Behavioral, pharmacological and molecular characterization of the saphenous nerve partial ligation: A new model of neuropathic pain

The highs and lows of cannabinoid receptor expression in disease: mechanisms and their therapeutic implications

Alterations in the endogenous cannabinoid system have been described in almost every category of disease. These changes can alternatively be protective or maladaptive, such as producing antinociception in neuropathic pain or fibrogenesis in liver disease, making the system an attractive therapeutic target. However, the challenge remains to selectively target the site of disease while sparing other areas, particularly mood and cognitive centers of the brain. Identifying regional changes in cannabinoid receptor-1 and -2 (CB(1)R and CB(2)R) expression is particularly important when considering endocannabinoid system-based therapies, because regional increases in cannabinoid receptor expression have been shown to increase potency and efficacy of exogenous agonists at sites of disease. Although there have been extensive descriptive studies of cannabinoid receptor expression changes in disease, the underlying mechanisms are only just beginning to unfold. Understanding these mechanisms is important and potentially relevant to therapeutics. In diseases for which cannabinoid receptors are protective, knowledge of the mechanisms of receptor up-regulation could be used to design therapies to regionally increase receptor expression and thus increase efficacy of an agonist. Alternatively, inhibition of harmful cannabinoid up-regulation could be an attractive alternative to global antagonism of the system. Here we review current findings on the mechanisms of cannabinoid receptor regulation in disease and discuss their therapeutic implications.

Design and evaluation of a novel fluorescent CB2 ligand as probe for receptor visualization in immune cells

Cannabinoid CB2 receptor has emerged as a very promising target over the last decades. We have synthesized and evaluated a new fluorescent probe designated NMP6 based on 6-methoxyisatin scaffold, which exhibited selectivity and K(i) value at hCB2 of 387 nM. We have demonstrated its ability to be an effective probe for visualization of CB2 receptor binding using confocal microscopy and a flow cytometry probe for the analysis of CB2 protein expression. Furthermore, NMP6 was easily obtained in two chemical steps from commercially available building blocks.

Antinociceptive effects induced through the stimulation of spinal cannabinoid type 2 receptors in chronically inflamed mice

The stimulation of spinal cannabinoid type 2 (CB(2)) receptors is a suitable strategy for the alleviation of experimental pain symptoms. Several reports have described the up-regulation of spinal cannabinoid CB(2) receptors in neuropathic settings together with the analgesic effects derived from their activation. Besides, we have recently reported in two murine bone cancer models that the intrathecal administration of cannabinoid CB(2) receptor agonists completely abolishes hyperalgesia and allodynia, whereas spinal cannabinoid CB(2) receptor expression remains unaltered. The present experiments were designed to measure the expression of spinal cannabinoid CB(2) receptors as well as the analgesic efficacy derived from their stimulation in mice chronically inflamed by the intraplantar injection of complete Freund's adjuvant 1 week before. Both spinal cannabinoid CB(2) receptors mRNA measured by real-time PCR and cannabinoid CB(2) receptor protein levels measured by western blot remained unaltered in inflamed mice. Besides, the intrathecal (i.t.) administration of the cannabinoid CB(2) receptor agonists AM1241, (R,S)-3-(2-Iodo-5-nitrobenzoyl)-1-(1-methyl-2-piperidinylmethyl)-1H-indole, (0.03-1 μg) and JWH 133, (6aR,10aR)-3-(1,1-Dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]pyran, (3-30 μg) dose-dependently blocked inflammatory thermal hyperalgesia and mechanical allodynia. The analgesic effects induced by both agonists were counteracted by the coadministration of the selective cannabinoid CB(2) receptor antagonist SR144528, 5-(4-chloro-3-methylphenyl)-1-[(4-methylphenyl)methyl]-N-[(1S,2S,4R)-1,3,3-trimethylbicyclo[2.2.1]hept-2-yl]-1H-pyrazole-3-carboxamide, (5 μg) but not by the cannabinoid CB(1) receptor antagonist AM251, N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide, (10 μg). The effects induced by AM1241 were also inhibited by the coadministration of the opioid receptor antagonist, naloxone (1 μg). These results demonstrate that effective analgesia can be achieved in chronic inflammatory settings through the stimulation of spinal cannabinoid CB(2) receptors even if this receptor population is not up-regulated.

Central and peripheral sites of action for CB₂ receptor mediated analgesic activity in chronic inflammatory and neuropathic pain models in rats

BACKGROUND AND PURPOSE

Cannabinoid CB₂ receptor activation by selective agonists has been shown to produce analgesic effects in preclinical models of inflammatory and neuropathic pain. However, mechanisms underlying CB₂-mediated analgesic effects remain largely unknown. The present study was conducted to elucidate the CB₂ receptor expression in 'pain relevant' tissues and the potential sites of action of CB₂ agonism in rats.

