TRPV1 agonist piperine but not olvanil enhances glutamatergic spontaneous excitatory transmission in rat spinal substantia gelatinosa neurons

Anesthetic- and Analgesic-Related Drugs Modulating Both Voltage-Gated Na+ and TRP Channels

Nociceptive information is transmitted by action potentials (APs) through primary afferent neurons from the periphery to the central nervous system. Voltage-gated Na+ channels are involved in this AP production, while transient receptor potential (TRP) channels, which are non-selective cation channels, are involved in receiving and transmitting nociceptive stimuli in the peripheral and central terminals of the primary afferent neurons. Peripheral terminal TRP vanilloid-1 (TRPV1), ankylin-1 (TRPA1) and melastatin-8 (TRPM8) activation produces APs, while central terminal TRP activation enhances the spontaneous release of L-glutamate from the terminal to spinal cord and brain stem lamina II neurons that play a pivotal role in modulating nociceptive transmission. There is much evidence demonstrating that chemical compounds involved in Na+ channel (or nerve AP conduction) inhibition modify TRP channel functions. Among these compounds are local anesthetics, anti-epileptics, α2-adrenoceptor agonists, antidepressants (all of which are used as analgesic adjuvants), general anesthetics, opioids, non-steroidal anti-inflammatory drugs and plant-derived compounds, many of which are involved in antinociception. This review mentions the modulation of Na+ channels and TRP channels including TRPV1, TRPA1 and TRPM8, both of which modulations are produced by pain-related compounds.

TRPV1: Receptor structure, activation, modulation and role in neuro-immune interactions and pain

In the 1990s, the identification of a non-selective ion channel, especially responsive to capsaicin, revolutionized the studies of somatosensation and pain that were to follow. The TRPV1 channel is expressed mainly in neuronal cells, more specifically, in sensory neurons responsible for the perception of noxious stimuli. However, its presence has also been detected in other non-neuronal cells, such as immune cells, β- pancreatic cells, muscle cells and adipocytes. Activation of the channel occurs in response to a wide range of stimuli, such as noxious heat, low pH, gasses, toxins, endocannabinoids, lipid-derived endovanilloid, and chemical agents, such as capsaicin and resiniferatoxin. This activation results in an influx of cations through the channel pore, especially calcium. Intracellular calcium triggers different responses in sensory neurons. Dephosphorylation of the TRPV1 channel leads to its desensitization, which disrupts its function, while its phosphorylation increases the channel's sensitization and contributes to the channel's rehabilitation after desensitization. Kinases, phosphoinositides, and calmodulin are the main signaling pathways responsible for the channel's regulation. Thus, in this review we provide an overview of TRPV1 discovery, its tissue expression as well as on the mechanisms by which TRPV1 activation (directly or indirectly) induces pain in different disease models.

Role of bradykinin and prostaglandin EP4 receptors in regulating TRPV1 channel sensitization in rat dorsal root ganglion neurons

Transient receptor potential vanilloid type-1 (TRPV1) channels play key roles in chronic pain conditions and are modulated by different inflammatory mediators to elicit heat sensitisation. Bradykinin is a 9-amino acid peptide chain that promotes inflammation. The aim of present study is to investigate how bradykinin and prostaglandin receptors (EP3 and EP4 ) modulate the sensitisation of TRPV1-mediated responses. Calcium imaging studies of rat dorsal root ganglion (DRG) neurons were employed to investigate the desensitizing responses of TRPV1 ion channels by capsaicin, and the re-sensitization of TRPV1 by bradykinin, then to explore the role EP3 and EP4 receptors in mediating these bradykinin-dependent effects. Immunocytochemistry was used to study the co-expression and distribution of EP4, TRPV1, COX-1 and B2 in rat DRG neurons. Desensitization was seen upon repeated capsaicin application, we show that bradykinin-mediated sensitization of capsaicin-evoked calcium responses in rat DRG neurons occurs is dependent on COX-1 activity and utilizes a pathway that involves EP4 but not EP3 receptors. Immunocytochemical techniques revealed that EP4, TRPV1, COX-1 and B2 proteins are expressed mainly in small diameter (<1000 μm2 ) cell bodies of rat DRG neurons which are typically nociceptors. The present study provides suggestive evidence for a potential signalling pathway through which bradykinin may regulate TRPV1 ion channel function via EP4 receptors. In addition to confirming existing knowledge, the anatomical distribution and colocalization of these proteins in DRG neurons as revealed by this study offer valuable insight.

(-)-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.

The histology, physiology, neurochemistry and circuitry of the substantia gelatinosa Rolandi (lamina II) in mammalian spinal cord

The substantia gelatinosa Rolandi (SGR) was first described about two centuries ago. In the following decades an enormous amount of information has permitted us to understand - at least in part - its role in the initial processing of pain and itch. Here, I will first provide a comprehensive picture of the histology, physiology, and neurochemistry of the normal SGR. Then, I will analytically discuss the SGR circuits that have been directly demonstrated or deductively envisaged in the course of the intensive research on this area of the spinal cord, with particular emphasis on the pathways connecting the primary afferent fibers and the intrinsic neurons. The perspective existence of neurochemically-defined sets of primary afferent neurons giving rise to these circuits will be also discussed, with the proposition that a cross-talk between different subsets of peptidergic fibers may be the structural and functional substrate of additional gating mechanisms in SGR. Finally, I highlight the role played by slow acting high molecular weight modulators in these gating mechanisms.

