Involvement of spinal PKA/CREB signaling pathway in the development of bone cancer pain

MAPK-ERK-CREB signaling pathway upregulates Nav1.6 in oxaliplatin-induced neuropathic pain in the rat

The voltage-gated sodium channel subtype Nav1.6 is involved in the electrophysiological changes of primary sensory neurons that occur in oxaliplatin-induced neuropathic pain, but its regulatory mechanism remains unclear. In this study, Western blot, RT-qPCR, immunofluorescence staining, chromatin immunoprecipitation were used to prove the mechanism of MAPK-ERK-CREB signaling pathway participating in oxaliplatin-induced neuropathic pain by regulating Nav1.6. The results showed that p-Raf1 and p-ERK, key molecules in MAPK/ERK pathway, and Nav1.6 were significantly increased in DRGs of oxaliplatin-induced neuropathic pain rats. Inhibition of p-Raf1 and p-ERK respectively not only reduced the expression of Nav1.6 protein in DRGs of OXA rats, but also caused a decrease in Nav1.6 mRNA, which led us to further explore the transcription factor CREB regulated by MAPK/ERK pathway. Results showed that CREB was co-distributed with Nav1.6. Inhibition of CREB resulted in decreased mRNA and protein expression of Nav1.6, and alleviated oxaliplatin-induced neuropathic pain. A chromatin immunoprecipitation experiment proved that OXA caused p-CREB to directly bind to the promoter region of Scn8A, which is the encoding gene for Nav1.6, and promote the transcription of Scn8A. In summary, in this study, we found that oxaliplatin can activate the MAPK/ERK pathway, which promotes the expression and activation of CREB and leads to an increase in Scn8A transcription, and then leads to an increase in Nav1.6 protein expression to enhance neuronal excitability and cause pain. This study provides an experimental basis for the molecular mechanism of sodium channel regulation in oxaliplatin-induced neuropathic pain.

GABAB Receptors and Pain

A substantial fraction of the human population suffers from chronic pain states, which often cannot be sufficiently treated with existing drugs. This calls for alternative targets and strategies for the development of novel analgesics. There is substantial evidence that the G protein-coupled GABA_B receptor is involved in the processing of pain signals and thus has long been considered a valuable target for the generation of analgesics to treat chronic pain. In this review, the contribution of GABA_B receptors to the generation and modulation of pain signals, their involvement in chronic pain states as well as their target suitability for the development of novel analgesics is discussed.

Repeated oxytocin prevents central sensitization by regulating synaptic plasticity via oxytocin receptor in a chronic migraine mouse model

BACKGROUND

Central sensitization is one of the characters of chronic migraine (CM). Aberrant synaptic plasticity can induce central sensitization. Oxytocin (OT), which is a hypothalamic hormone, plays an important antinociceptive role. However, the antinociceptive effect of OT and the underlying mechanism in CM remains unclear. Therefore, we explored the effect of OT on central sensitization in CM and its implying mechanism, focusing on synaptic plasticity.

METHODS

A CM mouse model was established by repeated intraperitoneal injection of nitroglycerin (NTG). Von Frey filaments and radiant heat were used to measure the nociceptive threshold. Repeated intranasal OT and intraperitoneal L368,899, an oxytocin receptor (OTR) antagonist, were administered to investigate the effect of OT and the role of OTR. The expression of calcitonin gene-related peptide (CGRP) and c-fos were measured to assess central sensitization. N-methyl D-aspartate receptor subtype 2B (NR2B)-regulated synaptic-associated proteins and synaptic plasticity were explored by western blot (WB), transmission electron microscope (TEM), and Golgi-Cox staining.

RESULTS

Our results showed that the OTR expression in the trigeminal nucleus caudalis (TNC) of CM mouse was significantly increased, and OTR was colocalized with the postsynaptic density protein 95 (PSD-95) in neurons. Repeated intranasal OT alleviated the NTG-induced hyperalgesia and prevented central sensitization in CM mouse. Additionally, the OT treatment inhibited the overexpression of phosphorylated NR2B and synaptic-associated proteins including PSD-95, synaptophysin-1 (syt-1), and synaptosomal-associated protein 25 (snap25) in the TNC of CM mouse and restored the abnormal synaptic structure. The protective effect of OT was prevented by L368,899. Furthermore, the expression of adenylyl cyclase 1 (AC1)/ protein kinase A (PKA)/ phosphorylation of cyclic adenosine monophosphate response element-binding protein (pCREB) pathway was depressed by OT and restored by L368,899.

CONCLUSIONS

Our findings demonstrate that repeated intranasal OT eliminates central sensitization by regulating synaptic plasticity via OTR in CM. The effect of OT has closely associated with the down-regulation of AC1/PKA/pCREB signaling pathway, which is activated in CM model. Repeated intranasal OT may be a potential candidate for CM prevention.

