Essential role of mitochondrial permeability transition in vanilloid receptor 1-dependent cell death of sensory neurons

Cytotoxic Effects of Cannabidiol on Neonatal Rat Cortical Neurons and Astrocytes: Potential Danger to Brain Development

The influence of cannabidiol (CBD) on brain development is inadequately understood. Since CBD is considered a non-intoxicating drug, it has attracted great interest concerning its potential medical applicability, including in pregnant women and children. Here, we elucidated the response of perinatal rat cortical neurons and astrocytes to CBD at submicromolar (0.1, 0.5, 1, 5 µM) concentrations attainable in humans. The effect of CBD was concentration- and time-dependent and cell-specific. In neurons, 0.1 µM CBD induced an early and transient change in mitochondrial membrane potential (ΔΨm), ATP depletion, and caspase-8 activation, followed by rapid ATP recovery and progressive activation of caspase-9 and caspase-3/7, resulting in early apoptotic cell death with reduction and shortening of dendrites, cell shrinkage, and chromatin condensation. The decrease in neuronal viability, ATP depletion, and caspase activation due to CBD exposure was prevented by transient receptor potential vanilloid 1 (TRPV1) antagonist. In astrocytes, 0.5 µM CBD caused an immediate short-term dysregulation of ΔΨm, followed by ATP depletion with transient activation of caspase-8 and progressive activation of caspase-9 and caspase-3/7, leading to early apoptosis and subsequent necroptosis. In astrocytes, both TRPV1 and cannabinoid receptor 1 (CB₁) antagonists protected viability and prevented apoptosis. Given that CBD is a non-intoxicating drug, our results clearly show that this is not the case during critical periods of brain development when it can significantly interfere with the endogenous cannabinoid system.

Resilience to capsaicin-induced mitochondrial damage in trigeminal ganglion neurons

Capsaicin is an agonist of transient receptor potential cation channel subfamily V member 1 (TRPV1). Strong TRPV1 stimulation with capsaicin causes mitochondrial damage in primary sensory neurons. However, the effect of repetitive and moderate exposure to capsaicin on the integrity of neuronal mitochondria remains largely unknown. Our electron microscopic analysis revealed that repetitive stimulation of the facial skin of mice with 10 mM capsaicin induced short-term damage to the mitochondria in small-sized trigeminal ganglion neurons. Further, capsaicin-treated mice exhibited decreased sensitivity to noxious heat stimulation, indicating TRPV1 dysfunction, in parallel with the mitochondrial damage in the trigeminal ganglion neurons. To analyze the capsaicin-induced mitochondrial damage and its relevant cellular events in detail, we performed cell-based assays using TRPV1-expressing PC12 cells. Dose-dependent capsaicin-mediated mitochondrial toxicity was observed. High doses of capsaicin caused rapid destruction of mitochondrial internal structure, while low doses induced mitochondrial swelling. Further, capsaicin induced a dose-dependent loss of mitochondria and autophagy-mediated degradation of mitochondria (mitophagy). Concomitantly, transcriptional upregulation of mitochondrial proteins, cytochrome c oxidase subunit IV, Mic60/Mitofilin, and voltage-dependent anion channel 1 was observed, which implied induction of mitochondrial biogenesis to compensate for the loss of mitochondria. Collectively, although trigeminal ganglion neurons transiently exhibit mitochondrial damage and TRPV1 dysfunction following moderate capsaicin exposure, they appear to be resilient to such a challenge. Our in vitro data show a dose-response relationship in capsaicin-mediated mitochondrial toxicity. We postulate that induction of mitophagy and mitochondrial biogenesis in response to capsaicin stimulation play important roles in repairing the damaged mitochondrial system.

TRPV1: Role in Skin and Skin Diseases and Potential Target for Improving Wound Healing

Skin is innervated by a multitude of sensory nerves that are important to the function of this barrier tissue in homeostasis and injury. The role of innervation and neuromediators has been previously reviewed so here we focus on the role of the transient receptor potential cation channel, subfamily V member 1 (TRPV1) in wound healing, with the intent of targeting it in treatment of non-healing wounds. TRPV1 structure and function as well as the outcomes of TRPV1-targeted therapies utilized in several diseases and tissues are summarized. In skin, keratinocytes, sebocytes, nociceptors, and several immune cells express TRPV1, making it an attractive focus area for treating wounds. Many intrinsic and extrinsic factors confound the function and targeting of TRPV1 and may lead to adverse or off-target effects. Therefore, a better understanding of what is known about the role of TRPV1 in skin and wound healing will inform future therapies to treat impaired and chronic wounds to improve healing.

TRPV1 Channel: A Noxious Signal Transducer That Affects Mitochondrial Function

The Transient Receptor Vanilloid 1 (TRPV1) or capsaicin receptor is a nonselective cation channel, which is abundantly expressed in nociceptors. This channel is an important transducer of several noxious stimuli, having a pivotal role in pain development. Several TRPV1 studies have focused on understanding its structure and function, as well as on the identification of compounds that regulate its activity. The intracellular roles of these channels have also been explored, highlighting TRPV1′s actions in the homeostasis of Ca²⁺ in organelles such as the mitochondria. These studies have evidenced how the activation of TRPV1 affects mitochondrial functions and how this organelle can regulate TRPV1-mediated nociception. The close relationship between this channel and mitochondria has been determined in neuronal and non-neuronal cells, demonstrating that TRPV1 activation strongly impacts on cell physiology. This review focuses on describing experimental evidence showing that TRPV1 influences mitochondrial function.

Fight fire with fire: Neurobiology of capsaicin-induced analgesia for chronic pain

Capsaicin, the pungent ingredient in chili peppers, produces intense burning pain in humans. Capsaicin selectively activates the transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptive primary afferents, and underpins the mechanism for capsaicin-induced burning pain. Paradoxically, capsaicin has long been used as an analgesic. The development of topical patches and injectable formulations containing capsaicin has led to application in clinical settings to treat chronic pain conditions, such as neuropathic pain and the potential to treat osteoarthritis. More detailed determination of the neurobiological mechanisms of capsaicin-induced analgesia should provide the logical rationale for capsaicin therapy and help to overcome the treatment's limitations, which include individual differences in treatment outcome and procedural discomfort. Low concentrations of capsaicin induce short-term defunctionalization of nociceptor terminals. This phenomenon is reversible within hours and, hence, likely does not account for the clinical benefit. By contrast, high concentrations of capsaicin lead to long-term defunctionalization mediated by the ablation of TRPV1-expressing afferent terminals, resulting in long-lasting analgesia persisting for several months. Recent studies have shown that capsaicin-induced Ca²⁺/calpain-mediated ablation of axonal terminals is necessary to produce long-lasting analgesia in a mouse model of neuropathic pain. In combination with calpain, axonal mitochondrial dysfunction and microtubule disorganization may also contribute to the longer-term effects of capsaicin. The analgesic effects subside over time in association with the regeneration of the ablated afferent terminals. Further determination of the neurobiological mechanisms of capsaicin-induced analgesia should lead to more efficacious non-opioidergic analgesic options with fewer adverse side effects.

Capsaicin: Physicochemical properties, cutaneous reactions and potential applications in painful and inflammatory conditions

Capsaicin is a natural protoalkaloid recognized as the main pungent component in hot peppers (Capsicum annuum L.). The capsaicin receptor is highly expressed in the unmyelinated type C nerve fibers originating from small diameter sensory neurons in dorsal root ganglia and cranial nerve ganglia correspondents. Capsaicin and related vanilloids have a variety of effects on primary sensory neurons function, from sensory neuron excitation characterized by local burning sensation and neurogenic inflammation, followed by conduction blockage accompanied by reversible ultrastructural changes of peripheral nociceptive endings (desensitization), going as far as irreversible degenerative changes (neurotoxicity). The main role in capsaicin-induced neurogenic inflammation relies on the capsaicin sensitive, small diameter primary sensory neurons, therefore its evaluation could be used as a diagnostic instrument in functional alterations of cutaneous sensory nerve fibers. Moreover, capsaicin-induced desensitization and neurotoxicity explain the analgesic/anti-nociceptive and anti-inflammatory effects of topical capsaicin and its potential use in the management of painful and inflammatory conditions. In this study, we describe the effects of capsaicin on neurogenic inflammation and nociception, as well as its potential diagnostic value and therapeutic impact in various conditions involving impairment of sensory nerve fibers.

