Search

This study aims to determine whether caveolin-1 (Cav-1) participates in the process of diabetic neuropathic pain by directly regulating the expression of toll-like receptor 4 (TLR4) and the subsequent phosphorylation of N-methyl-D-aspartate receptor 2B subunit (NR2B) in the spinal cord. Male Sprague-Dawley rats (120-150 g) were continuously fed with high-fat and high-sugar diet for 8 weeks, and received a single low-dose of intraperitoneal streptozocin injection in preparation for the type-II diabetes model. Then, these rats were divided into five groups according to the level of blood glucose, and the mechanical withdrawal threshold and thermal withdrawal latency values. The pain thresholds were measured at 3, 7, and 14 days after animal grouping. Then, eight rats were randomly chosen from each group and killed. Lumbar segments 4-6 of the spinal cord were removed for western blot analysis and immunofluorescence assay. Cav-1 was persistently upregulated in the spinal cord after diabetic neuropathic pain in rats. The downregulation of Cav-1 through the subcutaneous injection of Cav-1 inhibitor daidzein ameliorated the pain hypersensitivity and TLR4 expression in the spinal cord in diabetic neuropathic pain (DNP) rats. Furthermore, it was found that Cav-1 directly bound with TLR4, and the subsequent phosphorylation of NR2B in the spinal cord contributed to the modulation of DNP. These findings suggest that Cav-1 plays a vital role in DNP processing at least in part by directly regulating the expression of TLR4, and through the subsequent phosphorylation of NR2B in the spinal cord., (© 2019 Wiley Periodicals, Inc.)

Sickle cell disease (SCD) is one of the most common severe genetic diseases around the world. A majority of SCD patients experience intense pain, leading to hospitalization, and poor quality of life. Opioids form the bedrock of pain management, but their long-term use is associated with severe side effects including hyperalgesia, tolerance and addiction. Recently, excellent research has shown some new potential mechanisms that underlie SCD-associated pain. This review focused on how transient receptor potential vanilloid 1, endothelin-1/endothelin type A receptor, and cannabinoid receptors contributed to the pathophysiology of SCD-associated pain. Understanding these mechanisms may open a new avenue in managing SCD-associated pain and improving quality of life for SCD patients., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Earlier studies on genetically modified mice indicated that plasma membrane calcium ATPase 2 (PMCA2), a calcium extrusion pump, plays a novel and sex-dependent role in mechanical pain responses: female, but not male, PMCA2 +/- mice manifest increased mechanical pain compared to female PMCA2 +/+ mice. The goal of the present studies was to determine the contribution of ovarian steroids to the genotype- and sex-dependent manifestation of mechanical pain in PMCA2 +/+ versus PMCA2 +/- mice. Ovariectomy increased mechanical pain sensitivity and 17β-estradiol (E2) replacement restored it to basal levels in PMCA2 +/+ mice, but not in PMCA2 +/- littermates. Intrathecal administration of an estrogen receptor alpha (ERα) agonist induced ERα signaling in the dorsal horn (DH) of female PMCA2 +/+ mice, but was ineffective in PMCA2 +/- mice. In male PMCA2 +/+ and PMCA2 +/- mice, E2 treatment following orchidectomy did not recapitulate the genotype-dependent differential pain responses observed in females and the agonist did not elicit ERα signaling. These findings establish a novel, female-specific link between PMCA2, ERα and mechanical pain. It is postulated that PMCA2 is essential for adequate ERα signaling in the female DH and that impaired ERα signaling in the female PMCA2 +/- mice hinders the analgesic effects of E2 leading to increased sensitivity to mechanical stimuli.

Sickle cell disease is associated with acute painful episodes and chronic intractable pain. Endothelin-1, a known pain inducer, is elevated in the blood plasma of both sickle cell patients and mouse models of sickle cell disease. We show here that the levels of endothelin-1 and its endothelin type A receptor are increased in the dorsal root ganglia of a mouse model of sickle cell disease. Pharmacologic inhibition or neuron-specific knockdown of endothelin type A receptors in primary sensory neurons of dorsal root ganglia alleviated basal and post-hypoxia evoked pain hypersensitivities in sickle cell mice. Mechanistically, endothelin type A receptors contribute to sickle cell disease-associated pain likely through the activation of NF-κB-induced Nav1.8 channel upregulation in primary sensory neurons of sickle cell mice. Our findings suggest that endothelin type A receptor is a potential target for the management of sickle cell disease-associated pain, although this expectation needs to be further verified in clinical settings., (Copyright© 2018 Ferrata Storti Foundation.)

