Mirror-Image Pain Update: Complex Interactions Between Central and Peripheral Mechanisms.

The appearance of contralateral effects after unilateral injury has been shown in various experimental pain models, as well as in clinics. They consist of a diversity of phenomena in contralateral peripheral nerves, sensory ganglia, or spinal cord: from structural changes and altered gene or protein expression to functional consequences such as the development of mirror-image pain (MP). Although MP is a well-documented phenomenon, the exact molecular mechanism underlying the induction and maintenance of mirror-like spread of pain is still an unresolved challenge. MP has generally been explained by central sensitization mechanisms leading to facilitation of pain impulse transfer through neural connections between the two sides of the central nervous system. On the contrary, the peripheral nervous system (PNS) was usually regarded unlikely to evoke such a symmetrical phenomenon. However, recent findings provided evidence that events in the PNS could play a significant role in MP induction. This manuscript provides an updated and comprehensive synthesis of the MP phenomenon and summarizes the available data on the mechanisms. A more detailed focus is placed on reported evidence for peripheral mechanisms behind the MP phenomenon, which were not reviewed up to now.

Mirror image pain mediated by D2 receptor regulation of astrocytic Cx43 phosphorylation and channel opening.

Mirror image pain (MIP), a clinical syndrome of contralateral pain hypersensitivity caused by unilateral injury, has been identified in various neuropathological conditions. Gap junctional protein Connexin 43 (Cx43), its phosphorylation levels and dopamine D2 receptor (DRD2) play key integrating roles in pain processing. We presume D2DR activity may affect Cx43 hemichannel opening via Cx43 phosphorylation levels to regulate MIP. This study shows that spinal astrocytic Cx43 directly interacts with DRD2 to mediate MIP. DRD2 and Cx43 expression levels were asymmetrically elevated in bilateral spinal during MIP, and DRD2 modulated the opening of primary astrocytic Cx43 hemichannels. Furthermore, Cx43 phosphorylation at Ser373 was increased during MIP, but decreased in DRD2 knockout (KO) mice. Finally, activation of spinal protein kinase A (PKA) altered the expression of Cx43 and its phosphorylation bilaterally, thus reversing the analgesic effect in DRD2 KO mice. Together, these data reveal that spinal Cx43 phosphorylation and channel opening are regulated by DRD2 via PKA activation, and that spinal Cx43 and DRD2 are key molecular sensors mediating mirror image pain.

Intrathecal Administration of the α1 Adrenergic Antagonist Phentolamine Upregulates Spinal GLT-1 and Improves Mirror Image Pain in SNI Model Rats.

Mirror image pain (MIP) is a type of extraterritorial pain that results in contralateral pain or allodynia. Glutamate transporter-1 (GLT-1) is expressed in astrocytes and plays a role in maintaining low glutamate levels in the synaptic cleft. Previous studies have shown that GLT-1 dysfunction induces neuropathic pain. Our previous study revealed bilateral GLT-1 downregulation in the spinal cord of a spared nerve injury (SNI) rat. We hypothesized that spinal GLT-1 is involved in the mechanism of MIP. We also previously demonstrated noradrenergic GLT-1 regulation. Therefore, this study aimed to investigate the effect of an α1 adrenergic antagonist on the development of MIP. Rats were subjected to SNI. Changes in pain behavior and GLT-1 protein levels in the SNI rat spinal cords were then examined by intrathecal administration of the α1 adrenergic antagonist phentolamine, followed by von Frey test and western blotting. SNI resulted in the development of MIP and bilateral downregulation of GLT-1 protein in the rat spinal cord. Intrathecal phentolamine increased contralateral GLT-1 protein levels and partially ameliorated the 50% paw withdrawal threshold in the contralateral hind paw. Spinal GLT-1 upregulation by intrathecal phentolamine ameliorates MIP. GLT-1 plays a role in the development of MIPs.

Neuroinflammation in the anterior cingulate cortex: the potential supraspinal mechanism underlying the mirror-image pain following motor fiber injury.

