Intrathecal administration of clonidine attenuates spinal neuroimmune activation in a rat model of neuropathic pain with existing hyperalgesia

A Review of Chronic Pain and Device Interventions: Benefits and Future Directions

Chronic pain is a debilitating condition with a growing prevalence both in the USA and globally. The complex nature of this condition necessitates a multimodal approach to pain management that extends beyond the established pharmaceutical interventions currently employed. A variety of devices comprising both invasive and noninvasive approaches are available to patients, serving as adjuvants to existing regimens. The benefits of these interventions are notable for their lack of addiction potential, potential for patient autonomy regarding self-administration, minimal to no drug interaction, and overall relative safety. However, there remains a need for further research and more robust clinical trials to assess the true efficacy of these interventions and elucidate if there is an underlying physiological mechanism to their benefit in treating chronic pain or if their effect is predominantly placebo in nature. Regardless, the field of device-based intervention and treatment remains an evolving field with much promise for the future chronic pain management.

Melittin ameliorates motor function and prevents autophagy-induced cell death and astrogliosis in rat models of cerebellar ataxia induced by 3-acetylpyridine

BACKGROUND

Cerebellar ataxia (CA) is a form of ataxia that adversely affects the cerebellum. This study aims to investigate the therapeutic effects of melittin (MEL) on a 3-acetylpyridine-induced (3-AP) cerebellar ataxia (CA) rat model.

METHODS

Initially, CA rat models were generated by 3-AP administration followed by the subcutaneous injection of MEL. The open-field test was used for the evaluation of locomotion and anxiety. Immunohistochemistry was also conducted for the autophagy markers of LC3 and Beclin1. In the next step, the morphology of the astrocyte, the cell responsible for maintaining homeostasis in the CNS, was evaluated by the Sholl analysis.

RESULTS

The findings suggested that the administration of MEL in a 3-AP model of ataxia improved locomotion and anxiety (P < 0.001), decreased the expression of LC3 (P < 0.01) and Beclin1 (P < 0.05), increased astrocyte complexity (P < 0.05) and reduced astrocyte cell soma size (P < 0.001).

CONCLUSIONS

Overall, the findings imply that the MEL attenuates the 3-AP-induced autophagy, causes cell death and improves motor function. As such, it could be used as a therapeutic procedure for CA due to its neuroprotective effects.

Intrathecal Drug Delivery: Advances and Applications in the Management of Chronic Pain Patient

Advances in our understanding of the biology of spinal systems in organizing and defining the content of exteroceptive information upon which higher centers define the state of the organism and its role in the regulation of somatic and automatic output, defining the motor response of the organism, along with the unique biology and spatial organization of this space, have resulted in an increased focus on therapeutics targeted at this extracranial neuraxial space. Intrathecal (IT) drug delivery systems (IDDS) are well-established as an effective therapeutic approach to patients with chronic non-malignant or malignant pain and as a tool for management of patients with severe spasticity and to deliver therapeutics that address a myriad of spinal pathologies. The risk to benefit ratio of IDD makes it a useful interventional approach. While not without risks, this approach has a significant therapeutic safety margin when employed using drugs with a validated safety profile and by skilled practioners. The present review addresses current advances in our understanding of the biology and dynamics of the intrathecal space, therapeutic platforms, novel therapeutics, delivery technology, issues of safety and rational implementation of its therapy, with a particular emphasis upon the management of pain.

Ameliorative effects of escin on neuropathic pain induced by chronic constriction injury of sciatic nerve

ETHNOPHARMACOLOGY RELEVANCE

Escin is a natural mixture of triterpene saponins extracted from the seeds of Aesculus wilsonii Rehd. And has been reported to possess the therapeutic effects against neuropathic pain (NP). However, the underlying mechanisms remain unclear.

AIM OF THE STUDY

The present study aimed to investigate the therapeutic effects and explore the underlying mechanisms of escin on rats of NP induced by chronic constriction injury (CCI) of sciatic nerve.

