Analgesia mediated by a direct spinal action of narcotics

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Direct, long-term intrathecal application of therapeutics to the rodent CNS

Systemic application of therapeutics to the CNS tissue often results in subtherapeutic drug levels, because of restricted and selective penetration through the blood-brain barrier (BBB). Here, we give a detailed description of a standardized technique for intrathecal drug delivery in rodents, analogous to the technique used in humans. The intrathecal drug delivery method bypasses the BBB and thereby offers key advantages over oral or intravenous administration, such as maximized local drug doses with minimal systemic side effects. We describe how to deliver antibodies or drugs over several days or weeks from a s.c. minipump and a fine catheter inserted into the subdural space over the spinal cord (20 min operative time) or into the cisterna magna (10 min operative time). Drug levels can be sampled by quick and minimally invasive cerebrospinal fluid (CSF) collection from the cisterna magna (5 min procedure time). These techniques enable targeted application of any compound to the CNS for therapeutic studies in a wide range of CNS disease rodent models. Basic surgery skills are helpful for carrying out the procedures described in this protocol.

Efficacy of Intrathecal Morphine in a Model of Surgical Pain in Rats

Concerns over interactions between analgesics and experimental outcomes are a major reason for withholding opioids from rats undergoing surgical procedures. Only a fraction of morphine injected intravenously reaches receptors responsible for analgesia in the central nervous system. Intrathecal administration of morphine may represent a way to provide rats with analgesia while minimizing the amount of morphine injected. This study aimed to assess whether morphine injected intrathecally via direct lumbar puncture provides sufficient analgesia to rats exposed to acute surgical pain (caudal laparotomy).In an initial blinded, randomised study, pain-free rats received morphine subcutaneously (MSC, 3mg.kg-1, N = 6), intrathecally (MIT, 0.2mg.kg-1, N = 6); NaCl subcutaneously (NSC, N = 6) or intrathecally (NIT, N = 6). Previously validated pain behaviours, activity and Rat Grimace Scale (RGS) scores were recorded at baseline, 1, 2, 4 and 8h post-injection. Morphine-treated rats had similar behaviours to NaCl rats, but their RGS scores were significantly different over time and between treatments. In a second blinded study, rats (N = 28) were randomly allocated to one of the following four treatments (N = 7): MSC, 3mg.kg-1, surgery; MIT, 0.2mg.kg-1, surgery; NIT, surgery; NSC, sham surgery. Composite Pain Behaviours (CPB) and RGS were recorded as previously. CPB in MIT and MSC groups were not significantly different to NSC group. MSC and MIT rats displayed significantly lower RGS scores than NIT rats at 1 and 8h postoperatively. RGS scores for MIT and MSC rats were not significantly different at 1, 2, and 8h postoperatively. Intraclass correlation value amongst operators involved in RGS scoring (N = 9) was 0.913 for total RGS score. Intrathecal morphine was mostly indistinguishable from its subcutaneous counterpart, providing pain relief lasting up to 8 hours in a rat model of surgical pain. Further studies are warranted to clarify the relevance of the rat grimace scale for assessing pain in rats that have received opioid analgesics.

Determination of ED50 of hydromorphone for postoperative analgesia following cesarean delivery

BACKGROUND

Morphine is the most common opioid injected into the intrathecal space for postoperative analgesia following cesarean delivery, but ongoing medication shortages have resulted in limited availability. One proposed morphine alternative is hydromorphone. Studies investigating its use in post-cesarean analgesia are limited. This study was conducted to determine the median effective dose of intrathecal hydromorphone 12h postpartum.

METHODS

Twenty healthy women undergoing elective cesarean delivery were recruited into this study. Hydromorphone doses were determined using the up-down sequential method. The study dose of hydromorphone started at 6μg and was raised or lowered by 2μg depending on the 12-h efficacy of the preceding participant's dose. Pain scores of <3/10 were considered successful and the subsequent patient received a lower dose. Participants received 0.5% bupivacaine 12.5mg, fentanyl 25μg, and the study dose of hydromorphone as a single intrathecal injection.

RESULTS

Ten of 20 participants reported an effective hydromorphone dosage 12h post-injection. The median effective hydromorphone dosage was 4.6μg (95% CI 3.72 to 5.48μg) based on participants' reported visual analog pain scores of <3/10. No significant side effects or adverse outcomes were observed.

CONCLUSION

Intrathecal hydromorphone may be an effective alternative to morphine for post-cesarean pain management. The amount of intrathecal hydromorphone necessary to provide analgesia at 12h postoperatively may be significantly lower than doses currently in use. Further research should be performed to identify the optimal dose of intrathecal hydromorphone for post-surgical pain relief.

Potent spinal parenchymal AAV9-mediated gene delivery by subpial injection in adult rats and pigs

Effective in vivo use of adeno-associated virus (AAV)-based vectors to achieve gene-specific silencing or upregulation in the central nervous system has been limited by the inability to provide more than limited deep parenchymal expression in adult animals using delivery routes with the most clinical relevance (intravenous or intrathecal). Here, we demonstrate that the spinal pia membrane represents the primary barrier limiting effective AAV9 penetration into the spinal parenchyma after intrathecal AAV9 delivery. We develop a novel subpial AAV9 delivery technique and AAV9-dextran formulation. We use these in adult rats and pigs to show (i) potent spinal parenchymal transgene expression in white and gray matter including neurons, glial and endothelial cells after single bolus subpial AAV9 delivery; (ii) delivery to almost all apparent descending motor axons throughout the length of the spinal cord after cervical or thoracic subpial AAV9 injection; (iii) potent retrograde transgene expression in brain motor centers (motor cortex and brain stem); and (iv) the relative safety of this approach by defining normal neurological function for up to 6 months after AAV9 delivery. Thus, subpial delivery of AAV9 enables gene-based therapies with a wide range of potential experimental and clinical utilizations in adult animals and human patients.

Review article : The spinal administration of opioids in the treatment of acute and chronic pain: bolus doses, continuous infusion, intraventricular administration and implanted drug delivery systems

The spinal administration of opioids has been a significant addition to the previously available alternatives for pain control for patients with pain related to cancer. This article does not debate the more widespread use of these techniques in patients with nonmalignancy related pain. The incidence of destructive neurolytic procedures has fallen in most centres where spinal opioids have been administered. Some types of cancer pain have proved to be more effectively controlled than others, but in most circumstances potentially reversible procedures, such as outlined in this article, should be given a therapeutic trial before embarking on neuro-destructive procedures.1

It should be reemphasized however, that the spinal route of administration should be instituted only after systemically administered opioids have been shown to be ineffective or associated with intolerable side effects.

Baclofen as a Potential Analgesic Agent

It is proposed that analgesia might be achieved by pharmacological interruption of sensory input to the C.N.S. at the level of the spinal cord. On the basis of early reports that baclofen mimicked presynaptic inhibitory mechanisms in the spinal cord, baclofen was tested for potential analgesic activity in the mouse hot-plate test. Baclofen appeared to have some antinocisponsive activity in its own right and it potentiated the antinocisponsive effect of morphine. This observation has been confirmed and extended by several authors and the antinocisponsive activity appears to reside in the 1-isomer. Both spinal and supraspinal sites have been suggested for the locus of action in the C.N.S., whilst the underlying cellular mechanism may be inhibition of neurotransmitter release (via a bicuculline-refractory GABA receptor) or antagonism of the post-synaptic actions of substance P as a neurotransmitter.

