Intrathecal minocycline attenuates peripheral inflammation-induced hyperalgesia by inhibiting p38 MAPK in spinal microglia

Microglia and spinal cord synaptic plasticity in persistent pain

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

The activation of P2Y6 receptor in cultured spinal microglia induces the production of CCL2 through the MAP kinases-NF-κB pathway

Rat primary cultures of spinal microglia were stimulated by UTP, a known P2Y2/4 receptor agonist, which resulted in the production and release of the C-C chemokine CCL2 (monocyte chemoattractant protein-1; MCP-1) measured by real-time PCR and ELISA, respectively. In an in vitro preparation of rat spinal microglia, with regard to the P2Y subtypes, the expression of P2Y1, 2, 6, 12, 13 and P2Y14, but not P2Y4, were detected by RT-PCR. The subtype of microglial P2Y receptor which could be involved in the production of CCL2 was also determined. The UTP-induced production of CCL2 was significantly blocked by pretreatment with reactive blue 2 and suramin, nonselective P2Y receptor antagonists, and MRS2578, a selective P2Y6 receptor antagonist. By contrast, knockdown of the P2Y2 receptor by RNA interference had no effect. The stimulatory effect of UTP was inhibited by phospholipase C (PLC) inhibitor U73122 and Src tyrosine kinase inhibitor PP2. A potential role of mitogen activated protein kinases was suggested since UTP-induced CCL2 production was significantly blocked by both U0126 and SB 202190, which are potent inhibitors of extracellular signal-regulated kinase (ERK) and p38, respectively. Moreover, UTP-stimulated phosphorylation of these kinases involved the activation of the P2Y6 receptor. Lastly, activation of nuclear factor-κB (NF-κB) by UTP is likely to be essential in the expression of CCL2. Together, these findings suggest that stimulation of spinal microglia P2Y6 receptors induce the production of CCL2 through either PLC-mediated ERK or p38 phosphorylation and the subsequent activation of NF-κB.

Botulinum toxin A increases analgesic effects of morphine, counters development of morphine tolerance and modulates glia activation and μ opioid receptor expression in neuropathic mice

The use of botulinum neurotoxin type A (BoNT/A) against pain, with emphasis for its possible use in alleviating chronic pain, still represents an outstanding challenge for experimental research. In this study, we examined the effects of BoNT/A on morphine-induced tolerance during chronic morphine treatment in neuropathic CD1 mice subjected to sciatic nerve lesion according to the Chronic Constriction Injury (CCI) model of neuropathic pain. We measured the effects of BoNT/A on CCI-induced allodynia and hyperalgesia and on the expression of glial fibrillary acidic protein (GFAP, marker of astrocytes), complement receptor 3/cluster of differentiation 11b (CD11b, marker of microglia), and neuronal nuclei (NeuN) at the spinal cord level. We also analyzed the colocalized expression of GFAP, CD11b and NeuN with phosphorylated p-38 mitogen-activated protein kinase and with μ-opioid receptor (MOR). A single intraplantar injection of BoNT/A (15 pg/paw) into the injured hindpaw, the day before the beginning of chronic morphine treatment (9 days of twice daily injections of 40 mg/kg morphine), was able to counteract allodynia and enhancement of astrocytes expression/activation induced by CCI. In addition, BoNT/A increased the analgesic effect of morphine and countered morphine-induced tolerance during chronic morphine treatment. These effects were accompanied, in neurons, by re-expression of MORs that had been reduced by repeated morphine administration. The combinatory effects of BoNT/A and morphine could have relevant therapeutic implications for sufferers of chronic pain who could benefit of pain relief reducing tolerance due to repeated treatment with opiates.

Different peripheral tissue injury induces differential phenotypic changes of spinal activated microglia

The purpose of this study is to investigate the possible different cellular marker expression associated with spinal cord microglial activation in different pain models. Immunohistochemistry and western blotting analysis of CD45, CD68, and MHC class I antigen as well as CD11b and Iba-1 in the spinal cord were quantitatively compared among widely used three pain animal models, complete Freund's adjuvant (CFA) injection, formalin injection, and chronic constriction injury (CCI) models. The results showed that significant upregulated expressions of CD45 and MHC class I antigen in spinal microglia as well as morphological changes with increased staining with CD11b and Iba-1 were seen in CCI and formalin models and not found in CFA-induced inflammatory pain model. CD68 expression was only detected in CCI model. Our findings suggested that different peripheral tissue injuries produced differential phenotypic changes associated with spinal microglial activation; peripheral nerve injury might induce spinal microglia to acquire these immunomolecular phenotypic changes.

Spinal astrocytic activation contributes to mechanical allodynia in a rat chemotherapy-induced neuropathic pain model

Chemotherapy-induced neuropathic pain (CNP) is the major dose-limiting factor in cancer chemotherapy. However, the neural mechanisms underlying CNP remain enigmatic. Accumulating evidence implicates the involvement of spinal glia in some neuropathic pain models. In this study, using a vincristine-evoked CNP rat model with obvious mechanical allodynia, we found that spinal astrocyte rather than microglia was dramatically activated. The mechanical allodynia was dose-dependently attenuated by intrathecal administratration of L-α-aminoadipate (astrocytic specific inhibitor); whereas minocycline (microglial specific inhibitor) had no such effect, indicating that spinal astrocytic activation contributes to allodynia in CNP rat. Furthermore, oxidative stress mediated the development of spinal astrocytic activation, and activated astrocytes dramatically increased interleukin-1β expression which induced N-methyl-D-aspartic acid receptor (NMDAR) phosphorylation in spinal neurons to strengthen pain transmission. Taken together, our findings suggest that spinal activated astrocytes may be a crucial component of the pathophysiology of CNP and “Astrocyte-Cytokine-NMDAR-neuron” pathway may be one detailed neural mechanisms underlying CNP. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for treating CNP.

Analgesic activity of piracetam: effect on cytokine production and oxidative stress

Piracetam is a prototype of nootropic drugs used to improve cognitive impairment. However, recent studies suggest that piracetam can have analgesic and anti-inflammatory effects. Inflammatory pain is the result of a process that depends on neutrophil migration, cytokines and prostanoids release and oxidative stress. We analyze whether piracetam has anti-nociceptive effects and its mechanisms. Per oral pretreatment with piracetam reduced in a dose-dependent manner the overt pain-like behavior induced by acetic acid, phenyl-p-benzoquinone, formalin and complete Freund's adjuvant. Piracetam also diminished carrageenin-induced mechanical and thermal hyperalgesia, myeloperoxidase activity, and TNF-α-induced mechanical hyperalgesia. Piracetam presented analgesic effects as post-treatment and local paw treatment. The analgesic mechanisms of piracetam were related to inhibition of carrageenin- and TNF-α-induced production of IL-1β as well as prevention of carrageenin-induced decrease of reduced glutathione, ferric reducing ability and free radical scavenging ability in the paw. These results demonstrate that piracetam presents analgesic activity upon a variety of inflammatory stimuli by a mechanism dependent on inhibition of cytokine production and oxidative stress. Considering its safety and clinical use for cognitive function, it is possible that piracetam represents a novel perspective of analgesic.

Temperature-related effects of adenosine triphosphate-activated microglia on pro-inflammatory factors

BACKGROUND

Therapeutic hypothermia protects neurons after severe brain injury. Activated microglia produce several neurotoxic factors, such as pro-inflammatory cytokines and nitric oxide (NO), during neuron destruction. Hence, suppression of microglial release of these factors is thought to contribute partly to the neuroprotective effects of hypothermia. After brain insults, adenosine triphosphate (ATP) is released from injured cells and activates microglia. Here, we examined the acute effects of temperature on ATP-activated microglial production of inflammatory factors, and the possible involvement of p38 mitogen-activated protein kinase (p38) underlying such effects.

METHODS

Microglia were cultured with ATP at 33, 37, and 39°C, or with ATP in the presence of a p38 inhibitor, SB203580, at 37°C. Cytokine and NO levels, and p38 activation were measured.

RESULTS

Compared to 37°C, TNF-α was reduced at 33°C and augmented at 39°C for 1.5 h. IL-6 was reduced at 33°C for 6 h. NO was reduced at 33°C, but augmented at 39°C for 6 h. p38 was reduced at 33°C for 1 min. SB203580 inhibited ATP-induced TNF-α, IL-6, and NO production.

