Increased phosphorylation of cyclic AMP response element-binding protein (CREB) in the superficial dorsal horn neurons following partial sciatic nerve ligation

Time-Course Progression of Whole Transcriptome Expression Changes of Trigeminal Ganglia Compared to Dorsal Root Ganglia in Rats Exposed to Nerve Injury

Mechanisms underlying neuropathic pain (NP) are complex with multiple genes, their interactions, environmental and epigenetic factors being implicated. Transcriptional changes in the trigeminal (TG) and dorsal root (DRG) ganglia have been implicated in the development and maintenance of NP. Despite efforts to unravel molecular mechanisms of NP, many remain unknown. Also, most of the studies focused on the spinal system. Although the spinal and trigeminal systems share some of the molecular mechanisms, differences exist. We used RNA-sequencing technology to identify differentially expressed genes (DEGs) in the TG and DRG at baseline and 3 time points following the infraorbital or sciatic nerve injuries, respectively. Pathway analysis and comparison analysis were performed to identify differentially expressed pathways. Additionally, upstream regulator effects were investigated in the two systems. DEG (differentially expressed genes) analyses identified 3,225 genes to be differentially expressed between TG and DRG in naïve animals, 1,828 genes 4 days post injury, 5,644 at day 8 and 9,777 DEGs at 21 days postinjury. A comparison of top enriched canonical pathways revealed that a number of signaling pathway was significantly inhibited in the TG and activated in the DRG at 21 days postinjury. Finally, CORT upstream regulator was predicted to be inhibited in the TG while expression levels of the CSF1 upstream regulator were significantly elevated in the DRG at 21 days postinjury. This study provides a basis for further in-depth studies investigating transcriptional changes, pathways, and upstream regulation in TG and DRG in rats exposed to peripheral nerve injuries. PERSPECTIVE: Although trigeminal and dorsal root ganglia are homologs of each other, they respond differently to nerve injury and therefore treatment. Activation/inhibition of number of biological pathways appear to be ganglion/system specific suggesting that different approaches might be required to successfully treat neuropathies induced by injuries in spinal and trigeminal systems.

Over-expression of miR-3584-5p Represses Nav1.8 Channel Aggravating Neuropathic Pain caused by Chronic Constriction Injury

Nav1.8, a tetrodotoxin-resistant voltage-gated sodium channels (VGSCs) subtype encoded by SCN10A, which plays an important role in the production and transmission of peripheral neuropathic pain signals. Studies have shown that VGSCs may be key targets of MicroRNAs (miRNAs) in the regulation of neuropathic pain. In our study, bioinformatics analysis showed that the targeting relationship between miR-3584-5p and Nav1.8 was the most closely. The purpose of this study was to investigate the roles of miR-3584-5p and Nav1.8 in neuropathic pain. The effects of miR-3584-5p on chronic constriction injury (CCI)-induced neuropathic pain in rats was investigated by intrathecal injection of miR-3584-5p agomir (an agonist, 20 μM, 15 μL) or antagomir (an antagonist, 20 μM, 15 μL). The results showed that over-expression of miR-3584-5p aggravated neuronal injury by hematoxylin-eosin (H&E) staining and mechanical/thermal hypersensitivity in CCI rats. MiR-3584-5p indirectly inhibited the expression of Nav1.8 by up-regulating the expression of key proteins in the ERK5/CREB signaling pathway, and also inhibited the current density of the Nav1.8 channel, changed its channel dynamics characteristic, thereby accelerating the transmission of pain signals, and further aggravating pain. Similarly, in PC12 and SH-SY5Y cell cultures, miR-3584-5p increased the level of reactive oxygen species (ROS) and inhibited mitochondrial membrane potential (Δψm) in the mitochondrial pathway, decreased the ratio of apoptosis-related factor Bcl-2/Bax, and thus promoted neuronal apoptosis. In brief, over-expression of miR-3584-5p aggravates neuropathic pain by directly inhibiting the current density of Nav1.8 channel and altering its channel dynamics, or indirectly inhibiting Nav1.8 expression through ERK5/CREB pathway, and promoting apoptosis through mitochondrial pathway.

Fibromyalgia: Pathogenesis, Mechanisms, Diagnosis and Treatment Options Update

Fibromyalgia is a syndrome characterized by chronic and widespread musculoskeletal pain, often accompanied by other symptoms, such as fatigue, intestinal disorders and alterations in sleep and mood. It is estimated that two to eight percent of the world population is affected by fibromyalgia. From a medical point of view, this pathology still presents inexplicable aspects. It is known that fibromyalgia is caused by a central sensitization phenomenon characterized by the dysfunction of neuro-circuits, which involves the perception, transmission and processing of afferent nociceptive stimuli, with the prevalent manifestation of pain at the level of the locomotor system. In recent years, the pathogenesis of fibromyalgia has also been linked to other factors, such as inflammatory, immune, endocrine, genetic and psychosocial factors. A rheumatologist typically makes a diagnosis of fibromyalgia when the patient describes a history of pain spreading in all quadrants of the body for at least three months and when pain is caused by digital pressure in at least 11 out of 18 allogenic points, called tender points. Fibromyalgia does not involve organic damage, and several diagnostic approaches have been developed in recent years, including the analysis of genetic, epigenetic and serological biomarkers. Symptoms often begin after physical or emotional trauma, but in many cases, there appears to be no obvious trigger. Women are more prone to developing the disease than men. Unfortunately, the conventional medical therapies that target this pathology produce limited benefits. They remain largely pharmacological in nature and tend to treat the symptomatic aspects of various disorders reported by the patient. The statistics, however, highlight the fact that 90% of people with fibromyalgia also turn to complementary medicine to manage their symptoms.