The gate control theory (GCT) of pain proposes that pain- and touch-sensing neurons antagonize each other through spinal cord dorsal horn (DH) gating neurons. However, the exact neural circuits underlying the GCT remain largely elusive. Here, we identified a new population of deep layer DH (dDH) inhibitory interneurons that express the receptor tyrosine kinase Ret neonatally. These early RET+ dDH neurons receive excitatory as well as polysynaptic inhibitory inputs from touch- and/or pain-sensing afferents. In addition, they negatively regulate DH pain and touch pathways through both pre- and postsynaptic inhibition. Finally, specific ablation of early RET+ dDH neurons increases basal and chronic pain, whereas their acute activation reduces basal pain perception and relieves inflammatory and neuropathic pain. Taken together, our findings uncover a novel spinal circuit that mediates crosstalk between touch and pain pathways and suggest that some early RET+ dDH neurons could function as pain "gating" neurons., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Unlabelled: Persistent inflammation promotes internalization of synaptic GluR2-containing, Ca(2+)-impermeable AMPA receptors (AMPARs) and insertion of GluR1-containing, Ca(2+)-permeable AMPARs at extrasynaptic sites in dorsal horn neurons. Previously we have shown that internalization of synaptic GluR2-containing AMPARs requires activation of spinal cord protein kinase C alpha (PKCα), but molecular mechanisms that underlie altered trafficking of extrasynaptic AMPARs are unclear. Here, using antisense (AS) oligodeoxynucleotides (ODN) that specifically knock down PKCα, we found that a decrease in dorsal horn PKCα expression prevents complete Freund's adjuvant (CFA)-induced increase in functional expression of extrasynaptic Ca(2+)-permeable AMPARs in substantia gelatinosa (SG) neurons of the rat spinal cord. Augmented AMPA-induced currents and associated [Ca(2+)](i) transients were abolished, and the current rectification 1 day post-CFA was reversed. These changes were observed specifically in SG neurons characterized by intrinsic tonic firing properties, but not in those that exhibited strong adaptation. Finally, dorsal horn PKCα knockdown produced an antinociceptive effect on CFA-induced thermal and mechanical hypersensitivity during the maintenance period of inflammatory pain, indicating a role for PKCα in persistent inflammatory pain maintenance. Our results indicate that inflammation-induced trafficking of extrasynaptic Ca(2+)-permeable AMPARs in tonically firing SG neurons depends on PKCα, and that this PKCα-dependent trafficking may contribute to persistent inflammatory pain maintenance., Perspective: This study shows that PKCα knockdown blocks inflammation-induced upregulation of extrasynaptic Ca(2+)-permeable AMPARs in dorsal horn neurons and produces an antinociceptive effect during the maintenance period of inflammatory pain. These findings have potential implications for use of PKCα gene-silencing therapy to prevent and/or treat persistent inflammatory pain., (Copyright © 2013 American Pain Society. Published by Elsevier Inc. All rights reserved.)

Background: Spinal cord glutamate transporters clear synaptically released glutamate and maintain normal sensory transmission. However, their ultrastructural localization is unknown. Moreover, whether and how they participate in inflammatory pain has not been carefully studied., Methods: Immunogold labeling with electron microscopy was carried out to characterize synaptic and nonsynaptic localization of glutamate transporters in the superficial dorsal horn. Their expression and uptake activity after formalin- and complete Freund's adjuvant (CFA)-induced inflammation were evaluated by Western blot analysis and glutamate uptake assay. Effects of intrathecal glutamate transporter activator (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline and inhibitors (DL-threo-β-benzyloxyaspartate [TBOA], dihydrokainate, and DL-threo-β-hydroxyaspartate), or TBOA plus group III metabotropic glutamate receptor antagonist (RS)-α-methylserine-O-phosphate, on formalin- and CFA-induced inflammatory pain were examined., Results: In the superficial dorsal horn, excitatory amino acid carrier 1 is localized in presynaptic membrane, postsynaptic membrane, and axonal and dendritic membranes at nonsynaptic sites, whereas glutamate transporter-1 and glutamate/aspartate transporter are prominent in glial membranes. Although expression of these three spinal glutamate transporters was not altered 1 h after formalin injection or 6 h after CFA injection, glutamate uptake activity was decreased at these time points. Intrathecal (R)-(-)-5-methyl-1-nicotinoyl-2-pyrazoline had no effect on formalin-induced pain behaviors. In contrast, intrathecal TBOA, dihydrokainate, and DL-threo-β-hydroxyaspartate reduced formalin-evoked pain behaviors in the second phase. Intrathecal TBOA also attenuated CFA-induced thermal hyperalgesia at 6 h after CFA injection. The antinociceptive effects of TBOA were blocked by coadministration of (RS)-α-methylserine-O-phosphate., Conclusion: Our findings suggest that spinal glutamate transporter inhibition relieves inflammatory pain through activation of inhibitory presynaptic group III metabotropic glutamate receptors.

