mGluR5/ERK signaling regulated the phosphorylation and function of glycine receptor α1ins subunit in spinal dorsal horn of mice.

Inhibitory glycinergic transmission in adult spinal cord is primarily mediated by glycine receptors (GlyRs) containing the α1 subunit. Here, we found that α1ins, a longer α1 variant with 8 amino acids inserted into the intracellular large loop (IL) between transmembrane (TM)3 and TM4 domains, was expressed in the dorsal horn of the spinal cord, distributed at inhibitory synapses, and engaged in negative control over nociceptive signal transduction. Activation of metabotropic glutamate receptor 5 (mGluR5) specifically suppressed α1ins-mediated glycinergic transmission and evoked pain sensitization. Extracellular signal-regulated kinase (ERK) was critical for mGluR5 to inhibit α1ins. By binding to a D-docking site created by the 8-amino-acid insert within the TM3-TM4 loop of α1ins, the active ERK catalyzed α1ins phosphorylation at Ser380, which favored α1ins ubiquitination at Lys379 and led to α1ins endocytosis. Disruption of ERK interaction with α1ins blocked Ser380 phosphorylation, potentiated glycinergic synaptic currents, and alleviated inflammatory and neuropathic pain. These data thus unraveled a novel, to our knowledge, mechanism for the activity-dependent regulation of glycinergic neurotransmission.

Spinophilin-Targeted Protein Phosphatase-1 Alleviated Inflammatory Pain by Negative Control of MEK/ERK Signaling in Spinal Cord Dorsal Horn of Rats.

Protein phosphatase-1 (PP1), anchored by regulatory or targeting proteins at excitatory glutamatergic synapses, controls the phosphorylation of postsynaptic substrates and regulates the neurotransmission and plasticity. Here, we found that spinophilin, an actin-binding protein that targets PP1 at postsynaptic density, served as a scaffold for extracellular signal-regulated kinase (ERK) signaling components. Through the C-terminal PDZ domain, spinophilin directly interacted with ERK and its upstream mitogen-activated protein kinase kinase (MEK). PP1, recruited by spinophilin, gained access to and dephosphorylated these kinases, exerting a tonic inhibition of ERK signaling. The removal of PP1 inhibition by disturbing spinophilin/PP1 interaction allowed a restricted activation of MEK/ERK at synapses, which in turn augmented the synaptic transmission specifically mediated by GluN2B subunit-containing N-methyl-d-aspartate subtype of glutamate receptors. We provided evidence that in pain-related spinal cord dorsal horn, the scaffolding function of spinophilin played an important role in the negative control of ERK-dependent and GluN2B-dependent pain sensitization. Expression of wild-type spinophilin produced an effective analgesic action against chronic inflammatory pain induced by complete Freund's adjuvant in rats. SIGNIFICANCE STATEMENT: Extracellular signal-regulated kinase (ERK) relays the signals from multiple transmembrane receptors to a wide range of downstream effectors critical for the regulation of neuronal excitability and plasticity. The strength and duration of ERK signaling is spatiotemporally controlled by protein phosphatases. Sustained activation of ERK has been implicated in a variety of pathological processes. The current study revealed that spinophilin, a well characterized protein phosphatase 1 (PP1) synaptic targeting protein, was able to scaffold mitogen-activated protein kinase kinase (MEK) and ERK for dephosphorylation and inactivation by PP1. The loss of PP1 inhibition, as a result of spinophilin/PP1 dissociation, led to aberrant activation of MEK/ERK signaling, which had important implications for the exaggeration of NMDA receptor-dependent nociceptive synaptic transmission in spinal cord dorsal horn.

Peripheral inflammation activated focal adhesion kinase signaling in spinal dorsal horn of mice.

