Evidence for suppression of spinal glial activation by dexmedetomidine in a rat model of monoarthritis

Macrophages and glial cells: Innate immune drivers of inflammatory arthritic pain perception from peripheral joints to the central nervous system

Millions of people suffer from arthritis worldwide, consistently struggling with daily activities due to debilitating pain evoked by this disease. Perhaps the most intensively investigated type of inflammatory arthritis is rheumatoid arthritis (RA), where, despite considerable advances in research and clinical management, gaps regarding the neuroimmune interactions that guide inflammation and chronic pain in this disease remain to be clarified. The pain and inflammation associated with arthritis are not isolated to the joints, and inflammatory mechanisms induced by different immune and glial cells in other tissues may affect the development of chronic pain that results from the disease. This review aims to provide an overview of the state-of-the-art research on the roles that innate immune, and glial cells play in the onset and maintenance of arthritis-associated pain, reviewing nociceptive pathways from the joint through the dorsal root ganglion, spinal circuits, and different structures in the brain. We will focus on the cellular mechanisms related to neuroinflammation and pain, and treatments targeting these mechanisms from the periphery and the CNS. A comprehensive understanding of the role these cells play in peripheral inflammation and initiation of pain and the central pathways in the spinal cord and brain will facilitate identifying new targets and pathways to aide in developing therapeutic strategies to treat joint pain associated with RA.

Activation of locus coeruleus-spinal cord noradrenergic neurons alleviates neuropathic pain in mice via reducing neuroinflammation from astrocytes and microglia in spinal dorsal horn

Background

The noradrenergic neurons of locus coeruleus (LC) project to the spinal dorsal horn (SDH), and release norepinephrine (NE) to inhibit pain transmission. However, its effect on pathological pain and the cellular mechanism in the SDH remains unclear. This study aimed to explore the analgesic effects and the anti-neuroinflammation mechanism of LC-spinal cord noradrenergic pathway (LC:SC) in neuropathic pain (NP) mice with sciatic chronic constriction injury.

Methods

The Designer Receptors Exclusively Activated by Designer Drugs (DREADD) was used to selectively activate LC:SC. Noradrenergic neuron-specific retro–adeno-associated virus was injected to the spinal cord. Pain threshold, LC and wide dynamic range (WDR) neuron firing, neuroinflammation (microglia and astrocyte activation, cytokine expression), and α2AR expression in SDH were evaluated.

Results

Activation of LC:SC with DREADD increased the mechanical and thermal nociceptive thresholds and reduced the WDR neuron firing. LC:SC activation (daily, 7 days) downregulated TNF-α and IL-1β expression, upregulated IL-4 and IL-10 expression in SDH, and inhibited microglia and astrocytes activation in NP mice. Immunofluorescence double staining confirmed that LC:SC activation decreased the expression of cytokines in microglia of the SDH. In addition, the effects of LC:SC activation could be reversed by intrathecal injection of yohimbine. Immunofluorescence of SDH showed that NE receptor α2B-AR was highly expressed in microglia in CCI mice.

Conclusion

These findings indicate that selective activation of LC:SC alleviates NP in mice by increasing the release of NE and reducing neuroinflammation of astrocytes and microglia in SDH.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02489-9.

Dexmedetomidine alleviates hyperalgesia in arthritis rats through inhibition of the p38MAPK signaling pathway

BACKGROUND

Dexmedetomidine (DEX) has showed significant analgesic effects in neuropathic pain, but the underlying mechanism has remained elusive. Our present study aimed to explore the effect of DEX on hyperalgesia with the involvement of p38MAPK signaling pathway a rat model of monoarthritis (MA).

METHODS

MA rat model was induced by injection of Complete Freund's Adjuvant (CFA). Pathological changes of ma rats were observed by HE staining and Safranin-O/Fast Green staining. Ankle circumference, paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) was measured to judge the degree of hyperalgesia in MA rats. Immunohistochemistry and ELISA were applied to observe the degree of inflammation in rats. Western blot analysis was conducted to detect expression of p38MAPK signaling pathway-related factors. The mechanism of p38MAPK signaling pathway in MA rats was observed via treatment of Anisomycin or SB203580 combined with DEX.

RESULTS

After 8 h of CFA induction, joint swelling and hyperalgesia occurred in rats. There were obvious pathological changes in the joint cavity, the joint cavity space became narrow and synovial bursa became rough. A large number of inflammatory cell infiltration was observed under microscope. After injection of DEX and SB203580, PWT and PWL was prolonged, the expression of serum inflammatory factors was decreased, and the expression of p38MAPK signaling pathway-related factors was decreased; while all the detected indexes were recovered in MA rats after treated with DEX and Anisomycin.

CONCLUSIONS

Our study provided evidence that DEX could alleviate hyperalgesia in arthritis rats through inhibition of the p38MAPK signaling pathway.

Effect of Pregabalin Combined with Duloxetine and Tramadol on Allodynia in Chronic Postischemic Pain and Spinal Nerve Ligation Mouse Models

Although there are various drugs for Neuropathic pain (NP), the effects of single drugs are often not very satisfactory. The analgesic effects of different combinations of pregabalin, duloxetine, and tramadol or the combination of all three are still unclear. Mixtures of two or three drugs at low and high concentrations (7.5, 10, 15, and 20 mg/kg pregabalin; 7.5, 10, 15, and 30 mg/kg duloxetine; 5 and 10 mg/kg tramadol) were administered to chronic postischemic pain (CPIP) and spinal nerve ligation (SNL) model mice. The effects of these combinations of drugs on mechanical allodynia were investigated. The expression of the glial fibrillary acidic protein (GFAP) in the spinal cord and dorsal root ganglia (DRGs) was measured. The combination of pregabalin, duloxetine, and tramadol significantly alleviated mechanical hyperalgesia in mice with CPIP and SNL. After the administration of this drug combination, the expression of GFAP in the spinal cord and DRGs was lower in the CPIP and SNL model mice than in control mice. This result suggests that the combination of these three drugs may be advantageous for the treatment of NP because it can reduce side effects by preventing the overuse of a single drug class and exert increased analgesic effects via synergism.

