Role of protein kinase A in phosphorylation of NMDA receptor 1 subunits in dorsal horn and spinothalamic tract neurons after intradermal injection of capsaicin in rats

Protein Kinase A Inhibitor Attenuates the Antinociceptive Effect of NMDA-Receptor Channel Antagonists in the Capsaicin Test in Mice

Acute nociceptive pain in mice caused by subcutaneous (intraplantar) injection of TRPV1 ion channel agonist capsaicin (1.6 μg/mouse) and the effects of protein kinase A inhibitor H-89 (0.05 mg/mouse, intraplantar injection) and NMDA receptor channel antagonists MK-801 (7.5 and 15 μg/mouse, topical application) and hemantane (0.5 mg/mouse, topical application) on the pain were assessed. MK-801 and hemantane were found to reduce the duration of the pain response. H-89 did not significantly affect the pain in animals, but preliminary administration of this drug abolished the antinociceptive effect of MK-801 (7.5 μg/mouse) and weakens the effect of hemantane (0.5 mg/mouse).

Analysis of the Involvement of NMDA Receptors in Analgesia and Hypothermia Induced by the Activation of TRPV1 Ion Channels

NMDA glutamate receptors play an important role in normal and pathophysiological nociception. At the periphery, they can interact with TRPV1 ion channels. The blockade of TRPV1 ion channels decreases NMDA-induced hyperalgesia, and NMDA receptor antagonists suppress the pain response to the TRPV1 agonist capsaicin. Since TRPV1 ion channels and NMDA receptors can functionally interact at the periphery, it would be interesting to investigate the possibility that they interact in the CNS. A single subcutaneous injection of 1 mg/kg of capsaicin was found to raise the thermal pain threshold in the tail flick test in mice, which reproduces the spinal flexion reflex, owing to the ability of capsaicin to cause long-term desensitization of nociceptors. Preventive administration of either noncompetitive NMDA receptor antagonists (high-affinity MK-801 20 μg/kg and 0.5 mg/kg subcutaneously; low-affinity hemantane 40 mg/kg intraperitoneally) or the selective TRPV1 antagonist BCTC (20 mg/kg intraperitoneally) inhibit the capsaicin-induced increase in the pain threshold. Capsaicin (1 mg/kg, subcutaneous injection) induces transient hypothermia in mice, which is brought about by hypothalamus-triggered vegetative reactions. This effect is prevented by BCTC but not by the noncompetitive NMDA receptor antagonists.

Acute stress induces visceral hypersensitivity via glucocorticoid receptor-mediated membrane insertion of TRPM8: Involvement of a non-receptor tyrosine kinase Pyk2

BACKGROUND

Psychological stress is an important factor for the development and recurrence of irritable bowel syndrome (IBS). The mechanisms underlying stress-induced visceral hypersensitivity (VH), a key pathophysiological component in IBS, are still incompletely understood. We aimed to test whether transient receptor potential melastatin 8 (TRPM8) participates in acute stress-induced VH.

METHODS

Rats were subjected to 1-hour water avoidance stress (WAS). Visceral sensitivity was measured with visceromotor response to colorectal distension. Western blot and immunofluorescence were applied to evaluate the expression of GR and TRPM8 and activation of PKA, Akt, and PKC pathways.

RESULTS

WAS-caused VH depended on glucocorticoid receptors (GRs) and TRPM8 channels. In a dorsal root ganglion (DRG)-derived cell line, corticosterone rapidly (within 30 minutes) induced membrane expression of TRPM8. This effect was inhibited by GR antagonism and was mimicked by membrane-impermeable corticosterone. PKA, PI3K/Akt, and PKC pathways, which lied downstream of GR and acted in parallel to promote membrane expression of TRPM8, contributed to WAS-induced VH. The non-receptor tyrosine kinase Pyk2, which may serve as a convergence point for PKA, PI3K/Akt, and PKC pathways, facilitated membrane insertion of TRPM8 via tyrosine-phosphorylating TRPM8 in L6-S2 DRGs and participated in WAS-induced VH.

CONCLUSIONS

Collectively, acute stress-induced VH could involve membrane-bound GR-dependent enhancement of TRPM8 function in nociceptive DRG neurons. Mechanistically, Pyk2 could act as a key mediator that coordinates multiple protein kinase signaling and triggers phosphorylation and membrane insertion of TRPM8.

Tyrosine Kinase Inhibitors Reduce NMDA NR1 Subunit Expression, Nuclear Translocation, and Behavioral Pain Measures in Experimental Arthritis

In the lumbar spinal cord dorsal horn, release of afferent nerve glutamate activates the neurons that relay information about injury pain. Here, we examined the effects of protein tyrosine kinase (PTK) inhibition on NMDA receptor NR1 subunit protein expression and subcellular localization in an acute experimental arthritis model. PTK inhibitors genistein and lavendustin A reduced cellular histological translocation of NMDA NR1 in the spinal cord occurring after the inflammatory insult and the nociceptive behavioral responses to heat. The PTK inhibitors were administered into lumbar spinal cord by microdialysis, and secondary heat hyperalgesia was determined using the Hargreaves test. NMDA NR1 cellular protein expression and nuclear translocation were determined by immunocytochemical localization with light and electron microscopy, as well as with Western blot analysis utilizing both C- and N-terminal antibodies. Genistein and lavendustin A (but not inactive lavendustin B or diadzein) effectively reduced (i) pain related behavior, (ii) NMDA NR1 subunit expression increases in spinal cord, and (iii) the shift of NR1 from a cell membrane to a nuclear localization. Genistein pre-treatment reduced these events that occur in vivo within 4 h after inflammatory insult to the knee joint with kaolin and carrageenan (k/c). Cycloheximide reduced glutamate activated upregulation of NR1 content confirming synthesis of new protein in response to the inflammatory insult. In addition to this in vivo data, genistein or staurosporin inhibited upregulation of NMDA NR1 protein and nuclear translocation in vitro after treatment of human neuroblastoma clonal cell cultures (SH-SY5Y) with glutamate or NMDA (4 h). These studies provide evidence that inflammatory activation of peripheral nerves initiates increase in NMDA NR1 in the spinal cord coincident with development of pain related behaviors through glutamate non-receptor, PTK dependent cascades.