Focal adhesion kinase (FAK) is one of the nonreceptor protein tyrosine kinases critical for the dynamic regulation of cell adhesion structures. Recent studies have demonstrated that FAK is also localized at excitatory glutamatergic synapses and is involved in long-term modification of synaptic strength. However, whether FAK is engaged in nociceptive processing in the spinal dorsal horn remains unresolved. The current study shows that intraplantar injection of complete Freund's adjuvant (CFA) in mice significantly increases FAK autophosphorylation at Tyr397, indicating a close correlation of FAK activation with inflammatory pain. FAK activation depended on the activity of N-methyl-D-aspartate-subtype glutamate receptor (NMDAR) and metabotropic glutamate receptor (mGluR) because pharmacological inhibition of NMDAR or group I mGluR totally abolished FAK phosphorylation induced by CFA. The active FAK operated to stimulate extracellular signal-regulated kinase1/2 (ERK1/2), which boosted the protein expression of GluN2B subunit-containing NMDAR at the synaptosomal membrane fraction. Inhibition of FAK activity by spinal expression of a kinase-dead FAK(Y397F) mutant repressed ERK1/2 hyperactivity and reduced the synaptic concentration of NMDAR in CFA-injected mice. Electrophysiological recording demonstrated that intracellular loading of specific anti-FAK antibody significantly reduced the amplitudes of NMDAR-mediated excitatory postsynaptic currents on lamina II neurons from inflamed mice but not from naive mice. Behavioral tests showed that spinal expression of FAK(Y397F) generated a long-lasting alleviation of CFA-induced mechanical allodynia and thermal hyperalgesia. These data indicate that FAK might exaggerate NMDAR-mediated synaptic transmission in the spinal dorsal horn to sensitize nociceptive behaviors.

GABAergic inhibition regulated pain sensitization through STEP61 signaling in spinal dorsal horn of mice.

BACKGROUND: The reduction of γ-aminobutyric acid (GABA) type A receptor-mediated inhibition has long been implicated in spinal sensitization of nociceptive responses. However, it is largely unknown which signaling cascades in spinal dorsal horn neurons are initiated by the reduced inhibition to trigger pain hypersensitivity. METHODS: GABAergic inhibition was manipulated by intrathecal application of GABA type A receptor antagonist bicuculline in intact mice or by GABA type A receptor agonist muscimol in complete Freund's adjuvant-injected mice. Immunoblotting, coimmunoprecipitation, immunohistochemistry, and behavioral tests were used to explore the signaling pathways downstream of the altered GABAergic tone. RESULTS: The study data revealed that the 61-kD isoform of striatal-enriched protein phosphatase (STEP61) was a key molecule that relayed the signals from GABAergic neurotransmission. The authors found that STEP61 was highly expressed in dorsal horn neurons. Under physiological conditions, STEP61 tonically interacted with and negatively controlled the activities of extracellular signal-regulated kinase and Src-family protein tyrosine kinases member Fyn, two critical kinases involved in spinal sensitization. Once GABAergic inhibition was impaired, STEP61 interaction with its substrates was substantially disturbed, allowing for activation of extracellular signal-regulated kinase and Fyn (n = 4 to 6). The hyperactivities of extracellular signal-regulated kinase and Fyn, along with STEP61 dysregulation, caused the tyrosine phosphorylation and synaptic accumulation of GluN2B subunit-containing N-methyl-D-aspartate subtype of glutamate receptors (n = 6), leading to GluN2B receptor-dependent pain hypersensitivity. Overexpression of wild-type STEP61 to resume its enzymatic activity significantly blocked the mechanical allodynia evoked by bicuculline and more importantly, alleviated chronic inflammatory pain (n = 6 in each group). CONCLUSION: These data identified STEP61 as a key intermediary for GABAergic inhibition to regulate pain sensitization.

GPR39 regulated spinal glycinergic inhibition and mechanical inflammatory pain.

G protein-coupled receptor 39 (GPR39) senses the change of extracellular divalent zinc ion and signals through multiple G proteins to a broad spectrum of downstream effectors. Here, we found that GPR39 was prevalent at inhibitory synapses of spinal cord somatostatin-positive (SOM+) interneurons, a mechanosensitive subpopulation that is critical for the conveyance of mechanical pain. GPR39 complexed specifically with inhibitory glycine receptors (GlyRs) and helped maintain glycinergic transmission in a manner independent of G protein signalings. Targeted knockdown of GPR39 in SOM+ interneurons reduced the glycinergic inhibition and facilitated the excitatory output from SOM+ interneurons to spinoparabrachial neurons that engaged superspinal neural circuits encoding both the sensory discriminative and affective motivational domains of pain experience. Our data showed that pharmacological activation of GPR39 or augmenting GPR39 interaction with GlyRs at the spinal level effectively alleviated the sensory and affective pain induced by complete Freund's adjuvant and implicated GPR39 as a promising therapeutic target for the treatment of inflammatory mechanical pain.