The effects of anaesthetics and sedatives on brain inflammation

Microglia are involved in many dynamic processes in the central nervous system (CNS) including the development of inflammatory processes and neuromodulation. Several sedative, analgesic or anaesthetic drugs, such as opioids, ∝2-adrenergic agonists, ketamine, benzodiazepines and propofol can cause both neuroprotective and harmful effects on the brain. The purpose of this review is to present the main findings on the use of these drugs and the mechanisms involved in microglial activation. Alpha 2-adrenergic agonists, propofol and benzodiazepines have several pro- or anti-inflammatory effects on microglia. Long-term use of benzodiazepines and propofol causes neuroapoptotic effects and α2-adrenergic agonists may attenuate these effects. Conversely, morphine and fentanyl may have proinflammatory effects, causing behavioural changes in patients and changes in cell viability in vitro. Conversely, chronic administration of morphine induces CCL5 chemokine expression in microglial cells that promotes their survival.

Opposing functions of α- and β-adrenoceptors in the formation of processes by cultured astrocytes

Astrocytes are glial cells with numerous fine processes which are important for the functions of the central nervous system. The activation of β-adrenoceptors induces process formation of astrocytes via cyclic AMP (cAMP) signaling. However, the role of α-adrenoceptors in the astrocyte morphology has not been elucidated. Here, we examined it by using cultured astrocytes from neonatal rat spinal cords and cortices. Exposure of these cells to noradrenaline and the β-adrenoceptor agonist isoproterenol increased intracellular cAMP levels and induced the formation of processes. Noradrenaline-induced process formation was enhanced with the α1-adrenoceptor antagonist prazosin and α2-adrenoceptor antagonist atipamezole. Atipamezole also enhanced noradrenaline-induced cAMP elevation. Isoproterenol-induced process formation was not inhibited by the α1-adrenoceptor agonist phenylephrine but was inhibited by the α2-adrenoceptor agonist dexmedetomidine. Dexmedetomidine also inhibited process formation induced by the adenylate cyclase activator forskolin and the membrane-permeable cAMP analog dibutyryl-cAMP. Moreover, dexmedetomidine inhibited cAMP-independent process formation induced by adenosine or the Rho-associated kinase inhibitor Y27632. In the presence of propranolol, noradrenaline inhibited Y27632-induced process formation, which was abolished by prazosin or atipamezole. These results demonstrate that α-adrenoceptors inhibit both cAMP-dependent and -independent astrocytic process formation.

Role of Spinal Cord α2-Adrenoreceptors in Noradrenergic Inhibition of Nociceptive Transmission During Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a problem during cancer treatment and for cancer survivors but the central mechanisms underlying CIPN remain understudied. This study aims to determine if CIPN is associated with alterations of noradrenergic modulation of nociceptive transmission at the spinal cord. CIPN was induced in male Wistar rats by paclitaxel injections. One month after CIPN induction, the behavioral effects of the administration of reboxetine (noradrenaline reuptake inhibitor), clonidine (agonist of α₂-adrenoreceptors; α_2–AR) and atipamezole (antagonist of α_2–AR) were evaluated using the von Frey and cold plate tests. Furthermore, we measured the expression of the noradrenaline biosynthetic enzyme dopamine-β-hydroxylase (DBH) and of α_2–AR in the spinal dorsal horn. Reboxetine and clonidine reversed the behavioral signs of CIPN whereas the opposite occurred with atipamezole. In the 3 pharmacological approaches, a higher effect was detected in mechanical allodynia, the pain modality which is under descending noradrenergic control. DBH expression was increased at the spinal dorsal horn of paclitaxel-injected animals. The enhanced noradrenergic inhibition during CIPN may represent an adaptation of the descending noradrenergic pain control system to the increased arrival of peripheral nociceptive input. A potentiation of the α_2–AR mediated antinociception at the spinal cord may represent a therapeutic opportunity to face CIPN.

Dexmedetomidine inhibits the NF-κB pathway and NLRP3 inflammasome to attenuate papain-induced osteoarthritis in rats

Context: Dexmedetomidine (Dex) has been reported to have an anti-inflammatory effect. However, its role on osteoarthritis (OA) has not been explored. Objective: This study investigates the effect of Dex on OA rat model induced by papain. Materials and methods: The OA Wistar rat model was induced by intraluminal injection of 20 mL of papain mixed solution (4% papain 0.2 mL mixed with 0.03 mol L^-1 l-cysteine 0.1 mL) into the right knee joint. Two weeks after papain injection, OA rats were treated by intra-articular injection of Dex (5, 10, or 20 μg kg^-1) into the right knee (once a day, continuously for 4 weeks). Articular cartilage tissue was obtained after Dex treatment was completed. Results: The gait behavior scores (2.83 ± 0.49), PWMT (15.2 ± 1.78) and PTWL (14.81 ± 0.92) in H-DEX group were higher than that of OA group, while Mankin score (5.5 ± 0.81) was decreased (p < 0.05). Compared with the OA group, the IL-1β (153.11 ± 16.05 pg mg^-1), IL-18 (3.71 ± 0.7 pg mg^-1), IL-6 (14.15 ± 1.94 pg/mg) and TNF-α (40.45 ± 10.28 pg mg^-1) levels in H-DEX group were decreased (p < 0.05). MMP-13, NLRP3, and caspase-1 p10 expression in Dex groups were significantly lower than that of OA group (p < 0.05), while collagen II was increased (p < 0.05). p65 in the nucleus of Dex groups was significantly down-regulated than that of OA group (p < 0.05). Discussion and Conclusions: Dex can improve pain symptoms and cartilage tissue damage of OA rats, which may be related to its inhibition of the activation of NF-κB and NLRP3 inflammasome.