CONCLUSION

Lowering temperature rapidly reduced p38 activation and the subsequent p38-regulated production of pro-inflammatory cytokines and NO in ATP-activated microglia, suggesting that attenuation of early phase inflammatory responses via suppression of p38 in microglia is one possible neuroprotective mechanism of therapeutic hypothermia. Temperature elevation increased TNF-α and NO production in these cells. These temperature-dependent changes imply that monitoring of TNF-α and NO in the cerebrospinal fluid during the early phase might be useful as biomarkers for responses to therapeutic hypothermia and hyperthermia.

Spinal toll-like receptor signaling and nociceptive processing: regulatory balance between TIRAP and TRIF cascades mediated by TNF and IFNβ

Toll-like receptors (TLRs) play a pivotal role in inflammatory processes, and individual TLRs have been investigated in nociception. We examined overlapping and diverging roles of spinal TLRs and their associated adaptor proteins in nociceptive processing. Intrathecal (IT) TLR2, TLR3, or TLR4 ligands (-L) evoked persistent (7-day) tactile allodynia (TA) that was abolished in respective TLR-deficient mice. Using Tnf(-/-) mice, we found that IT TLR2 and TLR4 TA was tumor necrosis factor (TNF) dependent, whereas TLR3 was TNF-independent. In Toll-interleukin 1 receptor (TIR) domain-containing adaptor protein (TIRAP; Tirap(-/-)) mice (downstream to TLR2 and TLR4), allodynia after IT TLR2-L and TLR4-L was abolished. Unexpectedly, in TIR-domain-containing adapter-inducing interferon-β (Trif(lps2)) mice (downstream of TLR3 and TLR4), TLR3-L allodynia was abrogated, but intrathecal TLR4-L produced a persistent increase (>21days) in TA. Consistent with a role for interferon (IFN) β (downstream to TIR-domain-containing adapter-inducing IFNβ [TRIF]) in regulating recovery after IT TLR4-L, prolonged allodynia was noted in Ifnar1(-/-) mice. Further, IT IFNβ given to Trif(lps2) mice reduced TLR4 allodynia. Hence, spinal TIR domain-containing adaptor protein (TIRAP) and TRIF cascades differentially lead to robust TA by TNF-dependent and independent pathways, whereas activation of TRIF modulated processing through type I IFN receptors. Based on these results, we believe that processes leading to the activation of these spinal TLRs initiate TNF-dependent and -independent cascades, which contribute to the associated persistent pain state. In addition, TRIF pathways are able to modulate the TNF-dependent pain state through IFNβ.

Incorporation of monodisperse oligoethyleneglycol amino acids into anticonvulsant analogues of galanin and neuropeptide y provides peripherally acting analgesics

Delivery of neuropeptides into the central and/or peripheral nervous systems supports development of novel neurotherapeutics for the treatment of pain, epilepsy and other neurological diseases. Our previous work showed that the combination of lipidization and cationization applied to anticonvulsant neuropeptides galanin (GAL) and neuropeptide Y (NPY) improved their penetration across the blood-brain barrier yielding potent antiepileptic lead compounds, such as Gal-B2 (NAX 5055) or NPY-B2. To dissect peripheral and central actions of anticonvulsant neuropeptides, we rationally designed, synthesized and characterized GAL and NPY analogues containing monodisperse (discrete) oligoethyleneglycol-lysine (dPEG-Lys). The dPEGylated analogues Gal-B2-dPEG(24), Gal-R2-dPEG(24) and NPY-dPEG(24) displayed analgesic activities following systemic administration, while avoiding penetration into the brain. Gal-B2-dPEG(24) was synthesized by a stepwise deprotection of orthogonal 4-methoxytrityl and allyloxycarbonyl groups, and subsequent on-resin conjugations of dPEG(24) and palmitic acids, respectively. All the dPEGylated analogues exhibited substantially decreased hydrophobicity (expressed as logD values), increased in vitro serum stabilities and pronounced analgesia in the formalin and carrageenan inflammatory pain assays following systemic administration, while lacking apparent antiseizure activities. These results suggest that discrete PEGylation of neuropeptides offers an attractive strategy for developing neurotherapeutics with restricted penetration into the central nervous system.

Analgesic efficacy and safety of the novel p38 MAP kinase inhibitor, losmapimod, in patients with neuropathic pain following peripheral nerve injury: a double-blind, placebo-controlled study

BACKGROUND

Inhibitors of p38 mitogen-activated protein kinase are undergoing evaluation as a novel class of anti-rheumatic drugs, by virtue of their ability to suppress the production of pro-inflammatory cytokines. Emerging data suggests that they may also attenuate peripheral or central sensitization in neuropathic pain. A double-blind, placebo-controlled study was undertaken to evaluate the analgesic efficacy of losmapimod (GW856553), a novel p38α/β inhibitor, in subjects with neuropathic pain following traumatic peripheral nerve injury.

METHODS

One hundred and sixty-eight subjects with pain of at least moderate intensity (average daily score ≥4 on an 11-point pain intensity numeric rating scale; PI-NRS) at baseline were randomized to receive oral losmapimod, 7.5 mg BID or placebo for 28 days. Efficacy and safety assessments were undertaken at weekly clinic visits.

RESULTS

The mean treatment difference for the change in average daily pain score from baseline to week 4 of treatment based on the PI-NRS was -0.22 (95% CI -0.73, 0.28) in favour of losmapimod over placebo (p = 0.39). There were no statistically significant or clinically meaningful differences between the treatment groups over the 4-week dosing period for either the primary or secondary efficacy variables. There were no unexpected safety or tolerability findings following dosing with losmapimod.

CONCLUSIONS

Losmapimod could not be differentiated from placebo in terms of a primary analgesia response in patients with pain following peripheral nerve injury. The lack of response could reflect inadequate exposure at central sites of action or differences between rodent and human with respect to the target or neuropathic pain mechanisms.

Contribution of the spinal microglia to bee venom-induced inflammatory pain in conscious rats

It is well known that spinal glia plays a key role in the pathogenesis of pain. The present study was designed to determine the roles of spinal microglia in bee venom-induced persistent spontaneous nociception (PSN), mechanical hyperalgesia and inflammation. We determined the effects of microglia inhibitor minocycline on BV-induced PSN, mechanical hyperalgesia and inflammatory swelling. Pre-treatment with intrathecal administration of minocyline at different doses significantly inhibited BV-induced PSN and mechanical hyperalgesia, but had no effect on BV-induced inflammatory swelling. These data suggest that the activation of spinal microglia may play a key role in BV-induced nociception, but not inflammation.

Microglial cathepsin B contributes to the initiation of peripheral inflammation-induced chronic pain

Interleukin (IL)-1β and IL-18 play critical roles in the induction of chronic pain hypersensitivity. Their inactive forms are activated by caspase-1. However, little is known about the mechanism underlying the activation of pro-caspase-1. There is increasing evidence that cathepsin B (CatB), a typical lysosomal cysteine protease, is involved in the pro-caspase-1 activation and the subsequent maturation of IL-1β and IL-18. In this context, CatB is considered to be an important molecular target to control chronic pain. However, no information is currently available about the role of CatB in chronic pain hypersensitivity. We herein show that CatB deficiency or the intrathecal administration of CA-074Me, a specific CatB inhibitor, significantly inhibited the induction of complete Freund's adjuvant-induced tactile allodynia in mice without affecting peripheral inflammation. In contrast, CatB deficiency did not affect the nerve injury-induced tactile allodynia. Furthermore, CatB deficiency or CA-074Me treatment significantly inhibited the maturation and secretion of IL-1β and IL-18 by cultured microglia following treatment with the neuroactive glycoprotein chromogranin A (CGA), but not with ATP. Moreover, the IL-1β expression in spinal microglia and the induction of tactile allodynia following the intrathecal administration of CGA depended on CatB, whereas those induced by the intrathecal administration of ATP or lysophosphatidic acid were CatB independent. These results strongly suggest that CatB is an essential enzyme for the induction of chronic inflammatory pain through its activation of pro-caspase-1, which subsequently induces the maturation and secretion of IL-1β and IL-18 by spinal microglia. Therefore, CatB-specific inhibitors may represent a useful new strategy for treating inflammation-associated pain.