Reactive Astrocytes: Critical Players in the Development of Chronic Pain

Chronic pain is associated with long term plasticity of nociceptive pathways in the central nervous system. Astrocytes can profoundly affect synaptic function and increasing evidence has highlighted how altered astrocyte activity may contribute to the pathogenesis of chronic pain. In response to injury, astrocytes undergo a shift in form and function known as reactive astrogliosis, which affects their release of cytokines and gliotransmitters. These neuromodulatory substances have been implicated in driving the persistent changes in central nociceptive activity. Astrocytes also release lactate which neurons can use to produce energy during synaptic plasticity. Furthermore, recent research has provided insight into lactate's emerging role as a signaling molecule in the central nervous system, which may be involved in directly modulating neuronal and astrocytic activity. In this review, we present evidence for the involvement of astrocyte-derived tumor necrosis factor alpha in pain-associated plasticity, in addition to research suggesting the potential involvement of gliotransmitters D-serine and adenosine-5'-triphosphate. We also discuss work implicating astrocyte-neuron metabolic coupling, and the possible role of lactate, which has been sparsely studied in the context of chronic pain, in supporting pathological changes in central nociceptive activity.

Constitutive activity of GPR26 regulated by ubiquitin-dependent degradation and its antitumor role

G protein-coupled receptors (GPCRs) play important roles in many physiological functions and numerous diseases. In addition to the classic ligand-stimulated receptor activity, an increasing number of studies have established that many GPCRs function constitutively in a receptor dose-dependent manner. Previous observations showed that following gene transfection, little or no protein was detectable for certain GPCRs (designated apparent state A), such as GPR26, GPR39, GPR78, GPR133, GPR139, BRS3, and LGR5, which showed strong constitutive activities. When we lysed cells in the immediate presence of western blot loading buffer, a significant increase of protein levels was detected (actual state B), which was much closer to the true expression levels under physiological conditions. GPR26 was chosen for further functional experiments as the actual state B. We identified an important ubiquitination site, K286, as well as the ubiquitin ligase E3 homologous to the E6-associated protein carboxyl terminus domain containing 3 interacting with GPR26. The pronounced differences in the protein expression and constitutive activity of GPR26 were a consequence of the ubiquitin-mediated rapid degradation mechanism. Furthermore, we identified in vitro and in vivo antitumor activity associated with high expression levels and constitutive activity of GPR26 in liver cancer cells. Hence, GPR26 could act as an antitumor gene for hepatocellular carcinoma. This study also represents the actual state B of a batch of GPCRs that actually play potentially important roles in physiological functions by their constitutive activity, which is controlled by rapid ubiquitin-dependent degradation.

An Investigation of the Molecular Mechanisms Underlying the Analgesic Effect of Jakyak-Gamcho Decoction: A Network Pharmacology Study

Herbal drugs have drawn substantial interest as effective analgesic agents; however, their therapeutic mechanisms remain to be fully understood. To address this question, we performed a network pharmacology study to explore the system-level mechanisms that underlie the analgesic activity of Jakyak-Gamcho decoction (JGd; Shaoyao-Gancao-Tang in Chinese and Shakuyaku-Kanzo-To in Japanese), an herbal prescription consisting of Paeonia lactiflora Pallas and Glycyrrhiza uralensis Fischer. Based on comprehensive information regarding the pharmacological and chemical properties of the herbal constituents of JGd, we identified 57 active chemical compounds and their 70 pain-associated targets. The JGd targets were determined to be involved in the regulation of diverse biological activities as follows: calcium- and cytokine-mediated signalings, calcium ion concentration and homeostasis, cellular behaviors of muscle and neuronal cells, inflammatory response, and response to chemical, cytokine, drug, and oxidative stress. The targets were further enriched in various pain-associated signalings, including the PI3K-Akt, estrogen, ErbB, neurotrophin, neuroactive ligand-receptor interaction, HIF-1, serotonergic synapse, JAK-STAT, and cAMP pathways. Thus, these data provide a systematic basis to understand the molecular mechanisms underlying the analgesic activity of herbal drugs.