Transient Receptor Potential Vanilloid 1 Expression Mediates Capsaicin-Induced Cell Death

The transient receptor potential (TRP) ion channel family consists of a broad variety of non-selective cation channels that integrate environmental physicochemical signals for dynamic homeostatic control. Involved in a variety of cellular physiological processes, TRP channels are fundamental to the control of the cell life cycle. TRP channels from the vanilloid (TRPV) family have been directly implicated in cell death. TRPV1 is activated by pain-inducing stimuli, including inflammatory endovanilloids and pungent exovanilloids, such as capsaicin (CAP). TRPV1 activation by high doses of CAP (>10 μM) leads to necrosis, but also exhibits apoptotic characteristics. However, CAP dose-response studies are lacking in order to determine whether CAP-induced cell death occurs preferentially via necrosis or apoptosis. In addition, it is not known whether cytosolic Ca²⁺ and mitochondrial dysfunction participates in CAP-induced TRPV1-mediated cell death. By using TRPV1-transfected HeLa cells, we investigated the underlying mechanisms involved in CAP-induced TRPV1-mediated cell death, the dependence of CAP dose, and the participation of mitochondrial dysfunction and cytosolic Ca²⁺ increase. Together, our results contribute to elucidate the pathophysiological steps that follow after TRPV1 stimulation with CAP. Low concentrations of CAP (1 μM) induce cell death by a mechanism involving a TRPV1-mediated rapid and transient intracellular Ca²⁺ increase that stimulates plasma membrane depolarization, thereby compromising plasma membrane integrity and ultimately leading to cell death. Meanwhile, higher doses of CAP induce cell death via a TRPV1-independent mechanism, involving a slow and persistent intracellular Ca²⁺ increase that induces mitochondrial dysfunction, plasma membrane depolarization, plasma membrane loss of integrity, and ultimately, cell death.

Activation of the capsaicin-receptor TRPV1 by the acetaminophen metabolite N-arachidonoylaminophenol results in cytotoxicity

AIMS

The anandamide reuptake inhibitor N-arachidonoylaminophenol (AM404) and the reactive substance N-acetyl-p-benzoquinone imine (NAPQI) are both metabolites of acetaminophen and may contribute to acetaminophen-induced analgesia by acting at TRPV1 expressed in the peripheral or central nervous system. While NAPQI slowly sensitizes and activates TRPV1 by interacting with distinct intracellular cysteine residues, detailed properties of AM404 as an agonist of TRPV1 have not yet been reported on. We explored the effects of AM404 on recombinant human TRPV1 and in rodent dorsal root ganglion (DRG) neurons.

MATERIALS AND METHODS

HEK 293 cells expressing different isoforms of recombinant TRPV1 and rodent DRG neurons were employed for patch clamp and calcium imaging experiments. Cytotoxicity was assessed by propidium iodide and Annexin V staining on TRPV1-HEK 293 cells and with trypan blue staining on DRG neurons.

KEY FINDINGS

AM404 activates hTRPV1 at concentrations >1μM and in a concentration-dependent manner. AM404 also potentiates TRPV1-mediated currents evoked by heat and anandamide. Moreover, AM404-evoked currents are potentiated by NAPQI. While the partly capsaicin-insensitive rabbit (o) TRPV1 fails to respond to AM404, AM404-sensitivity is restored by insertion of the capsaicin binding-domain of rat TRPV1 into oTRPV1. In DRG neurons, AM404-evoked calcium influx as well as cell death is mediated by TRPV1.

SIGNIFICANCE

AM404 gates TRPV1 by interacting with the vanilloid-binding site, and TRPV1 is the main receptor for AM404 in DRG neurons. While direct activation of TRPV1 requires high concentrations of AM404, it is possible that synergistic effects of AM404 with further TRPV1-agonists may occur at clinically relevant concentrations.

Differential cytotoxicity and intracellular calcium-signalling following activation of the calcium-permeable ion channels TRPV1 and TRPA1

Several members of the transient receptor channel (TRP) family can mediate a calcium-dependent cytotoxicity. In sensory neurons, vanilloids like capsaicin induce neurotoxicity by activating TRPV1. The closely related ion channel TRPA1 is also activated by irritants, but it is unclear if and how TRPA1 mediates cell death. In the present study we explored cytotoxicity and intracellular calcium signalling resulting from activation of TRPV1 and TRPA1, either heterologously expressed in HEK 293 cells or in native mouse dorsal root ganglion (DRG) neurons. While activation of TRPV1 by the vanilloids capsaicin, resiniferatoxin and anandamide results in calcium-dependent cell death, activation by protons and the oxidant chloramine-T failed to reduce cell viability. The TRPA1-agonists acrolein, carvacrol and capsazepine all induced cytotoxicity, but this effect is independent of TRPA1. Activation of both TRPA1 and TRPV1 triggers a strong influx of external calcium, but also a strong calcium-release from intracellular stores most likely including the endoplasmic reticulum (ER). Activation of TRPV1, but not TRPA1 also results in a strong increase of mitochondrial calcium both in HEK 293 cells and mouse DRG neurons. Our data demonstrate that activation of TRPV1, but not TRPA1 mediates a calcium-dependent cell death. While both receptors mediate a release of calcium from intracellular stores, only activation of TRPV1 seems to mediate a robust and probably lethal increase in mitochondrial calcium.

Capsaicin Protects Against Oxidative Insults and Alleviates Behavioral Deficits in Rats with 6-OHDA-Induced Parkinson's Disease via Activation of TRPV1

Increasing evidence suggests that capsaicin may play a role in modulating neuronal function and controlling motor behavior. However, the underlying mechanism is still unclear and the activation of transient receptor potential vanilloid 1 (TRPV1) might be involved in. This study investigated the potential neuroprotective role of capsaicin in a rat model of 6-hydroxydopamine (6-OHDA)-induced Parkinson's disease (PD). Capsaicin was treated intraperitoneally for the 6-OHDA induced PD rats and the locomotor activity and abnormal involuntary movements were found alleviated. Besides, brain oxidative stress (lipid peroxidation, superoxide dismutase and catalase) was also assessed, and oxidative insults were investigated relieved. Both the expression of tyrosine hydroxylase and TRPV1 were increased in the striatal and substantia nigra areas of 6-OHDA induced rats after the treatment of capsaicin by the semi-quantitative analysis of Western Blot. And the immunostaining of substantia nigra further suggested that capsaicin might protect against dopaminergic neuronal loss. Our results showed that TRPV1 might be a novel therapeutic target for PD.

Molecular Surgery Concept from Bench to Bedside: A Focus on TRPV1+ Pain-Sensing Neurons

"Molecular neurosurgery" is emerging as a new medical concept, and is the combination of two partners: (i) a molecular neurosurgery agent, and (ii) the cognate receptor whose activation results in the selective elimination of a specific subset of neurons in which this receptor is endogenously expressed. In general, a molecular surgery agent is a selective and potent ligand, and the target is a specific cell type whose elimination is desired through the molecular surgery procedure. These target cells have the highest innate sensitivity to the molecular surgery agent usually due to the highest receptor density being in their plasma membrane. The interaction between the ligand and its receptor evokes an overactivity of the receptor. If the receptor is a ligand-activated non-selective cation channel, the overactivity of receptor leads to excess Ca²⁺ and Na⁺ influx into the cell and finally cell death. One of the best known examples of such an interaction is the effect of ultrapotent vanilloids on TRPV1-expressing pain-sensing neurons. One intrathecal resiniferatoxin (RTX) dose allows for the receptor-mediated removal of TRPV1+ neurons from the peripheral nervous system. The TRPV1 receptor-mediated ion influx induces necrotic processes, but only in pain-sensing neurons, and usually within an hour. Besides that, target-specific apoptotic processes are also induced. Thus, as a nano-surgery scalpel, RTX removes the neurons responsible for generating pain and inflammation from the peripheral nervous system providing an option in clinical management for the treatment of morphine-insensitive pain conditions. In the future, the molecular surgery concept can also be exploited in cancer research for selectively targeting the specific tumor cell.