Activation of different signals identified with glia cells contribute to the progression of hyperalgesia

Hyperalgesia results from a decreased pain threshold, often subsequent to peripheral tissue damage. Recent reports revealed several promising mechanisms of hyperalgesia, but many issues remain unclear. The glial activation accompanying inflammation of neurotransmission in the spinal cord might be related to the initiation and maintenance of hyperalgesia. The present study investigated the pharmacological pain-modifying effects of mitogen-associated protein kinase (MAPK)-related inhibitors identified with glia cells over time during inflammatory pain. A model of inflammatory pain was produced by injecting mustard oil (MO) into the hind paws of rats. Following MO injection, the changes in paws flinching as the early onset of pain and paw withdrawal latency (PWL) in response to thermal stimulation were measured as delayed-onset hyperalgesia. Before and after the MO injection, one of the inhibitors, a p38-MAPK (SB), nuclear factor (NF)-κB (PDTC), BDNF-trk-B (K252a), or JNK-1 (SP), was administered and flinching and PWL were measured. In the SB, PDTC, and k252a groups, early flinching following MO injection was moderately suppressed. Hyperalgesia was significantly suppressed in the left–right difference of PWL in animals receiving SB, k252a, or PDTC pre-treatment. In animals receiving post-treatment, the suppressive effects were most potent in the SP group. The present results revealed that microglial activation resulting from the release of the phosphatase p38-MAPK, the transcription factor NF-κB, and BDNF contributes to the early stage of inflammatory pain. Astrocyte activation accompanying JNK activation contributes to subsequent hyperalgesia. Activation of different signals identified with glia cells is thought to contribute to the progression of hyperalgesia, which represents an applicable finding for the treatment of hyperalgesia.

Possible role of spinal astrocytes in maintaining chronic pain sensitization: review of current evidence with focus on bFGF/JNK pathway

Although pain is regarded traditionally as neuronally mediated, recent progress shows an important role of spinal glial cells in persistent pain sensitization. Mounting evidence has implicated spinal microglia in the development of chronic pain (e.g. neuropathic pain after peripheral nerve injury). Less is known about the role of astrocytes in pain regulation. However, astrocytes have very close contact with synapses and maintain homeostasis in the extracellular environment. In this review, we provide evidence to support a role of spinal astrocytes in maintaining chronic pain. In particular, c-Jun N-terminal kinase (JNK) is activated persistently in spinal astrocytes in a neuropathic pain condition produced by spinal nerve ligation. This activation is required for the maintenance of neuropathic pain because spinal infusion of JNK inhibitors can reverse mechanical allodynia, a major symptom of neuropathic pain. Further study reveals that JNK is activated strongly in astrocytes by basic fibroblast growth factor (bFGF), an astroglial activator. Intrathecal infusion of bFGF also produces persistent mechanical allodynia. After peripheral nerve injury, bFGF might be produced by primary sensory neurons and spinal astrocytes because nerve injury produces robust bFGF upregulation in both cell types. Therefore, the bFGF/JNK pathway is an important signalling pathway in spinal astrocytes for chronic pain sensitization. Investigation of signaling mechanisms in spinal astrocytes will identify new molecular targets for the management of chronic pain.

Minocycline and fluorocitrate suppress spinal nociceptive signaling in intrathecal IL-1β-induced thermal hyperalgesic rats

We previously demonstrated that intrathecal IL-1β caused thermal hyperalgesia in rats. This study was conducted to examine the effects and cellular mechanisms of glial inhibitors on IL-1β-induced nociception in rats. The effects of minocycline (20 μg), fluorocitrate (1 nmol), and SB203580 (5 μg) on IL-1β (100 ng) treatment in rats were measured by nociceptive behaviors, western blotting of p38 mitogen-activated protein kinase (MAPK) and inducible nitric oxide synthase (iNOS) expression, cerebrospinal fluid nitric oxide (NO) levels, and immunohistochemical analyses. The results demonstrated that intrathecal IL-1β activated microglia and astrocytes, but not neurons, in the dorsal horn of the lumbar spinal cord, as evidenced by morphological changes and increased immunoreactivity, phosphorylated p38 (P-p38) MAPK, and iNOS expression; the activation of microglia and astrocytes peaked at 30 min and lasted for 6 h. The immunoreactivities of microglia and astrocytes were significantly increased at 30 min (6.6- and 2.7-fold, respectively) and 6 h (3.3- and 4.0-fold, respectively) following IL-1β injection, as compared with saline controls at 30 min (all P < 0.01). IL-1β induced P-p38 MAPK and iNOS expression predominantly in microglia and less in astrocytes. Minocycline, fluorocitrate, or SB203580 pretreatment suppressed this IL-1β-upregulated P-p38 MAPK mainly in microglia and iNOS mainly in astrocytes; minocycline exhibited the most potent effect. Minocycline and fluorocitrate pretreatment abrogated IL-1β-induced NO release and thermal hyperalgesia in rats. In conclusion, minocycline, fluorocitrate, and SB203580 effectively suppressed the IL-1β-induced central sensitization and hyperalgesia in rats.

Astrocytic CX43 hemichannels and gap junctions play a crucial role in development of chronic neuropathic pain following spinal cord injury

Chronic neuropathic pain is a frequent consequence of spinal cord injury (SCI). Yet despite recent advances, upstream releasing mechanisms and effective therapeutic options remain elusive. Previous studies have demonstrated that SCI results in excessive ATP release to the peritraumatic regions and that purinergic signaling, among glial cells, likely plays an essential role in facilitating inflammatory responses and nociceptive sensitization. We sought to assess the role of connexin 43 (Cx43) as a mediator of CNS inflammation and chronic pain. To determine the extent of Cx43 involvement in chronic pain, a weight-drop SCI was performed on transgenic mice with Cx43/Cx30 deletions. SCI induced robust and persistent neuropathic pain including heat hyperalgesia and mechanical allodynia in wild-type control mice, which developed after 4 weeks and was maintained after 8 weeks. Notably, SCI-induced heat hyperalgesia and mechanical allodynia were prevented in transgenic mice with Cx43/Cx30 deletions, but fully developed in transgenic mice with only Cx30 deletion. SCI-induced gliosis, detected as upregulation of glial fibrillary acidic protein in the spinal cord astrocytes at different stages of the injury, was also reduced in the knockout mice with Cx43/Cx30 deletions, when compared with littermate controls. In comparison, a standard regimen of post-SCI treatment of minocycline attenuated neuropathic pain to a significantly lesser degree than Cx43 deletion. These findings suggest Cx43 is critically linked to the development of central neuropathic pain following acute SCI. Since Cx43/Cx30 is expressed by astrocytes, these findings also support an important role of astrocytes in the development of chronic pain.

Contribution of microglia and astrocytes to the central sensitization, inflammatory and neuropathic pain in the juvenile rat

BACKGROUND

The development of pain after peripheral nerve and tissue injury involves not only neuronal pathways but also immune cells and glia. Central sensitization is thought to be a mechanism for such persistent pain, and ATP involves in the process. We examined the contribution of glia to neuronal excitation in the juvenile rat spinal dorsal horn which is subjected to neuropathic and inflammatory pain.

RESULTS

In rats subjected to neuropathic pain, immunoreactivity for the microglial marker OX42 was markedly increased. In contrast, in rats subjected to inflammatory pain, immunoreactivity for the astrocyte marker glial fibrillary acidic protein was increased slightly. Optically-recorded neuronal excitation induced by single-pulse stimulation to the dorsal root was augmented in rats subjected to neuropathic and inflammatory pain compared to control rats. The bath application of a glial inhibitor minocycline and a p38 mitogen-activated protein kinase inhibitor SB203580 inhibited the neuronal excitation in rats subjected to neuropathic pain. A specific P2X1,2,3,4 antagonist TNP-ATP largely inhibited the neuronal excitation only in rats subjected to neuropathic pain rats. In contrast, an astroglial toxin L-alpha-aminoadipate, a gap junction blocker carbenoxolone and c-Jun N-terminal kinase inhibitor SP600125 inhibited the neuronal excitation only in rats subjected to inflammatory pain. A greater number of cells in spinal cord slices from rats subjected to neuropathic pain showed Ca2+ signaling in response to puff application of ATP. This Ca2+ signaling was inhibited by minocycline and TNP-ATP.