Deficiency in the function of inhibitory interneurons contributes to glutamate-associated central sensitization through GABABR2-SynCAM1 signaling in chronic migraine rats

The occurrence of pain has always been closely related to a break in the balance between excitatory and inhibitory systems, and the internal relationship between these two systems has not been studied in the pathogenesis of chronic migraine (CM). In this study, we explored how inhibitory interneurons specifically modulate the glutamate-induced hyperexcitability in the periaqueductal gray (PAG) of CM rats. The CM model was established by repeated dural infusion of inflammatory soup (IS) in rats. Then, Baclofen, a gamma-aminobutyric acid type B receptor (GABABR) agonist; CGP35348, a GABABR antagonist; H89, a protein kinase A (PKA) inhibitor; and 8-Bromo-cAMP, a PKA agonist, were applied by intraventricular injection to investigate the detailed CM mechanism. Our results showed that GABABR2 mRNA and protein levels were significantly downregulated (P < .01) in the PAG of CM rats. Similarly, gamma-aminobutyric acid (GABA) and its synthetase glutamate decarboxylase 65/67 (GAD65/67) seriously decreased (P < .01), implying a deficit in the function of inhibitory interneurons in the PAG of CM rats. Afterward, the application of Baclofen and H89 alleviated the IS-evoked hyperalgesia and extenuated vesicular glutamate transporter 2 (VGLUT2), glutamate, calcitonin gene-related peptide (CGRP), and c-Fos expression by regulating the GABABR2/PKA/SynCAM1 pathway in the PAG of CM rats, while the application of CGP35348 and 8-Bromo-cAMP exactly exerted the opposite effect. Importantly, CGP35348 induced an elevation of CGRP, and VGLUT2 expression was relieved by H89. These data suggest that the loss in the function of inhibitory interneurons contributes to glutamate-associated central sensitization through the GABABR2/PKA/SynCAM1 pathway in the PAG of CM rats.

SNAP-25 Contributes to Neuropathic Pain by Regulation of VGLuT2 Expression in Rats

Synaptosomal-associated protein 25 (SNAP-25) plays an important role in neuropathic pain. However, the underlying mechanism is largely unknown. Vesicular glutamate transporter 2 (VGluT2) is an isoform of vesicular glutamate transporters that controls the storage and release of glutamate. In the present study, we found the expression levels of VGluT2 correlated with the upregulation of SNAP-25 in the spinal cord of rats following chronic constriction injury (CCI)-induced neuropathic pain. Cleavage of SNAP-25 by Botulinum toxin A (BoNT/A) attenuated mechanical allodynia, downregulated the expression of VGluT2 and reduced glutamate release. Overexpression of VGluT2 abolished the antinociceptive effect of BoNT/A. Upregulation of SNAP-25 in naive rats increased VGluT2 expression and induced pain-responsive behaviors. In pheochromocytoma (PC12) cells, the expression of VGluT2 was also depended on SNAP-25 dysregulation. Moreover, we found VGluT2 was involved in SNAP-25-mediated regulation of astrocyte expression and activation of the PKA/p-CREB pathway mediated the upregulation of SNAP-25 in neuropathic pain. The findings of our study indicate that VGluT2 contributes to the effect of SNAP-25 in maintaining the development of neuropathic pain and suggests a novel mechanism underlying SNAP-25 regulation of neuropathic pain.

P2Y12 receptor mediates microglial activation via RhoA/ROCK pathway in the trigeminal nucleus caudalis in a mouse model of chronic migraine

BACKGROUND

Microglial activation contributes to the development of chronic migraine (CM). The P2Y12 receptor (P2Y12R), a metabolic purinoceptor that is expressed on microglia in the central nervous system (CNS), has been indicated to play a critical role in the pathogenesis of chronic pain. However, whether it contributes to the mechanism of CM remains unknown. Thus, the present study investigated the precise details of microglial P2Y12R involvement in CM.

METHODS

Mice subjected to recurrent nitroglycerin (NTG) treatment were used as the CM model. Hyperalgesia were assessed by mechanical withdrawal threshold to electronic von Frey and thermal withdrawal latency to radiant heat. Western blot and immunohistochemical analyses were employed to detect the expression of P2Y12R, Iba-1, RhoA, and ROCK2 in the trigeminal nucleus caudalis (TNC). To confirm the role of P2Y12R and RhoA/ROCK in CM, we systemically administered P2Y12R antagonists (MRS2395 and clopidogrel) and a ROCK2 inhibitor (fasudil) and investigated their effects on microglial activation, c-fos, and calcitonin gene-related peptide (CGRP) expression in the TNC. To further confirm the effect of P2Y12R on microglial activation, we preincubated lipopolysaccharide (LPS)-treated BV-2 microglia with MRS2395 and clopidogrel. ELISA was used to evaluate the levels of inflammatory cytokines.

RESULTS

The protein levels of P2Y12R, GTP-RhoA, ROCK2, CGRP, c-fos, and inducible nitric oxide synthase (iNOS) in the TNC were increased after recurrent NTG injection. A double labeling study showed that P2Y12R was restricted to microglia in the TNC. MRS2395 and clopidogrel attenuated the development of tactile allodynia and suppressed the expression of CGRP, c-fos, and GTP-RhoA/ROCK2 in the TNC. Furthermore, fasudil also prevented hyperalgesia and suppressed the expression of CGRP in the TNC. In addition, inhibiting P2Y12R and ROCK2 activities suppressed NTG-induced microglial morphological changes (process retraction) and iNOS production in the TNC. In vitro, a double labeling study showed that P2Y12R was colocalized with BV-2 cells, and the levels of iNOS, IL-1β, and TNF-α in LPS-stimulated BV-2 microglia were reduced by P2Y12R inhibitors.

