Activation Of Nociceptive Neurons Research Articles

Systemic administration of resveratrol suppress the nociceptive neuronal activity of spinal trigeminal nucleus caudalis in rats.

Although a modulatory role has been reported for the red wine polyphenol resveratrol on several types of ion channels and excitatory synaptic transmission in the nervous system, the acute effects of resveratrol in vivo, particularly on nociceptive transmission of the trigeminal system, remain to be determined. The aim of the present study was to investigate whether acute intravenous resveratrol administration to rats attenuates the excitability of wide dynamic range (WDR) spinal trigeminal nucleus caudalis (SpVc) neurons in response to nociceptive and non-nociceptive mechanical stimulation in vivo. Extracellular single unit recordings were made from 18 SpVc neurons in response to orofacial mechanical stimulation of pentobarbital-anesthetized rats. Responses to both non-noxious and noxious mechanical stimuli were analyzed in the present study. The mean firing frequency of SpVc WDR neurons in response to both non-noxious and noxious mechanical stimuli was inhibited by resveratrol (0.5-2 mg/kg, i.v.) and maximum inhibition of the discharge frequency of both non-noxious and noxious mechanical stimuli was seen within 10 min. These inhibitory effects were reversed after approximately 20 min. The relative magnitude of inhibition by resveratrol of SpVc WDR neuronal discharge frequency was significantly greater for noxious than non-noxious stimulation. These results suggest that, in the absence of inflammatory or neuropathic pain, acute intravenous resveratrol administration suppresses trigeminal sensory transmission, including nociception, and so resveratrol may be used as a complementary and alternative medicine therapeutic agent for the treatment of trigeminal nociceptive pain, including hyperalgesia.

Antineuropathic profile of N-palmitoylethanolamine in a rat model of oxaliplatin-induced neurotoxicity.

Neurotoxicity is a main side effect of the anticancer drug oxaliplatin. The development of a neuropathic syndrome impairs quality of life and potentially results in chemotherapy dose reductions and/or early discontinuation. In the complex pattern of molecular and morphological alterations induced by oxaliplatin in the nervous system, an important activation of glia has been preclinically evidenced. N-Palmitoylethanolamine (PEA) modulates glial cells and exerts antinociceptive effects in several animal models. In order to improve the therapeutic chances for chemotherapy-dependent neuropathy management, the role of PEA was investigated in a rat model of oxaliplatin-induced neuropathy (2.4 mg kg-1 daily, intraperitoneally). On day 21, a single administration of PEA (30 mg kg-1 i.p.) was able to reduce oxaliplatin-dependent pain induced by mechanical and thermal stimuli. The repeated treatment with PEA (30 mg kg-1 daily i.p. for 21 days, from the first oxaliplatin injection) prevented lowering of pain threshold as well as increased pain on suprathreshold stimulation. Ex vivo histological and molecular analysis of dorsal root ganglia, peripheral nerves and spinal cord highlighted neuroprotective effects and glia-activation prevention induced by PEA repeated administration. The protective effect of PEA resulted in the normalization of the electrophysiological activity of the spinal nociceptive neurons. Finally, PEA did not alter the oxaliplatin-induced mortality of the human colon cancer cell line HT-29. The efficacy of PEA in neuropathic pain control and in preventing nervous tissue alteration candidates this endogenous compound as disease modifying agent. These characteristics, joined to the safety profile, suggest the usefulness of PEA in chemotherapy-induced neuropathy.