Mammalian target of rapamycin (mTOR) controls protein translation and has an important role in the mechanism of pain hypersensitivity under persistent pain conditions. However, its expression and localization in pain-related regions of the nervous system is not completely understood. Here, we examined the expression and distribution of mTOR, eukaryotic initiation factor 4E-binding protein1/2 (4E-BP1/2), p70 ribosomal S6 protein kinase (p70S6K), and their phosphorylated (active) counterparts in two major pain-related regions, the dorsal root ganglion (DRG) and spinal cord dorsal horn. Reverse transcriptase-polymerase chain reaction showed that mTOR, 4E-BP1, and p70S6K mRNA are expressed in the DRG and dorsal horn. Western blot analysis further confirmed the existence of their protein products in these two regions, but expression of their phosphorylated counterparts was very low in dorsal horn and was not detected in the DRG. Immunohistochemistry revealed mTOR and p70S6K in the DRG neurons. Quantitative analysis showed that approximately 26.1% (+/- 3.2%) of DRG neurons were positive for mTOR and 19.1% (+/- 1.9%) were positive for p70S6K. Most of these neurons were small-less than 600 microm(2) in cross-sectional area-and some co-labeled with substance P or isolectin B4. Surprisingly, 4E-BP1 was observed only in the DRG satellite glial cells. In the dorsal horn, mTOR, p70S6K, and 4E-BP1 were detected in neurons, but not in astrocytes or microglia. They were distributed in the whole dorsal horn, especially in the superficial dorsal horn. Immunostaining for their phosphorylated counterparts was very low or undetectable in DRG and dorsal horn. Behavioral study showed that intrathecal mTOR inhibitor, rapamycin, did not affect acute nocicepetive transmission. The results indicate that although mTOR, p70S6K, and 4E-BP1 are highly expressed in the DRG and dorsal horn, their activate forms are very low in both regions under normal conditions. Our findings support the view that mTOR and its downstream effectors do not play a key role in acute pain., (Copyright 2010 Elsevier B.V. All rights reserved.)

Activation of synaptic N-methyl-D-aspartic acid receptor and its intracellular downstream signals in dorsal horn neurons of spinal cord contribute to central sensitization, a mechanism that underlies the development and maintenance of pain hypersensitivity in persistent pain. However, the molecular process of this event is not understood completely. Recently, new studies suggest that peripheral inflammatory insults drive changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit trafficking via N-methyl-D-aspartic acid receptor-triggered activation of protein kinases in dorsal horn and raise the possibility that such changes might contribute to central sensitization in persistent pain. This review presents current evidence regarding the changes that occur in the trafficking of dorsal horn alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits GluR1 and GluR2 under persistent inflammatory pain conditions and discusses the potential mechanisms by which such changes participate in the development and maintenance of inflammatory pain.

Spinal cord alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) mediate acute spinal processing of nociceptive and non-nociceptive information, but whether and how their activation contributes to the central sensitization that underlies persistent inflammatory pain are still unclear. Here, we examined the role of spinal AMPARs in the development and maintenance of complete Freund's adjuvant (CFA)-induced persistent inflammatory pain. Intrathecal application of two selective non-competitive AMPAR antagonists, CFM-2 (25 and 50 microg) and GYKI 52466 (50 microg), significantly attenuated mechanical and thermal hypersensitivities on the ipsilateral hind paw at 2 and 24 h post-CFA injection. Neither CFM-2 nor GYKI 52466 affected the contralateral basal responses to thermal and mechanical stimuli. Locomotor activity was not altered in any of the drug-treated animals. CFA-induced inflammation did not change total expression or distribution of AMPAR subunits GluR1 and GluR2 in dorsal horn but did alter their subcellular distribution. The amount of GluR2 was markedly increased in the crude cytosolic fraction and decreased in the crude membrane fraction from the ipsilateral L4-5 dorsal horn at 24 h (but not at 2 h) post-CFA injection. Conversely, the level of GluR1 was significantly decreased in the crude cytosolic fraction and increased in the crude membrane fraction from the ipsilateral L4-5 dorsal horn at 24 h (but not at 2 h) post-CFA injection. These findings suggest that spinal AMPARs might participate in the central spinal mechanism of persistent inflammatory pain.

