Formalin-induced release of excitatory amino acids in the skin of the rat hindpaw

eEF2K regulates pain through translational control of BDNF

mRNA translation is integral to pain, yet the key regulatory factors and their target mRNAs are unclear. Here, we uncover a mechanism that bridges noxious insults to multiple phases of translational control in murine sensory neurons. We find that a painful cue triggers repression of peptide chain elongation through activation of elongation factor 2 kinase (eEF2K). Attenuated elongation is sensed by a ribosome-coupled mechanism that triggers the integrated stress response (ISR). Both eEF2K and the ISR are required for pain-associated behaviors in vivo. This pathway simultaneously induces biosynthesis of brain-derived neurotrophic factor (BDNF). Selective blockade of Bdnf translation has analgesic effects in vivo. Our data suggest that precise spatiotemporal regulation of Bdnf translation is critical for appropriate behavioral responses to painful stimuli. Overall, our results demonstrate that eEF2K resides at the nexus of an intricate regulatory network that links painful cues to multiple layers of translational control.

Experimental and pharmacoinformatic approaches unveil the neuropharmacological and analgesic potential of chloroform fraction of Roktoshirinchi (Achyranthes ferruginea Roxb.)

ETHNOPHARMACOLOGICAL RELEVANCE

Achyranthes ferruginea (A. ferruginea) Roxb. is a common plant used in traditional medicine in Asia and Africa. It has a variety of local names, including "Gulmanci" in Nigeria, "Dangar" in Pakistan, "Thola" in Ethiopia, and "Roktoshirinchi" in Bangladesh. It is edible and has several ethnomedical uses for a wide range of illnesses, including hysteria, dropsy, constipation, piles, boils, asthma, and shigellosis. However, the neuropharmacological and analgesic potential of A. ferruginea remains uninvestigated.

AIM OF THE STUDY

To assess the neuropharmacological and analgesic potential of A. ferruginea through a multifaceted approach encompassing both experimental and computational models.

MATERIALS AND METHODS

Methanol was used to extract the leaves of A. ferruginea. It was then fractionated with low to high polar solvents (n-hexane, chloroform, ethyl acetate, and water) to get different fractions, including chloroform fraction (CLF). The study selected CLF at different doses and conducted advanced chemical element and proximate analyses, as well as phytochemical profiling using GC-MS. Toxicological studies were done at 300 μg per rat per day for 14 days. Cholinesterase inhibitory potential was checked using an in-vitro colorimetric assay. Acetic acid-induced writhing (AAWT) and formalin-induced licking tests (FILT) were used to assess anti-nociceptive effects. The forced swim test (FST), tail suspension test (TST), elevated plus maze (EPM), hole board test (HBT), and light and dark box test (LDB) were among the behavioral tests used to assess depression and anxiolytic activity. Network pharmacology-based analysis was performed on selected compounds using the search tool for interacting chemicals-5 (STITCH 5), Swiss target prediction tool, and search tool for the retrieval of interacting genes and proteins (STRING) database to link their role with genes involved in neurological disorders through gene ontology and reactome analysis.

RESULTS

Qualitative chemical element analysis revealed the presence of 15 elements, including Na, K, Ca, Mg, P, and Zn. The moisture content, ash value, and organic matter were found to be 11.12, 11.03, and 88.97%, respectively. GC-MS data revealed that the CLF possesses 25 phytoconstituents. Toxicological studies suggested the CLF has no effects on normal growth, hematological and biochemical parameters, or cellular organs after 14 days at 300 μg per rat. The CLF markedly reduced the activity of both acetylcholinesterase and butyrylcholinesterase (IC50: 56.22 and 13.22 μg/mL, respectively). Promising dose-dependent analgesic activity (p < 0.05) was observed in chemically-induced pain models. The TST and FST showed a dose-dependent substantial reduction in immobility time due to the CLF. Treatment with CLF notably increased the number of open arm entries and time spent in the EPM test at doses of 200 and 400 mg/kg b.w. The CLF showed significant anxiolytic activity at 200 mg/kg b.w. in the HBT test, whereas a similar activity was observed at 400 mg/kg b.w. in the EPM test. A notable increase in the amount of time spent in the light compartment was observed in the LDB test by mice treated with CLF, suggesting an anxiolytic effect. A network pharmacology study demonstrated the relationship between the phytochemicals and a number of targets, such as PPARA, PPARG, CHRM1, and HTR2, which are connected to the shown bioactivities.

CONCLUSIONS

This study demonstrated the safety of A. ferruginea and its efficacy in attenuating cholinesterase inhibitory activity, central and peripheral pain, anxiety, and depression, warranting further exploration of its therapeutic potential.

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Group II metabotropic glutamate receptor activation suppresses ATP currents in rat dorsal root ganglion neurons

P2X3 receptors and group II metabotropic glutamate receptors (mGluRs) have been found to be expressed in primary sensory neurons. P2X3 receptors participate in a variety of pain processes, while the activation of mGluRs has an analgesic effect. However, it's still unclear whether there is a link between them in pain. Herein, we reported that the group II mGluR activation inhibited the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. Group II mGluR agonist LY354740 concentration-dependently decreased P2X3 receptor-mediated and α,β-methylene-ATP (α,β-meATP)-evoked inward currents in DRG neurons. LY354740 significantly suppressed the maximum response of P2X3 receptor to α,β-meATP, but did not change their affinity. Inhibition of ATP currents by LY354740 was blocked by the group II mGluR antagonist LY341495, also prevented by the intracellular dialysis of either the G_i/o protein inhibitor pertussis toxin, the cAMP analog 8-Br-cAMP, or the protein kinase A (PKA) inhibitor H-89. Moreover, LY354740 decreased α,β-meATP-induced membrane potential depolarization and action potential bursts in DRG neurons. Finally, intraplantar injection of LY354740 also relieved α,β-meATP-induced spontaneous nociceptive behaviors and mechanical allodynia in rats by activating peripheral group Ⅱ mGluRs. These results indicated that peripheral group II mGluR activation inhibited the functional activity of P2X3 receptors via a G_i/o protein and cAMP/PKA signaling pathway in rat DRG neurons, which revealed a novel mechanism underlying analgesic effects of peripheral group II mGluRs. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

The Interactions of Magnesium Sulfate and Cromoglycate in a Rat Model of Orofacial Pain; The Role of Magnesium on Mast Cell Degranulation in Neuroinflammation

Mast cell degranulation impacts the development of pain and inflammation during tissue injury. We investigated the antinociceptive effect of a combination of cromoglycate and magnesium in the orofacial model of pain and the histological profile of the effect of magnesium in orofacial pain. In male Wistar rats, formalin (1.5%, 100 µL) was injected subcutaneously into the right upper lip of rats after cromoglycate and/or magnesium. Pain was measured as the total time spent on pain-related behavior. Toluidine blue staining was used to visualize mast cells under the light microscope. In the formalin test, in phase 1, magnesium antagonized the antinociceptive effect of cromoglycate, while in phase 2, it potentiated or inhibited its effect. Magnesium significantly reduced mast cell degranulation in the acute phase by about 23% and in the second phase by about 40%. Pearson’s coefficient did not show a significant correlation between mast cell degranulation and pain under treatment with magnesium. The cromoglycate–magnesium sulfate combination may prevent the development of inflammatory orofacial pain. The effect of a combination of cromoglycate–magnesium sulfate depends on the nature of the pain and the individual effects of the drugs. Magnesium reduced orofacial inflammation in the periphery, and this effect did not significantly contribute to its analgesic effect.

