Extracellular ATP and cAMP signaling promote Piezo2-dependent mechanical allodynia after trigeminal nerve compression injury.

Trigeminal neuralgia (TN) is a type of severe paroxysmal neuropathic pain commonly triggered by mild mechanical stimulation in the orofacial area. Piezo2, a mechanically gated ion channel that mediates tactile allodynia in neuropathic pain, can be potentiated by a cyclic adenosine monophosphate (cAMP)-dependent signaling pathway that involves the exchange protein directly activated by cAMP 1 (Epac1). To study whether Piezo2-mediated mechanotransduction contributes to peripheral sensitization in a rat model of TN after trigeminal nerve compression injury, the expression of Piezo2 and activation of cAMP signal-related molecules in the trigeminal ganglion (TG) were detected. Changes in purinergic P2 receptors in the TG were also studied by RNA-seq. The expression of Piezo2, cAMP, and Epac1 in the TG of the TN animals increased after chronic compression of the trigeminal nerve root (CCT) for 21 days, but Piezo2 knockdown by shRNA in the TG attenuated orofacial mechanical allodynia. Purinergic P2 receptors P2X4, P2X7, P2Y1, and P2Y2 were significantly up-regulated after CCT injury. In vitro, Piezo2 expression in TG neurons was significantly increased by exogenous adenosine 5'-triphosphate (ATP) and Ca2+ ionophore ionomycin. ATP pre-treated TG neurons displayed elevated [Ca2+ ]i and faster increase in responding to blockage of Na+ /Ca2+ exchanger by KB-R7943. Furthermore, mechanical stimulation of cultured TG neurons led to sustained elevation in [Ca2+ ]i in ATP pre-treated TG neurons, which is much less in naïve TG neurons, or is significantly reduced by Piezo2 inhibitor GsMTx4. These results indicated a pivotal role of Piezo2 in peripheral mechanical allodynia in the rat CCT model. Extracellular ATP, Ca2+ influx, and the cAMP-to-Epac1 signaling pathway synergistically contribute to the pathogenesis and the persistence of mechanical allodynia.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion.

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

Comparison of MR findings of acute traumatic peripheral nerve injury and acute compressive neuropathy in a rat model.

PURPOSE: The treatment strategy is different for acute traumatic peripheral nerve injury and acute compressive neuropathy. This study aimed to compare magnetic resonance imaging (MRI) features of acute traumatic peripheral nerve injury and acute compressive neuropathy in a rat model. MATERIALS AND METHODS: Twenty female Sprague-Dawley rats were divided into two groups. In the crush injury group (n = 10), the unilateral sciatic nerve was crushed using forceps to represent acute traumatic peripheral nerve injury. In the compression injury group (n = 10), the unilateral sciatic nerve was ligated using silk to represent acute compressive neuropathy. The MRI of eight rats from each group were acquired on postoperative days 3 and 10. Fat-suppressed T2-weighted images were acquired. Changes in the injured nerve were divided into three grades. A Fisher's exact test was used to compare the changes in the nerves of the two groups. Histological staining and a western blot analysis were performed on one rat in each group on day 3. Neurofilament, myelin basic protein (MBP), and p75NTR staining were performed. Expression of neurofilament, MBP, p75NTR, and c-jun was evaluated by western blot analysis. RESULTS: MR neurography revealed substantial nerve changes in the compression injury group compared with the crush injury group at two-time points (p = 0.001 on day 3, p = 0.026 on day 10). The histopathological analysis indicated the destruction of the axon and myelin, mainly at the injury site and the distal portion of the injury in the crush injury group. It was prominent in the proximal portion, the injury site, and the distal portion of the injury in the compression injury group. The degree of axonal and myelin destruction was more pronounced in the compression injury group than in the crush injury group. CONCLUSION: MR neurography showed prominent and long-segmental changes associated with the injured nerve in acute compressive neuropathy compared with acute traumatic peripheral nerve injury.

IL-6 contributes to Nav1.3 up-regulation in trigeminal nerve following chronic constriction injury.