Involvement of medullary GABAergic system in extraterritorial neuropathic pain mechanisms associated with inferior alveolar nerve transection

In order to determine if the functional changes in the GABAergic system in the trigeminal spinal subnucleus caudalis (Vc) are involved in the mechanisms underlying extraterritorial neuropathic pain in the orofacial region following inferior alveolar nerve transection (IANX), mechanical noxious behavior, phosphorylated extracellular signal-regulated kinase (pERK) immunohistochemistry and single neuronal activity were analyzed in vesicular GABA transporter (VGAT)-VenusA rats expressing fluorescent protein and the VGAT in Vc neurons.The number of VGAT-VenusA positive neurons was significantly reduced in IANX rats than naive and sham rats at 7days after nerve transection. The number of VGAT-VenusA positive pERK-immunoreactive (IR) cells was significantly increased in IANX rats at 21days after IAN transection compared with naive and sham rats. The background activity and mechanical-evoked responses of Vc nociceptive neurons were significantly depressed after intrathecal application of the GABA receptor agonist muscimol in sham rats but not in IANX rats. Furthermore, the expression of potassium-chloride co-transporter 2 (KCC2) in the Vc was significantly reduced in IANX rats compared with sham rats. The head-withdrawal threshold (HWT) to mechanical stimulation of the whisker pad skin was significantly decreased in IANX rats compared with sham rats on days 7 and 21 after IANX. The significant reduction of the HWT and significant increase in the number of VGAT-VenusA negative pERK-IR cells were observed in KCC2 blocker R-DIOA-injected rats compared with vehicle-injected rats on day 21 after sham treatment.These findings revealed that GABAergic Vc neurons might be reduced in their number at the early period after IANX and the functional changes might occur in GABAergic neurons from inhibitory to excitatory at the late period after IANX, suggesting that the neuroplastic changes occur in the GABAergic neuronal network in the Vc due to morphological and functional changes at different time periods following IANX and resulting in the extraterritorial neuropathic pain in the orofacial region following trigeminal nerve injury.

Trigeminal neuralgia: peripheral and central mechanisms

BACKGROUND AND OBJECTIVES: Trigeminal neuralgia is one of the most common neuropathic pains that compromise head and neck. It manifests as shock or burning pain normally evoked by non-noxious facial stimulations. Its etiopathology is not totally understood, but it is known that different mechanisms contribute to the establishment and maintenance of pain. This study aimed to address current contexts of epidemiology, diagnosis, management and pathophysiological mechanisms underlying trigeminal neuralgia in peripheral and central nervous systems. CONTENTS: Inflammation and release of inflammatory mediators, neuropeptides and neurotrophic factors, as well as degenerative changes of nervous fibers caused by direct nervous injury are relevant peripheral mechanisms which lead to altered sensitivity of nociceptive neurons, development of spontaneous and exacerbated activity, allodynia and hyperalgesia. Among central mechanisms, exacerbated activation of central nociceptive neurons, neuroplasticity, changes in electrophysiological properties and neuronal hyperexcitability, in addition to changes in modulatory pain controls, lead to pain establishment and maintenance. CONCLUSION: Several mechanisms are involved in neuropathic pains, both in peripheral and central levels, although specific trigeminal neuralgia events are not totally described. Studies concerning its specific neurobiology are needed to understand functional and behavioral changes, which can contribute to trigeminal neuralgia clinical management and treatment.

Nociceptive spinal cord neurons of laminae I-III exhibit oxidative stress damage during diabetic neuropathy which is prevented by early antioxidant treatment with epigallocatechin-gallate (EGCG).

Spinal cord neurons located in laminae I-III respond to nociceptive stimuli and participate in the transmission of painful information to the brain. In the present study we evaluated if nociceptive laminae I-III neurons are affected by oxidative stress damage in a model of diabetic neuropathic pain (DNP), the streptozotocin-induced diabetic rat (STZ rat). Additionally, we evaluated the effects of a preventive antioxidant treatment with epigallocatechin-gallate (EGCG) in nociceptive neuronal activation and behavioural signs of DNP. Three days after diabetes induction, a treatment protocol of STZ rats with an aqueous solution of EGCG in the drinking water was initiated. Ten weeks after the onset of treatment, the spinal cords were immunoreacted against validated markers of oxidative stress damage (8-hydroxy-2'-deoxyguanosine; 8-OHdG) and of nociceptive neuronal activation (Fos). Mechanical hypersensitivity was assessed before and after EGCG treatment. Untreated STZ rats presented increased levels of 8-OHdG immunoreaction, higher numbers of Fos-immunoreacted neurons and high levels of co-localization of 8-OHdG and Fos in laminae I-III. Treatment with EGCG normalized the increase of the above mentioned parameters and ameliorated mechanical hypersensitivity. The present study shows that nociceptive neurons in spinal cord laminae I-III exhibit oxidative stress damage during diabetic neuropathy, which probably affects ascending pain transmission during DNP. The neurobiological mechanisms and translational perspectives of the beneficial effects of a preventive and sustained EGCG treatment in DNP need to be evaluated in the future.