Group II metabotropic glutamate receptor activation attenuates acid-sensing ion channel currents in rat primary sensory neurons

Acid-sensing ion channels (ASICs) play an important role in pain associated with tissue acidification. Peripheral inhibitory group II metabotropic glutamate receptors (mGluRs) have analgesic effects in a variety of pain conditions. Whether there is a link between ASICs and mGluRs in pain processes is still unclear. Herein, we show that the group II mGluR agonist LY354740 inhibited acid-evoked ASIC currents and action potentials in rat dorsal root ganglia neurons. LY354740 reduced the maximum current response to protons, but it did not change the sensitivity of ASICs to protons. LY354740 inhibited ASIC currents by activating group II mGluRs. We found that the inhibitory effect of LY354740 was blocked by intracellular application of the G_i/o protein inhibitor pertussis toxin and the cAMP analogue 8-Br-cAMP and mimicked by the protein kinase A (PKA) inhibitor H-89. LY354740 also inhibited ASIC3 currents in CHO cells coexpressing mGluR2 and ASIC3 but not in cells expressing ASIC3 alone. In addition, intraplantar injection of LY354740 dose-dependently alleviated acid-induced nociceptive behavior in rats through local group II mGluRs. Together, these results suggested that activation of peripheral group II mGluRs inhibited the functional activity of ASICs through a mechanism that depended on G_i/o proteins and the intracellular cAMP/PKA signaling pathway in rat dorsal root ganglia neurons. We propose that peripheral group II mGluRs are an important therapeutic target for ASIC-mediated pain.

A-kinase anchoring protein 79/150 coordinates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor sensitization in sensory neurons

Changes in sensory afferent activity contribute to the transition from acute to chronic pain. However, it is unlikely that a single sensory receptor is entirely responsible for persistent pain. It is more probable that extended changes to multiple receptor proteins expressed by afferent neurons support persistent pain. A-Kinase Anchoring Protein 79/150 (AKAP) is an intracellular scaffolding protein expressed in sensory neurons that spatially and temporally coordinates signaling events. Since AKAP scaffolds biochemical modifications of multiple TRP receptors linked to pain phenotypes, we probed for other ionotropic receptors that may be mediated by AKAP and contribute to persistent pain. Here, we identify a role for AKAP modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor (AMPA-R) functionality in sensory neurons. Pharmacological manipulation of distinct AMPA-R subunits significantly reduces persistent mechanical hypersensitivity observed during hyperalgesic priming. Stimulation of both protein kinases C and A (PKC, PKA, respectively) modulate AMPA-R subunit GluR1 and GluR2 phosphorylation and surface expression in an AKAP-dependent manner in primary cultures of DRG neurons. Furthermore, AKAP knock out reduces sensitized AMPA-R responsivity in DRG neurons. Collectively, these data indicate that AKAP scaffolds AMPA-R subunit organization in DRG neurons that may contribute to the transition from acute-to-chronic pain.

Central and peripheral pain intervention by Ophiorrhizarugosa leaves: Potential underlying mechanisms and insight into the role of pain modulators

ETHNOPHARMACOLOGICAL RELEVANCE

Ophiorrhiza rugosa var. prostrata is a traditional medicinal plant used by the indigenous and local tribes (Chakma, Marma and Tanchangya) of Bangladesh for the management of chest pain, body ache, and earache. However, the knowledge of anti-nociceptive and anti-inflammatory potentials of this plant is scarce.

AIM OF THE STUDY

Therefore, we scrutinized the anti-nociceptive and anti-inflammatory properties of O. rugosa leaves along with its possible mechanism(s) of action using chemical and heat-induced pain models.

METHODS AND MATERIALS

O. rugosa was extracted using 100% ethanol (EEOR) followed by exploring phytochemicals and assessing acute toxicity. To determine anti-nociceptive potentials, chemical-induced (acetic acid and formalin) and heat-induced (hot plate and tail immersion) nociceptive models were followed. To investigate the possible involvement of opioid receptors during formalin, hot plate, and tail immersion tests, naltrexone was administered whereas methylene blue and glibenclamide were used to explore cGMP involvement and ATP-sensitive K⁺ channel pathways, respectively. Moreover, the anti-inflammatory potential was assessed using the carrageenan-induced paw edema test model. Motor behaviours of EEOR were assessed by the open-field test. Finally, bioactive constituents (identified by GC-MS) from O. rugosa were subjected to molecular docking and ADME/t analysis to evaluate its potency and safety.

RESULTS

During chemical-induced and heat-induced pain models, EEOR exhibited significant and effective nociception suppression at all experimental doses (200 and 400 mg/kg). Also, the administration of naltrexone corroborated the association of opioid receptors with the anti-nociceptive activity by EEOR. Similarly, cGMP and ATP-sensitive K⁺ channel pathways were also found to be involved in the anti-nociceptive mechanism. Furthermore, significant and dose-dependent inhibition of inflammation induced by carrageenan was recorded for EEOR. Both doses of EEOR did not affect the animal's locomotor capacity in the open-field test. Besides, in silico test identified the key compounds (loliolide, harman, squalene, vitamin E, and gamma-sitosterol) that inhibited some particular receptors regarding pain and inflammation.

CONCLUSION

This research exposes central and peripheral pain intervention as well as anti-inflammatory activity of O. rugosa. Also, the identified compounds from this plant support its activities by effectively inhibiting anti-nociceptive and anti-inflammatory receptors. Overall, these outcomes valorize the ethnomedicinal efficacy of O. rugosa in managing various painful conditions.

Peripheral glutamate receptor and transient receptor potential channel mechanisms of craniofacial muscle pain

Temporomandibular joint disorder is a common chronic craniofacial pain condition, often involving persistent, widespread craniofacial muscle pain. Although the etiology of chronic muscle pain is not well known, sufficient clinical and preclinical information supports a contribution of trigeminal nociceptors to craniofacial muscle pain processing under various experimental and pathological conditions. Here, we review cellular and molecular mechanisms underlying sensitization of muscle nociceptive afferents. In particular, we summarize findings on pronociceptive roles of peripheral glutamate in humans, and we discuss mechanistic contributions of glutamate receptors, including N-methyl-D-aspartate receptors and metabotropic glutamate receptors, which have considerably increased our understanding of peripheral mechanisms of craniofacial muscle pain. Several members of the transient receptor potential (TRP) family, such as transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin 1, also play essential roles in the development of spontaneous pain and mechanical hypersensitivity in craniofacial muscles. Furthermore, glutamate receptors and TRP channels functionally and bi-directionally interact to modulate trigeminal nociceptors. Activation of glutamate receptors invokes protein kinase C, which leads to the phosphorylation of TRPV1. Sensitization of TRPV1 by inflammatory mediators and glutamate receptors in combination with endogenous ligands contributes to masseter hyperalgesia. The distinct intracellular signaling pathways through which both receptor systems engage and specific molecular regions of TRPV1 are offered as novel targets for the development of mechanism-based treatment strategies for myogenous craniofacial pain conditions.

Investigation of anti-inflammatory potential of 5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-thioxodihydropyrimidine-4,6(1H,5H)-dione compound

The aim of this study was to synthesise the novel di-tert-butylphenol compound, 5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2-thioxo-dihydropyrimidine-4,6(1H, 5H)-dione (LQFM218), and evaluate the potential anti-nociceptive and anti-inflammatory activities in acute (mice) models in vivo. The compound was tested on acute models of pain such as acetic acid-induced abdominal writhing, formalin-induced nociception and carrageenan-induced mechanical hyperalgesia. The anti-inflammatory activity was observed in paw oedema, carrageenan-induced pleurisy tests and inflammatory mediator quantification. Key findings: oral treatment with the LQFM218 (50, 100 or 200 mg/kg) reduced abdominal writhing (18.8%, 31.6% and 48.3%). The dose intermediate (100 mg/kg) reduced the nociception in the second phase of the formalin test (61.4%), and also showed anti-hyperalgic activity in carrageenan-induced mechanical hyperalgesia (until 42.3%). In acute inflammation models, the treatment of mice LQFM218 (100 mg/kg) reduced the paw oedema all the time (33.8%, 42.6%, 37.4% and 36%) and in pleurisy test reduced: polymorphonuclear cell migration (35.4%), myeloperoxidase activity (52.2%) and the levels of inflammatory mediators such as PGE₂ (23.0%), TNF-α (67.6%) and IL-1β (53.4%). The present study showed that LQFM218 effectively reduced the nociception and inflammation in different models, and its mechanism might be related to the reduction of PGE₂ and pro-inflammatory cytokines. These findings show LQFM218 as a potential anti-inflammatory drug.

