanation for the increased activity in MCS and symptom masking by previous chemical exposure. Activation of this receptor by certain mycotoxins may account for some cases of sick building syndrome, a frequent precursor of MCS. Twelve types of evidence implicate the vanilloid receptor as the major target of chemicals, including volatile organic solvents (but not pesticides) in MCS.

Extensive co-localization and heteromultimer formation of the vanilloid receptor-like protein TRPV2 and the capsaicin receptor TRPV1 in the adult rat cerebral cortex

The capsaicin receptor TRPV1, a member of the transient receptor potential (TRP) family of calcium-selective ion channels, responds to noxious stimuli and is predominantly expressed in nociceptive neurons. The homologous receptor TRPV2 shows wide tissue distribution including some sensory neurons, where it is proposed to function as a heat sensor or a growth-factor-activated channel. Members of the TRP family of channels have been shown to interact, resulting in hybrid channels with new properties. We examined the possibility of multimer formation between TRPV1 and TRPV2, using biochemical techniques. We present evidence that TRPV1 and TRPV2 can heteromultimerize efficiently in vitro. By using immunohistochemistry we detected co-localization of the two receptors in rat dorsal root ganglia. TRPC4 transcripts are also detected in capsaicin-sensitive dorsal root ganglia neurons. We extended the search for TRPV1-TRPV2 co-localization in the brain, where we detected extensive co-expression of the two receptors in the IV, V and VI layer neurons of the adult rat cerebral cortex. Co-immunoprecipitation experiments confirmed the interaction of the two receptors in vivo, indicating heteromultimer formation in native tissue. Formation of heteromultimers between vanilloid receptors may increase the functional diversity of this receptor family.

Pharmacological characterization of the TRPV1 receptor antagonist JYL1421 (SC0030) in vitro and in vivo in the rat

The TRPV1 capsaicin receptor is an integrator molecule on primary afferent neurones participating in inflammatory and nociceptive processes. The present paper characterizes the effects of JYL1421 (SC0030), a TRPV1 receptor antagonist, on capsaicin-evoked responses both in vitro and in vivo in the rat. JYL1421 concentration-dependently (0.1-2 microM) inhibited capsaicin-evoked substance P, calcitonin gene-related peptide and somatostatin release from isolated tracheae, while only 2 microM resulted in a significant inhibition of electrically induced neuropeptide release. Capsazepine (0.1-2 microM), as a reference compound, similarly diminished both capsaicin-evoked and electrically evoked peptide release. JYL1421 concentration-dependently decreased capsaicin-induced Ca(2+) accumulation in cultured trigeminal ganglion cells, while capsazepine was much less effective. In vivo 2 mg/kg i.p. JYL1421, but not capsazepine, inhibited capsaicin-induced hypothermia, eye wiping movements and reflex hypotension (a component of the pulmonary chemoreflex or Bezold-Jarisch reflex). Based on these data JYL1421 is a more selective and in most models also a more potent TRPV1 receptor antagonist than capsazepine, therefore it may promote the assessment of the (patho)physiological roles of the TRPV1 receptor.

Systemic and site-specific effects of A-425619, a selective TRPV1 receptor antagonist, on wide dynamic range neurons in CFA-treated and uninjured rats

Systemic administration of A-425619, a potent and selective TRPV1 receptor antagonist that does not readily enter the CNS, produces antinociception in several rat models of pathological nociception, including complete Freund's adjuvant (CFA)-induced thermal hyperalgesia. To further understand the peripheral mechanisms of TRPV1-related antinociception, we examined the effects of systemic and site-specific injections of A-425619 on evoked and spontaneous firing of spinal wide dynamic range (WDR) neurons in uninjured rats and rats with peripheral inflammation (CFA; 48 h). In uninjured rats, capsaicin-evoked (1 microg) WDR activity was completely blocked by intraplantar administration of A-425619 (3-100 nmol). Systemic injection of A-425619 (3-30 micromol/kg, iv) reduced WDR responses to thermal stimulation in both CFA-inflamed (47 degrees C) and uninjured (52 degrees C) rats. However, the efficacy of A-425619 to attenuate thermal-evoked WDR activity was significantly greater (P < 0.01) in CFA-treated rats. Both intradorsal root ganglion (DRG; L5; 20 nmol) and intraplantar (30-300 nmol) injection of A-425619 reduced WDR responses to thermal stimulation. While the effectiveness of A-425619 was similar between CFA-inflamed and uninjured rats after intraplantar injection, the effects of A-425619 after intra-DRG injection were enhanced in the inflamed rats (compared with the uninjured rats). Spontaneous WDR discharges were unaltered by systemic or site-specific injections of A-425619. Thus noxious thermal stimulation triggers the transmission of TRPV1-related signals to spinal WDR neurons in both inflamed and uninjured animals. The apparent increase in TRPV1 signaling to WDR neurons after injury may be the result of changes to the distribution/sensitization of peripheral TRPV1 receptors.