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.

Anti-nociceptive and desensitizing effects of olvanil on capsaicin-induced thermal hyperalgesia in the rat

BACKGROUND

Olvanil (NE 19550) is a non-pungent synthetic analogue of capsaicin, the natural pungent ingredient of capsicum which activates the transient receptor potential vanilloid type-1 (TRPV1) channel and was developed as a potential analgesic compound. Olvanil has potent anti-hyperalgesic effects in several experimental models of chronic pain. Here we report the inhibitory effects of olvanil on nociceptive processing using cultured dorsal root ganglion (DRG) neurons and compare the effects of capsaicin and olvanil on thermal nociceptive processing in vivo; potential contributions of the cannabinoid CB1 receptor to olvanil's anti-hyperalgesic effects were also investigated.

METHODS

A hot plate analgesia meter was used to evaluate the anti-nociceptive effects of olvanil on capsaicin-induced thermal hyperalgesia and the role played by CB1 receptors in mediating these effects. Single cell calcium imaging studies of DRG neurons were employed to determine the desensitizing effects of olvanil on capsaicin-evoked calcium responses. Statistical analysis used Student's t test or one way ANOVA followed by Dunnett's post-hoc test as appropriate.

RESULTS

Both olvanil (100 nM) and capsaicin (100 nM) produced significant increases in intracellular calcium concentrations [Ca(2+)]i in cultured DRG neurons. Olvanil was able to desensitise TRPV1 responses to further capsaicin exposure more effectively than capsaicin. Intraplantar injection of capsaicin (0.1, 0.3 and 1 μg) produced a robust TRPV1-dependant thermal hyperalgesia in rats, whilst olvanil (0.1, 0.3 and 1 μg) produced no hyperalgesia, emphasizing its lack of pungency. The highest dose of olvanil significantly reduced the hyperalgesic effects of capsaicin in vivo. Intraplantar injection of the selective cannabinoid CB1 receptor antagonist rimonabant (1 μg) altered neither capsaicin-induced thermal hyperalgesia nor the desensitizing properties of olvanil, indicating a lack of involvement of CB1 receptors.

CONCLUSIONS

Olvanil is effective in reducing capsaicin-induced thermal hyperalgesia, probably via directly desensitizing TRPV1 channels in a CB1 receptor-independent fashion. The results presented clearly support the potential for olvanil in the development of new topical analgesic preparations for treating chronic pain conditions while avoiding the unwanted side effects of capsaicin treatments.

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.

Medial prefrontal cortex TRPV1 channels modulate the baroreflex cardiac activity in rats

BACKGROUND AND PURPOSE

The ventral portion of the medial prefrontal cortex (vMPFC) comprises the infralimbic (IL), prelimbic (PL) and dorsopenducular (DP) cortices. The IL and PL regions facilitate the baroreceptor reflex arc. This facilitatory effect on the baroreflex is thought to be mediated by vMPFC glutamatergic transmission, through NMDA receptors. The glutamatergic transmission can be modulated by other neurotransmitters, such as the endocannabinoids, which are agonists of the TRPV1 receptor. TRPV1 channels facilitate glutamatergic transmission in the brain. Thus, we hypothesized that TRPV1 receptors in the vMPFC enhance the cardiac baroreflex response.

EXPERIMENTAL APPROACH

Stainless steel guide cannulae were bilaterally implanted into the vMPFC of male Wistar rats. Afterwards, a catheter was inserted into the femoral artery, for recording MAP and HR, and into the femoral vein for assessing baroreflex activation.

KEY RESULTS

Microinjections of the TRPV1 receptor antagonists capsazepine and 6-iodo-nordihydrocapsaicin (6-IODO) into the vMPFC reduced the cardiac baroreflex activity in unanaesthetized rats. Capsaicin microinjected into the vMPFC increased the cardiac baroreflex activity in unanaesthetized rats. When an ineffective dose of the TRPV1 receptor antagonist 6-IODO was used, the capsaicin-induced increase in the cardiac baroreflex response was abolished. The higher doses of capsaicin administered into the vMPFC after the ineffective dose of 6-IODO displaced the dose-response curve of the baroreflex parameters to the right, with no alteration in the maximum effect of capsaicin.

CONCLUSIONS AND IMPLICATIONS

The results of the present study show that stimulation of the TRPV1 receptors in the vMPFC increases the cardiac baroreceptor reflex response.

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.