Primary culture of the rat spinal dorsal horn: a tool to investigate the effects of inflammatory stimulation on the afferent somatosensory system

One maladaptive consequence of inflammatory stimulation of the afferent somatosensory system is the manifestation of inflammatory pain. We established and characterized a neuroglial primary culture of the rat superficial dorsal horn (SDH) of the spinal cord to test responses of this structure to neurochemical, somatosensory, or inflammatory stimulation. Primary cultures of the rat SDH consist of neurons (43%), oligodendrocytes (35%), astrocytes (13%), and microglial cells (9%). Neurons of the SDH responded to cooling (7%), heating (18%), glutamate (80%), substance P (43%), prostaglandin E₂ (8%), and KCl (100%) with transient increases in the intracellular calcium [Ca²⁺]_i. Short-term stimulation of SDH primary cultures with LPS (10 μg/ml, 2 h) caused increased expression of pro-inflammatory cytokines, inflammatory transcription factors, and inducible enzymes responsible for inflammatory prostaglandin E₂ synthesis. At the protein level, increased concentrations of tumor necrosis factor-α (TNFα) and interleukin-6 (IL-6) were measured in the supernatants of LPS-stimulated SDH cultures and enhanced TNFα and IL-6 immunoreactivity was observed specifically in microglial cells. LPS-exposed microglial cells further showed increased nuclear immunoreactivity for the inflammatory transcription factors NFκB, NF-IL6, and pCREB, indicative of their activation. The short-term exposure to LPS further caused a reduction in the strength of substance P as opposed to glutamate-evoked Ca²⁺-signals in SDH neurons. However, long-term stimulation with a low dose of LPS (0.01 μg/ml, 24 h) resulted in a significant enhancement of glutamate-induced Ca²⁺ transients in SDH neurons, while substance P-evoked Ca²⁺ signals were not influenced. Our data suggest a critical role for microglial cells in the initiation of inflammatory processes within the SDH of the spinal cord, which are accompanied by a modulation of neuronal responses.

Expression of the pacemaker channel HCN4 in excitatory interneurons in the dorsal horn of the murine spinal cord

In the central nervous system, hyperpolarization-activated, cyclic nucleotide-gated (HCN1–4) channels have been implicated in neuronal excitability and synaptic transmission. It has been reported that HCN channels are expressed in the spinal cord, but knowledge about their physiological roles, as well as their distribution profiles, appear to be limited. We generated a transgenic mouse in which the expression of HCN4 can be reversibly knocked down using a genetic tetracycline-dependent switch and conducted genetically validated immunohistochemistry for HCN4. We found that the somata of HCN4-immunoreactive (IR) cells were largely restricted to the ventral part of the inner lamina II and lamina III. Many of these cells were either parvalbumin- or protein kinase Cγ (PKCγ)-IR. By using two different mouse strains in which reporters are expressed only in inhibitory neurons, we determined that the vast majority of HCN4-IR cells were excitatory neurons. Mechanical and thermal noxious stimulation did not induce c-Fos expression in HCN4-IR cells. PKCγ-neurons in this area are known to play a pivotal role in the polysynaptic pathway between tactile afferents and nociceptive projection cells that contributes to tactile allodynia. Therefore, pharmacological and/or genetic manipulations of HCN4-expressing neurons may provide a novel therapeutic strategy for the pain relief of tactile allodynia.

Up-regulation of astrocyte excitatory amino acid transporter 2 alleviates central sensitization in a rat model of chronic migraine

Central sensitization is the potential pathogenesis of chronic migraine (CM) and is related to persistent neuronal hyperexcitability. Dysfunction of excitatory amino acid transporter 2 (EAAT2) leads to the accumulation of glutamate in the synaptic cleft, which may contribute to central sensitization by overactivating glutamate N-methyl-D-aspartate receptors and enhancing synaptic plasticity. However, the therapeutic potential of CM by targeting glutamate clearance remains largely unexplored. The purpose of this study was to investigate the role of EAAT2 in CM central sensitization and explore the effect of EAAT2 expression enhancer LDN-212320 in CM rats. The glutamate concentration was measured by high-performance liquid chromatography in a rat model of CM. Then, q-PCR and western blots were performed to detect EAAT2 expression, and the immunoreactivity of astrocytes was detected by immunofluorescence staining. To understand the effect of EAAT2 on central sensitization of CM, mechanical and thermal hyperalgesia and central sensitization-associated proteins were examined after administration of LDN-212320. In addition, the expression of synaptic-associated proteins was examined and Golgi-Cox staining was used to observe the dendritic spine density of trigeminal nucleus caudalis neurons. Also, the synaptic ultrastructure was observed by transmission electron microscope (TEM) to explore the changes of synaptic plasticity. In our study, elevated glutamate concentration and decreased EAAT2 expression were found in the trigeminal nucleus caudalis of CM rats, administration of LDN-212320 greatly up-regulated the protein expression of EAAT2, alleviated hyperalgesia, decreased the concentration of glutamate and the activation of astrocytes. Furthermore, reductions in calcitonin gene-related peptide, substance P(SP), and phosphorylated NR2B were examined after administration of LDN-212320. Moreover evaluation of the synaptic structure, synaptic plasticity-, and central sensitization-related proteins indicated that EAAT2 might participate in the CM central sensitization process by regulating synaptic plasticity. Taken together, up-regulation of EAAT2 expression has a protective effect in CM rats, and LDN-212320 may have clinical therapeutic potential. Cover Image for this issue: https://doi.org/10.1111/jnc.14769.

Network and pathway-based analysis of microRNA role in neuropathic pain in rat models

The molecular mechanisms underlying neuropathic pain (NP) remain poorly understood. Emerging evidence has suggested the role of microRNAs (miRNAs) in the initiation and development of NP, but the specific effects of miRNAs in NP are largely unknown. Here, we use network- and pathway-based methods to investigate NP-induced miRNA changes and their biological functions by conducting a systematic search through multiple electronic databases. Thirty-seven articles meet the inclusion criteria. Venn analysis and target gene forecasting are performed and the results indicate that 167 overlapping target genes are co-regulated by five down-regulated miRNAs (rno-miR-183, rno-miR-96, rno-miR-30b, rno-miR-150 and rno-miR-206). Protein-protein interaction network analysis shows that 77 genes exhibit interactions, with cyclic adenosine monophosphate (cAMP)-dependent protein kinase catalytic subunit beta (degree = 11) and cAMP-response element binding protein 1 (degree = 10) having the highest connectivity degree. Gene ontology analysis shows that these target genes are enriched in neuron part, neuron projection, somatodendritic compartment and nervous system development. Moreover, analysis of Kyoto Encyclopedia of Genes and Genomes reveals that three pathways, namely, axon guidance, circadian entrainment and insulin secretion, are significantly enriched. In addition, rno-miR-183, rno-miR-96, rno-miR-30b, rno-miR-150 and rno-miR-206 are consistently down-regulated in the NP models, thus constituting the potential biomarkers of this disease. Characterizing these miRNAs and their target genes paves way for their future use in clinical practice.