BACKGROUND: Peripheral nerve inflammation or lesion can affect contralateral healthy structures, and thus result in mirror-image pain. Supraspinal structures play important roles in the occurrence of mirror pain. The anterior cingulate cortex (ACC) is a first-order cortical region that responds to painful stimuli. In the present study, we systematically investigate and compare the neuroimmune changes in the bilateral ACC region using unilateral- (spared nerve injury, SNI) and mirror-(L5 ventral root transection, L5-VRT) pain models, aiming to explore the potential supraspinal neuroimmune mechanism underlying the mirror-image pain. METHODS: The up-and-down method with von Frey hairs was used to measure the mechanical allodynia. Viral injections for the designer receptors exclusively activated by designer drugs (DREADD) were used to modulate ACC glutamatergic neurons. Immunohistochemistry, immunofluorescence, western blotting, protein microarray were used to detect the regulation of inflammatory signaling. RESULTS: Increased expressions of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL-6) and chemokine CX3CL1 in ACC induced by unilateral nerve injury were observed on the contralateral side in the SNI group but on the bilateral side in the L5-VRT group, representing a stronger immune response to L5-VRT surgery. In remote ACC, both SNI and L5-VRT induced robust bilateral increase in the protein level of Nav1.6 (SCN8A), a major voltage-gated sodium channel (VGSC) that regulates neuronal activity in the mammalian nervous system. However, the L5-VRT-induced Nav1.6 response occurred at PO 3d, earlier than the SNI-induced one, 7 days after surgery. Modulating ACC glutamatergic neurons via DREADD-Gq or DREADD-Gi greatly changed the ACC CX3CL1 levels and the mechanical paw withdrawal threshold. Neutralization of endogenous ACC CX3CL1 by contralateral anti-CX3CL1 antibody attenuated the induction and the maintenance of mechanical allodynia and eliminated the upregulation of CX3CL1, TNF-α and Nav1.6 protein levels in ACC induced by SNI. Furthermore, contralateral ACC anti-CX3CL1 also inhibited the expression of ipsilateral spinal c-Fos, Iba1, CD11b, TNF-α and IL-6. CONCLUSIONS: The descending facilitation function mediated by CX3CL1 and its downstream cascade may play a pivotal role, leading to enhanced pain sensitization and even mirror-image pain. Strategies that target chemokine-mediated ACC hyperexcitability may lead to novel therapies for the treatment of neuropathic pain.

Complete Freund's adjuvant-induced protein dysregulation correlated with mirror image pain as assessed by quantitative proteomics of the mouse spinal cord.

Inflammation or trauma occurring on one side of the body can cause pathological pain on the contralateral noninjured side in a phenomenon called mirror-image pain (MIP). Although some potential mechanisms involved in MIP have been reported, including those involving the immune system and glial cells as well as neural mechanisms, the molecular mechanisms are not well understood. In this study, we aimed to understand the molecular mechanisms in MIP using quantitative proteomics and whole-cell patch clamp recordings. Behavioral test results showed that complete Freund's adjuvant could induce MIP in the mice. The results of isobaric tags for relative and absolute quantification (iTRAQ) quantitative proteomics showed that 108 proteins were dysregulated, and these proteins may represent potential targets. Furthermore, bioinformatics analysis was applied to explore the potential molecular mechanisms during MIP after complete Freund's adjuvant (CFA) treatment. Parallel reaction monitoring (PRM) results showed that PKCδ and seven other dysregulated proteins were related to MIP after CFA treatment. Patch clamp recording results showed that CFA treatment could increase intrinsic excitability and spontaneous firing in spinal cord neurons during MIP. In summary, we found that CFA could induce MIP. The results of proteomic research on the spinal cord after CFA treatment could provide new insight into the molecular mechanisms of MIP. Moreover, the neuronal activity of spinal cord neurons was upregulated during MIP after CFA treatment. In summary, the results of the spinal cord proteomic profile provide a potential molecular mechanism for understanding MIP.

Contralateral Projection of Anterior Cingulate Cortex Contributes to Mirror-Image Pain.