PC1, a non-peptide PKR1-preferring antagonist, reduces pain behavior and spinal neuronal sensitization in neuropathic mice

Peripheral neuropathy is characterized by abnormal pain responses triggered by the release of several mediators and neuronal hyperexcitability at the spinal cord level. Emerging evidence indicates that the enhanced activity of dorsal horn neurons requires communication with glia and microglia, cells that are physiologically involved in neuronal wellbeing. Prokineticins (PKs), which include PK1 and PK2, represent a novel family of chemokines characterized by a unique structural motif comprising five disulfide bonds. They are expressed in the peripheral and central nervous system. PKs bind two G protein coupled receptors, PKR1 and PKR2, and participate in the regulation of several biological processes, including pain sensation. This study aimed to investigate the anti-nociceptive effect of PC1, a non-peptide PKR1-preferring antagonist, in a mouse model of neuropathic pain. To do this, we assessed the activity of spinal cord nociceptive neurons as well as astrocyte and microglia phenotypes after repeated administration of PC1 in vivo. PC1 treatment strongly delayed the development of thermal hyperalgesia and tactile and mechanical allodynia. It also reduced spinal microglial and glial activation 8 days post injury in spared nerve injury (SNI) mice. Neuropathic mice showed an increased level of PK2 protein in the spinal cord, mostly in astrocytes. PC1 treatment completely reversed the increased responsiveness to mechanical stimuli, the decreased threshold of neuronal activation, and the increased spontaneous activity that were observed in nociceptive specific (NS) neurons of SNI mice.

A selective α2 B adrenoceptor agonist (A-1262543) and duloxetine modulate nociceptive neurones in the medial prefrontal cortex, but not in the spinal cord of neuropathic rats.

The noradrenergic system contributes to pain modulation, but the roles of its specific adrenoceptors are still being defined. We have identified a novel, potent (rat EC50 = 4.3 nM) and selective α2B receptor agonist, A-1262543, to further explore this adrenoceptor subtype's contribution to pathological nociception. Systemic administration of A-1262543 (1-10 mg/kg, intraperitoneal) dose-dependently attenuated mechanical allodynia in animals with a spinal nerve ligation injury. To further explore its mechanism of action, the activity of nociceptive neurones in the spinal cord and medial prefrontal cortex (mPFC) were examined after injection of 3 mg/kg of A-1262543 (intravenous, i.v.). These effects were compared with duloxetine (3 mg/kg, i.v.), a dual noradrenaline (NA) and serotonin (5-HT) reuptake inhibitor. Systemic administration of A-1262543 or duloxetine did not alter the spontaneous or evoked firing of spinal wide dynamic range and nociceptive-specific neurones in the neuropathic rats, indicating that neither compound engaged spinal, peripheral or descending pathways. In contrast to the lack of effect on spinal neurones, both A-1262543 and duloxetine reduced the evoked and spontaneous firing of 'pain-responsive' (PR) neurones in the mPFC. Duloxetine, but not A-1262543, also inhibited the firing of pain non-responsive (nPR) neurones in the mPFC probably reflecting duloxetine's contribution to modulating non-pain endpoints. These data highlight that activation of the α2B adrenoceptor as well as inhibiting NA and 5-HT reuptake can result in modulating the ascending nociceptive system, and in particular, dampening the firing of PR neurones in the mPFC.

Identification of a functional interaction of HMGB1 with Receptor for Advanced Glycation End-products in a model of neuropathic pain.