Rescue of Noradrenergic System as a Novel Pharmacological Strategy in the Treatment of Chronic Pain: Focus on Microglia Activation

Different types of pain can evolve toward a chronic condition characterized by hyperalgesia and allodynia, with an abnormal response to normal or even innocuous stimuli, respectively. A key role in endogenous analgesia is recognized to descending noradrenergic pathways that originate from the locus coeruleus and project to the dorsal horn of the spinal cord. Impairment of this system is associated with pain chronicization. More recently, activation of glial cells, in particular microglia, toward a pro-inflammatory state has also been implicated in the transition from acute to chronic pain. Both α2- and β2-adrenergic receptors are expressed in microglia, and their activation leads to acquisition of an anti-inflammatory phenotype. This review analyses in more detail the interconnection between descending noradrenergic system and neuroinflammation, focusing on drugs that, by rescuing the noradrenergic control, exert also an anti-inflammatory effect, ultimately leading to analgesia. More specifically, the potential efficacy in the treatment of neuropathic pain of different drugs will be analyzed. On one side, drugs acting as inhibitors of the reuptake of serotonin and noradrenaline, such as duloxetine and venlafaxine, and on the other, tapentadol, inhibitor of the reuptake of noradrenaline, and agonist of the µ-opioid receptor.

Thymulin treatment attenuates inflammatory pain by modulating spinal cellular and molecular signaling pathways

Thymulin is a peptide hormone which is mainly produced by thymic epithelial cells and it has immune-modulatory and anti-inflammatory effects. In this study, we investigated the effects of different doses and various timings of thymulin intraperitoneal administration on spinal microglial activity and intracellular pathways in an inflammatory rat model of Complete Freund's adjuvant (CFA). Thymulin treatment was implemented following CFA-induced inflammation for 21 days. After conducting behavioral tests (edema and hyperalgesia), the cellular and molecular aspects were examined to detect the thymulin effect on inflammatory factors and microglial activity. We demonstrated that thymulin treatment notably reduced thermal hyperalgesia and paw edema induced by CFA. Furthermore, molecular investigations showed that thymulin reduced CFA-induced activation of microglia cells, phosphorylation of p38 MAPK and the production of spinal pro-inflammatory cytokines (TNF-α, IL-6) during the study. Our results suggest that thymulin treatment attenuates CFA-induced inflammation. This effect may be mediated by inhibition of spinal microglia and production of central inflammatory mediators which seems to be associated with the ability of thymulin to reduce p38 MAPK phosphorylation. These data provide evidence of the anti-hyperalgesic effect of thymulin on inflammatory pain and characterize some of the underlying spinal mechanisms.

Additive analgesic effect of dexmedetomidine and dezocine administered intrathecally in a mouse pain model

Background

It is known that dexmedetomidine can reduce opioid requirements and that there is a synergistic effect when dexmedetomidine and morphine (a full mu opioid receptor agonist) are administered together. However, it was unclear whether a synergistic or additive effect would be observed when dexmedetomidine was co-administered with a partial mu opioid receptor agonist. The present study was designed to elucidate such effects by intrathecally co-administering dexmedetomidine and dezocine, a partial mu receptor agonist, in a mouse pain model.

Methods

C57 mice (N = 165) were randomly divided into 19 groups. The tail flick test was adopted to measure the antinociceptive effects of the tested agents. The mice were divided into saline and drug groups to investigate the dose-dependent analgesic effects. Each drug was administered at fixed doses alone and in combination with one of three doses of a second drug.

Results

Dezocine (0.3125 - 1.25 μg) and dexmedetomidine (0.04 - 1 μg) both enhanced the tail withdrawal latency in dose-dependent fashions. Dexmedetomidine (0.04 - 1 μg) enhanced the analgesic effect of dezocine. Dezocine (0.3125 - 1.25 μg) enhanced the analgesic effect of dexmedetomidine. Compared with the individual drug effects, the combined effects of dezocine (0.625 μg) and dexmedetomidine (0.04 μg) were more potent 15 - 60 min after injection, but they remained similar to the sum of the effects of the two individual drugs.

Conclusions

Dexmedetomidine and dezocine produce an additive analgesic effect on acute nociception when administered simultaneously.

Assessing disease-modifying effects of norepinephrine in Down syndrome and Alzheimer's disease

Building upon the knowledge that a number of important brain circuits undergo significant degeneration in Alzheimer's disease, numerous recent studies suggest that the norepinephrine-ergic system in the brainstem undergoes significant alterations early in the course of both Alzheimer's disease and Down syndrome. Massive projections from locus coeruleus neurons to almost the entire brain, extensive innervation of brain capillaries, and widespread distribution of noradrenergic receptors enable the norepinephrine-ergic system to play a crucial role in neural processes, including cognitive function. These anatomical and functional characteristics support the role of the norepinephrine-ergic system as an important target for developing new therapies for cognitive dysfunction. Careful neuropathological examinations using postmortem samples from individuals with Alzheimer's disease have implicated the role of the norepinephrine-ergic system in the etiopathogenesis of Alzheimer's disease. Furthermore, numerous studies have supported the existence of a strong interaction between norepinephrine-ergic and neuroimmune systems. We explore the interaction between the two systems that could play a role in the disease-modifying effects of norepinephrine in Alzheimer's disease and Down syndrome.