Ca2+ /calmodulin-dependent protein kinase II in spinal dorsal horn contributes to the pain hypersensitivity induced by γ-aminobutyric acid type a receptor inhibition.

The fast inhibitory synaptic transmission mediated by the γ-aminobutyric acid type A receptor (GABAA R) within spinal dorsal horn exerts a gating control over the synaptic conveyance of nociceptive information from the periphery to higher brain regions. Although a large body of evidence has demonstrated that the impairment of GABAergic inhibition alone is sufficient to elicit pain hypersensitivity in intact animals, the underlying mechanisms remain to be characterized. The present study shows that Ca(2+) /calmodulin-dependent protein kinase II (CaMKII) is an important signaling protein downstream of reduced GABAergic inhibition. We found that pharmacological removal of inhibition by intrathecal application of the GABAA R antagonist bicuculline significantly enhanced the autophosphorylation of CaMKII at Thr286 in spinal dorsal horn of mice. In addition to increased CaMKII activity, bicuculline also promoted CaMKII interaction with N-methyl-D-aspartate (NMDA)-subtype glutamate receptors and induced the translocation of CaMKII from cytosolic compartments to the synaptosomal membrane fraction. Immunoblotting analysis revealed that the phosphorylation levels of NMDA receptor NR2B subunit at Ser1303 and of AMPA-subtype glutamate receptor GluR1 subunit at Ser831, two important CaMKII phosphorylation sites, were substantially enhanced after bicuculline application. Behavioral tests illustrated that intrathecal administration of the CaMKII inhibitor KN-93, NMDA receptor antagonist D-APV, or AMPA receptor antagonist GYKI 52466 effectively ameliorated the mechanical allodynia evoked by bicuculline. These data thus indicate that CaMKII signaling is critical for the reduced inhibition to evoke spinal sensitization.

cAMP-dependent protein kinase activated Fyn in spinal dorsal horn to regulate NMDA receptor function during inflammatory pain.

Selective inhibition of GluN2B-containing NMDA receptor (GluN2BR) in spinal dorsal horn effectively alleviates inflammatory pain, suggesting the up-regulation of GluN2BR function involved in central sensitization. Previous studies have demonstrated that the increase in GluN2BR synaptic expression serves as a key step to enhance GluN2BR function after intradermal injection of Complete Freund's Adjuvant (CFA). Here, we showed that cAMP-dependent protein kinase (PKA) played an important role in redistributing GluN2BR at synapses, because inhibition of PKA activity impaired GluN2BR accumulation at post-synaptic density (PSD)-enriched fraction in CFA-injected mice, and direct stimulation of PKA in naïve mice mimicked the effect of CFA by recruiting GluN2BR at PSD fraction to evoke pain sensitization. Analysis of PKA-initiated signalings unraveled that PKA was able to activate Src-family protein tyrosine kinases member Fyn, possibly by disrupting Fyn association with its inhibitory partner striatal-enriched protein tyrosine phosphatase 61. The active Fyn then promoted GluN2B phosphorylation at Tyr1472, a molecular event known to prevent GluN2BR endocytosis. As a result, pharmacological or genetic manipulation of Fyn activity greatly depressed GluN2BR accumulation at PSD-enriched fraction and ameliorated mechanical allodynia induced by PKA. Our data thus elucidated a critical role of PKA/Fyn/GluN2B signaling in triggering GluN2BR hyperfunction and pain hypersensitivity.

NR2B phosphorylation at tyrosine 1472 in spinal dorsal horn contributed to N-methyl-D-aspartate-induced pain hypersensitivity in mice.

