Tyrosine Phosphorylation of Kir3 following ��-Opioid Receptor Activation of p38 MAPK Causes Heterologous Desensitization

Prior studies showed that tyrosine 12 phosphorylation in the N-terminal, cytoplasmic domain of the G-protein-gated inwardly rectifying potassium channel, K_ir3.1 facilitates channel deactivation by increasing intrinsic GTPase activity of the channel. Using a phosphoselective antibody directed against this residue (pY12), we now report that partial sciatic nerve ligation increased pY12-K_ir3.1-immunoreactivity (ir) in the ipsilateral dorsal horn of wild-type mice, but not in mice lacking the κ-opioid receptor (KOR) or lacking the G-protein receptor kinase 3 (GRK3) genes. Treatment of AtT-20 cells stably expressing KOR-GFP with the selective KOR agonist U50,488 increased both phospho-p38-ir and pY12-K_ir3.1-ir. The U50,488-induced increase in pY12-K_ir3.1-ir was blocked by the p38 inhibitor SB203580. Cells expressing KOR(S369A)-GFP did not increase either phospho-p38-ir or pY12-K_ir3.1-ir following U50,488 treatment. Whole cell voltage clamp of AtT-20 cells expressing KOR-GFP demonstrated that p38 activation by U50,488 reduced somatostatin-evoked K_ir3 currents. This heterologous desensitization was blocked by SB203580 and was not evident in cells expressing KOR(S369A)-GFP. Tyrosine phosphorylation of K_ir3.1 was likely mediated by p38 MAPK activation of Src kinase. U50,488 also increased (pY418)Src-ir; this increase was blocked by SB203580 and not evident in KOR(S369A)-GFP expressing AtT20 cells; the Src inhibitor PP2 blocked the U50,488-induced increase in pY12-K_ir3.1-ir; and the heterologous desensitization of K_ir3 currents was blocked by PP2. These results suggest that KOR causes phosphorylation of Y12-K_ir3.1 and channel inhibition through a GRK3-, p38 MAPK- and Src-dependent mechanism. Reduced inward potassium current following nerve ligation would increase dorsal horn neuronal excitability and may contribute to the neuropathic pain response.     

Molecular mechanism of changes in the morphine-induced pharmacological actions under chronic pain-like state: suppression of dopaminergic transmission in the brain

In the present study, we demonstrated whether a neuropathic pain-like state induced by sciatic nerve ligation in rodents could cause a long-lasting change in intracellular signaling in both supraspinal and spinal cord related to the suppression of morphine's effect. Mice with sciatic nerve ligation exhibited a significant suppression of the morphine-induced antinociception. Under this condition, phosphorylated-conventional protein kinase C-like immunoreactivity (p-cPKC-IR) and phosphorylated-micro-opioid receptor (p-MOR)-IR were clearly increased on the ipsilateral side in the dorsal horn of the spinal cord of nerve-ligated mice. It is of interest to note that astroglial hypertrophy as well as its proliferation was also noted in this area of sciatic nerve-ligated mice. Like nerve injury, the increase in cPKC activities and astroglial hypertrophy/proliferation in this region was observed by repeated morphine treatment. These findings suggest that the phosphorylation of both cPKC and MOR in the dorsal horn of the spinal cord by sciatic nerve ligation may play a substantial role in the suppression of morphine-induced antinociception under a neuropathic pain-like state. Sciatic nerve injury also caused a significant inhibition of MOR-mediated G-protein activation onto GABAergic neurons and a dramatic reduction in ERK activities onto dopaminergic neurons in the ventral tegmental area (VTA) regulating the rewarding effect of opioids. Furthermore, we found that the inhibition of ERK cascade in the VTA by treatment with specific inhibitors suppressed the morphine-induced rewarding effect in normal mice. These findings provide evidence that the direct reduction in MOR function and the persistent decrease in ERK activity of dopaminergic neurons in the VTA may contribute to the suppression of the morphine-induced rewarding effect under a neuropathic pain-like state. Conclusively, our recent findings provide novel evidences for the mechanism underlying the less sensitivity to opioids under a neuropathic pain-like state.     