Baclofen is compared with two other antinocisponsive agents (morphine and clonidine) in terms of its effects on release of neurotransmitters. Consideration is given to the possibility that a further characteristic is shared by these compounds, namely that following chronic administration, abrupt cessation of therapy might be accompanied by withdrawal or ‘rebound’ effects.

Evaluation of Postoperative Anti-nociceptive Efficacy of Intrathecal Dexketoprofen in Rats

BACKGROUND

Some studies have suggested that the intrathecal use of cyclooxygenase enzyme inhibitors provides an anti-nociceptive effect. Therefore, the occurrence of side effects seen in systemic usage can be eliminated.

AIMS

The primary objective of this experimental, randomized, controlled trial was to test the hypothesis asserting that intrathecal dexketoprofen trometamol would demonstrate an analgesic effect during postoperative period.

STUDY DESIGN

Animal experimentation.

METHODS

Forty rats were randomized into 4 groups 7 days after intrathecal catheterization; the following drugs were given through catheter lumens: Group Lidocaine (Group L): Lidocaine 20 μg; Group Lidocaine-Morphine (Group LM): Lidocaine 20 μg and morphine 0.5 μgr; Group Lidocaine-Dexketoprofen (Group LD): Lidocaine 20 μg and dexketoprofen trometamol 100 μg; and Group Dexketoprofen (Group D): Dexketoprofen trometamol 100 μg. Paw incision was achieved under ether inhalation. To measure analgesic potential, hot plate and tail immersion tests were used as nociceptive tests during the postoperative period.

RESULTS

The mean reaction times detected in groups during hot plate and tail immersion tests were shortest in Group L at 15, 30, 45, 60, 75, 90, 105, and 120 minutes after start of surgery (p<0.01, all others). In the groups using dexketoprofen, as in the morphine group, longer reaction times were detected than in the lidocaine group at all measurement times except 120 minutes (p<0.01).

CONCLUSION

Intrathecal dexketoprofen in the optimal perioperative pain management is effective, and can be administered as an adjuvant in clinics after neurotoxicity studies in animals, and effective dose studies in volunteers.

Inhibition of CaMKIV relieves streptozotocin-induced diabetic neuropathic pain through regulation of HMGB1

Background

The pathogenesis of diabetic neuropathic pain is complicated and its underlying mechanisms remain unclear. Calmodulin-dependent protein kinases (CaMKs) IV (CaMKIV), one of CaMKs, regulates several transcription factors in pain mechanisms. High-mobility group box 1 (HMGB1) is a key mediator in diabetic neuropathic pain. This study aims to find the roles and mechanisms of CaMIV in diabetic neuropathic pain.

Methods

Diabetic animal models were constructed by injecting with streptozotocin (STZ) intraperitoneally. They were randomly divided into seven groups (n = 6 per group): Naive, Normal Saline, STZ, STZ + Sham, STZ + DMSO and STZ + KN93 (an inhibitor of CaMKIV) (50 μg), STZ + KN93 (100 μg), which received KN93 (50 or 100 μg) intrathecally after the administration of STZ. Phospho-CaMKIV (pCaMKIV) and HMGB1 expression in rat dorsal root ganglion (DRG) and RAW264.7 cell line were measured by western blot. Distribution of pCaMKIV immune reactivity in different subpopulations of DRG neurons was measured by double-immunofluorescence staining.

Results

The pCaMKIV and HMGB1 in DRG significantly increased after STZ administration, and pCaMKIV can regulate the expression of HMGB1 based on both cellular and animal models. Pretreatment with CaMKIV inhibitor attenuated STZ-induced mechanical allodynia and thermal hyperalgesia, as well as reduced HMGB1 expression in the DRG.

Conclusions

This study demonstrates that CaMKIV can relieve STZ-induced diabetic neuropathic pain. The mechanism of this function depended on the process: pCaMKIV localized in the nuclei of DRG neurons and regulated HMGB1 which was an important mediator of neuropathic pain. These findings reported CaMKIV may be a potential target or important node in relieving diabetic neuropathic pain.

Nanofibers used for the delivery of analgesics

Nanofibers are extremely advantageous for drug delivery because of their high surface area-to-volume ratios, high porosities and 3D open porous structures. Local delivery of analgesics by using nanofibers allows site-specificity and requires a lower overall drug dosage with lower adverse side effects. Different analgesics have been loaded onto various nanofibers, including those that are natural, synthetic and copolymer, for various medical applications. Analgesics can also be singly or coaxially loaded onto nanofibers to enhance clinical applications. In particular, analgesic-eluting nanofibers provide additional benefits to preventing wound adhesion and scar formation. This paper reviews current research and breakthrough discoveries on the innovative application of analgesic-loaded nanofibers that will alter the clinical therapy of pain.

Implantable Devices: Assessment and Perioperative Management

Technology is the prominent feature of the twenty-first century, including in medicine. There are very few organs that cannot be stimulated, shocked, or infused. With more and more implantable devices being approved for clinical use, anesthesiologists have to regularly take care of patients who have these devices. An understanding of the devices, the associated comorbidities, and the perioperative risks is crucial for safe management of these patients. Cardiac devices are discussed in some detail; neurostimulators and other implantable devices are briefly described. The principles of assessment and management are similar for all patients with implanted devices.

HSV vector-mediated GAD67 suppresses neuropathic pain induced by perineural HIV gp120 in rats through inhibition of ROS and Wnt5a

Human immunodeficiency virus (HIV)-related neuropathic pain is a debilitating chronic condition that is severe and unrelenting. Despite the extensive research, the exact neuropathological mechanisms remain unknown, which hinders our ability to develop effective treatments. Loss of GABAergic tone may play an important role in the neuropathic pain state. Glutamic acid decarboxylase 67 (GAD67) is one of isoforms that catalyze GABA synthesis. Here, we used recombinant herpes simplex virus (HSV-1) vectors that encode gad1 gene to evaluate the therapeutic potential of GAD67 in peripheral HIV gp120-induced neuropathic pain in rats. We found that 1) subcutaneous inoculation of the HSV vectors expressing GAD67 attenuated mechanical allodynia in the model of HIV gp120-induced neuropathic pain, 2) the anti-allodynic effect of GAD67 was reduced by GABA-A and-B receptors antagonists, 3) HSV vectors expressing GAD67 reversed the lowered GABA-IR expression, and 4) the HSV vectors expressing GAD67 suppressed the upregulated mitochondrial superoxide and Wnt5a in the spinal dorsal horn. Taken together, our studies support the concept that recovering GABAergic tone by the HSV vectors may reverse HIV-associated neuropathic pain through suppressing mitochondrial superoxide and Wnt5a. Our studies provide validation of HSV-mediated GAD67 gene therapy in the treatment of HIV-related neuropathic pain.