Rescue of HSP70 in Spinal Neurons Alleviates Opioids-Induced Hyperalgesia via the Suppression of Endoplasmic Reticulum Stress in Rodents

A major unresolved issue in treating pain is the paradoxical hyperalgesia produced by the gold-standard analgesic morphine and other opioids. Endoplasmic reticulum (ER) stress has been shown to contribute to neuropathic or inflammatory pain, but its roles in opioids-induced hyperalgesia (OIH) are elusive. Here, we provide the first direct evidence that ER stress is a significant driver of OIH. GRP78, the ER stress marker, is markedly upregulated in neurons in the spinal cord after chronic morphine treatment. At the same time, morphine induces the activation of three arms of unfolded protein response (UPR): inositol-requiring enzyme 1α/X-box binding protein 1 (IRE1α/XBP1), protein kinase RNA-like ER kinase/eukaryotic initiation factor 2 subunit alpha (PERK/eIF2α), and activating transcription factor 6 (ATF6). Notably, we found that inhibition on either IRE1α/XBP1 or ATF6, but not on PERK/eIF2α could attenuate the development of OIH. Consequently, ER stress induced by morphine enhances PKA-mediated phosphorylation of NMDA receptor subunit 1(NR1) and leads to OIH. We further showed that heat shock protein 70 (HSP70), a molecular chaperone involved in protein folding in ER, is heavily released from spinal neurons after morphine treatment upon the control of K_ATP channel. Glibenclamide, a classic K_ATP channel blocker that inhibits the efflux of HSP70 from cytoplasm to extracellular environment, or HSP70 overexpression in neurons, could markedly suppress morphine-induced ER stress and hyperalgesia. Taken together, our findings uncover the induction process and the central role of ER stress in the development of OIH and support a novel strategy for anti-OIH treatment.

Effect of bucladesine, pentoxifylline, and H-89 as cyclic adenosine monophosphate analog, phosphodiesterase, and protein kinase A inhibitor on acute pain

The aim of this study was to determine the effects of cyclic adenosine monophosphate (cAMP) and its dependent pathway on thermal nociception in a mouse model of acute pain. Here, we studied the effect of H-89 (protein kinase A inhibitor), bucladesine (Db-cAMP) (membrane-permeable analog of cAMP), and pentoxifylline (PTX; nonspecific phosphodiesterase (PDE) inhibitor) on pain sensation. Different doses of H-89 (0.05, 0.1, and 0.5 mg/100 g), PTX (5, 10, and 20 mg/100 g), and Db-cAMP (50, 100, and 300 nm/mouse) were administered intraperitoneally (I.p.) 15 min before a tail-flick test. In combination groups, we injected the first and the second compounds 30 and 15 min before the tail-flick test, respectively. I.p. administration of H-89 and PTX significantly decreased the thermal-induced pain sensation in their low applied doses. Db-cAMP, however, decreased the pain sensation in a dose-dependent manner. The highest applied dose of H-89 (0.5 mg/100 g) attenuated the antinociceptive effect of Db-cAMP in doses of 50 and 100 nm/mouse. Surprisingly, Db-cAMP decreased the antinociceptive effect of the lowest dose of H-89 (0.05 mg/100 g). All applied doses of PTX reduced the effect of 0.05 mg/100 g H-89 on pain sensation; however, the highest dose of H-89 compromised the antinociceptive effect of 20 mg/100 g dose of PTX. Co-administration of Db-cAMP and PTX increased the antinociceptive effect of each compound on thermal-induced pain. In conclusion, PTX, H-89, and Db-cAMP affect the thermal-induced pain by probably interacting with intracellular cAMP and cGMP signaling pathways and cyclic nucleotide-dependent protein kinases.

Quantitative Thermal Testing Profiles as a Predictor of Treatment Response to Topical Capsaicin in Patients with Localized Neuropathic Pain

There are no reliable predictors of response to treatment with capsaicin. Given that capsaicin application causes heat sensation, differences in quantitative thermal testing (QTT) profiles may predict treatment response. The aim of this study was to determine whether different QTT profiles could predict treatment outcomes in patients with localized peripheral neuropathic pain (PeLNP). We obtained from medical records QTT results and treatment outcomes of 55 patients treated between 2010 and 2013. Warm sensation threshold (WST) and heat pain threshold (HPT) values were assessed at baseline at the treatment site and in the asymptomatic, contralateral area. Responders were defined as those who achieved a > 30% decrease in pain lasting > 30 days. Two distinct groups were identified based on differences in QTT profiles. Most patients (27/31; 87.1%) with a homogenous profile were nonresponders. By contrast, more than half of the patients (13/24, 54.2%) with a nonhomogenous profile were responders (p = 0.0028). A nonhomogenous QTT profile appears to be predictive of response to capsaicin. We hypothesize patients with a partial loss of cutaneous nerve fibers or receptors are more likely to respond. By contrast, when severe nerve damage or normal cutaneous sensations are present, the pain is likely due to central sensitization and thus not responsive to capsaicin. Prospective studies with larger patient samples are needed to confirm this hypothesis.

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

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

Capsaicin, Nociception and Pain

Capsaicin, the pungent ingredient of the hot chili pepper, is known to act on the transient receptor potential cation channel vanilloid subfamily member 1 (TRPV1). TRPV1 is involved in somatic and visceral peripheral inflammation, in the modulation of nociceptive inputs to spinal cord and brain stem centers, as well as the integration of diverse painful stimuli. In this review, we first describe the chemical and pharmacological properties of capsaicin and its derivatives in relation to their analgesic properties. We then consider the biochemical and functional characteristics of TRPV1, focusing on its distribution and biological effects within the somatosensory and viscerosensory nociceptive systems. Finally, we discuss the use of capsaicin as an agonist of TRPV1 to model acute inflammation in slices and other ex vivo preparations.

Effects of the dimeric PSD-95 inhibitor UCCB01-144 in mouse models of pain, cognition and motor function

NMDAR antagonism shows analgesic action in humans and animal pain models, but disrupts cognitive and motor functions. NMDAR-dependent NO production requires tethering of the NMDAR to neuronal NO synthase (nNOS) by the postsynaptic density protein-95 (PSD-95). Perturbing the NMDAR/PSD-95/nNOS interaction has therefore been proposed as an alternative analgesic mechanism. We recently reported that UCCB01-125, a dimeric PSD-95 inhibitor with limited blood-brain-barrier permeability, reduced mechanical hypersensitivity in the complete Freund's adjuvant (CFA) inflammatory pain model, without disrupting cognitive or motor functions. Here, we investigated the analgesic efficacy in the CFA model of UCCB01-144, a PSD-95 inhibitor with improved blood-brain-barrier permeability. To extend the comparison of UCCB01-125 and UCCB01-144, we also tested both compounds in the spared nerve injury (SNI) model of neuropathic pain. Potential cognitive effects of UCCB01-144 were examined using the social transmission of food preference (STFP) test and the V-maze test, and motor coordination was assessed with the rotarod test. UCCB01-144 (10mg/kg) reversed CFA-induced mechanical hypersensitivity after 1h, and completely normalised sensitivity after 24h. In the SNI model, UCCB01-144 (30mg/kg) partially reversed hypersensitivity after 1h, but no effect was observed after 24h. UCCB01-125 did not affect SNI-induced hypersensitivity. Rotarod performance was unaffected by UCCB01-144, but 30mg/kg UCCB01-144 impaired performance in the STFP test. Collectively, UCCB01-144 reversed both CFA and SNI-induced hypersensitivity, but the efficacy in the SNI model was only transient. This suggests that enhanced BBB permeability of PSD-95 inhibitors improves the analgesic action in neuropathic pain states.