Direct evidence for spinal cord microglia in the development of a neuropathic pain-like state in mice

The present study was undertaken to further investigate the role of glial cells in the development of the neuropathic pain-like state induced by sciatic nerve ligation in mice. At 7 days after sciatic nerve ligation, the immunoreactivities (IRs) of the specific astrocyte marker glial fibrillary acidic protein (GFAP) and the specific microglial marker OX-42, but not the specific oligodendrocyte marker O4, were increased on the ipsilateral side of the spinal cord dorsal horn in nerve-ligated mice compared with that on the contralateral side. Furthermore, a single intrathecal injection of activated spinal cord microglia, but not astrocytes, caused thermal hyperalgesia in naive mice. Furthermore, 5-bromo-2'-deoxyuridine (BrdU)-positive cells on the ipsilateral dorsal horn of the spinal cord were significantly increased at 7 days after nerve ligation and were highly co-localized with another microglia marker, ionized calcium-binding adaptor molecule 1 (Iba1), but neither with GFAP nor a specific neural nuclei marker, NeuN, in the spinal dorsal horn of nerve-ligated mice. The present data strongly support the idea that spinal cord astrocytes and microglia are activated under the neuropathic pain-like state, and that the proliferated and activated microglia directly contribute to the development of a neuropathic pain-like state in mice.     

Effect of aging on the substance P receptor, NK-1, in the spinal cord of rats with peripheral nerve injury

Substance P (SP) levels in the spinal cords of very old rats are less than the levels in younger rats (Bergman et al., 1996). After injury to a peripheral nerve in young rats, immunoreactivity (ir) to the SP receptor, NK-1 (neurokinin-1), increases in the spinal cord ipsilateral to the injury and the increases are correlated with the development of thermal hyperalgesia (Goff et al., 1998). Thus we postulated that aged rats might display an increased sensitivity to thermal stimulation before peripheral nerve injury and that they might respond differently to injury than do younger rats. To test this hypothesis, we used the Bennett and Xie model (1988) of chronic constriction injury (CCI) to the sciatic nerve to induce a neuropathic pain condition. We investigated the effect of age on changes in NK-1 ir in superficial layers of the dorsal horn and on numbers of NK ir cells in deeper laminae at the L4-L5 levels of the spinal cord after CCI. NK-1 receptors were tagged immunohistochemically and their distribution quantified by use of computer-assisted image analysis. NK-1 ir changes were related to alterations in thermal and tactile sensitivity that developed after CCI in young, mature and aged (4-6, 14-16, and 24-26 months) Fischer F344 BNF1 hybrid rats. No differences in thermal or tactile sensitivity of young and aged rats were seen in the absence of nerve injury. After injury, aged rats developed thermal hyperalgesia and tactile allodynia more slowly than did the younger rats. NK-1 receptor ir and numbers of NK-1 ir cells in the dorsal horn increased with time post-injury in all three groups. NK-1 ir increases were correlated with the development of thermal hyperalgesia in those rats that displayed hyperalgesia. However, some rats developed an increased threshold to thermal stimuli (analgesia) and that also was correlated with increases in NK-1 ir. Thus NK-1 ir extent, while correlated with thermal sensitivity in the absence of injury, is not a specific marker for disturbances in one particular sensory modality; rather it increases with peripheral nerve injury per se.     

The distribution and origin of VIP in the spinal cord of six mammalian species

The distribution of VIP-immunoreactivity was studied in the spinal cord and dorsal root ganglia of 6 mammalian species. Immunoreactive fibres and cell bodies were most apparent in the dorsal horn, dorsolateral funiculus, intermediolateral cell columns and the area around the central canal. The distribution of VIP immunoreactivity was similar in all species studied, mouse, rat, guinea pig, cat, horse and the marmoset monkey. There were fewer VIP fibres in the dorsal horn of cervical and thoracic segments than in lumbosacral segments. Using radioimmunoassay this gradient increase was quantitatively most marked in the sacral spinal cord of the cat. In dorsal root ganglia few nerve cell bodies but numerous fibres were present. A dual origin for VIP in the spinal cord is suggested: (A) Extrinsic, from dorsal root afferent fibres since immunoreactivity was decreased in dorsally rhizotomized animals (cats and rats) and in capsaicin pretreated rats (microinjection of dorsal root ganglia). (B) From local cell bodies intrinsic to the spinal cord which became visible after colchicine pretreatment of rats.     