CONCLUSIONS

These results directly support the notion that microglia is more involved in neuropathic pain and astrocyte in inflammatory pain.

Involvement of microglia and interleukin-18 in the induction of long-term potentiation of spinal nociceptive responses induced by tetanic sciatic stimulation

OBJECTIVE

The present study aimed to investigate the potential roles of spinal microglia and downstream molecules in the induction of spinal long-term potentiation (LTP) and mechanical allodynia by tetanic stimulation of the sciatic nerve (TSS).

METHODS

Spinal LTP was induced in adult male Sprague-Dawley rats by tetanic stimulation of the sciatic nerve (0.5 ms, 100 Hz, 40 V, 10 trains of 2-s duration at 10-s intervals). Mechanical allodynia was determined using von Frey hairs. Immunohistochemical staining and Western blot were used to detect changes in glial expression of interleukin-18 (IL-18) and IL-18 receptor (IL-18R).

RESULTS

TSS induced LTP of C-fiber-evoked field potentials in the spinal cord. Intrathecal administration of the microglial inhibitor minocycline (200 μg/20 μL) 1 h before TSS completely blocked the induction of spinal LTP. Furthermore, after intrathecal injection of minocycline (200 μg/20 μL) by lumbar puncture 1 h before TSS, administration of minocycline for 7 consecutive days (once per day) partly inhibited bilateral allodynia. Immunohistochemistry showed that minocycline inhibited the sequential activation of microglia and astrocytes, and IL-18 was predominantly colocalized with the microglial marker Iba-1 in the spinal superficial dorsal horn. Western blot revealed that repeated intrathecal injection of minocycline significantly inhibited the increased expression of IL-18 and IL-18Rs in microglia induced by TSS.

CONCLUSION

The IL-18 signaling pathway in microglia is involved in TSS-induced spinal LTP and mechanical allodynia.

The effect of minocycline on the masticatory movements following the inferior alveolar nerve transection in freely moving rats

Background

To determine the effects of inferior alveolar nerve transection (IAN-X) on masticatory movements in freely moving rats and to test if microglial cells in the trigeminal principal sensory nucleus (prV) or motor nucleus (motV) may be involved in modulation of mastication, the effects of microglial cell inhibitor minocycline (MC) on masticatory jaw movements, microglia (Iba1) immunohistochemistry and the masticatory jaw movements and related masticatory muscle EMG activities were studied in IAN-X rats.

Results

The number of Iba1-immunoreactive (IR) cells both in prV and motV was significantly larger in IAN-X rats compared with sham rats on day 3 after IAN-X. The intraperitoneal (i.p.) administration of MC caused a significant reduction of the number of Iba1-IR cells both in prV and motV that was evident on day 14 after IAN-X. Furthermore, a significant reduction of the number of Iba1-IR cells could be observed in motV but not in prV after microinjection (m.i.) of MC into the motV of IAN-X rats. The rats also exhibited a significant decrease in the head-withdrawal threshold on the side ipsilateral to the IAN-X compared to the threshold before IAN-X and it lasted to day 14. In addition, IAN-X markedly affected the ability to rat to carry out mastication. The number of complete masticatory sequences was significantly decreased. Furthermore, the total masticatory sequence time and food preparatory (PP) period duration was significantly elongated in compared to sham rats. Although IAN-X significantly affected the total number of chewing cycles within the RC period of a masticatory sequence, it had no effect on the duration of the chewing cycles. On the other hand, systemic administration of MC (both i.p. and m.i.) in IAN-X rats significantly improved decreased head-withdrawal threshold and the impaired masticatory jaw movements.

Conclusions

The present findings reveal that the strong modulation of masticatory jaw movements occurs following microglial cell activation after IAN-X, and the modulation recovers after inhibition of the microglial cell activation by MC, suggesting that microglial cell activation in the motV as well as in the prV has a pivotal role in modulating mastication following trigeminal nerve injury associated with orofacial neuropathic pain.

Botulinum neurotoxin A enhances the analgesic effects on inflammatory pain and antagonizes tolerance induced by morphine in mice

Over the recent years compelling evidence has accumulated indicating that botulinum neurotoxin serotype A (BoNT/A) results in analgesic effects on neuropathic as well as inflammatory pain, both in humans and in animal models. In the present study, the pharmacological interaction of BoNT/A with morphine in fighting inflammatory pain was investigated in mice using the formalin test. Moreover, the effects of BoNT/A on the tolerance-induced by chronic administration of morphine were tested and the behavioral effects were correlated with immunofluorescence staining of glial fibrillary acidic protein, the specific marker of astrocytes, at the spinal cord level. An ineffective dose of BoNT/A (2 pg/paw) combined with an ineffective dose of morphine (1 mg/kg) exerted a significant analgesic action both during the early and the late phases of formalin test. A single intraplantar injection of BoNT/A (15 pg/paw; i.pl.), administered the day before the beginning of chronic morphine treatment (7 days of s.c. injections of 20 mg/kg), was able to counteract the occurrence of tolerance to morphine. Moreover, BoNT/A reduces the enhancement of the expression of astrocytes induced by inflammatory formalin pain. Side effects of opiates, including the development of tolerance during repeated use, may limit their therapeutic use, the possibility of using BoNT/A for lowering the effective dose of morphine and preventing the development of opioid tolerance would have relevant implications in terms of potential therapeutic perspectives.

Tetracyclines and pain

Tetracyclines are natural or semi-synthetic bacteriostatic agents which have been used since late 1940s against a wide range of gram-positive and gram-negative bacteria and atypical organisms such as chlamydia, mycoplasmas, rickettsia, and protozoan parasites. After the discovery of the first tetracyclines, a second generation of compounds was sought in order to improve water solubility for parenteral administration or to enhance bioavailability after oral administration. This approach resulted in the development of doxycycline and minocycline in the 1970s. Doxycycline was included in the World Health Organization Model List of Essential Medicines either as antibacterial or to prevent malaria or to treat patients with this disease. Additional development led to the third generation of tetracyclines, being tigecycline the only medicine of this class to date. Besides antibacterial activities, the anti-inflammatory, antihypernociceptive and neuroprotective activities of tetracyclines began to be widely studied in the late 1990s. Indeed, there has been an increasing interest in investigating the effects induced by minocycline as this liposoluble derivative is known to cross the blood-brain barrier to the greatest extent. Minocycline induces antihypernociceptive effects in a wide range of animal models of nociceptive, inflammatory and neuropathic pain. In this study, we discuss the antihypernociceptive activity of tetracyclines and summarise its underlying cellular and molecular mechanisms.

Diverse sensory inputs permit priming in the acidic saline model of hyperalgesia

BACKGROUND

The rodent acidic saline model of hyperalgesia uses repeat injections of acidic saline in the right lateral gastrocnemius muscle, spaced five days apart, to induce a persistent decrease in hindpaw withdrawal thresholds. The objective of this study was to determine if alternate injection sites would permit development of hyperalgesia.

METHODS

The location of the first muscle injection was varied between 3 groups of rats to include the right lateral gastrocnemius, the right medial gastrocnemius or the left lateral gastrocnemius. All second injections were placed in the right lateral gastrocnemius.

RESULTS

As reported by others, placing both injections in the right lateral gastrocnemius produced a significant reduction in paw withdrawal thresholds 24 hours after the second injection (p < 0.05). Relocating the first injection to the right medial gastrocnemius or the left lateral gastrocnemius also produced significant reductions in paw withdrawal thresholds (p < 0.05 for both). Hyperalgesia was also observed if the first muscle injection was replaced with a systemic injection of lipopolysaccharide. Further experiments tested whether glia cells may contribute to the priming process. Pretreatment with minocycline prior to the first injection completely blocked the development of hyperalgesia but was ineffective if injected before the second injection.

CONCLUSIONS

These data indicate that anatomically diverse peripheral stimuli can converge within the central nervous system to produce hyperalgesia.

Shifts in cell-type expression accompany a diminishing role of spinal p38-mapkinase activation over time during prolonged postoperative pain

BACKGROUND

Surgery often causes prolonged postoperative pain, the mechanisms of which are unknown. The authors investigated the role of p38, a pain-associated mitogen-activated protein kinase, in induction and maintenance of such pain.