Unlabelled: In certain patients with neuropathic pain, the pain is dependent on activity in the sympathetic nervous system. To investigate whether the spared nerve injury model (SNI) produced by injury to the tibial and common peroneal nerves and leaving the sural nerve intact is a model for sympathetically maintained pain, we measured the effects of surgical sympathectomy on the resulting mechanical allodynia, mechanical hyperalgesia, and cold allodynia. Decreases of paw withdrawal thresholds to von Frey filament stimuli and increases in duration of paw withdrawal to pinprick or acetone stimuli were observed in the ipsilateral paw after SNI, compared with their pre-SNI baselines. Compared with sham surgery, surgical lumbar sympathectomy had no effect on the mechanical allodynia and mechanical hyperalgesia induced by SNI. However, the sympathectomy significantly attenuated the cold allodynia induced by SNI. These results suggest that the allodynia and hyperalgesia to mechanical stimuli in the SNI model is not sympathetically maintained. However, the sympathetic nervous system may be involved, in part, in the mechanisms of cold allodynia in the SNI model., Perspective: The results of our study suggest that the SNI model is not an appropriate model of sympathetically maintained mechanical allodynia and hyperalgesia but may be useful to study the mechanisms of cold allodynia associated with sympathetically maintained pain states.

Glutamate is a neurotransmitter critical for spinal excitatory synaptic transmission and for generation and maintenance of spinal states of pain hypersensitivity via activation of glutamate receptors. Understanding the regulation of synaptically and non-synaptically released glutamate associated with pathological pain is important in exploring novel molecular mechanisms and developing therapeutic strategies of pathological pain. The glutamate transporter system is the primary mechanism for the inactivation of synaptically released glutamate and the maintenance of glutamate homeostasis. Recent studies demonstrated that spinal glutamate transporter inhibition relieved pathological pain, suggesting that the spinal glutamate transporter might serve as a therapeutic target for treatment of pathological pain. However, the exact function of glutamate transporter in pathological pain is not completely understood. This report will review the evidence for the role of the spinal glutamate transporter during normal sensory transmission and pathological pain conditions and discuss potential mechanisms by which spinal glutamate transporter is involved in pathological pain.

Glutamate is a major excitatory neurotransmitter in primary afferent terminals and is critical for normal spinal excitatory synaptic transmission. However, little is known about the regulation of synaptically released glutamate in the spinal cord under physiologic conditions. The sodium-dependent, high-affinity glutamate transporters are the primary mechanism for the clearance of synaptically released glutamate. In the present study, we found that intrathecal injection of glutamate transporter blockers DL-threo-beta-benzyloxyaspartate (TBOA) and dihydrokainate produced significant and dose-dependent spontaneous nociceptive behaviors, such as licking, shaking, and caudally directed biting, phenomena similar to the behaviors caused by intrathecal glutamate receptor agonists. Intrathecal TBOA also led to remarkable hypersensitivity in response to thermal and mechanical stimuli. These behavioral responses could be significantly blocked by intrathecal injection of the NMDA receptor antagonists MK-801 and AP-5, the non-NMDA receptor antagonist CNQX or the nitric oxide synthase inhibitor L-NAME. In vivo microdialysis analysis showed short-term elevation of extracellular glutamate concentration in the spinal cord after intrathecal injection of TBOA. Furthermore, topical application of TBOA on the dorsal surface of the spinal cord resulted in a significant elevation of extracellular glutamate concentration demonstrated by in vivo glutamate voltametry. The present study indicates that defective spinal glutamate uptake caused by inhibition of glutamate transporters leads to excessive glutamate accumulation in the spinal cord. The latter results in persistent over-activation of synaptic glutamate receptors, producing spontaneous nociceptive behaviors and sensory hypersensitivity. Our results suggest that glutamate uptake through spinal glutamate transporters is critical for maintaining normal sensory transmission under physiologic conditions.