MATERIALS AND METHODS

Rats were treated with escin (7, 14, and 28 mg/kg, i. g.) daily from the third day after the surgery (day 0) for consecutive 14 days. Regular behavior and thermal threshold were measured on days 0, 3, 5, 7, 10 and 14. Investigations into mechanisms involved measurement of inflammatory factors and biochemical factors in dorsal root ganglion (DRG). Inflammatory pain responses and nerve injuries were induced by the CCI model. Tonic pain model and acute inflammatory model induced by formalin or carrageenan were established to evaluated the pharmacological effects of escin on acute inflammatory pain. Corresponding behaviors were monitored and relevant gene expression such as c-fos, mu opioid receptor (MOR) and KCNK1 were detected by qRT-PCR. Investigate the neuroprotective effects of escin on PC12 cell injury induced by lipopolysaccharide (LPS). Cell morphology was observed under inverted microscope and neuroprotective effect of escin on cell activity was assessed by MTT assay.

RESULTS

Escin could widen thermal threshold, downregulate the concentration of inflammatory factors like tumor necrosis factor (TNF)-α and interleukin (IL)-1β, suppress the gene expression of toll-like receptor 4 (TLR4), nuclear factor κB (NF-κB), decrease the level of glial fibrillary acidic protein (GFAP) and nerve growth factor (NGF) remarkably. In addition, escin significantly lowered the duration of licking, numbers of flinches and increase in paw edema, showing great therapeutic effects on inflammatory pain responses. Moreover, the activity of injured PC12 cells was significantly improved after escin administrated.

CONCLUSION

Escin exerted the ameliorative effects on NP induced by CCI which may be related to downregulating the release of pro-inflammatory cytokines, suppressing TLR-4/NF-κB signal pathway, thereafter decreasing the level of GFAP and NGF.

Crotoxin Conjugated to SBA-15 Nanostructured Mesoporous Silica Induces Long-Last Analgesic Effect in the Neuropathic Pain Model in Mice

Neuropathic pain is a disease caused by structural and functional plasticity in central and peripheral sensory pathways that produce alterations in nociceptive processing. Currently, pharmacological treatment for this condition remains a challenge. Crotoxin (CTX), the main neurotoxin of Crotalus durissus terrificus rattlesnake venom, has well described prolonged anti-inflammatory and antinociceptive activities. In spite of its potential benefits, the toxicity of CTX remains a limiting factor for its use. SBA-15 is an inert nanostructured mesoporous silica that, when used as a vehicle, may reduce toxicity and potentiate the activity of different compounds. Based on this, we propose to conjugate crotoxin with SBA-15 (CTX:SBA-15) in order to investigate if when adsorbed to silica, CTX would have its toxicity reduced and its analgesic effect enhanced in neuropathic pain induced by the partial sciatic nerve ligation (PSNL) model. SBA-15 enabled an increase of 35% of CTX dosage. Treatment with CTX:SBA-15 induced a long-lasting reduction of mechanical hypernociception, without modifying the previously known pathways involved in antinociception. Moreover, CTX:SBA-15 reduced IL-6 and increased IL-10 levels in the spinal cord. Surprisingly, the antinociceptive effect of CTX:SBA-15 was also observed after oral administration. These data indicate the potential use of the CTX:SBA-15 complex for neuropathic pain control and corroborates the protective potential of SBA-15.

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.

Intrathecal drug delivery for pain management: recent advances and future developments

Introduction: Chronic pain conditions of malignant and non-malignant etiology afflict a large group of the population and pose a vast economic burden on society. Intrathecal drug therapy is a viable treatment option in such patients who have failed conservative medical measures and less invasive pain management procedures. However, the clinical growth of intrathecal therapy in managing intractable chronic pain conditions continues to face many challenges and is likely underutilized secondary to its high-complexity and lack of understanding. Areas covered: This review will briefly discuss the history of intrathecal drug delivery systems (IDDS), cerebrospinal fluid (CSF) flow dynamics, types of IDDS, indications and patient profile suitable for this therapy, and risks and complications related to IDDS. We will also discuss challenges faced by physicians utilizing this therapy and the future changes that are needed for making this treatment modality more efficacious. Expert opinion: IDDS offer an effective therapy for pain control in patients suffering from chronic intractable pain conditions. These devices provide a safer alternative to oral opioid medications with reduced systemic side effects. Adherence to best practices and continued clinical and basic science research is important to ensure continuing success of this therapy.

