Zingerone enhances glutamatergic spontaneous excitatory transmission by activating TRPA1 but not TRPV1 channels in the adult rat substantia gelatinosa

Zingerone Alleviates Morphine Tolerance and Dependence in Mice by Reducing Oxidative Stress-Mediated NLRP3 Inflammasome Activation

Morphine (MPH) is widely used for pain management; however, long-term MPH therapy results in antinociceptive tolerance and physical dependence, limiting its clinical use. Zingerone (ZIN) is a natural phenolic compound with neuroprotective effects. We investigated the effects of single and repeated doses of ZIN on MPH-induced tolerance, dependence, and underlying biochemical mechanisms. After a dose-response experiment, tolerance was developed to MPH (10 mg/kg, i.p.) for seven days. In the single-dose study, ZIN was administered on day seven. In the repeated-dose study, ZIN was administered for seven days. Naloxone (5 mg/kg, i.p., 120 min after MPH) was injected to assess withdrawal signs on day seven. The levels of thiobarbituric acid reactive substances (TBARS), nitric oxide (NO), total thiol (TT), and glutathione peroxidase (GPx) were measured in the prefrontal cortex. The protein levels of interleukin-1 beta (IL-1β) and NLRP3-ASC-Caspase-1 axis were assessed by ELISA and Western blotting, respectively. Results showed that ZIN (100 mg/kg) had no antinociceptive activity, and subsequent experiments were performed at this dose. Repeated ZIN reversed MPH antinociceptive tolerance, whereas single ZIN did not. Single and repeated ZIN attenuated naloxone-induced jumping. In addition, repeated ZIN significantly inhibited weight loss. Repeated ZIN suppressed the MPH-induced increase in TBARS, NO, IL-1β, NLRP3, ASC, and Caspase-1. It also inhibited MPH-induced TT and GPx reduction. In contrast, single ZIN had no effect. Findings suggest that ZIN reduces MPH-induced tolerance and dependence by suppressing oxidative stress and NLRP3 inflammasome activation. This study provides a novel therapeutic approach to reduce the side effects of MPH.

Regulation of transient receptor potential channels by traditional Chinese medicines and their active ingredients

In recent years, activation of thermal transient receptor potential (TRP) ion channels at a range of temperatures has received widespread attention as a target for traditional Chinese medicine (TCM) to regulate body temperature and relieve pain. Discovery of transient receptor potential vanilloid 1 (TRPV1) was awarded a Nobel Prize, reflecting the importance of these channels. Here, the regulatory effects of TCMs and their active ingredients on TRP ion channels are reviewed, and future directions for research on the cold, hot, warm, cool, and neutral natures of TCMs are considered. In herbs with cold, hot, warm, cool, and neutral natures, we found 29 TCMs with regulatory effects on TRP ion channels, including Cinnamomi Cortex, Capsici Fructus, Rhei Radix et Rhizoma, Macleayae cordatae Herba, Menthae Haplocalycis Herba, and Rhodiolae Crenulatae Radix et Rhizoma. Although some progress has been made in understanding the regulation of TRP ion channels by TCMs and their ingredients, the molecular mechanism by which TCMs have this effect remains to be further studied. We hope this review will provide a reference for further research on the cold, hot, warm, cool, and neutral natures of TCMs.

Zingerone Modulates Neuronal Voltage-Gated Na+ and L-Type Ca2+ Currents

Zingerone (ZO), a nontoxic methoxyphenol, has been demonstrated to exert various important biological effects. However, its action on varying types of ionic currents and how they concert in neuronal cells remain incompletely understood. With the aid of patch clamp technology, we investigated the effects of ZO on the amplitude, gating, and hysteresis of plasmalemmal ionic currents from both pituitary tumor (GH₃) cells and hippocampal (mHippoE-14) neurons. The exposure of the GH₃ cells to ZO differentially diminished the peak and late components of the I_Na. Using a double ramp pulse, the amplitude of the I_Na(P) was measured, and the appearance of a hysteresis loop was observed. Moreover, ZO reversed the tefluthrin-mediated augmentation of the hysteretic strength of the I_Na(P) and led to a reduction in the I_Ca,L. As a double ramp pulse was applied, two types of voltage-dependent hysteresis loops were identified in the I_Ca,L, and the replacement with BaCl₂-attenuated hysteresis of the I_Ca,L enhanced the I_Ca,L amplitude along with the current amplitude (i.e., the I_Ba). The hysteretic magnitude of the I_Ca,L activated by the double pulse was attenuated by ZO. The peak and late I_Na in the hippocampal mHippoE-14 neurons was also differentially inhibited by ZO. In addition to acting on the production of reactive oxygen species, ZO produced effects on multiple ionic currents demonstrated herein that, considered together, may significantly impact the functional activities of neuronal cells.