Recent studies indicate that the release of high mobility group box 1 (HMGB1) following nerve injury may play a central role in the pathogenesis of neuropathic pain. HMGB1 is known to influence cellular responses within the nervous system via two distinct receptor families; the Receptor for Advanced Glycation End-products (RAGE) and Toll-like receptors (TLRs). The degree to which HMGB1 activates a receptor is thought to be dependent upon the oxidative state of the ligand, resulting in the functional isoforms of all-thiol HMGB1 (at-HMGB1) acting through RAGE, and disufide HMGB1 (ds-HMGB1) interacting with TLR4. Though it is known that dorsal root ganglia (DRG) sensory neurons exposed to HMGB1 and TLR4 agonists can influence excitation, the degree to which at-HMGB1 signaling through neuronal RAGE contributes to neuropathic pain is unknown. Here we demonstrate that at-HMGB1 activation of nociceptive neurons is dependent on RAGE and not TLR4. To distinguish the possible role of RAGE on neuropathic pain, we characterized the changes in RAGE mRNA expression up to one month after tibial nerve injury (TNI). RAGE mRNA expression in lumbar dorsal root ganglion (DRG) is substantially increased by post-injury day (PID) 28 when compared with sham injured rodents. Protein expression at PID28 confirms this injury-induced event in the DRG. Moreover, a single exposure to monoclonal antibody to RAGE (RAGE Ab) failed to abrogate pain behavior at PID 7, 14 and 21. However, RAGE Ab administration produced reversal of mechanical hyperalgesia on PID28. Thus, at-HMGB1 activation through RAGE may be responsible for sensory neuron sensitization and mechanical hyperalgesia associated with chronic neuropathic pain states.

The nucleus raphe magnus OFF-cells are involved in diffuse noxious inhibitory controls

Diffuse noxious inhibitory controls (DNIC) are very powerful long-lasting descending inhibitory controls which are pivotal in modulating the activity of spinal and trigeminal nociceptive neurons. DNIC are subserved by a loop involving supraspinal structures such as the lateral parabrachial nucleus and the subnucleus reticularis dorsalis. Surprisingly, though, whether the nucleus raphe magnus (NRM), another supraspinal area which is long known to be important in pain modulation, is involved in DNIC is still a matter of discussion. Here, we reassessed the role of the NRM neurons in DNIC by electrophysiologically recording from wide dynamic range (WDR) neurons in the trigeminal subnucleus oralis and pharmacologically manipulating the NRM OFF- and ON-cells. In control conditions, C-fiber-evoked responses in trigeminal WDR neurons are inhibited by a conditioning noxious heat stimulation applied to the hindpaw. We show that inactivating the NRM by microinjecting the GABAA receptor agonist, muscimol, both facilitates C-fiber-evoked responses of trigeminal WDR neurons and strongly attenuates their inhibition by heat applied to the hindpaw. Interestingly, selective blockade of ON-cells by microinjecting the broad-spectrum excitatory amino acid antagonist, kynurenate, into the NRM neither affects C-fiber-evoked responses nor attenuates DNIC of trigeminal WDR neurons. These results indicate that the NRM tonically inhibits trigeminal nociceptive inputs and is involved in the neuronal network underlying DNIC. Moreover, within NRM, OFF-cells might be more specifically involved in both the tonic and phasic descending inhibitory controls of trigeminal nociception.

Coolness both underlies and protects against the painfulness of the thermal grill illusion