Calcium influx via N-methyl-D-aspartate (NMDA)-subtype glutamate receptors (NMDARs) regulates the intracellular trafficking of NMDARs, leading to long-lasting modification of NMDAR-mediated synaptic transmission that is involved in development, learning, and synaptic plasticity. The present study investigated the contribution of such NMDAR-dependent synaptic trafficking in spinal dorsal horn to the induction of pain hypersensitivity. Our data showed that direct activation of NMDARs by intrathecal NMDA application elicited pronounced mechanical allodynia in intact mice, which was concurrent with a specific increase in the abundance of NMDAR subunits NR1 and NR2B at the postsynaptic density (PSD)-enriched fraction. Selective inhibition of NR2B-containing NMDARs (NR2BR) by ifenprodil dose dependently attenuated the mechanical allodynia in NMDA-injected mice, suggesting the importance of NR2BR synaptic accumulation in NMDA-induced pain sensitization. The NR2BR redistribution at synapses after NMDA challenge was associated with a significant increase in NR2B phosphorylation at Tyr1472, a catalytic site by Src family protein tyrosine kinases (SFKs) that has been shown to prevent NR2B endocytosis. Intrathecal injection of a specific SFKs inhibitor, PP2, to block NR2B tyrosine phosphorylation eliminated NMDA-induced NR2BR synaptic expression and also attenuated the mechanical allodynia. These data suggested that activation of spinal NMDARs was able to accumulate NR2BR at synapses via SFK signaling, which might exaggerate NMDAR-dependent nociceptive transmission and contribute to NMDA-induced nociceptive behavioral hyperresponsiveness.

BDNF modulated KCC2 ubiquitylation in spinal cord dorsal horn of mice.

The K+-Cl- co-transporter 2 (KCC2) is a neuron-specific Cl- extruder in the dorsal horn of spinal cord. The low intracellular Cl- concentration established by KCC2 is critical for GABAergic and glycinergic systems to generate synaptic inhibition. Peripheral nerve lesions have been shown to cause KCC2 dysfunction in adult spinal cord through brain-derived neurotrophic factor (BDNF) signaling, which switches the hyperpolarizing inhibitory transmission to be depolarizing and excitatory. However, the mechanisms by which BDNF impairs KCC2 function remain to be elucidated. Here we found that BDNF treatment enhanced KCC2 ubiquitination in the dorsal horn of adult mice, a post-translational modification that leads to KCC2 degradation. Our data showed that spinal BDNF application promoted KCC2 interaction with Casitas B-lineage lymphoma b (Cbl-b), one of the E3 ubiquitin ligases that are involved in the spinal processing of nociceptive information. Knockdown of Cbl-b expression decreased KCC2 ubiquitination level and attenuated the pain hypersensitivity induced by BDNF. Spared nerve injury significantly increased KCC2 ubiquitination, which could be reversed by inhibition of TrkB receptor. Our data implicated that KCC2 was one of the important pain-related substrates of Cbl-b and that ubiquitin modification contributed to BDNF-induced KCC2 hypofunction in the spinal cord.

Functional expression of glycine receptors in DRG neurons of mice.

Glycine receptor is one of the chloride-permeable ion channels composed of combinations of four α subunits and one ß subunit. In adult spinal cord, the glycine receptor α1 subunit is crucial for the generation of inhibitory neurotransmission. The reduced glycinergic inhibition is regarded as one of the key spinal mechanisms underlying pathological pain symptoms. However, the expression and function of glycine receptors in the peripheral system are largely unknown as yet. Here we found that glycine receptor α1 subunit was prevalent in the dorsal root ganglia (DRG) neurons as well as in the sciatic nerves of adult mice. Intraganglionar or intraplantar injection of glycine receptor antagonist strychnine caused the hypersensitivity to mechanical, thermal and cold stimuli, suggesting the functional importance of peripheral glycine receptors in the control of nociceptive signal transmission. Our data showed that peripheral inflammation induced by formalin decreased the expression of glycine receptor α1 subunit on the plasma membrane of DRG neurons, which was attributed to the activation of protein kinase C signaling. Intraplantar application of glycine receptor agonist glycine or positive modulator divalent zinc ion alleviated the first-phase painful behaviors induced by formalin. These data suggested that peripheral glycine receptor might serve as an effective target for pain therapy.