Background: To verify the hypothesis that the nature of trigeminal neuralgia (TN) is an ectopic impulse induced by sodium channel modulated by cytokines, we conducted an animal study using the infraorbital nerve chronic constriction injury (CCI) model in rats.Method: The expression of Nav1.3 or IL-6 in the infraorbital nerve (ION) and trigeminal ganglion (TG) were detected by western blot and immunocytochemistry after administration of antisense oligodeoxynucleotide sequence (AS), IL-6 or Anti-IL-6.Results: With intrathecal administration of AS or mismatch oligodeoxynucleotide sequence (MM) in the CCI rats, the Nav1.3-IR in ION and TG accounted for 2.2 ± 0.51% and 8.5 ± 3.1% in AS+CCI group vs. 6.9 ± 1.3% and 38.7 ± 4.8% in MM+CCI group (p < 0.05), respectively. While with local administration of IL-6 in those with sham operation, it accounted for 7.4 ± 2.1% and 45.5 ± 3.4% in IL-6+ sham group vs. 1.9 ± 0.67% and 8.1 ± 1.3% in vehicle+sham group (p < 0.05); with local administration of anti-IL-6 in CCI rats, 4.5 ± 0.78% and 32.1 ± 9.6% in Anti-IL-6+ CCI group vs 8.9 ± 2.1% and 61.4 ± 11.2% in vehicle+CCI group (p < 0.05).Discussion: We believe that the emergence of Nav1.3 from the compressed trigeminal nerve might be an important structural basis for the development of the ectopic excitability on the axon and IL-6 may play a role of necessary precondition.

Effect of cGMP-activated aquaporin 1 on TRPV4 in rats with allodynia induced by chronic compression of the dorsal root ganglion.

BACKGROUND: The aim of this study was to investigate the effects of aquaporin 1 (AQP1) knockdown on allodynia in rats with chronic compression of the dorsal root ganglia (DRG) and the role of TRPV4 in these effects. METHODS: Adult male Wistar rats were subjected to chronic compression of the dorsal root ganglia (CCD) via surgery. Behavioral tests were performed to calculate the paw withdrawal mechanical threshold (PWMT). Gene silence was induced by injecting rats with lentivirus expressing AQP1 short hairpin RNA (shRNA, Lv-shAQP1). Western blot analyses were performed to examine AQP1 and TRPV4 protein expression. The concentration of cyclic guanosine monophosphate (cGMP) was determined via enzyme-linked immunosorbent assay. RESULTS: AQP1 protein levels in DRG neurons were significantly increased in CCD rats and were accompanied by a decrease in the PWMT. Lentivirus-mediated RNA interference of AQP1 decreased AQP1 protein expression in CCD rats and normalized their PWMT, but not in rats infected with lentivirus-expressing negative control short hairpin RNA. Furthermore, AQP1 was identified as a cGMP-gated channel. cGMP concentration was upregulated in CCD rats. This effect was attenuated by treatment with a cGMP inhibitor. Additionally, the cGMP inhibitor decreased the mechanical allodynia and AQP1 protein expression in CCD rats. Finally, levels of TRPV4 expression were upregulated in DRG neurons and the L4/L5 spinal cord following surgery, and these effects were reversed by treatment with Lv-shAQP1 or a cGMP inhibitor. CONCLUSION: AQP1 plays a vital role in CCD-induced allodynia as Lv-shAQP1 significantly reduced the allodynia in CCD rats by inhibiting TRPV4 expression.

Involvement of advillin in somatosensory neuron subtype-specific axon regeneration and neuropathic pain.