The Regulatory Mechanisms and Therapeutic Potential of MicroRNAs: From Chronic Pain to Morphine Tolerance

Chronic pain, including cancer-related pain, is a pain condition often caused by inflammation or dysfunctional nerves. Chronic pain treatment poses a significant health care challenge, where opioids especially morphine are widely used and patients often develop tolerance over time with aggravated pain. microRNA (miRNA) is known to play important roles in regulating gene expressions in the nervous system to affect neuronal network plasticity related to algogenesis and the developing of morphine tolerance. In this article, we reviewed studies conducted in rodent animal models investigating the mechanisms of miRNAs regulation in chronic pain with different phenotypes and morphine tolerance. In addition, the potential of targeting miRNAs for chronic pain and morphine tolerance treatment is also reviewed. Finally, we point out the directions of the future research in chronic pain and morphine tolerance.

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

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

Comorbid anxiety-like behavior and locus coeruleus impairment in diabetic peripheral neuropathy: A comparative study with the chronic constriction injury model

Anxiety frequently appears in patients with diabetic neuropathic pain, a highly prevalent clinical condition. However, the neurobiological mechanisms of this comorbidity are poorly known. Anxiogenic phenotype has been associated with alterations of the noradrenergic locus coeruleus (LC) after peripheral nerve entrapment. We have examined the sensorial (pain) and affective (anxiety) behaviors, and the LC activity in streptozotocin (STZ)-induced diabetic rats. A comparative study with the chronic constriction injury (CCI) model of sciatic nerve was also carried out. Diabetic nociceptive hypersensitivity was observed to appear gradually, reaching their maximum at fourth week. In contrast, CCI displayed a sharp decrease in their sensorial threshold at seventh day. In both models, anxiety-like phenotype was evident after four weeks but not earlier, coincident with the LC alterations. Indeed, STZ animals showed reduced LC firing activity, tyrosine hydroxylase, pCREB and noradrenaline transporter levels, contrary to observed in CCI animals. However, in both models, enhanced LC alpha2-adrenoceptor sensitivity was presented at this time point. This study demonstrated that diabetes induced anxiety-like behavior comorbid with LC impairment at long-term. However, the nociceptive sensitivity time-course, as well as the LC functions, showed distinct features compared to the CCI model, indicating that specific neuroplastic mechanisms are at play in every model.

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.

The Central Analgesic Mechanism of YM-58483 in Attenuating Neuropathic Pain in Rats

Calcium channel antagonists are commonly used to treat neuropathic pain. Their analgesic effects rely on inhibiting long-term potentiation, and neurotransmitters release in the spinal cord. Store-operated Ca(2+)channels (SOCCs) are highly Ca(2+)-selective cation channels broadly expressed in non-excitable cells and some excitable cells. Recent studies have shown that the potent inhibitor of SOCCs, YM-58483, has analgesic effects on neuropathic pain, but its mechanism is unclear. This experiment performed on spinal nerve ligation (SNL)-induced neuropathic pain model in rats tries to explore the mechanism, whereby YM-58483 attenuates neuropathic pain. The left L5 was ligated to produce the SNL neuropathic pain model in male Sprague-Dawley rats. The withdrawal threshold of rats was measured by the up-down method and Hargreaves' method before and after intrathecal administration of YM-58483 and vehicle. The SOCCs in the spinal dorsal horn were located by immunofluorescence. The expression of phosphorylated ERK and phosphorylated CREB, CD11b, and GFAP proteins in spinal level was tested by Western blot, while the release of proinflammatory cytokines (IL-1β, TNF-α, PGE2) was measured by enzyme-linked immunosorbent assay (ELISA). Intrathecal YM-58483 at the concentration of 300 μM (1.5 nmol) and 1000 μM (10 nmol) produced a significant central analgesic effect on the SNL rats, compared with control + vehicle (n = 7, P < 0.001). However, both could not prevent the development of neuropathic pain, compared with normal + saline (P < 0.001). Immunofluorescent staining revealed that Orai1 and STIM1 (the two key components of SOCCs) were located in the spinal dorsal horn neurons. Western blot showed that YM-58483 could decrease the levels of P-ERK and P-CREB (n = 10, #P < 0.05), without affecting the expression of CD11b and GFAP (n = 10, #P > 0.05). YM-58483 also inhibited the release of spinal cord IL-1β, TNF-α, and PGE2, compared with control + vehicle (n = 5, #P < 0.001). The analgesic mechanism of YM-58483 may be via inhibiting central ERK/CREB signaling in the neurons and decreasing central IL-1β, TNF-α, and PGE2 release to reduce neuronal excitability in the spinal dorsal horn of the SNL rats.

Mechanical Antiallodynic Effect of Intrathecal Nefopam in a Rat Neuropathic Pain Model

Nefopam has a pharmacologic profile distinct from that of opioids or other anti-inflammatory drugs. Several recent studies demonstrate that nefopam has a mechanism of action similar to those of anti-depressants and anticonvulsants for treating neuropathic pain. The present study investigates the mechanical antiallodynic effect of nefopam using immunohistochemical study and western blot analysis in a rat neuropathic pain model. Twenty-eight male Sprague-Dawley rats were subjected to left fifth lumbar (L5) spinal nerve ligation and intrathecal catheter implantation, procedures which were not performed on the 7 male Sprague-Dawley rats in the sham surgery group (group S). Nefopam, either 10 or 100 µg/kg (group N10 or N100, respectively), and normal saline (group C) were intrathecally administered into the catheter every day for 14 days. The mechanical allodynic threshold of intrathecal nefopam was measured using a dynamic plantar aesthesiometer. Immunohistochemistry targeting cluster of differentiation molecule 11b (CD11b) and glial fibrillary acidic protein (GFAP) was performed on the harvested spinal cord at the level of L5. Extracellular signal-regulated kinase 1/2 (ERK 1/2) and cyclic adenosine monophosphate response element binding protein (CREB) were measured using western blot analysis. The N10 and N100 groups showed improved mechanical allodynic threshold, reduced CD11b and GFAP expression, and attenuated ERK 1/2 and CREB in the affected L5 spinal cord. In conclusion, intrathecal nefopam reduced mechanical allodynia in a rat neuropathic pain model. Its mechanical antiallodynic effect is associated with inhibition of glial activation and suppression of the transcription factors' mitogen-activated protein kinases in the spinal cord.