Metabolism and Functions of Amino Acids in the Skin

Amino acids are the building blocks of all proteins, including the most abundant fibrous proteins in the skin, as keratins, collagen and elastin. Sagging and wrinkled skin are features of chronic sun-damaged and aged uncared skin, and they are mainly associated with the deterioration of collagen and elastic fibers. The maintenance of skin structures by self-repair processes is essential to skin health. Thus, amino acids significantly impact the appearance of the skin. Amino acids are important nutrients required for (a) wound healing promotion and repair of the damaged skin; (b) acid-base balance and water retention in cellular layers, such as stratum corneum; (c) protection against sunlight damage; (d) maintenance of an appropriate skin microbiome. This review highlights the contribution of all proteinogenic amino acids and some related metabolites to the skin structures as constituents of the main cutaneous proteins or as signaling molecules for the regulation and determination of skin physiology.

Sensitization of glutamate receptor-mediated pain behaviour via nerve growth factor-dependent phosphorylation of transient receptor potential V1 under inflammatory conditions

BACKGROUND AND PURPOSE

Glutamate and metabotropic glutamate (mGlu) receptors on primary sensory neurons are crucial in modulating pain sensitivity. However, it is unclear how inflammation affects mGlu receptor-mediated nociceptive responses. We therefore investigated the effects of mGlu_1/5 receptor agonists on pain-related behaviour during persistent inflammation and their underlying mechanisms.

EXPERIMENTAL APPROACH

Effects of a mGlu_1/5 receptor agonist on pain-related behaviour during inflammation was assessed in mice. Intracellular calcium responses, membrane current responses, and protein expression in primary sensory neurons were examined using cultured dorsal root ganglion (DRG) neurons, dissociated from wild-type and gene knockout mice.

KEY RESULTS

Persistent inflammation induced by complete Freund's adjuvant increased the duration of mGlu_1/5 receptor-mediated pain behaviour, which was antagonized by inhibition of nerve growth factor (NGF)-tropomyosin receptor kinase A (TrkA) signalling. Calcium imaging revealed that NGF treatment increased the number of cultured DRG neurons responding to mGlu_1/5 receptor activation. Stimulation of mGlu_1/5 receptors in NGF-treated DRG neurons induced inward currents through TRPV1 channels in association with PLC but not with IP₃ receptors. NGF treatment also increased the number of neurons responding to a DAG analogue via TRPV1 channel activation. Furthermore, NGF up-regulated expression of TRPV1 and A-kinase anchoring protein 5 (AKAP5), resulting in increased AKAP5-dependent TRPV1 phosphorylation. AKAP5 knockout mice did not exhibit mGlu_1/5 receptor-mediated excitation in NGF-treated DRG neurons or pain response facilitation under inflammatory conditions.

CONCLUSIONS AND IMPLICATIONS

NGF augments glutamate- and mGlu_1/5 receptor-mediated excitation of nociceptive neurons by AKAP5-dependent phosphorylation of TRPV1 channels, potentiating hypersensitivity to glutamate in inflamed tissues.

Glutamate receptors and glutamatergic signalling in the peripheral nerves

In the peripheral nervous system, the vast majority of axons are accommodated within the fibre bundles that constitute the peripheral nerves. Axons within the nerves are in close contact with myelinating glia, the Schwann cells that are ideally placed to respond to, and possibly shape, axonal activity. The mechanisms of intercellular communication in the peripheral nerves may involve direct contact between the cells, as well as signalling via diffusible substances. Neurotransmitter glutamate has been proposed as a candidate extracellular molecule mediating the cross-talk between cells in the peripheral nerves. Two types of experimental findings support this idea: first, glutamate has been detected in the nerves and can be released upon electrical or chemical stimulation of the nerves; second, axons and Schwann cells in the peripheral nerves express glutamate receptors. Yet, the studies providing direct experimental evidence that intercellular glutamatergic signalling takes place in the peripheral nerves during physiological or pathological conditions are largely missing. Remarkably, in the central nervous system, axons and myelinating glia are involved in glutamatergic signalling. This signalling occurs via different mechanisms, the most intriguing of which is fast synaptic communication between axons and oligodendrocyte precursor cells. Glutamate receptors and/or synaptic axon-glia signalling are involved in regulation of proliferation, migration, and differentiation of oligodendrocyte precursor cells, survival of oligodendrocytes, and re-myelination of axons after damage. Does synaptic signalling exist between axons and Schwann cells in the peripheral nerves? What is the functional role of glutamate receptors in the peripheral nerves? Is activation of glutamate receptors in the nerves beneficial or harmful during diseases? In this review, we summarise the limited information regarding glutamate release and glutamate receptors in the peripheral nerves and speculate about possible mechanisms of glutamatergic signalling in the nerves. We highlight the necessity of further research on this topic because it should help to understand the mechanisms of peripheral nervous system development and nerve regeneration during diseases.

Anti-inflammatory and antinociceptive effects of active ingredients in the essential oils from Gynura procumbens, a traditional medicine and a new and popular food material

ETHNOPHARMACOLOGICAL RELEVANCE

Gynura procumbens is applied topically for the treatment of traumatic injuries in South China.

MATERIALS AND METHODS

This study was conducted to identify the active ingredients in the essential oils from Gynura procumbens (GPEO) by Gas Chromatography-Mass Spectrometer (GC-MS) and to elucidate the mechanisms underlying the anti-inflammatory and antinociceptive effects in vivo and in vitro.

RESULTS

A reduction in dswelling and pain were observed in mice treated with GPEO or its active ingredients (α-pinene, 3-carene, and limonene) compared with mice treated with a solvent control. GPEO or its three active ingredients had potent pharmacological effects on COX-2 overexpression and LPS-induced migration of Raw264.7 macrophages. All three ingredients inhibited nociceptive stimulus-induced inflammatory infiltrates and COX-2 overexpression, which could be responsible for the anti-inflammatory effect of GPEO. However, only 3-carene produced an antinociceptive effect.

CONCLUSION

Consistent with the traditional usage in Southern China, GPEO may serve as a promising potent external therapeutic agent for the treatment of chronic pain.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Vesicular glutamate transporter isoforms: The essential players in the somatosensory systems

In nervous system, glutamate transmission is crucial for centripetal conveyance and cortical perception of sensory signals of different modalities, which necessitates vesicular glutamate transporters 1-3 (VGLUT 1-3), the three homologous membrane-bound protein isoforms, to load glutamate into the presysnaptic vesicles. These VGLUTs, especially VGLUT1 and VGLUT2, selectively label and define functionally distinct neuronal subpopulations at each relay level of the neural hierarchies comprising spinal and trigeminal sensory systems. In this review, by scrutinizing each structure of the organism's fundamental hierarchies including dorsal root/trigeminal ganglia, spinal dorsal horn/trigeminal sensory nuclear complex, somatosensory thalamic nuclei and primary somatosensory cortex, we summarize and characterize in detail within each relay the neuronal clusters expressing distinct VGLUT protein/transcript isoforms, with respect to their regional distribution features (complementary distribution in some structures), axonal terminations/peripheral innervations and physiological functions. Equally important, the distribution pattern and characteristics of VGLUT1/VGLUT2 axon terminals within these structures are also epitomized. Finally, the correlation of a particular VGLUT isoform and its physiological role, disclosed thus far largely via studying the peripheral receptors, is generalized by referring to reports on global and conditioned VGLUT-knockout mice. Also, researches on VGLUTs relating to future direction are tentatively proposed, such as unveiling the elusive differences between distinct VGLUTs in mechanism and/or pharmacokinetics at ionic/molecular level, and developing VGLUT-based pain killers.