Ca2+ and calpain mediate capsaicin-induced ablation of axonal terminals expressing transient receptor potential vanilloid 1

Capsaicin is an ingredient in spicy peppers that produces burning pain by activating transient receptor potential vanilloid 1 (TRPV1), a Ca²⁺-permeable ion channel in nociceptors. Capsaicin has also been used as an analgesic, and its topical administration is approved for the treatment of certain pain conditions. The mechanisms underlying capsaicin-induced analgesia likely involve reversible ablation of nociceptor terminals. However, the mechanisms underlying these effects are not well understood. To visualize TRPV1-lineage axons, a genetically engineered mouse model was used in which a fluorophore is expressed under the TRPV1 promoter. Using a combination of these TRPV1-lineage reporter mice and primary afferent cultures, we monitored capsaicin-induced effects on afferent terminals in real time. We found that Ca²⁺ influx through TRPV1 is necessary for capsaicin-induced ablation of nociceptive terminals. Although capsaicin-induced mitochondrial Ca²⁺ uptake was TRPV1-dependent, dissipation of the mitochondrial membrane potential, inhibition of the mitochondrial transition permeability pore, and scavengers of reactive oxygen species did not attenuate capsaicin-induced ablation. In contrast, MDL28170, an inhibitor of the Ca²⁺-dependent protease calpain, diminished ablation. Furthermore, overexpression of calpastatin, an endogenous inhibitor of calpain, or knockdown of calpain 2 also decreased ablation. Quantitative assessment of TRPV1-lineage afferents in the epidermis of the hind paws of the reporter mice showed that EGTA and MDL28170 diminished capsaicin-induced ablation. Moreover, MDL28170 prevented capsaicin-induced thermal hypoalgesia. These results suggest that TRPV1/Ca²⁺/calpain-dependent signaling plays a dominant role in capsaicin-induced ablation of nociceptive terminals and further our understanding of the molecular mechanisms underlying the effects of capsaicin on nociceptors.

Crosstalk between calcium and reactive oxygen species signaling in cancer

The interplay between Ca²⁺ and reactive oxygen species (ROS) signaling pathways is well established, with reciprocal regulation occurring at a number of subcellular locations. Many Ca²⁺ channels at the cell surface and intracellular organelles, including the endoplasmic reticulum and mitochondria are regulated by redox modifications. In turn, Ca²⁺ signaling can influence the cellular generation of ROS, from sources such as NADPH oxidases and mitochondria. This relationship has been explored in great depth during the process of apoptosis, where surges of Ca²⁺ and ROS are important mediators of cell death. More recently, coordinated and localized Ca²⁺ and ROS transients appear to play a major role in a vast variety of pro-survival signaling pathways that may be crucial for both physiological and pathophysiological functions. While much work is required to firmly establish this Ca²⁺-ROS relationship in cancer, existing evidence from other disease models suggests this crosstalk is likely of significant importance in tumorigenesis. In this review, we describe the regulation of Ca²⁺ channels and transporters by oxidants and discuss the potential consequences of the ROS-Ca²⁺ interplay in tumor cells.

Use of Capsaicin to Treat Pain: Mechanistic and Therapeutic Considerations

Capsaicin is the pungent ingredient of chili peppers and is approved as a topical treatment of neuropathic pain. The analgesia lasts for several months after a single treatment. Capsaicin selectively activates TRPV1, a Ca²⁺-permeable cationic ion channel that is enriched in the terminals of certain nociceptors. Activation is followed by a prolonged decreased response to noxious stimuli. Interest also exists in the use of injectable capsaicin as a treatment for focal pain conditions, such as arthritis and other musculoskeletal conditions. Recently injection of capsaicin showed therapeutic efficacy in patients with Morton’s neuroma, a painful foot condition associated with compression of one of the digital nerves. The relief of pain was associated with no change in tactile sensibility. Though injection evokes short term pain, the brief systemic exposure and potential to establish long term analgesia without other sensory changes creates an attractive clinical profile. Short-term and long-term effects arise from both functional and structural changes in nociceptive terminals. In this review, we discuss how local administration of capsaicin may induce ablation of nociceptive terminals and the clinical implications.

Capsaicin: Current Understanding of Its Mechanisms and Therapy of Pain and Other Pre-Clinical and Clinical Uses

In this review, we discuss the importance of capsaicin to the current understanding of neuronal modulation of pain and explore the mechanisms of capsaicin-induced pain. We will focus on the analgesic effects of capsaicin and its clinical applicability in treating pain. Furthermore, we will draw attention to the rationale for other clinical therapeutic uses and implications of capsaicin in diseases such as obesity, diabetes, cardiovascular conditions, cancer, airway diseases, itch, gastric, and urological disorders.

Different Contribution of Redox-Sensitive Transient Receptor Potential Channels to Acetaminophen-Induced Death of Human Hepatoma Cell Line

Acetaminophen (APAP) is a safe analgesic antipyretic drug at prescribed doses. Its overdose, however, can cause life-threatening liver damage. Though, involvement of oxidative stress is widely acknowledged in APAP-induced hepatocellular death, the mechanism of this increased oxidative stress and the associated alterations in Ca²⁺ homeostasis are still unclear. Among members of transient receptor potential (TRP) channels activated in response to oxidative stress, we here identify that redox-sensitive TRPV1, TRPC1, TRPM2, and TRPM7 channels underlie Ca²⁺ entry and downstream cellular damages induced by APAP in human hepatoma (HepG2) cells. Our data indicate that APAP treatment of HepG2 cells resulted in increased reactive oxygen species (ROS) production, glutathione (GSH) depletion, and Ca²⁺ entry leading to increased apoptotic cell death. These responses were significantly suppressed by pretreatment with the ROS scavengers N-acetyl-L-cysteine (NAC) and 4,5-dihydroxy-1,3-benzene disulfonic acid disodium salt monohydrate (Tiron), and also by preincubation of cells with the glutathione inducer Dimethylfumarate (DMF). TRP subtype-targeted pharmacological blockers and siRNAs strategy revealed that suppression of either TRPV1, TRPC1, TRPM2, or TRPM7 reduced APAP-induced ROS formation, Ca²⁺ influx, and cell death; the effects of suppression of TRPV1 or TRPC1, known to be activated by oxidative cysteine modifications, were stronger than those of TRPM2 or TRPM7. Interestingly, TRPV1 and TRPC1 were labeled by the cysteine-selective modification reagent, 5,5′-dithiobis (2-nitrobenzoic acid)-2biotin (DTNB-2Bio), and this was attenuated by pretreatment with APAP, suggesting that APAP and/or its oxidized metabolites act directly on the modification target cysteine residues of TRPV1 and TRPC1 proteins. In human liver tissue, TRPV1, TRPC1, TRPM2, and TRPM7 channels transcripts were localized mainly to hepatocytes and Kupffer cells. Our findings strongly suggest that APAP-induced Ca²⁺ entry and subsequent hepatocellular death are regulated by multiple redox-activated cation channels, among which TRPV1 and TRPC1 play a prominent role.