Blockade of Toll-Like Receptors (TLR2, TLR4) Attenuates Pain and Potentiates Buprenorphine Analgesia in a Rat Neuropathic Pain Model

Accumulating evidence indicates that microglial TLR2 and TLR4 play a significant role in nociception. Experiments were conducted to evaluate the contribution of TLR2 and TLR4 and their adaptor molecules to neuropathy and their ability to amplify opioid effectiveness. Behavioral tests (von Frey's and cold plate) and biochemical (Western blot and qRT-PCR) analysis of spinal cord and DRG tissue were conducted after chronic constriction injury (CCI) to the sciatic nerve. Repeated intrathecal administration of LPS-RS (TLR2 and TLR4 antagonist) and LPS-RS Ultrapure (TLR4 antagonist) attenuated allodynia and hyperalgesia. Biochemical analysis revealed time-dependent upregulation of mRNA and/or protein levels of TLR2 and TLR4 and MyD88 and TRIF adaptor molecules, which was paralleled by an increase in IBA-1/CD40-positive cells under neuropathy. LPS-RS and LPS-RS Ultrapure similarly influenced opioid analgesia by enhancing the effectiveness of buprenorphine but not morphine. Summing up, in light of their upregulation over the course of pain, both TLR2 and TLR4 may indeed play a significant role in neuropathy, which could be linked to the observed activation of IBA-1/CD40-positive cells. Blockade of TLR2 and TLR4 produced analgesia and enhanced buprenorphine's effectiveness, which suggests that they may be a putative target for future pharmacological pain relief tools, especially for opioid rotation, when the effect of morphine is tolerated.

Pain outcomes in children who received intrathecal vs intravenous opioids for pain control following major urologic surgery: a retrospective review

BACKGROUND

Intrathecal (IT) opioid administration has been associated with postoperative benefits including reduced pain and opioid use in children. However, the postoperative benefits and risks of IT opioid administration during major urologic surgery in children remain unclear.

AIM

The aim of this study was to compare postoperative pain and adverse event outcomes among children who received IT vs intravenous (IV) opioids during major urologic surgery.

METHODS

We reviewed the medical records of children 3-17 years of age who underwent ureteroneocystostomy or pyeloplasty between 2006 and 2012. Electronically captured anesthetic and surgical data, postanesthesia care recovery unit (PACU) and nursing flowsheets, and daily progress notes through hospital discharge were reviewed. Analgesic techniques (i.e., IT or IV patient/nurse controlled opioids), all analgesic drugs and doses were recorded. Outcome measures included pain scores, need for rescue analgesics, opioid-related adverse events, and their treatments.

RESULTS

Seventy-seven children received IT opioids and 51 received IV opioids. More children in the IV group required rescue analgesics and had higher pain scores at PACU discharge. Children in the IV group required rescue opioids more frequently than the IT group from 0 to 8 h and 8 to 16 h after PACU discharge, but rates were similar by 16-24 h 70% of children in IT group transitioned directly to oral opioids. Seven IT placements were considered as failed due to early need for rescue opioids. Four (8%) of the IV group and seven (9%) of the IT group experienced oxygen desaturation. Two of these, both in IT group required naloxone and one was admitted to ICU for observation. The IT group experienced a higher incidence of pruritus, constipation and hypotension.

CONCLUSION

We observed better postoperative pain control in children who received IT vs IV opioids for the first 16 h with no discernible difference thereafter. The intrathecal group experienced higher incidences of pruritus, constipation, and hypotension.

A Prospective, Randomized, Single-Blinded, Head-to-Head Long-Term Outcome Study, Comparing Intrathecal (IT) Boluses With Continuous Infusion Trialing Techniques Prior to Implantation of Drug Delivery Systems (DDS) for the Treatment of Severe Intractable Chronic Nonmalignant Pain

OBJECTIVE

The study aims to compare intrathecal (IT) boluses to continuous infusion trialing techniques prior to implantation of drug delivery systems (DDS) for the treatment of severe intractable chronic nonmalignant pain.

DESIGN

This is a prospective, randomized, head-to-head long-term outcome study.

MATERIALS AND METHODS

Forty patients with comparable patient demographics were randomly assigned to two cohorts. Cohort A trialed with intermittent boluses; Cohort B trialed with continuous infusion. One patient failed trial in each group. Nineteen patients were implanted in each group. Follow-up was for 36 months with intervals at 6, 12, 18, 24, 36 months. The Brief Pain Inventory was used was used for assessment.

OUTCOME MEASURES

We used the Brief Pain Inventory to measure pain (worst and average), physical function (walking, normal work, and general activity), behavioral function (mood, sleep, and relations with others), IT dose, and oral opioid use.

RESULTS

We observed statistically significant reduction in pain and improvement of function in both cohorts following DDS implantation throughout the observation period. The IT dose remained virtually unchanged throughout as well, with overall limited dose escalation. Oral opioid use was significantly reduced. There was no statistically significant difference in prediction of trial success or long-term outcomes between the two cohorts.

CONCLUSION

Low-dose IT opioids via DDS can provide significant and long-lasting reduction in pain, and improvement in function (physical and behavioral) for patients with chronic nonmalignant pain. The two trialing techniques tested, intermittent boluses, and continuous infusion delivered intrathecally showed no clinical significance difference in terms of predicting trial success or long-term outcomes.

Neuraxial opioid-induced itch and its pharmacological antagonism

Given its profound analgesic nature, neuraxial opioids are frequently used for pain management. Unfortunately, the high incident rate of itch/pruritus after spinal administration of opioid analgesics reported in postoperative and obstetric patients greatly diminishes patient satisfaction and thus the value of the analgesics. Many endeavors to solve the mystery behind neuraxial opioid-induced itch had not been successful, as the pharmacological antagonism other than the blockade of mu opioid receptors remains elusive. Nevertheless, as the characteristics of all opioid receptor subtypes have become more understood, more studies have shed light on the potential effective treatments. This review discusses the mechanisms underlying neuraxial opioid-induced itch and compares pharmacological evidence in nonhuman primates with clinical findings across diverse drugs. Both nonhuman primate and human studies corroborate that mixed mu/kappa opioid partial agonists seem to be the most effective drugs in ameliorating neuraxial opioid-induced itch while retaining neuraxial opioid-induced analgesia.

Adenosine Monophosphate-activated Protein Kinase Regulates Interleukin-1β Expression and Glial Glutamate Transporter Function in Rodents with Neuropathic Pain

BACKGROUND

Neuroinflammation and dysfunctional glial glutamate transporters (GTs) in the spinal dorsal horn are implicated in the genesis of neuropathic pain. The authors determined whether adenosine monophosphate-activated protein kinase (AMPK) in the spinal dorsal horn regulates these processes in rodents with neuropathic pain.

METHODS

Hind paw withdrawal responses to radiant heat and mechanical stimuli were used to assess nociceptive behaviors. Spinal markers related to neuroinflammation and glial GTs were determined by Western blotting. AMPK activities were manipulated pharmacologically and genetically. Regulation of glial GTs was determined by measuring protein expression and activities of glial GTs.