Anandamide acts as an intracellular messenger amplifying Ca2+ influx via TRPV1 channels

The endocannabinoid anandamide is able to interact with the transient receptor potential vanilloid 1 (TRPV1) channels at a molecular level. As yet, endogenously produced anandamide has not been shown to activate TRPV1, but this is of importance to understand the physiological function of this interaction. Here, we show that intracellular Ca2+ mobilization via the purinergic receptor agonist ATP, the muscarinic receptor agonist carbachol or the Ca(2+)-ATPase inhibitor thapsigargin leads to formation of anandamide, and subsequent TRPV1-dependent Ca2+ influx in transfected cells and sensory neurons of rat dorsal root ganglia (DRG). Anandamide metabolism and efflux from the cell tonically limit TRPV1-mediated Ca2+ entry. In DRG neurons, this mechanism was found to lead to TRPV1-mediated currents that were enhanced by selective blockade of anandamide cellular efflux. Thus, endogenous anandamide is formed on stimulation of metabotropic receptors coupled to the phospholipase C/inositol 1,4,5-triphosphate pathway and then signals to TRPV1 channels. This novel intracellular function of anandamide may precede its action at cannabinoid receptors, and might be relevant to its control over neurotransmitter release.

Lipids as regulators of the activity of transient receptor potential type V1 (TRPV1) channels

After 7 years from its cloning, the transient receptor potential vanilloid type-1 (TRPV1) channel remains the sole membrane receptor mediating the pharmacological effects of the hot chilli pepper pungent component, capsaicin, and of the Euphorbia toxin, resiniferatoxin. Yet, this ion channel represents one of the most complex examples of how the activity of a protein can be regulated. Among the several chemicophysical stimuli that can modulate TRPV1 permeability to cations, endogenous lipids appear to play a major role, either as allosteric effectors or as direct agonists, or both. Furthermore, the capability of some mediators, such as the endocannabinoid anandamide, or the eicosanoid precursors 12- and 5-hydroperoxy-eicosatetraenoic acids, to activate TRPV1 receptors provides a striking example of the "site-dependent" and "metabolic" functional plasticity, respectively, typical of bioactive lipids. In this article, the multi-faceted and most recently discovered aspects of TRPV1 regulation are reviewed, with particular emphasis on the interaction between these membrane channels and some lipid molecules.

Activation of transient receptor potential vanilloid 1 (TRPV1) by resiniferatoxin

Transient receptor potential vanilloid 1 (TRPV1) is a Ca(2+) permeable non-selective cation channel activated by physical and chemical stimuli. Resiniferatoxin (RTX), an ultrapotent agonist of TRPV1, is under investigation for treatment of urinary bladder hyper-reflexia and chronic pain conditions. Here, we have determined the characteristics of RTX-induced responses in cells expressing native and cloned rat TRPV1. Whole-cell currents increase with repeated application of submaximal concentrations of RTX until a maximal response is attained and do not deactivate even after prolonged washout. Interestingly, the rate of activation and block by capsazepine of RTX-induced currents are significantly slower than for capsaicin-induced currents. RTX-induced whole-cell currents are outwardly rectifying, but to a lesser extent than capsaicin-induced currents. RTX-induced single channel currents exhibit multiple conductance states and outward rectification. The open probability (P(o)) of RTX-induced currents is higher at all potentials as compared to capsaicin-induced currents, which showed a strong voltage-dependent decrease at negative potentials. Single-channel kinetic analyses reveal that open-time distribution of RTX-induced currents can be fitted with three exponential components at negative and positive potentials. The areas of distribution of the longer open time constants are significantly larger than capsaicin-induced currents. The closed-time distribution of RTX-induced currents can be fitted with three exponential components as compared to capsaicin-induced currents, which require four exponential components. Current-clamp experiments reveal that low concentrations of RTX caused a slow and sustained depolarization beyond threshold while generating few action potentials. Concentrations of capsaicin required for the same extent of depolarization generated a significantly greater number of action potentials. These properties of RTX may play a role in its clinical usefulness.