Advillin is a sensory neuron-specific actin-binding protein expressed at high levels in all types of somatosensory neurons in early development. However, the precise role of advillin in adulthood is largely unknown. Here we reveal advillin expression restricted to isolectin B4-positive (IB4+) neurons in the adult dorsal root ganglia (DRG). Advillin knockout (KO) specifically impaired axonal regeneration in adult IB4+ DRG neurons. During axon regeneration, advillin was expressed at the very tips of filopodia and modulated growth cone formation by interacting with and regulating focal-adhesion-related proteins. The advillin-containing focal-adhesion protein complex was shed from neurite tips during neurite retraction and was detectable in cerebrospinal fluid in experimental autoimmune encephalomyelitis, oxaliplatin-induced peripheral neuropathy, and chronic constriction injury of the sciatic nerve. In addition, advillin KO disturbed experimental autoimmune encephalomyelitis-induced neural plasticity in the spinal-cord dorsal horn and aggravated neuropathic pain. Our study highlights a role for advillin in growth cone formation, axon regeneration, and neuropathic pain associated with IB4+ DRG neurons in adulthood.

Reversal of Fatty Infiltration After Suprascapular Nerve Compression Release Is Dependent on UCP1 Expression in Mice.

BACKGROUND: In large rotator cuff tears, retraction of the supraspinatus muscle creates suprascapular nerve traction and compression. However, suprascapular nerve transection, when used in previous models, is different from chronic compression of the suprascapular nerve in patients. To define the role of suprascapular nerve chronic injury in rotator cuff muscle atrophy and fatty infiltration, we developed a novel reversible suprascapular nerve compression mouse model. QUESTIONS/PURPOSES: We asked: (1) Can suprascapular nerve injury be induced by compression but reversed after compression release? (2) Can muscle fatty infiltration be induced by suprascapular nerve compression and reversed after compression release? (3) Is white fat browning involved in fatty infiltration resorption? METHODS: Mice in a common strain of C57BL/6J were randomly assigned to suprascapular nerve transection (n = 10), nerve compression (n = 10), nerve compression and release (n = 10), or sham control (n = 10) groups. To study the role or white fat browning on muscle fatty infiltration, additional UCP1 reporter mice (n = 4 for nerve compression and n = 4 for nerve compression release) and knockout mice (n = 4 for nerve compression and n = 4 for nerve compression release) were used. Nerve injury was testified using osmium tetroxide staining and neural muscular junction staining and then semiquantified by counting the degenerating axons and disrupted junctions. Muscle fatty infiltration was evaluated using Oil Red O staining and then semiquantified by measuring the area fraction of fat. Immunofluorescent and Oil Red O staining on UCP1 transgenic mice was conducted to testify whether white fat browning was involved in fatty infiltration resorption. Ratios of UCP1 positively stained area and fat area to muscle cross-section area were measured to semiquantify UCP1 expression and fatty infiltration in muscle by blinded reviewers. Analysis of variance with Tukey post hoc comparisons was used for statistical analysis between groups. RESULTS: Suprascapular nerve injury was induced by compression but reversed after release. The ratios of degenerating axons were: sham control: 6% ± 3% (95% confidence interval [CI], 3%-10%); nerve compression: 58% ± 10% (95% CI, 45%-70% versus sham, p < 0.001); and nerve compression and release: 15% ± 9% (95% CI, 5%-26% versus sham, p = 0.050). The supraspinatus muscle percentage area of fatty infiltration increased after 6 weeks of nerve compression (19% ± 1%; 95% CI, 18%-20%; p < 0.001) but showed no difference after compression release for 6 weeks (5% ± 3%; 95% CI, 1%-10%; p = 0.054) compared with sham (2% ± 1%; 95% CI, 1%-3%). However, the fat area fraction in UCP1 knockout mice did not change after nerve compression release (6% ± 1%; 95% CI, 4%-8% at 2 weeks after compression and 5% ± 0.32%; 95% CI, 4%-6% after 2 weeks of release; p = 0.1095). CONCLUSIONS: We developed a clinically relevant, reversible suprascapular nerve compression mouse model. Fatty infiltration resorption after compression release was mediated through white fat browning. CLINICAL RELEVANCE: If the mechanism of browning of white fat in rotator cuff muscle fatty infiltration can be confirmed in humans, a UCP1 agonist may be an effective treatment for patients with suprascapular nerve injury.

Differential gene expression in trigeminal ganglia of male and female rats following chronic constriction of the infraorbital nerve.