Emerging Role of Spinal Cord TRPV1 in Pain Exacerbation

TRPV1 is well known as a sensor ion channel that transduces a potentially harmful environment into electrical depolarization of the peripheral terminal of the nociceptive primary afferents. Although TRPV1 is also expressed in central regions of the nervous system, its roles in the area remain unclear. A series of recent reports on the spinal cord synapses have provided evidence that TRPV1 plays an important role in synaptic transmission in the pain pathway. Particularly, in pathologic pain states, TRPV1 in the central terminal of sensory neurons and interneurons is suggested to commonly contribute to pain exacerbation. These observations may lead to insights regarding novel synaptic mechanisms revealing veiled roles of spinal cord TRPV1 and may offer another opportunity to modulate pathological pain by controlling TRPV1. In this review, we introduce historical perspectives of this view and details of the recent promising results. We also focus on extended issues and unsolved problems to fully understand the role of TRPV1 in pathological pain. Together with recent findings, further efforts for fine analysis of TRPV1's plastic roles in pain synapses at different levels in the central nervous system will promote a better understanding of pathologic pain mechanisms and assist in developing novel analgesic strategies.

Anandamide-induced endoplasmic reticulum stress and apoptosis are mediated by oxidative stress in non-melanoma skin cancer: Receptor-independent endocannabinoid signaling

Endocannabinoids are neuromodulatory lipids that regulate central and peripheral physiological functions. Endocannabinoids have emerged as effective antitumor drugs due to their ability to induce apoptosis in various cancer studies. The G-protein coupled cannabinoid receptors (CB1 and CB2) and the TRPV1 ion channel were reported to mediate the antiproliferative activity of endocannabinoids. However, receptor-independent effects also account for their activity. Our previous studies showed that the antiproliferative activity of anandamide (AEA) was regulated by cyclooxygenase-2 (COX-2) via induction of endoplasmic reticulum (ER) stress. We also determined that AEA induced oxidative stress. However, the role of oxidative stress, the cannabinoid receptors, and TRPV1 in AEA-induced ER stress-apoptosis was unclear. Therefore, the current study examines the role of oxidative stress in ER stress-apoptosis and investigates whether this effect is modulated by CB1, CB2, or TRPV1. In non-melanoma skin cancer (NMSC) cells, AEA reduced the total intracellular level of glutathione and induced oxidative stress. To evaluate the importance of oxidative stress in AEA-induced cell death, the antioxidants, N-acetylcysteine (NAC) and Trolox, were utilized. Each antioxidant ameliorated the antiproliferative effect of AEA. Furthermore, Trolox inhibited AEA-induced CHOP10 expression and caspase 3 activity, indicating that oxidative stress was required for AEA-induced ER stress-apoptosis. On the other hand, selective blockade of CB1, CB2, and TRPV1 did not inhibit AEA-induced oxidative stress or ER stress-apoptosis. These findings suggest that AEA-induced ER stress-apoptosis in NMSC cells is mediated by oxidative stress through a receptor-independent mechanism. Hence, receptor-independent AEA signaling pathways may be targeted to eliminate NMSC. © 2015 Wiley Periodicals, Inc.

TRPV1 and Endocannabinoids: Emerging Molecular Signals that Modulate Mammalian Vision

Transient Receptor Potential Vanilloid 1 (TRPV1) subunits form a polymodal cation channel responsive to capsaicin, heat, acidity and endogenous metabolites of polyunsaturated fatty acids. While originally reported to serve as a pain and heat detector in the peripheral nervous system, TRPV1 has been implicated in the modulation of blood flow and osmoregulation but also neurotransmission, postsynaptic neuronal excitability and synaptic plasticity within the central nervous system. In addition to its central role in nociception, evidence is accumulating that TRPV1 contributes to stimulus transduction and/or processing in other sensory modalities, including thermosensation, mechanotransduction and vision. For example, TRPV1, in conjunction with intrinsic cannabinoid signaling, might contribute to retinal ganglion cell (RGC) axonal transport and excitability, cytokine release from microglial cells and regulation of retinal vasculature. While excessive TRPV1 activity was proposed to induce RGC excitotoxicity, physiological TRPV1 activity might serve a neuroprotective function within the complex context of retinal endocannabinoid signaling. In this review we evaluate the current evidence for localization and function of TRPV1 channels within the mammalian retina and explore the potential interaction of this intriguing nociceptor with endogenous agonists and modulators.

Properties of capsaicinoids for the control of fungi and oomycetes pathogenic to pepper

Capsaicinoids are pungent compounds found in pepper (Capsicum spp.) fruits. Capsaicin showed antimicrobial activity in plate assays against seven isolates of five species of fungi and nine isolates of two species of oomycetes. The general trend was that oomycetes were more inhibited than fungi. Assays of capsaicin biosynthetic precursors suggest that the lateral chain of capsaicinoids has more inhibitory activity than the phenolic part. In planta tests of capsaicinoids (capsaicin and N-vanillylnonanamide) applied to the roots demonstrated that these compounds conferred protection against the pathogenic fungus Verticillium dahliae and induced both chitinase activity and expression of several defence-related genes, such as CASC1, CACHI2 and CABGLU. N-Vanillylnonanamide infiltrated into cotyledons confers systemic protection to the upper leaves of pepper against the fungal pathogen Botrytis cinerea. In wild-type tomato plants such cotyledon infiltration has no protective effect, but is effective in the Never-ripe tomato mutant impaired in ethylene response. A similar effect was observed in tomato after salicylic acid infiltration.

Differential cellular localization of antioxidant enzymes in the trigeminal ganglion

Because of its high oxygen demands, neural tissue is predisposed to oxidative stress. Here, our aim was to clarify the cellular localization of antioxidant enzymes in the trigeminal ganglion. We found that the transcriptional factor Sox10 is localized exclusively in satellite glial cells (SGCs) in the adult trigeminal ganglion. The use of transgenic mice that express the fluorescent protein Venus under the Sox10 promoter enabled us to distinguish between neurons and SGCs. Although both superoxide dismutases 1 and 2 were present in the neurons, only superoxide dismutase 1 was identified in SGCs. The enzymes relevant to hydrogen peroxide degradation displayed differential cellular localization, such that neurons were endowed with glutathione peroxidase 1 and thioredoxin 2, and catalase and thioredoxin 2 were present in SGCs. Our immunohistochemical finding showed that only SGCs were labeled by the oxidative damage marker 8-hydroxy-2'-deoxyguanosine, which indicates that the antioxidant systems of SGCs were less potent. The transient receptor potential vanilloid subfamily member 1 (TRPV1), the capsaicin receptor, is implicated in inflammatory hyperalgesia, and we demonstrated that topical capsaicin application causes short-lasting mechanical hyperalgesia in the face. Our cell-based assay revealed that TRPV1 agonist stimulation in the presence of TRPV1 overexpression caused reactive oxygen species-mediated caspase-3 activation. Moreover, capsaicin induced the cellular demise of primary TRPV1-positive trigeminal ganglion neurons in a dose-dependent manner, and this effect was inhibited by a free radical scavenger and a pancaspase inhibitor. This study delineates the localization of antioxidative stress-related enzymes in the trigeminal ganglion and reveals the importance of the pivotal role of reactive oxygen species in the TRPV1-mediated caspase-dependent cell death of trigeminal ganglion neurons. Therapeutic measures for antioxidative stress should be taken to prevent damage to trigeminal primary sensory neurons in inflammatory pain disorders.