Arvanil, a hybrid endocannabinoid and vanilloid compound, behaves as an antihyperkinetic agent in a rat model of Huntington's disease

The present study was designed to examine whether arvanil (N-arachidonoyl-vanillyl-amide), an endocannabinoid/vanilloid structural "hybrid", might provide symptom relief in the rat model of Huntington's disease (HD) generated by bilateral intrastriatal application of 3-nitropropionic acid (3-NP), where previous evidence suggests that hybrid cannabinoid/vanilloid compounds might be effective. As expected, arvanil did reduce ambulation, stereotypic activity, and number of hole entries, and increased the inactivity, in control rats. It was also active in 3-NP-lesioned rats, where, despite its lowering effects on stereotypic activity and number of hole entries, arvanil reduced the hyperkinesia (increased ambulation) typical of these rats, and also increased the inactivity, these two effects being more moderate than those found in control rats. Arvanil caused its antihyperkinetic effects in 3-NP-lesioned rats presumably by enhancing excitatory transmission at the globus pallidus, since it increased glutamate content in this region. This contrasts with its effects in control rats where arvanil enhanced GABA transmission at the globus pallidus. In summary, arvanil does alleviate hyperkinesia typical of HD, although it also affects locomotion in normal rats. Nevertheless, considering the lack of efficacious pharmacological treatments in this basal ganglia disorder, our findings might provide the basis for the development of more specific drugs against HD.

The role of the vanilloid (capsaicin) receptor (TRPV1) in physiology and pathology

The cloning of the vanilloid receptor 1 opened a floodgate for discoveries regarding the function of this complex molecule. It has been found that, in addition to heat, protons and vanilloids, this receptor also responds to various endogenous ligands. Furthermore, it has been also emerged that, through associations with other molecules, the vanilloid receptor 1 plays an important role in the integration of various stimuli and modulation of cellular excitability. Although, originally, the vanilloid receptor 1 was associated with nociceptive primary afferent fibres, it has been gradually revealed that it is broadly expressed in the brain, epidermis and visceral cells. The expression pattern of the vanilloid receptor 1 indicates that it could be involved in various physiological functions and in the pathomechanisms of diverse diseases. Here, we summarise the molecular, pharmacological and physiological characteristics, and putative functions, of the vanilloid receptor 1, and discuss the therapeutic potential of this molecule.

Rectal instillation of butyrate provides a novel clinically relevant model of noninflammatory colonic hypersensitivity in rats

BACKGROUND & AIMS

The treatment of irritable bowel syndrome (IBS), characterized by abdominal pain and bloating, is empirical and often poorly efficient. Research lacks suitable models for studying the pathophysiologic mechanisms of the colonic hypersensitivity and new pharmacologic targets. The present study aimed to develop a novel model of colonic hypersensitivity possessing several of the characteristics encountered in patients with IBS.

METHODS

Rats received enemas of a butyrate solution (8-1000 mmol/L) twice daily for 3 days. A time course was determined for colonic hypersensitivity (colorectal distention test) and referred cutaneous lumbar hyperalgesia (von Frey hairs). Macroscopic and histologic analyses were performed on colonic mucosa. The efficacy of morphine, U50488H (a kappa opioid agonist), and trimebutine on the 2 pain parameters was determined. Finally, the involvement of peptidergic C-fibers was evaluated using capsaicin-pretreated animals and treatments with calcitonin gene-related peptide (CGRP) and neurokinin 1 receptor antagonists.

RESULTS

Butyrate enemas induced a sustained, concentration-dependent colonic hypersensitivity and, to a lesser extent, a referred cutaneous mechanical hyperalgesia, particularly in female rats, but no macroscopic and histologic modifications of the colonic mucosa, as observed in patients with IBS. Both pain parameters were sensitive to morphine, U50488H, trimebutine, neonatal capsaicin treatment, and the CGRP receptor antagonist but not to the neurokinin 1 receptor antagonist.