BACKGROUND: The mechanisms underlying sex-based differences in pain and analgesia are poorly understood. In this study, we investigated gene expression changes in trigeminal ganglia (TG) of male and female rats exposed to infraorbital nerve chronic constriction injury (IoN-CCI). METHODS: Somatosensory assessments were performed prior to IoN-CCI and at selected time points postsurgery. Selected gene expression changes were examined with real-time quantitative polymerase chain reaction (RT-PCR) in ipsilateral TG at 21 days postsurgery. RESULTS: Rats exposed to IoN-CCI developed significant mechanical allodynia and hyperalgesia on days 19 and 21 postsurgery. During this period, females developed significantly more allodynia but not hyperalgesia compared to males. At 21 days postsurgery, expression levels of 44 of the 84 investigated pain-related genes in ipsilateral TG were significantly regulated relative to naïve rats in either sex. Csf1 and Cx3cr1 were up-regulated in both sexes, but the magnitude of regulation was significantly higher in females (p = 0.02 and p = 0.001, respectively). Htr1a and Scn9a were down-regulated in both sexes, but the down-regulation was significantly more pronounced in males (p = 0.04 and p = 0.02, respectively). Additionally, Cck, Il1a, Pla2g1b and Tnf genes were significantly regulated in females but not in males, and Chrna4 gene was significantly down-regulated in males but not in females. CONCLUSIONS: Our findings suggest sex-dependent gene regulation in response to nerve injury, which may contribute to sex dimorphism of trigeminal neuropathic pain. Further studies are needed to establish gene expression changes over time and correlate these with hormonal and other physiological parameters in male and female. SIGNIFICANCE: We present novel sex-specific transcriptional regulation in trigeminal ganglia that may contribute to male-/female-based differences in trigeminal neuropathic pain. These findings are expected to open new research horizons, particularly in male versus female targeted therapeutic regimens.

Nerve Root Compression Increases Spinal Astrocytic Vimentin in Parallel With Sustained Pain and Endothelial Vimentin in Association With Spinal Vascular Reestablishment.

STUDY DESIGN: Temporal immunohistochemistry analysis of spinal cord tissue from a rat model of cervical radiculopathy. OBJECTIVE: The goal was to measure spinal endothelial and astrocytic vimentin expression after a painful nerve root compression to define spinal cellular expression of vimentin in the context of pain. SUMMARY OF BACKGROUND DATA: The intermediate filament, vimentin, is expressed in a variety of cell types in the spinal cord and is modulated in response to neural pathologies. Early after nerve root compression spinal astrocytes become activated and blood-spinal cord barrier (BSCB) breakdown occurs in parallel with development of pain-related behaviors; these spinal responses remain activated as does the presence of pain. In addition to vimentin, glial fibrillary acidic protein (GFAP) expression is a hallmark of astrocyte activation. In contrast, vascular endothelial cells down-regulate vimentin expression in parallel with vascular breakdown. It is not known whether spinal astrocytes and endothelial cells modulate their expression of vimentin in response to a painful neural injury. METHODS: Mechanical hyperalgesia was measured and spinal cord tissue was harvested at days 1 and 7 after a unilateral nerve root compression in rats. Vimentin was coimmunolabeled with GFAP to label astrocytes and von Willebrand factor (VWF) for endothelial cells in the spinal cord on the side of injury. RESULTS: Spinal astrocytic vimentin increases by day 7 after nerve root compression, corresponding to when mechanical hyperalgesia is maintained. Spinal endothelial vimentin increases as early as day 1 after a painful compression and is even more robust at day 7. CONCLUSION: The delayed elevation in spinal astrocytic vimentin corresponding to sustained mechanical hyperalgesia supports its having a relationship with pain maintenance. Further, since BSCB integrity has been shown to be reestablished by day 7 after a painful compression, endothelial expressed vimentin may help to fortify spinal vasculature contributing to BSCB stability. LEVEL OF EVIDENCE: N/A.

Effect of TRPV4-p38 MAPK Pathway on Neuropathic Pain in Rats with Chronic Compression of the Dorsal Root Ganglion.