Antinociceptive Effects of Cardamonin in Mice: Possible Involvement of TRPV₁, Glutamate, and Opioid Receptors

Pain is one of the most common cause for hospital visits. It plays an important role in inflammation and serves as a warning sign to avoid further injury. Analgesics are used to manage pain and provide comfort to patients. However, prolonged usage of pain treatments like opioids and NSAIDs are accompanied with undesirable side effects. Therefore, research to identify novel compounds that produce analgesia with lesser side effects are necessary. The present study investigated the antinociceptive potentials of a natural compound, cardamonin, isolated from Boesenbergia rotunda (L) Mansf. using chemical and thermal models of nociception. Our findings showed that intraperitoneal and oral administration of cardamonin (0.3, 1, 3, and 10 mg/kg) produced significant and dose-dependent inhibition of pain in abdominal writhing responses induced by acetic acid. The present study also demonstrated that cardamonin produced significant analgesia in formalin-, capsaicin-, and glutamate-induced paw licking tests. In the thermal-induced nociception model, cardamonin exhibited significant increase in response latency time of animals subjected to hot-plate thermal stimuli. The rota-rod assessment confirmed that the antinociceptive activities elicited by cardamonin was not related to muscle relaxant or sedative effects of the compound. In conclusion, the present findings showed that cardamonin exerted significant peripheral and central antinociception through chemical- and thermal-induced nociception in mice through the involvement of TRPV₁, glutamate, and opioid receptors.

New βN-octadecanoyl-5-hydroxytryptamide: antinociceptive effect and possible mechanism of action in mice

The present study examined the potential antinociceptive activity of C18 5-HT (^βN-octadecanoyl-5-hydroxytryptamide) using chemical and thermal nociception models in mice. Orally administered C18 5-HT (0.1, 1 and 10 mg/kg) produced significant dose-dependent antinociceptive effects in formalin-, capsaicin- and glutamate-induced licking models. This compound also induced a significant increase in the response to thermal stimuli in the hot plate test, and its antinociceptive effect was not related to muscle relaxant or sedative actions. In a thermal hyperalgesia model, C18 5-HT presented an anti-hyperalgesic profile as evidenced by the increase in the response time of the animals. Furthermore, intraperitoneal (i.p) pretreatment with naloxone (a non-selective opioid receptor antagonist, 1 mg/kg), ondansetron (serotoninergic receptor antagonist (5-HT3 subtype), 0.5 mg/kg) or AM241 (CB1 cannabinoid receptor antagonist, 1 mg/kg) reversed the antinociceptive effects of C18 5-HT in the hot plate model. In the formalin-induced licking model, pretreatment with naloxone reversed the antinociceptive effects of C18 5-HT, as demonstrated by an increase in the paw licking response when compared with the C18 5-HT-treated group. These findings suggest that C18 5-HT has peripheral and central antinociceptive effects and that its mechanism of action involves, ate least in part, opioid, serotoninergic and cannabinoid pathways.

Chemical characterization and pharmacological assessment of polysaccharide free, standardized cashew gum extract (Anacardium occidentale L.)

ETHNOPHARMACOLOGICAL RELEVANCE

The cashew gum (Anacardium occidentale L.) is used in traditional Brazilian medicine in the treatment of inflammatory conditions, asthma, diabetes, and gastrointestinal disturbances.

AIM OF THE STUDY

In the present study, we aimed at forming a chemical characterization and investigation of the antinociceptive and anti-inflammatory activities of the aqueous extract of cashew gum without the presence of polysaccharides in its composition (CGE).

MATERIALS AND METHODS

The CGE was obtained after the precipitation and removal of polysaccharides through the use of acetone. After, the acetone was removed by rotaevaporation, and the concentrated extract was lyophilized. The chemical characterization of CGE was performed by liquid chromatography mass spectrometry (LC-MS) and tandem mass spectrometry (MS/MS) analyses. Mice were used for the evaluation of the antinociceptive and anti-inflammatory activities. CGE was analyzed via the Irwin test, acetic acid-induced writhing test, formalin-induced pain test, and carrageenan-induced paw edema test. The motor activity or probable sedation was verified through the chimney, open-field, and sodium pentobarbital-induced sleep tests. We investigated if the analgesic and anti-inflammatory effects of CGE depend of reduction in PGE₂ levels, were performed the carrageenan or PGE₂-induced hyperalgesia tests.

RESULTS

The chemical characterization of CGE showed the presence of anacardic acids as the predominant phytoconstituents. The treatment with CGE (75, 150, and 300mg/kg, p.o.) inhibited the number of writhing in a dose-dependent manner. With an intermediate dose, CGE did not cause motor impairment with the chimney test or alterations in either the open-field or sodium pentobarbital-induced sleep. In the formalin-induced pain test, CGE (150mg/kg, p.o.) produced an antinociceptive effect only in the first phase of the test, suggesting anti-inflammatory activity. With the same dosage, CGE also reduced the carrageenan-induced paw edema at all hours of the test, confirming its anti-inflammatory effect. Furthermore, CGE (150mg/kg, p.o.) presented an antihyperalgic effect at all hours of the carrageenan-induced hyperalgesia test. However, this dose of CGE was not able to reduce the hyperalgesia induced by PGE₂, suggesting that the anti-inflammatory effect of this extract depends on the reduction in the PGE₂ levels.

CONCLUSION

The anacardic acids are the predominant phytoconstituents identified in the CGE. The action mechanisms of CGE suggest the reduction in the PGE₂ levels. These findings support the use of cashew gum in popular medicine and demonstrate that part of its antinociceptive and anti-inflammatory effects should also be attributed to the presence of anacardic acids in its composition, independent of the presence of polysaccharides.

Ethnopharmacological Evaluation of Breu Essential Oils from Protium Species Administered by Inhalation

Background

Breu is an aromatic oleoresin which has been used by Amazonian traditional communities as a remedy for headaches and migraines by burning and inhaling the smoke produced during its combustion. This study evaluated the antinociceptive and sedative activities of formulations containing breu essential oils administered by inhalation.

Methods

Five different formulations (A–E) containing breu essential oils were evaluated for their sedative and antinociceptive activities in mice. They were delivered for 20 minutes using an inhalation chamber coupled with a nebulizer and the air inside was collected by static headspace and analyzed by GC-FID.

Results

All nebulized formulations had similar chemical compositions and major compounds as the original essential oils. None of them resulted in significant increase in response time during the hot plate test. In the formalin test, Formulation E showed a significant inhibition of licking responses in the early (46.8%) and late (60.2%) phases. Formulation B was effective (36.9%) in the first phase and Formulation D (37.9%) in the second. None of the formulations presented sedative effects.

Conclusion

Breu essential oils, when inhaled, may present antinociceptive and anti-inflammatory properties without sedation. Additionally, nebulization proved to be an efficient method for administration of formulations containing these essential oils.