Valproate reduces neuroinflammation and neuronal death in a rat chronic constriction injury model

Valproate (VPA) is a well-known drug for treating epilepsy and mania, but its action in neuropathic pain is unclear. We used a chronic constriction injury (CCI) model to explore whether VPA prevents neuropathic pain-mediated inflammation and neuronal death. Rats were treated with or without VPA. CCI + VPA rats were intraperitoneally injected with VPA (300 mg/kg/day) from postoperative day (POD) 1 to 14. We measured paw withdrawal latency (PWL) and paw withdrawal threshold (PWT) 1 day before surgery and 1, 3, 7, 14 days after CCI and harvested the sciatic nerves (SN), spinal cord (SC) and dorsal root ganglia (DRG) on POD 3, 7, and 14. PWL and PWT were reduced in CCI rats, but increased in CCI + VPA rats on POD 7 and POD 14. VPA lowered CCI-induced inflammatory proteins (pNFκB, iNOS and COX-2), pro-apoptotic proteins (pAKT/AKT and pGSK-3β/GSK-3β), proinflammatory cytokines (TNF-α and IL-1β) and nuclear pNFκB activation in the SN, DRG and SC in CCI rats. COX-2 and pGSK-3 proteins were decreased by VPA on immunofluorescence analysis. VPA attenuated CCI-induced thermal and mechanical pain behaviors in rats in correlation with anti-neuroinflammation action involving reduction of pNFκB/iNOS/COX-2 activation and inhibition of pAKT/pGSK-3β-mediated neuronal death from injury to peripheral nerves.

Breaking barriers to novel analgesic drug development

Acute and chronic pain complaints, although common, are generally poorly served by existing therapies. This unmet clinical need reflects a failure to develop novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms, and the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities for developing novel therapeutic strategies and revisiting existing targets, including modulating ion channels, enzymes and G-protein-coupled receptors.

The Proteomics of Intrathecal Analgesic agents for Chronic Pain

Chronic pain remains a challenging clinical problem with a growing socio-economic burden for the state. Its prevalence is high and many of the patients are of work age. Our knowledge regarding the pathophysiology of chronic pain is poor. The consensus view is that the central nervous system plays a key role in the persistence of pain after an initiating event has long ceased. However the specifics of this biological response to an initiating event remains unclear. There is a growing body of evidence to support the concept that a central neuroimmune response is initiated and a number of small peptides have been implicated in this process following cerebrospinal fluid analysis in patients with chronic pain. This central biosynthetic peptide response leads to a process called central sensitization. Therapy is aimed at modulating and even inhibiting this response. However current pharmacological therapeutic options are limited in efficacy with significant deleterious side effect profiles. Proteomic studies extend single molecule analysis by identifying the components of biological networks and pathways and defining their interactions. This tool offers the potential to provide a molecular overview of the biological processes involved in chronic pain. It will also facilitate examination of gene-drug interactions. This technique offers a mechanism of defining the central biological responses that result in chronic pain and this information may facilitate the development of better therapies.

Medial plantar nerve ligation as a novel model of neuropathic pain in mice: pharmacological and molecular characterization

Peripheral neuropathic pain is a consequence of an injury/disease of the peripheral nerves. The mechanisms involved in its pathophysiology are not entirely understood. To better understand the mechanisms involved in the development of peripheral nerve injury-induced neuropathic pain, more experimental models are required. Here, we developed a novel peripheral neuropathic pain model in mice by using a minimally invasive surgery and medial plantar nerve ligation (MPNL). After MPNL, mechanical allodynia was established, and mice quickly recovered from the surgery without any significant motor impairment. MPNL causes an increased expression of ATF-3 in the sensory neurons. At 14 days after surgery, gabapentin was capable of reversing the mechanical allodynia, whereas anti-inflammatory drugs and opioids were ineffective. MPNL-induced neuropathic pain was mediated by glial cells activation and the production of TNF-α and IL-6 in the spinal cord. These results indicate MPNL as a reasonable animal model for the study of peripheral neuropathic pain, presenting analgesic pharmacological predictivity to clinically used drugs. The results also showed molecular phenotypic changes similar to other peripheral neuropathic pain models, with the advantage of a lack of motor impairment. These features indicate that MPNL might be more appropriate for the study of neuropathic pain than classical models.