We investigated the contributions of warm and cool signals in generating the thermal grill illusion (TGI), a phenomenon in which interlaced warm and cool bars generate an experience of burning, and under some conditions painful, heat. Each subject underwent 3 runs, 2 of which tested the effects of preadapting subjects to the grill’s warm or cool bars (while the interlaced bars were thermally neutral) on the subsequent intensity of the illusion. In a control run, all bars were neutral during the adaptation phase. Thermal visual analogue scale ratings during the warm and cool adaptation periods revealed significant and equivalent adaptation to the 2 temperatures. Adaptation to the grill’s cool bars significantly reduced pain and perceived thermal intensity of the TGI, compared to the control condition, while adaptation to the grill’s warm bars had little effect. These results suggest that the cool stimulus triggers the pain signals that produce the illusion. The inability of warm adaptation to attenuate the TGI is at odds with theories suggesting that the illusion depends upon a simple addition of warm and cool signals. While the grill’s cool bars are necessary for the TGI’s painfulness, we also observed that the more often a participant reported feeling coolness or coldness, the less pain he or she experienced from the TGI. These results are consistent with research showing that cool temperatures generate activity in both thermoreceptive-specific, pain-inhibitory neurons and nociceptive dorsal horn neurons.

Artemin growth factor increases nicotinic cholinergic receptor subunit expression and activity in nociceptive sensory neurons.

BackgroundArtemin (Artn), a member of the glial cell line-derived growth factor (GDNF) family, supports the development and function of a subpopulation of peptidergic, TRPV1-positive sensory neurons. Artn (enovin, neublastin) is elevated in inflamed tissue and its injection in skin causes transient thermal hyperalgesia. A genome wide expression analysis of trigeminal ganglia of mice that overexpress Artn in the skin (ART-OE mice) showed elevation in nicotinic acetylcholine receptor (nAChR) subunits, suggesting these ion channels contribute to Artn-induced sensitivity. Here we have used gene expression, immunolabeling, patch clamp electrophysiology and behavioral testing assays to investigate the link between Artn, nicotinic subunit expression and thermal hypersensitivity.ResultsReverse transcriptase-PCR validation showed increased levels of mRNAs encoding the nAChR subunits α3 (13.3-fold), β3 (4-fold) and β4 (7.7-fold) in trigeminal ganglia and α3 (4-fold) and β4 (2.8-fold) in dorsal root ganglia (DRG) of ART-OE mice. Sensory ganglia of ART-OE mice had increased immunoreactivity for nAChRα3 and exhibited increased overlap in labeling with GFRα3-positive neurons. Patch clamp analysis of back-labeled cutaneous afferents showed that while the majority of nicotine-evoked currents in DRG neurons had biophysical and pharmacological properties of α7-subunit containing nAChRs, the Artn-induced increase in α3 and β4 subunits resulted in functional channels. Behavioral analysis of ART-OE and wildtype mice showed that Artn-induced thermal hyperalgesia can be blocked by mecamylamine or hexamethonium. Complete Freund’s adjuvant (CFA) inflammation of paw skin, which causes an increase in Artn in the skin, also increased the level of nAChR mRNAs in DRG. Finally, the increase in nAChRs transcription was not dependent on the Artn-induced increase in TRPV1 or TRPA1 in ART-OE mice since nAChRs were elevated in ganglia of TRPV1/TRPA1 double knockout mice.ConclusionsThese findings suggest that Artn regulates the expression and composition of nAChRs in GFRα3 nociceptors and that these changes contribute to the thermal hypersensitivity that develops in response to Artn injection and perhaps to inflammation.

Fos and pERK immunoreactivity in spinal cord slices: Comparative analysis of in vitro models for testing putative antinociceptive molecules

To detect central neuron activation, expression of the transcription factor Fos and phosphorylation of the protein kinase ERK (pERK) can be visualized by immunocytochemistry. These approaches have been extensively used to quantify the activation of nociceptive neurons in the spinal dorsal horn (DH) following peripheral stimulation in vivo. Here we propose an alternative and simplified in vitro model to investigate Fos and pERK expression based on the stimulation of acutely dissected spinal cord slices to mimic acute inflammatory changes in DH. Transverse slices were obtained from postnatal (P8-P12) CD1 mice and were treated for 5 min with capsaicin (CAP, 2 μM). CAP induces a strong release of glutamate from primary afferent terminals which, in turn, excites spinal DH neurons. Since ERK phosphorylation and Fos expression occur following different time frames, two distinct protocols were used to detect their activation. Thus, for studying Fos immunoreactivity CAP-treated slices were left for 3h in Krebs solution after stimulation. Instead, for studying pERK immunoreactivity slices were maintained in Krebs solution for only 15 min after stimulation. Both Fos and pERK were significantly up-regulated following CAP challenge. To validate our model we tested the efficacy of octreotide (OCT, 1 μM) in preventing the CAP effect on Fos and pERK expression. OCT is a synthetic antinociceptive analog of somatostatin, one of the neuropeptides involved in the negative modulation of pain signals in DH. After CAP, OCT reduced the response to both Fos and pERK. Our data validate the use of Fos and pERK immunoreactivity in vitro to investigate the activation of spinal nociceptive pathways and testing potentially antinociceptive molecules.