Plasticity and emerging role of BKCa channels in nociceptive control in neuropathic pain

Large‐conductance Ca²⁺‐activated K⁺ (BK_Ca, MaxiK) channels are important for the regulation of neuronal excitability. Peripheral nerve injury causes plasticity of primary afferent neurons and spinal dorsal horn neurons, leading to central sensitization and neuropathic pain. However, little is known about changes in the BK_Ca channels in the dorsal root ganglion (DRG) and spinal dorsal horn and their role in the control of nociception in neuropathic pain. Here we show that L5 and L6 spinal nerve ligation in rats resulted in a substantial reduction in both the mRNA and protein levels of BK_Ca channels in the DRG but not in the spinal cord. Nerve injury primarily reduced the BK_Ca channel immunoreactivity in small‐ and medium‐sized DRG neurons. Furthermore, although the BK_Ca channel immunoreactivity was decreased in the lateral dorsal horn, there was an increase in the BK_Ca channel immunoreactivity present on dorsal horn neurons near the dorsal root entry zone. Blocking the BK_Ca channel with iberiotoxin at the spinal level significantly reduced the mechanical nociceptive withdrawal threshold in control and nerve‐injured rats. Intrathecal injection of the BK_Ca channel opener [1,3‐dihydro‐1‐[2‐hydroxy‐5‐(trifluoromethyl)phenyl]‐5‐(trifluoromethyl)‐2H‐benzimidazol‐2‐one] dose dependently reversed allodynia and hyperalgesia in nerve‐ligated rats but it had no significant effect on nociception in control rats. Our study provides novel information that nerve injury suppresses BK_Ca channel expression in the DRG and induces a redistribution of BK_Ca channels in the spinal dorsal horn. BK_Ca channels are increasingly involved in the control of sensory input in neuropathic pain and may represent a new target for neuropathic pain treatment.     

Effect of Treadmill Exercise on Serotonin Immunoreactivity in Medullary Raphe Nuclei and Spinal Cord Following Sciatic Nerve Transection in Rats

The serotoninergic system modulates nociceptive and locomotor spinal cord circuits. Exercise improves motor function and changes dopaminergic, noradrenergic, and serotonergic central systems. However, the direct relationship between serotonin, peripheral nerve lesion and aerobic treadmill exercise has not been studied. Using immunohistochemistry and optic densitometry, this study showed that the sciatic nerve transection increased the serotoninergic immunoreactivity in neuronal cytoplasm of the magnus raphe nuclei of trained and sedentary rats. In the dorsal raphe nucleus the increase only occurred in sedentary-sham-operated rats. In the spinal cord of trained, transected rats, the ventral horn showed significant changes, while the change in dorsal horn was insignificant. Von Frey’s test indicated analgesia in all exercise-trained rats. The sciatic nerve functional index indicated recovery in the trained group. Thus, both the aerobic treadmill exercise training and the nervous lesion appear to contribute to changes in serotonin immunoreactivity.     

Direct evidence for the involvement of brain-derived neurotrophic factor in the development of a neuropathic pain-like state in mice

Thermal hyperalgesia and tactile allodynia induced by sciatic nerve ligation were completely suppressed by repeated intrathecal (i.t.) injection of a TrkB/Fc chimera protein, which sequesters endogenous brain-derived neurotrophic factor (BDNF). In addition, BDNF heterozygous (+/-) knockout mice exhibited a significant suppression of nerve ligation-induced thermal hyperalgesia and tactile allodynia compared with wild-type mice. After nerve ligation, BDNF-like immunoreactivity on the superficial laminae of the ipsilateral side of the spinal dorsal horn was clearly increased compared with that of the contralateral side. It should be noted that a single i.t. injection of BDNF produced a long-lasting thermal hyperalgesia and tactile allodynia in normal mice, and these responses were abolished by i.t. pre-treatment with either a Trk-dependent tyrosine kinase inhibitor K-252a or a selective protein kinase C (PKC) inhibitor Ro-32-0432. Supporting these findings, we demonstrated here for the first time that the increase in intracellular Ca2+ concentration by application of BDNF in cultured mouse spinal neurons was abolished by pre-treatment with either K-252a or Ro-32-0432. Taken together, these findings suggest that the binding of spinally released BDNF to TrkB by nerve ligation may activate PKC within the spinal cord, resulting in the development of a neuropathic pain-like state in mice.     