Spinal D1-like dopamine receptors modulate NMDA receptor-induced hyperexcitability and NR1 subunit phosphorylation at serine 889

Activation of the N-methyl-d-aspartate receptor (NMDAR) in dorsal horn neurons is recognized as a fundamental mechanism of central sensitization and pathologic pain. This study assessed the influence of dopaminergic, D1-like receptor-mediated input to the spinal dorsal horn on NMDAR function. Spinal superfusion with selective NMDAR agonist cis-ACPD significantly increased C-fiber-evoked field potentials in rats subjected to spinal nerve ligation (SNL), but not in sham-operated rats. Simultaneous application of D1LR antagonist SCH 23390 dramatically reduced hyperexcitability induced by cis-ACPD. Furthermore, cis-ACPD-induced hyperexcitability seen in nerve-ligated rats could be mimicked in unin-jured rats during stimulation of D1LRs by agonist SKF 38393 at subthreshold concentration. Phosphorylation of NMDAR subunit NR1 at serine 889 at postsynaptic sites was found to be increased in dorsal horn neurons 90 min after SNL, as assessed by increased co-localization with postsynaptic marker PSD-95. Increased NR1 phosphorylation was attenuated in the presence of SCH 23390 in the spinal superfusate. The present results support that D1LRs regulate most basic determinants of NMDAR function in dorsal horn neurons, suggesting a potential mechanism whereby dopaminergic input to the dorsal horn can modulate central sensitization and pathologic pain.

Unraveling the role of Mg(++) in osteoarthritis

Mg(++) is widely involved in human physiological processes that may play key roles in the generation and progression of diseases. Osteoarthritis (OA) is a complex joint disorder characterized by articular cartilage degradation, abnormal mineralization and inflammation. Magnesium deficiency is considered to be a major risk factor for OA development and progression. Magnesium deficiency is active in several pathways that have been implicated in OA, including increased inflammatory mediators, cartilage damage, defective chondrocyte biosynthesis, aberrant calcification and a weakened effect of analgesics. Abundant in vitro and in vivo evidence in animal models now suggests that the nutritional supplementation or local infiltration of Mg(++) represent effective therapies for OA. The goal of this review is to summarize the current understanding of the role of Mg(++) in OA with particular emphasis on the related molecular mechanisms involved in OA progression.

Excitatory amino acid glutamate: role in peripheral nociceptive transduction and inflammation in experimental and clinical osteoarthritis

Although a large proportion of patients with osteoarthritis (OA) show inflammation in their affected joints, the pathological role of inflammation in the development and progression of OA has yet to be clarified. Glutamate is considered an excitatory amino acid (EAA) neurotransmitter in the mammalian central nervous system (CNS). There are cellular membrane glutamate receptors and transporters for signal input modulation and termination as well as vesicular glutamate transporters (VGLUTs) for signal output through exocytotic release. Glutamate been shown to mediate intercellular communications in bone cells in a manner similar to synaptic transmission within the CNS. Glutamate-mediated events may also contribute to the pathogenesis and ongoing processes of peripheral nociceptive transduction and inflammation of experimental arthritis models as well as human arthritic conditions. This review will discuss the differential roles of glutamate signaling and blockade in peripheral neuronal and non-neuronal joint tissues, including bone remodeling systems and their potentials to impact OA-related inflammation and progression. This will serve to identify several potential targets to direct novel therapies for OA. Future studies will further elucidate the role of glutamate in the development and progression of OA, as well as its association with the clinical features of the disease.

Endogenous BDNF augments NMDA receptor phosphorylation in the spinal cord via PLCγ, PKC, and PI3K/Akt pathways during colitis

Background

Spinal central sensitization is an important process in the generation and maintenance of visceral hypersensitivity. The release of brain-derived neurotrophic factor (BDNF) from the primary afferent neurons to the spinal cord contributes to spinal neuronal plasticity and increases neuronal activity and synaptic efficacy. The N-Methyl-D-aspartic acid (NMDA) receptor possesses ion channel properties, and its activity is modulated by phosphorylation of its subunits including the NMDA receptor 1 (NR1).

Methods

Colonic inflammation was induced by a single dose of intracolonic instillation of tri-nitrobenzene sulfonic acid (TNBS). NR1 phosphorylation by BDNF in vivo and in culture was examined by western blot and immunohistochemistry. Signal transduction was studied by direct examination and use of specific inhibitors.

Results

During colitis, the level of NR1 phospho-Ser⁸⁹⁶ was increased in the dorsal horn region of the L1 and S1 spinal cord; this increase was attenuated by injection of BDNF neutralizing antibody to colitic animals (36 μg/kg, intravenous (i.v.)) and was also reduced in BDNF^+/− rat treated with TNBS. Signal transduction examination showed that the extracellular signal-regulated kinase (ERK) activation was not involved in BDNF-induced NR1 phosphorylation. In contrast, the phosphatidylinositol 3-kinase (PI3K)/Akt pathway mediated BDNF-induced NR1 phosphorylation in vivo and in culture; this is an additional pathway to the phospholipase C-gamma (PLCγ) and the protein kinase C (PKC) that was widely considered to phosphorylate NR1 at Ser⁸⁹⁶. In spinal cord culture, the inhibitors to PLC (U73122), PKC (bisindolylmaleimide I), and PI3K (LY294002), but not MEK (PD98059) blocked BDNF-induced NR1 phosphorylation. In animals with colitis, treatment with LY294002 (50 μg/kg, i.v.) blocked the Akt activity as well as NR1 phosphorylation at Ser⁸⁹⁶ in the spinal cord.

Conclusion

BDNF participates in colitis-induced spinal central sensitization by up-regulating NR1 phosphorylation at Ser⁸⁹⁶. The PI3K/Akt pathway, in addition to PLCγ and PKC, mediates BDNF action in the spinal cord during colitis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0371-z) contains supplementary material, which is available to authorized users.

Supression of chronic central pain by superoxide dismutase in rats with spinal cord injury: Inhibition of the NMDA receptor implicated

Superoxide dismutase (SOD) is used to manage chronic pain, including neuropathic and inflammatory pain. However, data regarding the clinical effectiveness are conflicting and the neurophysiological mechanism of SOD has yet to be elucidated. The aim of the present study was to investigate whether SOD relieved chronic central pain (CCP) following spinal cord injury (SCI) and the possible underlying mechanisms. A CCP model was established using the Allen method and the CCP of the rats was measured using the paw withdrawal threshold. SOD was administered intraperitoneally following the establishment of CCP as a result of SCI. The results demonstrated that SOD relieved CCP in rats following SCI. In addition, the expression of spinal phosphorylated N-methyl-D-aspartate(NMDA) receptor subunit 1 (pNR-1) was inhibited in the CCP rats that had been treated with SOD. These observations indicated that SOD reduced mechanical allodynia and attenuated the enhancement of spinal pNR1 expression in rats with CCP. In addition, the results indicated that superoxide, produced via xanthine oxidase, and the participation of superoxide and nitric oxide (NO) as a precursor of peroxynitrite in NMDA, were involved in the mediation of central sensitization. Therefore, the observations support the hypothesis that SOD may have a potential therapeutic role for the treatment of CCP following SCI via the manipulation of superoxide and NO.