The aim of this study was to investigate the relationships among TRPV4, p38, and neuropathic pain in a rat model of chronic compression of the dorsal root ganglion. Mechanical allodynia appeared after CCD surgery, enhanced via the intrathecal injection of 4α-phorbol 12,13-didecanoate (4α-PDD, an agonist of TRPV4) and anisomycin (an agonist of p38), but was suppressed by Ruthenium Red (RR, an inhibitor of TRPV4) and SB203580 (an inhibitor of p38). The protein expressions of p38 and P-p38 were upregulated by 4α-PDD and anisomycin injection but reduced by RR and SB203580. Moreover, TRPV4 was upregulated by 4α-PDD and SB203580 and downregulated by RR and anisomycin. In DRG tissues, the numbers of TRPV4- or p38-positive small neurons were significantly changed in CCD rats, increased by the agonists, and decreased by the inhibitors. The amplitudes of ectopic discharges were increased by 4α-PDD and anisomycin but decreased by RR and SB203580. Collectively, these results support the link between TRPV4 and p38 and their intermediary role for neuropathic pain in rats with chronic compression of the dorsal root ganglion.

Calbindin-D-28K like immunoreactivity in superficial dorsal horn neurons and effects of sciatic chronic constriction injury.

The neuropathic pain that results from peripheral nerve injury is associated with alterations in the properties of neurons in the superficial spinal laminae. Chronic constriction injury (CCI) of the rat sciatic nerve increases excitatory synaptic drive to excitatory neurons in the substantia gelatinosa while limiting that to inhibitory neurons. Since the calcium-binding protein calbindin D-28K has been associated with excitatory neurons, we examined whether CCI altered the properties of neurons expressing calbindin-like immunoreactivity (Cal+). These account for 30% of the neurons in lamina I and II. Calbindin did not co-localize with any particular electrophysiological phenotype of neuron; in substantia gelatinosa, it was found in some tonic, delay, irregular, phasic and transient firing neurons and in some cells that displayed central, radial or vertical morphology. When neuronal phenotype was defined more precisely in terms of both morphology and electrophysiological properties, no strong correlation with calbindin expression was found. The frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSC) in calbindin negative (Cal-) neurons was greater than that in Cal+ neurons. CCI did not alter the proportion of Cal+ neurons in the dorsal horn. Although CCI promoted a fourfold increase in sEPSC frequency in Cal+ neurons, sEPSC amplitude was reduced by 22% and charge transfer per second was unchanged. Since synaptic drive to Cal+ neurons is weak and there is no firm correlation between neuronal phenotype and calbindin expression, it is doubtful whether these neurons play a major role in the generation of central sensitization.

NMDA receptor triggered molecular cascade underlies compression-induced rapid dendritic spine plasticity in cortical neurons.

Compression causes the reduction of dendritic spines of underlying adult cortical pyramidal neurons but the mechanisms remain at large. Using a rat epidural cerebral compression model, dendritic spines on the more superficial-lying layer III pyramidal neurons were found quickly reduced in 12h, while those on the deep-located layer V pyramidal neurons were reduced slightly later, starting 1day following compression. No change in the synaptic vesicle markers synaptophysin and vesicular glutamate transporter 1 suggest no change in afferents. Postsynaptically, N-methyl-d-aspartate (NMDA) receptor trafficking to synaptic membrane was detected in 10min and lasting to 1day after compression. Translocation of calcineurin to synapses and enhancement of its enzymatic activity were detected within 10min as well. These suggest that compression rapidly activated NMDA receptors to increase postsynaptic calcium, which then activated the phosphatase calcineurin. In line with this, dephosphorylation and activation of the actin severing protein cofilin, and the consequent depolymerization of actin were all identified in the compressed cortex within matching time frames. Antagonizing NMDA receptors with MK801 before compression prevented this cascade of events, including NR1 mobilization, calcineurin activation and actin depolymerization, in the affected cortex. Morphologically, MK801 pretreatment prevented the loss of dendritic spines on the compressed cortical pyramidal neurons as well. In short, we demonstrated, for the first time, mechanisms underlying the rapid compression-induced cortical neuronal dendritic spine plasticity. In addition, the mechanical force of compression appears to activate NMDA receptors to initiate a rapid postsynaptic molecular cascade to trim dendritic spines on the compressed cortical pyramidal neurons within half a day.