坐骨神经结扎后大鼠背根神经节和脊髓CGRP表达的变化

目的研究大鼠坐骨神经结扎后降钙素基因相关肽(calcitoningene-relatedpeptide,CGRP)表达变化。方法SD大鼠随机分为假手术对照组和坐骨神经结扎组,实验组结扎后分别存活1、3、5、7、14、21和28d(n=8),免疫荧光(双标法)和免疫组织化学(SABC法)观察术后不同时间点CGRP和NGF在坐骨神经、背根神经节(dorsalrootganglion,DRG)和脊髓的表达变化,Westernblot结合图像分析技术对不同时间的变化进行定量测定。结果术后1d结扎远端坐骨神经内NGF大量堆积,持续到28d仍高于正常。结扎后7dDRG内CGRP阳性细胞百分率减少,持续到28d仍低于正常;结扎后14d脊髓后角CGRP下降,28d仍低于正常,各时间点脊髓前角CGRP表达未见明显变化。结论神经结扎可导致DRG和脊髓后角的CGRP表达下调,可能与靶源性的NGF来源减少有关。     

Differential Changes in Neuronal Excitability in the Spinal Dorsal Horn After Spinal Nerve Ligation in Rats

Previous studies demonstrated that peripheral nerve injury induced excessive neuronal response and glial activation in the spinal cord dorsal horn, and such change has been proposed to reflect the development and maintenance of neuropathic pain states. The aim of this study was to examine neuronal excitability and glial activation in the spinal dorsal horn after peripheral nerve injury. We examined noxious heat stimulation-induced c-Fos protein-like immunoreactivity (Fos-LI) neuron profiles in fourth-to-sixth lumbar (L4–L6) level spinal dorsal horn neurons after fifth lumbar spinal nerve ligation (L5 SNL). Immunofluorescence labeling of OX-42 and GFAP was also performed in histological sections of the spinal cord. A significant increase in the number of Fos-LI neuron profiles in the spinal dorsal horn at the L4 level was found at 3 days after SNL, but returned to a level similar to that in sham-operated controls by 14 days after injury. As expected, a decrease in the number of Fos-LI neuron profiles in the spinal dorsal horn at the L5 level was found at 3 days after SNL. However, these profiles had reappeared in large numbers by 14 and 21 days after injury. Immunofluorescence labeling of OX-42 and GFAP indicated sequential activation of microglia and astrocytes in the spinal dorsal horn. We conclude that nerve injury causes differential changes in neuronal excitability in the spinal dorsal horn, which may coincide with glial activation. These changes may play a substantial role in the pathogenesis of neuropathic pain after peripheral nerve injury.     

Involvement of gicerin, a cell adhesion molecule, in development and regeneration of chick sciatic nerve

We have examined the role of gicerin, an immunoglobulin superfamily cell adhesion molecule, in chick sciatic nerves during development and regeneration. Gicerin was expressed in the spinal cord, dorsal root ganglion (DRG) and sciatic nerves in embryos, but declined after hatching. Neurite extensions from explant cultures of the DRG were promoted on gicerin's ligands, which were inhibited by an anti-gicerin antibody. Furthermore, gicerin expression was upregulated in the regenerating sciatic nerves, DRG and dorsal horn of the spinal cord after injury to the sciatic nerve. These results indicate that gicerin might participate in the development and regeneration of sciatic nerves.     

Effect of aging on the substance P receptor,NK–1, in the spinal cord of rats with peripheral nerve injury