Chronic pain, particularly neuropathic pain, is notoriously difficult to treat. NMDA receptor antagonists are effective in reducing pain hypersensitivity in animal models and clinical settings but are associated with an unacceptable level of side-effects. Recent studies of the role of a family of synaptic membrane-associated guanylate kinase proteins in chronic pain provide new insights into central mechanisms of chronic pain that could result in new biochemical targets for its treatment.

Intraplantar formalin injection is widely used as an experimental model of tonic pain. We investigated the role of endogenous micro-opioid receptor mechanisms in formalin-induced nocifensive behavior in mice. The flinching response induced by formalin (2%, 20 microl) was studied in mice with normal (wild type, n = 8) and absent (homozygous micro-opioid receptor knockout, n = 8) micro-opioid receptor levels. The flinch responses were counted every 5 min for 60 min post-formalin injection. Lumbar spinal cord (L4, 5) was harvested 2 h post-formalin injection to examine c-Fos expression using immunohistochemistry. The effects of naloxone (5 mg/kg, sc) administered 30 min before the intraplantar formalin injection on the flinching response of wild-type mice (n = 7) were also recorded. The second-phase formalin response (10-60 min after formalin) was higher in homozygous micro-opioid receptor knockout mice compared to the wild-type mice (P < 0.01). Naloxone administration in wild-type mice before formalin injection resulted in pain behavior similar to that observed in homozygous micro-opioid receptor knockout mice (P > 0.05). The c-Fos expression induced by formalin injection in the knockout mice was not different from that observed in wild-type mice. Our results suggest that the endogenous micro-opioid system is activated by intraplantar formalin injection and exerts a tonic inhibitory effect on the pain behavior. These results suggest an important modulatory role of endogenous micro-opioid receptor mechanisms in tonic pain states.

PSD-93/chapsin-110 is a neuronal PDZ domain-containing protein that binds to and clusters the N-methyl-D-aspartate receptor (NMDAR) at synapses in the central nervous system. It also assembles a specific set of signaling proteins around the NMDAR and mediates downstream signaling by the NMDAR. Thus, PSD-93/chapsin-110 might be involved in many physiological and pathophysiological actions triggered via the activation of the NMDAR. In the current study, we report that abundant PSD-93/chapsin-110 protein was detected in rat spinal cord, particularly in the superficial dorsal horn. The rats injected intrathecally with PSD-93/chapsin-110 antisense oligodeoxynucleotide every 24 h for 4 days displayed not only a remarkable decrease in spinal cord PSD-93/chapsin-110 expression but also a significant reduction in the paw withdrawal responses to thermal and mechanical stimuli during complete Freund's adjuvant-induced inflammatory pain and peripheral nerve injury-induced neuropathic pain. In contrast, the rats injected intrathecally with PSD-93/chapsin-110 missense oligodeoxynucleotide did not exhibit these changes. We also found that pretreatment with PSD-93/chapsin-110 antisense oligodeoxynucleotide did not change the locomotor activity or the responses to acute noxious thermal and mechanical stimuli in intact rats. The present results indicate that the deficiency of spinal cord PSD-93/chapsin-110 protein significantly attenuates thermal and mechanical hyperalgesia in complete Freund's adjuvant- or peripheral nerve injury-induced chronic pain. This suggests that spinal cord PSD-93/chapsin-110 might be involved in the central mechanism of chronic pain. Our work might provide a new target for the therapy of chronic pain.

Modification of synaptic NMDA receptor (NMDAR) expression influences NMDAR-mediated synaptic function and associated persistent pain. NMDARs directly bind to a family of membrane-associated guanylate kinases (MAGUKs) that regulate surface and synaptic NMDAR trafficking in the CNS. We report here that postsynaptic density-93 protein (PSD-93), a postsynaptic neuronal MAGUK, is expressed abundantly in spinal dorsal horn and forebrain, where it colocalizes and interacts with NMDAR subunits NR2A and NR2B. Targeted disruption of the PSD-93 gene reduces not only surface NR2A and NR2B expression but also NMDAR-mediated excitatory postsynaptic currents and potentials, without affecting surface AMPA receptor expression or its synaptic function, in the regions mentioned above. Furthermore, mice lacking PSD-93 exhibit blunted NMDAR-dependent persistent pain induced by peripheral nerve injury or injection of Complete Freund's Adjuvant, although they display intact nociceptive responsiveness to acute pain. PSD-93 appears to be important for NMDAR synaptic targeting and function and to be a potential biochemical target for the treatment of persistent pain.