CD200R1 agonist attenuates glial activation, inflammatory reactions, and hypersensitivity immediately after its intrathecal application in a rat neuropathic pain model

Background

Interaction of CD200 with its receptor CD200R has an immunoregulatory role and attenuates various types of neuroinflammatory diseases.

Methods

Immunofluorescence staining, western blot analysis, and RT-PCR were used to investigate the modulatory effects of CD200 fusion protein (CD200Fc) on activation of microglia and astrocytes as well as synthesis of pro- (TNF, IL-1β, IL-6) and anti-inflammatory (IL-4, IL-10) cytokines in the L4–L5 spinal cord segments in relation to behavioral signs of neuropathic pain after unilateral sterile chronic constriction injury (sCCI) of the sciatic nerve. Withdrawal thresholds for mechanical hypersensitivity and latencies for thermal hypersensitivity were measured in hind paws 1 day before operation; 1, 3, and 7 days after sCCI operation; and then 5 and 24 h after intrathecal application of artificial cerebrospinal fluid or CD200Fc.

Results

Seven days from sCCI operation and 5 h from intrathecal application, CD200Fc reduced mechanical and thermal hypersensitivity when compared with control animals. Simultaneously, CD200Fc attenuated activation of glial cells and decreased proinflammatory and increased anti-inflammatory cytokine messenger RNA (mRNA) levels. Administration of CD200Fc also diminished elevation of CD200 and CD200R proteins as a concomitant reaction of the modulatory system to increased neuroinflammatory reactions after nerve injury. The anti-inflammatory effect of CD200Fc dropped at 24 h after intrathecal application.

Conclusions

Intrathecal administration of the CD200R1 agonist CD200Fc induces very rapid suppression of neuroinflammatory reactions associated with glial activation and neuropathic pain development. This may constitute a promising and novel therapeutic approach for the treatment of neuropathic pain.

Disruption of Spinal Noradrenergic Activation Delays Recovery of Acute Incision-Induced Hypersensitivity and Increases Spinal Glial Activation in the Rat

Results of clinical studies suggest that descending inhibitory controls from the brainstem are important for speeding recovery from pain after surgery. We examined the effects of destroying spinally projecting noradrenergic neurons via intrathecally administered antibody to dopamine β-hydroxylase conjugated to saporin (DβH-saporin) on recovery in an acute incisional pain model. Mechanical and thermal paw withdrawal thresholds and nonevoked spontaneous guarding scores were tested for several weeks postoperatively and analyzed using mixed effects growth curve modeling. DβH-saporin treatment resulted in a significant prolongation in the duration of mechanical and to a lesser degree thermal hypersensitivity in the ipsilateral paw of incised rats but did not increase the duration of spontaneous guarding. DβH-saporin treatment was also associated with increased microglial and astrocyte activation in the ipsilateral spinal cord 21 days after incision compared with immunoglobulin G-saporin treated controls. Chronic intrathecal administration of the α2 adrenergic receptor antagonist atipamezole (50-200 μg/d) produced similar effects. These data suggest that spinally projecting noradrenergic pathways and spinal α2 adrenergic receptor activation are important for speeding recovery from hypersensitivity after surgical incision possibly by reducing spinal glial activation. Interventions that augment the noradrenergic system might be important to speed recovery from pain after surgery.

PERSPECTIVE

Endogenous descending spinal noradrenergic activation promotes resolution of incision-induced hypersensitivity and inhibits spinal microglial and astrocyte activation in part through α2 adrenergic receptors.