RESULTS

AMPK activities were reduced in the spinal dorsal horn of rats (n = 5) with thermal hyperalgesia induced by nerve injury, which were accompanied with the activation of astrocytes, increased production of interleukin-1β and activities of glycogen synthase kinase 3β, and suppressed protein expression of glial glutamate transporter-1. Thermal hyperalgesia was reversed by spinal activation of AMPK in neuropathic rats (n = 10) and induced by inhibiting spinal AMPK in naive rats (n = 7 to 8). Spinal AMPKα knockdown (n = 6) and AMPKα1 conditional knockout (n = 6) induced thermal hyperalgesia and mechanical allodynia. These genetic alterations mimicked the changes of molecular markers induced by nerve injury. Pharmacological activation of AMPK enhanced glial GT activity in mice with neuropathic pain (n = 8) and attenuated glial glutamate transporter-1 internalization induced by interleukin-1β (n = 4).

CONCLUSIONS

These findings suggest that enhancing spinal AMPK activities could be an effective approach for the treatment of neuropathic pain.

Role of spinal 5-HT5A, and 5-HT1A/1B/1D, receptors in neuropathic pain induced by spinal nerve ligation in rats

Serotonin (5-HT) participates in pain modulation by interacting with different 5-HT receptors. The role of 5-HT5A receptor in neuropathic pain has not previously studied. The purpose of this study was to investigate: A) the role of 5-HT5A receptors in rats subjected to spinal nerve injury; B) the expression of 5-HT5A receptors in dorsal spinal cord and dorsal root ganglia (DRG). Neuropathic pain was induced by L5/L6 spinal nerve ligation. Tactile allodynia in neuropathic rats was assessed with von Frey filaments. Western blot methodology was used to determine 5-HT5A receptor protein expression. Intrathecal administration (on day 14th) of 5-HT (10-100 nmol) or 5-carboxamidotryptamine (5-CT, 0.03-0.3 nmol) reversed nerve injury-induced tactile allodynia. Intrathecal non-selective (methiothepin, 0.1-0.8 nmol) and selective (SB-699551, 1-10 nmol) 5-HT5A receptor antagonists reduced, by ~60% and ~25%, respectively, the antiallodynic effect of 5-HT (100 nmol) or 5-CT (0.3 nmol). Moreover, both selective 5-HT1A and 5-HT1B/1D receptor antagonists, WAY-100635 (0.3-1 nmol) and GR-127935 (0.3-1 nmol), respectively, partially diminished the antiallodynic effect of 5-HT or 5-CT by about 30%. Injection of antagonists, by themselves, did not affect allodynia. 5-HT5A receptors were expressed in the ipsilateral dorsal lumbar spinal cord and DRG and L5/L6 spinal nerve ligation did not modify 5-HT5A receptor protein expression in those sites. Results suggest that 5-HT5A receptors reduce pain processing in the spinal cord and that 5-HT and 5-CT reduce neuropathic pain through activation of 5-HT5A and 5-HT1A/1B/1D receptors. These receptors could be an important part of the descending pain inhibitory system.

Blocking the GABA transporter GAT-1 ameliorates spinal GABAergic disinhibition and neuropathic pain induced by paclitaxel

Paclitaxel is a chemotherapeutic agent widely used for treating carcinomas. Patients receiving paclitaxel often develop neuropathic pain and have a reduced quality of life which hinders the use of this life-saving drug. In this study, we determined the role of GABA transporters in the genesis of paclitaxel-induced neuropathic pain using behavioral tests, electrophysiology, and biochemical techniques. We found that tonic GABA receptor activities in the spinal dorsal horn were reduced in rats with neuropathic pain induced by paclitaxel. In normal controls, tonic GABA receptor activities were mainly controlled by the GABA transporter GAT-1 but not GAT-3. In the spinal dorsal horn, GAT-1 was expressed at presynaptic terminals and astrocytes while GAT-3 was only expressed in astrocytes. In rats with paclitaxel-induced neuropathic pain, the protein expression of GAT-1 was increased while GAT-3 was decreased. This was concurrently associated with an increase in global GABA uptake. The paclitaxel-induced attenuation of GABAergic tonic inhibition was ameliorated by blocking GAT-1 but not GAT-3 transporters. Paclitaxel-induced neuropathic pain was significantly attenuated by the intrathecal injection of a GAT-1 inhibitor. These findings suggest that targeting GAT-1 transporters for reversing disinhibition in the spinal dorsal horn may be a useful approach for treating paclitaxel-induced neuropathic pain. Patients receiving paclitaxel for cancer therapy often develop neuropathic pain and have a reduced quality of life. In this study, we demonstrated that animals treated with paclitaxel develop neuropathic pain, have enhancements of GABA transporter-1 protein expression and global GABA uptake, as well as suppression of GABAergic tonic inhibition in the spinal dorsal horn. Pharmacological inhibition of GABA transporter-1 ameliorates the paclitaxel-induced suppression of GABAergic tonic inhibition and neuropathic pain. Thus, targeting GAT-1 transporters for reversing GABAergic disinhibition in the spinal dorsal horn could be a useful approach for treating paclitaxel-induced neuropathic pain.

Neurobiological studies of chronic pain and analgesia: Rationale and refinements

Chronic pain is a complex condition for which the need for specialized research and therapies has been recognized internationally. This review summarizes the context for the international call for expansion of pain research to improve our understanding of the mechanisms underlying pain in order to achieve improvements in pain management. The methods for conducting sensory assessment in animal models are discussed and the development of animal models of chronic pain is specifically reviewed, with an emphasis on ongoing refinements to more closely mimic a variety of human pain conditions. Pharmacological correspondences between pre-clinical pain models and the human clinical experience are noted. A discussion of the 3Rs Framework (Replacement, Reduction, Refinement) and how each may be considered in pain research is featured. Finally, suggestions are provided for engaging principal investigators, IACUC reviewers, and institutions in the development of strong partnerships to simultaneously expand our knowledge of the mechanisms underlying pain and analgesia while ensuring the humane use of animals in research.

Paclitaxel induces acute pain via directly activating toll like receptor 4

Paclitaxel, a powerful anti-neoplastic drug, often causes pathological pain, which significantly reduces the quality of life in patients. Paclitaxel-induced pain includes pain that occurs immediately after paclitaxel treatment (paclitaxel-associated acute pain syndrome, P-APS) and pain that persists for weeks to years after cessation of paclitaxel treatment (paclitaxel induced chronic neuropathic pain). Mechanisms underlying P-APS remain unknown. In this study, we found that paclitaxel causes acute pain in rodents in a dose-dependent manner. The paclitaxel-induced acute pain occurs within 2 hrs after a single intravenous injection of paclitaxel. This is accompanied by low levels of paclitaxel penetrating into the cerebral spinal fluid and spinal dorsal horn. We demonstrated that an intrathecal injection of paclitaxel induces mechanical allodynia in a dose-dependent manner. Paclitaxel causes activation of toll like receptor 4 (TLR4) in the spinal dorsal horn and dorsal root ganglions. Through activating TLR4, paclitaxel increases glutamatergic synaptic activities and reduces glial glutamate transporter activities in the dorsal horn. Activations of TLR4 are necessary in the genesis of paclitaxel-induced acute pain. The cellular and molecular signaling pathways revealed in this study could provide rationales for the development of analgesics and management strategies for P-APS in patients.