Analgesic action of adenosine A1 receptor involves the dephosphorylation of glycine receptor α1ins subunit in spinal dorsal horn of mice.

Glycine receptor α1ins subunit is located at inhibitory synapses in the superficial dorsal horn of adult spinal cord and is engaged in the glycinergic inhibition of nociceptive neuronal excitability and transmission. The α1ins phosphorylation at Ser380 by extracellular signal-regulated kinase (ERK) has been shown to decrease glycinergic synaptic currents and contribute to spinal disinhibition. Here we found that peripheral inflammation induced by Complete Freund's Adjuvant increased Ser380 phosphorylation in spinal cord dorsal horn of mice, which was repressed by specific activation of adenosine A1 receptor (A1R). Protein phosphatase-1 (PP1), a ubiquitously-distributed serine/threonine phosphatase, was required for A1R to reduce Ser380 phosphorylation. Our data showed that Gßγ dimer, when released after activation of Gi protein-coupled A1R, interacted with PP1 and directed this phosphatase to α1ins, allowing for the full dephosphorylation of Ser380 residue. Sequestration of Gßγ dimer by viral expression of the C-terminal tail of ß-adrenergic receptor kinase (ßARKct) dissociated PP1 from α1ins complex, leading to robust Ser380 phosphorylation. Meanwhile, Gßγ inhibition compromised the ability of A1R to alleviate inflammatory pain. The inhibitory effect of A1R on Ser380 phosphorylation was also attributed to the inactivation of ERK in CFA mice. Our data thus identified glycine receptor α1ins subunit as an important target for adenosinergic suppression of inflammatory pain.

SNAP25/syntaxin4/VAMP2/Munc18-1 Complexes in Spinal Dorsal Horn Contributed to Inflammatory Pain.

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) have been implicated in the trafficking of postsynaptic glutamate receptors, including N-methyl-d-aspartate (NMDA)-subtype glutamate receptors (NMDARs) that are critical for nociceptive plasticity and behavioral sensitization. However, the components of SNAREs complex involved in spinal nociceptive processing remain largely unknown. Here we found that SNAP25, syntaxin4, VAMP2 and Munc18-1 were localized at postsynaptic sites and formed the complex in the superficial lamina of spinal cord dorsal horn of rats. The complex formation between these SNAREs components were accelerated after intraplantar injection of complete Freund's adjuvant (CFA), pharmacological removal of GABAergic inhibition or activation of NMDAR in intact rats. The increased SNAP25/syntaxin4/VAMP2/Munc18-1 interaction facilitated the surface delivery and synaptic accumulation of NMDAR during inflammatory pain. Disruption of the molecular interaction between SNAP25 with its SNARE partners by using a blocking peptide derived from the C-terminus of SNAP25 effectively repressed the surface and synaptic accumulation of GluN2B-containing NMDARs in CFA-injected rats. This peptide also alleviated inflammatory mechanical allodynia and thermal hypersensitivity. These data suggested that SNAREs complex assembly in spinal cord dorsal horn was involved in the inflammatory pain hypersensitivity through promoting NMDAR synaptic trafficking.

Ubiquitination and functional modification of GluN2B subunit-containing NMDA receptors by Cbl-b in the spinal cord dorsal horn.