Anti-nociceptive effect of dexmedetomidine in a rat model of monoarthritis via suppression of the TLR4/NF-κB p65 pathway

As a therapeutic target for neuropathic pain, the anti-nociceptive effects of α 2-adrenoceptors (α2AR) have attracted attention. Dexmedetomidine (DEX), a potent and highly selective α2AR agonist, has exhibited significant analgesic effects in neuropathic pain, but the underlying mechanism has remained elusive. The present study investigated the effect of DEX on Toll-like receptor (TLR)4 and nuclear factor (NF)-κB p65 expression, as well as the production of pro-inflammatory cytokines. The rat monoarthritis (MA) model was induced by intra-articular injection of complete Freund's adjuvant (CFA) at the ankle joint. After induction of MA, the rats were intrathecally treated with normal saline or DEX (2.5 µg) for 3 consecutive days. The concentration of interleukin-1β and -6 as well as tumor necrosis factor-α was examined by ELISA. The expression levels of TLR4 and NF-κB p65 were determined by western blot analysis and immunohistochemistry. The results indicated that the pro-inflammatory cytokines TLR4 and NF-κB p65 were significantly upregulated in MA rats. DEX treatment markedly reduced mechanical and thermal hyperalgesia, suppressed MA-induced elevation of the pro-inflammatory cytokines and inhibited the TLR4/NF-κB p65 pathway, while these effects were blocked by pre-treatment with the selective α2AR antagonist BRL44408 (15 µg) at 30 min prior to CFA injection. These results suggested that DEX has an anti-nociceptive effect via suppressing the TLR4/NF-κB p65 pathway.

Systemic dexmedetomidine attenuates mechanical allodynia through extracellular sign db type 2 diabetic mice

Painful diabetic neuropathy (PDN) is a common complication of diabetes mellitus. However, the treatment for PDN is limited in clinical practice. In the present study, we investigated the effect of systemic administration dexmedetomidine (DEX), a selective alpha 2 adrenergic receptor (α2AR) agonist, on mechanical allodynia and its underlying mechanism in db/db mice, an animal model of type 2 diabetes mellitus. Our data demonstrated that db/db mice develop mechanical allodynia at the early stage of diabetes. During the period of mechanical allodynia, we detected increased release of norepinephrine (NE) and decreased levels of α_2A-Adrenoceptors in db/db mice. Immunohistochemistry showed that the α_2A-Adrenoceptor is predominantly expressed in neurons in the spinal cord. Acute injection of dexmedetomidine significantly decreased mechanical allodynia, which was blocked by its selective antagonist BRL44408. Furthermore, the upregulation of pERK1 and pERK2 in db/db mice were attenuated by preadministration of dexmedetomidine. We provide the first evidence that the functional alternation of spinal noradrenergic system might underlie exaggerated nociception in PDN. Systemic dexmedetomidine inhibits the mechanical allodynia which is related to ERK signaling pathway in type 2 diabetes, implying that the α₂-Adrenoceptor might be a potential therapeutic strategy for PDN.

Protective effects of dexmedetomidine combined with flurbiprofen axetil on remifentanil-induced hyperalgesia: A randomized controlled trial

High dosages of intra-operative remifentanil are associated with opioid-induced hyperalgesia (OIH). The aim of the present study was to investigate the effect of combined dexmedetomidine and flurbiprofen axetil treatment on remifentanil-induced hyperalgesia. Patients with an American Society of Anesthesiologists physical status of I-II who were diagnosed with hysteromyoma and scheduled for laparoscopic assisted vaginal hysterectomy (LAVH) were randomly divided into three groups. Group hyperalgesia (Group H, n=29) received intra-operative remifentanil, Group hyperalgesia and dexmedetomidine (Group HD, n=28) received remifentanil and a continuous infusion of dexmedetomidine, and Group hyperalgesia, dexmedetomidine and flurbiprofen axetil (Group HDF, n=29) received remifentanil, flurbiprofen axetil and dexmedetomidine. Mechanical pain thresholds were measured during the preoperative visit and postoperatively at 1, 6 and 24-h time points. Visual analog scale (VAS) scores, time to analgesic requirement, total sufentanil consumption and side effects were assessed postoperatively. Mechanical pain threshold at the incision site was significantly lower in Group H compared with Groups HD and HDF (both P<0.05), and significantly higher in Group HDF than in Group HD (P<0.05). The area of secondary hyperalgesia at the incision site was greater in Group H than in the other two groups (both P<0.05), and significantly smaller in Group HDF compared with Group HD (P<0.05). VAS scores and total sufentanil consumption were significantly higher in Group H compared with the other two groups (both P<0.05), and were significantly lower in Group HDF compared with Group HD (P<0.05). Dexmedetomidine combined with flurbiprofen axetil exhibits synergetic effects in the prevention of remifentanil-induced hyperalgesia in patients undergoing LAVH.

Diacerein reduces joint damage, pain behavior and inhibits transient receptor potential vanilloid 1, matrix metalloproteinase and glial cells in rat spinal cord

AIM

To investigate the antinociceptive, antiedematogenic and chondroprotective effects of diacerein (DIA) in a model of joint inflammation induced by complete Freund's adjuvant (CFA), as well as to investigate the involvement of metalloproteinase (MMP)-9, transient receptor potential vanilloid 1 (TRPV1) and glial cells in DIA's action mechanism.

METHODS

Complete Freund's adjuvant was injected into the knee joint of male rats. We observed mechanical and cold hypersensitivity, vocalization and spontaneous pain-related behaviors, as well as edema of the knee. Tissue samples of the knee were stained with Cason`s technique and the thickness of the condilus cartilage was measured. Immunohistochemical analysis was performed on the spinal cord using anti-GFAP (glial fibrillary acidic protein), anti-MMP and anti-TRPV1 antibodies. Sections of the dorsal horns of the spinal cord were captured and an optical density was obtained.