METHODS

Male rats were subjected to the skin-muscle incision retraction procedure at the saphenous region; the procedure causes ~4 weeks of secondary tactile hyperalgesia in the ipsilateral plantar region, indicating central sensitization. The spinal cord was sectioned from L3 and L4 + L5 vertebral segments and stained for activated p38 (P-p38) at postoperative day 3 (POD 3), just as secondary hyperalgesia develops; at PODs 10-12, the time of maximum hyperalgesia; and at POD 35, after the resolution of hyperalgesia. Some sections were costained for microglia, astrocytes, and neurons. Intrathecal injections of a P-p38 inhibitor were performed at POD 2 or POD 9, and subsequent changes in pain were monitored.

RESULTS

Skin-muscle incision retraction increased the numbers of dorsal horn P-p38 positive cells in L3 by ~3-fold and in L4 + L5 by ~7-fold from POD 3 to PODs 11-12. This increase was accompanied by a shift from microglia to neurons, resulting in a ~20-fold increase in P-p38-positive neurons in L4-L5 over this time. No P-p38 was detected in astrocytes. A P-p38 inhibitor given at POD 2 prevented development of secondary hypersensitivity, but when given at POD 9 the same dose gave weak relief of pain for less than 3 h.

CONCLUSIONS

Spinal P-p38 mitogen-activated protein kinase, activated after incision retraction, is important for the induction of prolonged pain, but despite increased pain near the time of maximum pain, its functional importance for the maintenance of pain is not great.

Blockade of microglial activation reduces mechanical allodynia in rats with compression of the trigeminal ganglion

The present study investigated the role of microglia and p38 MAPK in the development of mechanical allodynia in rats with compression of the trigeminal ganglion. Male Sprague-Dawley rats weighing 250-260 g were used. Under pentobarbital sodium anesthesia, the animals were mounted onto a stereotaxic frame and given injections of 4% agar solution (10 μL) to compress the trigeminal ganglion. The air-puff thresholds significantly decreased after compression of the trigeminal ganglion. On postoperative day 14, immunoreactivity to both OX-42 and p-p38 MAPK was up-regulated in the medullary dorsal horn as compared to the sham group. P-p38 MAPK was found to be co-localized with OX-42, but not with NeuN, a neuronal cell marker, or with GFAP, an astroglial cell marker. Intracisternal administration of 100 μg of minocycline significantly inhibited both mechanical allodynia and activation of microglia produced by compression of the trigeminal ganglion. Intracisternal administration of 0.1, 1, or 10 μg of SB203580, a p38 MAPK inhibitor, also significantly decreased mechanical allodynia and p38 MAPK activation in the trigeminal ganglion-compressed group. These results suggest that activation of p38 MAPK in the microglia is an important step in the development of mechanical allodynia in rats with compression of the trigeminal ganglion and that the targeted blockade of microglial p38 MAPK pathway is a potentially important new treatment strategy for trigeminal neuralgia-like nociception.

Spinal microglia initiate and maintain hyperalgesia in a rat model of chronic pancreatitis

BACKGROUND & AIMS

The chronic, persistent pain associated with chronic pancreatitis (CP) has many characteristics of neuropathic pain, initiated and maintained by the activation of spinal microglia. We investigated whether activated microglia in the thoracic spinal cord contribute to chronic pain in a rat model of CP.

METHODS

CP was induced in Sprague-Dawley rats by an intraductal injection of 2% trinitrobenzene sulfonic acid. Hyperalgesia was assessed by the measurement of mechanical sensitivity of the abdomen and nocifensive behavior to electrical stimulation of the pancreas. Three weeks after induction of CP, spinal samples were analyzed by immunostaining and immunoblot analyses for levels of CD11 (a marker of microglia, determined with the antibody OX42) and phosphorylated p38 (P-p38, a marker of activation of p38 mitogen-activated protein kinase signaling). We examined the effects of minocycline (inhibitor of microglia) and fractalkine (microglia-activating factor) on visceral hyperalgesia in rats with CP.

RESULTS

Rats with CP had increased sensitivity and nociceptive behaviors to mechanical probing of the abdomen and electrical stimulation of the pancreas. The dorsal horn of the thoracic spinal cords of rats with CP contained activated microglia (based on increased staining with OX42), with an ameboid appearance. Levels of P-p38 increased in rats with CP and colocalized with OX42-positive cells. Intrathecal injection of minocycline reversed and prevented the increase of nocifensive behaviors and levels of P-p38 in rats with CP. Fractalkine induced hyperalgesia in rats without CP, which was blocked by minocycline.

CONCLUSIONS

Activated spinal microglia have important roles in maintaining and initiating chronic pain in a rat model of CP. Microglia might be a target for treatment of hyperalgesia caused by pancreatic inflammation.

Priming of adult pain responses by neonatal pain experience: maintenance by central neuroimmune activity

Adult brain connectivity is shaped by the balance of sensory inputs in early life. In the case of pain pathways, it is less clear whether nociceptive inputs in infancy can have a lasting influence upon central pain processing and adult pain sensitivity. Here, we show that adult pain responses in the rat are 'primed' by tissue injury in the neonatal period. Rats that experience hind-paw incision injury at 3 days of age, display an increased magnitude and duration of hyperalgesia following incision in adulthood when compared with those with no early life pain experience. This priming of spinal reflex sensitivity was measured by both reductions in behavioural withdrawal thresholds and increased flexor muscle electromyographic responses to graded suprathreshold hind-paw stimuli in the 4 weeks following adult incision. Prior neonatal injury also 'primed' the spinal microglial response to adult injury, resulting in an increased intensity, spatial distribution and duration of ionized calcium-binding adaptor molecule-1-positive microglial reactivity in the dorsal horn. Intrathecal minocycline at the time of adult injury selectively prevented both the hyperalgesia and early microglial reactivity associated with prior neonatal injury. The enhanced neuroimmune response seen in neonatally primed animals could also be demonstrated in the absence of peripheral tissue injury by direct electrical stimulation of tibial nerve fibres, confirming that centrally mediated mechanisms contribute to these long-term effects. These data suggest that early life injury may predispose individuals to enhanced sensitivity to painful events.

Spatial and temporal activation of spinal glial cells: role of gliopathy in central neuropathic pain following spinal cord injury in rats

In the spinal cord, neuron and glial cells actively interact and contribute to neurofunction. Surprisingly, both cell types have similar receptors, transporters and ion channels and also produce similar neurotransmitters and cytokines. The neuroanatomical and neurochemical similarities work synergistically to maintain physiological homeostasis in the normal spinal cord. However, in trauma or disease states, spinal glia become activated, dorsal horn neurons become hyperexcitable contributing to sensitized neuronal-glial circuits. The maladaptive spinal circuits directly affect synaptic excitability, including activation of intracellular downstream cascades that result in enhanced evoked and spontaneous activity in dorsal horn neurons with the result that abnormal pain syndromes develop. Recent literature reported that spinal cord injury produces glial activation in the dorsal horn; however, the majority of glial activation studies after SCI have focused on transient and/or acute time points, from a few hours to 1 month, and peri-lesion sites, a few millimeters rostral and caudal to the lesion site. In addition, thoracic spinal cord injury produces activation of astrocytes and microglia that contributes to dorsal horn neuronal hyperexcitability and central neuropathic pain in above-level, at-level and below-level segments remote from the lesion in the spinal cord. The cellular and molecular events of glial activation are not simple events, rather they are the consequence of a combination of several neurochemical and neurophysiological changes following SCI. The ionic imbalances, neuroinflammation and alterations of cell cycle proteins after SCI are predominant components for neuroanatomical and neurochemical changes that result in glial activation. More importantly, SCI induced release of glutamate, proinflammatory cytokines, ATP, reactive oxygen species (ROS) and neurotrophic factors trigger activation of postsynaptic neuron and glial cells via their own receptors and channels that, in turn, contribute to neuronal-neuronal and neuronal-glial interaction as well as microglia-astrocytic interactions. However, a systematic review of temporal and spatial glial activation following SCI has not been done. In this review, we describe time and regional dependence of glial activation and describe activation mechanisms in various SCI models in rats. These data are placed in the broader context of glial activation mechanisms and chronic pain states. Our work in the context of work by others in SCI models demonstrates that dysfunctional glia, a condition called "gliopathy", is a key contributor in the underlying cellular mechanisms contributing to neuropathic pain.