Trpv1 and Trpa1 are not essential for Psickle-like activity in red cells of the SAD mouse model of sickle cell disease

The molecular identity of Psickle, the deoxygenation-activated cation conductance of the human sickle erythrocyte, remains unknown. We observed in human sickle red cells that inhibitors of TRPA1 and TRPV1 inhibited Psickle, whereas a TRPV1 agonist activated a Psickle-like cation current. These observations prompted us to test the roles of TRPV1 and TRPA1 in Psickle in red cells of the SAD mouse model of sickle cell disease. We generated SAD mice genetically deficient in either TRPV1 or TRPA1. SAD;Trpv1^-/- and SAD;Trpa1^-/- mice were indistinguishable in appearance, hematological indices, and osmotic fragility from SAD mice. We found that deoxygenation-activated cation currents remained robust in SAD;Trpa1^-/- and SAD;Trpv1^-/- mice. In addition, ⁴⁵Ca²⁺ influx into SAD mouse red cells during prolonged deoxygenation was not reduced in red cells from SAD;Trpa1^-/- and SAD;Trpv1^-/- mice. We conclude that the nonspecific cation channels TRPA1 and TRPV1 are not required for deoxygenation to stimulate Psickle-like activity in red cells of the SAD mouse model of sickle cell disease. (159).

Investigating the Efficacy of Zingerone on Mesothelioma and the Role of TRPV1 in This Effect

Mesothelioma is a highly lethal cancer developing in the lung, heart, and abdominal membranes. Zingerone, a capsaicin-like bioactive compound, has been shown to have anticancer properties. Transient Receptor Potential Vanilloid 1 (TRPV1) is an ion channel involving in the cytotoxicity of capsaicin. In the present study, we aimed at determining the cytotoxicity of zingerone on a mesothelioma cell line and to evaluate the role of TRPV1 in this effect. For this purpose, H2452 was used as the mesothelioma cell line and MTS was performed to calculate zingerone cytotoxicity. Moreover, TRPV1 was inhibited by capsazepeine while TRPV1 production was reduced through shRNA treatment. Besides, wound healing and clonogenic assays were performed to measure the migration and colony forming abilities, respectively. As a result, IC₅₀ value of zingerone was calculated as 11.49 mM. Capsazepine treatment or lowered TRPV1 gene expression did not appear to affect zingerone cytotoxicity (p > 0.05) even though the migration rate and colony forming abilities of the zingerone treated cells decreased significantly compared to the control (p < 0.05). Therefore, we concluded that zingerone was less cytotoxic to H2452 cells than the most cancer types and TRPV1 did not seem to have a role in its cytotoxicity.

Zingerone Targets Status Epilepticus by Blocking Hippocampal Neurodegeneration via Regulation of Redox Imbalance, Inflammation and Apoptosis

Epilepsy is an intricate neurological disease where the neurons are severely affected, leading to the mortality of millions worldwide. Status epilepticus (SE), induced by lithium chloride (LiCl) and pilocarpine, is the most accepted model for epilepsy. The current work aims to unravel the mechanisms underlying the anti-epileptic efficacy of zingerone (an active ingredient of ginger), which has beneficial pharmacological activities on seizure-induced behavioral, histological, neurochemical, and molecular patterns in mice. Zingerone restored cognitive function by diminishing seizure activity, escape latency, and subsequent hippocampal damage manifested in histology. Seizures are associated with local inflammation, redox imbalance, and neural loss, confirmed by the present study of SE, and was attenuated by zingerone treatment. Nuclear factor-kappa B and its downstream signaling molecules (TNF-α, IL-1β, IL-6, NO, MPO) were activated in the LiCl-and-pilocarpine-induced group leading to inflammatory signaling, which was substantially ameliorated by zingerone treatment. The intrinsic apoptotic process was triggered subsequent to SE, as demonstrated by augmentation of cleaved caspase-3, downregulation of Bcl-2. However, zingerone treatment downregulated caspase-3 and upregulated Bcl-2, increasing cell survival and decreasing hippocampal neural death, deciphering involvement of apoptosis in SE. Therefore, zingerone plays an essential role in neuroprotection, probably by precluding oxidative stress, inflammation, and obstructing the mitochondrial pathway of apoptosis.