Glial cell line-derived neurotrophic factor-mediated enhancement of noradrenergic descending inhibition in the locus coeruleus exerts prolonged analgesia in neuropathic pain

BACKGROUND AND PURPOSE

The locus coeruleus (LC) is the principal nucleus containing the noradrenergic neurons and is a major endogenous source of pain modulation in the brain. Glial cell line-derived neurotrophic factor (GDNF), a well-established neurotrophic factor for noradrenergic neurons, is a major pain modulator in the spinal cord and primary sensory neurons. However, it is unknown whether GDNF is involved in pain modulation in the LC.

EXPERIMENTAL APPROACH

Rats with chronic constriction injury (CCI) of the left sciatic nerve were used as a model of neuropathic pain. GDNF was injected into the left LC of rats with CCI for 3 consecutive days and changes in mechanical allodynia and thermal hyperalgesia were assessed. The α2 -adrenoceptor antagonist yohimbine was injected intrathecally to assess the involvement of descending inhibition in GDNF-mediated analgesia. The MEK inhibitor U0126 was used to investigate whether the ERK signalling pathway plays a role in the analgesic effects of GDNF.

KEY RESULTS

Both mechanical allodynia and thermal hyperalgesia were attenuated 24 h after the first GDNF injection. GDNF increased the noradrenaline content in the dorsal spinal cord. The analgesic effects continued for at least 3 days after the last injection. Yohimbine abolished these effects of GDNF. The analgesic effects of GDNF were partly, but significantly, inhibited by prior injection of U0126 into the LC.

CONCLUSIONS AND IMPLICATIONS

GDNF injection into the LC exerts prolonged analgesic effects on neuropathic pain in rats by enhancing descending noradrenergic inhibition.

Alpha-adrenoceptor modulation in central nervous system trauma: pain, spasms, and paralysis--an unlucky triad

Many researchers have attempted to pharmacologically modulate the adrenergic system to control locomotion, pain, and spasms after central nervous system (CNS) trauma, although such efforts have led to conflicting results. Despite this, multiple studies highlight that α-adrenoceptors (α-ARs) are promising therapeutic targets because in the CNS, they are involved in reactivity to stressors and regulation of locomotion, pain, and spasms. These functions can be activated by direct modulation of these receptors on neuronal networks in the brain and the spinal cord. In addition, these multifunctional receptors are also broadly expressed on immune cells. This suggests that they might play a key role in modulating immunological responses, which may be crucial in treating spinal cord injury and traumatic brain injury as both diseases are characterized by a strong inflammatory component. Reducing the proinflammatory response will create a more permissive environment for axon regeneration and may support neuromodulation in combination therapies. However, pharmacological interventions are hindered by adrenergic system complexity and the even more complicated anatomical and physiological changes in the CNS after trauma. This review is the first concise overview of the pros and cons of α-AR modulation in the context of CNS trauma.

Analgesic effects of naringenin in rats with spinal nerve ligation-induced neuropathic pain

Naringenin, a flavonoid abundant in citrus fruits, such as grapefruits, has been reported to possess anti-inflammatory properties. The present study aimed to investigate the analgesic potential of naringenin in L5 spinal nerve ligation (SNL)-induced peripheral neuropathic pain and the underlying mechanisms associated with neuroinflammation. Different doses of naringenin or saline were administered intrathecally once daily for 11 consecutive days, from 3 days prior to surgery to 7 days after surgery. Pain development was assessed 1 day prior to and 7-14 days after surgery in terms of mechanical withdrawal threshold and thermal withdrawal latency. Astrocytic and microglial activation and production of inflammatory mediators were determined on day 14 after surgery. The results demonstrated that naringenin dose-dependently attenuated the mechanical allodynia and thermal hyperalgesia induced by SNL. Furthermore, naringenin significantly inhibited SNL-induced activation of glial cells (astrocytes and microglia). Morover, the upregulated expression of inflammatory mediators in neuropathic pain was significantly inhibited by naringenin. Our findings suggested that repeated administration of naringenin may alleviate neuropathic pain, possibly through inhibiting neuroinflammation.