Fish do not feel pain and its implications for understanding phenomenal consciousness

Phenomenal consciousness or the subjective experience of feeling sensory stimuli is fundamental to human existence. Because of the ubiquity of their subjective experiences, humans seem to readily accept the anthropomorphic extension of these mental states to other animals. Humans will typically extrapolate feelings of pain to animals if they respond physiologically and behaviourally to noxious stimuli. The alternative view that fish instead respond to noxious stimuli reflexly and with a limited behavioural repertoire is defended within the context of our current understanding of the neuroanatomy and neurophysiology of mental states. Consequently, a set of fundamental properties of neural tissue necessary for feeling pain or experiencing affective states in vertebrates is proposed. While mammals and birds possess the prerequisite neural architecture for phenomenal consciousness, it is concluded that fish lack these essential characteristics and hence do not feel pain.

Inhibition of intracellular signaling pathways NF-κB and MEK1/2 attenuates neuropathic pain development and enhances morphine analgesia

BACKGROUND

Neuropathic pain is clinically challenging because it is resistant to alleviation by morphine. The nuclear factor κB (NF-κB) and mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathways may be involved in the development of neuropathic pain. The aim of our study was to examine the influence of a chronic, intrathecal administration of parthenolide (PTL, inhibitor of NF-κB) and U0126 (inhibitor of MEK1/2) on nociception and morphine effectiveness in a rat model of neuropathy.

METHODS

The chronic constriction injury of the sciatic nerve in Wistar rats was performed. PTL and U0126 were injected chronic intrathecally and morphine was injected once at day 7. To evaluate allodynia and hyperalgesia, the von Frey and cold plate tests were used, respectively. The experiments were carried out according to IASP rules. Using qRT-PCR we analyzed mRNAs of μ-(mor), δ-(dor) and κ-(kor)-opioid receptors in the lumbar spinal cord after drugs administration.

RESULTS

The administration of PTL and U0126 decreased allodynia and hyperalgesia and significantly potentiated morphine effect. The mor, dor and kor mRNAs were down-regulated 7 days after injury in the ipsilateral spinal cord. The PTL and U0126 significantly up-regulated the mRNA levels of all opioid receptors. The levels of mor and dor mRNAs were much higher compared to those in naïve, but only the kor levels returned to control values.

CONCLUSIONS

These results indicate that the inhibition of the NF-κB pathway has better analgesic effects. Both inhibitors similarly potentiate morphine analgesia, which parallels the up-regulation of both mor and dor mRNAs expression spinal levels of the model of neuropathy.

Reversible paralysis and loss of deep pain sensation after topical intrathecal morphine administration following durotomy

OBJECTIVE

To report 2 dogs that developed temporary pelvic limb paralysis with loss of deep pain sensation after topical intrathecal morphine administration during spinal surgery.

STUDY DESIGN

Clinical report.

ANIMALS

Dogs (n = 2).

METHODS

A 5-year-old castrated male French Bulldog with a subarachnoid diverticulum at T9-T10 and a 9.5-year-old castrated male Belgian Shepherd dog with a herniated disc at T12-T13 and intradural component, had Gelfoam soaked with morphine placed over the dura mater defect.

RESULTS

Pelvic limb paralysis and loss of deep pain sensation was noticed immediately after recovery from anesthesia. After intravenous naloxone administration, both dogs immediately regained ambulation and normal pain sensation; however the effect was temporary lasting only a few hours. Permanent resolution of clinical signs occurred 24 hours after surgery.

CONCLUSIONS

Topical intrathecal morphine administration resulted in temporary pelvic limb paralysis and loss of deep pain sensation. This route of administration should be used cautiously until further determination of the efficacy and adverse effects associated with topical intrathecal morphine administration.

Mechanical allodynia induced by nucleoside reverse transcriptase inhibitor is suppressed by p55TNFSR mediated by herpes simplex virus vector through the SDF1α/CXCR4 system in rats

BACKGROUND

In the human immunodeficiency virus (HIV)-associated sensory neuropathy, neuropathic pain associated with the use of nucleoside reverse transcriptase inhibitors (NRTIs) in patients with HIV/acquired immunodeficiency syndrome is clinically common. While evidence demonstrates that neuropathic pain is influenced by neuroinflammatory events that include the proinflammatory molecules, tumor necrosis factor-α (TNF-α), stromal cell-derived factor 1-α (SDF1-α), and C-X-C chemokine receptor type 4 (CXCR4), the detailed mechanisms by which NRTIs contribute to the development of neuropathic pain are not known. In this study, we investigated the role of these proinflammatory molecules in the dorsal root ganglion (DRG) and the spinal dorsal horn in NRTIs-mediated neuropathic pain state.

METHODS

Neuropathic pain was induced by intraperitoneal administration of 2',3'-dideoxycytidine (ddC, one of the NRTIs). Mechanical threshold was tested using von Frey filament fibers. Nonreplicating herpes simplex virus (HSV) vectors expressing p55 TNF soluble receptor (p55TNFSR) were inoculated into hindpaw of rats. The expression of TNF-α, SDF1-α, and CXCR4 in both the lumbar spinal cord and the L4/5 DRG was examined using Western blots. Intrathecal CXCR4 antagonist was administered.

RESULTS

The present study demonstrated that (1) systemic ddC induced upregulation of TNF-α, SDF1-α, and CXCR4 in both the lumbar spinal cord and the L4/5 DRG; (2) p55TNFSR mediated by a nonreplicating HSV vector reversed mechanical allodynia induced by systemic ddC; (3) intrathecal administration of the CXCR4 antagonist AMD3100 increased mechanical threshold; and (4) HSV vector expressing p55TNFSR reversed upregulation of TNF-α, SDF1-α, and CXCR4 induced by ddC in the lumbar spinal dorsal horn and the DRG.

CONCLUSIONS

Our studies demonstrate that TNF-α through the SDF1/CXCR4 system is involved in the NRTIs-related neuropathic pain state and that blocking the signaling of these proinflammatory molecules is able to reduce NRTIs-related neuropathic pain. These results provide a novel mechanism-based approach (gene therapy) to treating HIV-associated neuropathic pain.

Contributions of peripheral, spinal, and supraspinal actions to analgesia

Pain signaling involves several main compartments that can be considered as potential sites for analgesic drug actions. When drugs are given systemically, they can act at spinal, supraspinal and peripheral sites, and several methods have been developed for identifying where they act. These include (1) localized delivery of drugs to specific sites (via intracerebral, intrathecal, and intraplantar injections), (2) systemic delivery of drugs with localized delivery of antagonists for the receptor on which the drug acts or for a system recruited by the drug, (3) use of peripherally restricted analogs, and (4) use of conditional knockout technology to selectively deplete receptors on nociceptors. Delivery of drugs simultaneously to several sites (spinal/supraspinal, peripheral/spinal, and peripheral/supraspinal) reveals "self-synergy" between sites for some agents. Knowledge of peripheral contributions to drug actions is important because of the potential to develop peripherally restricted analgesics (with a diminished side effect profile due to not entering the central nervous system), the potential to deliver drugs peripherally (e.g. topically) to act on sensory nerve endings and adjacent tissue (with a diminished side effect profile due to limited systemic absorption), and the potential to use combinations of topical and oral drug regimens to obtain improved pain relief (without increasing the side effect burden). This review considers methods used for compartmental analysis, and results of such site analysis for several major classes of analgesic drugs that are in current use.