Peripheral NMDA Receptor/NO System Blockage Inhibits Itch Responses Induced by Chloroquine in Mice

Intradermal administration of chloroquine (CQ) provokes scratching behavior in mice. Chloroquine-induced itch is histamine-independent and we have reported that the nitric oxide (NO)/cyclic guanosine monophosphate (cGMP) pathway is involved in CQ-induced scratching behavior in mice. Previous studies have demonstrated that activation of N-methyl-d-aspartate receptors (NMDARs) induces NO production. Here we show that NMDAR antagonists significantly decrease CQ-induced scratching in mice while a non-effective dose of an NMDAR agonist potentiates the scratching behavior provoked by sub-effective doses of CQ. In contrast, combined pre-treatment with sub-effective doses of an NMDAR antagonist, MK-801, and the NO synthase inhibitor, L-N-nitro arginine methyl ester (L-NAME), decreases CQ-induced scrat-ching behavior. While intradermal administration of CQ significantly increases the concentration of intradermal nitrite, the end product of NO metabolism, effective doses of intraperitoneal and intradermal MK-801 significantly decrease intradermal nitrite levels. Likewise, administration of an effective dose of L-NAME significantly decreases CQ-induced nitrite production. We conclude that the NMDA/NO pathway in the skin modulates CQ-induced scratching behavior.

Characterization of anti-inflammatory effect and possible mechanism of action of Tibouchina granulosa

OBJECTIVES

Tibouchina granulosa, popularly known as 'quaresmeira', belong to a genus widely used in the traditional medicine as infusions from their leaves. Other species of Tibouchina are used as antibacterial, antioxidant or antileishmanial. In this work, our objectives were to investigate the biological effects of T. granulosa in models of acute inflammation.

METHODS

Chemical analysis showed the presence of proanthocyanidins and flavonoids. Infusions from leaves of T. granulosa (1, 3, 10, 30 or 100 mg/kg) were orally administered to mice, and the anti-inflammatory effects were evaluated by the formalin-induced licking response, inhibition of carrageenan-induced cell migration into subcutaneous air pouch (SAP) and inhibition of inflammatory mediator production in inflammatory exudate collected from SAP.

KEY FINDINGS

Our data indicate that tested doses of T. granulosa infusion reduced cell migration, protein extravasated to SAP and cytokine production (i.e. TNF-α and IL-10). All doses also inhibited the first and second phase of formalin-induced licking response.

CONCLUSIONS

Taken together, our results indicate that leaves of T. granulosa present anti-inflammatory effect and can be useful in the preparation of new phytomedicines.

Rat odontoblasts may use glutamate to signal dentin injury

Accumulating evidence indicates that odontoblasts act as sensor cells, capable of triggering action potentials in adjacent pulpal nociceptive axons, suggesting a paracrine signaling via a currently unknown mediator. Since glutamate can mediate signaling by non-neuronal cells, and peripheral axons may express glutamate receptors (GluR), we hypothesized that the expression of high levels of glutamate, and of sensory receptors in odontoblasts, combined with an expression of GluR in adjacent pulpal axons, is the morphological basis for odontoblastic sensory signaling. To test this hypothesis, we investigated the expression of glutamate, the thermo- and mechanosensitive ion channels transient receptor potential vanilloid 1 (TRPV1), transient receptor potential ankyrin 1 (TRPA1), and TWIK-1-related K+channel (TREK-1), and the glutamate receptor mGluR5, in a normal rat dental pulp, and following dentin injury. We also examined the glutamate release from odontoblast in cell culture. Odontoblasts were enriched with glutamate, at the level as high as in adjacent pulpal axons, and showed immunoreactivity for TRPV1, TRPA1, and TREK-1. Pulpal sensory axons adjacent to odontoblasts expressed mGluR5. Both the levels of glutamate in odontoblasts, and the expression of mGluR5 in nearby axons, were upregulated following dentin injury. The extracellular glutamate concentration was increased significantly after treating of odontoblast cell line with calcium permeable ionophore, suggesting glutamate release from odontoblasts. These findings lend morphological support to the hypothesis that odontoblasts contain glutamate as a potential neuroactive substance that may activate adjacent pulpal axons, and thus contribute to dental pain and hypersensitivity.

Group III mGluR8 negatively modulates TRPA1

Several lines of evidence indicate group III metabotropic glutamate receptors (mGluRs) have systemic anti-hyperalgesic effects. We hypothesized this could occur through modulation of TRP channels on nociceptors. This study used a multifaceted approach to examine the interaction between group III mGluRs (mGluR8) and transient receptor potential ankyrin 1 (TRPA1) on cutaneous nociceptors in rats. Ca²⁺ imaging studies demonstrated co-localization and functional coupling of TRPA1 and mGluR8, since 1μM (S)-3,4-dicarboxyphenylglycine (DCPG) (mGluR8 agonist) significantly reduced Ca²⁺ mobilization produced by 30μM mustard oil (MO), a TRPA1 agonist. Behavioral studies demonstrated that 10mM MO produced mechanical hypersensitivity when topically applied to the hind paw, significantly decreasing paw withdrawal threshold (PWT) from 15g to 6g. However, administration of 30μM DCPG prior to 10mM MO reversed this hypersensitivity such that PWT was not significantly different from baseline. At the single-fiber level, compared to vehicle, 30μM MO significantly increased nociceptor activity and decreased mechanical threshold. However, 30μM DCPG reversed both of these MO-induced effects. Furthermore, DCPG significantly reduced the number of MO-induced mechanically sensitive fibers. Inhibition of protein kinase A (PKA) using Rp-cyclic 3',5'-hydrogen phosphorothioate adenosine triethylammonium salt (RpCAMPS) (PKA inhibitor, 1 and 10μM) significantly reduced MO-induced Ca²⁺ mobilization. Taken together, these results show that group III mGluRs negatively modulate TRPA1 activity on cutaneous nociceptors. Furthermore, it is likely that this modulation occurs intracellularly at the level of the cAMP/PKA pathway. This study demonstrates that group III agonists may be effective in the treatment of mechanical hypersensitivity which can develop as a result of inflammation, nerve injury, chemotherapy and other disease states.

A-kinase anchoring protein 79/150 coordinates metabotropic glutamate receptor sensitization of peripheral sensory neurons

Glutamate serves as the primary excitatory neurotransmitter in the nervous system. Previous studies have identified a role for glutamate and group I metabotropic receptors as targets for study in peripheral inflammatory pain. However, the coordination of signaling events that transpire from receptor activation to afferent neuronal sensitization has not been explored. Herein, we identify that scaffolding protein A-kinase anchoring protein 79/150 (AKAP150) coordinates increased peripheral thermal sensitivity after group I metabotropic receptor (mGluR5) activation. In both acute and persistent models of thermal somatosensory behavior, we report that mGluR5 sensitization requires AKAP150 expression. Furthermore, electrophysiological approaches designed to record afferent neuronal activity reveal that mGluR5 sensitization also requires functional AKAP150 expression. In dissociated primary afferent neurons, mGluR5 activation increases TRPV1 responses in an AKAP-dependent manner through a mechanism that induces AKAP association with TRPV1. Experimental results presented herein identify a mechanism of receptor-driven scaffolding association with ion channel targets. Importantly, this mechanism could prove significant in the search for therapeutic targets that repress episodes of acute pain from becoming chronic in nature.