Uliginosin B, a natural phloroglucinol derivative, presents a multimediated antinociceptive effect in mice

Objective

Investigate the involvement of monoaminergic and glutamatergic systems on the antinociceptive and ataxic effects of uliginosin B, which we have already demonstrated to be a promising molecular scaffold to develop new analgesic drugs.

Methods

Uliginosin B was obtained from hexane extract of aerial parts of Hypericum polyanthemum by chromatographic methods. Uliginosin B antinociceptive and motor coordination effects were evaluated in mice by using hot-plate (15 and 90 mg/kg, i.p.) and rotarod (90 mg/kg, i.p.) tests, respectively. The mechanism of action was investigated through pretreatments with prazosin 1 mg/kg intraperitoneal (α1 receptor antagonist), yohimbine 5 mg/kg intraperitoneal (α2 receptor antagonist), pCPA 300 mg/kg intraperitoneal (serotonin synthesis inhibitor) and MK-801 0.25 mg/kg intraperitoneal (N-methyl-D-aspartic acid receptor antagonist).

Key findings

The antinociceptive effect of uliginosin B (15 and 90 mg/kg, i.p.) was reduced significantly by pCPA and MK-801. Prazosin and yohimbine improved the antinociceptive effect of the highest dose (90 mg/kg, i.p.) of uliginosin B only. The ataxic effect of uliginosin B (90 mg/kg, i.p.) was completely prevented by pretreatment with pCPA or MK-801, but it was unaffected by pretreatment with prazosin or yohimbine.

Conclusion

These data confirm the contribution of monoaminergic neurotransmission as well as provide the first evidence of glutamatergic neurotransmission contribution to the uliginosin B effects.

Endogenous PI3K/Akt and NMDAR act independently in the regulation of CREB activity in lumbosacral spinal cord in cystitis

The integral interaction of signaling components in the regulation of visceral inflammation-induced central sensitization in the spinal cord has not been well studied. Here we report that phosphoinositide 3-kinase (PI3K)-dependent Akt activation and N-methyl-d-aspartic acid receptor (NMDAR) in lumbosacral spinal cord independently regulate the activation of cAMP response element-binding protein (CREB) in vivo in a rat visceral pain model of cystitis induced by intraperitoneal injection of cyclophosphamide (CYP). We demonstrate that suppression of endogenous PI3K/Akt activity with a potent PI3K inhibitor LY294002 reverses CYP-induced phosphorylation of CREB, however, it has no effect on CYP-induced phosphorylation of NR1 at Ser(897) and Ser(896); conversely, inhibition of NMDAR in vivo with MK801 fails to block CYP-induced Akt activation but significantly attenuates CYP-induced CREB phosphorylation in lumbosacral spinal cord. This novel interrelationship of PI3K/Akt, NMDAR, and CREB activation in lumbosacral spinal cord is further confirmed in an ex vivo spinal slice culture system exposed to an excitatory neurotransmitter calcitonin gene-related peptide (CGRP). Consistently we found that CGRP-triggered CREB activation can be blocked by both PI3K inhibitor LY294002 and NMDAR antagonists MK801 and D-AP5. However, CGRP-triggered Akt activation cannot be blocked by MK801 or D-AP5; vice versa, LY294002 pretreatment that suppresses the Akt activity fails to reverse CGRP-elicited NR1 phosphorylation. These results suggest that PI3K/Akt and NMDAR independently regulate spinal plasticity in visceral pain model, and target of a single pathway is necessary but not sufficient in treatment of visceral hypersensitivity.

Sigma 1 receptor: a new therapeutic target for pain

Sigma 1 receptor (σ₁ receptor) is a unique ligand-regulated molecular chaperone located mainly in the endoplasmic reticulum and the plasma membrane. σ₁ receptor is activated under stress or pathological conditions and interacts with several neurotransmitter receptors and ion channels to modulate their function. The effects reported preclinically with σ₁ receptor ligands are consistent with a role for σ₁ receptor in central sensitization and pain hypersensitivity and suggest a potential therapeutic use of σ₁ receptor antagonists for the management of neuropathic pain as monotherapy. Moreover, data support their use in opioid adjuvant therapy: combination of σ₁ receptor antagonists and opioids results in potentiation of opioid analgesia, without significant increases in opioid-related unwanted effects. Results from clinical trials using selective σ₁ receptor antagonists in several pain conditions are eagerly awaited to ascertain the potential of σ₁ receptor modulation in pain therapy.

Regular physical activity prevents development of chronic pain and activation of central neurons

Chronic musculoskeletal pain is a significant health problem and is associated with increases in pain during acute physical activity. Regular physical activity is protective against many chronic diseases; however, it is unknown if it plays a role in development of chronic pain. The current study induced physical activity by placing running wheels in home cages of mice for 5 days or 8 wk and compared these to sedentary mice without running wheels in their home cages. Chronic muscle pain was induced by repeated intramuscular injection of pH 4.0 saline, exercise-enhanced pain was induced by combining a 2-h fatiguing exercise task with a low-dose muscle inflammation (0.03% carrageenan), and acute muscle inflammation was induced by 3% carrageenan. We tested the responses of the paw (response frequency) and muscle (withdrawal threshold) to nociceptive stimuli. Because the rostral ventromedial medulla (RVM) is involved in exercise-induced analgesia and chronic muscle pain, we tested for changes in phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor in the RVM. We demonstrate that regular physical activity prevents the development of chronic muscle pain and exercise-induced muscle pain by reducing phosphorylation of the NR1 subunit of the NMDA receptor in the central nervous system. However, regular physical activity has no effect on development of acute pain. Thus physical inactivity is a risk factor for development of chronic pain and may set the nervous system to respond in an exaggerated way to low-intensity muscle insults.

Unravelling the mystery of capsaicin: a tool to understand and treat pain

A large number of pharmacological studies have used capsaicin as a tool to activate many physiological systems, with an emphasis on pain research but also including functions such as the cardiovascular system, the respiratory system, and the urinary tract. Understanding the actions of capsaicin led to the discovery its receptor, transient receptor potential (TRP) vanilloid subfamily member 1 (TRPV1), part of the superfamily of TRP receptors, sensing external events. This receptor is found on key fine sensory afferents, and so the use of capsaicin to selectively activate pain afferents has been exploited in animal studies, human psychophysics, and imaging studies. Its effects depend on the dose and route of administration and may include sensitization, desensitization, withdrawal of afferent nerve terminals, or even overt death of afferent fibers. The ability of capsaicin to generate central hypersensitivity has been valuable in understanding the consequences and mechanisms behind enhanced central processing of pain. In addition, capsaicin has been used as a therapeutic agent when applied topically, and antagonists of the TRPV1 receptor have been developed. Overall, the numerous uses for capsaicin are clear; hence, the rationale of this review is to bring together and discuss the different types of studies that exploit these actions to shed light upon capsaicin working both as a tool to understand pain but also as a treatment for chronic pain. This review will discuss the various actions of capsaicin and how it lends itself to these different purposes.