Substance P (SP) levels in the spinal cords of very old rats are less than the levels in younger rats (Bergman et al., 1996). After injury to a peripheral nerve in young rats, immunoreactivity (ir) to the SP receptor, NK–1 (neurokinin-1), increases in the spinal cord ipsilateral to the injury and the increases are correlated with the development of thermal hyperalgesia (Goff et al., 1998). Thus we postulated that aged rats might display an increased sensitivity to thermal stimulation before peripheral nerve injury and that they might respond differently to injury than do younger rats. To test this hypothesis, we used the Bennett and Xie model (1988) of chronic constriction injury (CCI) to the sciatic nerve to induce a neuropathic pain condition. We investigated the effect of age on changes in NK-1 ir in superficial layers of the dorsal horn and on numbers of NK ir cells in deeper laminae at the L4-L5 levels of the spinal cord after CCI. NK-1 receptors were tagged immunohistochemically and their distribution quantified by use of computer-assisted image analysis. NK-1 ir changes were related to alterations in thermal and tactile sensitivity that developed after CCI in young, mature and aged (4-6, 14-16, and 24-26 months) Fischer F344 BNF1 hybrid rats. No differences in thermal or tactile sensitivity of young and aged rats were seen in the absence of nerve injury. After injury, aged rats developed thermal hyperalgesia and tactile allodynia more slowly than did the younger rats. NK-1 receptor ir and numbers of NK-1 ir cells in the dorsal horn increased with time post-injury in all three groups. NK-1 ir increases were correlated with the development of thermal hyperalgesia in those rats that displayed hyperalgesia. However, some rats developed an increased threshold to thermal stimuli (analgesia) and that also was correlated with increases in NK-1 ir. Thus NK-1 ir extent, while correlated with thermal sensitivity in the absence of injury, is not a specific marker for disturbances in one particular sensory modality; rather it increases with peripheral nerve injury per se.     

Role of spinal voltage-dependent calcium channel alpha 2 delta-1 subunit in the expression of a neuropathic pain-like state in mice

The present study was undertaken to investigate the role of spinal voltage-dependent calcium channel alpha(2)delta-1 subunit in the expression of a neuropathic pain-like state induced by partial sciatic nerve ligation in mice. In cultured spinal neurons, gabapentin (GBP), which displays the inhibitory effect of alpha(2)delta-1 subunit, suppressed the extracellular Ca(2+) influx induced by KCl, whereas it failed to inhibit the intracellular Ca(2+) release induced by inositol-1,4,5-triphosphate. Seven days after sciatic nerve ligation, the protein level of alpha(2)delta-1 subunit in the ipsilateral spinal cord was clearly increased compared to that observed in sham-operated mice. In addition, the mRNA level of alpha(2)delta-1 subunit was significantly increased in the dorsal root ganglion, but not in the spinal cord, of nerve-ligated mice. Under these conditions, a marked decrease in the latency of paw-withdrawal against a thermal stimulation and tactile stimulation, induced by sciatic nerve ligation was abolished by repeated intrathecal (i.t.) treatment with GBP. Additionally, the persistent reduction in the nociceptive threshold by i.t. treatment with GBP at the early stage of the neuropathic pain-like state was maintained for 7 days even after GBP withdrawal. It is of interest to note that a single i.t. post-injection of GBP showed a marked and transient inhibitory effect on the developed neuropathic pain-like state, whereas repeated i.t. post-treatment with GBP produced a persistent inhibitory effect during the treatment. In conclusion, we propose here that the neuropathic pain-like state with sciatic nerve ligation is associated with the increased level of the alpha(2)delta-1 subunit of Ca(2+) channels at the sensory nerve terminal in the spinal dorsal horn of mice. Furthermore, the present data provide evidence that the neuropathic pain may be effectively controlled by repeated treatment with GBP at the early stage.     

Bombesin-like peptides in rat spinal cord: Biochemical characterization,localization and mechanism of release

Bombesin-like peptides were characterized in rat spinal cord by radio-immunoassay. The density of bombesin-like peptides was eight-fold greater in the dorsal than ventral horn or white matter of the spinal cord. Using high pressure liquid chromatography fractionation techniques, the main peak of immunoreactivity coeluted with synthetic bombesin. Also, the mechanism of release spinal cord peptides was determined. K⁺ and veratridine stimulated release of immunoreactive bombesin in a Ca⁺⁺-dependent mechanism. These data indicate that bombesin-like peptides may function as a unique class of neuroregulatory agents in mammalian spinal cord.     