RESULTS

Complete Freund's adjuvant induced mechanical and thermal hypersensitivity, as well as joint edema and changes in the synovial membrane and cartilage. DIA (30 mg/kg, orally, daily) significantly inhibited mechanical (58 ± 10-87 ± 3%) and thermal (66 ± 12-87 ± 8%) hypersensitivity, vocalization (83 ± 5-41 ± 11%), spontaneous pain score, joint swelling (60 ± 6-40 ± 9%), as well as the histological changes induced by CFA. In addition, DIA inhibited astrocyte activation, and prevented the increase of MMP-9 and TRPV1 expression in the spinal cord of the animals subjected to CFA injections.

CONCLUSIONS

In short, this study shows that DIA reduces joint damage and hypersensitivity associated with inflammation induced by CFA through the inhibition of astroglial activation and decreases the expression of TRPV1 and MMP-9 in the rat spinal cord.

Adrenergic regulation of innate immunity: a review

The sympathetic nervous system has a major role in the brain-immune cross-talk, but few information exist on the sympathoadrenergic regulation of innate immune system. The aim of this review is to summarize available knowledge regarding the sympathetic modulation of the innate immune response, providing a rational background for the possible repurposing of adrenergic drugs as immunomodulating agents. The cells of immune system express adrenoceptors (AR), which represent the target for noradrenaline and adrenaline. In human neutrophils, adrenaline and noradrenaline inhibit migration, CD11b/CD18 expression, and oxidative metabolism, possibly through β-AR, although the role of α₁- and α₂-AR requires further investigation. Natural Killer express β-AR, which are usually inhibitory. Monocytes express β-AR and their activation is usually antiinflammatory. On murine Dentritic cells (DC), β-AR mediate sympathetic influence on DC-T cells interactions. In human DC β₂-AR may affect Th1/2 differentiation of CD4+ T cells. In microglia and in astrocytes, β₂-AR dysregulation may contribute to neuroinflammation in autoimmune and neurodegenerative disease. In conclusion, extensive evidence supports a critical role for adrenergic mechanisms in the regulation of innate immunity, in peripheral tissues as well as in the CNS. Sympathoadrenergic pathways in the innate immune system may represent novel antiinflammatory and immunomodulating targets with significant therapeutic potential.

Individual differences in acute pain-induced endogenous analgesia predict time to resolution of postoperative pain in the rat

BACKGROUND

Chronic postsurgical pain, a significant public health problem, occurs in 10 to 50% of patients undergoing major surgery. Acute pain induces endogenous analgesia termed conditioned pain modulation (CPM), and the strength of CPM preoperatively predicts the likelihood of chronic postsurgical pain. The relation between CPM and recovery from surgery has not been examined in preclinical models.

METHODS

CPM was assessed in individual rats and correlated with each animal's time course of recovery of hypersensitivity after partial spinal nerve ligation. The role of descending noradrenergic pathways in the spinal cord to mechanisms of CPM and recovery was tested using idazoxan to block noradrenergic receptors or antidopamine β-hydroxylase-conjugated saporin to ablate these pathways. Behavioral hypersensitivity, static weight bearing, and spinal glial activation were measured after partial spinal nerve ligation.

RESULTS

The strength of CPM varied over two-fold between individuals and was directly correlated with the slope of recovery from hypersensitivity after surgery (P < 0.0001; r = 0.660). CPM induced the release of norepinephrine in the spinal cord and was partially blocked by intrathecal idazoxan or dopamine β-hydroxylase-saporin. Dopamine β-hydroxylase-saporin also slowed recovery and enhanced spinal glial activation after partial spinal nerve ligation surgery. Ongoing activation of these pathways was critical to sustained recovery because intrathecal dopamine β-hydroxylase-saporin given 7 weeks after recovery reinstituted hypersensitivity, while having no effect in animals without previous surgery.

CONCLUSION

Collectively, these studies provide a clear back-translation from clinical observations of CPM and chronic postsurgical pain and suggest that the ability to engage ongoing descending endogenous noradrenergic signaling may be critical in determining time course of recovery from hypersensitivity after surgery.

Dexmedetomidine inhibits Tetrodotoxin-resistant Nav1.8 sodium channel activity through Gi/o-dependent pathway in rat dorsal root ganglion neurons

Background

Systemically administered dexmedetomidine (DEX), a selective α2 adrenergic receptor (α2-AR) agonists, produces analgesia and sedation. Peripherally restricted α2-AR antagonist could block the analgesic effect of systemic DEX on neuropathic pain, with no effect on sedation, indicating peripheral analgesic effect of DEX. Tetrodotoxin-resistant (TTX-R) sodium channel Na_v1.8 play important roles in the conduction of nociceptive sensation. Both α2-AR and Nav1.8 are found in small nociceptive DRG neurons. We, therefore, investigated the effects of DEX on the Na_v1.8 currents in acutely dissociated small-diameter DRG neurons.

Results

Whole-cell patch-clamp recordings demonstrated that DEX concentration-dependently suppressed TTX-R Na_v1.8 currents in small-diameter lumbar DRG neurons. DEX also shifted the steady-state inactivation curves of Na_v1.8 in a hyperpolarizing direction and increased the threshold of action potential and decrease electrical and chemical stimuli-evoked firings in small-diameter DRG neurons. The α2-AR antagonist yohimbine or α2_A-AR antagonist BRL44408 but not α2_B-AR antagonist imiloxan blocked the inhibition of Na_v1.8 currents by DEX. Immunohistochemistry results showed that Na_v1.8 was predominantly expressed in peripherin-positive small-diameter DRG neurons, and some of them were α2_A-AR-positive ones. Our electrophysiological recordings also demonstrated that DEX-induced inhibition of Na_v1.8 currents was prevented by intracellular application of G-protein inhibitor GDPβ-s or G_i/o proteins inhibitor pertussis toxin (PTX), and bath application of adenylate cyclase (AC) activator forskolin or membrane-permeable cAMP analogue 8-Bromo-cAMP (8-Br-cAMP). PKA inhibitor Rp-cAMP could mimic DEX-induced inhibition of Na_v1.8 currents.