EXPERIMENTAL APPROACH

Expression of cannabinoid receptor mRNA was evaluated by quantitative RT-PCR in dorsal root ganglia (DRGs), spinal cords, paws and several brain regions of sham, chronic inflammatory pain (CFA) and neuropathic pain (spinal nerve ligation, SNL) rats. The sites of CB₂ mediated antinociception were evaluated in vivo following intra-DRG, intrathecal (i.t.) or intraplantar (i.paw) administration of potent CB₂-selective agonists A-836339 and AM1241.

KEY RESULTS

CB₂ receptor gene expression was significantly up-regulated in DRGs (SNL and CFA), spinal cords (SNL) or paws (CFA) ipsilateral to injury under inflammatory and neuropathic pain conditions. Systemic A-836339 and AM1241 produced dose-dependent efficacy in both inflammatory and neuropathic pain models. Local administration of CB₂ agonists also produced significant analgesic effects in SNL (intra-DRG and i.t.) and CFA (intra-DRG) pain models. In contrast to A-836339, i.paw administration of AM-1241 dose-relatedly reversed the CFA-induced thermal hyperalgesia, suggesting that different mechanisms may be contributing to its in vivo properties.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that both DRG and spinal cord are important sites contributing to CB₂ receptor-mediated analgesia and that the changes in CB₂ receptor expression play a crucial role for the sites of action in regulating pain perception.

Activation of cannabinoid receptors by the pentacyclic triterpene α,β-amyrin inhibits inflammatory and neuropathic persistent pain in mice

In this study, we report that α,β-amyrin, a plant-derived pentacyclic triterpene, reduced persistent inflammatory and neuropathic hyperalgesia in mice by a direct activation of the CB(1) and CB(2) cannabinoid receptors (CB(1)R and CB(2)R). The oral treatment with α,β-amyrin (30 mg/kg) significantly reduced mechanical and thermal hyperalgesia and inflammation induced by complete Freund's adjuvant (CFA) and by partial sciatic nerve ligation (PSNL). The pretreatment with either CB(1)R or CB(2)R antagonists and the knockdown gene of the receptors significantly reverted the antinociceptive effect of α,β-amyrin. Of note, binding studies showed that α,β-amyrin directly bound with very high affinity to CB(1)R (K(i)=0.133 nM) and with a lower affinity to CB(2)R (K(i)=1989 nM). Interestingly, α,β-amyrin, ACEA (CB(1)R agonist), or JWH-133 (CB(2)R agonist), at doses that caused antinociception, failed to provoke any behavioral disturbance, as measured in the tetrad assay. In addition, α,β-amyrin largely decreased interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), keratinocyte-derived chemokine (KC) and interleukin 6 (IL-6) levels, and myeloperoxidase activity. Likewise, α,β-amyrin prevented the activation of the transcriptional factors: nuclear factor κB (NF-κB) and cyclic adenosine monophosphate response element binding (CREB) and the expression of cyclooxygenase 2 in mice footpads and spinal cords. The present results demonstrated that α,β-amyrin exhibits long-lasting antinociceptive and anti-inflammatory properties in 2 models of persistent nociception via activation of cannabinoid receptors and by inhibiting the production of cytokines and expression of NF-κB, CREB and cyclooxygenase 2.

Activation of the galanin receptor 2 in the periphery reverses nerve injury-induced allodynia

BACKGROUND

Galanin is expressed at low levels in the intact sensory neurons of the dorsal root ganglia with a dramatic increase after peripheral nerve injury. The neuropeptide is also expressed in primary afferent terminals in the dorsal horn, spinal inter-neurons and in a number of brain regions known to modulate nociception. Intrathecal administration of galanin modulates sensory responses in a dose-dependent manner with inhibition at high doses. To date it is unclear which of the galanin receptors mediates the anti-nociceptive effects of the neuropeptide and whether their actions are peripherally and/or centrally mediated. In the present study we investigated the effects of direct administration into the receptive field of galanin and the galanin receptor-2/3-agonist Gal2-11 on nociceptive primary afferent mechanical responses in intact rats and mice and in the partial saphenous nerve injury (PSNI) model of neuropathic pain.