Calcitonin gene-related peptide-receptor component protein expression in the uterine cervix, lumbosacral spinal cord, and dorsal root ganglia

The neuropeptide calcitonin gene-related peptide (CGRP) may play a role in neurogenic inflammation, tissue remodeling of the uterine cervix, promoting vasodilation, parturition, and processing of sensory information in the spinal cord. CGRP-immunoreactive nerves of the cervix and spinal cord have been studied but cellular identification of the CGRP receptor has received little attention. CGRP-receptor component protein (CGRP-RCP) is a small protein associated with the CGRP receptor; thus, immunostaining for the CGRP-RCP can be used to identify sites of the CGRP receptor. We determined sites of CGRP-RCP immunoreactivity relative to the presence of CGRP-ir nerve fibers in the female rat uterine cervix, spinal cord, and dorsal root ganglia. CGRP-RCP immunoreactivity was expressed in the dorsal horn of the spinal cord, venules of the uterine cervix, and perikarya of sensory neurons in dorsal root ganglia. CGRP-immunoreactive fibers were adjacent to CGRP-RCP-immunoreactive vessels in the cervix and among CGRP-RCP-immunoreactive structures in the dorsal horn of the spinal cord. This suggests CGRP-RCP is associated with structures innervated by CGRP nerves and these interactions may be changed in tissues in response to an appropriate stimulus.     

Complete sciatic nerve transection induces increase of neuropeptide Y-like immunoreactivity in primary sensory neurons and spinal cord of frogs

Neuropeptide Y (NPY) was immunohistochemically investigated in the frog spinal cord and dorsal root ganglia after axotomy. In normal ganglia, moderate NPY-like immunoreactivity (NPY-IR) prevailed in large and medium cells. In the spinal cord, the NPY-IR was densest in the dorsal part of the lateral funiculus. Other fibers and neurons NPY-IR were observed in the dorsal and ventral terminal fields and mediolateral band. NPY-IR fibers were also found in the ventral horn and in the ventral and lateral funiculi. The sciatic nerve transection increased the NPY-IR in large and medium neurons of the ipsilateral and contralateral dorsal root ganglia at 3 and 7 days, but no clear change was found at 15 days. In the spinal cord, there was a bilateral increase in the NPY-IR of the dorsal part of the lateral funiculus. In the ipsilateral side, the NPY-IR was increased at 3 and 7 days but was decreased at 15 days. In the contralateral side, a significant reduction at 15 days occurred. These findings seem to favor the role of NPY in the modulation of pain-related information in frogs, suggesting that this role of NPY may have appeared early in vertebrate evolution.     

ErbB transmembrane tyrosine kinase receptors are expressed by sensory and motor neurons projecting into sciatic nerve.

Adult spinal cord motor and dorsal root ganglion (DRG) sensory neurons express multiple neuregulin-1 (NRG-1) isoforms that act as axon-associated factors promoting neuromuscular junction formation and Schwann cell proliferation and differentiation. NRG-1 isoforms are also expressed by muscle and Schwann cells, suggesting that motor and sensory neurons are themselves acted on by NRG-1 isoforms produced by their peripheral targets. To test this hypothesis, we examined the expression of the NRG-1 receptor subunits erbB2, erbB3, and erbB4 in rat lumbar DRG and spinal cord. All three erbB receptors are expressed in these tissues. Sciatic nerve transection, an injury that induces Schwann cell expression of NRG-1, alters erbB expression in DRG and cord. Virtually all DRG neurons are erbB2- and erbB3-immunoreactive, with erbB4 also detectable in many neurons. In spinal cord white matter, erbB2 and erbB4 antibodies produce dense punctate staining, whereas the erbB3 antibody primarily labels glial cell bodies. Spinal cord dorsal and ventral horn neurons, including alpha-motor neurons, exhibit erbB2, erbB3, and erbB4 immunoreactivity. Spinal cord ventral horn also contains a population of small erbB3+/S100beta+/GFAP- cells (GFAP-negative astrocytes or oligodendrocytes). We conclude that sensory and motor neurons projecting into sciatic nerve express multiple erbB receptors and are potentially NRG-1 responsive.     