Long-Term Activation of Group I Metabotropic Glutamate Receptors Increases Functional TRPV1-Expressing Neurons in Mouse Dorsal Root Ganglia

Damaged tissues release glutamate and other chemical mediators for several hours. These chemical mediators contribute to modulation of pruritus and pain. Herein, we investigated the effects of long-term activation of excitatory glutamate receptors on functional expression of transient receptor potential vaniloid type 1 (TRPV1) in dorsal root ganglion (DRG) neurons and then on thermal pain behavior. In order to detect the TRPV1-mediated responses in cultured DRG neurons, we monitored intracellular calcium responses to capsaicin, a TRPV1 agonist, with Fura-2. Long-term (4 h) treatment with glutamate receptor agonists (glutamate, quisqualate or DHPG) increased the proportion of neurons responding to capsaicin through activation of metabotropic glutamate receptor mGluR1, and only partially through the activation of mGluR5; engagement of these receptors was evident in neurons responding to allylisothiocyanate (AITC), a transient receptor potential ankyrin type 1 (TRPA1) agonist. Increase in the proportion was suppressed by phospholipase C (PLC), protein kinase C, mitogen/extracellular signal-regulated kinase, p38 mitogen-activated protein kinase or transcription inhibitors. Whole-cell recording was performed to record TRPV1-mediated membrane current; TRPV1 current density significantly increased in the AITC-sensitive neurons after the quisqualate treatment. To elucidate the physiological significance of this phenomenon, a hot plate test was performed. Intraplantar injection of quisqualate or DHPG induced heat hyperalgesia that lasted for 4 h post injection. This chronic hyperalgesia was attenuated by treatment with either mGluR1 or mGluR5 antagonists. These results suggest that long-term activation of mGluR1/5 by peripherally released glutamate may increase the number of neurons expressing functional TRPV1 in DRG, which may be strongly associated with chronic hyperalgesia.

Potentiation of acid-sensing ion channel activity by peripheral group I metabotropic glutamate receptor signaling

Glutamate activates peripheral group I metabotropic glutamate receptors (mGluRs) and contributes to inflammatory pain. However, it is still not clear the mechanisms are involved in group I mGluR-mediated peripheral sensitization. Herein, we report that group I mGluRs signaling sensitizes acid-sensing ion channels (ASICs) in dorsal root ganglion (DRG) neurons and contributes to acidosis-evoked pain. DHPG, a selective group I mGluR agonist, can potentiate the functional activity of ASICs, which mediated the proton-induced events. DHPG concentration-dependently increased proton-gated currents in DRG neurons. It shifted the proton concentration-response curve upwards, with a 47.3±7.0% increase of the maximal current response to proton. Group I mGluRs, especially mGluR5, mediated the potentiation of DHPG via an intracellular cascade. DHPG potentiation of proton-gated currents disappeared after inhibition of intracellular Gq/11 proteins, PLCβ, PKC or PICK1 signaling. Moreover, DHPG enhanced proton-evoked membrane excitability of rat DRG neurons and increased the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, peripherally administration of DHPG dose-dependently exacerbated nociceptive responses to intraplantar injection of acetic acid in rats. Potentiation of ASIC activity by group I mGluR signaling in rat DRG neurons revealed a novel peripheral mechanism underlying group I mGluRs involvement in hyperalgesia.

Glutaminase Increases in Rat Dorsal Root Ganglion Neurons after Unilateral Adjuvant-Induced Hind Paw Inflammation

Glutamate is a neurotransmitter used at both the peripheral and central terminals of nociceptive primary sensory neurons, yet little is known concerning regulation of glutamate metabolism during peripheral inflammation. Glutaminase (GLS) is an enzyme of the glutamate-glutamine cycle that converts glutamine into glutamate for neurotransmission and is implicated in producing elevated levels of glutamate in central and peripheral terminals. A potential mechanism for increased levels of glutamate is an elevation in GLS expression. We assessed GLS expression after unilateral hind paw inflammation by measuring GLS immunoreactivity (ir) with quantitative image analysis of L4 dorsal root ganglion (DRG) neurons after one, two, four, and eight days of adjuvant-induced arthritis (AIA) compared to saline injected controls. No significant elevation in GLS-ir occurred in the DRG ipsilateral to the inflamed hind paw after one or two days of AIA. After four days AIA, GLS-ir was elevated significantly in all sizes of DRG neurons. After eight days AIA, GLS-ir remained elevated in small (<400 µm²), presumably nociceptive neurons. Western blot analysis of the L4 DRG at day four AIA confirmed the elevated GLS-ir. The present study indicates that GLS expression is increased in the chronic stage of inflammation and may be a target for chronic pain therapy.

The role of TRPA1 in muscle pain and mechanical hypersensitivity under inflammatory conditions in rats

Transient receptor potential cation channel, subfamily A, member 1 (TRPA1) is expressed in muscle afferents and direct activation of these receptors induces acute mechanical hypersensitivity. However, the functional role of TRPA1 under pathological muscle pain conditions and mechanisms by which TRPA1 mediate muscle pain and hyperalgesia are not clearly understood. Two rodent behavioral models validated to assess craniofacial muscle pain conditions were used to study ATP- and N-Methyl-D-aspartate (NMDA)-induced acute mechanical hypersensitivity and complete Freund's adjuvant (CFA)-induced persistent mechanical hypersensitivity. The rat grimace scale (RGS) was utilized to assess inflammation-induced spontaneous muscle pain. Behavioral pharmacology experiments were performed to assess the effects of AP18, a selective TRPA1 antagonist under these conditions. TRPA1 expression levels in trigeminal ganglia (TG) were examined before and after CFA treatment in the rat masseter muscle. Pre-treatment of the muscle with AP18 dose-dependently blocked the development of acute mechanical hypersensitivity induced by NMDA and α,β-methylene adenosine triphosphate (αβmeATP), a specific agonist for NMDA and P2X3 receptor, respectively. CFA-induced mechanical hypersensitivity and spontaneous muscle pain responses were significantly reversed by post-treatment of the muscle with AP18 when CFA effects were most prominent. CFA-induced myositis was accompanied by significant up-regulation of TRPA1 expression in TG. Our findings showed that TRPA1 in muscle afferents plays an important role in the development of acute mechanical hypersensitivity and in the maintenance of persistent muscle pain and hypersensitivity. Our data suggested that TRPA1 may serve as a downstream target of pro-nociceptive ion channels, such as P2X3 and NMDA receptors in masseter afferents, and that increased TRPA1 expression under inflammatory conditions may contribute to the maintenance of persistent muscle pain and mechanical hyperalgesia. Mechanistic studies elucidating transcriptional or post-translational regulation of TRPA1 expression under pathological pain conditions should provide important basic information to further advance the treatment of craniofacial muscle pain conditions.

Expression of vesicular glutamate transporters in transient receptor potential melastatin 8 (TRPM8)-positive dental afferents in the mouse

Transient receptor potential melastatin 8 (TRPM8) is activated by innocuous cool and noxious cold and plays a crucial role in cold-induced acute pain and pain hypersensitivity. To help understand the mechanism of TRPM8-mediated cold perception under normal and pathologic conditions, we used light microscopic immunohistochemistry and Western blot analysis in mice expressing a genetically encoded axonal tracer in TRPM8-positive (+) neurons. We investigated the coexpression of TRPM8 and vesicular glutamate transporter 1 (VGLUT1) and VGLUT2 in the trigeminal ganglion (TG) and the dental pulp before and after inducing pulpal inflammation. Many TRPM8+ neurons in the TG and axons in the dental pulp expressed VGLUT2, while none expressed VGLUT1. TRPM8+ axons were dense in the pulp horn and peripheral pulp and also frequently observed in the dentinal tubules. Following pulpal inflammation, the proportion of VGLUT2+ and of VGLUT2+/TRPM8+ neurons increased significantly, whereas that of TRPM8+ neurons remained unchanged. Our findings suggest the existence of VGLUT2 (but not VGLUT1)-mediated glutamate signaling in TRPM8+ neurons possibly underlying the cold-induced acute pain and hypersensitivity to cold following pulpal inflammation.