Linoleic acid stimulates [Ca2+]i increase in rat pancreatic beta-cells through both membrane receptor- and intracellular metabolite-mediated pathways

The role of the free fatty acid (FFA) receptor and the intracellular metabolites of linoleic acid (LA) in LA-stimulated increase in cytosolic free calcium concentration ([Ca²⁺]i) was investigated. [Ca²⁺]i was measured using Fura-2 as indicator in rat pancreatic β-cells in primary culture. LA (20 µM for 2 min) stimulated a transient peak increase followed by a minor plateau increase in [Ca²⁺]i. Elongation of LA stimulation up to 10 min induced a strong and long-lasting elevation in [Ca²⁺]i. Activation of FFA receptors by the non-metabolic agonist GW9508 (40 µM for 10 min) resulted in an increase in [Ca²⁺]i similar to that of 2-min LA treatment. Inhibition of acyl-CoA synthetases by Triacsin C suppressed the strong and long-lasting increase in [Ca²⁺]i. The increase in [Ca²⁺]i induced by 2 min LA or GW9508 were fully eliminated by exhaustion of endoplasmic reticulum (ER) Ca²⁺ stores or by inhibition of phospholipase C (PLC). Removal of extracellular Ca²⁺ did not influence the transient peak increase in [Ca²⁺]i stimulated by 2 min LA or GW9508. The strong and long-lasting increase in [Ca²⁺]i induced by 10 min LA was only partially suppressed by extracellular Ca²⁺ removal or thapsigargin pretreatment, whereas remaining elevation in [Ca²⁺]i was eliminated after exhaustion of mitochondrial Ca²⁺ using triphenyltin. In conclusion, LA stimulates Ca²⁺ release from ER through activation of the FFA receptor coupled to PLC and mobilizes mitochondrial Ca²⁺ by intracellular metabolites in β-cells.

Neuroprotection induced by N-acetylcysteine against cytosolic glutathione depletion-induced Ca2+ influx in dorsal root ganglion neurons of mice: role of TRPV1 channels

Glutathione (GSH) and N-acetylcysteine (NAC) are thiol-containing antioxidants, and also act through a direct reaction with free radicals. Transient receptor potential vanilloid 1 (TRPV1) is the principal transduction channel serving as a polymodal detector. Despite the importance of oxidative stress in pain sensitivity, its role in TRPV1 modulation is poorly understood. NAC may also have a regulator role on TRPV1 channel activity in the dorsal root ganglion (DRG) neuron. Therefore, we tested the effects of GSH and NAC on TRPV1 channel current, Ca(2+) influx, oxidative stress and caspase activity in the DRG of mice. DRG neurons were freshly isolated from mice and the neurons were incubated for 6 and 24h with buthionine sulfoximine (BSO). Pretreatment of cultured DRG neurons with NAC, results in a protection against oxidative damages. This neuroprotection is associated with the attenuation of a Ca(2+) influx triggered by oxidative agents such as H2O2, 5,5'-dithiobis-(2-nitrobenzoic acid) and GSH depletion via BSO. Here, we demonstrate the contribution of cytosolic factors (related to thiol group depletion) on the activation of TRPV1 channels in this mechanism. TRPV1 channels are activated by various agents including capsaicin (CAP), the pungent component of hot chili peppers, and are blocked by capsazepine. An oxidative environment also increased CAP-evoked TRPV1 currents in the neurons. When NAC and GSH were included in the patch pipette as well as extracellularly in the chamber, TRPV1 channels were not activated by CAP and H2O2. TRPV1 inhibitors, 2-aminoethyl diphenylborinate and N-(p-amylcinnamoyl)anthranilic acid strongly reduced BSO-induced oxidative toxicity and Ca(2+) influx, in a manner similar to pretreatment with NAC and GSH. Caspase-3 and -9 activities of all groups were not changed by the agonists or antagonists. In conclusion, in our experimental model, TRPV1 channels are involved in the oxidative stress-induced neuronal death, and negative modulation of this channel activity by GSH and NAC pretreatment may account for their neuroprotective activity against oxidative stress.

Transient receptor potential vanilloid 1 as a therapeutic target in analgesia

INTRODUCTION

The selective excitatory action of capsaicin followed by long-term chemoanalgesia due to an action on the 'capsaicin receptor' of C-polymodal nociceptors, cloned 15 years ago, opened up fascinating perspectives for a class of nociceptor blocking analgesics.

AREAS COVERED

The TRPV1/capsaicin receptor is an integrative, chemoceptive, noxious heat-gated cation channel also gated by several endogenous ligands and sensitized by phosphorylation through intracellular cascades triggered from receptors of bradykinin, prostanoids, NGF and interactions with TRPA1. In this review, types of sensory receptors and unique mechanisms for blocking nociceptor action, e.g., 'pore dilation' intracellular acidosis and the long-term function-related mitochondrial swelling at the nerve terminals and sensory neurons are summarized. In humans the 8% capsaicin dermal patch is already in usage for nondiabetic neuropathic pain and two topical preparations of civamide have also been approved recently for cluster headache and osteoarthritis. Evidence for epidermal nerve terminal loss in humans after topical applications and misleading results on sensory neuron death evoked by TRPV1 agonism in animals are discussed.

EXPERT OPINION

The unique 'multisteric' gating of TRPV1 channel which is opened and modulated in various conformational changes to natural stimuli differs from the operation of canonical ligand-gated channels and makes it suitable to initiate development of second generation of TRPV1 antagonists without on-target side effects of hyperthermia and risk of burn injury.

Mechanism of capsaicin receptor TRPV1-mediated toxicity in pain-sensing neurons focusing on the effects of Na(+)/Ca(2+) fluxes and the Ca(2+)-binding protein calretinin

Transient receptor potential vanilloid subtype 1 (TRPV1) receptor is a pain-sensing, ligand-gated, non-selective cation channel expressed in peripheral sensory neurons. Prolonged activation of TRPV1 by capsaicin leads to cell swelling and formation of membrane blebs in rat dorsal root ganglion (DRG) neurons. Similar results were obtained in NIH3T3 fibroblast cells stably expressing TRPV1. Here, we assessed the contribution of Ca(2+) and Na(+) ions to TRPV1-mediated changes. Cell swelling was caused by a substantial influx of extracellular Na(+) via TRPV1 channels, causing concomitant transport of water. In the absence of extracellular Na(+), the membrane blebbing was completely inhibited, but Ca(2+) influx did not change under these conditions. Na(+) influx was modulated by the intracellular Ca(2+) concentration ([Ca(2+)]i). Elevation of [Ca(2+)]i by ionomycin sensitized/activated TRPV1 channels causing cell swelling in TRPV1-positive cells. In the absence of extracellular Ca(2+), capsaicin caused only little increase in [Ca(2+)]i indicating that the increase in [Ca(2+)]i observed after capsaicin application is derived essentially from extracellular Ca(2+) and not from internal Ca(2+) stores. In the absence of extracellular Ca(2+) also the process of cell swelling was considerably slower. Calretinin is a Ca(2+) buffer protein, which is expressed in a subset of TRPV1-positive neurons. Calretinin decreased the amplitude, but slowed down the decay of Ca(2+) signals evoked by ionomycin. Cells co-expressing TRPV1 and calretinin were less sensitive to TRPV1-mediated, capsaicin-induced volume increases. In TRPV1-expressing NIH3T3 cells, calretinin decreased the capsaicin-induced Ca(2+) and Na(+) influx. Swelling and formation of membrane blebs resulted in impaired plasma membrane integrity finally leading to cell death. Our results hint towards a mechanistic explanation for the apoptosis-independent capsaicin-evoked neuronal loss and additionally reveal a protective effect of calretinin; we propose that the Ca(2+)-buffering capacity of calretinin reduces the susceptibility of calretinin-expressing DRG neurons against cell swelling/death caused by overstimulation of TRPV1 channels. This article is part of a Special Issue entitled:12th European Symposium on Calcium.