Localization of Endomorphin-2-Like Immunoreactivity in the Rat Medulla and Spinal Cord

Martin-Schild, S., J. E. Zadina, A. A. Gerall, S. Vigh and A. J. Kastin. Localization of endomorphin-2-like immunoreactivity in the rat medulla and spinal cord. Peptides 18(10) 1641–1649, 1997.—Endomorphin-1 (Tyr-Pro-Trp-Phe-NH₂) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH₂) are endogenous ligands that have greater affinity and selectivity for the μ-opiate receptor than any other known mammalian peptide. A polyclonal antiserum, screened for specificity to endomorphin-2 by immunodot-blot assay and preabsorption controls, was used for localization of this peptide. Immunocytochemistry performed on the brainstem, spinal cord, and sensory ganglia of rats by the avidin–biotin–peroxidase method revealed a continuous dense aggregation of endomorphin-2-like immunoreactive varicose fibers in the superficial laminae of the dorsal horn of the medulla and spinal cord. Immunoreactive fibers were detected in the dorsal root as well as within the dorsal root ganglia. The results suggest that endomorphin-2 is synthesized in primary sensory neurons in ganglia, transported to the superficial dorsal horn, and released near neurons expressing μ receptors. Its distribution appears to represent a functional unit likely to be associated with modulation of nociceptive stimuli.     

Differential expression of microRNAs in mouse pain models

Background

MicroRNAs (miRNAs) are short non-coding RNAs that inhibit translation of target genes by binding to their mRNAs. The expression of numerous brain-specific miRNAs with a high degree of temporal and spatial specificity suggests that miRNAs play an important role in gene regulation in health and disease. Here we investigate the time course gene expression profile of miR-1, -16, and -206 in mouse dorsal root ganglion (DRG), and spinal cord dorsal horn under inflammatory and neuropathic pain conditions as well as following acute noxious stimulation.

Results

Quantitative real-time polymerase chain reaction analyses showed that the mature form of miR-1, -16 and -206, is expressed in DRG and the dorsal horn of the spinal cord. Moreover, CFA-induced inflammation significantly reduced miRs-1 and -16 expression in DRG whereas miR-206 was downregulated in a time dependent manner. Conversely, in the spinal dorsal horn all three miRNAs monitored were upregulated. After sciatic nerve partial ligation, miR-1 and -206 were downregulated in DRG with no change in the spinal dorsal horn. On the other hand, axotomy increases the relative expression of miR-1, -16, and 206 in a time-dependent fashion while in the dorsal horn there was a significant downregulation of miR-1. Acute noxious stimulation with capsaicin also increased the expression of miR-1 and -16 in DRG cells but, on the other hand, in the spinal dorsal horn only a high dose of capsaicin was able to downregulate miR-206 expression.

Conclusions

Our results indicate that miRNAs may participate in the regulatory mechanisms of genes associated with the pathophysiology of chronic pain as well as the nociceptive processing following acute noxious stimulation. We found substantial evidence that miRNAs are differentially regulated in DRG and the dorsal horn of the spinal cord under different pain states. Therefore, miRNA expression in the nociceptive system shows not only temporal and spatial specificity but is also stimulus-dependent.

     

福尔马林致痛后大鼠中脑导水管周围灰质甲啡肽及脊髓背角P物质含量的变化

本文用免疫组化方法结合计算机图像处理技术观察大鼠后脚掌皮下注射福尔马林后脊髓背角Ｐ物质免疫阳性反应（ＳＰＬＩ）变化的节段性分布及中脑导水管周围灰质（ＰＡＧ）内甲啡肽样免疫阳性反应（ＭＥＬＩ）的变化。结果显示,注射福尔马林后,脊髓腰段（Ｌ１－２,Ｌ４－５）背角ＳＰＬＩ显著增强（Ｐ＜．０５）,３０ｍｉｎ组与６０ｍｉｎ组相比较无显著变化（Ｐ＞０．０５）;胸脊髓（Ｔ８）无显著变化（Ｐ＞０．０５）;颈脊髓背角ＳＰＬＩ有增强趋势（０．０５＜Ｐ＜０．１）;ＰＡＧ中ＭＥＬＩ减弱,腹外侧部３０ｍｉｎ组比６０ｍｉｎ组变化更大（Ｐ＜０．０５）。ＰＡＧ中ＭＥＬＩ与脊髓背角ＳＰＬＩ变化的时相关系提示福尔马林致痛引起的脊髓背角Ｐ物质的增多可能与ＰＡＧ中甲啡肽及阿片受体活动有关。