Conclusions

We established a functional link between α2-AR and Na_v1.8 in primary sensory neurons utilizing the G_i/o/AC/cAMP/PKA pathway, which probably mediating peripheral analgesia of DEX.

Inflammatory Response in Patients under Coronary Artery Bypass Grafting Surgery and Clinical Implications: A Review of the Relevance of Dexmedetomidine Use

Despite the fact that coronary artery bypass grafting surgery (CABG) with cardiopulmonary bypass (CPB) prolongs life and reduces symptoms in patients with severe coronary artery diseases, these benefits are accompanied by increased risks. Morbidity associated with cardiopulmonary bypass can be attributed to the generalized inflammatory response induced by blood-xenosurfaces interactions during extracorporeal circulation and the ischemia/reperfusion implications, including exacerbated inflammatory response resembling the systemic inflammatory response syndrome (SIRS). The use of specific anesthetic agents with anti-inflammatory activity can modulate the deleterious inflammatory response. Consequently, anti-inflammatory anesthetics may accelerate postoperative recovery and better outcomes than classical anesthetics. It is known that the stress response to surgery can be attenuated by sympatholytic effects caused by activation of central (α-)2-adrenergic receptor, leading to reductions in blood pressure and heart rate, and more recently, that they can have anti-inflammatory properties. This paper discusses the clinical significance of the dexmedetomidine use, a selective (α-)2-adrenergic agonist, as a coadjuvant in general anesthesia. Actually, dexmedetomidine use is not in anesthetic routine, but this drug can be considered a particularly promising agent in perioperative multiple organ protection.

Intrathecal antagonism of microglial TLR4 reduces inflammatory damage to blood-spinal cord barrier following ischemia/reperfusion injury in rats

BACKGROUND

Inflammatory reaction in blood-spinal cord barrier (BSCB) plays a crucial role in ischemia/reperfusion (I/R) injury. It has been shown that microglia could be activated through Toll-like receptors (TLRs). Therefore, we hypothesize that TLR4 is involved in the microglial activation and BSCB disruption after I/R.

RESULTS

To verify our hypothesis, we analyzed the behavioral data, changes of BSCB permeability, as well as expressions of microglial marker Iba-1 and TLR4 in spinal cord I/R model induced by 14 min aortic occlusion. Double immunostaining reveals that after I/R, Iba-1 immunoreactivity increased gradually 12 h after reperfusion and maintained at a such level throughout 36 h. Such increasing pattern of Iba-1 expression is consistent with the increases in Evan's Blue (EB) extravasation, spinal water content and mechanical allodynia demonstrated by lowed withdrawal threshold to Von Frey filaments. Moreover, double immunostaining suggested that TLR4 was highly expressed in microglia. Intrathecal infusion of minocycline and TAK-242 (TLR4 inhibitor) treatment attenuated I/R-induced allodynia and BSCB leakage. In contrast, LPS induced TLR4 expression aggregated above-mentioned injuries. Furthermore, the nuclear factor-kappa B (NF-κB) activity has a similar profile as TLR4 activity. It is consisted with the results of NF-κB mRNA and protein expression changes and activation of downstream cytokine, IL-1β. Expectedly, intrathecal infusion of pyrrolidine dithiocarbamate (PDTC), a NF-κB inhibitor, showed similar protective effects as minocycline and TAK-242. In addition, our data show that TLR4 closely involved in I/R-induced inflammatory damage induced neuronal apoptosis. Significantly, neutralizing TLR4 function largely reduced neuronal apoptosis determined by NeuN immunoreactivity in ventral gray matter and increased percentage of double-label cells with cleaved caspase3, whereas LPS reversed these effects. Similarly, inhibitions of microglia and NF-κB with minocycline or PDTC treatment accordingly perform the same protective effects on I/R injury.

CONCLUSION

The results indicate that compromised BSCB caused by I/R injury lead to spinal microglial activation and TLR4, its membrane-bound receptor, up-regulation, which then initiate neuro-inflammation and neuro-apoptosis via NF-κB/ IL-1β pathway. To inhibit the positive feedback loop of TLR4-microglia-NF-κB/ IL-1β pathway by minocycline, TAK-242 (TLR4 inhibitor) and pyrrolidine dithiocarbamate (PDTC, NF-κB inhibitor) may provide new targets for treating I/R injury in clinic.

Inhibiting spinal neuron-astrocytic activation correlates with synergistic analgesia of dexmedetomidine and ropivacaine

Background

This study aims to identify that intrathecal (i.t.) injection of dexmedetomidine (Dex) and ropivacaine (Ropi) induces synergistic analgesia on chronic inflammatory pain and is accompanied with corresponding “neuron-astrocytic” alterations.

Methods

Male, adult Sprague-Dawley rats were randomly divided into sham, control and i.t. medication groups. The analgesia profiles of i.t. Dex, Ropi, and their combination detected by Hargreaves heat test were investigated on the subcutaneous (s.c.) injection of complete Freund adjuvant (CFA) induced chronic pain in rat and their synergistic analgesia was confirmed by using isobolographic analysis. During consecutive daily administration, pain behavior was daily recorded, and immunohistochemical staining was applied to investigate the number of Fos-immunoreactive (Fos-ir) neurons on hour 2 and day 1, 3 and 7, and the expression of glial fibrillary acidic protein (GFAP) within the spinal dorsal horn (SDH) on day 1, 3, 5 and 7 after s.c. injection of CFA, respectively, and then Western blot to examine spinal GFAP and β-actin levels on day 3 and 7.

Results

i.t. Dex or Ropi displayed a short-term analgesia in a dose-dependent manner, and consecutive daily administrations of their combination showed synergistic analgesia and remarkably down-regulated neuronal and astrocytic activations indicated by decreases in the number of Fos-ir neurons and the GFAP expression within the SDH, respectively.