Long-term limb nerve injury often leads to mirror-image pain (MIP), an abnormal pain sensation in the limb contralateral to the injury. Although it is clear that MIP is mediated in part by central nociception processing, the underlying mechanisms remain poorly understood. The anterior cingulate cortex (ACC) is a key brain region that receives relayed peripheral nociceptive information from the contralateral limb. In this study, we induced MIP in male mice, in which a unilateral chronic constrictive injury of the sciatic nerve (CCI) induced a decreased nociceptive threshold in both hind limbs and an increased number of c-Fos-expressing neurons in the ACC both contralateral and ipsilateral to the injured limb. Using viral-mediated projection mapping, we observed that a portion of ACC neurons formed monosynaptic connections with contralateral ACC neurons. Furthermore, the number of cross-callosal projection ACC neurons that exhibited c-Fos signal was increased in MIP-expressing mice, suggesting enhanced transmission between ACC neurons of the two hemispheres. Moreover, selective inhibition of the cross-callosal projection ACC neurons contralateral to the injured limb normalized the nociceptive sensation of the uninjured limb without affecting the increased nociceptive sensation of the injured limb in CCI mice. In contrast, inhibition of the non-cross-callosal projection ACC neurons contralateral to the injury normalized the nociceptive sensation of the injured limb without affecting the MIP exhibited in the uninjured limb. These results reveal a circuit mechanism, namely, the cross-callosal projection of ACC between two hemispheres, that contributes to MIP and possibly other forms of contralateral migration of pain sensation.SIGNIFICANCE STATEMENT Mirror-image pain (MIP) refers to the increased pain sensitivity of the contralateral body part in patients with chronic pain. This pathology requires central processing, yet the mechanisms are less known. Here, we demonstrate that the cross-callosal projection neurons in the anterior cingulate cortex (ACC) contralateral to the injury contribute to MIP exhibited in the uninjured limb, but do not affect nociceptive sensation of the injured limb. In contrast, the non-cross-callosal projection neurons in the ACC contralateral to the injury contribute to nociceptive sensation of the injured limb, but do not affect MIP exhibited in the uninjured limb. Our study depicts a novel cross-callosal projection of ACC that contributes to MIP, providing a central mechanism for MIP in chronic pain state.

Bell's Palsy-Retroauricular Pain Threshold.

Background and objectives: Non-motor symptoms in the form of increased sensitivity are often associated with the onset of idiopathic Bell's palsy (IBP). The aims were to determine whether the pain threshold in the retroauricular regions (RAR) in IBP patients and the time of its occurrence is related to IBP severity. Materials and Methods: The study was conducted among 220 respondents (142 IBP patients, 78 healthy subjects (HS)). The degree of IBP was graded using the House-Brackmann and Sunnybrook Grading Scales (II-mild dysfunction, VI-total paralysis), whereas the pain thresholds were measured using the digital pressure algometer. Results: We found no difference in the degree of the pain threshold between the right and left RAR in the HS group. IBP patients belonging to groups II, III, IV, and V had lower pain thresholds in both RARs than HS and IBP patients belonging to group VI. There was no difference in the degree of pain threshold in RAR between the affected and unaffected side in IBP patients. The incidence of retroauricular pain that precedes paralysis and ceases after its occurrence in groups II and III of IBP patients is noticeably lower and the incidence of retroauricular pain that occurred only after the onset of paralysis is more frequent. Also, we found that the incidence of retroauricular pain that precedes paralysis and ceases after its occurrence in groups V and VI of IBP patients was more frequent. Conclusions: The degree of pain threshold lowering in RAR (bilaterally) is inversely related to the severity of IBP. We suggest that the occurrence of retroauricular pain before the onset of facial weakness is associated with higher severity of IBP while the occurrence after the onset is associated with lower severity of IBP.

Evolution of mirror-image pain in temporomandibular joint osteoarthritis mouse model

Abstract Mirror-image pain is a kind of pain that occurs on the contralateral side, but its pathogenesis remains unclear. Objective To develop an osteoarthritis mouse model for investigating mirror-image pain through observing nocifensive behaviors, histological changes, and nociceptive activity at days 3, 7, 14, 21, and 28 after the chemical induction of unilateral temporomandibular joint (TMJ) osteoarthritis. Methodology We randomly divided 6-week-old mice into sham and complete Freund adjuvant groups. To induce nocifensive behaviors, we applied 0.04 g of von Frey filament, 10 psi of air puff, and cold acetone on both sides of whisker pads at different days. The histology of TMJ on both sides was observed by hematoxylin/eosin staining and microcomputed tomography scanning. Furthermore, the nociceptive activity was evaluated using the phosphorylated cyclic AMP response element binding protein (pCREB) and a microglia marker at different days in the trigeminal subnucleus caudalis. Results Nocifensive behaviors against mechanical and temperature stimuli on the contralateral side became stronger than the baseline on day 28, in agreement with the elevation of the pCREB and the microglia marker in the trigeminal subnucleus caudalis. Thus, hypernociception on the contralateral side occurred at day 28. Conclusions Clearly, the TMJ model with unilateral osteoarthritis exhibited mirror-image pain. Therefore, this model is useful in investigating the pathogenesis of pain and in developing treatments.