Function of mGlu1 receptors in the modulation of nociceptive processing in the thalamus

As postsynaptic metabotropic subtype 1 (mGlu1) receptors are present in the thalamus, we have investigated the effect of potentiating and antagonising mGlu1 receptors on responses of thalamic neurones to noxious sensory stimulation. Extracellular recordings were made in vivo with multi-barrel iontophoretic electrodes from single neurones in the thalamus of urethane-anaesthetised rats. Responses to iontophoretic applications of the Group I mGlu agonist 3,5-dihydroxy-phenylglycine (DHPG) were selectively potentiated by co-application of the mGlu1 positive allosteric modulator Ro67-4853, whereas they were selectively reduced upon co-application of the mGlu1 receptor orthosteric antagonist LY367385. This indicates that thalamic DHPG responses are mediated primarily via mGlu1 receptors, consistent with the high postsynaptic levels of this receptor in the thalamus. Furthermore, potentiation of DHPG responses by Ro67-4853 were greater when the initial DHPG response was of a low magnitude. Ro67-4853 also potentiated responses of thalamic neurones to noxious thermal stimulation, whilst having little effect on the baseline activity of nociceptive neurones. By contrast, nociceptive responses were reduced by LY367385. In a further series of experiments we found that inactivation of somatosensory cortex by cooling resulted in a reduction of thalamic nociceptive responses. These results underline the importance of mGlu1 receptors in the processing of sensory information in the thalamus, particularly with respect to nociceptive responses. Furthermore, the involvement of mGlu1 receptors may reflect the activity of descending cortico-thalamic afferents.

Tramadol and Propentofylline Coadministration Exerted Synergistic Effects on Rat Spinal Nerve Ligation-Induced Neuropathic Pain

Neuropathic pain is an intractable clinical problem. Drug treatments such as tramadol have been reported to effectively decrease neuropathic pain by inhibiting the activity of nociceptive neurons. It has also been reported that modulating glial activation could also prevent or reverse neuropathic pain via the administration of a glial modulator or inhibitor, such as propentofylline. Thus far, there has been no clinical strategy incorporating both neuronal and glial participation for treating neuropathic pain. Therefore, the present research study was designed to assess whether coadministration of tramadol and propentofylline, as neuronal and glial activation inhibitors, respectively, would exert a synergistic effect on the reduction of rat spinal nerve ligation (SNL)-induced neuropathic pain. Rats underwent SNL surgery to induce neuropathic pain. Pain behavioral tests were conducted to ascertain the effect of drugs on SNL-induced mechanical allodynia with von-Frey hairs. Proinflammatory factor interleukin-1β (IL-1β) expression was also detected by Real-time RT-PCR. Intrathecal tramadol and propentofylline administered alone relieved SNL-induced mechanical allodynia in a dose-dependent manner. Tramadol and propentofylline coadministration exerted a more potent effect in a synergistic and dose dependent manner than the intrathecal administration of either drug alone. Real-time RT-PCR demonstrated IL-1β up-expression in the ipsilateral spinal dorsal horn after the lesion, which was significantly decreased by tramadol and propentofylline coadministration. Inhibiting proinflammatory factor IL-1β contributed to the synergistic effects of tramadol and propentofylline coadministration on rat peripheral nerve injury-induced neuropathic pain. Thus, our study provided a rationale for utilizing a novel strategy for treating neuropathic pain by blocking the proinflammatory factor related pathways in the central nervous system.