CONCLUSIONS

These data demonstrate that microglial P2Y12R in the TNC plays a critical role in the pathogenesis of CM by regulating microglial activation in the TNC via RhoA/ROCK pathway.

The endocannabinoid system: Novel targets for treating cancer induced bone pain

Treating Cancer-induced bone pain (CIBP) continues to be a major clinical challenge and underlying mechanisms of CIBP remain unclear. Recently, emerging body of evidence suggested the endocannabinoid system (ECS) may play essential roles in CIBP. Here, we summarized the current understanding of the antinociceptive mechanisms of endocannabinoids in CIBP and discussed the beneficial effects of endocannabinoid for CIBP treatment. Targeting non-selective cannabinoid 1 receptors or selective cannabinoid 2 receptors, and modulation of peripheral AEA and 2-AG, as well as the inhibition the function of fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) have produced analgesic effects in animal models of CIBP. Management of ECS therefore appears to be a promising way for the treatment of CIBP in terms of efficacy and safety. Further clinical studies are encouraged to confirm the possible translation to humans of the very promising results already obtained in the preclinical studies.

Antinociceptive effects of Ginsenoside Rb1 in a rat model of cancer-induced bone pain

Ginsenoside Rb1 (GRb1) is a major ingredient of ginseng, a traditional medicine that has been used for thousands of years. Previous studies have reported that GRb1 had anti-inflammatory, antioxidant and neuroprotective effects. The current study aimed to evaluate the antinociceptive effects of GRb1 in a rat model of cancer-induced bone pain (CIBP) established by intratibial injection of Walker 256 cells. Intraperitoneal injection (i.p.) of GRb1 (5 and 10 mg/kg, but not 1 mg/kg) partially and transiently reversed the mechanical allodynia and thermal hyperalgesia in CIBP rats at 14 days following surgery when the pain behavior is established. Furthermore, repeated administration of GRb1 demonstrated persistent analgesic effect. Additionally, the protein expression and immunoreactivity of iba1, which is the maker of microglia, was significantly suppressed in CIBP rats treated with GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days compared with CIBP rats treated with a vehicle. Furthermore, upregulation of spinal interleukin (IL)-1β, IL-6 and tumor necrosis factor-α were also significantly inhibited by the treatment of GRb1 (i.p., 10 mg/kg) from day 12 for three consecutive days. Together, these results indicated that GRb1 may attenuate CIBP via inhibiting the activation of microglia and glial-derived proinflammatory cytokines.

Galanin receptor 1 plays an antinociceptive effect via inhibiting PKA activation in the nucleus accumbens of rats with neuropathic pain

Galanin and galanin receptors (GalRs) have been reported to be involved in the transmission and modulation of nociceptive information in the central nervous system (CNS). However, the underlying mechanism of the antinociception of GalRs in neuropathic pain remains unclear. This study investigated the antinociception induced by galanin receptor 1 (GalR1) via protein kinase A (PKA) signaling pathway in the nucleus accumbens (NAc) of rats with neuropathic pain. A mononeuropathy model was replicated by ligation of the left sciatic nerve, following which the expression of phospho-PKA (p-PKA) in the NAc were markedly up-regulated at 14(th) and 28(th) day after ligation of sciatic nerve, and p-PKA expression was down-regulated by intra-NAc injection of GalR1 agonist M617, but the GalR1 antagonist M35 did not have an effect. We also found that M35 in the NAc blocked the M617-induced increase in the hind paw withdrawal latencies (HWLs) of rats with mononeuropathy, but M35 alone had no effect on HWLs, and PKA inhibitor H-89 attenuated the M617-induced an increase in the HWLs. These results suggested that GalR1 induced an antinociception via inhibiting PKA activation, implying that GalR agonists may be potential and potent therapeutic options to treat chronic neuropathic pain.

Effects of CREB1 gene silencing on cognitive dysfunction by mediating PKA-CREB signaling pathway in mice with vascular dementia

Background

As a form of dementia primarily affecting the elderly, vascular dementia (VD) is characterized by changes in the supply of blood to the brain, resulting in cognitive impairment. The aim of the present study was to explore the effects involved with cyclic adenosine monophosphate (cAMP) response element-binding (CREB)1 gene silencing on cognitive dysfunction through meditation of the protein kinase A (PKA)-CREB signaling pathway in mice with VD.