Sodium hydrosulfide relieves neuropathic pain in chronic constriction injured rats

Aberrant neuronal activity in injured peripheral nerves is believed to be an important factor in the development of neuropathic pain (NPP). Channel protein pCREB of that activity has been shown to mitigate the onset of associated molecular events in the nervous system, and sodium hydrosulfide (NaHS) could inhibit the expression of pCREB. However, whether NaHS could relieve the pain, it needs further experimental research. Furthermore, the clinical potential that NaHS was used to relieve pain was limited so it would be required. To address these issues, the rats of sciatic nerve chronic constriction injury (CCI) were given intraperitoneal injection of NaHS containing hydrogen sulfide (H₂S). The experimental results showed that NaHS inhibited the reduction of paw withdrawal thermal latency (PWTL), mechanical withdrawal threshold (MWT), and the level of pCREB in CCI rats in a dose-dependent manner and they were greatly decreased in NaHS_M group (P < 0.05). NaHS alleviates chronic neuropathic pain by inhibiting expression of pCREB in the spinal cord of Sprague-Dawley rats.

Tumor necrosis factor-alpha induces expression of C/EBP-beta in primary afferent neurons following nerve injury

CCAAT/enhancer binding protein-beta (C/EBP-beta) is a transcription factor that belongs to the C/EBP family. To understand the role of C/EBP-beta in the peripheral nervous system, we investigated the expression of C/EBP-beta in the dorsal root ganglion. C/EBP-beta was weakly detected in nuclei of naive dorsal root ganglion (DRG) neurons. Spinal nerve ligation increased the expression of C/EBP-beta in L4 and L5 DRG neurons. Treatment with anti-TNF-alpha prevented SNL-induced pain hypersensitivity and C/EBP-beta expression in the DRG. Injection of TNF-alpha into the sciatic nerve produced transient pain hypersensitivity and induction of C/EBP-beta expression in the DRG. These results demonstrate that C/EBP-beta is activated in the DRG neurons by a TNF-alpha-dependent manner and might be involved in the activation of primary afferent neurons after nerve injury.

Local translation and retrograde axonal transport of CREB regulates IL-6-induced nociceptive plasticity

Transcriptional regulation of genes by cyclic AMP response element binding protein (CREB) is essential for the maintenance of long-term memory. Moreover, retrograde axonal trafficking of CREB in response to nerve growth factor (NGF) is critical for the survival of developing primary sensory neurons. We have previously demonstrated that hindpaw injection of interleukin-6 (IL-6) induces mechanical hypersensitivity and hyperalgesic priming that is prevented by the local injection of protein synthesis inhibitors. However, proteins that are locally synthesized that might lead to this effect have not been identified. We hypothesized that retrograde axonal trafficking of nascently synthesized CREB might link local, activity-dependent translation to nociceptive plasticity. To test this hypothesis, we determined if IL-6 enhances the expression of CREB and if it subsequently undergoes retrograde axonal transport. IL-6 treatment of sensory neurons in vitro caused an increase in CREB protein and in vivo treatment evoked an increase in CREB in the sciatic nerve consistent with retrograde transport. Importantly, co-injection of IL-6 with the methionine analogue azido-homoalanine (AHA), to assess nascently synthesized proteins, revealed an increase in CREB containing AHA in the sciatic nerve 2 hrs post injection, indicating retrograde transport of nascently synthesized CREB. Behaviorally, blockade of retrograde transport by disruption of microtubules or inhibition of dynein or intrathecal injection of cAMP response element (CRE) consensus sequence DNA oligonucleotides, which act as decoys for CREB DNA binding, prevented the development of IL-6-induced mechanical hypersensitivity and hyperalgesic priming. Consistent with previous studies in inflammatory models, intraplantar IL-6 enhanced the expression of BDNF in dorsal root ganglion (DRG). This effect was blocked by inhibition of retrograde axonal transport and by intrathecal CRE oligonucleotides. Collectively, these findings point to a novel mechanism of axonal translation and retrograde trafficking linking locally-generated signals to long-term nociceptive sensitization.

The inhibitory effect of somatostatin receptor activation on bee venom-evoked nociceptive behavior and pCREB expression in rats

The present study examined nociceptive behaviors and the expression of phosphorylated cAMP response element-binding protein (pCREB) in the dorsal horn of the lumbar spinal cord and the dorsal root ganglion (DRG) evoked by bee venom (BV). The effect of intraplantar preapplication of the somatostatin analog octreotide on nociceptive behaviors and pCREB expression was also examined. Subcutaneous injection of BV into the rat unilateral hindpaw pad induced significant spontaneous nociceptive behaviors, primary mechanical allodynia, primary thermal hyperalgesia, and mirror-thermal hyperalgesia, as well as an increase in pCREB expression in the lumbar spinal dorsal horn and DRG. Octreotide pretreatment significantly attenuated the BV-induced lifting/licking response and mechanical allodynia. Local injection of octreotide also significantly reduced pCREB expression in the lumbar spinal dorsal horn and DRG. Furthermore, pretreatment with cyclosomatostatin, a somatostatin receptor antagonist, reversed the octreotide-induced inhibition of the lifting/licking response, mechanical allodynia, and the expression of pCREB. These results suggest that BV can induce nociceptive responses and somatostatin receptors are involved in mediating the antinociception, which provides new evidence for peripheral analgesic action of somatostatin in an inflammatory pain state.