Ca(2+) influx mediates the TRPV4-NO pathway in neuropathic hyperalgesia following chronic compression of the dorsal root ganglion.

Chronic compression of the dorsal root ganglion (DRG) (CCD) in rats is a typical model of neuropathic pain. TRPV4 contributed to mechanical allodynia induced by the CCD model. Our previous study demonstrated that TRPV4 enhances neuropathic hyperalgesia through a NO-cGMP-PKG cascade. However, the underlying mechanism(s) is still largely unknown. Therefore, the aim of the present study was to test whether TRPV4-mediated Ca(2+) influx is involved in the TRPV4-NO pathway. Regulation of intracellular calcium concentration by intrathecal injection of TRPV4-targeted siRNA significantly decreased the behavioural hyperalgesia, NF-κB activity, and NO content in CCD rats. Intraperitoneal (i.p.) injection of mibefradil significantly induced dose-dependent increases in the paw withdrawal latency (PWL) and mechanical withdrawal thresholds (MWT), as well as decreases in NF-κB activity and NO content in DRG of CCD rats. Moreover, pre-treatment with 4α-PDD attenuated the suppressive effects of mibefradil on CCD-induced neuropathic hyperalgesia, NF-κB activity, and NO production. The data showed that TRPV4-mediated Ca(2+) influx might be engaged in the TRPV4-NO pathway in neuropathic hyperalgesia in the CCD model.

VEGF-A promotes both pro-angiogenic and neurotrophic capacities for nerve recovery after compressive neuropathy in rats.

Nerve recovery following injury is usually incomplete, leaving functional deficits. Our aim was to investigate the neural changes in pro-angiogenic, pro-inflammatory and apoptotic factors during and after chronic nerve compression (CNC). Nerve function was impaired after CNC and was progressively restored after nerve decompression, while nerve blood flow was elevated. While the expression of the pro-inflammatory and pro-angiogenic cytokines IL-6, TNF-α and VEGF-A was high during and after CNC, we observed that inhibition of VEGF-A receptors strongly counteracted the angiogenic response induced by the ex vivo CNC. Activation of the pro-survival transcription factor nuclear factor-kappa B (NF-κB) increased during CNC, returning to control levels after nerve decompression. After nerve decompression, the downregulation of Mdm2 correlated well with an increased expression of pro-apoptotic transcription factor p53. All together, we bring novel evidence that CNC activates transcription factors such as NF-κB and p53, which are key effectors of the cellular stress response, suggesting a neuroprotective process associated with an increased VEGF-A-mediated neurotrophic effect. Our results highlight the role of pro-angiogenic and pro-inflammatory cytokines during CNC that are reinforced by increasing neurotrophic capacity during recovery to promote nerve regeneration.

Evaluation of pain behavior and calcitonin gene-related peptide immunoreactive sensory nerve fibers in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus in rats.