Polyanalgesic Consensus Conference 2012: recommendations for the management of pain by intrathecal (intraspinal) drug delivery: report of an interdisciplinary expert panel

INTRODUCTION

The use of intrathecal (IT) infusion of analgesic medications to treat patients with chronic refractory pain has increased since its inception in the 1980s, and the need for clinical research in IT therapy is ongoing. The Polyanalgesic Consensus Conference (PACC) panel of experts convened in 2000, 2003, and 2007 to make recommendations on the rational use of IT analgesics based on preclinical and clinical literature and clinical experiences.

METHODS

The PACC panel convened again in 2011 to update the standard of care for IT therapies to reflect current knowledge gleaned from literature and clinical experience. A thorough literature search was performed, and information from this search was provided to panel members. Analysis of published literature was coupled with the clinical experience of panel members to form recommendations regarding the use of IT analgesics to treat chronic pain.

RESULTS

After a review of literature published from 2007 to 2011 and discussions of clinical experience, the panel created updated algorithms for the rational use of IT medications for the treatment of neuropathic pain and nociceptive pain.

CONCLUSIONS

The advent of new algorithmic tracks for neuropathic and nociceptive pain is an important step in improving patient care. The panel encourages continued research and development, including the development of new drugs, devices, and safety recommendations to improve the care of patients with chronic pain.

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.

Treatment of rat spinal cord injury with the neurotrophic factor albumin-oleic acid: translational application for paralysis, spasticity and pain

Sensorimotor dysfunction following incomplete spinal cord injury (iSCI) is often characterized by the debilitating symptoms of paralysis, spasticity and pain, which require treatment with novel pleiotropic pharmacological agents. Previous in vitro studies suggest that Albumin (Alb) and Oleic Acid (OA) may play a role together as an endogenous neurotrophic factor. Although Alb can promote basic recovery of motor function after iSCI, the therapeutic effect of OA or Alb-OA on a known translational measure of SCI associated with symptoms of spasticity and change in nociception has not been studied. Following T9 spinal contusion injury in Wistar rats, intrathecal treatment with: i) Saline, ii) Alb (0.4 nanomoles), iii) OA (80 nanomoles), iv) Alb-Elaidic acid (0.4/80 nanomoles), or v) Alb-OA (0.4/80 nanomoles) were evaluated on basic motor function, temporal summation of noxious reflex activity, and with a new test of descending modulation of spinal activity below the SCI up to one month after injury. Albumin, OA and Alb-OA treatment inhibited nociceptive Tibialis Anterior (TA) reflex activity. Moreover Alb-OA synergistically promoted early recovery of locomotor activity to 50 ± 10% of control and promoted de novo phasic descending inhibition of TA noxious reflex activity to 47 ± 5% following non-invasive electrical conditioning stimulation applied above the iSCI. Spinal L4-L5 immunohistochemistry demonstrated a unique increase in serotonin fibre innervation up to 4.2 ± 1.1 and 2.3 ± 0.3 fold within the dorsal and ventral horn respectively with Alb-OA treatment when compared to uninjured tissue, in addition to a reduction in NR1 NMDA receptor phosphorylation and microglia reactivity. Early recovery of voluntary motor function accompanied with tonic and de novo phasic descending inhibition of nociceptive TA flexor reflex activity following Alb-OA treatment, mediated via known endogenous spinal mechanisms of action, suggests a clinical application of this novel neurotrophic factor for the treatment of paralysis, spasticity and pain.

Effects of clonidine on bilateral pain behaviors and inflammatory response in rats under the state of neuropathic pain