Exercise-induced pain requires NMDA receptor activation in the medullary raphe nuclei

PURPOSE

Pain in response to physical activity is common in people with chronic musculoskeletal pain and is likely a barrier to regular exercise, which would lead to a sedentary lifestyle. We recently developed a model of exercise-induced pain that is associated with increased activation of neurons in the medullary raphe nuclei, i.e., the nucleus raphe obscurus (NRO) and nucleus raphe pallidus (NRP). Because the NRO and NRP not only modulate motor output but also respond to noxious stimuli, we hypothesized that the NRO and NRP were key nuclei in the interaction between pain and exercise. We tested whether exercise enhances hyperalgesia through activation of N-methyl D-aspartate (NMDA) receptors in the NRO/NRP.

METHODS

Muscle insult was induced by two injections of pH 5.0 saline 5 d apart into one gastrocnemius muscle. We initially tested whether hyperalgesia developed in mice injected with acidic saline (pH 5.0) into the gastrocnemius muscle immediately after a 30-min or 2-h exercise task or 2 h after a 2-h exercise task. Next, we tested whether blockade of NMDA receptors in the NRO/NRP during the exercise task prevented the development of exercise-induced hyperalgesia. Finally, we evaluated changes in phosphorylation of the NR1 subunit of the NMDA receptor (pNR1) after the exercise task at times in which muscle insult was given in behavioral experiments, i.e., immediately after a 30-min or 2-h exercise task or 2 h after the 2-h exercise task.

RESULTS

All exercise conditions enhanced nociception (hyperalgesia) after combining with two injections of pH 5.0 saline. Microinjection of AP5 (1.0-0.1 nmol; 2-amino-5-phophonopenanoate) dose-dependently prevented the development of exercise-induced hyperalgesia. All exercise conditions increased pNR1 in the NRO and NRP.

CONCLUSIONS

Thus, exercise-induced pain in sedentary mice is associated with increased phosphorylation and activation of NMDA receptors in the NRO/NRP, suggesting that changes in central excitability mediate an interaction between unaccustomed exercise and pain.

Central sensitization in the trigeminal nucleus caudalis produced by a conjugate of substance P and the A subunit of cholera toxin

Individuals with chronic craniofacial pain experience symptoms that are consistent with central sensitization. In fact, central sensitization may constitute the major disease process in these conditions, particularly if the original injury has healed or the condition is idiopathic. To understand central sensitization we have developed a conjugate of substance P and cholera toxin (SP-CTA). SP-CTA is selectively taken up by cells that express neurokinin receptors. Twenty-four hours following intracisternal administration of SP-CTA, wild-type rats and mice demonstrated signs of persistent background nociception, but when tested for facial cold sensitivity, they did not differ from controls. However, treating the SP-CTA-injected animals with naloxone exposed cold hypersensitivity in the face. Mu-opioid receptor knockout mice treated with SP-CTA demonstrated hypersensitivity without naloxone treatment. These findings suggest that central sensitization leads to activation of an endogenous opioid system. The data also demonstrate that the intracisternal administration of SP-CTA in rodents is a useful model for studying central sensitization as a disease process without having to induce a peripheral injury.

PERSPECTIVE

Central sensitization is a concern in many craniofacial pain conditions. In this project, we utilize a conjugate of substance P and the catalytic subunit of cholera toxin to induce central sensitization in the nucleus caudalis of rodents. The data indicate that the injected animals become hypersensitive in the face.

CNS animal fMRI in pain and analgesia

Animal imaging of brain systems offers exciting opportunities to better understand the neurobiology of pain and analgesia. Overall functional studies have lagged behind human studies as a result of technical issues including the use of anesthesia. Now that many of these issues have been overcome including the possibility of imaging awake animals, there are new opportunities to study whole brain systems neurobiology of acute and chronic pain as well as analgesic effects on brain systems de novo (using pharmacological MRI) or testing in animal models of pain. Understanding brain networks in these areas may provide new insights into translational science, and use neural networks as a "language of translation" between preclinical to clinical models. In this review we evaluate the role of functional and anatomical imaging in furthering our understanding in pain and analgesia.

An increase in spinal dehydroepiandrosterone sulfate (DHEAS) enhances NMDA-induced pain via phosphorylation of the NR1 subunit in mice: involvement of the sigma-1 receptor

Our laboratory has recently demonstrated that an increase in the spinal neurosteroid, dehydroepiandrosterone sulfate (DHEAS) facilitates nociception via the activation of sigma-1 receptors and/or the allosteric inhibition GABA(A) receptors. Several lines of evidence have suggested that DHEAS positively modulates N-methyl-d-aspartate (NMDA) receptor activity within the central nervous system. Moreover, we have demonstrated that the activation of sigma-1 receptors increases NMDA receptor activity. Since NMDA receptors play a key role in the enhancement of pain perception, the present study was designed to determine whether spinally administered DHEAS modulates NMDA receptor-mediated nociceptive activity and whether this effect is mediated by sigma-1 or GABA(A) receptors. Intrathecal (i.t.) DHEAS was found to significantly potentiate i.t. NMDA-induced spontaneous pain behaviors. Subsequent immunohistochemical analysis demonstrated that i.t. DHEAS also increased protein kinase C (PKC)- and protein kinase A (PKA)-dependent phosphorylation of the NMDA receptor subunit NR1 (pNR1), which was used as a marker of NMDA receptor sensitization. The sigma-1 receptor antagonist, BD-1047, but not the GABA(A) receptor agonist, muscimol, dose-dependently suppressed DHEAS's facilitatory effect on NMDA-induced nociception and pNR1 expression. In addition, pretreatment with either a PKC or PKA blocker significantly reduced the facilitatory effect of DHEAS on NMDA-induced nociception. Conversely the GABA(A) receptor antagonist, bicuculline did not affect NMDA-induced pain behavior or pNR1 expression. The results of this study suggest that the DHEAS-induced enhancement of NMDA-mediated nociception is dependent on an increase in PKC- and PKA-dependent pNR1. Moreover, this effect of DHEAS on NMDA receptor activity is mediated by the activation of spinal sigma-1 receptors and not through the inhibition of GABA(A) receptors.