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

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

PKC-Dependent Signaling Pathways within PAG and Thalamus Contribute to the Nitric Oxide-Induced Nociceptive Behavior

Nitric oxide (NO) is an important molecule involved in nociceptive processing in the central nervous system. The release of NO within the spinal cord has long been implicated in the mechanisms underlying exaggerated pain sensitivity, and administration of NO donors can induce hyperalgesia. To elucidate the supraspinal mechanism responsible for NO-induced nociceptive hypersensitivity, we investigated the modulation of protein kinase C (PKC) and downstream effectors following treatment with the NO donors nitroglycerin and sodium nitroprusside. Both compounds induced a prolonged cold allodynia and heat hyperalgesia, increased levels of c-Fos and IL-1β, and activated NF-κB within periaqueductal grey matter and thalamus. Simultaneously, an increased expression and phosphorylation of PKC γ and ε were detected. To clarify the cellular mechanism involved in the NO-induced hypernociception, we examined the expression of transcription factors that act as PKC downstream effectors. A dramatic hyperphosphorylation of CREB and STAT1 was observed. The i.c.v. administration of the PKC blocker calphostin C prevented the NO-induced hypernociception, the hyperphosphorylation of CREB and STAT1, and partially reduced NF-κB activation. Conversely, the increase of IL-1β was unmodified by calphostin C. These results suggest the relevance of cerebral PKC-mediated CREB and STAT1 activation in the NO donor-induced nociceptive behavior.

Microglia and spinal cord synaptic plasticity in persistent pain

Microglia are regarded as macrophages in the central nervous system (CNS) and play an important role in neuroinflammation in the CNS. Microglial activation has been strongly implicated in neurodegeneration in the brain. Increasing evidence also suggests an important role of spinal cord microglia in the genesis of persistent pain, by releasing the proinflammatory cytokines tumor necrosis factor-alpha (TNFα), Interleukine-1beta (IL-1β), and brain derived neurotrophic factor (BDNF). In this review, we discuss the recent findings illustrating the importance of microglial mediators in regulating synaptic plasticity of the excitatory and inhibitory pain circuits in the spinal cord, leading to enhanced pain states. Insights into microglial-neuronal interactions in the spinal cord dorsal horn will not only further our understanding of neural plasticity but may also lead to novel therapeutics for chronic pain management.

Antinociceptive effects of oxymatrine from Sophora flavescens, through regulation of NR2B-containing NMDA receptor-ERK/CREB signaling in a mice model of neuropathic pain

PURPOSE

In this study we investigated antinociceptive effects of oxymatrine through regulation of NR2B-containing NMDA receptor-ERK/CREB signaling in a chronic neuropathic pain model induced by chronic constrictive injury (CCI) of the sciatic nerve.

METHODS

The von Frey and plantar tests were performed to assess the degree of mechanical and thermal changes respectively. Immunohistochemistry assay was used to evaluate the expressions of NR2B. Western blotting assay were used to evaluate the expressions of NR2B, tERK, p-ERK, tCREB and p-CREB.

RESULTS

The intraperitoneal administration of OMT (160, 80 mg/kg) could prevent the development of mechanical allodynia, thermal hyperalgesia induced by CCI. Intraperitoneal administration of OMT decreased the mean IOD of NR2B in the dorsal horn and expression of NR2B, p-ERK and p-CREB protein.

CONCLUSION

Regulation of NMDA NR2B receptor-ERK/CREB signaling maybe the targets for the antinociceptive effects of OMT on a chronic neuropathic pain model induced by chronic constrictive injury of the sciatic nerve.

Reversal of NO-induced nociceptive hypersensitivity by St. John's wort and hypericin: NF-κB, CREB and STAT1 as molecular targets

RATIONALE

Hypericum perforatum, popularly called St. John's wort (SJW), is a medicinal plant mainly used as antidepressant with a favorable safety profile than standard antidepressants. Some studies have also documented other SJW bioactivities, including pain modulation.

OBJECTIVES

The aim of this study was to demonstrate the capability of SJW to relieve nitric oxide (NO)-induced nociceptive hypersensitivity and identify the effective component.

METHODS

Nociceptive hypersensitivity induced by administration of the NO donors nitroglycerin (GTN) and sodium nitroprusside (SNP) was assessed by cold and hot plate tests. The cellular pathways and molecular targets involved were investigated by Western blotting.

RESULTS

GTN and SNP produced a prolonged allodynia and hyperalgesia in mice. A single oral administration of low doses of an SJW dried extract or purified hypericin reversed the NO donor-induced nociceptive behavior whereas hyperforin and flavoinoids were ineffective. Investigating into the cellular pathways involved, an increased CREB and STAT1 phosphorylation, and activation of NF-κB were detected within PAG and thalamus following NO donors' administration. These cellular events were prevented by SJW or hypericin. Since hypericin showed PKC blocking properties, a role of PKC as an upstream modulator of these transcription factors was hypothesized. NO donors increased expression and phosphorylation of protein kinase C (PKC) γ and ε isoforms, molecular events prevented by SJW or hypericin.

CONCLUSIONS

SJW reversed NO-induced nociceptive hypersensitivity through the blockade of a supraspinal signaling pathway involving a PKC-dependent CREB, STAT1 and NF-κB activation due to presence of hypericin. These data indicate SJW/hypericin as a therapeutic perspective for pain treatment.

Intrathecal administration of cyclic AMP response element-binding protein-antisense oligonucleotide attenuates neuropathic pain after peripheral nerve injury and decreases the expression of N-methyl-D-aspartic receptors in mice

The aim of the present study was to determine whether the cAMP response element binding protein (CREB) contributes to neuropathic pain during development stage. Adult (7-8 weeks old) male C57BL/6 mice weighing 20-25 g were used. Intrathecal catheter implantation and chronic constriction of the sciatic nerve of the animals were performed. Western blotting and reverse transcription PCR experiments were carried out. Our study demonstrated that the expression of spinal NMDAR after peripheral nerve injury was modulated by central CREB. Chronic constriction nerve injury (CCI) in mice induced thermal hyperalgesia and mechanical allodynia. The increase of NR1 and NR2B subunits of the NMDAR was significantly diminished by intrathecal administration of the CREB antisense oligonucleotide against CREB and pCREB. Additionally, nociceptive behavior induced by CCI was attenuated by intrathecal administration of the CREB antisense oligonucleotide during the period of injection, and the above effects of relieving pain lasted at least 12 days following the last injection. Our results suggested that central functional pCREB may contribute to the development of neuropathic pain and regulate the expression of the NR1 and NR2B subunits of the NMDAR in the process.