Structural mechanisms underlying activation of TRPV1 channels by pungent compounds in gingers

BACKGROUND AND PURPOSE

Like chili peppers, gingers produce pungent stimuli by a group of vanilloid compounds that activate the nociceptive transient receptor potential vanilloid 1 (TRPV1) ion channel. How these compounds interact with TRPV1 remains unclear.

EXPERIMENTAL APPROACH

We used computational structural modelling, functional tests (electrophysiology and calcium imaging), and mutagenesis to investigate the structural mechanisms underlying ligand-channel interactions.

KEY RESULTS

The potency of three principal pungent compounds from ginger -shogaol, gingerol, and zingerone-depends on the same two residues in the TRPV1 channel that form a hydrogen bond with the chili pepper pungent compound, capsaicin. Computational modelling revealed binding poses of these ginger compounds similar to those of capsaicin, including a "head-down tail-up" orientation, two specific hydrogen bonds, and important contributions of van der Waals interactions by the aliphatic tail. Our study also identified a novel horizontal binding pose of zingerone that allows it to directly interact with the channel pore when bound inside the ligand-binding pocket. These observations offer a molecular level explanation for how unique structures in the ginger compounds affect their channel activation potency.

CONCLUSIONS AND IMPLICATIONS

Mechanistic insights into the interactions of ginger compounds and the TRPV1 cation channel should help guide drug discovery efforts to modulate nociception.

(-)-menthol increases excitatory transmission by activating both TRPM8 and TRPA1 channels in mouse spinal lamina II layer

(-)-menthol, a major form of menthol, is one of the most commonly used chemicals. Many studies have demonstrated that (-)-menthol produces analgesic action through peripheral mechanisms which are mainly mediated by activation of TRPM8. Moreover, intrathecal injection of menthol induces analgesia as well. However, the central actions and mechanisms of (-)-menthol remain unclear. Here, we have investigated the action of (-)-menthol on excitatory synaptic transmission in spinal lamina II layer which plays a pivotal role in modulating nociceptive transmission from the periphery by using patch-clamp technique in mice spinal cord. We found that (-)-menthol increased miniature excitatory postsynaptic current frequency. The frequency increases which (-)-menthol induced were in a dose-dependent manner (EC50: 0.1079 mM). However, neither genetic knockout nor pharmacological inhibition of TRPM8 could block (-)-menthol-induced effects entirely. Furthermore, this increase was also impaired by TRPA1 antagonist HC030031, but abolished utterly by co-application of TRPM8 and TRPA1 antagonist. Our results indicate that (-)-menthol increases the excitatory synaptic transmission by activating either TRPA1 or TRPM8 channels in spinal lamina II layer.

Moieties of plant-derived compounds responsible for outward current production and TRPA1 activation in rat spinal substantia gelatinosa

BACKGROUND

Transient receptor potential ankyrin-1 (TRPA1) channels expressed in the central terminal of dorsal root ganglion neurons in the spinal substantia gelatinosa (SG) play a role in modulating nociceptive transmission. Although plant-derived compounds exhibiting antinociception (such as eugenol, carvacrol and thymol) activate TRPA1 channels to enhance spontaneous excitatory transmission while hyperpolarizing membranes in SG neurons without TRPA1 activation, specific chemical moieties involved in synaptic modulation are unknown.

METHODS

We examined the effects of other plant-derived compounds (guaiacol, vanillin, vanillic acid and p-cymene) on holding current and spontaneous excitatory transmission at -70 mV by applying the whole-cell patch-clamp technique to SG neurons in adult rat spinal cord slices.