This study was conducted to investigate the effects of clonidine on bilateral pain behaviors and inflammatory responses in neuropathic pain induced by partial sciatic nerve ligation (PSNL), and to better understand whether the antinociception of clonidine was related to α(2)-adrenoceptor mechanisms. Rats were divided randomly into five groups: sham-operation with saline, i.p.; PSNL with clonidine (0.2mg/kg) or saline, i.p.; PSNL with yohimbine (2mg/kg) followed by clonidine (0.2mg/kg), i.p.; and PSNL with naloxone (0.3mg/kg) followed by clonidine (0.2mg/kg), i.p. On post-operative days 1, 3, 7, 14, and 21, both ipsilateral and contralateral pain behaviors were measured. In rats receiving antagonists, bilateral behavioral changes were measured on day 14. Bilateral paw pressure threshold and paw withdrawal latencies were measured, and the extent of glial activation was dertermined by measuring macrophage antigen complex-1 (Mac-1) and glial fibrillary acidic protein (GFAP). Additionally, the levels of tumor necrosis factor α (TNF-α) and interleukin (IL)-6 were determined. PSNL induced bilateral behavioral hyperalgesia, with the ipsilateral level displaying a higher extent of behavior changes than the contralateral side. In addition, the glial activation markers and cytokine production were augmented bilaterally. Clonidine caused significant attenuation of bilateral mechanical allodynia and thermal hyperalgesia, accompanied by inhibition of glial activation and the expression of cytokines. The effects of clonidine were blocked by the α(2)-adrenoceptor antagonist yohimbine and partially reversed by the μ-opioid receptor antagonist naloxone. These data suggest that the bilateral antinoceptive effects of clonidine might mediate through immunomodulation by acting on α(2)-adrenoceptor in rats undergoing neuropathic pain.

Chemical stimulation of the ST36 acupoint reduces both formalin-induced nociceptive behaviors and spinal astrocyte activation via spinal alpha-2 adrenoceptors

Spinal astrocytes have emerged as important mechanistic contributors to pathological and chronic pain. Recently, we have demonstrated that injection of diluted bee venom (DBV) into the Zusanli (ST36) acupoint produces a potent anti-nociceptive effect via the activation of spinal alpha-2 adrenoceptors. However, it is unclear if this anti-nociceptive effect is associated with alterations in spinal astrocytes. Thus, the present study was designed to determine: (1) whether DBV's anti-nociceptive effect in the formalin test involves suppression of spinal astrocyte activation; (2) whether DBV-induced astrocyte inhibition is mediated by spinal alpha-2 adrenoceptors; and (3) whether this glial modulation is potentiated by intrathecal administration of the glial metabolic inhibitor, fluorocitrate (FC) in combination with DBV injection. DBV was injected directly into the ST36 acupoint, and spinal expression of the astrocytic marker, glial fibrillary acidic protein (GFAP), was assessed together with effects on formalin-induced nociception. DBV treatment reduced pain responses in the late phase of the formalin test and significantly blocked the formalin-evoked increase in spinal GFAP expression. These effects of DBV were prevented by intrathecal pretreatment with selective alpha-2A and alpha-2C adrenoceptor antagonists. Moreover, low dose intrathecal injection of FC in conjunction with low dose DBV injection into the ST36 acupoint synergistically suppressed pain responses and GFAP expression. These results demonstrate that DBV stimulation of the ST36 acupoint inhibits the formalin-induced activation of spinal astrocytes and nociceptive behaviors in this inflammatory pain model and this inhibition is associated with the activation of spinal alpha-2 adrenoceptors.

Conditioned place preference reveals tonic pain in an animal model of central pain

A limitation of animal models of central pain is their inability to recapitulate all clinical characteristics of the human condition. Specifically, many animal models rely on reflexive measures of hypersensitivity and ignore, or cannot assess, spontaneous pain, the hallmark characteristic of central pain in humans. Here, we adopt a conditioned place preference paradigm to test if animals with lesions in the anterolateral quadrant of the spinal cord develop signs consistent with spontaneous pain. This paradigm relies on the fact that pain relief is rewarding to animals, and has been used previously to show that animals with peripheral nerve injury develop tonic pain. With the use of 2 analgesic treatments commonly used to treat patients with central pain (clonidine infusion and motor cortex stimulation), we demonstrate that analgesic treatments are rewarding to animals with spinal cord lesions but not sham-operated controls. These findings are consistent with the conclusion that animals with spinal cord injury suffer from tonic pain.

PERSPECTIVE

The hallmark characteristic of central pain in humans is spontaneous pain. Animal models of central pain rely on reflexive measures of hypersensitivity and do not assess spontaneous pain. Demonstrating that animals with spinal cord injury suffer from tonic pain is important to study the etiology of central pain.