Methods

Both the Morris water maze test and the step down test were applied to assess the cognitive function of the mice with VD. Immunohistochemical and TUNEL staining techniques were employed to evaluate the positive expression rates of the protein CREB1 and Cleaved Caspase-3, as well as neuronal apoptosis among hippocampal tissues in a respective manner. Flow cytometry was applied to determine the proliferation index and apoptosis rate of the hippocampal cells among each group. Reverse transcription quantitative polymerase chain reaction and Western blot analysis methods were applied to detect the expressions of cAMP, PKA and CREB in hippocampal cells.

Results

Compared with the normal group, all the other groups exhibited impaired cognitive function, reduced cell numbers in the CAI area, positive expressions of CREB1 as well as positive optical density (OD) values. Furthermore, increased Cleaved Caspase-3 positive expression, OD value, proliferation index, apoptosis rate of hippocampal cells and neurons, were observed in the other groups when compared with the normal group, as well as lower expressions of cAMP, PKA and CREB1 and p-CREB1 (the shCREB1–1, H89 and shCREB1–1 + H89 groups < the VD group).

Conclusion

The key findings of the present study demonstrated that CREB1 gene silencing results in aggravated VD that occurs as a result of inhibiting the PKA-CREB signaling pathway, thus exasperating cognitive dysfunction.

The role and mechanism of glutamic NMDA receptor in the mechanical hyperalgesia in diabetic rats

OBJECTIVES

Some studies have shown that painful neuropathy is a common and costly complication of both type 1 and type 2 diabetes mellitus, and glutamate is involved in the process although the mechanisms are not clear. The purpose of the present study was to investigate the effect of N-methyl-D-aspartate (NMDA) receptor on mechanical hyperalgesia in diabetic rats and the possible mechanism.

METHODS

Diabetic rat model was established by intraperitoneal injection of streptozotocin (STZ, 1%, 70 mg/kg) once, and evaluated by the change in the fasting blood glucose. The mechanical hyperalgesia was estimated by mechanical withdrawal threshold (MWT) using a set of calibrated Von Frey's filaments. In addition, the expressions of phosphorylated NMDA NR1 and phosphorylated cAMP response element binding protein (pCREB) in L4/L5 dorsal horns of spinal cord were observed.

RESULTS

Behavioral results showed that MK-801, an antagonist of NMDA receptor, could reduce the proportion of mechanical hyperalgesia in diabetic rats from 76.67 to 20.00%. Meanwhile, the mean MWTs in STZ group or saline-treated STZ group decreased significantly at 3-8 week, while, the MWTs in MK-801 treated STZ group were significant higher than those in STZ or saline-treated STZ group. In addition, the expressions of NMDA NR1 and pCREB in L4/5 dorsal horns of spinal cord were significant higher in diabetic rats, and MK-801 down-regulated their expressions partly.

CONCLUSION

All these results suggested that NMDA receptor and pCREB in the spinal cord were involved in the regulation of mechanical hyperalgesia in diabetic rats.

The Role of Spinal GABAB Receptors in Cancer-Induced Bone Pain in Rats

Cancer-induced bone pain (CIBP) remains a major challenge in advanced cancer patients because of our lack of understanding of its mechanisms. Previous studies have shown the vital role of γ-aminobutyric acid B receptors (GABABRs) in regulating nociception and various neuropathic pain models have shown diminished activity of GABABRs. However, the role of spinal GABABRs in CIBP remains largely unknown. In this study, we investigated the specific cellular mechanisms of GABABRs in the development and maintenance of CIBP in rats. Our behavioral results show that acute as well as chronic intrathecal treatment with baclofen, a GABABR agonist, significantly attenuated CIBP-induced mechanical allodynia and ambulatory pain. The expression levels of GABABRs were significantly decreased in a time-dependent manner and colocalized mostly with neurons and a minority with astrocytes and microglia. Chronic treatment with baclofen restored the expression of GABABRs and markedly inhibited the activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase and the cAMP-response element-binding protein signaling pathway.

PERSPECTIVE

Our findings provide, to our knowledge, the first evidence that downregulation of GABABRs contribute to the development and maintenance of CIBP and restored diminished GABABRs attenuate CIBP-induced pain behaviors at least partially by inhibiting the protein kinase/cAMP-response element-binding protein signaling pathway. Therefore, spinal GABABR may become a potential therapeutic target for the management of CIBP.