Anti-inflammatory activity of Choisya ternata Kunth essential oil, ternanthranin, and its two synthetic analogs (methyl and propyl N-methylanthranilates)

Choisya ternata Kunth (Rutaceae) is native to North America where it is popularly known as "Mexican orange". In this study, the anti-inflammatory effects of the essential oil (EO) obtained from the leaves of C. ternata, one of its minor components (ternanthranin-ISOAN) and its two synthetic analogues (methyl and propyl N-methylanthranilate--MAN and PAN) were evaluated. Mice pretreated with the EO (EO) obtained from C. ternata leaves (3-100 mg/kg, p.o.), ISOAN, MAN or PAN (1-30 mg/kg, p.o.) and the reference drugs, morphine (1 mg/kg, p.o.) and acetylsalicylic acid (ASA, 100 mg/kg, p.o.), were evaluated in inflammation models such as formalin and subcutaneous air pouch models, with measurement of cell migration, exudate volume, protein extravasation, nitric oxide and pro-inflammatory cytokines. The EO from C. ternata significantly inhibited the time that the animals spent licking the formalin-injected paw in the second phase of the model at their higher doses (30 and 100 mg/kg, respectively). An inhibition of the inflammatory reaction induced after subcutaneous carrageenan injection into air pouch was also observed. In this model, the EO significantly reduced cell migration, exudate volume, protein extravased, and the increase in levels of inflammatory mediators (nitric oxide, TNF-α and IL-1β). ISOAN, MAN and PAN behaved in the same fashion at much smaller doses. Also, these molecules were able to show significant effects in the reduction of paw edema (at all tested doses) when the phlogistic agent was carrageenan, bradykinin, 5-HT, PGE2, C48/80 or 12-O-tetradecanoylphorbol-acetate (TPA). None of the tested doses had any effect in reducing histamine-induced edema. Our results indicate that the EO from C. ternata and anthranilate derivatives demonstrates an anti-inflammatory effect.

Inhibitors of glutamate release from breast cancer cells; new targets for cancer-induced bone-pain

Glutamate is an important signaling molecule in a wide variety of tissues. Aberrant glutamatergic signaling disrupts normal tissue homeostasis and induces several disruptive pathological conditions including pain. Breast cancer cells secrete high levels of glutamate and often metastasize to bone. Exogenous glutamate can disrupt normal bone turnover and may be responsible for cancer-induced bone pain (CIBP). CIBP is a significant co-morbidity that affects quality of life for many advanced-stage breast cancer patients. Current treatment options are commonly accompanied by serious side-effects that negatively impact patient care. Identifying small molecule inhibitors of glutamate release from aggressive breast cancer cells advances a novel, mechanistic approach to targeting CIBP that could advance treatment for several pathological conditions. Using high-throughput screening, we investigated the ability of approximately 30,000 compounds from the Canadian Compound Collection to reduce glutamate release from MDA-MB-231 breast cancer cells. This line is known to secrete high levels of glutamate and has been demonstrated to induce CIBP by this mechanism. Positive chemical hits were based on the potency of each molecule relative to a known pharmacological inhibitor of glutamate release, sulfasalazine. Efficacy was confirmed and drug-like molecules were identified as potent inhibitors of glutamate secretion from MDA-MB-231, MCF-7 and Mat-Ly-Lu cells.

Biphasic modulation by mGlu5 receptors of TRPV1-mediated intracellular calcium elevation in sensory neurons contributes to heat sensitivity

BACKGROUND AND PURPOSE

Elevation of glutamate, an excitatory amino acid, during inflammation and injury plays a crucial role in the reception and transmission of sensory information via ionotropic and metabotropic receptors. This study aimed to investigate the mechanisms underlying the biphasic effects of metabotropic glutamate mGlu5 receptor activation on responses to noxious heat.

EXPERIMENTAL APPROACH

We assessed the effects of intraplantar quisqualate, a non-selective glutamate receptor agonist, on heat and mechanical pain behaviours in mice. In addition, the effects of quisqualate on the intracellular calcium response and on membrane currents mediated by TRPV1 channels, were examined in cultured dorsal root ganglion neurons from mice.

KEY RESULTS

Activation of mGlu5 receptors in hind paw transiently increased, then decreased, the response to noxious heat. In sensory neurons, activation of mGlu5 receptors potentiated TRPV1-mediated intracellular calcium elevation, while terminating activation of mGlu5 receptors depressed it. TRPV1-induced currents were potentiated by activation of mGlu5 receptors under voltage clamp conditions and these disappeared after washout. However, voltage-gated calcium currents were inhibited by the mGlu5 receptor agonist, even after washout.

CONCLUSIONS AND IMPLICATIONS

These results suggest that, in sensory neurons, mGlu5 receptors biphasically modulate TRPV1-mediated intracellular calcium response via transient potentiation of TRPV1 channel-induced currents and persistent inhibition of voltage-gated calcium currents, contributing to heat hyper- and hypoalgesia.

TRPs in taste and chemesthesis

TRP channels are expressed in taste buds, nerve fibers, and keratinocytes in the oronasal cavity. These channels play integral roles in transducing chemical stimuli, giving rise to sensations of taste, irritation, warmth, coolness, and pungency. Specifically, TRPM5 acts downstream of taste receptors in the taste transduction pathway. TRPM5 channels convert taste-evoked intracellular Ca(2+) release into membrane depolarization to trigger taste transmitter secretion. PKD2L1 is expressed in acid-sensitive (sour) taste bud cells but is unlikely to be the transducer for sour taste. TRPV1 is a receptor for pungent chemical stimuli such as capsaicin and for several irritants (chemesthesis). It is controversial whether TRPV1 is present in the taste buds and plays a direct role in taste. Instead, TRPV1 is expressed in non-gustatory sensory afferent fibers and in keratinocytes of the oronasal cavity. In many sensory fibers and epithelial cells lining the oronasal cavity, TRPA1 is also co-expressed with TRPV1. As with TRPV1, TRPA1 transduces a wide variety of irritants and, in combination with TRPV1, assures that there is a broad response to noxious chemical stimuli. Other TRP channels, including TRPM8, TRPV3, and TRPV4, play less prominent roles in chemesthesis and no known role in taste, per se. The pungency of foods and beverages is likely highly influenced by the temperature at which they are consumed, their acidity, and, for beverages, their carbonation. All these factors modulate the activity of TRP channels in taste buds and in the oronasal mucosa.

Expression of vesicular glutamate transporters VGLUT1 and VGLUT2 in the rat dental pulp and trigeminal ganglion following inflammation

Background

There is increasing evidence that peripheral glutamate signaling mechanism is involved in the nociceptive transmission during pathological conditions. However, little is known about the glutamate signaling mechanism and related specific type of vesicular glutamate transporter (VGLUT) in the dental pulp following inflammation. To address this issue, we investigated expression and protein levels of VGLUT1 and VGLUT2 in the dental pulp and trigeminal ganglion (TG) following complete Freund’s adjuvant (CFA) application to the rat dental pulp by light microscopic immunohistochemistry and Western blot analysis.

Results

The density of VGLUT2− immunopositive (+) axons in the dental pulp and the number of VGLUT2+ soma in the TG increased significantly in the CFA-treated group, compared to control group. The protein levels of VGLUT2 in the dental pulp and TG were also significantly higher in the CFA-treated group than control group by Western blot analysis. The density of VGLUT1+ axons in the dental pulp and soma in the TG remained unchanged in the CFA-treated group.

Conclusions

These findings suggest that glutamate signaling that is mediated by VGLUT2 in the pulpal axons may be enhanced in the inflamed dental pulp, which may contribute to pulpal axon sensitization leading to hyperalgesia following inflammation.