Peripheral 5-HT3 mediates mirror-image pain by a cross-talk with acid-sensing ion channel 3.

Mirror-image pain (MIP), which occurs along with complex regional pain syndrome, rheumatoid arthritis and chronic migraine, is characterized by increased pain sensitivity of healthy body regions other than the actual injured or inflamed sites. A high level of peripheral inflammation may activate central or peripheral glia, triggering mirror-image pain. However, which receptors mediate inflammatory signals to contribute glial activation remains unclear. Intraplantarly injecting mice with 5-hydroxytryptamine (5-HT) or acidic buffer (proton) caused only unilateral hyperalgesia, but co-injection of 5-HT/acid induced bilateral hyperalgesia (MIP). Blocking 5-HT3 or acid-sensing ion channel 3 (ASIC3) abolished satellite glial activation, inhibiting MIP. Interestingly, intraplantar administration of a 5-HT3 agonist induced MIP, and 5-HT3-mediated MIP can be reversed by a 5-HT3 antagonist or an ASIC3 blocker. Similar results were found using a ASIC3 agonist. Furthermore, 5-HT3 was observed to co-localize with ASIC3 in DRG neurons; 5-HT3 activation-induced an increase in intracellular calcium that was inhibited by an ASIC3 blocker and vice versa. A cross-talk between 5-HT3 and ASIC3 mediates satellite glial activation, thereby triggering mirror-image pain.

[Analgesic effect and central mechanisms of CQ prescription on cancer invasion induced mirror image pain in model mice].

This study aimed to analyze the analgesic effect and related central mechanisms of CQ prescription on cancer invasion induced mirror image pain (CIIMIP)in model mice.In the study, male BALB/c mice were randomly divided into normal group, operation control group (injected with 0.2 mL inactivated S180 sarcoma cell sap), model group (injected with 0.2 mL S180 sarcoma cell sap on the right leg near the greater trochanter of femur) and CQ prescription low dose group (intraperitoneally injected with CQ prescription 100 mg•kg⁻¹ on the basis of model mice), CQ prescription middle dose group (intraperitoneally injected with CQ prescription 150 mg•kg⁻¹ on the basis of model mice), and CQ prescription high dose group (intraperitoneally injected with CQ prescription 200 mg•kg⁻¹ on the basis of model mice). Mechanical withdraw threshold (MWT) of the mirror image lateral hind paws were evaluated by Von Frey hairs before modeling and after surgery. The levels of glutamate (Glu), gamma aminobutyric acid (GABA), glycine (Gly), and taurine (Tau) in the L3-L5 spinal cord were measured by the high performance liquid chromatography-fluorescence detector (HPLC-FLD); AimPlex detection technology with multiple factors was used to detect the levels of regulated on activation in normal T-cell expressed and secreted (RANTES), monocyte chemoattractant protein (MCP-3) in the L3-L5 spinal cord. Then we observed the influence of GABAa receptor antagonist (Bicuculline) on analgesic effect of CQ prescription.The results indicated that CQ prescription could remarkably increase MWT of model mice(P<0.01, P<0.05), decrease the level of Glu(P<0.01, P<0.05), improve the levels of GABA, Gly, Tau(P<0.01, P<0.05), lower the ratio of Glu/GABA(P<0.01, P<0.05), and reduce the levels of RANTES, MCP-3(P<0.05) in the L3-L5 spinal cord, and GABAa receptor antagonist significantly blocked the analgesic effect of CQ prescription at two time points(P<0.05).This study showed that CQ prescription had significant analgesic effect on CIIMIP model mice, and its mechanism was associated with regulating the balance between excitability amino acid(EAA) and inhibitory amino acid (IAA) transmitters in central nervous system, partially activating GABAa receptor, and reducing the release of RANTES and MCP-3 in the spinal cord.