Inhibition of the PKCγ-ε pathway relieves from meningeal nociception in an animal model: an innovative perspective for migraine therapy?

There is convincing evidence that nitric oxide (NO) may be a causative factor in the pathogenesis of migraine. We investigated the consequences of NO donors' administration on meningeal processes related to the development of migraine pain in an animal model of meningeal nociception. The administration in mice of the NO donors nitroglycerin (GTN) and sodium nitroprusside (SNP) produced a delayed meningeal upregulation of interleukin-1ß and inducible NO synthase. A thermal allodynia and hyperalgesia devoid of side effects was produced 1 to 4 h after administration. To clarify the cellular pathways modulated by GTN and SNP, we examined the expression of cellular factors involved in pain modulation, such as protein kinase C (PKC) and its downstream effectors. Western blotting experiments showed an upregulation and increased phosphorylation of PKCγ and PKCε within dura mater after NO donors' administration. A dramatic PKC-dependent increase of the phosphorylation of cyclic AMP response element binding protein (CREB) and signal transducer and activator of transcription (STAT)-1 was observed, along with an activation of the nuclear factor-κB (NF-κB) pathway, as reflected by a reduction of the inhibitory protein-κ-Bα (IκBα). Furthermore, the PKC blocker, Calphostin C, prevented the GTN and SNP-induced pain hypersensitivity. These results suggest the relevance of the PKC-mediated pathway in the induction of meningeal nociception and might help clarify the etiopathology of migraines. We can suggest PKC as a new target for migraine pain.

Characterization of hyperpolarization-activated currents in deep dorsal horn neurons of neonate mouse spinal cord in vitro

Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces analgesia acting at peripheral sites. However, little is known about the role of this current in central pain-processing structures. The aim of the present work was to characterize the Ih in deep dorsal horn neurons and to assess the role of the current in the transmission of somatosensory signals across spinal circuits. To these purpose in vitro preparations of the spinal cord from mice pups were used in combination with whole cell recordings to characterize the current in native neurons. Extracellular recordings from sensory and motor pathways were performed to assess the role of the current in spinal somatosensory processing. Cesium chloride and ZD7288 were used as current blockers. Most deep dorsal horn neurons showed a functional Ih that was blocked by ZD7288 and cesium. Ih blockade caused hyperpolarization, increased input resistance and potentiation of synaptic responses. Excitatory effects of Ih blockade on synaptic transmission were confirmed in projecting anterolateral axons and ventral roots. Ih modulation by cAMP produced a rightward shift in the voltage dependency curve and blocked excitatory effects of ZD7288 on sensory pathways. Results indicate that Ih currents play a stabilizing role in the spinal cord controlling transmission across sensory and motor spinal pathways via cellular effects on input resistance and excitability. In addition, results suggest that current modulation may alter significantly the role of the current in somatosensory processing.

Targeted deletion of LPA5 identifies novel roles for lysophosphatidic acid signaling in development of neuropathic pain

Lysophosphatidic acid (LPA) is a bioactive lipid that serves as an extracellular signaling molecule acting through cognate G protein-coupled receptors designated LPA(1-6) that mediate a wide range of both normal and pathological effects. Previously, LPA(1), a G(αi)-coupled receptor (which also couples to other G(α) proteins) to reduce cAMP, was shown to be essential for the initiation of neuropathic pain in the partial sciatic nerve ligation (PSNL) mouse model. Subsequent gene expression studies identified LPA(5), a G(α12/13)- and G(q)-coupled receptor that increases cAMP, in a subset of dorsal root ganglion neurons and also within neurons of the spinal cord dorsal horn in a pattern complementing, yet distinct from LPA(1), suggesting its possible involvement in neuropathic pain. We therefore generated an Lpar5 null mutant by targeted deletion followed by PSNL challenge. Homozygous null mutants did not show obvious base-line phenotypic defects. However, following PSNL, LPA(5)-deficient mice were protected from developing neuropathic pain. They also showed reduced phosphorylated cAMP response element-binding protein expression within neurons of the dorsal horn despite continued up-regulation of the characteristic pain-related markers Caα(2)δ(1) and glial fibrillary acidic protein, results that were distinct from those previously observed for LPA(1) deletion. These data expand the influences of LPA signaling in neuropathic pain through a second LPA receptor subtype, LPA(5), involving a mechanistically distinct downstream signaling pathway compared with LPA(1).

Calcium/calmodulin dependent kinase II contributes to persistent central neuropathic pain following spinal cord injury

Chronic central neuropathic pain after central nervous system injuries remains refractory to therapeutic interventions. A novel approach would be to target key intracellular signaling proteins that are known to contribute to continued activation by phosphorylation of kinases, transcription factors, and/or receptors that contribute to changes in membrane excitability. We demonstrate that one signaling kinase, calcium/calmodulin-dependent kinase II (CaMKII), is critical in maintaining aberrant dorsal horn neuron hyperexcitability in the neuropathic pain condition after spinal cord injury (SCI). After contusion SCI at spinal level T10, activated CaMKII (phosphorylated, pCaMKII) expression is significantly upregulated in the T7/8 spinal dorsal horn in neurons, but not glial cells, and in oligodendrocytes in the dorsal column in the same rats that displayed at-level mechanical allodynia. Furthermore, identified spinothalamic neurons demonstrated significant increases of pCaMKII after SCI compared to sham-treated control animals. However, neither astrocytes nor microglia showed pCaMKII expression in either sham-treated or SCI rats. To demonstrate causality, treatment of SCI rats with KN-93, which prevents CaMKII activation, significantly attenuated at-level mechanical allodynia and aberrant wide dynamic range neuronal activity evoked by brush, pressure, and pinch stimuli and a graded series of von Frey stimuli, respectively. Persistent CaMKII activation contributes to chronic central neuropathic pain by mechanisms that involve maintained hyperexcitability of wide dynamic range dorsal horn neurons. Furthermore, targeting key signaling proteins is a novel, useful therapeutic strategy for treating chronic central neuropathic pain.