Up-regulation of Glutamate in Painful Human Supraspinatus Tendon Tears

BACKGROUND

Pain related to rotator cuff tendinopathy is a common problem, but little is known regarding the origin and cause of pain from the tendon substance. No study to date has looked at the association between tissue changes and patient outcomes.

PURPOSE

To describe the peripheral neuronal phenotype in painful rotator cuff tears and to determine correlations between tissue changes and clinical outcome measures.

STUDY DESIGN

Controlled laboratory study.

METHODS

Tissue samples of the supraspinatus were taken from patients undergoing surgery to repair a rotator cuff tendon tear. Patients were classified as having small/medium or large/massive tears. Control tissue was obtained from patients undergoing surgery for posttraumatic shoulder instability. Immunohistochemical techniques were performed using antibodies to known nociceptive and neuronal markers as well as general tissue structural markers.

RESULTS

There was no correlation between tissue changes and patient-reported outcomes. A significant increase in the expression of glutamate was seen in tendon tears. There were differences in the expression of metabotropic and ionotropic glutamate receptors. Expression changes were also observed for markers of the sensory and autonomic systems; however, no differences were found in neurotrophins.

CONCLUSION

Glutamate and the glutaminergic system play a key role in painful human tendon tears; however, the exact role is still uncertain, as glutamate is highly involved in both pain and metabolic pathways.

CLINICAL RELEVANCE

This study has identified a number of markers that could be potential therapeutic targets.

Nociceptive primary afferents: they have a mind of their own

Nociceptive primary afferents have three surprising properties: they are highly complex in their expression of neurotransmitters and receptors and most probably participate in autocrine and paracrine interactions; they are capable of exerting tonic and activity-dependent inhibitory control over incoming nociceptive input; they can generate signals in the form of dorsal root reflexes that are transmitted antidromically out to the periphery and these signals can result in neurogenic inflammation in the innervated tissue. Thus, nociceptive primary afferents are highly complicated structures, capable of modifying input before it is ever transmitted to the central nervous system and capable of altering the tissue they innervate.

Glutamate, substance P, and calcitonin gene-related peptide cooperate in inflammation-induced heat hyperalgesia

The transient receptor potential cation channel subfamily V member 1 (TRPV1) is known as a thermosensor and integrator of inflammation-induced hyperalgesia. TRPV1 is expressed in a subpopulation of primary afferent neurons that express several different neurotransmitters. The role of the TRPV1 channel in the development of hyperalgesia is established, but the role of the neurotransmitter glutamate, used partially by the same neuronal population and thus probably mediating the response, is still under investigation. We have used a Trpv1-Cre mouse line in which we either ablated Trpv1-Cre expressing neurons or induced vesicular glutamate transporter 2 (Vglut2) deficiency in Trpv1-Cre expressing neurons and investigated specific states of hyperalgesia after persistent inflammation. Furthermore, by pharmacologic inhibition of substance P (SP) or calcitonin gene-related peptide (CGRP) signaling in Vglut2-deficient mice, we also evaluated the contribution of SP or CGRP to inflammation-induced hyperalgesia, with or without the presence of vesicular glutamate transporter 2 (VGLUT2)-mediated glutamatergic transmission in Trpv1-Cre neurons. This examination, together with c-Fos analyses, showed that VGLUT2-mediated glutamatergic transmission in Trpv1-Cre afferents together with SP or CGRP is essential for the development of the heat hyperalgesia associated with persistent inflammation. Additionally, SP-, CGRP-, and VGLUT2-mediated transmission together were found to play a role in the development of mechanical hyperalgesia after persistent inflammation.

Evaluation of analgesic and anti-inflammatory activities of Hydrocotyle umbellata L., Araliaceae (acariçoba) in mice

The Hydrocotyle umbellata L. is a specimen of the Araliaceae family popularly known as acariçoba. Its indications in folk medicine include treatment of skin ulcers, and rheumatism. The aim of this study was to evaluate the antinociceptive and anti-inflammatory activities of the ethanolic extract from acariçoba's underground parts (EEA). EEA reduced the nociceptive response of the animals as evaluated in the acetic acid-induced writhing test and in both phases of formalin test. EEA also presented a supraspinal analgesic activity by increasing the pain latency in the hot plate test. Moreover, EEA reduced the leukocytes migration and plasma extravasation to pleural cavity in the carrageenan-induced pleurisy, besides reducing the edema induced by carrageenan until the second hour and also the edema induced by dextran. In conclusion our results showed that EEA of H. umbellata L. presents analgesic and anti-inflammatory activities, and that a blockade of activity or reduction in the release of different mediators, such as histamine and serotonin, could be involved in these pharmacologic effects.

Inhibition of breast cancer-cell glutamate release with sulfasalazine limits cancer-induced bone pain

Cancer in bone is frequently a result of metastases from distant sites, particularly from the breast, lung, and prostate. Pain is a common and often severe pathological feature of cancers in bone, and is a significant impediment to the maintenance of quality of life of patients living with bone metastases. Cancer cell lines have been demonstrated to release significant amounts of the neurotransmitter and cell-signalling molecule l-glutamate via the system xC(-) cystine/glutamate antiporter. We have developed a novel mouse model of breast cancer bone metastases to investigate the impact of inhibiting cancer cell glutamate transporters on nociceptive behaviour. Immunodeficient mice were inoculated intrafemorally with the human breast adenocarcinoma cell line MDA-MB-231, then treated 14days later via mini-osmotic pumps inserted intraperitoneally with sulfasalazine, (S)-4-carboxyphenylglycine, or vehicle. Both sulfasalazine and (S)-4-carboxyphenylglycine attenuated in vitro cancer cell glutamate release in a dose-dependent manner via the system xC(-) transporter. Animals treated with sulfasalazine displayed reduced nociceptive behaviours and an extended time until the onset of behavioural evidence of pain. Animals treated with a lower dose of (S)-4-carboxyphenylglycine did not display this reduction in nociceptive behaviour. These results suggest that a reduction in glutamate secretion from cancers in bone with the system xC(-) inhibitor sulfasalazine may provide some benefit for treating the often severe and intractable pain associated with bone metastases.

Peripheral metabotropic glutamate receptor subtype 5 contributes to inflammation-induced hypersensitivity of the rat temporomandibular joint

Temporomandibular disorders (TMD) comprise an assortment of clinical conditions characterized by pain in the temporomandibular joint (TMJ). TMD patients have a variety of symptoms, including jaw movement disorder and TMJ pain. Metabotropic glutamate receptor subtype 5 (mGluR5) was reported to be involved in pain processing in several animal models of neuropathic and inflammatory pain. In this study, the head withdrawal threshold and mGluR5 expression were investigated in rats with complete Freund's adjuvant (CFA)-induced TMJ inflammatory pain. CFA injection into the TMJ significantly decreased the mechanical head withdrawal thresholds relative to vehicle injection, and the effects were blocked by pre-injection of 2-methyl-6-(phenylethynyl)-pyridine (MPEP). mGluR5 expression in the trigeminal ganglion was predominantly increased in the CFA-injected group compared with the normal control group. Pretreatment with MPEP, a selective mGluR5 antagonist, reduced mGluR5 expression in the trigeminal ganglion compared with the CFA group, as measured by immunohistochemistry, western blotting, and RT-PCR. Significant differences in the proportion or intensity of mGluR5 expression were found in animals with inflammation versus control animals at the examined time point. These findings indicate a role for peripheral mGluR5 in CFA-induced nociceptive behavior and TMJ inflammation. Peripheral application of mGluR5 antagonists could provide therapeutic benefits for inflammatory TMJ pain.