Conclusion

i.t. co-delivery of Dex and Ropi shows synergistic analgesia on the chronic inflammatory pain, in which spinal “neuron-astrocytic activation” mechanism may play an important role.

New therapeutic uses for an alpha2 adrenergic receptor agonist--dexmedetomidine in pain management

Dexmedetomidine was initially approved for clinical use as a sedative. Its development in pain management has been limited. Dexmedetomidine has analgesic effects and analgesic-sparing properties, especially for patients with obstructed airways. Mixing dexmedetomidine with local anesthetics is a promising new avenue to enhance local anesthetics' effectiveness. Peripheral, spinal and supraspinal α(2A)-ARs are responsible for the analgesic function of dexmedetomidine. Animal studies have shown that antinociceptive synergism results from co-application of dexmedetomidine and opioids or local anesthetics. Dexmedetomidine has potential adverse effects such as hypotension and bradycardia. Therefore, dexmedetomidine is contraindicated for patients suffering from bradycardia or using β-adrenergic antagonists. Clinical trials of dexmedetomidine in chronic pain or hyperalgesia are lack.

Involvement of spinal microglia and interleukin-18 in the anti-nociceptive effect of dexmedetomidine in rats subjected to CCI

Dexmedetomidine, a selective alpha 2-adrenoceptor (α2AR) agonist, has provided significant analgesia in neuropathic pain. However, its underlying molecular mechanism has not been fully elucidated. In the present study, we found that intrathecal administration of dexmedetomidine alleviated mechanical allodynia induced by chronic constriction injury (CCI), and pretreatment with BRL44408 significantly reversed the dexmedetomidine-induced anti-nociceptive effect. Western blotting revealed that dexmedetomidine reduced the activation of microglia and the upregulation of interleukin-18 (IL-18) protein expression in the ipsilateral lumbar spinal dorsal horn, while BRL44408 pretreatment significantly blocked these effects of dexmedetomidine. Immunocytochemistry/immunohistochemistry indicated that the α2A-adrenoceptor was localised to microglia in primary culture, and IL-18 predominantly colocalised with the microglial marker Iba-1 in the dorsal horn of the spinal cord. These results suggest that the IL-18 signalling pathway in microglia may be involved in the anti-nociceptive effect of dexmedetomidine in rats subjected to CCI.

Interruption of spinal cord microglial signaling by alpha-2 agonist dexmedetomidine in a murine model of delayed paraplegia

BACKGROUND

Despite investigation into preventable pharmacologic adjuncts, paraplegia continues to complicate thoracoabdominal aortic interventions. The alpha 2a adrenergic receptor agonist, dexmedetomidine, has been shown to preserve neurologic function and neuronal viability in a murine model of spinal cord ischemia reperfusion, although the mechanism remains elusive. We hypothesize that dexmedetomidine will blunt postischemic inflammation in vivo following thoracic aortic occlusion with in vitro demonstration of microglial inhibition following lipopolysaccharide (LPS) stimulation.

METHODS

Adult male C57BL/6 mice underwent 4 minutes of aortic occlusion. Mice received 25 μg/kg intraperitoneal dexmedetomidine (n = 8) or 0.9% normal saline (n = 7) at reperfusion and 12-hour intervals postoperatively until 48 hours. Additionally, sham mice (n = 3), which had aortic arch exposed with no occlusion, were included for comparison. Functional scoring was done at 6 hours following surgery and 12-hour intervals until 60 hours when spinal cords were removed and examined for neuronal viability and cytokine production. Additional analysis of microglia activation was done in 12 hours following surgery. Age- and sex-matched mice had spinal cord removed for microglial isolation culture. Cells were grown to confluence and stimulated with toll-like receptor-4 agonist LPS 100 ng/mL in presence of dexmedetomidine or vehicle control for 24 hours. Microglia and media were then removed for analysis of protein expression.

RESULTS

Dexmedetomidine treatment at reperfusion significantly preserved neurologic function with mice in treatment group having a Basso Score of 6.3 in comparison to 2.3 in ischemic control group. Treatment was associated with a significant reduction in microglia activation and in interleukin-6 production. Microglial cells in isolation when stimulated with LPS had an increased production of proinflammatory cytokines and markers of activation. Treatment with dexmedetomidine significantly attenuated microglial activation and proinflammatory cytokine production in vitro with a greater than twofold reduction in tumor necrosis factor-α.

CONCLUSIONS

Alpha 2a agonist, dexmedetomidine treatment at reperfusion preserved neurologic function and neuronal viability. Furthermore, dexmedetomidine treatment resulted in an attenuation of microglial activation and proinflammatory cytokine production both in vivo and in vitro following LPS stimulation. This finding lends insight into the mechanism of paralysis following thoracic aortic interventions and may guide future pharmacologic targets for attenuating spinal cord ischemia and reperfusion.

Analgesic effects of dexmedetomidine in vincristine-evoked painful neuropathic rats

Dexmedetomidine, which is a selective α2-adrenoceptor agonist, was recently introduced into clinical practice for its analgesic properties. The purpose of this study was to evaluate the effects of dexmedetomidine in a vincristine-evoked neuropathic rat models. Sprague-Dawley rats were injected intraperitoneally with vincristine or saline (0.1 mg/kg/day) using a 5-day-on, 2-day-off schedule for 2 weeks. Saline and dexmedetomidine (12.5, 25, 50, and 100 µg/kg) were injected to rats developed allodynia 14 days after vincristine injection, respectively. We evaluated allodynia at before, 15, 30, 60, 90, 120, 180, and 240 min, and 24 hr after intraperitoneal drug (normal saline or dexmedetomidine) injection. Saline treatment did not show any differences for all the allodynia. Maximal paw withdrawal thresholds to mechanical stimuli were 3.0 ± 0.4, 9.1 ± 1.9, 13.0 ± 3.6, 16.6 ± 2.4, and 24.4 ± 1.6 g at saline, 12.5, 25, 50, and 100 µg/kg dexmedetomidine injection, respectively. Minimal withdrawal frequency to cold stimuli were 73.3 ± 4.2, 57.1 ± 6.8, 34.3 ± 5.7, 20.0 ± 6.2, and 14.3 ± 9.5 g at saline, 12.5, 25, 50, and 100 µg/kg dexmedetomidine injection, respectively. Dexmedetomidine shows a dose-dependent antiallodynic effect on mechanical and cold stimuli in vincristine-evoked neuropathic rat models (P < 0.05).