N-methyl-d-aspartate (NMDA) glutamate receptors (NMDARs) containing GluN2B subunits are prevalent early after birth in most brain regions in rodents. Upon synapse maturation, GluN2B is progressively removed from synapses, which affects NMDAR function and synaptic plasticity. Aberrant recruitment of GluN2B into mature synapses has been implicated in several neuropathologies that afflict adults. We found that the E3 ubiquitin ligase Cbl-b was enriched in the spinal cord dorsal horn neurons of mice and rats and suppressed GluN2B abundance during development and inflammatory pain. Cbl-b abundance increased from postnatal day 1 (P1) to P14, a critical time period for synapse maturation. Through its N-terminal tyrosine kinase binding domain, Cbl-b interacted with GluN2B. Ubiquitination of GluN2B by Cbl-b decreased the synaptic transmission mediated by GluN2B-containing NMDARs. Knocking down Cbl-b in vivo during P1 to P14 led to sustained retention of GluN2B at dorsal horn synapses, suggesting that Cbl-b limits the synaptic abundance of GluN2B in adult mice. However, peripheral inflammation induced by intraplantar injection of complete Freund's adjuvant resulted in the dephosphorylation of Cbl-b at Tyr363, which impaired its binding to and ubiquitylation of GluN2B, enabling the reappearance of GluN2B-containing NMDARs at synapses. Expression of a phosphomimic Cbl-b mutant in the dorsal horn suppressed both GluN2B-mediated synaptic currents and manifestations of pain induced by inflammation. The findings indicate a ubiquitin-mediated developmental switch in NMDAR subunit composition that is dysregulated by inflammation, which can enhance nociception.

Spinophilin negatively controlled the function of transient receptor potential vanilloid 1 in dorsal root ganglia neurons of mice.

Protein phosphatase-1 (PP1) is ubiquitously distributed in the nervous system and catalyzes the dephosphorylation of numerous substrates. The specificity and efficacy of PP1-mediated dephosphorylation depend on scaffolding proteins that anchor PP1 to the close vicinity of substrates. Spinophilin is one of the scaffolding proteins which are able to direct PP1 into postsynaptic density and regulate the synaptic transmission and plasticity. Here we found that spinophilin was enriched in dorsal root ganglia (DRG) neurons and engaged in the modification of nociceptive signaling processing. Disturbing spinophilin/PP1 interaction in DRG neurons led to the enhanced sensitivity of mice to heat and mechanical stimuli. The transient receptor potential vanilloid 1 (TRPV1) was identified as an important target for spinophilin modification. Our data showed that spinophilin physically interacted with TRPV1 and facilitated PP1 dephosphorylation of TRPV1 at Ser502. Disruption of spinophilin/PP1 complex enhanced Ser502 phosphorylation and boosted TRPV1 expression on plasma membrane. Peripheral inflammation induced by formalin disturbed spinophilin/PP1 interaction, which removed PP1-mediated inhibition and caused a marked increase of TRPV1 phosphorylation. Viral expression of wild-type spinophilin in DRG neurons repressed TRPV1 phosphorylation and alleviated formalin-induced inflammatory pain. These data suggested that spinophilin/PP1 complex negatively controlled TRPV1 function in DRG neurons.

A synthetic peptide disturbing GluN2A/SHP1 interaction in dorsal root ganglion attenuated pathological pain.

Src Homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1) interacts specifically with GluN2A subunit of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in spinal cord dorsal horn. This molecular interaction is involved in the development of GluN2A-dependent spinal sensitization of nociceptive behaviors. Intrathecal application of a GluN2A-derived polypeptide (short for pep-GluN2A) has been shown to disturb spinal GluN2A/SHP1 interaction and inhibit inflammatory pain. Here we found that SHP1 was also located at dorsal root ganglion (DRG) neurons and formed complexes with GluN2A subunit. Peripheral inflammation activated SHP1 in DRG neurons, which promoted GluN2A tyrosine phosphorylation. The SHP1 binding to GluN2A facilitated the glutamate release from primary afferent fibers and exaggerated nociceptive synaptic transmission onto postsynaptic spinal cord neurons. Our data showed that intradermal application of pep-GluN2A disrupted GluN2A/SHP1 interaction in DRG neurons, attenuated the ability of GluN2A subunit-containing NMDA receptors to regulate the presynaptic glutamate release and more importantly, alleviated the pain hypersensitivity caused by carrageenan, complete Freund's adjuvant and formalin. The neuropathic pain induced by spared nerve injury was also ameliorated by intradermal pep-GluN2A application. These data suggested that disruption of GluN2A/SHP1 interaction in DRG neurons generated an effective analgesic action against pathological pain.