Bilateral tactile hypersensitivity and neuroimmune responses after spared nerve injury in mice lacking vasoactive intestinal peptide.

Vasoactive intestinal peptide (VIP) is one of the neuropeptides showing the strongest up-regulation in the nociceptive pathway after peripheral nerve injury and has been proposed to support neuropathic pain. Nevertheless, the story may be more complicated considering the known suppressive effects of the peptide on the immune reactivity of microglial cells, which have been heavily implicated in the onset and maintenance of pain after nerve injury. We here used mice deficient in VIP and the model of spared nerve injury, characterized by persistent tactile hypersensitivity. While tactile hypersensitivity developed similarly to wild type mice for the ipsilateral hindpaw, only transgenic mice showed a mirror-image tactile hypersensitivity in the contralateral hindpaw. This exacerbated neuropathic pain phenotype appeared to be mediated through a local mechanism acting at the level of the lumbar spinal cord as a distant nerve lesion in the front limb did not lead to hindpaw hypersensitivity in VIP-deficient mice. Innocuous tactile hindpaw stimulation was found to increase a neuronal activation marker in the bilateral superficial laminae of the lumbar dorsal horn of VIP-deficient, but not wild type mice, after SNI. A deeper study into the immune responsiveness to the nerve lesion also proved that VIP-deficient mice had a stronger early pro-inflammatory cytokine response and a more pronounced microglial reactivity compared to wild type controls. The latter was also observed at four weeks after spared nerve injury, a time at which bilateral tactile hypersensitivity persisted in VIP-deficient mice. These data suggest an action of VIP in neuropathic states that is more complicated than previously assumed. Future research is now needed for a deeper understanding of the relative contribution of receptors and fiber populations involved in the VIP-neuropathic pain link.

Constriction of the buccal branch of the facial nerve produces unilateral craniofacial allodynia.

Despite pain being a sensory experience, studies of spinal cord ventral root damage have demonstrated that motor neuron injury can induce neuropathic pain. Whether injury of cranial motor nerves can also produce nociceptive hypersensitivity has not been addressed. Herein, we demonstrate that chronic constriction injury (CCI) of the buccal branch of the facial nerve results in long-lasting, unilateral allodynia in the rat. An anterograde and retrograde tracer (3000MW tetramethylrhodamine-conjugated dextran) was not transported to the trigeminal ganglion when applied to the injury site, but was transported to the facial nucleus, indicating that this nerve branch is not composed of trigeminal sensory neurons. Finally, intracisterna magna injection of interleukin-1 (IL-1) receptor antagonist reversed allodynia, implicating the pro-inflammatory cytokine IL-1 in the maintenance of neuropathic pain induced by facial nerve CCI. These data extend the prior evidence that selective injury to motor axons can enhance pain to supraspinal circuits by demonstrating that injury of a facial nerve with predominantly motor axons is sufficient for neuropathic pain, and that the resultant pain has a neuroimmune component.

The role of spinal interleukin-1ß and astrocyte connexin 43 in the development of mirror-image pain in an inflammatory pain model.

Although we have recently demonstrated that carrageenan-induced inflammation upregulates the expression of spinal interleukin (IL)-1ß, which inhibits spinal astrocyte activation and results in the delayed development of Mirror-Image Pain (MIP), little is known regarding the mechanisms that underlie how spinal IL-1ß inhibits the astrocyte activation. In this study, we examined the effect of spinal IL-1ß on astrocyte gap junctions (GJ) and the development of MIP. Following unilateral carrageenan (CA) injection, mechanical allodynia (MA) was evaluated at various time points. Immunohistochemistry and Western blot analysis were used to determine changes in the expression of GFAP and connexins (Cx) in the spinal cord dorsal horn. Carrageenan rats showed a delayed onset of contralateral MA, which mimicked the temporal expression pattern of spinal Cx43 (an astrocyte gap junctional protein) and GFAP. Intrathecal administration of an interleukin-1 receptor antagonist (IL-1ra) twice-a-day on post-carrageenan injection days 0 to 3 caused a significant increase in contralateral MA and spinal Cx43 and GFAP expression. In addition, co-administration of IL-1ß with IL-1ra blocked the IL-1ra-induced increase in contralateral MA and the upregulated expression of spinal Cx43 and GFAP. Finally, co-administration of carbenoxolone (CBX; a GJ decoupler) or Gap26 (a specific Cx43 mimetic blocking peptide) with IL-1ra significantly blocked the IL-1ra-induced early development of contralateral MA and the associated upregulation of spinal Cx43 and GFAP expression. These results demonstrate that spinal IL-1ß suppresses Cx43 expression and astrocyte activation during the early phase of CA-induced inflammation resulting in the delayed onset of contralateral MA. These findings imply that spinal IL-1ß can inhibit astrocyte activation and regulate the time of induction of contralateral MA through modulation of spinal Cx43 expression.