Novel signaling of dynorphin at κ-opioid receptor/bradykinin B2 receptor heterodimers

The κ-opioid receptor (KOR) and bradykinin B2 receptor (B2R) are involved in a variety of important physiological processes and share many similar characteristics in terms of their distribution and functions in the nervous system. We first demonstrated the endogenous expression of KOR and B2R in human SH-SY5Y cells and their co-localization on the membrane of human embryonic kidney 293 (HEK293) cells. Bioluminescence and fluorescence resonance energy transfer and the proximity ligation assay were exploited to demonstrate the formation of functional KOR and B2R heteromers in transfected cells. KOR/B2R heteromers triggered dynorphin A (1-13)-induced Gαs/protein kinase A signaling pathway activity, including upregulation of intracellular cAMP levels and cAMP-response element luciferase reporter activity, resulting in increased cAMP-response element-binding protein (CREB) phosphorylation, which could be dampened by the protein kinase A (PKA) inhibitor H89. This indicated that the co-existence of KOR and B2R is critical for CREB phosphorylation. In addition, dynorphin A (1-13) induced a significantly higher rate of proliferation in HEK293-KOR/B2R and human SH-SY5Y cells than in the control group. These results indicate that KOR can form a heterodimer with B2R and this leads to increased protein kinase A activity by the CREB signaling pathway, leading to a significant increase in cell proliferation. The nature of this signaling pathway has significant implications for the role of dynorphin in the regulation of neuroprotective effects.

Inhibition of the cAMP/PKA/CREB Pathway Contributes to the Analgesic Effects of Electroacupuncture in the Anterior Cingulate Cortex in a Rat Pain Memory Model

Pain memory is considered as endopathic factor underlying stubborn chronic pain. Our previous study demonstrated that electroacupuncture (EA) can alleviate retrieval of pain memory. This study was designed to observe the different effects between EA and indomethacin (a kind of nonsteroid anti-inflammatory drugs, NSAIDs) in a rat pain memory model. To explore the critical role of protein kinase A (PKA) in pain memory, a PKA inhibitor was microinjected into anterior cingulate cortex (ACC) in model rats. We further investigated the roles of the cyclic adenosine monophosphate (cAMP), PKA, cAMP response element-binding protein (CREB), and cAMP/PKA/CREB pathway in pain memory to explore the potential molecular mechanism. The results showed that EA alleviates the retrieval of pain memory while indomethacin failed. Intra-ACC microinjection of a PKA inhibitor blocked the occurrence of pain memory. EA reduced the activation of cAMP, PKA, and CREB and the coexpression levels of cAMP/PKA and PKA/CREB in the ACC of pain memory model rats, but indomethacin failed. The present findings identified a critical role of PKA in ACC in retrieval of pain memory. We propose that the proper mechanism of EA on pain memory is possibly due to the partial inhibition of cAMP/PKA/CREB signaling pathway by EA.

The Walker 256 Breast Cancer Cell- Induced Bone Pain Model in Rats

The majority of patients with terminal breast cancer show signs of bone metastasis, the most common cause of pain in cancer. Clinically available drug treatment options for the relief of cancer-associated bone pain are limited due to either inadequate pain relief and/or dose-limiting side-effects. One of the major hurdles in understanding the mechanism by which breast cancer causes pain after metastasis to the bones is the lack of suitable preclinical models. Until the late twentieth century, all animal models of cancer induced bone pain involved systemic injection of cancer cells into animals, which caused severe deterioration of animal health due to widespread metastasis. In this mini-review we have discussed details of a recently developed and highly efficient preclinical model of breast cancer induced bone pain: Walker 256 cancer cell- induced bone pain in rats. The model involves direct localized injection of cancer cells into a single tibia in rats, which avoids widespread metastasis of cancer cells and hence animals maintain good health throughout the experimental period. This model closely mimics the human pathophysiology of breast cancer induced bone pain and has great potential to aid in the process of drug discovery for treating this intractable pain condition.

CREB-regulated transcription coactivator 1 enhances CREB-dependent gene expression in spinal cord to maintain the bone cancer pain in mice

Background

cAMP response element binding protein (CREB)-dependent gene expression plays an important role in central sensitization. CREB-regulated transcription coactivator 1 (CRTC1) dramatically increases CREB-mediated transcriptional activity. Brain-derived neurotrophic factor, N-methyl-d-aspartate receptor subunit 2B, and miRNA-212/132, which are highly CREB responsive, function downstream from CREB/CRTC1 to mediate activity-dependent synaptic plasticity and in turn loops back to amplify CREB/CRTC1 signaling. This study aimed to investigate the role of spinal CRTC1 in the maintenance of bone cancer pain using an RNA interference method.

Results

Osteosarcoma cells were implanted into the intramedullary space of the right femurs of C3H/HeNCrlVr mice to induce bone cancer pain. Western blotting was applied to examine the expression of spinal phospho-Ser133 CREB and CRTC1. We further investigated effects of repeated intrathecal administration with Adenoviruses expressing CRTC1-small interfering RNA (siRNA) on nociceptive behaviors and on the upregulation of CREB/CRTC1-target genes associated with bone cancer pain. Inoculation of osteosarcoma cells induced progressive mechanical allodynia and spontaneous pain, and resulted in upregulation of spinal p-CREB and CRTC1. Repeated intrathecal administration with Adenoviruses expressing CRTC1-siRNA attenuated bone cancer–evoked pain behaviors, and reduced CREB/CRTC1-target genes expression in spinal cord, including BDNF, NR2B, and miR-212/132.