Ubiquitination and inhibition of glycine receptor by HUWE1 in spinal cord dorsal horn.

Glycine receptors (GlyRs) are pentameric proteins that consist of α (α1-α4) subunits and/or ß subunit. In the spinal cord of adult animals, the majority of inhibitory glycinergic neurotransmission is mediated by α1 subunit-containing GlyRs. The reduced glycinergic inhibition (disinhibition) is proposed to increase the excitabilities and spontaneous activities of spinal nociceptive neurons during pathological pain. However, the molecular mechanisms by which peripheral lesions impair GlyRs-α1-mediated synaptic inhibition remain largely unknown. Here we found that activity-dependent ubiquitination of GlyRs-α1 subunit might contribute to glycinergic disinhibition after peripheral inflammation. Our data showed that HUWE1 (HECT, UBA, WWE domain containing 1), an E3 ubiquitin ligase, located at spinal synapses and specifically interacted with GlyRs-α1 subunit. By ubiquitinating GlyRs-α1, HUWE1 reduced the surface expression of GlyRs-α1 through endocytic pathway. In the dorsal horn of Complete Freund's Adjuvant-injected mice, shRNA-mediated knockdown of HUWE1 blunted GlyRs-α1 ubiquitination, potentiated glycinergic synaptic transmission and attenuated inflammatory pain. These data implicated that ubiquitin modification of GlyRs-α1 represented an important way for peripheral inflammation to reduce spinal glycinergic inhibition and that interference with HUWE1 activity generated analgesic action by resuming GlyRs-α1-mediated synaptic transmission.

Inhibition of protein tyrosine phosphatase 1B in spinal cord dorsal horn of rats attenuated diabetic neuropathic pain.

Protein tyrosine phosphatase 1B (PTP1B) has been shown to dephosphorylate and inactivate insulin receptors, which contributes to the pathogenesis of diabetes. Neuropathic pain is one of the severe complications that results from diabetic neuropathy. However, whether PTP1B was involved in the development of diabetic neuropathic pain is largely unknown. The current study illustrated that PTP1B was located in spinal cord dorsal horn neurons of Sprague-Dawley rats. Western blot analysis demonstrated that the diabetic neuropathic pain induced by intraperitoneal injection of streptozotocin was associated with an increased protein expression and a dynamic redistribution of spinal PTP1B into excitatory glutamatergic synapses. We found that PTP1B operated to stimulate Src kinase and enhance the tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors. The siRNA-mediated knockdown of PTP1B in streptozotocin-injected rats repressed Src activity, decreased NMDA receptor phosphorylation and alleviated the thermal hyperalgesia and mechanical allodynia. A similar analgesia against diabetic neuropathic pain was also achieved when PTP1B activity was manipulated by a chemical PTP Inhibitor or PTP1B(C215S) mutant. These data revealed a regulated expression of PTP1B in spinal cord dorsal horn of rats after diabetic neuropathy, and demonstrated that inhibition of PTP1B was beneficial for the treatment of pain hypersensitivity related to diabetes.

Enhanced Activities of δ Subunit-containing GABAA Receptors Blocked Spinal Long-term Potentiation and Attenuated Formalin-induced Spontaneous Pain.