Resolvins: Endogenously-Generated Potent Painkilling Substances and their Therapeutic Perspectives

The efficacy of many of pain-relieving drugs is based on mechanisms by which the drugs interfere with the body’s natural pain-mediating pathways. By contrast, although it is less popular, other drugs including opioids exert more powerful analgesic actions by augmenting endogenous inhibitory neural circuits for pain mediation. Recently, a novel endogenous pain-inhibitory principle was suggested and is now attracting both scientific and clinical attentions. The central players for the actions are particular body lipids: resolvins. Although research is in the preclinical phase, multiple hypotheses have actively been matured regarding the potency and molecular and neural processes of the analgesic effects of these substances. Consistently, accumulating experimental evidence has been demonstrating that treatment with these lipid substances is strongly effective at controlling diverse types of pain. Treatment of resolvins does not appear to disturb the body homeostasis as severely as many other therapeutic agents that interrupt the body’s natural signaling flow, which enables us to predict their fewer adverse effects. This paper serves as a review of currently documented painkilling actions of resolvins, summarizes the potential cellular and receptor-mediated mechanisms to date, and discusses the many clinical uses for these therapeutic lipids that have not yet been tested. Future scientific efforts will more concentrate to unveil such aspects of the substances and to construct clear proofs of concept for pain relief.

Phosphoproteomics and bioinformatics analyses of spinal cord proteins in rats with morphine tolerance

Introduction

Morphine is the most effective pain-relieving drug, but it can cause unwanted side effects. Direct neuraxial administration of morphine to spinal cord not only can provide effective, reliable pain relief but also can prevent the development of supraspinal side effects. However, repeated neuraxial administration of morphine may still lead to morphine tolerance.

Methods

To better understand the mechanism that causes morphine tolerance, we induced tolerance in rats at the spinal cord level by giving them twice-daily injections of morphine (20 µg/10 µL) for 4 days. We confirmed tolerance by measuring paw withdrawal latencies and maximal possible analgesic effect of morphine on day 5. We then carried out phosphoproteomic analysis to investigate the global phosphorylation of spinal proteins associated with morphine tolerance. Finally, pull-down assays were used to identify phosphorylated types and sites of 14-3-3 proteins, and bioinformatics was applied to predict biological networks impacted by the morphine-regulated proteins.

Results

Our proteomics data showed that repeated morphine treatment altered phosphorylation of 10 proteins in the spinal cord. Pull-down assays identified 2 serine/threonine phosphorylated sites in 14-3-3 proteins. Bioinformatics further revealed that morphine impacted on cytoskeletal reorganization, neuroplasticity, protein folding and modulation, signal transduction and biomolecular metabolism.

Conclusions

Repeated morphine administration may affect multiple biological networks by altering protein phosphorylation. These data may provide insight into the mechanism that underlies the development of morphine tolerance.

Glycogen synthase kinase 3 beta regulates glial glutamate transporter protein expression in the spinal dorsal horn in rats with neuropathic pain

Dysfunctional glial glutamate transporters and over production of pro-inflammatory cytokines (including interleukin-1β, IL-1β) are two prominent mechanisms by which glial cells enhance neuronal activities in the spinal dorsal horn in neuropathic pain conditions. Endogenous molecules regulating production of IL-1β and glial glutamate functions remain poorly understood. In this study, we revealed a dynamic alteration of GSK3β activities and its role in regulating glial glutamate transporter 1 (GLT-1) protein expression in the spinal dorsal horn and nociceptive behaviors following the nerve injury. Specifically, GSK3β was expressed in both neurons and astrocytes in the spinal dorsal horn. GSK3β activities were suppressed on day 3 but increased on day 10 following the nerve injury. In parallel, protein expression of GLT-1 in the spinal dorsal horn was enhanced on day 3 but reduced on day 10. In contrast to these time-dependent changes, the activation of astrocytes and over-production of IL-1β were found on both day 3 and day 10. Meanwhile, thermal hyperalgesia was observed from day 2 through day 10 and mechanical allodynia from day 4 through day 10. Pre-emptive pharmacological inhibition of GSK3β activities significantly ameliorated thermal hyperalgesia and mechanical allodynia at the late stage but did not have effects at the early stage. These were accompanied with the suppression of GSK3β activities, prevention of decreased GLT-1 protein expression, inhibition of astrocytic activation, and reduction of IL-1β in the spinal dorsal horn on day 10. These data indicate that the increased GSK3β activity in the spinal dorsal horn is attributable to the downregulation of GLT-1 protein expression in neuropathic rats at the late stage. Further, we also demonstrated that the nerve-injury-induced thermal hyperalgesia on day 10 was transiently suppressed by pharmacological inhibition of GSK3β. Our study suggests that GSK3β may be a potential target for the development of analgesics for chronic neuropathic pain.

Ventral tegmental site of opiate reward: antagonism by a hydrophilic opiate receptor blocker

The ventral tegmental area has been suggested as a possible site of action for the rewarding effect of opiates on the basis of the fact that local morphine injections in this but not other regions are rewarding. That this is a necessary and not just a sufficient site of opiate rewarding action was suggested by the fact that diallyl-normorphinium bromide, a hydrophilic opiate blocker, caused compensatory increases in intravenous heroin self-administration when injected into the ventral tegmental area but not other brain regions.

Effects of general anesthetics on substance P release and c-Fos expression in the spinal dorsal horn

BACKGROUND

The authors examined in vivo the effects of general anesthetics on evoked substance P release (primary afferent excitability) and c-Fos expression (neuronal activation) in superficial dorsal horn.

METHODS

Rats received saline, propofol (100 mg/kg), pentobarbital (50 mg/kg), isoflurane (2 minimum alveolar concentration), nitrous oxide (66%), or fentanyl (30 μg/kg). During anesthesia, rats received intraplantar 5% formalin (50 μl) to left hind paw. Ten minutes later, rats underwent transcardial perfusion with 4% paraformaldehyde. Substance P release from small primary afferents was assessed by incidence of neurokinin 1 receptor internalization in the superficial dorsal horn. In separate studies, rats were sacrificed after 2 h and c-Fos expression measured.

RESULTS

Intraplantar formalin-induced robust neurokinin 1 receptor internalization in ipsilateral dorsal horn (ipsilateral: 54 ± 6% [mean ± SEM], contralateral: 12 ± 2%; P < 0.05; n = 4). Fentanyl, but not propofol, pentobarbital, isoflurane, nor nitrous oxide alone inhibited neurokinin 1 receptor internalization. However, 2 minimum alveolar concentration isoflurane + nitrous oxide reduced neurokinin 1 receptor internalization (27 ± 3%; P < 0.05; n = 5). All agents reduced c-Fos expression (control: 34 ± 4, fentanyl: 8 ± 2, isoflurane: 12 ± 3, nitrous oxide: 11 ± 2, isoflurane + nitrous oxide: 12 ± 1, pentobarbital: 11 ± 2, propofol: 13 ± 3; P < 0.05; n = 3).