A time-dependent history of mood disorders in a murine model of neuropathic pain

BACKGROUND

Chronic pain is clinically associated with the development of affective disorders. However, studies in animal models of neuropathic pain are contradictory and the relationship with mood disorders remains unclear. In this study, we aimed to characterize the affective consequences of neuropathic pain over time and to study potential underlying mechanisms.

METHODS

Neuropathic pain was induced by inserting a polyethylene cuff around the main branch of the right sciatic nerve in C57BL/6J mice. Anxiety- and depression-related behaviors were assessed over 2 months, using a battery of tests, such as elevated plus maze, marble burying, novelty suppressed feeding, splash test, and forced swimming test. Plasma corticosterone levels were assessed by radioimmunoassay. We also investigated changes in cyclic adenosine monophosphate response element (CRE) activity using CRE-LacZ transgenic mice.

RESULTS

Mice developed anxiety-related behavior 4 weeks after induction of the neuropathy, and depression-related behaviors were observed after 6 to 8 weeks. Control and neuropathic mice did not differ for basal or stress-induced levels of corticosterone or for hypothalamic-pituitary-adrenal axis negative feedback. After 8 weeks, the CRE-mediated activity decreased in the outer granule layer of dentate gyrus of neuropathic mice but not in the amygdala or in the anterior cingulate cortex.

CONCLUSIONS

Our results demonstrate that the affective consequences of neuropathic pain evolve over time, independently from the hypothalamic-pituitary-adrenal axis, which remains unaffected. CRE-mediated transcription within a limbic structure was altered at later time points of the neuropathy. These experiments provide a preclinical model to study time-dependent development of mood disorders and the underlying mechanism in a neuropathic pain context.

Cannabinoid agonists inhibit neuropathic pain induced by brachial plexus avulsion in mice by affecting glial cells and MAP kinases

Background

Many studies have shown the antinociceptive effects of cannabinoid (CB) agonists in different models of pain. Herein, we have investigated their relevance in neuropathic pain induced by brachial plexus avulsion (BPA) in mice.

Methodology/Principal Findings

Mice underwent BPA or sham surgery. The mRNA levels and protein expression of CB₁ and CB₂ receptors were assessed by RT-PCR and immunohistochemistry, respectively. The activation of glial cells, MAP kinases and transcription factors were evaluated by immunohistochemistry. The antinociceptive properties induced by cannabinoid agonists were assessed on the 5^th and 30^th days after surgery. We observed a marked increase in CB₁ and CB₂ receptor mRNA and protein expression in the spinal cord and dorsal root ganglion, either at the 5^th or 30^th day after surgery. BPA also induced a marked activation of p38 and JNK MAP kinases (on the 30^th day), glial cells, such as microglia and astrocytes, and the transcription factors CREB and NF-κB (at the 5^th and 30^th days) in the spinal cord. Systemic treatment with cannabinoid agonists reduced mechanical allodynia on both the 5^th and 30^th days after surgery, but the greatest results were observed by using central routes of administration, especially at the 30^th day. Treatment with WIN 55,212-2 prevented the activation of both glial cells and MAP kinases, associated with an enhancement of CREB and NF-κB activation.

Conclusions/Significance

Our results indicate a relevant role for cannabinoid agonists in BPA, reinforcing their potential therapeutic relevance for the management of chronic pain states.

Targeting pain where it resides ... In the brain

Chronic pain is estimated to be the third most prevalent health problem in the world. Although scientists have made great strides in our understanding of the molecular mechanisms through which chronic pain develops, this knowledge has not been translated into new therapies. In this issue of Science Translational Medicine, Wang and colleagues report on the development of a selective antagonist of type 1 adenylate cyclase (AC1), which is induced in subsets of neurons in the central nervous system during the development of neuropathic pain. Blockade of AC1 significantly alleviates the mechanical hypersensitivity that occurs in a mouse model of neuropathic pain without affecting acute pain responsiveness or cognitive and motor function. These features make AC1 a potential therapeutic target for the treatment of neuropathic pain.

Changes in serotoninergic and noradrenergic descending pain pathways during painful diabetic neuropathy: the preventive action of IGF1

Painful diabetic neuropathy (PDN) induces neuronal hyperactivity at the spinal cord and periaqueductal gray (PAG), a key area in descending nociceptive modulation. Since the PAG uses relay stations at serotoninergic and noradrenergic brainstem areas, we determined the serotonin and noradrenaline levels at the spinal cord of streptozotocin-diabetic rats and at those brainstem areas (serotoninergic rostroventromedial medulla and noradrenergic A(5) and A(7) cell groups). Since, during diabetes, the levels of insulin growth factor 1 (IGF1) decrease, reducing its neurotrophic effect in the brain, we also studied the effects of IGF1 treatment. One week after diabetes induction, subcutaneous injections of IGF1 (2.5mg/kg) were performed during 3 weeks. Body weights, glycemia, and mechanical nociception were weekly evaluated until the end of the study, the time when the animals were subjected to a modified formalin test to study chemical allodynia. Serotonin and noradrenaline levels were quantified by ELISA at the spinal cord, whereas at the brainstem, the quantification was performed by immunohistochemistry against, respectively, tryptophan hydroxylase (TpH) or tyrosine hydroxylase (TH). STZ-diabetic rats exhibited mechanical hyperalgesia and chemical allodynia, along with higher spinal levels of serotonin and noradrenaline and higher numbers of neurons expressing TpH at the RVM and TH at the A(5) noradrenergic cell group. Treatment with IGF1 prevented the behavioral signs of PDN and reversed the neuronal hyperactivity at the spinal cord and ventrolateral PAG and the neurochemical changes at the spinal cord and at the brainstem. Based on the facilitatory role of serotoninergic and noradrenergic descending modulation during chronic pain, the increased serotonin and noradrenaline innervation of the dorsal horn in STZ-diabetic rats may probably account for enhanced pain during PDN. The benefits of IGF1 in PDN are probably due to blockade of the increased peripheral input to the somatosensory system, but direct central actions cannot be discarded. The value of IGF1 in PDN treatment deserves further evaluation.