Fos expression induced by activation of NMDA and neurokinin-1 receptors in the trigeminal subnucleus caudalis in vitro: role of protein kinases

Activity-induced neuronal plasticity is partly facilitated by the expression of the immediate-early gene c-fos and the resulting transcription factor Fos. Expression of Fos is associated with nociceptive afferent activation, but a detailed stimulation-transcription pathway for Fos expression has not yet been determined in the trigeminal system. This study utilized a novel in vitro model to determine whether Fos expression can be induced in trigeminal subnucleus caudalis by NMDA or neurokinin-1 receptor activation, and whether inhibition of intracellular kinases has any effect on Fos expression induced by activation of these receptors. Brainstems of male Wistar rats were excised and maintained in artificial cerebrospinal fluid at 37°C. NMDA or the specific neurokinin-1 receptor agonist [Sar(9),Met(O(2))(11)]-SP was applied. These agonists were subsequently tested in the presence of the protein kinase A inhibitor Rp-cAMP or protein kinase C inhibitor chelerythrine chloride. In all experiments the sodium channel blocker tetrodotoxin was used to prevent indirect neuronal activation. Brainstems were processed immunocytochemically for Fos expression, and positive cells were counted in the trigeminal subnucleus caudalis. NMDA and [Sar(9),Met(O(2))(11)]-SP significantly increased Fos expression, but these increases could be prevented by chelerythrine chloride. Rp-cAMP had no effect on Fos induced by NMDA but caused a significant reduction in Fos induced by [Sar(9),Met(O(2))(11)]-SP. These data demonstrate that in trigeminal subnucleus caudalis activation of either NK1 or NMDA receptors alone induces Fos expression; protein kinases A and C are involved in NK1R-induced Fos while protein kinase A is not required for NMDA receptor-induced Fos.

Changes in expression of NMDA-NR1 receptor subunits in the rostral ventromedial medulla modulate pain behaviors

NMDA receptors have an important role in pain facilitation in rostral ventromedial medulla (RVM) and the NR1 subunit is essential for its function. Studies suggest that the NMDA receptors in RVM are critical to modulate both cutaneous and muscle hypersensitivity induced by repeated intramuscular acid injections. We propose that increased expression of the NR1 subunit in the RVM is critical for the full development of hypersensitivity. To test this we used recombinant lentiviruses to over-express the NR1 subunit in the RVM and measured nociceptive sensitivity to cutaneous and muscle stimuli. We also downregulated the expression of NR1 in the RVM and measured the hyperalgesia produced by repeated-acid injections. Increasing the expression of NR1 in the RVM reduces cutaneous and muscle withdrawal threshold, and decreasing the expression of NR1 in the RVM increases the muscle withdrawal threshold and prevents the development of hyperalgesia in an animal model of muscle pain. These results suggest that the NR1 subunits in the RVM are critical for modulating NMDA receptor function, which in turn sets the 'tone' of the nervous system's response to noxious stimuli and tissue injury.

Phosphorylation of spinal N-methyl-d-aspartate receptor NR1 subunits by extracellular signal-regulated kinase in dorsal horn neurons and microglia contributes to diabetes-induced painful neuropathy

The N-methyl-d-aspartate receptor (NMDAR) contributes to central sensitization in the spinal cord, a phenomenon which comprises various pathophysiological mechanisms responsible for neuropathic pain-like signs in animal models. NMDAR function is modulated by post-translational modifications including phosphorylation, and this is proposed to underlie its involvement in the production of pain hypersensitivity. As in diabetic patients, streptozotocin-induced diabetic rats exhibit or not somatic mechanical hyperalgesia; these rats were named DH and DNH respectively. At three weeks of diabetes, we present evidence that somatic mechanical hyperalgesia was correlated with an enhanced phosphorylation of the NMDAR NR1 subunit (pNR1) in the rat spinal cord. This increase was not found in normal and DNH rats, suggesting that this regulation was specific to hyperalgesia. Double immunofluorescence studies revealed that the numbers of pNR1-immunoreactive neurons and microglial cells were significantly increased in all laminae (I-II and III-VI) of the dorsal horn from DH animals. Western-blots analysis showed no change in NR1 protein levels, whatever the behavioural and glycemic status of the animals. Chronic intrathecal treatment (5μg/rat/day for 7days) by U0126 and MK801, which blocked MEK (an upstream kinase of extracellular signal-regulated protein kinase: ERK) and the NMDAR respectively, simultaneously suppressed somatic mechanical hyperalgesia developed by diabetic rats and decreased pNR1. These results indicate for the first time that increased expression of pNR1 is regulated by ERK and the NMDAR via a feedforward mechanism in spinal neurons and microglia and represents one mechanism involved in central sensitization and somatic mechanical hyperalgesia after diabetes.

Reactive nitroxidative species and nociceptive processing: determining the roles for nitric oxide, superoxide, and peroxynitrite in pain

Pain is a multidimensional perception and is modified at distinct regions of the neuroaxis. During enhanced pain, neuroplastic changes occur in the spinal and supraspinal nociceptive modulating centers and may result in a hypersensitive state termed central sensitization, which is thought to contribute to chronic pain states. Central sensitization culminates in hyperexcitability of dorsal horn nociceptive neurons resulting in increased nociceptive transmission and pain perception. This state is associated with enhanced nociceptive signaling, spinal glutamate-mediated N-methyl-D: -aspartate receptor activation, neuroimmune activation, nitroxidative stress, and supraspinal descending facilitation. The nitroxidative species considered for their role in nociception and central sensitization include nitric oxide (NO), superoxide ([Formula: see text]), and peroxynitrite (ONOO(-)). Nitroxidative species are implicated during persistent but not normal nociceptive processing. This review examines the role of nitroxidative species in pain through a discussion of their contributions to central sensitization and the underlying mechanisms. Future directions for nitroxidative pain research are also addressed. As more selective pharmacologic agents are developed to target nitroxidative species, the exact role of nitroxidative species in pain states will be better characterized and should offer promising alternatives to available pain management options.

Synaptic Plasticity and Pain: Role of Ionotropic Glutamate Receptors

Pain hypersensitivity that develops after tissue or nerve injury is dependent both on peripheral processes in the affected tissue and on enhanced neuronal responses in the central nervous system, including the dorsal horn of the spinal cord. It has become increasingly clear that strengthening of glutamatergic sensory synapses, such as those established in the dorsal horn by nociceptive thin-caliber primary afferent fibers, is a major contributor to sensitization of neuronal responses that leads to pain hypersensitivity. Here, the authors review recent findings on the roles of ionotropic glutamate receptors in synaptic plasticity in the dorsal horn in relation to acute and persistent pain.

Facilitation of synaptic transmission and pain responses by CGRP in the amygdala of normal rats

Calcitonin gene-related peptide (CGRP) plays an important role in peripheral and central sensitization. CGRP also is a key molecule in the spino-parabrachial-amygdaloid pain pathway. Blockade of CGRP1 receptors in the spinal cord or in the amygdala has antinociceptive effects in different pain models. Here we studied the electrophysiological mechanisms of behavioral effects of CGRP in the amygdala in normal animals without tissue injury.Whole-cell patch-clamp recordings of neurons in the latero-capsular division of the central nucleus of the amygdala (CeLC) in rat brain slices showed that CGRP (100 nM) increased excitatory postsynaptic currents (EPSCs) at the parabrachio-amygdaloid (PB-CeLC) synapse, the exclusive source of CGRP in the amygdala. Consistent with a postsynaptic mechanism of action, CGRP increased amplitude, but not frequency, of miniature EPSCs and did not affect paired-pulse facilitation. CGRP also increased neuronal excitability. CGRP-induced synaptic facilitation was reversed by an NMDA receptor antagonist (AP5, 50 microM) or a PKA inhibitor (KT5720, 1 microM), but not by a PKC inhibitor (GF109203X, 1 microM). Stereotaxic administration of CGRP (10 microM, concentration in microdialysis probe) into the CeLC by microdialysis in awake rats increased audible and ultrasonic vocalizations and decreased hindlimb withdrawal thresholds. Behavioral effects of CGRP were largely blocked by KT5720 (100 microM) but not by GF109203X (100 microM).The results show that CGRP in the amygdala exacerbates nocifensive and affective behavioral responses in normal animals through PKA- and NMDA receptor-dependent postsynaptic facilitation. Thus, increased CGRP levels in the amygdala might trigger pain in the absence of tissue injury.