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

Abstract: Group I metabotropic glutamate receptors (group I mGluRs) have been implicated in several central nervous system diseases including chronic pain. It is known that activation of group I mGluRs results in the production of inositol triphosphate (IP3) and diacylglycerol that leads to activation of extracellular signal-regulated kinases (ERKs) and an increase in neuronal excitability, but how group I mGluRs mediate this process remains unclear. We previously reported that Orai1 is responsible for store-operated calcium entry and plays a key role in central sensitization. However, how Orai1 is activated under physiological conditions is unknown. Here, we tested the hypothesis that group I mGluRs recruit Orai1 as part of its downstream signaling pathway in dorsal horn neurons. We demonstrate that neurotransmitter glutamate induces STIM1 puncta formation, which is not mediated by N-Methyl-D-aspartate (NMDA) or α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. Glutamate-induced Ca2+ entry in the presence of NMDA or AMPA receptor antagonists is eliminated in Orai1-deficient neurons. Dihydroxyphenylglycine (DHPG) (an agonist of group I mGluRs)-induced Ca2+ entry is abolished by Orai1 deficiency, but not affected by knocking down of transient receptor potential cation channel 1 (TRPC1) or TRPC3. Dihydroxyphenylglycine-induced activation of ERKs and modulation of neuronal excitability are abolished in cultured Orai1-deficient neurons. Moreover, DHPG-induced nociceptive behavior is markedly reduced in Orai1-deficient mice. Our findings reveal previously unknown functional coupling between Orai1 and group I mGluRs and shed light on the mechanism underlying group I mGluRs-mediated neuronal plasticity. [ABSTRACT FROM AUTHOR]

Previous studies demonstrated that Loperamide, originally known as an anti-diarrheal drug, is a promising analgesic agent primarily targeting mu-opioid receptors. However some evidences suggested that non-opioid mechanisms may be contributing to its analgesic effect. In the present study, Loperamide was identified as a Nav1.7 blocker in a pilot screen. In HEK293 cells expressing Nav1.7 sodium channels, Loperamide blocked the resting state of Nav1.7 channels (IC 50 = 1.86 ± 0.11 μM) dose-dependently and reversibly. Loperamide produced a 10.4 mV of hyperpolarizing shift for the steady-state inactivation of Nav1.7 channels without apparent effect on the voltage-dependent activation. The drug displayed a mild use- and state-dependent inhibition on Nav1.7 channels, which was removed by the local anesthetic-insensitive construct Nav1.7-F1737A. Inhibition of Nav1.7 at resting state was not altered significantly by the F1737A mutation. Compared to its effects on Nav1.7, Loperamide exhibited higher potency on recombinant Nav1.8 channels in ND7/23 cells (IC 50 = 0.60 ± 0.10 μM) and weaker potency on Nav1.9 channels (3.48 ± 0.33 μM). Notably more pronounced inhibition was observed in the native Nav1.8 channels (0.11 ± 0.08 μM) in DRG neurons. Once mu-opioid receptor was antagonized by Naloxone in DRG neurons, potency of Loperamide on Nav1.8 was identical to that of recombinant Nav1.8 channels. The inhibition on Nav channels may be the main mechanism of Loperamide for pain relief beyond mu-opioid receptor. In the meanwhile, the opioid receptor pathway may also influence the blocking effect of Loperamide on sodium channels, implying a cross-talk between sodium channels and opioid receptors in pain processing. [ABSTRACT FROM AUTHOR]