Analgesic effect and central mechanisms of CQ prescription on cancer invasion induced mirror image pain in model mice / 中国中药杂志

This study aimed to analyze the analgesic effect and related central mechanisms of CQ prescription on cancer invasion induced mirror image pain (CIIMIP)in model mice.In the study, male BALB/c mice were randomly divided into normal group, operation control group (injected with 0.2 mL inactivated S180 sarcoma cell sap), model group (injected with 0.2 mL S180 sarcoma cell sap on the right leg near the greater trochanter of femur) and CQ prescription low dose group (intraperitoneally injected with CQ prescription 100 mg•kg⁻¹ on the basis of model mice), CQ prescription middle dose group (intraperitoneally injected with CQ prescription 150 mg•kg⁻¹ on the basis of model mice), and CQ prescription high dose group (intraperitoneally injected with CQ prescription 200 mg•kg⁻¹ on the basis of model mice). Mechanical withdraw threshold (MWT) of the mirror image lateral hind paws were evaluated by Von Frey hairs before modeling and after surgery. The levels of glutamate (Glu), gamma aminobutyric acid (GABA), glycine (Gly), and taurine (Tau) in the L3-L5 spinal cord were measured by the high performance liquid chromatography-fluorescence detector (HPLC-FLD); AimPlex detection technology with multiple factors was used to detect the levels of regulated on activation in normal T-cell expressed and secreted (RANTES), monocyte chemoattractant protein (MCP-3) in the L3-L5 spinal cord. Then we observed the influence of GABAa receptor antagonist (Bicuculline) on analgesic effect of CQ prescription.The results indicated that CQ prescription could remarkably increase MWT of model mice(P<0.01, P<0.05), decrease the level of Glu(P<0.01, P<0.05), improve the levels of GABA, Gly, Tau(P<0.01, P<0.05), lower the ratio of Glu/GABA(P<0.01, P<0.05), and reduce the levels of RANTES, MCP-3(P<0.05) in the L3-L5 spinal cord, and GABAa receptor antagonist significantly blocked the analgesic effect of CQ prescription at two time points(P<0.05).This study showed that CQ prescription had significant analgesic effect on CIIMIP model mice, and its mechanism was associated with regulating the balance between excitability amino acid(EAA) and inhibitory amino acid (IAA) transmitters in central nervous system, partially activating GABAa receptor, and reducing the release of RANTES and MCP-3 in the spinal cord.

Effect of preemptive local injection of ropivocaine with dexmedetomidine on mirror pain in rats and its mechanism.

OBJECTIVE: To observe the effect of preemptive local injection of ropivocaine with dexmedetomidine on activation of glial cells and on the mirror pain in rats and its mechanism. METHODS: A total of 48 adult male Sprague-Dawley rats (weighing 180 g-220 g) were included in the study and randomized into 3 groups, Group S, Group R, and Group RD1. A rat model of persistent postoperative pain evoked by skin/muscle incision and retraction was established in the three groups. Before procedures and nerve extraction, Group S (n = 16) was injected 0.9% saline locally; Group R (n = 16) was injected 0.5% ropivocaine locally, and Group RD1 (n = 16) was injected 0.5% ropivocaine in combined with 1 µg dexmedetomidine locally. After the model being established in the three groups, 8 rats were used for behavior test until 28 d, and dorsal root ganglions (DRGs) of the other 8 rats were harvested on the 3rd day after surgery. Immunofluorescent and transmission electron microscopy were used to observe the activation of glial cells in DRG, and the behavior test results in the three groups were compared. RESULTS: The results showed that mechanical pain threshold in ipsilateral hind-paws of the Group S, Group R, Group RD1 animals dropped to (3.640 ± 1.963) g, (5.827 ± 1.204) g, (7.482) ± 1.412 g at 3 d respectively; while in contralateral paws dropped to (7.100 ± 1.789) g, (17.687 ± 1.112) g, (16.213 ± 1.345) g on the 3 d respectively. Immunofluorescent showed that the glial cells were activated in bilateral side DRG after surgery in 3 groups, but ipsilateral paws expressed more active glial cells than contralateral paws. Transmission electron microscopy showed that mitochondria swelling/vacuolization and lysosomes were more obvious in ipsilateral paws than contralateral paws, but Group RD1 formula could reduce glial cells activity, mitochondria swelling/vacuolization and the amount of lysosomes. CONCLUSIONS: Local injection of ropivocaine and/or dexmedetomidine can effectively inhibit the activation of glial cells in DRG, mitigate the pathological changes of neuron in DRG and reduce mirror image pain.