Regulation of AMPA receptors in spinal nociception

The functional properties of α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptors in different brain regions, such as hippocampus and cerebellum, have been well studied in vitro and in vivo. The AMPA receptors present a unique characteristic in the mechanisms of subunit regulation during LTP (long-term potentiation) and LTD (long-term depression), which are involved in the trafficking, altered composition and phosphorylation of AMPA receptor subunits. Accumulated data have demonstrated that spinal AMPA receptors play a critical role in the mechanism of both acute and persistent pain. However, less is known about the biochemical regulation of AMPA receptor subunits in the spinal cord in response to painful stimuli. Recent studies have shown that some important regulatory processes, such as the trafficking of AMPA receptor subunit, subunit compositional changes, phosphorylation of AMPA receptor subunits, and their interaction with partner proteins may contribute to spinal nociceptive transmission. Of all these regulation processes, the phosphorylation of AMPA receptor subunits is the most important since it may trigger or affect other cellular processes. Therefore, these study results may suggest an effective strategy in developing novel analgesics targeting AMPA receptor subunit regulation that may be useful in treating persistent and chronic pain without unacceptable side effects in the clinics.

The effects of sympathetic outflow on upregulation of vanilloid receptors TRPV(1) in primary afferent neurons evoked by intradermal capsaicin

The vanilloid receptor TRPV(1) is a key nociceptive molecule located in primary afferent nociceptive neurons in dorsal root ganglia (DRG) for initiating neurogenic inflammation and pain. Our recent study demonstrates that up-regulation of TRPV(1) receptors by intradermal injection of capsaicin is modulated by activation of the protein kinase C (PKC) cascade. Neurogenic inflammation and pain resulting from capsaicin injection are sympathetically dependent, responding to norepinephrine, adenosine 5'-triphosphate (ATP) and/or neuropeptide Y released from sympathetic efferents. In a rat model of acute neurogenic inflammatory pain produced by capsaicin injection, we used immunofluorescence and Western blots combined with pharmacology and surgical sympathectomies to analyze whether the capsaicin-evoked up-regulation of TRPV(1) in DRG neurons is affected by sympathetic outflow by way of activating the PKC cascade. Sympathetic denervation reduced significantly the capsaicin-evoked expressions of TRPV(1), calcitonin gene-related peptide and/or phosphorylated PKC and their co-expression. These reductions could be restored by exogenous pretreatment with an analog of ATP, alpha,beta-methylene ATP. Inhibition of PKC with chelerythrine chloride prevented the ATP effect. Consistent results were obtained from experiments in which capsaicin-evoked changes in cutaneous inflammation (vasodilation and edema) were examined after sympathetic denervation, and the effects of the above pharmacological manipulations were evaluated. Our findings suggest that the capsaicin-evoked up-regulation of TRPV(1) receptors in DRG neurons is modulated sympathetically by the action of ATP released from sympathetic efferents to activate the PKC cascade. Thus, this study proposes a potential new mechanism of sympathetic modulation of neurogenic inflammation.

A role of periaqueductal grey NR2B-containing NMDA receptor in mediating persistent inflammatory pain

The midbrain periaqueductal grey (PAG) is a structure known for its roles in pain transmission and modulation. Noxious stimuli potentiate the glutamate synaptic transmission and enhance glutamate NMDA receptor expression in the PAG. However, little is known about roles of NMDA receptor subunits in the PAG in processing the persistent inflammatory pain. The present study was undertaken to investigate NR2A- and NR2B-containing NMDA receptors in the PAG and their modulation to the peripheral painful inflammation. Noxious stimuli induced by hind-paw injection of complete Freund's adjuvant (CFA) caused up-regulation of NR2B-containing NMDA receptors in the PAG, while NR2A-containing NMDA receptors were not altered. Whole-cell patch-clamp recordings revealed that NMDA receptor mediated mEPSCs were increased significantly in the PAG synapse during the chronic phases of inflammatory pain in mice. PAG local infusion of Ro 25-6981, an NR2B antagonist, notably prolonged the paw withdrawal latency to thermal radian heat stimuli bilaterally in rats. Hyperoside (Hyp), one of the flavonoids compound isolated from Rhododendron ponticum L., significantly reversed up-regulation of NR2B-containing NMDA receptors in the PAG and exhibited analgesic activities against persistent inflammatory stimuli in mice. Our findings provide strong evidence that up-regulation of NR2B-containing NMDA receptors in the PAG involves in the modulation to the peripheral persistent inflammatory pain.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Intra-articular magnesium sulfate (MgSO4) reduces experimental osteoarthritis and nociception: association with attenuation of N-methyl-D-aspartate (NMDA) receptor subunit 1 phosphorylation and apoptosis in rat chondrocytes

OBJECTIVE

To study the effects of intra-articular injection of magnesium sulfate (MgSO(4)) on the development of osteoarthritis (OA) and to examine concomitant changes in the nociceptive behavior of rats.

METHODS

OA was induced in Wistar rats with intra-articular injection of collagenase (500 U) in the right knee; the left knee was left untreated. In the OA+MgSO(4) group (n=7), the treated knee was injected with 500-microg (0.1-ml) MgSO(4) twice a week for 5 consecutive weeks starting at 1 week after collagenase injection; in the OA group (n=7), the same knee was injected with the same amount of physiological normal saline. In the MgSO(4) group (n=6), naïve rats received only MgSO(4) injections; in the control group (n=6), naïve rats received only physiological normal saline injections. Nociceptive behavior (mechanical allodynia and thermal hyperalgesia) on OA development was measured before and at 1, 2, 4, 6, and 8 weeks after collagenase injection, following which the animals were sacrificed. Gross morphology and histopathology were examined in the femoral condyles, tibial plateau, and synovia. Immunohistochemical analysis was performed to examine the effect of MgSO(4) on N-methyl-D-aspartate (NMDA) receptor subunit 1 phosphorylation (p-NR1) and apoptosis in the articular cartilage chondrocytes.

RESULTS

OA rats receiving intra-articular MgSO(4) injections showed a significantly lower degree of cartilage degeneration than the rats receiving saline injections. MgSO(4) treatment also suppressed synovitis. Mechanical allodynia and thermal hyperalgesia showed significant improvement in the OA+MgSO(4) group as compared to the OA group. Moreover, MgSO(4) attenuated p-NR1 and chondrocyte apoptosis in OA-affected cartilage.