RESULTS

None of the compounds affected the frequency or amplitude of spontaneous excitatory postsynaptic current. Guaiacol and vanillic acid had no effect on holding currents, while vanillin and p-cymene produced an inward and outward current, respectively, in some neurons tested. Synaptic modulation was also observed within the same neuron as the activities of eugenol, carvacrol, thymol, and the chemically-related plant-derived compound zingerone occurred.

CONCLUSION

A substituted group in eugenol and zingerone, but not in guaiacol, vanillin or vanillic acid, as well as an OH bound to the benzene ring of carvacrol and thymol, but not p-cymene, play a role in producing outward current and TRPA1 activation. Thus, the binding of such chemical moeties to the benzene ring of plant-derived compounds appears necessary to modulate nociceptive transmission in the SG. This information provides insight for the development of new analgesics based on plant-derived compounds.

Enhancement by citral of glutamatergic spontaneous excitatory transmission in adult rat substantia gelatinosa neurons

Although citral, which is abundantly present in lemongrass, has various actions including antinociception, how citral affects synaptic transmission has not been examined as yet. Citral activates in heterologous cells transient receptor potential vanilloid-1, ankyrin-1, and melastatin-8 (TRPV1, TRPA1, and TRPM8, respectively) channels, the activation of which in the spinal lamina II [substantia gelatinosa (SG)] increases the spontaneous release of L-glutamate from nerve terminals. It remains to be examined what types of transient receptor potential channel in native neurons are activated by citral. With a focus on transient receptor potential activation, we examined the effect of citral on glutamatergic spontaneous excitatory transmission using the whole-cell patch-clamp technique to SG neurons in adult rat spinal cord slices. Bath-applied citral for 3 min increased the frequency of spontaneous excitatory postsynaptic current in a concentration-dependent manner (half-maximal effective concentration=0.58 mM), with a small increase in its amplitude. The spontaneous excitatory postsynaptic current frequency increase produced by citral was repeated at a time interval of 30 min, albeit this action recovered with a slow time course after washout. The presynaptic effect of citral was inhibited by TRPA1 antagonist HC-030031, but not by voltage-gated Na-channel blocker tetrodotoxin, TRPV1 antagonist capsazepine, and TRPM8 antagonist BCTC. It is concluded that citral increases spontaneous L-glutamate release in SG neurons by activating TRPA1 channels. Considering that the SG plays a pivotal role in modulating nociceptive transmission from the periphery, the citral activity could contribute toward at least a part of the modulation.

Differential Activation of TRP Channels in the Adult Rat Spinal Substantia Gelatinosa by Stereoisomers of Plant-Derived Chemicals

Activation of TRPV1, TRPA1 or TRPM8 channel expressed in the central terminal of dorsal root ganglion (DRG) neuron increases the spontaneous release of l-glutamate onto spinal dorsal horn lamina II (substantia gelatinosa; SG) neurons which play a pivotal role in regulating nociceptive transmission. The TRP channels are activated by various plant-derived chemicals. Although stereoisomers activate or modulate ion channels in a distinct manner, this phenomenon is not fully addressed for TRP channels. By applying the whole-cell patch-clamp technique to SG neurons of adult rat spinal cord slices, we found out that all of plant-derived chemicals, carvacrol, thymol, carvone and cineole, increase the frequency of spontaneous excitatory postsynaptic current, a measure of the spontaneous release of l-glutamate from nerve terminals, by activating TRP channels. The presynaptic activities were different between stereoisomers (carvacrol and thymol; (-)-carvone and (+)-carvone; 1,8-cineole and 1,4-cineole) in the extent or the types of TRP channels activated, indicating that TRP channels in the SG are activated by stereoisomers in a distinct manner. This result could serve to know the properties of the central terminal TRP channels that are targets of drugs for alleviating pain.

Neurodegeneration and the Brain Tumor Microenvironment. [corrected]