CONCLUSION

General anesthetics at anesthetic concentrations block spinal neuron activation through a mechanism that is independent of an effect on small primary afferent peptide release. The effect of fentanyl alone and the synergistic effect of isoflurane and nitrous oxide on substance P release suggest a correlative rationale for the therapeutic use of these anesthetic protocols by blocking nociceptive afferent transmitter release and preventing the initiation of cascade, which is immediately postsynaptic to the primary afferent.

Intrathecal drug administration in chronic pain syndromes

Chronic pain may recur after initial response to strong opioids in both patients with cancer and patients without cancer or therapy may be complicated by intolerable side effects. When minimally invasive interventional pain management techniques also fail to provide satisfactory pain relief, continuous intrathecal analgesic administration may be considered. Only 3 products have been officially approved for long-term intrathecal administration: morphine, baclofen, and ziconotide. The efficacy of intrathecal ziconotide for the management of patients with severe chronic refractory noncancer pain was illustrated in 3 placebo-controlled trials. A randomized study showed this treatment option to be effective over a short follow-up period for patients with pain due to cancer or AIDS. The efficacy of intrathecal opioid administration for the management of chronic noncancer pain is mainly derived from prospective and retrospective noncontrolled trials. The effect of intrathecal morphine administration in patients with pain due to cancer was compared with oral or transdermal treatment in a randomized controlled trial, which found better pain control and fewer side effects with intrathecal opioids. Other evidence is derived from cohort studies. Side effects of chronic intrathecal therapy may either be technical (catheter or pump malfunction) or biological (infection). The most troublesome complication is, however, the possibility of granuloma formation at the catheter tip that may induce neurological damage. Given limited studies, the evidence for intrathecal drug administration in patients suffering from cancer-related pain is more compelling than that of chronic noncancer pain.

Randomised, double-blind controlled trial by dose reduction of implanted intrathecal morphine delivery in chronic non-cancer pain

OBJECTIVE

This study aimed to investigate the efficacy of intrathecal morphine in the long term by hypothesising that a reduction of the intrathecal opioid dose following long-term administration would increase the level of pain intensity.

DESIGN

Randomised, double-blind, controlled, parallel group trial.

SETTING

Department of Pain Management, Russells Hall Hospital, Dudley, UK.

PARTICIPANTS

24 patients with non-cancer pain implanted with morphine reservoirs were assessed for eligibility.

INTERVENTIONS

Participants were randomly allocated to one of two parallel groups in which one of the groups had no change in morphine dose and the other group had a small reduction (20%) in dosage every week during a 10-week follow-up.

OUTCOME

Primary outcomes were visual analogue scale (VAS) pain score change and withdrawal from the study due to lack of efficacy.

RESULTS

9 of the patients assessed for eligibility declined to participate in the study. 15 patients were randomised to control (n=5) or intervention (n=10) and included in an intention-to-treat analysis. Owing to worsening of pain, seven patients withdrew from the study prematurely. None knew prior to withdrawal which arm of the study they were in, but all turned out to be in the dose-reduction arm. The calculation of dropout rates between groups indicated a significant statistical difference (p=0.026) and recruitment was ceased. The VAS change between baseline and the last observation was smaller in the control group (median, Mdn=11) than in the intervention group (Mdn=30.5), although not statistically significant, Z=-1.839, p=0.070; r=-0.47. Within groups, VAS was significantly lower at baseline (Mdn=49.5) than at the last observation (Mdn=77.5) for the reduction group, Z=-2.805, p=0.002; r=-0.627 but not for the control group (p=0.188).

CONCLUSIONS

This double-blind randomised controlled trial of chronic intrathecal morphine administration suggests the effectiveness of this therapy for the management of chronic non-cancer pain. However, owing to the small number of patients completing the study (n=8), further studies are warranted.

TRIAL REGISTRATION

International Standard Randomised Controlled Trials Centre (ISRCTN 33733462).

Biodegradable poly([D,L]-lactide-co-glycolide) nanofibers for the sustainable delivery of lidocaine into the epidural space after laminectomy

AIM

We developed biodegradable, lidocaine-embedded poly([D,L]-lactide-co-glycolide) nanofibers for epidural analgesia to reduce the severe pain in rats after laminectomies.

MATERIALS & METHODS

Nanofibers were prepared by an electrospinning process and were introduced into the epidural space of rats after laminectomy. The lidocaine concentration, postoperative bodyweight change and amount of food/water intake were monitored to evaluate the analgesic effectiveness of the drug-eluting nanofibers.

RESULTS

It was demonstrated that the nanofibers provided a sustained release of lidocaine for more than 2 weeks, and the local pharmaceutical concentration was much higher than the concentration in plasma. Rats that received laminectomies without nanofibers exhibited the greatest bodyweight reduction. The food/water intake and activity performance were significantly higher in rats receiving laminectomies with nanofibers than in rats without nanofibers.

CONCLUSION

The results of this study suggest that the lidocaine-loaded nanofibers can provide an easy, practical and safe means of achieving effective postlaminectomy analgesia.

Peripheral antinociceptive action of mangiferin in mouse models of experimental pain: role of endogenous opioids, K(ATP)-channels and adenosine

This study aimed to assess the possible systemic antinociceptive activity of mangiferin and to clarify the underlying mechanism, using the acute models of chemical (acetic acid, formalin, and capsaicin) and thermal (hot-plate and tail-flick) nociception in mice. Mangiferin at oral doses of 10 to 100 mg/kg evidenced significant antinociception against chemogenic pain in the test models of acetic acid-induced visceral pain and in formalin- and capsaicin-induced neuro-inflammatory pain, in a naloxone-sensitive manner, suggesting the participation of endogenous opiates in its mechanism. In capsaicin test, the antinociceptive effect of mangiferin (30 mg/kg) was not modified by respective competitive and non-competitive transient receptor potential vanilloid 1 (TRPV1) antagonists, capsazepine and ruthenium red, or by pretreatment with L-NAME, a non-selective nitric oxide synthase inhibitor, or by ODQ, an inhibitor of soluble guanylyl cyclase. However, mangiferin effect was significantly reversed by glibenclamide, a blocker of K(ATP) channels and in animals pretreated with 8-phenyltheophylline, an adenosine receptor antagonist. Mangiferin failed to modify the thermal nociception in hot-plate and tail-flick test models, suggesting that its analgesic effect is only peripheral but not central. The orally administered mangiferin (10-100 mg/kg) was well tolerated and did not impair the ambulation or the motor coordination of mice in respective open-field and rota-rod tests, indicating that the observed antinociception was unrelated to sedation or motor abnormality. The findings of this study suggest that mangiferin has a peripheral antinociceptive action through mechanisms that involve endogenous opioids, K(ATP)-channels and adenosine receptors.