CONCLUSIONS

These results present our noninflammatory model of chronic colonic hypersensitivity as a useful novel tool for studying IBS. The CGRP receptor antagonist-induced reduction of colonic hypersensitivity suggests that CGRP receptors may provide a promising target for treatment of IBS.

Activation of TRPV1 in the VTA excites dopaminergic neurons and increases chemical- and noxious-induced dopamine release in the nucleus accumbens

Dopamine (DA)-containing neurons of the ventral tegmental area (VTA) provide dopaminergic input to the nucleus accumbens and to the prefrontal cortex within the mesolimbic pathway. In the present study, we combined electrophysiological recordings and microdialysis techniques to investigate the function of transient receptor potential vanilloid 1 (TRPV1) channel in the VTA. In brain slices, application of the TRPV1 receptor agonist capsaicin increased the firing rate of rat dopamine neurons and in a proportion of tested cells (44%) it also induced a bursting behavior. The effects of capsaicin were concentration dependent. The increase in neuronal firing was dependent on enhanced glutamatergic transmission since it was blocked by the superfusion of the ionotropic glutamate antagonists, CNQX and AP5. Interestingly, microinjection of capsaicin into the VTA and noxious tail stimulation transiently enhanced dopamine release into the nucleus accumbens. Both the in vitro and in vivo effects were mediated by TRPV1 activation in the VTA since they were reduced by co-perfusion of the selective TRPV1 receptor antagonist iodoresineferatoxin. Our data suggest a novel role for TRPV1 channels in the mesencephalon of rat, namely activation of the DA system following a peripheral noxious stimulation.

Expression and distribution of vanilloid receptor 1 (TRPV1) in the adult rat brain

The vanilloid receptor (TRPV1 or VR1) is a molecular integrator of various painful stimuli, including capsaicin, acid, and high temperature. It can also be activated by endogenous ligands, like the cannabinoid 1 receptor (CB1) agonist anandamide. TRPV1 is well characterized at the terminals of sensory nerves involved in the pain pathway. There is also evidence that TRPV1 is expressed in the brain but little is known about its function. Here, using commercially available specific antibodies to investigate the localization of TRPV1 in the brain of the rat, we report that TRPV1 was expressed in hippocampus, cortex, cerebellum, olfactory bulb, mesencephalon and hindbrain. Immunohistochemical analyses showed high expression in the cell bodies and dendrites of neurons in the hippocampus and in the cortex. To address the question of subcellular localization, immunoelectronmicroscopy was used. TRPV1-like staining was detected in the synapses (mostly, but not exclusively in post-synaptic dendritic spines), on the end feet of astrocytes and in pericytes. In summary, TRPV1 expression shows wide distribution in the brain of the rat, being found in astrocytes and pericytes as well as in neurons. Its localization is consistent with multiple functions within the central nervous system, including the regulation of brain vasculature.

Involvement of cannabinoid receptors in gut motility and visceral perception

From a historical perspective to the present day, all the evidence suggests that activation of cannabinoid receptors (CBRs) is beneficial for gut discomfort and pain, which are symptoms related to dysmotility and visceral perception. CBRs comprise G-protein coupled receptors that are predominantly in enteric and central neurones (CB1R) and immune cells (CB2R). In the last decade, evidence obtained from the use of selective agonists and inverse agonists/antagonists indicates that manipulation of CB1R can alter (1) sensory processing from the gut, (2) brain integration of brain-gut axis, (3) extrinsic control of the gut and (4) intrinsic control by the enteric nervous system. The extent to which activation of CB1R is most critical at these different levels is related to the region of the GI tract. The upper GI tract is strongly influenced by CB1R activation on central vagal pathways, whereas intestinal peristalsis can be modified by CB1R activation in the absence of extrinsic input. Actions at multiple levels make the CB1R a target for the treatment of functional bowel disorders, such as IBS. Since low-grade inflammation may act as a trigger for occurrence of IBS, CB2R modulation could be beneficial, but there is little supporting evidence for this yet. The challenge is to accomplish CBR activation while minimizing adverse effects and abuse liabilities. Potential therapeutic strategies involve increasing signaling by endocannabinoids (EC). The pathways involved in the biosynthesis, uptake and degradation of EC provide opportunities for modulation of CB1R and some recent evidence with inhibitors of EC uptake and metabolism suggest that these could be exploited for therapeutic gain.