Knockdown of L calcium channel subtypes: differential effects in neuropathic pain

The maintenance of chronic pain states requires the regulation of gene expression, which relies on an influx of calcium. Calcium influx through neuronal L-type voltage-gated calcium channels (LTCs) plays a pivotal role in excitation-transcription coupling, but the involvement of LTCs in chronic pain remains unclear. We used a peptide nucleic acid (transportan 10-PNA conjugates)-based antisense strategy to investigate the role of the LTC subtypes Ca(V)1.2 and Ca(V)1.3 in long-term pain sensitization in a rat model of neuropathy (spinal nerve ligation). Our results demonstrate that specific knockdown of Ca(V)1.2 in the spinal dorsal horn reversed the neuropathy-associated mechanical hypersensitivity and the hyperexcitability and increased responsiveness of dorsal horn neurons. Intrathecal application of anti-Ca(V)1.2 siRNAs confirmed the preceding results. We also demonstrated an upregulation of Ca(V)1.2 mRNA and protein in neuropathic animals concomitant to specific Ca(V)1.2-dependent phosphorylation of the cAMP response element (CRE)-binding protein (CREB) transcription factor. Moreover, spinal nerve ligation animals showed enhanced transcription of the CREB/CRE-dependent gene COX-2 (cyclooxygenase 2), which also depends strictly on Ca(V)1.2 activation. We propose that L-type calcium channels in the spinal dorsal horn play an important role in pain processing, and that the maintenance of chronic neuropathic pain depends specifically on channels comprising Ca(V)1.2.

SIP30 is regulated by ERK in peripheral nerve injury-induced neuropathic pain

ERK plays an important role in chronic neuropathic pain. However, the underlying mechanism is largely unknown. Here we show that in chronic constriction injury-treated rat spinal cords, up-regulation of SIP30 (SNAP25-interacting protein 30), which is involved in the development and maintenance of chronic constriction injury-induced neuropathic pain, correlates with ERK activation and that the up-regulation of SIP30 is suppressed by intrathecal delivery of the MEK inhibitor U0126. In PC12 cells, up-regulation of SIP30 by nerve growth factor is also dependent on ERK activation. We found that there is an ERK-responsive region in the rat sip30 promoter. Activation of ERK promotes the recruitment of the transcription factor cyclic AMP-response element-binding protein to the sip30 gene promoter. Taken together, our results provide a potential downstream target of ERK activation-mediated neuropathic pain.

The role of peptides in central sensitization

Peptides released in the spinal cord from the central terminals of nociceptors contribute to the persistent hyperalgesia that defines the clinical experience of chronic pain. Using substance P (SP) and calcitonin gene-related peptide (CGRP) as examples, this review addresses the multiple mechanisms through which peptidergic neurotransmission contributes to the development and maintenance of chronic pain. Activation of CGRP receptors on terminals of primary afferent neurons facilitates transmitter release and receptors on spinal neurons increases glutamate activation of AMPA receptors. Both effects are mediated by cAMP-dependent mechanisms. Substance P activates neurokinin receptors (3 subtypes) which couple to phospholipase C and the generation of the intracellular messengers whose downstream effects include depolarizing the membrane and facilitating the function of AMPA and NMDA receptors. Activation of neurokinin-1 receptors also increases the synthesis of prostaglandins whereas activation of neurokinin-3 receptors increases the synthesis of nitric oxide. Both products act as retrograde messengers across synapses and facilitate nociceptive signaling in the spinal cord. Whereas these cellular effects of CGRP and SP at the level of the spinal cord contribute to the development of increased synaptic strength between nociceptors and spinal neurons in the pathway for pain, the different intracellular signaling pathways also activate different transcription factors. The activated transcription factors initiate changes in the expression of genes that contribute to long-term changes in the excitability of spinal and maintain hyperalgesia.

Chronic pain alters drug self-administration: implications for addiction and pain mechanisms

This review article focuses on the impact that the presence of pain has on drug self-administration in rodents, and the potential for using self-administration to study both addiction and pain, as well as their interaction. The literature on the effects of noxious input to the brain on both spinal and supraspinal neuronal activity is reviewed as well as the evidence that human and rodent neurobiology is affected similarly by noxious stimulation. The convergence of peripheral input to somatosensory systems with limbic forebrain structures is briefly discussed in the context of how the activity of one system may influence activity within the other system. Finally, the literature on how pain influences drug-seeking behaviors in rodents is reviewed, with a final discussion of how these techniques might be able to contribute to the development of novel analgesic treatments that minimize addiction and tolerance.

Neurochemical characterization of neuronal populations expressing protein kinase C gamma isoform in the spinal cord and gracile nucleus of the rat

Protein kinase C gamma (PKCgamma) is widely distributed throughout the CNS and is thought to play a role in long term hyper-excitability in nociceptive neurones. Here, we provide the first report of PKCgamma cells in the dorsal column nuclei of the adult rat. Retrograde labeling of PKCgamma cells from the thalamus with choleragenoid revealed that 25% of the PKCgamma positive gracile cells projected to the thalamus. Further, we have characterized the distribution of PKCgamma within gracile nucleus in terms of colocalization with various neurotransmitter receptors or enzymes and calcium binding proteins, and compared this with PKCgamma colocalization in cells of laminae I-III of the spinal cord. We show that approximately 90% of the PKCgamma cells in the gracile nucleus and 60% in the dorsal horn were immuno-positive for the AMPA receptor subunit glutamate 2/3 (GluR2/3). Little coexpression was seen with neurokinin 1 receptor, nitric oxide synthase (NOS) and the AMPA receptor subunit GluR1, markers of distinct neuronal subpopulations. In the spinal cord, a quarter of PKCgamma cells expressed calbindin, but very few cells did so in the gracile nucleus. Electrical stimulation at c-fiber strength of the normal or injured sciatic nerve was used to induce c-fos as a marker of postsynaptic activation in the spinal cord and gracile nucleus. Quantitative analysis of the number of PKCgamma positive gracile cells that expressed also c-fos increased from none to 24% after injury, indicating an alteration in the sensory activation pattern in these neurones after injury. C-fos was not induced in inner lamina II following c-fiber electrical stimulation of the intact or axotomized sciatic nerve, indicating no such plasticity at the spinal cord level. As dorsal column nuclei cells may contribute to allodynia after peripheral nerve injury, pharmacological modulation of PKCgamma activity may therefore be a possible way to ameliorate neuropathic pain after peripheral nerve injury.