STUDY DESIGN: Animal study. OBJECTIVE: To evaluate pain behavior and neuropeptide changes in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus (NP) in rats. SUMMARY OF BACKGROUND DATA: The pathomechanisms of lumbar disc herniation pain have not been fully elucidated. Pain-associated neuropeptides, including substance P and calcitonin gene-related peptide (CGRP), are produced in dorsal root ganglion neurons and transported to spinal dorsal horn nerve terminals where they function in pain transmission. However, changes in CGRP-immunoreactive (IR) sensory nerve terminals have not been reported in models of disc herniation. This study evaluated pain-related behavior and changes in CGRP-IR terminals in the spinal dorsal horn after combined sciatic nerve compression and NP application. METHODS: Five groups of rats underwent either sciatic nerve compression with NP (n = 20), application of NP only (n = 20), nerve compression only (n = 20), and sham operation with neither compression nor NP (n = 20) or no operation (controls, n = 20). Mechanical hyperalgesia was measured every second day for 3 weeks. CGRP-IR terminals in each spinal dorsal horn lamina were examined 7 and 14 days postsurgery. Pain behavior and CGRP immunoreactivity were compared among the 5 groups. RESULTS: Mechanical hyperalgesia was found in the NP only, nerve compression only, and the NP with nerve compression groups (P ≤ 0.05). CGRP-IR nerve terminals in the superficial laminae (I and II) and the deep laminae (III-VI) significantly increased in the NP only, nerve compression only, and NP with nerve compression groups compared with control and sham groups (P ≤ 0.05). Significant mechanical hyperalgesia and increased CGRP-IR nerve terminals were found in the NP with nerve compression group compared with the NP only and nerve compression only groups (P ≤ 0.05). CONCLUSION: Our results indicate that nerve compression plus NP application produces the most pain-related behavior. CGRP-IR nerve terminals increased in laminae I and II that transmit pain and in laminae III to VI that transmit proprioception. Findings suggest that nerve compression plus NP application induces changes in CGRP expression in the superficial and deep laminae, and these changes are partly responsible for disc herniation pain.

Role of colchicine-induced microtubule depolymerization in hyperalgesia via TRPV4 in rats with chronic compression of the dorsal root ganglion.

The aim of this study is to investigate the effect of microtubule depolymerization by colchicine on hyperalgesia mediated by transient receptor potential vanilloid 4 (TRPV4) in a neuropathic pain model of chronic compression of the dorsal root ganglion (DRG) (hereafter termed CCD) in rat. Intrathecal administration of microtubule-depolymerizing agent, colchicine, attenuated the activated effect of 4alpha-phorbol 12, 13-didecanoate (4alpha-PDD, TRPV4 specific agonist) on mechanical and thermal hyperalgesia in CCD rats. This observation is in agreement with our in vitro experiments with DRG cells that showed a significant attenuation of 4alpha-PDD-activated Ca(2+)-influx and substance P (SP) release with the colchicine treatment. We conclude that microtubule depolymerization by colchicine can regulate pain sensitivity by depressing the hyperalgesia mediated by TRPV4.

Bilateral elevation of interleukin-6 protein and mRNA in both lumbar and cervical dorsal root ganglia following unilateral chronic compression injury of the sciatic nerve.

BACKGROUND: Current research implicates interleukin (IL)-6 as a key component of the nervous-system response to injury with various effects. METHODS: We used unilateral chronic constriction injury (CCI) of rat sciatic nerve as a model for neuropathic pain. Immunofluorescence, ELISA, western blotting and in situ hybridization were used to investigate bilateral changes in IL-6 protein and mRNA in both lumbar (L4-L5) and cervical (C7-C8) dorsal root ganglia (DRG) following CCI. The operated (CCI) and sham-operated (sham) rats were assessed after 1, 3, 7, and 14 days. Withdrawal thresholds for mechanical hyperalgesia and latencies for thermal hyperalgesia were measured in both ipsilateral and contralateral hind and fore paws. RESULTS: The ipsilateral hind paws of all CCI rats displayed a decreased threshold of mechanical hyperalgesia and withdrawal latency of thermal hyperalgesia, while the contralateral hind and fore paws of both sides exhibited no significant changes in mechanical or thermal sensitivity. No significant behavioral changes were found in the hind and fore paws on either side of the sham rats, except for thermal hypersensitivity, which was present bilaterally at 3 days. Unilateral CCI of the sciatic nerve induced a bilateral increase in IL-6 immunostaining in the neuronal bodies and satellite glial cells (SGC) surrounding neurons of both lumbar and cervical DRG, compared with those of naive control rats. This bilateral increase in IL-6 protein levels was confirmed by ELISA and western blotting. More intense staining for IL-6 mRNA was detected in lumbar and cervical DRG from both sides of rats following CCI. The DRG removed from sham rats displayed a similar pattern of staining for IL-6 protein and mRNA as found in naive DRG, but there was a higher staining intensity in SGC. CONCLUSIONS: Bilateral elevation of IL-6 protein and mRNA is not limited to DRG homonymous to the injured nerve, but also extended to DRG that are heteronymous to the injured nerve. The results for IL-6 suggest that the neuroinflammatory reaction of DRG to nerve injury is propagated alongside the neuroaxis from the lumbar to the remote cervical segments. This is probably related to conditioning of cervical DRG neurons to injury.