Malignant brain tumors are characterized by destructive growth and neuronal cell death making them one of the most devastating diseases. Neurodegenerative actions of malignant gliomas resemble mechanisms also found in many neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and amyotrophic lateral sclerosis. Recent data demonstrate that gliomas seize neuronal glutamate signaling for their own growth advantage. Excessive glutamate release via the glutamate/cystine antiporter xCT (system xc-, SLC7a11) renders cancer cells resistant to chemotherapeutics and create the tumor microenvironment toxic for neurons. In particular the glutamate/cystine antiporter xCT takes center stage in neurodegenerative processes and sets this transporter a potential prime target for cancer therapy. Noteworthy is the finding, that reactive oxygen species (ROS) activate transient receptor potential (TRP) channels and thereby TRP channels can potentiate glutamate release. Yet another important biological feature of the xCT/glutamate system is its modulatory effect on the tumor microenvironment with impact on host cells and the cancer stem cell niche. The EMA and FDA-approved drug sulfasalazine (SAS) presents a lead compound for xCT inhibition, although so far clinical trials on glioblastomas with SAS were ambiguous. Here, we critically analyze the mechanisms of action of xCT antiporter on malignant gliomas and in the tumor microenvironment. Deciphering the impact of xCT and glutamate and its relation to TRP channels in brain tumors pave the way for developing important cancer microenvironmental modulators and drugable lead targets.

Integrating TRPV1 Receptor Function with Capsaicin Psychophysics

Capsaicin is a naturally occurring vanilloid that causes a hot, pungent sensation in the human oral cavity. This trigeminal stimulus activates TRPV1 receptors and stimulates an influx of cations into sensory cells. TRPV1 receptors function as homotetramers that also respond to heat, proinflammatory substances, lipoxygenase products, resiniferatoxin, endocannabinoids, protons, and peptide toxins. Kinase-mediated phosphorylation of TRPV1 leads to increased sensitivity to both chemical and thermal stimuli. In contrast, desensitization occurs via a calcium-dependent mechanism that results in receptor dephosphorylation. Human psychophysical studies have shown that capsaicin is detected at nanomole amounts and causes desensitization in the oral cavity. Psychophysical studies further indicate that desensitization can be temporarily reversed in the oral cavity if stimulation with capsaicin is resumed at short interstimulus intervals. Pretreatment of lingual epithelium with capsaicin modulates the perception of several primary taste qualities. Also, sweet taste stimuli may decrease the intensity of capsaicin perception in the oral cavity. In addition, capsaicin perception and hedonic responses may be modified by diet. Psychophysical studies with capsaicin are consistent with recent findings that have identified TRPV1 channel modulation by phosphorylation and interactions with membrane inositol phospholipids. Future studies will further clarify the importance of capsaicin and its receptor in human health and nutrition.

1,8- and 1,4-cineole enhance spontaneous excitatory transmission by activating different types of transient receptor potential channels in the rat spinal substantia gelatinosa

Although transient receptor potential (TRP) channels expressed in the spinal substantia gelatinosa play a role in modulating nociceptive transmission, their properties have not been fully examined yet. In order to address this issue, the effects of 1,8-cineole and its stereoisomer 1,4-cineole on excitatory transmission were examined by applying the whole-cell patch-clamp technique to substantia gelatinosa neurons in adult rat spinal cord slices. Miniature excitatory postsynaptic current frequency was increased by 1,8- and 1,4-cineole. The cineole activities were repeated and resistant to voltage-gated Na⁺ -channel blocker tetrodotoxin. The 1,8-cineole activity was inhibited by TRP ankyrin-1 (TRPA1) antagonists (HC-030031 and mecamylamine) but not TRP vanilloid-1 (TRPV1) antagonists (capsazepine and SB-366791), whereas the 1,4-cineole activity was depressed by the TRPV1 but not TRPA1 antagonists. Although 1,8- and 1,4-cineole reportedly activate TRP melastatin-8 (TRPM8) channels, their activities were unaffected by TRPM8 antagonist 4-(3-chloro-2-pyridinyl)-N-[4-(1,1-dimethylethyl)phenyl]-1-piperazinecarboxamide. Monosynaptically evoked C-fiber, but not Aδ-fiber excitatory postsynaptic current amplitude, was reduced by 1,8- and 1,4-cineole. These results indicate that 1,8- and 1,4-cineole increase spontaneous l-glutamate release from nerve terminals by activating TRPA1 and TRPV1 channels, respectively, while inhibiting C-fiber but not Aδ-fiber evoked l-glutamate release. This difference between 1,8- and 1,4-cineole may serve to know the properties of TRP channels located in the central terminals of primary-afferent neurons. The spinal dorsal horn lamina II (substantia gelatinosa; SG) plays a pivotal role in regulating nociceptive transmission from the periphery. We found out in the SG that 1,4- and 1,8-cineole activate TRPV1 and TRPA1 channels, respectively, located in primary-afferent, possibly C-fiber, central terminals. This difference may serve to know the properties of TRP channels expressed in the central terminals.