Spinal matrix metalloproteinase 3 mediates inflammatory hyperalgesia via a tumor necrosis factor-dependent mechanism

Matrix metalloproteinases (MMPs) have been implicated in the modulation of synaptic plasticity, glial activation, and long-term potentiation in the CNS. Here we demonstrate for the first time a mechanism for the regulation of nociceptive processing by spinal MMP-3 during peripheral inflammation. We first determined by western blotting that the catalytic (active) form of MMP-3 (cMMP-3) is increased in lumbar spinal cord following peripheral inflammation in rats. The peripheral inflammation-induced thermal hyperalgesia and tactile hypersensitivity was transiently (2-3 h) attenuated by intrathecal (IT) pretreatment with either an MMP-3 inhibitor (NNGH), or a broad spectrum MMP inhibitor (GM6001). In addition, IT delivery of cMMP-3 evoked hypersensitivity, whereas the pro (enzymatically inactive) form of MMP-3 did not. This suggests a pro-algesic effect of spinal MMP-3 mediated by an enzymatic mechanism. This cMMP-3-induced hypersensitivity is concurrent with increased tumor necrosis factor (TNF) in the spinal cord. The hypersensitivity behavior is prevented by intrathecal etanercept (TNF blockade). Treatment with cMMP-3 resulted in an increase in TNF release from spinal primary microglial, but not astrocyte cultures. These findings thus present direct evidence implicating MMP-3 in the coordination of spinal nociceptive processing via a spinal TNF-dependent mechanism.

Central sensitization of nociceptive neurons in rat medullary dorsal horn involves purinergic P2X7 receptors

Central sensitization is a crucial process underlying the increased neuronal excitability of nociceptive pathways following peripheral tissue injury and inflammation. Our previous findings have suggested that extracellular adenosine 5'-triphosphate (ATP) molecules acting at purinergic receptors located on presynaptic terminals (e.g., P2X2/3, P2X3 subunits) and glial cells are involved in the glutamatergic-dependent central sensitization induced in medullary dorsal horn (MDH) nociceptive neurons by application to the tooth pulp of the inflammatory irritant mustard oil (MO). Since growing evidence indicates that activation of P2X7 receptors located on glia is involved in chronic inflammatory and neuropathic pain, the aim of the present study was to test in vivo for P2X7 receptor involvement in this acute inflammatory pain model. Experiments were carried out in anesthetized Sprague-Dawley male rats. Single unit recordings were made in MDH functionally identified nociceptive neurons for which mechanoreceptive field, mechanical activation threshold and responses to noxious stimuli were tested. We found that continuous intrathecal (i.t.) superfusion over MDH of the potent P2X7 receptor antagonists brilliant blue G and periodated oxidized ATP could each significantly attenuate the MO-induced MDH central sensitization. MDH central sensitization could also be produced by i.t. superfusion of ATP and even more effectively by the P2X7 receptor agonist benzoylbenzoyl ATP. Superfusion of the microglial blocker minocycline abolished the MO-induced MDH central sensitization, consistent with reports that dorsal horn P2X7 receptors are mostly expressed on microglia. In control experiments, superfusion over MDH of vehicle did not produce any significant changes. These novel findings suggest that activation of P2X7 receptors in vivo may be involved in the development of central sensitization in an acute inflammatory pain model.

Spinal astrocytic activation is involved in a virally-induced rat model of neuropathic pain

Postherpetic neuralgia (PHN), the most common complication of herpes zoster (HZ), plays a major role in decreased life quality of HZ patients. However, the neural mechanisms underlying PHN remain unclear. Here, using a PHN rat model at 2 weeks after varicella zoster virus infection, we found that spinal astrocytes were dramatically activated. The mechanical allodynia and spinal central sensitization were significantly attenuated by intrathecally injected L-α-aminoadipate (astrocytic specific inhibitor) whereas minocycline (microglial specific inhibitor) had no effect, which indicated that spinal astrocyte but not microglia contributed to the chronic pain in PHN rat. Further study was taken to investigate the molecular mechanism of astrocyte-incudced allodynia in PHN rat at post-infection 2 weeks. Results showed that nitric oxide (NO) produced by inducible nitric oxide synthase mediated the development of spinal astrocytic activation, and activated astrocytes dramatically increased interleukin-1β expression which induced N-methyl-D-aspartic acid receptor (NMDAR) phosphorylation in spinal dorsal horn neurons to strengthen pain transmission. Taken together, these results suggest that spinal activated astrocytes may be one of the most important factors in the pathophysiology of PHN and “NO-Astrocyte-Cytokine-NMDAR-Neuron” pathway may be the detailed neural mechanisms underlying PHN. Thus, inhibiting spinal astrocytic activation may represent a novel therapeutic strategy for clinical management of PHN.

Effect of Electroacupuncture on Activation of p38MAPK in Spinal Dorsal Horn in Rats with Complete Freund's Adjuvant-Induced Inflammatory Pain

Activation of mitogen-activated protein kinases (MAPKs), especially p38 MAPK, plays an important role in the development of central sensitization related to persistent inflammatory pain. Electroacupuncture (EA) is well known to relieve persistent inflammatory pain. However, little is known about relationship between EA and p38 MAPK. Inflammatory pain rat model was induced by intraplantar injection of complete Freund's adjuvant (CFA). Male adult SD rats were randomly divided into the saline group, CFA group, and CFA + EA group. EA (constant saquare wave, 2 Hz and 100 Hz alternating frequencies, intensities ranging from 1 to 2 mA) was applied to bilateral “Zusanli” (ST 36) and “Kunlun” acupoints (BL 60) for 30 min, once per day. The paw edema and paw withdrawal threshold (PWT) were measured at preinjection and days postinjection 1, 3, and 14. Spinal p-p38MAPK- immunoreactivty (p-p38MAPK-IR) cells were detected by immunohistochemistry at postinjection day 3 and 14. EA significantly inhibited paw edema at postinjection days 14 and increased PWT at postinjection days 3 and 14. Moreover, the increasing number of spinal p-p38MAPK-IR cells which was induced by CFA injection was suppressed by EA stimulation. These results indicate that anti-inflammatory and analgesic effect of EA might be associated with its inhibition of spinal p38 MAPK activation and thereby provide a potential mechanism for the treatment of inflammatory pain by EA.

Post-injury administration of minocycline: an effective treatment for nerve-injury induced neuropathic pain

Neuropathic pain is an intractable clinical problem, affecting millions of people worldwide. Preemptive administration of minocycline has been confirmed useful for treating neuropathic pain by inhibiting spinal microglia activation and consequently lowering proinflammatory cytokine expression. However, most patients with neuropathic pain have no chance to receive preemptive treatment and it remains unclear whether there is a therapeutic time window for post treatment with minocycline. The present study is to confirm the effect and the therapeutic time window of intrathecal minocycline on spinal nerve ligation (SNL)-induced neuropathic pain after lesion. Behavioral test and immunohistochemistry are utilized to determine the variation of mechanical allodynia and microglia phosphorylated-p38 (p-p38) expression respectively after intrathecal minocycline. Results showed that post-injury intrathecal minocycline attenuated mechanical allodynia effectively together with inhibiting spinal microglia p-p38 expression on post operative day (POD) 1, POD 3 and POD 7. Additionally, results from POD 10 and POD 21 showed that intrathecal minocycline suppressed spinal microglia p-p38 expression but without any effects on reversing mechanical allodynia. It is concluded that post-injury intrathecal minocycline is an effective therapeutic intervention for treating SNL-induced neuropathic pain by inhibiting spinal microglia activation. Accordingly, there is indeed a therapeutic time window for post-injury intrathecal minocycline, which is the initiation stage of neuropathic pain development.