CONCLUSIONS

Our results indicate that local intra-articular administration of MgSO(4) following collagenase injection in an experimental rat OA model (1) modulates chondrocyte metabolism through inhibition of cell NMDA receptor phosphorylation and apoptosis, (2) attenuates the development of OA, and (3) concomitantly reduces nociception.

Ionotropic glutamate receptors in spinal nociceptive processing

Glutamate is the predominant excitatory transmitter used by primary afferent synapses and intrinsic neurons in the spinal cord dorsal horn. Accordingly, ionotropic glutamate receptors mediate basal spinal transmission of sensory, including nociceptive, information that is relayed to supraspinal centers. However, it has become gradually more evident that these receptors are also crucially involved in short- and long-term plasticity of spinal nociceptive transmission, and that such plasticity have an important role in the pain hypersensitivity that may result from tissue or nerve injury. This review will cover recent findings on pre- and postsynaptic regulation of synaptic function by ionotropic glutamate receptors in the dorsal horn and how such mechanisms contribute to acute and chronic pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

The role of TRPV1 receptors in pain evoked by noxious thermal and chemical stimuli

Transient receptor potential receptors (TRP) on primary afferent neurons respond to noxious and/or thermal stimuli. TRPV1 receptors can be activated by noxious heat, acid, capsaicin and resiniferatoxin, leading to burning pain or itch mediated by discharges in C polymodal and Adelta mechano-heat nociceptors and in central neurons, including spinothalamic tract (STT) cells. Central nociceptive transmission involves both non-NMDA and NMDA receptors, and inhibitory interneurons as well as projection neurons contribute to the neural interactions. Behavioral consequences of intradermal injection of capsaicin include pain, as well as primary and secondary hyperalgesia and allodynia. Primary hyperalgesia depends on sensitization of peripheral nociceptors, whereas, secondary hyperalgesia and allodynia result from sensitization of central nociceptive neurons, such as STT cells. Central sensitization is associated with enhanced responses to excitatory amino acids and decreased responses to inhibitory amino acids. The mechanism of the increase in responses to excitatory amino acids includes phosphorylation of NR1 subunits of NMDA receptors and GluR1 subunits of AMPA receptors. Central sensitization depends on activation of several protein kinases and other enzymes, such as nitric oxide synthase. This process is regulated by protein phosphatases. Central sensitization can be regarded as a spinal cord form of long-term potentiation.

Enhanced pre-synaptic glutamate release in deep-dorsal horn contributes to calcium channel alpha-2-delta-1 protein-mediated spinal sensitization and behavioral hypersensitivity

Nerve injury-induced expression of the spinal calcium channel alpha-2-delta-1 subunit (Ca_vα₂δ₁) has been shown to mediate behavioral hypersensitivity through a yet identified mechanism. We examined if this neuroplasticity modulates behavioral hypersensitivity by regulating spinal glutamatergic neurotransmission in injury-free transgenic mice overexpressing the Ca_vα₂δ₁ proteins in neuronal tissues. The transgenic mice exhibited hypersensitivity to mechanical stimulation (allodynia) similar to the spinal nerve ligation injury model. Intrathecally delivered antagonists for N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxyl-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptors, but not for the metabotropic glutamate receptors, caused a dose-dependent allodynia reversal in the transgenic mice without changing the behavioral sensitivity in wild-type mice. This suggests that elevated spinal Ca_vα₂δ₁ mediates allodynia through a pathway involving activation of selective glutamate receptors. To determine if this is mediated by enhanced spinal neuronal excitability or pre-synaptic glutamate release in deep-dorsal horn, we examined wide-dynamic-range (WDR) neuron excitability with extracellular recording and glutamate-mediated excitatory postsynaptic currents with whole-cell patch recording in deep-dorsal horn of the Ca_vα₂δ₁ transgenic mice. Our data indicated that overexpression of Ca_vα₂δ₁ in neuronal tissues led to increased frequency, but not amplitude, of miniature excitatory post synaptic currents mediated mainly by AMPA/kainate receptors at physiological membrane potentials, and also by NMDA receptors upon depolarization, without changing the excitability of WDR neurons to high intensity stimulation. Together, these findings support a mechanism of Ca_vα₂δ₁-mediated spinal sensitization in which elevated Ca_vα₂δ₁ causes increased pre-synaptic glutamate release that leads to reduced excitation thresholds of post-synaptic dorsal horn neurons to innocuous stimuli. This spinal sensitization mechanism may mediate at least partially the neuropathic pain states derived from increased pre-synaptic Ca_vα₂δ₁ expression.

Serotonin depletion increases nociception-evoked trigeminal NMDA receptor phosphorylation

OBJECTIVE

To investigate the effect of serotonin depletion on phosphorylation and expression of NR1 subunit of N-methyl-D-aspartate (NMDA) receptor in trigeminal nucleus caudalis (TNC), and on trigeminal nociception evoked by dural inflammation.

BACKGROUND

Migraine is associated with low serotonin condition and an increased neuronal excitability. NMDA receptor is implicated in central plasticity change that leads to neural sensitization. Alteration in NMDA receptor phosphorylation or expression in TNC may be responsible for increased trigeminal nociception in serotonin-depleted state.

METHODS

Adult male Wistar rats were separated into normal and low serotonin groups. Serotonin was depleted by intraperitoneal injection with para-chlorophenylalanine 3 days before the experiment. Trigeminal nociception was induced by applying inflammatory soup on exposed dura. Two hours after induction, phosphorylated NR1, NR1, and Fos expressions were studied in TNC by immunohistochemistry.

RESULTS

Dural application of inflammatory soup led to the activation of trigeminal nociceptive system as well as the phosphorylation of NR1, which were further enhanced in the low serotonin condition. There was a strong relationship between NR1 phosphorylation and trigeminal nociception. However, neither meningeal inflammation nor serotonin depletion altered NR1 expression.

CONCLUSIONS

Low serotonin condition facilitates dural inflammation-induced NR1 phosphorylation and trigeminal nociception. It is suggested that the mechanism of nociceptive facilitation in serotonin-depleted state may involve the increase in NR1 phosphorylation rather than the upregulation of NR1 subunit of NMDA receptor.

Role of interleukin-1beta during pain and inflammation

The cytokine cascade in pain and inflammatory processes is a tremendously complex system, involving glial, immune, and neuronal cell interactions. IL-1beta is a pro-inflammatory cytokine that has been implicated in pain, inflammation and autoimmune conditions. This review will focus on studies that shed light on the critical role of IL-1beta in various pain states, including the role of the intracellular complex, the inflammasome, which regulates IL-1beta production. Evidence will be presented demonstrating the importance of IL-1beta in both the induction of pain and in the maintenance of pain in chronic states, such as after nerve injury. Additionally, the involvement of IL-1beta as a key mediator in the interaction between glia and neurons in pain states will be discussed. Taken together, the evidence presented in the current review showing the importance of IL-1beta in animal and human pain states, suggests that blockade of IL-1beta be considered as a therapeutic opportunity.