Reduced number, G protein coupling, and antinociceptive efficacy of spinal mu-opioid receptors in diabetic rats are reversed by nerve growth factor

This study investigated putative mechanisms of impaired spinal opioid antinociception such as a downregulation of mu-opioid receptor (MOR) number, coupling, and efficacy in rats with advanced (12 weeks) streptozotocin (STZ)-induced diabetes. Intravenous injection of STZ (45 mg/kg) in Wistar rats led to selective degeneration of insulin-producing pancreatic ß-cells, elevated blood glucose, and mechanical hyperalgesia. In these animals, dose-dependent and naloxone-reversible intrathecal fentanyl antinociception was significantly impaired and associated with a loss in MOR immunoreactivity of calcitonin gene-related peptide-immunoreactive (CGRP-IR) sensory nerve terminals, membrane-bound MOR binding sites, and MOR-stimulated G protein coupling within the dorsal horn of the spinal cord. Intrathecal delivery of nerve growth factor (NGF) in diabetic animals normalized spinal MOR number and G protein coupling and rescued spinal fentanyl-induced antinociception. These findings identify for the first time a loss in functional MOR on central terminals of sensory neurons as a contributing factor for the impaired spinal opioid responsiveness during advanced STZ-induced diabetes that can be reversed by NGF. Moreover, they support growing evidence of a distinct regulation of opioid responsiveness during various painful states of disease (eg, arthritis, cancer, neuropathy) and may give novel therapeutic incentives.

PERSPECTIVE

In diabetic neuropathy a loss in sensory neuron mu-opioid receptor number and coupling contributes to impaired spinal opioid antinociception that can be reversed by NGF. These findings support growing evidence of a distinct regulation of opioid responsiveness during various painful diseases and may give novel therapeutic incentives.

Gene-based approaches in pain research and exploration of new therapeutic targets and strategies

Large panel of gene-based techniques is used for many years specifically in the pain research field. From the first identification (cloning) of some "mythic" genes, such as those encoding opioid or capsaicin receptors allowing then the creation of first-generation knockout mice, to the today conditional (time, tissue, cell-type and even pathology-dependent) and regulatable modulation of a gene function, these approaches largely contributed to fundamental leaps forward in our understanding of the function of some proteins and of their interest as possible druggable targets. Perhaps one of the most remarkable evolution in the last years is the passage of these approaches from the bench to the patient; whether it concerns the identification of genes involved in inherited pain insensibility/susceptibility, the search for genetic markers of pain types, the individual pharmacogenomics or even the first gene therapy trials. From many possible variants of gene-grounded techniques used in pain research we focus here on gene knockouts and some recent developments, on viral vectors-based gene transfer and on transgenic models for the tracing of pain pathways. Through these selected examples we attempted to emphasize the immense potential of these approaches and their already well-recognized contribution in both the basic and clinical pain research.

Antinociceptive and anti-inflammatory effects of the monoterpene α,β-epoxy-carvone in mice

α,β-Epoxy-carvone (EC) is a monoterpene found in the essential oils of many species of plants. It can also be obtained by organic synthesis. EC exerts a depressant effect on the central nervous system and is also known to have anticonvulsant, antimicrobial and antioxidant effects. The present study investigated the antinociceptive and anti-inflammatory effects of EC. Intraperitoneal administration of EC at doses of 100, 200 or 300 mg/kg promoted a significant antinociceptive effect, as shown in the acetic acid-induced abdominal writhing test. EC also provoked a reduction in formalin-induced nociception in the first (300 mg/kg) and second phases (200 and 300 mg/kg). In the hot-plate test, an increase in response latency was found at 30 min (at 100, 200 and 300 mg/kg), and at 60 and 120 min (at 300 mg/kg) following administration of EC, an effect that was reversed by naloxone. Intraperitoneal administration of EC (300 mg/kg) inhibited the increased vascular permeability provoked by acetic acid. These findings suggest that EC inhibited the acute inflammatory reaction, with a pronounced peripheral and central antinociceptive effect in mice that is probably associated with activation of the opioidergic system, which appears to play a role in the antinociceptive activity induced by EC.

Characteristics of distribution of morphine and metabolites in cerebrospinal fluid and plasma with chronic intrathecal morphine infusion in humans

BACKGROUND

Despite widespread use of chronic intrathecal (IT) infusions of morphine, there is little systematic human work evaluating the steady state morphine concentrations or cerebrospinal (CSF) chemistry after long-term IT morphine delivery. We sought to address these issues in patients receiving chronic IT morphine infusion.

METHODS

Pain patients with implanted catheters and pumps (range: 127 to 2165 days), receiving a stable dosing (>1 week) of IT morphine by infusion, were entered into the study. The following sequence was performed: (1) estimation of pain score; (2) radiograph localization of catheter tip; (3) percutaneous sampling of lumbar CSF at the L4 to 5 or L5-S1 space. CSF/plasma samples were assayed for chemistry, and morphine and its 3/6 glucuronide metabolites (M3G, M6G) by liquid chromatography mass spectrometry.

RESULTS

Nineteen patients were enrolled. CSF samples were obtained from 16 subjects. Three patients were not included in the primary analysis because 1 catheter was epidural, 1 catheter was fractured, and 1 had a granuloma at the catheter tip. Of the 13 sampled patients, the range of daily doses, rates, and concentrations were 1.6 to 25 mg/d and 0.1 to 1 mL/d, 5 to 50 mg/mL, respectively. The principal observations were as follows: (i) morphine, M3G, and M6G were present in the CSF and plasma and showed a significant regression slope when plotted versus daily dose; (ii) in contrast, the regression slope of the group ratio morphine:M3G:M6G plotted versus daily dose in CSF or plasma was not different from zero; (iii) plotting "normalized" CSF analyte concentration (e.g., concentration at site/daily IT morphine dose) against the segmental distance of the sampling site from the catheter tip revealed a significant decline in concentration of morphine, but not of conjugates as a function of distance from the catheter tip; (iv) plotting CSF protein, glucose, and red and white cell counts versus daily morphine dose or morphine concentration at the sampling site revealed no significant regression; and (v) patients with a catheter failure or a granuloma showed reduced concentrations of morphine in their CSF.

CONCLUSION

Chronic infusion of morphine shows high concentrations, which correlate with the infusion dose and the proximity of the sampling site to the infusion site with no effects on CSF chemistry.

Complement factor C5a and C5a receptor contribute to morphine tolerance and withdrawal-induced hyperalgesia in rats

Morphine is a potent opioid analgesic. However, the repeated use of morphine causes tolerance and hyperalgesia. Neuroinflammation has been reported to be involved in morphine tolerance and withdrawal-induced hyperalgesia. The complement system is a crucial effector mechanism of immune responses. The present study investigated the roles of complement factor C5a and C5a receptor (C5aR) in the development of morphine tolerance and withdrawal-induced hyperalgesia. In the present study, the levels of C5a and C5aR were increased in the L5 lumbar spinal cords of morphine-tolerant rats. The administration of C5a promoted the development of hyperalgesia and the expression of spinal antinociceptive tolerance to intrathecal morphine in both mechanical and thermal test. However, these phenomena caused by morphine were significantly attenuated by the C5aR antagonist PMX53. These results suggest that complement activation within the spinal cord is involved in morphine tolerance and withdrawal-induced hyperalgesia. C5a and C5aR may serve as novel targets for the control of morphine tolerance and withdrawal-induced hyperalgesia.