Capsaicin induces apoptosis in SCC-4 human tongue cancer cells through mitochondria-dependent and -independent pathways

Although there have been advances in the fields of surgery, radiotherapy, and chemotherapy of tongue cancer, the cure rates are still not substantially satisfactory. Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide) is the major pungent ingredient of hot chili pepper and has been reported to have an antitumor effect on many human cancer cell types. The molecular mechanisms of the antitumor effect of capsaicin are not yet completely understood. Herein, we investigated whether capsaicin induces apoptosis in human tongue cancer cells. Capsaicin decreased the percentage of viable cells in a dose-dependent manner in human tongue cancer SCC-4 cells. In addition, capsaicin produced DNA fragmentation, decreased the DNA contents (sub-G1 phase), and induced G0/G1 phase arrest in SCC-4 cells. We demonstrated that capsaicin-induced apoptosis is associated with an increase in reactive oxygen species and Ca²⁺ generations and a disruption of the mitochondrial transmenbrane potential (ΔΨ(m)). Treatment with capsaicin induced a dramatic increase in caspase-3 and -9 activities, as assessed by flow cytometric methods. A possible mechanism of capsaicin-induced apoptosis is involved in the activation of caspase-3 (one of the apoptosis-executing enzyme). Confocal laser microscope examination also showed that capsaicin induced the releases of AIF, ATF-4, and GADD153 from mitochondria of SCC-4 cells.

TRPV1-dependent and -independent alterations in the limbic cortex of neuropathic mice: impact on glial caspases and pain perception

During neuropathic pain, caspases are activated in the limbic cortex. We investigated the role of TRPV1 channels and glial caspases in the mouse prelimbic and infralimbic (PL-IL) cortex after spared nerve injury (SNI). Reverse transcriptase-polymerase chain reaction, western blots, and immunfluorescence showed overexpression of several caspases in the PL-IL cortex 7 days postinjury. Caspase-3 release and upregulation of AMPA receptors in microglia, caspase-1 and IL-1β release in astrocytes, and upregulation of Il-1 receptor-1, TRPV1, and VGluT1 in glutamatergic neurons, were also observed. Of these alterations, only those in astrocytes persisted in SNI Trpv1(-/-) mice. A pan-caspase inhibitor, injected into the PL-IL cortex, reduced mechanical allodynia, this effect being reduced but not abolished in Trpv1(-/-) mice. Single-unit extracellular recordings in vivo following electrical stimulation of basolateral amygdala or application of pressure on the hind paw, showed increased excitatory pyramidal neuron activity in the SNI PL-IL cortex, which also contained higher levels of the endocannabinoid 2-arachidonoylglycerol. Intra-PL-IL cortex injection of mGluR5 and NMDA receptor antagonists and AMPA exacerbated, whereas TRPV1 and AMPA receptor antagonists and a CB(1) agonist inhibited, allodynia. We suggest that SNI triggers both TRPV1-dependent and independent glutamate- and caspase-mediated cross-talk among IL-PL cortex neurons and glia, which either participates or counteracts pain.

Capsaicinoids, chloropicrin and sulfur mustard: possibilities for exposure biomarkers

Incapacitating and irritating agents produce temporary disability persisting for hours to days after the exposure. One can be exposed to these agents occupationally in industrial or other working environments. Also general public can be exposed in special circumstances, like industrial accidents or riots. Incapacitating and irritating agents discussed in this review are chloropicrin and capsaicinoids. In addition, we include sulfur mustard, which is an old chemical warfare agent and known to cause severe long-lasting injuries or even death. Chloropicrin that was used as a warfare agent in the World War I is currently used mainly as a pesticide. Capsaicinoids, components of hot pepper plants, are used by police and other law enforcement personnel as riot control agents. Toxicity of these chemicals is associated particularly with the respiratory tract, eyes, and skin. Their acute effects are relatively well known but the knowledge of putative long-term effects is almost non-existent. Also, mechanisms of effects at cellular level are not fully understood. There is a need for further research to get better idea of health risks, particularly of long-term and low-level exposures to these chemicals. For this, exposure biomarkers are essential. Validated exposure biomarkers for capsaicinoids, chloropicrin, and sulfur mustard do not exist so far. Metabolites and macromolecular adducts have been suggested biomarkers for sulfur mustard and these can already be measured qualitatively, but quantitative biomarkers await further development and validation. The purpose of this review is, based on the existing mechanistic and toxicokinetic information, to shed light on the possibilities for developing biomarkers for exposure biomonitoring of these compounds. It is also of interest to find ideas for early effect biomarkers considering the need for studies on subchronic and chronic toxicity.

Capsaicin induces apoptosis through ubiquitin-proteasome system dysfunction

Capsaicin is an active component of red pepper having an antiproliferative effect in a variety of cancer cells, which recent evidence suggests due to its ability to induce apoptosis. However, the molecular mechanisms through which capsaicin induces apoptosis are not well understood. Here we demonstrate that capsaicin-induced apoptosis is mediated via the inhibition cellular proteasome function. Treatment of capsaicin to mouse neuro 2a cells results in the inhibition of proteasome activity in a dose- and time-dependent manner that seems to correlate with its effect on cell death. The effect of capsaicin on cellular proteasome function is indirect and probably mediated via the generation of oxidative stress. Exposure of capsaicin also causes increased accumulation of ubiquitinated proteins as wells as various target substrates of proteasome like p53 and Bax and p27. Like many other classical proteasome inhibitors, capsaicin also triggers the intrinsic pathway of apoptosis involving mitochondria and induces neurite outgrowth. Our results strongly support for the use of capsaicin as an anticancer drug.

Tumor tissue-derived formaldehyde and acidic microenvironment synergistically induce bone cancer pain

BACKGROUND

There is current interest in understanding the molecular mechanisms of tumor-induced bone pain. Accumulated evidence shows that endogenous formaldehyde concentrations are elevated in the blood or urine of patients with breast, prostate or bladder cancer. These cancers are frequently associated with cancer pain especially after bone metastasis. It is well known that transient receptor potential vanilloid receptor 1 (TRPV1) participates in cancer pain. The present study aims to demonstrate that the tumor tissue-derived endogenous formaldehyde induces bone cancer pain via TRPV1 activation under tumor acidic environment.

METHODOLOGY/PRINCIPAL FINDINGS

Endogenous formaldehyde concentration increased significantly in the cultured breast cancer cell lines in vitro, in the bone marrow of breast MRMT-1 bone cancer pain model in rats and in tissues from breast cancer and lung cancer patients in vivo. Low concentrations (1 approximately 5 mM) of formaldehyde induced pain responses in rat via TRPV1 and this pain response could be significantly enhanced by pH 6.0 (mimicking the acidic tumor microenvironment). Formaldehyde at low concentrations (1 mM to 100 mM) induced a concentration-dependent increase of [Ca(2+)]i in the freshly isolated rat dorsal root ganglion neurons and TRPV1-transfected CHO cells. Furthermore, electrophysiological experiments showed that low concentration formaldehyde-elicited TRPV1 currents could be significantly potentiated by low pH (6.0). TRPV1 antagonists and formaldehyde scavengers attenuated bone cancer pain responses.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that cancer tissues directly secrete endogenous formaldehyde, and this formaldehyde at low concentration induces metastatic bone cancer pain through TRPV1 activation especially under tumor acidic environment.

Pharmacology of capsaicin-, anandamide-, and N-arachidonoyl-dopamine-evoked cell death in a homogeneous transient receptor potential vanilloid subtype 1 receptor population

BACKGROUND

Transient receptor potential vanilloid subtype 1 (TRPV1) receptor is a primary pain-sensing relay at peripheral sensory nerve endings and is also widespread in the brain, where it is implicated in neurodegeneration. Previous studies of TRPV1 neurotoxicity have utilized heterogeneous receptor populations, non-selective ligands, or non-neuronal cell types. Here, we explored the pharmacology of TRPV1-induced cytotoxicity in a homogeneous, neurone-like cellular environment.

METHODS

Cell death was examined in a human neurone-like cell line, stably expressing recombinant human TRPV1. Cytotoxicity was quantified in terms of nuclear morphology and mitochondrial complex II activity. Immunocytochemical markers of apoptotic cell death were also examined.