Mirror-image pain after nerve reconstruction in rats is related to enhanced density of epidermal peptidergic nerve fibers.

Mirror-image pain is a phenomenon in which unprovoked pain is detected on the uninjured contralateral side after unilateral nerve injury. Although it has been implicated that enhanced production of nerve growth factor (NGF) in the contralateral dorsal root ganglion is important in the development of mirror-image pain, it is not known if this is related to enhanced expression of nociceptive fibers in the contralateral skin. Mechanical and thermal sensitivity in the contralateral hind paw was measured at four different time points (5, 10, 20 and 30weeks) after transection and immediate end-to-end reconstruction of the sciatic nerve in rats. These findings were compared to the density of epidermal (peptidergic and non-peptidergic) nerve fibers on the contralateral hind paw. Mechanical hypersensitivity of the contralateral hind paw was observed at 10weeks PO, a time point in which both subgroups of epidermal nerve fibers reached control values. Thermal hypersensitivity was observed with simultaneous increase in the density of epidermal peptidergic nerve fibers of the contralateral hind paw at 20weeks PO. Both thermal sensitivity and the density of epidermal nerve fibers returned to control values 30weeks PO. We conclude that changes in skin innervation and sensitivity are present on the uninjured corresponding side in a transient pain model. Therefore, the contralateral side cannot serve as control. Moreover, the current study confirms the involvement of the peripheral nervous system in the development of mirror-image pain.

Mas-related G-protein-coupled receptor c agonist bovine adrenal medulla 8-22 attenuates bone cancer pain in mice.

OBJECTIVES: The aim of this study is to investigate the effects of Mas-related G-protein-coupled receptor C (MrgC) agonist bovine adrenal medulla 8-22 (BAM8-22) on bone cancer pain and mirror-image pain. METHODS: Bone cancer pain was induced by intramedullary injection of NC2472 fibrosarcoma cells in the mice. BAM8-22 and/or anti-MrgC antibody were injected intrathecally at day 14 after bone cancer induction and their effects on pain behaviors were detected. The pain behaviours were assessed by the number of spontaneous foot lifts and paw withdrawal mechanical threshold (PWMT) tests. MrgC expression was detected using western blot analysis and immunofluorescence assay. RESULTS: There were increased bone cancer pain and mirror-image pain in the tumor-bearing mice while not in the sham-treated mice. BAM8-22 attenuated bone cancer pain in mice dose dependently with the highest effects at 2 hr after BAM8-22 administration, and anti-MrgC antibody reversed the effects of BAM8-22. However, intrathecal administration of BAM8-22 did not affect the mirror-image pain. Furthermore, BAM8-22 stimulated the expression of MrgC in the spinal dorsal horn. CONCLUSIONS: MrgC agonist BAM8-22 could attenuate bone cancer pain in mice. This study may provide a novel strategy for the treatment of bone cancer pain.