Action of thymol on spontaneous excitatory transmission in adult rat spinal substantia gelatinosa neurons

Thymol, which is contained in thyme essential oil, has various actions including antinociception and nerve conduction inhibition. Although thymol activates transient receptor potential (TRP) channels expressed in heterologous cells, it remains to be examined whether this is so in native neurons. It has not yet been examined how thymol affects synaptic transmission. In order to know how thymol modulates excitatory transmission with a focus on TRP activation, we investigated its effect on glutamatergic spontaneous excitatory transmission in lamina II (substantia gelatinosa; SG) neurons with which nerve terminals expressing TRP channels make synaptic contacts. The experiment was performed by using the blind whole-cell patch-clamp technique in adult rat spinal cord slices. Superfusing thymol (1 mM) for 3 min reversibly increased the frequency of spontaneous excitatory postsynaptic current (sEPSC) with a minimal increase in its amplitude in all neurons examined. Seventy-seven% of the neurons produced an outward current at a holding potential of -70 mV. The sEPSC frequency increase and outward current produced by thymol were concentration-dependent with almost the same half-maximal effective concentration (EC50) values of 0.18 and 0.14 mM, respectively. These activities were repeated at a time interval of 30 min, although the sEPSC frequency increase but not outward current recovered with a slow time course. Voltage-gated Na(+)-channel blocker tetrodotoxin did not affect the thymol activities. The sEPSC frequency increase was inhibited by TRPA1 antagonist HC-030031 but not TRPV1 and TRPM8 antagonist (capsazepine and BCTC, respectively), while these antagonists had no effect on the outward current. This was so, albeit the two thymol activities had similar EC50 values. It is concluded that thymol increases the spontaneous release of L-glutamate onto SG neurons by activating TRPA1 channels while producing an outward current without TRP activation. Considering that the SG plays a pivotal role in modulating nociceptive transmission from the periphery, these actions of thymol could contribute to at least a part of its antinociceptive effect.

TRP Channels Involved in Spontaneous L-Glutamate Release Enhancement in the Adult Rat Spinal Substantia Gelatinosa

The spinal substantia gelatinosa (SG) plays a pivotal role in modulating nociceptive transmission through dorsal root ganglion (DRG) neurons from the periphery. TRP channels such as TRPV1 and TRPA1 channels expressed in the SG are involved in the regulation of the nociceptive transmission. On the other hand, the TRP channels located in the peripheral terminals of the DRG neurons are activated by nociceptive stimuli given to the periphery and also by plant-derived chemicals, which generates a membrane depolarization. The chemicals also activate the TRP channels in the SG. In this review, we introduce how synaptic transmissions in the SG neurons are affected by various plant-derived chemicals and suggest that the peripheral and central TRP channels may differ in property from each other.

TRPA1

The transient receptor potential ankyrin subtype 1 protein (TRPA1) is a nonselective cation channel permeable to Ca(2+), Na(+), and K(+). TRPA1 is a promiscuous chemical nocisensor that is also involved in noxious cold and mechanical sensation. It is present in a subpopulation of Aδ- and C-fiber nociceptive sensory neurons as well as in other sensory cells including epithelial cells. In primary sensory neurons, Ca(2+) and Na(+) flowing through TRPA1 into the cell cause membrane depolarization, action potential discharge, and neurotransmitter release both at peripheral and central neural projections. In addition to being activated by cysteine and lysine reactive electrophiles and oxidants, TRPA1 is indirectly activated by pro-inflammatory agents via the phospholipase C signaling pathway, in which cytosolic Ca(2+) is an important regulator of channel gating. The finding that non-electrophilic compounds, including menthol and cannabinoids, activate TRPA1 may provide templates for the design of non-tissue damaging activators to fine-tune the activity of TRPA1 and raises the possibility that endogenous ligands sharing binding sites with such non-electrophiles exist and regulate TRPA1 channel activity. TRPA1 is promising as a drug target for novel treatments of pain, itch, and sensory hyperreactivity in visceral organs including the airways, bladder, and gastrointestinal tract.