Conclusions

Upregulation of CRTC1 enhancing CREB-dependent gene transcription in spinal cord may play an important role in bone cancer pain. Inhibition of spinal CRTC1 expression reduced bone cancer pain. Interruption to the positive feedback circuit between CREB/CRTC1 and its targets may contribute to the analgesic effects. These findings may provide further insight into the mechanisms and treatment of bone cancer pain.

Positive feedback regulation between microRNA-132 and CREB in spinal cord contributes to bone cancer pain in mice

BACKGROUND

cAMP response element-binding protein (CREB)-dependent gene expression plays an important role in central sensitization. CREB-regulated transcription coactivator 1 (CRTC1) dramatically increase CREB-mediated transcriptional activity. microRNA-132 (miR-132), which is highly CREB-responsive, functions downstream from CREB/CRTC1 to mediate activity-dependent synaptic plasticity and in turn loops back to amplify CREB/CRTC1 signalling. This study aimed to investigate the positive feedback regulation between miR-132 and CREB in spinal cord in the maintenance of bone cancer pain.

METHODS

Osteosarcoma cells were implanted into the intramedullary space of the right femurs of C3H/HeNCrlVr mice to induce bone cancer pain. We further investigated effects of repeated intrathecal administration with Adenoviruses expressing CREB-siRNA or miR-132 antisense locked nucleic acid (LNA), respectively, on nociceptive behaviours and on the activity of CREB/CRTC1 signalling.

RESULTS

Intramedullary inoculation of osteosarcoma cells resulted in up-regulation of spinal p-CREB, CRTC1 and CREB-target genes (NR2B and miR-132). Repeated intrathecal administration with Adenoviruses expressing CREB-siRNA or miR-132 LNA-AS, respectively, attenuated bone cancer-evoked pain behaviours, reduced the activity of CREB/CRTC1 signalling and down-regulated CREB-target gene NR2B expression in spinal cord.

CONCLUSIONS

These findings suggest that activation of spinal CREB/CRTC1 signalling may play an important role in bone cancer pain. Interruption to the positive feedback regulation between CREB/CRTC1 and its target gene miR-132 can effectively relieved the bone cancer-induced mechanical allodynia and spontaneous pain. WHAT DOES THIS STUDY ADD?: The positive feedback regulation between CREB/CRTC1 and its target gene miR-132 in spinal cord plays an important role in bone cancer pain.

Downregulation of connexin36 in mouse spinal dorsal horn neurons leads to mechanical allodynia

Connexin36 (Cx36), a component of neuronal gap junctions, is crucial for interneuronal communication and regulation. Gap junction dysfunction underlies neurological disorders, including chronic pain. Following a peripheral nerve injury, Cx36 expression in the ipsilateral spinal dorsal horn was markedly decreased over time, which paralleled the time course of hind paw tactile allodynia. Intrathecal (i.t.) injection of Cx36 siRNA (1 and 5 pg) significantly reduced the expression of Cx36 protein in the lumbar spinal cord, peaking 3 days after the injection, which corresponded with the onset of hind paw tactile allodynia. It is possible that some of the tactile allodynia resulting from Cx36 downregulation could be mediated through excitatory neuromodulators, such as glutamate and substance P. The Cx36 knockdown-evoked tactile allodynia was significantly attenuated by i.t. treatment with the N-methyl-D-aspartate glutamate receptor antagonist MK-801 but not the substance P receptor antagonist CP96345. Immunohistochemistry showed that Cx36 was colocalized with glycine transporter-2, a marker for inhibitory glycinergic spinal interneurons, but not with glutamate decarboxylase 67, a marker for inhibitory GABAergic spinal interneurons. The results indicate that spinal inhibition through glycinergic interneurons is reduced, leading to increased glutamatergic neurotransmission, as a result of Cx36 downregulation. The current data suggest that gap junction dysfunction underlies neuropathic pain and further suggest a novel target for the development of analgesics.

Epigenetic upregulation of Cdk5 in the dorsal horn contributes to neuropathic pain in rats

Numerous reports have shown that cyclin-dependent kinase 5 (Cdk5), a proline-directed serine/threonine kinase, critically contributes to the induction and maintenance of chronic pain induced by peripheral inflammation and nerve injury. Recent evidence has also suggested the critical role of an epigenetic mechanism in the setting of chronic pain. The present study aims to elucidate the cyclic AMP response element-binding protein (CREB)-mediated upregulation of Cdk5 and its functional significance in rats with neuropathic pain induced by chronic constriction injury (CCI) in the sciatic nerve. Significantly increased expression of Cdk5 was observed in the dorsal horn of rats with CCI, and intrathecal delivery of Cdk5 inhibitor roscovitine significantly attenuated the mechanical allodynia in these rats. Phosphorylation of CREB and its occupancy in the Cdk5 promoter region was also increased in the dorsal horn, which led to increased histone H4 acetylation in the Cdk5 promoter region and the upregulated transcription of Cdk5. Inhibition of CREB activity attenuated the upregulation of Cdk5 and alleviated the mechanical allodynia in rats with CCI. These results demonstrated a CREB-mediated epigenetic upregulation of Cdk5 in the dorsal horn, which critically contributed to the maintenance of painful behavior in the rats with neuropathic pain.