The development of opioid-induced analgesic tolerance and hyperalgesia is a clinical challenge for managing chronic pain. Adaptive changes in protein translation in the nervous system are thought to promote opioid tolerance and hyperalgesia; however, how opioids drive such changes remains elusive. Here, we report that mammalian target of rapamycin (mTOR), which governs most protein translation, was activated in rat spinal dorsal horn neurons after repeated intrathecal morphine injections. Activation was triggered through μ opioid receptor and mediated by intracellular PI3K/Akt. Spinal mTOR inhibition blocked both induction and maintenance of morphine tolerance and hyperalgesia, without affecting basal pain perception or locomotor functions. These effects were attributed to the attenuation of morphine-induced increases in translation initiation activity, nascent protein synthesis, and expression of some known key tolerance-associated proteins, including neuronal NOS (nNOS), in dorsal horn. Moreover, elevating spinal mTOR activity by knocking down the mTOR-negative regulator TSC2 reduced morphine analgesia, produced pain hypersensitivity, and increased spinal nNOS expression. Our findings implicate the μ opioid receptor-triggered PI3K/Akt/mTOR pathway in promoting morphine-induced spinal protein translation changes and associated morphine tolerance and hyperalgesia. These data suggest that mTOR inhibitors could be explored for prevention and/or reduction of opioid tolerance in chronic pain management. [ABSTRACT FROM AUTHOR]

Abstract: Protein interacting with C kinase 1 (PICK1) is a PDZ-containing protein that binds to AMPA receptor (AMPAR) GluR2 subunit and protein kinase Cα (PKCα) in the central neurons. It functions as a targeting and transport protein, presents the activated form of PKCα to synaptic GluR2, and participates in synaptic AMPAR trafficking in the nervous system. Thus, PICK1 might be involved in many physiological and pathological processes triggered via the activation of AMPARs. We report herein that PICK1 knockout mice display impaired mechanical and thermal pain hypersensitivities during complete Freund’s adjuvant (CFA)-induced inflammatory pain maintenance. Acute transient knockdown of spinal cord PICK1 through intrathecal injection of PICK1 antisense oligodeoxynucleotide had a similar effect. In contrast, knockout and knockdown of spinal cord PICK1 did not affect incision-induced guarding pain behaviors or mechanical or thermal pain hypersensitivities. We also found that PICK1 is highly expressed in dorsal horn, where it interacts with GluR2 and PKCα. Injection of CFA into a hind paw, but not a hind paw incision, increased PKCα-mediated GluR2 phosphorylation at Ser880 and GluR2 internalization in dorsal horn. These increases were absent when spinal cord PICK1 was deficient. Given that dorsal horn PKCα-mediated GluR2 phosphorylation at Ser880 and GluR2 internalization contribute to the maintenance of CFA-induced inflammatory pain, our findings suggest that spinal PICK1 may participate in the maintenance of persistent inflammatory pain, but not in incision-induced post-operative pain, through promoting PKCα-mediated GluR2 phosphorylation and internalization in dorsal horn neurons. [Copyright &y& Elsevier]

Abstract: Nitric oxide (NO) acts as a neurotransmitter or neuromodulator involving in the modulation of thermal and/or inflammatory hyperalgesia. The neuronal nitric oxide synthase (nNOS) is a key enzyme for NO production in normal neuronal tissues, but its functional role in chronic pain remains unclear. The present study combined a genetic strategy with a pharmacologic approach to address the role of nNOS in the central mechanism of complete Freund''s adjuvant (CFA)-induced chronic inflammatory pain. Targeted disruption of the nNOS gene significantly reduced CFA-induced mechanical pain hypersensitivity during the maintenance (but not the development) of inflammatory pain, while it failed to attenuate either development or maintenance of CFA-induced thermal pain hypersensitivity. Intraperitoneal administration of L-NG-nitro-arginine methyl ester (L-NAME), a non-specific NOS inhibitor, blocked CFA-evoked thermal and mechanical pain hypersensitivity at both development (2h) and maintenance (24h) phase in wild type mice, but had no effect in the knockout mice. Furthermore, intrathecal injection of either L-NAME or 7-nitroindazole, a selective nNOS inhibitor, markedly attenuated mechanical pain hypersensitivity at both 2 and 24h after CFA injection. Finally, spinal cord nNOS (but not endothelial NOS or inducible NOS) expression was up-regulated at 24h after CFA injection, occurring mainly in the ipsilateral superficial dorsal horn. Together, these data indicate that spinal cord nNOS may be essential for the maintenance of mechanical pain hypersensitivity and that it may also be sufficient for the development of mechanical pain hypersensitivity and for the development and maintenance of thermal pain hypersensitivity after chronic inflammation. Our findings suggest that spinal cord nNOS might play a critical role in central mechanisms of the development and/or maintenance of chronic inflammatory pain. [Copyright &y& Elsevier]