No relevant modulation of TRPV1-mediated trigeminal pain by intranasal carbon dioxide in healthy humans

BackgroundNasal insufflation of CO2 has been shown to exert antinociceptive respectively antihyperalgesic effects in animal pain models using topical capsaicin with activation of TRPV1-receptor positive nociceptive neurons. Clinical benefit from CO2 inhalation in patients with craniofacial pain caused by a putative activation of TRPV1 receptor positive trigeminal neurons has also been reported. These effects are probably mediated via an activation of TRPV1 receptor - positive neurons in the nasal mucosa with subsequent central inhibitory effects (such as conditioned pain modulation). In this study, we aimed to examine the effects of intranasal CO2 on a human model of craniofacial pain elicited by nasal application of capsaicin.MethodsIn a first experiment, 48 healthy volunteers without previous craniofacial pain received intranasal capsaicin to provoke trigeminal pain elicited by activation of TRVP1 positive nociceptive neurons. Then, CO2 or air was insufflated alternatingly into the nasal cavity at a flow rate of 1 l/min for 60 sec each. In the subsequent experiment, all participants were randomized into 2 groups of 24 each and received either continuous nasal insufflation of CO2 or placebo for 18:40 min after nociceptive stimulation with intranasal capsaicin. In both experiments, pain was rated on a numerical rating scale every 60 sec.ResultsContrary to previous animal studies, the effects of CO2 on experimental trigeminal pain were only marginal. In the first experiment, CO2 reduced pain ratings only minimally by 5.3% compared to air if given alternatingly with significant results for the main factor GROUP (F1,47 = 4.438; p = 0.041) and the interaction term TIME*GROUP (F2.6,121.2 = 3.3; p = 0.029) in the repeated-measures ANOVA. However, these effects were abrogated after continuous insufflation of CO2 or placebo with no significant changes for the main factors or the interaction term.ConclusionsAlthough mild modulatory effects of low-flow intranasal CO2 could be seen in this human model of TRPV-1 mediated activation of nociceptive trigeminal neurons, utility is limited as observed changes in pain ratings are clinically non-significant.

Palmitoylethanolamide Reduces Formalin-Induced Neuropathic-Like Behaviour Through Spinal Glial/Microglial Phenotypical Changes in Mice

Palmitoylethanolamide (PEA) is an endogenous cannabinoid-like compound in the central nervous system, which can modulate several functions in different pathological states, such as inflammation and pain response. We have here investigated the effect of PEA (5-10 mg/kg, intraperitoneally) on mechanical allodynia and thermal hyperalgesia 3 and 7 days following peripheral injection of formalin. Formalin induced a significant decrease of thermal and mechanical threshold in the injected and contralateral paw. PEA chronic treatment (once per day) significantly reduced mechanical allodynia and thermal hyperalgesia in a dose-dependent manner. Consistently, in vivo electrophysiological analysis revealed a significant increase of the duration and frequency, and a rapid decrease in the onset of evoked activity of the spinal nociceptive neurons 7 days after formalin. PEA normalized the electrophysiological parameters in a dose-dependent manner. Moreover, we investigated PEA effect on the glial/microglial phenotypical changes associated with spinal neuronal sensitization. We found that formalin induced a significant microglia and glia activation normalized by PEA, together with increased expression of glial interleukin 10. Finally, primary microglial cell cultures, conditioned with PEA or vehicle, where transplanted in naive and formalin-treated mice, and nociceptive neurons were recorded. We observed that only PEA-conditioned cells normalized the activity of sensitized nociceptive neurons. In conclusion these data confirm the potent anti-inflammatory and anti-allodynic effect of PEA, and highlight a possible targeted microglial/glial effect of this drug in the spinal cord.