TRP ion channels in the nervous system

The transient receptor potential (TRP) superfamily comprises a group of non-selective cation channels that sense and respond to changes in their local environments. TRP channels are found in many eukaryotes, from yeast to mammals. They are a diverse group of proteins organized into six families: classical (TRPC), vanilloid (TRPV), melastatin (TRPM), muclopins (TRPML), polycystin (TRPP), and ANKTM1 (TRPA). In the peripheral nervous system, stimuli including temperature, pressure, inflammatory agents, and receptor activation effect TRP-mediated responses. In the central nervous system, TRPs participate in neurite outgrowth, receptor signalling and excitotoxic cell death resulting from anoxia. TRP channels are emerging as essential cellular switches that allow animals to respond to their environments.

Endovanilloids. Putative endogenous ligands of transient receptor potential vanilloid 1 channels

Endovanilloids are defined as endogenous ligands of the transient receptor potential vanilloid type 1 (TRPV1) protein, a nonselective cation channel that belongs to the large family of TRP ion channels, and is activated by the pungent ingredient of hot chilli peppers, capsaicin. TRPV1 is expressed in some nociceptor efferent neurons, where it acts as a molecular sensor of noxious heat and low pH. However, the presence of these channels in various regions of the central nervous system, where they are not likely to be targeted by these noxious stimuli, suggests the existence of endovanilloids. Three different classes of endogenous lipids have been found recently that can activate TRPV1, i.e. unsaturated N-acyldopamines, lipoxygenase products of arachidonic acid and the endocannabinoid anandamide with some of its congeners. To classify a molecule as an endovanilloid, the compound should be formed or released in an activity-dependent manner in sufficient amounts to evoke a TRPV1-mediated response by direct activation of the channel. To control TRPV1 signaling, endovanilloids should be inactivated within a short time-span. In this review, we will discuss, for each of the proposed endogenous ligands of TRPV1, their ability to act as endovanilloids in light of the criteria mentioned above.

Biochemical pharmacology of the vanilloid receptor TRPV1. An update

There is mounting evidence that the vanilloid (capsaicin) receptor; transient receptor potential channel, vanilloid subfamily member 1 (TRPV1), is subjected to multiple interacting levels of control. The first level is by reversible phosphorylation catalyzed by intrinsic kinases (e.g. protein kinase A and C) and phosphatases (e.g. calcineurin), which plays a pivotal role in receptor sensitization vs. tachyphylaxis. In addition, this mechanism links TRPV1 to intracellular signaling by various important endogenous as well as exogenous substances such as bradykinin, ethanol, nicotin and insulin. It is not clear, however, whether phosphorylation per se is sufficient to liberate TRPV1 under the inhibitory control of phosphatydylinositol-4,5-bisphosphate. The second level of control is by forming TRPV1 heteromers and their association with putative regulatory proteins. The next level of regulation is by subcellular compartmentalization. The membrane form of TRPV1 functions as a nonselective cation channel. On the endoplasmic reticulum, TRPV1 is present in two differentially regulated forms, one of which is inositol triphosphate-dependent whereas the other is not. These three TRPV1 compartments provide a versatile regulation of intracellular Ca(2+) levels. Last, there is a complex and poorly understood regulation of TRPV1 activity via control of gene expression. Factors that downregulate TRPV1 expression include vanilloid treatment and growth factor (notably, nerve growth factor) deprivation. By contrast, TRPV1 appears to be upregulated during inflammatory conditions. Interestingly, following experimental nerve injury and in animal models of diabetic neuropathy TRPV1 is present on neurons that do not normally express TRPV1. Combined, these findings imply an important role for aberrant TRPV1 expression in the development of neuropathic pain and hyperalgesia. In humans, disease-related changes in TRPV1 expression have already been described (e.g. inflammatory bowel disease and irritable bowel syndrome). The mechanisms that regulate TRPV1 gene expression under pathological conditions are unknown but a better understanding of these pathways has obvious implications for rational drug development.