Attenuation of spinal cord ischemia-reperfusion injury by specific α-2a receptor activation with dexmedetomidine

BACKGROUND

Despite surgical adjuncts, paralysis remains a devastating complication after thoracoabdominal aortic interventions. Dexmedetomidine, a selective α-2a agonist commonly used for sedation in the critical care setting, has been shown to have protective effects against ischemia-reperfusion injuries in multiple organ systems. We hypothesized that treatment with dexmedetomidine would attenuate spinal cord ischemia-reperfusion injury via α-2a receptor activation.

METHODS

Adult C57BL/6 mice underwent sternotomy, followed by occlusion of the aortic arch for 4 minutes. Eight experimental mice received pretreatment with intraperitoneal dexmedetomidine (25 μg/kg) and at 12-hour intervals after reperfusion. Eight control mice received an equivalent amount of 0.9% normal saline. Five mice underwent the same procedure with dexmedetomidine (25 μg/kg) and atipamezole (250 μg/kg), an α-2a receptor antagonist. Functional analysis of the mice was obtained at 12-hour intervals and scored using the Basso Mouse Scale for Locomotion until 60 hours. All mice were euthanized at 60 hours. Their spinal cords were removed en bloc and were stained with hematoxylin and eosin to assess cytoarchitecture and neuronal viability.

RESULTS

Mice treated with the α-2a agonist demonstrated preserved motor function compared with ischemic controls and with mice treated with the α-2a antagonist in addition to the agonist. Functional differences in the dexmedetomidine group were statistically significant from 24 hours through the remainder of the experiment (P < .05). In addition, the treated mice had preserved cytoarchitecture, decreased vacuolization, and improved neuronal viability compared with ischemic control mice and mice concurrently treated with atipamezole, the dexmedetomidine α-2a antagonist.

CONCLUSIONS

Treatment of mice with the α-2a agonist dexmedetomidine preserves motor function and neuronal viability after aortic cross-clamping. In addition, mice exhibited almost complete reversal of the protective effect with the administration of the α-2a receptor antagonist atipamezole. Dexmedetomidine appears to attenuate spinal cord ischemia-reperfusion injury via α-2a receptor-mediated agonism.

CLINICAL RELEVANCE

There remains a significant risk of paraplegia after thoracoabdominal aortic interventions. This complication is devastating to the patient and the health care system. Pharmacologic adjuncts to further decrease this complication have been studied; however, few viable options exist. The α-2a agonists have been shown to improve outcomes after strokes but have not been studied in spinal cord ischemia. We show that dexmedetomidine, a commonly used α-2a agonist in the operating room, can preserve neurologic function in mice after aortic cross-clamping. Although the protective mechanism of dexmedetomidine remains unknown, it might prove to be beneficial in reducing the incidence of paraplegia after aortic interventions.

Depletion of endogenous noradrenaline does not prevent spinal cord plasticity following peripheral nerve injury

The present study examined the role of endogenous noradrenaline on glial and neuronal plasticity in the spinal cord in rats after peripheral nerve injury. An intrathecal injection of dopamine-β-hydroxylase antibody conjugated to saporin (DβH-saporin) completely depleted noradrenergic axons in the spinal cord and also reduced noradrenergic neurons in the locus coeruleus (A6) and A5 noradrenergic nucleus in the brainstem and noradrenergic axons in the paraventricular nucleus of the hypothalamus. DβH-saporin treatment itself did not alter mechanical withdrawal threshold, but enhanced mechanical hypersensitivity and intrathecal clonidine analgesia after L5-L6 spinal nerve ligation. In the spinal dorsal horn of spinal nerve ligation rats, DβH-saporin treatment increased choline acetyltransferase immunoreactivity as well as immunoreactivity in microglia of ionized calcium binding adaptor molecule 1[IBA1] and in astrocytes of glial fibrillary acidic protein, and brain-derived nerve growth factor content. DβH-saporin treatment did not, however, alter the fractional release of acetylcholine from terminals by dexmedetomidine after nerve injury. These results suggest that endogenous tone of noradrenergic fibers is not necessary for the plasticity of α2-adrenoceptor analgesia and glial activation after nerve injury, but might play an inhibitory role on glial activation.

PERSPECTIVE

This study demonstrates that endogenous noradrenaline modulates plasticity of glia and cholinergic neurons in the spinal cord after peripheral nerve injury and hence influences the pathophysiology of spinal cord changes associated with neuropathic pain.

Adrenergic modulation of immune cells: an update

Sympathoadrenergic pathways are crucial to the communication between the nervous system and the immune system. The present review addresses emerging issues in the adrenergic modulation of immune cells, including: the specific pattern of adrenoceptor expression on immune cells and their role and changes upon cell differentiation and activation; the production and utilization of noradrenaline and adrenaline by immune cells themselves; the dysregulation of adrenergic immune mechanisms in disease and their potential as novel therapeutic targets. A wide array of sympathoadrenergic therapeutics is currently used for non-immune indications, and could represent an attractive source of non-conventional immunomodulating agents.