Magnesium and memantine do not inhibit nociceptive neuronal activity in the trigeminocervical complex of the rat

Experimental studies with NMDA receptor antagonists, such as magnesium and memantine, have demonstrated the ability of these substances to inhibit nociceptive trigeminal neurotransmission in electrophysiological studies. Despite these promising experimental results clinical trials results have been less than clear. To investigate further this contrast using the open channel blockers, magnesium and memantine, we studied their effects in a model of nociceptive trigeminovascular activation.

Magnesium and memantine do not inhibit nociceptive neuronal activity in the trigeminocervical complex of the rat

Magnesium and memantine do not inhibit nociceptive neuronal activity in the trigeminocervical complex of the rat

Role of nociceptive arcuate nucleus neurons in chloroquine-induced pruritic behaviors in mice

Despite its clinical importance, the underlying central mechanisms of pruritic behaviors are poorly understood. To investigate the role of nociceptive arcuate nucleus neurons in chloroquine-induced pruritic behaviors in mice, we tested the effect of arcuate nucleus neurons and interscapular brown adipose tissue (IBAT) on itch produced by intradermal injection of chloroquine in the nape of the neck. Our results provide several lines of evidence for an important role of nociceptive arcuate nucleus neurons in chloroquine-induced pruritic behavior: (1) Intradermal microinjection of chloroquine resulted in a dramatic increase in itch behaviors accompanied by the activation of c-Fos positive neurons in arcuate nucleus; (2) Microinjection of chloroquine significantly increased IBAT temperature in the mice. These findings suggested that chloroquine-induced pruritic behaviors were associated with the activity of nociceptive arcuate nucleus neurons.

Two Mechanisms Involved in Trigeminal CGRP Release: Implications for Migraine Treatment

The goal of this study was to better understand the cellular mechanisms involved in proton stimulation of calcitonin gene-related peptide (CGRP) secretion from cultured trigeminal neurons by investigating the effects of 2 antimigraine therapies, onabotulinumtoxinA and rizatriptan. Stimulated CGRP release from peripheral and central terminating processes of trigeminal ganglia neurons is implicated in migraine pathology by promoting inflammation and nociception. Based on models of migraine pathology, several inflammatory molecules including protons are thought to facilitate sensitization and activation of trigeminal nociceptive neurons and stimulate CGRP secretion. Despite the reported efficacy of triptans and onabotulinumtoxinA to treat acute and chronic migraine, respectively, a substantial number of migraineurs do not get adequate relief with these therapies. A possible explanation is that triptans and onabotulinumtoxinA are not able to block proton-mediated CGRP secretion. CGRP secretion from cultured primary trigeminal ganglia neurons was quantitated by radioimmunoassay while intracellular calcium and sodium levels were measured in neurons via live cell imaging using Fura-2 AM and SBFI AM, respectively. The expression of acid-sensing ion channel 3 (ASIC3) was determined by immunocytochemistry and Western blot analysis. In addition, the involvement of ASICs in mediating proton stimulation of CGRP was investigated using the potent and selective ASIC3 inhibitor APETx2. While KCl caused a significant increase in CGRP secretion that was significantly repressed by treatment with ethylene glycol tetraacetic acid (EGTA), onabotulinumtoxinA, and rizatriptan, the stimulatory effect of protons (pH 5.5) was not suppressed by EGTA, onabotulinumtoxinA, or rizatriptan. In addition, while KCl caused a transient increase in intracellular calcium levels that was blocked by EGTA, no appreciable change in calcium levels was observed with proton treatment. However, protons did significantly increase the intracellular level of sodium ions. Under our culture conditions, ASIC3 was shown to be expressed in most trigeminal ganglion neurons. Importantly, proton stimulation of CGRP secretion was repressed by pretreatment with the ASIC3 inhibitor APETx2, but not the transient receptor potential vanilloid-1 antagonist capsazepine. Our findings provide evidence that proton regulated release of CGRP from trigeminal neurons utilizes a different mechanism than the calcium and synaptosomal-associated protein 25-dependent pathways that are inhibited by the antimigraine therapies, rizatriptan and onabotulinumtoxinA.