PKA is required for the modulation of spinal nociceptive information related to ephrinB-EphB signaling in mice

EphB receptors and their ephrinB ligands are implicated in modulating of spinal nociceptive information processing. Here, we investigated whether protein kinase A (PKA), acts as a downstream effector, participates in the modulation spinal nociceptive information related to ephrinB-EphB signaling. Intrathecal injection of ephrinB2-Fc caused thermal hyperalgesia and mechanical allodynia, which were accompanied by increased expression of spinal PKA catalytic subunit (PKAca) and phosphorylated cAMP-response element-binding protein (p-CREB). Pre-treatment with H89, a PKA inhibitor, prevented the activation of CREB by ephrinB2-Fc. Inhibition of spinal PKA signaling prevented and reversed pain behaviors induced by the intrathecal injection of ephrinB2-Fc. Furthermore, blockade of the EphB receptors by intrathecal injection of EphB2-Fc reduced formalin-induced inflammatory, chronic constrictive injury (CCI)-induced neuropathic, and tibia bone cavity tumor cell implantation (TCI)-induced bone cancer pain behaviors, which were accompanied by decreased expression of spinal PKAca and p-CREB. Overall, these results confirmed the important involvement of PKA in the modulation of spinal nociceptive information related to ephrinBs-EphBs signaling. This finding may have important implications for exploring the roles and mechanisms of ephrinB-EphB signaling in physiologic and pathologic pain.

RhoA/ROCK pathway mediates p38 MAPK activation and morphological changes downstream of P2Y12/13 receptors in spinal microglia in neuropathic pain

Recent studies have indicated an important role of ATP receptors in spinal microglia, such as P2Y12 or P2Y13, in the development of chronic pain. However, intracellular signaling cascade of these receptors have not been clearly elucidated. We found that intrathecal injection of 2-(methylthio)adenosine 5'-diphosphate (2Me-SADP) induced mechanical hypersensitivity and p38 mitogen-activated protein kinase (MAPK) phosphorylation in the spinal cord. Intrathecal administration of P2Y12/P2Y13 antagonists and Rho-associated coiled-coil-containing protein kinase (ROCK) inhibitor H1152 suppressed not only p38 MAPK phosphorylation, but also mechanical hypersensitivity induced by 2Me-SADP. In the rat peripheral nerve injury model, intrathecal administration of antagonists for the P2Y12/P2Y13 receptor suppressed activation of p38 MAPK in the spinal cord. In addition, subarachnoidal injection of H1152 also attenuated nerve injury-induced spinal p38 MAPK phosphorylation and neuropathic pain behavior, suggesting an essential role of ROCK in nerve injury-induced p38 MAPK activation. We also found that the antagonists of the P2Y12/P2Y13 receptor and H1152 had inhibitory effects on the morphological changes of microglia such as retraction of processes in both 2Me-SADP and nerve injured rats. In contrast these treatments had no effect on the number of Iba1-positive cells in the nerve injury model. Collectively, our results have demonstrated roles of ROCK in the spinal microglia that is involved in p38 MAPK activation and the morphological changes. Inhibition of ROCK signaling may offer a novel target for the development of a neuropathic pain treatment.

Evidence for involvement of spinal RANTES in the antinociceptive effects of triptolide, a diterpene triepoxide, in a rat model of bone cancer pain

It has been shown that triptolide has beneficial effects in the treatment of neuropathic pain, but its effects on bone cancer pain (BCP) remain unclear. In this study, we aimed to explore the potential role of spinal regulated activation of normal T cell expressed and secreted (RANTES) in the antinociceptive effects of triptolide on BCP. A BCP model was induced by injecting Walker 256 mammary gland carcinoma cells into the intramedullary space of rat tibia. Intrathecal administration of triptolide (0.5, 1, 2 μg) could dose-dependently alleviate mechanical hyperalgesia and spontaneous pain. In addition, there were also concomitant decreases in RANTES mRNA and protein expression levels in spinal dorsal horn. These results suggest that the antinociceptive effects of triptolide are related with inhibition of spinal RANTES expression in BCP rats. The findings of this study may provide a promising drug for the treatment of BCP.