RESULTS

The TRPV1-selective agonist capsaicin, and the endovanilloids anandamide and N-arachidonoyl-dopamine (NADA), induced TRPV1-dependent delayed cell death in a concentration- and time-dependent manner. Capsaicin exposure time was significantly correlated with potency (r(2)=0.91, P=0.01). Release of cytochrome c from mitochondria, activation of caspase-3, and condensed nuclear chromatin were evident 6 h after capsaicin exposure, but cytotoxicity was unaffected by a pan-caspase inhibitor (zVAD-fmk, 50 microM).

CONCLUSIONS

We conclude that capsaicin, anandamide, and NADA can initiate TRPV1-dependent delayed cell death in neurone-like cells. This is an apoptosis-like process, but independent of caspase activity.

Mechanism of oxidative DNA damage induced by capsaicin, a principal ingredient of hot chili pepper

Although capsaicin exhibits antitumor activity, carcinogenic potential has also been reported. To clarify the mechanism for expression of potential carcinogenicity of capsaicin, we examined DNA damage induced by capsaicin in the presence of metal ion and various kinds of cytochrome P450 (CYP) using 32P-5'-end-labeled DNA fragments. Capsaicin induced Cu(II)-mediated DNA damage efficiently in the presence of CYP1A2 and partially in the presence of 2D6. CYP1A2-treated capsaicin caused double-base lesions at 5'-TG-3', 5'-GC-3' and CG of the 5'-ACG-3' sequence complementary to codon 273, a hotspot of p53 gene. DNA damage was inhibited by catalase and bathocuproine, a Cu(I) chelator, suggesting that reactive species derived from the reaction of H2O2 with Cu(I) participate in DNA damage. Formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine was significantly increased by CYP1A2-treated capsaicin in the presence of Cu(II). Therefore, we conclude that Cu(II)-mediated oxidative DNA damage by CYP-treated capsaicin seems to be relevant for the expression of its carcinogenicity.

Fas-associated factor 1 is a negative regulator in capsaicin induced cancer cell apoptosis

Vanilloid receptor1 (VR1/TRPV1) is expressed on peripheral nerves and involved in sensing of temperature and pain. Recent reports have demonstrated that tumor cells express TRPV1 and that capsaicin (CP), a ligand for TRPV1, induces apoptosis in cancer cells. The mechanism underlying CP-induced tumor cell apoptosis remains unclear. Here, we investigated the role of TRPV1 in tumor apoptosis using TRPV1-expressing cancer cell lines. We demonstrate that iodo-resiniferatoxin (I-RTX), an antagonist of TRPV1 does not inhibit CP mediated apoptosis nor is it cytotoxic by itself, but acts as a partial agonist and shows synergistic effect with CP. We further demonstrate that CP treatment degrades Fas-associated factor1 (FAF1); a TRPV1 associated protein. Moreover, using RNA interference with small inhibitory RNAs (siRNA) for FAF1 we observed that down-regulation of FAF1 by siRNA makes the cell susceptible to enhanced apoptosis with CP. In summary, our data shows for the first time that the underlying mechanisms of CP-induced cancer cell apoptosis involves FAF1, a TRPV1 associated protein and serves as an important foundation for further understanding of anticancer activity of CP.

Submembraneous microtubule cytoskeleton: biochemical and functional interplay of TRP channels with the cytoskeleton

Much work has focused on the electrophysiological properties of transient receptor potential channels. Recently, a novel aspect of importance emerged: the interplay of transient receptor potential channels with the cytoskeleton. Recent data suggest a direct interaction and functional repercussion for both binding partners. The bi-directionality of physical and functional interaction renders therefore, the cytoskeleton a potent integration point of complex biological signalling events, from both the cytoplasm and the extracellular space. In this minireview, we focus mostly on the interaction of the cytoskeleton with transient receptor potential vanilloid channels. Thereby, we point out the functional importance of cytoskeleton components both as modulator and as modulated downstream effector. The resulting implications for patho-biological situations are discussed.

Vanilloid-induced conduction analgesia: selective, dose-dependent, long-lasting, with a low level of potential neurotoxicity

Vanilloid agonists (capsaicin, resiniferatoxin, [RTX]) applied to the peripheral nerves provide conduction blockade. In contrast to the analgesic component of conduction anesthesia produced by local anesthetics, vanilloid agonists provide conduction analgesia not associated with suppression of motor or sensory functions not related to pain. Vanilloid agonists provide conduction analgesia selectively because their effect on the nerve trunks is limited to C- and ADelta-fibers. RTX is much more potent than capsaicin and has a wider therapeutic window. In rat experiments, perineural RTX produced a long-lasting thermal and mechanical hypoalgesia with a very wide separation between effective concentrations (from 0.00003% to 0.001%) providing an effect lasting from several hours to several weeks. A nerve block with RTX prevented the development of thermal and mechanical hyperalgesia as well as pain behavior in a model of incisional pain. RTX-induced conduction blockade has an inherent drawback of TRPV1 agonists, the initial excitation (pain); therefore, a local anesthetic should be injected to prevent it. When RTX was applied to the rat's sciatic nerve in doses necessary to provide conduction analgesia, the frequency of unmyelinated fiber degeneration was more than an order of magnitude lower than that with the therapeutic concentration of lidocaine. These promising results should be confirmed by experiments in species other than rodents (pigs, sheep). Taken together, the data indicate possible clinical applicability of vanilloid-induced conduction analgesia.

TRPM8 activation suppresses cellular viability in human melanoma

The transient receptor potential melastatin subfamily (TRPM), which is a mammalian homologue of cell death-regulated genes in Caenorhabditis elegans and Drosophila, has potential roles in the process of the cell cycle and regulation of Ca2+signaling. Among this subfamily, TRPM8 (also known as Trp-p8) is a Ca2+-permeable channel that was originally identified as a prostate-specific gene upregulated in tumors. Here we showed that the TRPM8 channel was expressed in human melanoma G-361 cells, and activation of the channel produced sustainable Ca2+influx. The application of menthol, an agonist for TRPM8 channel, elevated cytosolic Ca2+concentration in a concentration-dependent manner with an EC50value of 286 μM in melanoma cells. Menthol-induced responses were significantly abolished by the removal of external Ca2+. Moreover, inward currents at a holding potential of −60 mV in melanoma cells were markedly potentiated by the addition of 300 μM menthol. The most striking finding was that the viability of melanoma cells was dose-dependently depressed in the presence of menthol. These results reveal that a functional TRPM8 protein is expressed in human melanoma cells to involve the mechanism underlying tumor progression via the Ca2+handling pathway, providing us with a novel target of drug development for malignant melanoma.

TRPV1 mediates cell death in rat synovial fibroblasts through calcium entry-dependent ROS production and mitochondrial depolarization

Synoviocyte hyperplasia is critical for rheumatoid arthritis, therefore, potentially an important target for therapeutics. It was found in this work that a TRPV1 agonist capsaicin, and acidic solution (pH 5.5) induced increases in cytosolic calcium concentration ([Ca(2+)](c)) and reactive oxygen species (ROS) production in synoviocytes isolated from a rat model of collagen-induced arthritis. The increases in both [Ca(2+)](c) and ROS production were completely abolished in calcium-free buffer or by a TRPV1 antagonist capsazepine. Further experiments revealed that capsaicin and pH 5.5 solution caused mitochondrial membrane depolarization and reduction in cell viability; such effects were inhibited by capsazepine, or the NAD(P)H oxidase inhibitor diphenylene iodonium. Both capsaicin and pH 5.5 buffer induced apoptosis as shown by nuclear condensation and fragmentation. Furthermore, RT-PCR readily detected TRPV1 mRNA expression in the isolated synoviocytes. Taken together, these data indicated that TRPV1 activation triggered synoviocyte death by [Ca(2+)](c) elevation, ROS production, and mitochondrial membrane depolarization.