The δ subunit-containing γ-Aminobutyric acid type A receptors (δ-GABAARs) are located at extrasynaptic sites and persistently active in the control of neuronal excitability. Here we recorded primary afferent C fiber-evoked field potentials in the superficial dorsal horn of rat spinal cords in vivo and investigated the possible influence of δ-GABAARs activities on nociceptive synaptic transmission. We found that δ-GABAARs-preferring agonist 4,5,6,7-tetrahydroisoxazolol [4,5-c] pyridine-3-ol (THIP), when topically applied onto spinal cord dorsum, inhibited the basal synaptic responses in a dose-dependent manner. Low-frequency stimulation (LFS) of sciatic nerves elicited long-term potentiation (LTP) of C fiber transmission, a synaptic correlate of central sensitization. Pretreatment with THIP before LFS delivery blocked the induction of LTP. When applied at 30 min and 180 min post-LFS, THIP reduced the magnitudes of established LTP. Intraplantar injection of formalin naturally evoked LTP in anesthetized rats. Spinal administration of THIP not only reversed formalin-induced LTP, but alleviated the spontaneous painful behaviors and mechanical hyperalgesia. Biochemical analysis demonstrated that δ-GABAARs activation by THIP decreased the synaptic expression and phosphorylation of AMPA receptor GluA1 subunit in formalin-injected rats, and meanwhile, increased synaptic GluA2 content, allowing the switch of GluA2-lacking AMPA receptors to GluA2-containing ones at synapses. THIP also suppressed the synaptic accumulation and phosphorylation of NMDA receptor GluN1 subunit in formalin-injected rats. Our data suggested that enhanced δ-GABAARs activities blunted the initiation and maintenance of spinal LTP, which correlated with the amelioration of central sensitization of nociceptive behaviors.

Disruption of SHP1/NMDA receptor signaling in spinal cord dorsal horn alleviated inflammatory pain.

Src-homology 2 domain-containing protein tyrosine phosphatase-1 (SHP1) is one of the non-receptor-like phosphatases that are highly enriched in hematopoietic cells. Although accumulating evidence has implicated the protein tyrosine phosphatases in the regulation of nociceptive transmission and plasticity, it is largely unknown whether SHP1 was expressed in pain-related spinal cord dorsal horn and engaged in the synaptic modification of nociceptive signals. Here we found that SHP1 was present in spinal neurons of rats and functionally coupled to GluN2A subunit-containing N-methyl-d-aspartate subtype of glutamate receptors, one of the key players in central sensitization of nociceptive behaviors. SHP1 interacted with a membrane-proximal region within the cytoplasmic tail of GluN2A. This interaction was necessary to stimulate SHP1 activity and more importantly, restrict SHP1 signaling to specifically enhance the tyrosine phosphorylation of GluN2A during inflammatory pain. Electrophysiological and behavioral studies showed that SHP1 binding potentiated GluN2A currents and evoked GluN2A-dependent pain hypersensitivity. The siRNA-mediated knockdown of SHP1 or interference with SHP1/GluN2A interaction by a synthetic peptide alleviated inflammatory pain induced by either Complete Freund's Adjuvant or formalin. Our data implicated that SHP1 was a specific enhancer of GluN2A-mediated nociceptive synaptic transmission in spinal cord dorsal horn, and manipulation of SHP1 activity may serve as an effective strategy for the treatment of inflammatory pain.

Activity-dependent Synaptic Recruitment of Neuroligin 1 in Spinal Dorsal Horn Contributed to Inflammatory Pain.

Neuroligin 1 (NLGN1), a cell adhesion molecule present at excitatory glutamatergic synapses, has been shown to be critical for synaptic specialization and N-methyl-d-aspartate (NMDA)-subtype glutamate receptor-dependent synaptic plasticity. Whether and how NLGN1 is engaged in nociceptive behavioral sensitization remains largely unknown. In this study, we found an activity-dependent regulation of NLGN1 synaptic expression in pain-related spinal cord dorsal horns of mice. The enhancement of neuronal activity by pharmacological activation of NMDA receptor (NMDAR) or removal of GABAergic inhibition in intact mice significantly increased NLGN1 concentration at synaptosomal membrane fraction. Intraplantar injection of complete Freund's adjuvant (CFA) also increased the NLGN1 expression at synapses. NMDAR might act through Ca2+/calmodulin-dependent protein kinase II (CaMKII) and Src-family protein tyrosine kinase member Fyn to induce the synaptic redistribution of NLGN1. We also found that one of the important roles of NLGN1 was to facilitate the clustering of NMDAR at synapses. The NLGN1-targeting siRNA suppressed the synaptic expression of GluN2B-containing NMDAR in CFA-injected mice and meanwhile, attenuated the inflammatory mechanical allodynia and thermal hypersensitivity. These data suggested that tissue injury-induced synaptic redistribution of NLGN1 was involved in the development of pain hypersensitivity through facilitating the synaptic incorporation of NMDARs.