An animal model for trigeminal neuralgia by compression of the trigeminal nerve root.

BACKGROUND: Microvascular compression of the trigeminal nerve root is a major cause of most trigeminal neuralgia (TN) in patients; however, no reliable animal model to further study the pathogenesis of TN currently exists. OBJECTIVE: Our objective was to establish a novel and practical animal model for TN by chronic compression of the trigeminal (CCT) nerve root in rats, which would provide a better animal model to mimic the clinical feature of TN on the research of the pathogenesis of TN. STUDY DESIGN: A randomized, double blind, controlled animal trial. METHODS: Sixteen adult male Sprague-Dawley rats (200-220 g) were randomly divided into 2 groups: one group that received chronic compression of the trigeminal nerve root (the CCT group, n=8) and another group that received sham operation without compression (the sham operation group, n=8). A small plastic filament was retrogressively inserted into the intracalvarium from the inferior orbital fissure until it reached the trigeminal nerve root for compression in CCT group. Animal behaviors were observed for 4 weeks after operation. Immunohistochemistry of glial fibrillary acidic protein (GFAP), isolectin B4 (IB4), substance P (SP) and calcitonin gene-related peptide (CGRP) were performed in the trigeminal root entry zone (TREZ) and medullary dorsal horn (MDH). RESULTS: The orofacial mechanical allodynia and heat hyperalgesia in the CCT rats were obviously increased after the operation and lasted for 28 days. Increased face-grooming behavior was also observed in the CCT rats and continued for over 21 days, returning to baseline by day 28. Immunohistochemistry for GFAP in the TREZ revealed a progressive extension of astrocytic processes in the ipsilateral TREZ of rats in the CCT group. Furthermore, the IB4 positive immunoreactive nonpeptidergic C-fiber terminals in the MDH were reduced for 4 weeks after the operation. Both SP and CGRP, expressed in the peptidergic C-fiber terminals, were found to be decreased in the ipsilateral MDH of CCT animals after the trigeminal nerve root injury. LIMITATIONS: CCT animal model with a plastic filament only imitated the mechanical compression of the trigeminal root but not to display the complex vascular physiological feature as the microvascular in the TN patient. CONCLUSIONS: The chronic compression of the trigeminal nerve root in rats effectively induced persistent orofacial neuropathic pain behaviors, and it would provide a novel and practical animal model for future research on the pathogenesis of TN.

Rhabdomyolysis and peroneal nerve compression associated with thyroid hormone withdrawal in the setting of remnant ablation: review of the literature.

OBJECTIVE: To review the putative mechanisms whereby hypothyroidism is associated with severe myopathy, neural injury, and acute compartment syndrome and report a case of nontraumatic common peroneal nerve compression associated with hypothyroidism-induced rhabdomyolysis in a patient with diabetes prepared for remnant ablation after thyroidectomy for differentiated thyroid carcinoma. METHODS: We performed a review of the English-language literature on the PubMed database using the terms hypothyroidism, muscle disease, hypothyroid myopathy, rhabdomyolysis, compression neuropathy, and acute compartment syndrome. RESULTS: Myopathy occurs frequently among patients with overt hypothyroidism; however, severe myoneural injury seems to be precipitated or accompanied by comorbid conditions. Focal peroneal neuropathy may be related to hypothyroidism-induced extrinsic compression from severe myopathy and soft tissue swelling in a narrowed fascial compartment. CONCLUSION: Severe short-term iatrogenic hypothyroidism may lead to severe myopathy and compression nerve injury in patients with underlying diabetic neuropathy. We recommend avoidance of withdrawal of thyroid hormone for purposes of remnant ablation among patients with preexisting diabetic neuropathy.