Roles of transient receptor potential vanilloid subtype 1 and cannabinoid type 1 receptors in the brain: neuroprotection versus neurotoxicity

Transient receptor potential vanilloid subtype 1 (TRPV1), also known as vanilloid receptor 1 (VR1), is a nonselective cation channel that is activated by a variety of ligands, such as exogenous capsaicin (CAP) or endogenous anandamide (AEA), as well as products of lipoxygenases. Cannabinoid type 1 (CB1) receptor belongs to the G protein-coupled receptor superfamily and is activated by cannabinoids such as AEA and exogenous Delta-9-tetrahydrocannabinol (THC). TRPV1 and CB1 receptors are widely expressed in the brain and play many significant roles in various brain regions; however, the issue of whether TRPV1 or CB1 receptors mediate neuroprotection or neurotoxicity remains controversial. Furthermore, functional crosstalk between these two receptors has been recently reported. It is therefore timely to review current knowledge regarding the functions of these two receptors and to consider new directions of investigation on their roles in the brain.

The TRPV1 receptor is associated with preferential stress in large dorsal root ganglion neurons in early diabetic sensory neuropathy

Chronic diabetic neuropathy is associated with peripheral demyelination and degeneration of nerve fibers. The mechanism(s) underlying neuronal injury in diabetic sensory neuropathy remain poorly understood. Recently, we reported increased expression and function of transient receptor potential vanilloid 1 (TRPV1) in large dorsal root ganglion (DRG) neurons in diabetic sensory neuropathy. In this study, we examined the effects of TRPV1 activation on cell injury pathways in this subpopulation of neurons in the streptozotocin-induced diabetic rat model. Large DRG neurons from diabetic (6-8 weeks) rats displayed increased oxidative stress and activation of cell injury markers compared with healthy controls. Capsaicin (CAP) treatment induced decreased labeling of MitoTracker Red and increased cytosolic cytochrome c and activation of caspase 3 in large neurons isolated from diabetic rats. CAP treatment also induced oxidative stress in large diabetic DRG neurons, which was blocked by pre-treatment with caspase or calpain inhibitor. In addition, both mu-calpain expression and calpain activity were significantly increased in DRG neurons from diabetic rats after CAP treatment. Treatment with capsazepine, a competitive TRPV1 antagonist, markedly reduced these abnormalities in vitro and prevented activation of cell injury in large DRG neurons in diabetic rats in vivo. These results suggest that activation of the TRPV1 receptor activates pathways associated with caspase-dependent and calpain-dependent stress in large DRG neurons in STZ-diabetic rats. Activation of the TRPV1 receptor may contribute to preferential neuronal stress in large DRG neurons relatively early in diabetic sensory neuropathy.

Activation of apoptosis, but not necrosis, during Mycobacterium tuberculosis infection correlated with decreased bacterial growth: role of TNF-alpha, IL-10, caspases and phospholipase A2

Monocyte/macrophage cell death is an important event during mycobacterial infection. To get insights about the influence of mononuclear phagocyte maturation in this event we compared the response to Mycobacterium tuberculosis (Mtb) infection of fresh isolated monocytes and monocyte-derived macrophages (MDM) from healthy tuberculin positive individuals. Both monocytes and MDM underwent apoptosis, however, there was a higher numbers of apoptotic macrophages with active Caspases 8 and 9. We also compared Mtb-induced cell death in U937 pro-monocytes and PMA-differentiated cells (U937D). In response to Mtb infection, U937D cells underwent apoptosis and promonocytes both apoptosis and necrosis. There were high number of U937D cells producing TNF-alpha and high number of IL-10+ promonocytes. These evidences suggest that U937 could be a valid model to study the mechanisms that rule Mtb-induced cell death. Experiments with the cell line and fresh isolated mononuclear cells with pharmacological inhibitors showed that induction of necrosis involved calcium and cAMP signals resulting in IL-10 production. Necrosis also correlated with Caspase 3, PLA2 activity and bacterial growth. In U937D cells and monocytes from healthy donors there was activation of calcium, TNF-alpha and Caspase 8 activation and decreased bacterial load. Understanding the mechanisms that control the dichotomy events between apoptosis and necrosis/oncosis associated with cell maturity might open new strategies to better control the course of mycobacterial infections.

Activation of recombinant human TRPV1 receptors expressed in SH-SY5Y human neuroblastoma cells increases [Ca2+]i, initiates neurotransmitter release and promotes delayed cell death

The transient receptor potential (TRP) vanilloid receptor subtype 1 (TRPV1) is a ligand-gated, Ca(2+)-permeable ion channel in the TRP superfamily of channels. We report the establishment of the first neuronal model expressing recombinant human TRPV1 (SH-SY5Y(hTRPV1)). SH-SY5Y human neuroblastoma cells were stably transfected with hTRPV1 using the Amaxa Biosystem (hTRPV1 in pIREShyg2 with hygromycin selection). Capsaicin, olvanil, resiniferatoxin and the endocannabinoid anandamide increased [Ca(2+)](i) with potency (EC(50)) values of 2.9 nmol/L, 34.7 nmol/L, 0.9 nmol/L and 4.6 micromol/L, respectively. The putative endovanilloid N-arachidonoyl-dopamine increased [Ca(2+)](i) but this response did not reach a maximum. Capsaicin, anandamide, resiniferatoxin and olvanil mediated increases in [Ca(2+)](i) were inhibited by the TRPV1 antagonists capsazepine and iodo-resiniferatoxin with potencies (K(B)) of approximately 70 nmol/L and 2 nmol/L, respectively. Capsaicin stimulated the release of pre-labelled [(3)H]noradrenaline from monolayers of SH-SY5Y(hTRPV1) cells with an EC(50) of 0.6 nmol/L indicating amplification between [Ca(2+)](i) and release. In a perfusion system, we simultaneously measured [(3)H]noradrenaline release and [Ca(2+)](i) and observed that increased [Ca(2+)](i) preceded transmitter release. Capsaicin treatment also produced a cytotoxic response (EC(50) 155 nmol/L) that was antagonist-sensitive and mirrored the [Ca(2+)](I) response. This model displays pharmacology consistent with TRPV1 heterologously expressed in standard non-neuronal cells and native neuronal cultures. The advantage of SH-SY5Y(hTRPV1) is the ability of hTRPV1 to couple to neuronal biochemical machinery and produce large quantities of cells.

Rapid modulation of micro-opioid receptor signaling in primary sensory neurons

Management of pain by opioid analgesics is confounded by central adverse effects that limit clinical dosages. Consequently, there is considerable interest to understand peripheral analgesic effects of opioids. The actions of opioids on peripheral sensory neurons have been difficult to study because of a general lack of effect of opioid agonists on nociceptor function in culture despite documented presence of opioid receptors. In this study, the micro-opioid receptor agonist, [D-Ala(2),N-MePhe(4),Gly-ol(5)]-enkephalin (DAMGO), did not alter guanosine 5'-O-(3-[(35)S]thio)-triphosphate (GTPgamma[(35)S]) binding, adenylyl cyclase activity, or neuropeptide release in primary cultures of rat trigeminal ganglion (TG). However, after brief exposure to bradykinin (BK), DAMGO stimulated GTPgamma[(35)S] binding and inhibited both prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity and BK/PGE(2)-stimulated neuropeptide release. The effect of BK was blocked by the B(2) antagonist HOE 140 [D-Arg[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-bradykinin], but not by the B(1) antagonist, Lys-[Leu8]des-Arg9-BK, and was mimicked by the protease-activated receptor-2 agonist, Ser-Leu-Ile-Gly-Arg-Leu-NH(2), and by activation of protein kinase C (PKC) or by administration of arachidonic acid (AA). The enhanced responsiveness of micro-opioid receptor signaling by BK priming was blocked by both cyclooxygenase and PKC inhibitors; however, the effect of AA was blocked only by a cyclooxygenase inhibitor. The results indicate that micro-opioid receptor signaling in primary sensory TG neurons is enhanced by activation of phospholipase C-coupled receptors via a cyclooxygenase-dependent AA metabolite that is downstream of PKC.