The response of spinal microglia to chemotherapy-evoked painful peripheral neuropathies is distinct from that evoked by traumatic nerve injuries

Painful peripheral neuropathies produced by nerve trauma are accompanied by substantial axonal degeneration and by a response in spinal cord microglia that is characterized by hypertrophy and increased expression of several intracellular and cell-surface markers, including ionizing calcium-binding adapter molecule 1 (Iba1) and Cd11b (a complement receptor 3 antigen recognized by the OX42 antibody). The microglia response has been hypothesized to be essential for the pathogenesis of the neuropathic pain state. In contrast, the painful peripheral neuropathies produced by low doses of cancer chemotherapeutics do not produce degeneration of axons in the peripheral nerve, although they do cause partial degeneration of the sensory axons' distal-most tips, that is the intraepidermal nerve fibers that form the axons' terminal receptor arbors. The question thus arises as to whether the relatively minor and distal axonal injury characterizing the chemotherapy-evoked neuropathies is sufficient to evoke the microglial response that is seen after traumatic nerve injury. We examined the lumbar spinal cord of rats with painful peripheral neuropathies due to the anti-neoplastic agents, paclitaxel, vincristine, and oxaliplatin, and the anti-retroviral agent, 2',3'-dideoxycytidine (ddC), and compared them to rats with a complete sciatic nerve transection and the partial sciatic nerve injury produced in the chronic constriction injury model (CCI). As expected, microglia hypertrophy and increased expression of Iba1 were pronounced in the nerve transection and CCI animals. However, there was no microglia hypertrophy or increased Iba1 staining in the animals treated with paclitaxel, vincristine, oxaliplatin, or ddC. These results suggest that the mechanisms that produce neuropathic pain after exposure to chemotherapeutics may be fundamentally different than those operating after nerve trauma.

Anti-inflammatory properties of doxycycline and minocycline in experimental models: an in vivo and in vitro comparative study

AIMS AND METHODS

Minocycline (Mino) and doxycycline (Dox) are second generation tetracyclines known to present several other effects, which are independent from their antimicrobial activities. We studied in a comparative way the anti-inflammatory effects of Mino and Dox, on acute models of peripheral inflammation in rodents (formalin test and peritonitis in mice, and carrageenan-induced paw oedema in rats). Immunohistochemical assays for TNF-alpha and iNOS in rat paws of carrageenan-induced oedema were also carried out as well as in vitro assays for myeloperoxidase (MPO) and lactate dehydrogenase (LDH). Furthermore, antioxidant activities were evaluated by the DPPH assay.

RESULTS

In the formalin test although Mino and Dox (1, 5, 10 and 25 mg/kg, i.p.) inhibited the first phase, they acted predominantly on the second phase of the test, where inhibition of the licking time close to 80% were observed. Mino and Dox were very efficacious in reducing the carrageenan-induced paw oedema in rats (10, 25 and 50 mg/kg, i.p.) and carrageenan-induced leucocyte migration (1 and 5 mg/kg, i.p.) to mice peritoneal cavities. Besides, they also significantly inhibited MPO and LDH releases at doses ranging from 0.001 to 1 μg/ml. Thus, in general, the anti-inflammatory activity of Dox was higher as compared to that of Mino, although the radical scavenging activity of Mino was of a magnitude 10 times higher.

CONCLUSIONS

Our data indicate that anti-inflammatory and antioxidant effects, involve the inhibition of iNOS and TNF-alpha, among other properties, and these encourage clinical studies of these compounds for new therapeutic applications, especially those were inflammation plays a role.

Minocycline completely reverses mechanical hyperalgesia in diabetic rats through microglia-induced changes in the expression of the potassium chloride co-transporter 2 (KCC2) at the spinal cord

AIM

neuronal hyperactivity at the spinal cord during mechanical hyperalgesia induced by diabetes may result from a decrease in the local expression of the potassium chloride co-transporter 2 (KCC2), which shifts the action of the neurotransmitter γ-amminobutiric acid (GABA) from inhibitory to excitatory. In this study, we evaluated the effects of spinal microglia inhibition or brain-derived neurotrophic factor (BDNF) blockade on KCC2 expression, spinal neuronal activity and mechanically induced pain responses of streptozotocin (STZ)-diabetic rats.

METHODS

four weeks after induction of diabetes, the STZ-diabetic rats received daily intrathecal injections, for 3 days, of minocycline (microglia inhibitor), TrkB/Fc (BDNF sequester) or saline. Behavioural responses to mechanical nociceptive stimulation of STZ-diabetic rats were evaluated by the Randall-Selitto test. The lumbar spinal cord was immunoreacted against the Fos protein (marker of neuronal activation) or KCC2, which was also quantified by western blotting. BDNF levels at the spinal cord were quantified by an enzyme-linked immunosorbent assay (ELISA).

RESULTS

minocycline treatment reversed the mechanical hyperalgesia, increased Fos expression and decreased the KCC2 expression detected in STZ-diabetic rats to control levels. Treatment with TrkB/Fc was less effective, inducing moderate effects in mechanical hyperalgesia and Fos expression and only a partial correction of KCC2 expression. BDNF levels were not increased in STZ-diabetic rats.

CONCLUSIONS

this study demonstrates that the microglial activation at the spinal cord contributes to mechanical hyperalgesia and spinal neuronal hyperactivity induced by diabetes, apparently by regulating the KCC2 expression. These effects do not seem to be mediated by BDNF, which is an important difference from other chronic pain conditions. New targets directed to prevent spinal microglia activation should be considered for the treatment of mechanical hyperalgesia induced by diabetes.

Effects of minocycline on Na+ currents in rat dorsal root ganglion neurons

Minocycline is an inhibitor of microglial activation and proliferation. Minocycline suppresses pain-related behaviors in many different pain states, which correlates closely with its inhibition of microglial activation and subsequent release of pro-inflammatory mediators in the spinal cord. Na(+) channels in dorsal root ganglion (DRG) neurons are implicated in the generation of inflammatory and neuropathic pain. To elucidate a possible peripheral mechanism of minocycline analgesia, effects of minocycline on tetrodotoxin-sensitive and tetrodotoxin-resistant Na(+) currents in rat DRG neurons were investigated. Minocycline potently inhibited both types of Na(+) currents with IC(50) values of 350 nM and 410 nM, respectively. The inhibition was accompanied by a depolarizing shift of the activation voltage. However, minocycline slowed the inactivation and speeded up the recovery from inactivation. These results suggest minocycline may exert analgesia peripherally thorough Na(+) channel inhibition in the primary afferent neurons as well as centrally through microglial inhibition in the spinal cord.

Interactions between the immune and nervous systems in pain

Immune cells and glia interact with neurons to alter pain sensitivity and to mediate the transition from acute to chronic pain. In response to injury, resident immune cells are activated and blood-borne immune cells are recruited to the site of injury. Immune cells not only contribute to immune protection but also initiate the sensitization of peripheral nociceptors. Through the synthesis and release of inflammatory mediators and interactions with neurotransmitters and their receptors, the immune cells, glia and neurons form an integrated network that coordinates immune responses and modulates the excitability of pain pathways. The immune system also reduces sensitization by producing immune-derived analgesic and anti-inflammatory or proresolution agents. A greater understanding of the role of the immune system in pain processing and modulation reveals potential targets for analgesic drug development and new therapeutic opportunities for managing chronic pain.

Characterization of the acute and persistent pain state present in K/BxN serum transfer arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune arthritis that affects approximately 1% of the population. Synovial inflammation cannot fully explain the level of pain reported by patients and facilitation of pain processing at the spinal level has been implicated. We characterized the K/BxN serum transfer arthritis model as a model of joint inflammation-induced pain and examined pharmacologic responsiveness and spinal glia activation. Mechanical allodynia developed congruently with joint swelling. Surprisingly, allodynia persisted after resolution of inflammation. At the peak of joint inflammation (days 4-10), hypersensitivity was attenuated with i.p. etanercept, gabapentin, and ketorolac. Following resolution of synovial inflammation (days 19-23), only gabapentin relieved allodynia. The superficial dorsal horn of arthritic mice displayed increased staining of microglia at early and late time points, but astrocyte staining increased only during the inflammatory phase. ATF3, a marker of nerve injury, was significantly increased in the lumbar dorsal root ganglia during the late phase (day 28). Hence, serum transfer in the K/BxN serum transfer arthritis model produces a persistent pain state, where the allodynia during the inflammatory state is attenuated by TNF and prostaglandin inhibitors, and the pharmacology and histochemistry data suggest a transition from an inflammatory state to a state that resembles a neuropathic condition over time. Therefore, the K/BxN serum transfer model represents a multifaceted model for studies exploring pain mechanisms in conditions of joint inflammation and may serve as a platform for exploring novel treatment strategies for pain in human arthritic conditions.