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

Transient receptor potential (TRP) channels are thought to play a role in regulating nociceptive transmission to spinal substantia gelatinosa (SG) neurons. It remains to be unveiled whether the TRP channels in the central nervous system are different in property from those involved in receiving nociceptive stimuli in the peripheral nervous system. We examined the effect of the vanilloid compound zingerone, which activates TRPV1 channels in the cell body of a primary afferent neuron, on glutamatergic excitatory transmission in the SG neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. Bath-applied zingerone reversibly and concentration-dependently increased spontaneous excitatory postsynaptic current (EPSC) frequency. This effect was accompanied by an inward current at -70 mV that was resistant to glutamate receptor antagonists. These zingerone effects were repeated and persisted in Na(+)-channel blocker tetrodotoxin-, La(3+)-, or IP3-induced Ca(2+)-release inhibitor 2-aminoethoxydiphenyl borate-containing or Ca(2+)-free Krebs solution. Zingerone activity was resistant to the selective TRPV1 antagonist capsazepine but sensitive to the nonselective TRP antagonist ruthenium red, the TRPA1 antagonist HC-030031, and the Ca(2+)-induced Ca(2+)-release inhibitor dantrolene. TRPA1 agonist allyl isothiocyanate but not capsaicin inhibited the facilitatory effect of zingerone. On the other hand, zingerone reduced monosynaptically evoked EPSC amplitudes, as did TRPA1 agonists. Like allyl isothiocyanate, zingerone enhanced GABAergic spontaneous inhibitory transmission in a manner sensitive to tetrodotoxin. We conclude that zingerone presynaptically facilitates spontaneous excitatory transmission, probably through Ca(2+)-induced Ca(2+)-release mechanisms, and produces a membrane depolarization in SG neurons by activating TRPA1 but not TRPV1 channels.

Inhibition by capsaicin and its related vanilloids of compound action potentials in frog sciatic nerves

AIMS

Although capsaicin not only activates transient receptor potential vanilloid-1 (TRPV1) channels but also inhibits nerve conduction, the latter action has not yet been fully examined. The purpose of the present study was to know whether various vanilloids have an inhibitory action similar to that of capsaicin and further to compare their actions with that of local anesthetic procaine.

MAIN METHODS

Fast-conducting compound action potentials (CAPs) were recorded from frog sciatic nerve fibers by using the air-gap method.

KEY FINDINGS

Capsaicin reversibly and concentration-dependently reduced the peak amplitude of the CAP. TRPV1 antagonist capsazepine did not affect the capsaicin activity, and powerful TRPV1 agonist resiniferatoxin had no effect on CAPs, indicating no involvement of TRPV1 channels. Capsaicin analogs and other various vanilloids also inhibited CAPs in a concentration-dependent manner. An efficacy sequence of these inhibitions was capsaicin=dihydrocapsaicin>capsiate>eugenol>guaiacol≥zingerone≥vanillin>vanillylamine. Vanillic acid had almost no effect on CAPs; olvanil and curcumin appeared to be effective less than capsaicin. Capsaicin and eugenol were, respectively, ten- and two-fold effective more than procaine in CAP inhibition, while each of guaiacol, zingerone and vanillin was five-fold effective less than procaine.

SIGNIFICANCE

Various vanilloids exhibit CAP inhibition, the extent of which is determined by the property of the side chain bound to the vanillyl group, and some of them are more effective than procaine. These results may serve to unveil molecular mechanisms for capsaicin-induced conduction block and to develop antinociceptive drugs related to capsaicin.

Presynaptic enhancement by eugenol of spontaneous excitatory transmission in rat spinal substantia gelatinosa neurons is mediated by transient receptor potential A1 channels

Eugenol, which is contained in several plants including clove, has been widely used as an analgesic and anti-inflammatory drug in the dental clinic. Eugenol also has anesthetic effects and produces sedation and the reduction of convulsion threshold. These benefits have been partly attributed to the effects of eugenol on neural tissues, such as inhibition of voltage-gated ion channels. As expected from the fact that eugenol is a vanilloid compound, this drug activates transient receptor potential (TRP) V1 channels in the peripheral nervous system (PNS). Although eugenol affects synaptic transmission in the central nervous system (CNS), this has not yet been fully examined. We investigated how eugenol affects spontaneous glutamatergic excitatory transmission in substantia gelatinosa (SG; lamina II of Rexed) neurons of adult rat spinal cord slices by use of the blind whole-cell patch-clamp technique. Bath-applied eugenol reversibly enhanced spontaneous excitatory transmission and produced an outward current at -70 mV in SG neurons. The former action was due to a large increase in the frequency of spontaneous excitatory postsynaptic current (sEPSC) with a small increase in the amplitude. These actions of eugenol were seen by its repeated application and resistant to a voltage-gated Na(+) channel blocker tetrodotoxin. The effect of eugenol on sEPSC frequency was concentration-dependent with an EC(50) value of 3.8 mM and unaffected by a TRPV1 antagonist capsazepine, whereas inhibited by a nonspecific TRP antagonist ruthenium red and a TRPA1 antagonist HC-030031. On the other hand, the eugenol-induced outward current was not affected by these TRP antagonists. It is concluded that eugenol activates TRPA1 channels in the SG, leading to an increase in the spontaneous release of L-glutamate to SG neurons, and that eugenol also produces a membrane hyperpolarization that is not mediated by TRP channels. Eugenol is suggested to activate different types of TRP channel between the PNS and CNS.