Roles of neurochemicals in central nerve system of mirror image pain in cancer invasion pain model and effects of gabapentin on them / 中国生化药物杂志

Objective To study the roles of neurochemicals as Glu, GABA in the spinal cord and SP, DynA1-13 in the cerebral cortex of mirror image pain in cancer invasion pain model and the effects of gabapentin on them.Methods Male BALB/c mices were randomly divided into native group, sham group (injected inactivated S180 sarcoma cell sap), model group (injected 0.2 mL of S180 sarcoma cell sap on the right leg near the greater trochanter of femur) and GBP group (intraperitoneally injected gabapentin 120 mg/kg on the basis of model mice).Mechanical withdraw threshold of the ipsilateral and contralateral hind paw were evaluated by Von Frey hairs before and after surgery.The levels of Glu and GABA in the L3-L5 spinal cord were measured by the high performance liquid chromatography-fluorescence detector ( HPLC-FLD ) and radioimmunoassay was used to detect the concentrations of SP and DynA1-13 in the cerebral cortex.Results The mechanical withdraw threshold of contralateral mirror sites in model mice appeared same trend and approximate degree of decline, following the generation of cancer invasion pain of ipsilateral hind paw.Compared with native group, the concentrations of Glu in the spinal cord and SP in the cerebral cortex in model group were significantly increased (P<0.05, P<0.01), and the levels of GABA in the spinal cord and Dyn A1-13 in the cerebral cortex in model group were significantly decreased (P<0.05, P<0.01).Gabapentin could significantly increase the bilateral mechanical withdraw threshold of model mice and the analgesic effect could maintain to 240 min after administration (P<0.05 or P<0.01).Moreover, gabapentin could reverse the changes of above neurochemicals in the central nervous system of mirror image pain in cancer invasion pain model mice (P<0.01 or P<0.05).Conclusion The mirror image pain phenomenon does exist in the cancer invasion pain model mice induced by S180 sarcoma.The mechanism of mirror image pain occurr and preserve in cancer invasion pain model may involve the changes of Glu, GABA in the spinal cord and SP, Dyn A1-13 in the cerebral cortex, through which gabapentin can relieve mirror image pain in cancer invasion pain model.

Effect of preemptive local injection of ropivocaine with dexmedetomidine on mirror pain in rats and its mechanism

Objective: To observe the effect of preemptive local injection of ropivocaine with dexmedetomidine on activation of glial cells and on the mirror pain in rats and its mechanism. Methods: A total of 48 adult male Sprague-Dawley rats (weighing 180 g-220 g) were included in the study and randomized into 3 groups, Group S, Group R, and Group RD

Effect of preemptive local injection of ropivocaine with dexmedetomidine on mirror pain in rats and its mechanism

OBJECTIVE@#To observe the effect of preemptive local injection of ropivocaine with dexmedetomidine on activation of glial cells and on the mirror pain in rats and its mechanism.@*METHODS@#A total of 48 adult male Sprague-Dawley rats (weighing 180 g-220 g) were included in the study and randomized into 3 groups, Group S, Group R, and Group RD1. A rat model of persistent postoperative pain evoked by skin/muscle incision and retraction was established in the three groups. Before procedures and nerve extraction, Group S (n = 16) was injected 0.9% saline locally; Group R (n = 16) was injected 0.5% ropivocaine locally, and Group RD1 (n = 16) was injected 0.5% ropivocaine in combined with 1 μg dexmedetomidine locally. After the model being established in the three groups, 8 rats were used for behavior test until 28 d, and dorsal root ganglions (DRGs) of the other 8 rats were harvested on the 3rd day after surgery. Immunofluorescent and transmission electron microscopy were used to observe the activation of glial cells in DRG, and the behavior test results in the three groups were compared.@*RESULTS@#The results showed that mechanical pain threshold in ipsilateral hind-paws of the Group S, Group R, Group RD1 animals dropped to (3.640 ± 1.963) g, (5.827 ± 1.204) g, (7.482) ± 1.412 g at 3 d respectively; while in contralateral paws dropped to (7.100 ± 1.789) g, (17.687 ± 1.112) g, (16.213 ± 1.345) g on the 3 d respectively. Immunofluorescent showed that the glial cells were activated in bilateral side DRG after surgery in 3 groups, but ipsilateral paws expressed more active glial cells than contralateral paws. Transmission electron microscopy showed that mitochondria swelling/vacuolization and lysosomes were more obvious in ipsilateral paws than contralateral paws, but Group RD1 formula could reduce glial cells activity, mitochondria swelling/vacuolization and the amount of lysosomes.@*CONCLUSIONS@#Local injection of ropivocaine and/or dexmedetomidine can effectively inhibit the activation of glial cells in DRG, mitigate the pathological changes of neuron in DRG and reduce mirror image pain.