Minocycline prevents impaired glial glutamate uptake in the spinal sensory synapses of neuropathic rats

Activation of glutamate receptors and glial cells in the spinal dorsal horn are two fundamental processes involved in the pathogenesis of various pain conditions, including neuropathic pain induced by injury to the peripheral or central nervous systems. Numerous studies have demonstrated that minocycline treatment attenuates allodynic and hyperalgesic behaviors induced by tissue inflammation or nerve injury. However, the synaptic mechanisms by which minocycline prevents hyperalgesia are not fully understood. We recently reported that deficient glutamate uptake by glial glutamate transporters (GTs) is key for the enhanced activation of N-methyl-d-aspartate (NMDA) receptors in the spinal sensory synapses of rats receiving partial sciatic nerve ligation (pSNL). In this study, we investigated how minocycline affects activation of NMDA receptors in the spinal sensory synapses in rats with pSNL by whole cell recordings of NMDA currents in spinal laminea I and II neurons from spinal slices. The effects of minocycline treatments on the dorsal horn expression of glial GTs and astrocyte marker glial fibrillary acidic protein (GFAP) were analyzed by immunohistochemistry. We demonstrated that normalized activation of NMDA receptors in synapses activated by both weak and strong peripheral input in the spinal dorsal horn is temporally associated with attenuated mechanical allodynia in rats with pSNL receiving intraperitoneal injection of minocycline. Minocycline ameliorated both the downregulation of glial GT expression and the activation of astrocytes induced by pSNL in the spinal dorsal horn. We further revealed that preventing deficient glial glutamate uptake at the synapse is crucial for preserving the normalized activation of NMDA receptors in the spinal sensory synapses in pSNL rats treated with minocycline. Our studies suggest that glial GTs may be a potential target for the development of analgesics.

Minocycline inhibits the enhancement of antidromic primary afferent stimulation-evoked vasodilation following intradermal capsaicin injection

Neurogenic inflammation is induced by inflammatory mediators released in peripheral tissue from primary afferent nociceptors. Our previous studies suggest that neurogenic inflammation induced by intradermal injection of capsaicin results from the enhancement of dorsal root reflexes (DRRs), which involve antidromic activation of dorsal root ganglion (DRG) neurons. Numerous studies have reported the important role of glial modulation in pain. However, it remains unclear whether glial cells participate in the process of neurogenic inflammation-induced pain. Here we tested the role of DRG satellite glial cells (SGCs) in this process in anesthetized rats by administration of a glial inhibitor, minocycline. Electrical stimuli (ES, frequency 10 Hz; duration 1 ms; strength 3 mA) were applied to the cut distal ends of the L4-5 dorsal roots. The stimuli evoked antidromic action potentials designed to mimic DRRs. Local cutaneous blood flow in the hindpaw was measured using a Doppler flow meter. Antidromic ES for 10 min evoked a significant vasodilation that could be inhibited dose-dependently by local administration of the calcitonin gene-related peptide receptor antagonist, CGRP8-37. Pretreatment with capsaicin intradermally injected into the hindpaw 2h before the ES enhanced greatly the vasodilation evoked by antidromic ES, and this enhancement could be reversed by minocycline pretreatment. Our findings support the view that neurogenic inflammation following capsaicin injection involves antidromic activation of DRG neurons via the generation of DRRs. Inhibition of neurogenic inflammation by minocycline is suggested to be associated with its inhibitory effect on SGCs that are possibly activated following capsaicin injection.

Release of prostaglandin E(2) and nitric oxide from spinal microglia is dependent on activation of p38 mitogen-activated protein kinase

BACKGROUND

The spinal release of prostaglandins (PGs), nitric oxide (NO), and cytokines has been implicated in spinal nociceptive processing. Microglia represent a possible cell of origin for these proexcitatory mediators. Spinal microglia possess Toll-like receptor 4 (TLR4) and neurokinin 1 (NK1) receptors, and both receptors play a significant role in peripheral nerve injury- and inflammation-induced spinal sensitization. Accordingly, we examined the properties of the cascades activated by the respective targets, which led to the release of PGE(2) and an increase in nitrite (NO(2)(-)) (a marker of NO) from cultured rat spinal microglia.

METHODS

Spinal microglia isolated from Sprague-Dawley neonatal rats were cultured with lipopolysaccharide (LPS) or substance P (SP) alone, with LPS in combination with SP, and with LPS in the presence of each inhibitor of cyclooxygenase (COX), NO synthase 2 (NOS2) or p38 mitogen-activated protein kinase (p38), or minocycline for 24 hours and 48 hours. Concentrations of PGE(2) and NO(2)(-) in culture supernatants were measured using an enzyme immunoassay and a colorimetric assay, respectively.

RESULTS

Application of LPS (a TLR4 ligand, 0.1 to 10 ng/mL) to cultured microglia produced a dose- and time-dependent increase in PGE(2) and NO(2)(-) production, whereas no effects were observed after incubation with SP (an NK1 agonist, up to 10(-5) M) alone or in combination with LPS. Antagonist studies with SC-560 (COX-1 inhibitor) and SC-236 (COX-2 inhibitor) showed that LPS-induced PGE(2) release was generated from both COX-1 and COX-2. LPS-induced NO release was suppressed by 1400W, an inhibitor of NOS2. Minocycline, an agent blocking microglial activation, and SB203580, an inhibitor of p38, both attenuated the LPS-induced PGE(2) and NO release. The 1400W, at the doses that suppressed NO release, also blocked increased PGE(2) release.

CONCLUSIONS

Our findings suggest that (a) activation of spinal microglia via TLR4 but not NK1 receptors produces PGE(2) and NO release from these cells; (b) the evoked PGE(2) release is generated by both COX-1 and COX-2, and (c) the COX-PGE(2) pathway is regulated by p38 and NOS2. Taken together with our previous in vivo work, the current findings emphasize that p38 in spinal microglia is a key player in regulating production of pronociceptive molecules, such as PGE(2) and NO.

Key role for spinal dorsal horn microglial kinin B1 receptor in early diabetic pain neuropathy

Background

The pro-nociceptive kinin B₁ receptor (B₁R) is upregulated on sensory C-fibres, astrocytes and microglia in the spinal cord of streptozotocin (STZ)-diabetic rat. This study aims at defining the role of microglial kinin B₁R in diabetic pain neuropathy.

Methods

Sprague-Dawley rats were made diabetic with STZ (65 mg/kg, i.p.), and 4 days later, two specific inhibitors of microglial cells (fluorocitrate, 1 nmol, i.t.; minocycline, 10 mg/kg, i.p.) were administered to assess the impact on thermal hyperalgesia, allodynia and mRNA expression (qRT-PCR) of B₁R and pro-inflammatory markers. Spinal B₁R binding sites ((¹²⁵I)-HPP-desArg¹⁰-Hoe 140) were also measured by quantitative autoradiography. Inhibition of microglia was confirmed by confocal microscopy with the specific marker Iba-1. Effects of intrathecal and/or systemic administration of B₁R agonist (des-Arg⁹-BK) and antagonists (SSR240612 and R-715) were measured on neuropathic pain manifestations.

Results

STZ-diabetic rats displayed significant tactile and cold allodynia compared with control rats. Intrathecal or peripheral blockade of B₁R or inhibition of microglia reversed time-dependently tactile and cold allodynia in diabetic rats without affecting basal values in control rats. Microglia inhibition also abolished thermal hyperalgesia and the enhanced allodynia induced by intrathecal des-Arg⁹-BK without affecting hyperglycemia in STZ rats. The enhanced mRNA expression (B₁R, IL-1β, TNF-α, TRPV1) and Iba-1 immunoreactivity in the STZ spinal cord were normalized by fluorocitrate or minocycline, yet B₁R binding sites were reduced by 38%.

Conclusion

The upregulation of kinin B₁R in spinal dorsal horn microglia by pro-inflammatory cytokines is proposed as a crucial mechanism in early pain neuropathy in STZ-diabetic rats.