RESULTS

Exogenous galanin altered the responses of mechano-nociceptive C-fibre afferents in a dose-dependent manner in both naive and nerve injured animals, with low concentrations facilitating and high concentrations markedly inhibiting mechano-nociceptor activity. Further, use of the galanin fragment Gal2-11 confirmed that the effects of galanin were mediated by activation of galanin receptor-2 (GalR2). The inhibitory effects of peripheral GalR2 activation were further supported by our demonstration that after PSNI, mechano-sensitive nociceptors in galanin over-expressing transgenic mice had significantly higher thresholds than in wild type animals, associated with a marked reduction in spontaneous neuronal firing and C-fibre barrage into the spinal cord.

CONCLUSIONS

These findings are consistent with the hypothesis that the high level of endogenous galanin in injured primary afferents activates peripheral GalR2, which leads to an increase in C-fibre mechanical activation thresholds and a marked reduction in evoked and ongoing nociceptive responses.

Crystal structure of a soluble form of human monoglyceride lipase in complex with an inhibitor at 1.35 Å resolution

A high-resolution structure of a ligand-bound, soluble form of human monoglyceride lipase (MGL) is presented. The structure highlights a novel conformation of the regulatory lid-domain present in the lipase family as well as the binding mode of a pharmaceutically relevant reversible inhibitor. Analysis of the structure lacking the inhibitor indicates that the closed conformation can accommodate the native substrate 2-arachidonoyl glycerol. A model is proposed in which MGL undergoes conformational and electrostatic changes during the catalytic cycle ultimately resulting in its dissociation from the membrane upon completion of the cycle. In addition, the study outlines a successful approach to transform membrane associated proteins, which tend to aggregate upon purification, into a monomeric and soluble form.

Antinociceptive effect of intrathecal cannabinoid receptor agonist WIN 55,212-2 in a rat bone tumor pain model

Bone tumor pain is a poorly controlled pain comprising background and severe pain on moving or weight-bearing postures that decreases the quality of life for cancer patients; thus, more effective analgesics are clearly needed. This study evaluated the efficacy of a cannabinoid (CB) receptor agonist (WIN 55,212-2) on bone tumor pain in the spinal cords of rats, and clarified the roles of the CB1 and CB2 receptors in WIN 55,212-2-induced antinociception at the spinal level. Bone tumor pain was induced by injecting MRMT-1 tumor cells (1×10(5)) into the right tibias of female Sprague-Dawley rats under sevoflurane anesthesia. Bone tumor development was monitored radiologically. Under sevoflurane anesthesia, a polyethylene catheter was inserted into the intrathecal space for drug administration. To assess pain, the withdrawal threshold was measured by applying a von Frey filament to the tumor cell inoculation site. The effect of intrathecal WIN 55,212-2 was investigated. Next, the WIN 55,212-2-mediated antinociception was reversed using CB1 (AM 251) and CB2 (AM 630) receptor antagonists. The intratibial injection of MRMT-1 tumor cells produced radiologically confirmed bone tumors. The paw withdrawal threshold decreased significantly (mechanical allodynia) with tumor development; however, intrathecal WIN 55,212-2 dose-dependently increased the withdrawal threshold. The antinociceptive effect of WIN 55,212-2 was reversed by both CB1 and CB2 receptor antagonists. Intrathecal WIN 55,212-2 reduced bone tumor-related pain behavior mediated via spinal CB1 and CB2 receptors. Therefore, spinal CB receptor agonists may be novel analgesics in the treatment of bone tumor pain.

Opioid receptors and opioid peptide-producing leukocytes in inflammatory pain--basic and therapeutic aspects

This review summarizes recent findings on neuro-immune mechanisms underlying opioid-mediated inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms by immune cell-derived opioid peptides. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generators of impulses relaying nociceptive information towards the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. This is in part achieved by endogenously released immune cell-derived opioid peptides within inflamed tissue. In addition, exogenous opioid receptor ligands that selectively modulate primary afferent function and do not cross the blood-brain barrier, avoid centrally mediated untoward side effects of conventional analgesics (e.g., opioids, anticonvulsants). This article discusses peripheral opioid receptors and their signaling pathways, opioid peptide-producing/secreting inflammatory cells and arising therapeutic perspectives.

Electroacupuncture increases CB2 receptor expression on keratinocytes and infiltrating inflammatory cells in inflamed skin tissues of rats

Endogenous cannabinoids and peripheral cannabinoid CB2 receptors (CB2Rs) are involved in the antinociceptive effect of electroacupuncture (EA) on inflammatory pain. However, it remains unclear about how EA affects the expression and distribution patterns of peripheral CB2Rs in inflamed skin tissues. To study this, inflammatory pain was induced by local injection of complete Freund's adjuvant into the hindpaw of rats. The mRNA and protein levels of CB2Rs were quantified by using RTPCR and Western blotting, respectively. The distribution of CB2Rs on keratinocytes and immune cells recruited to the inflamed skin tissues was determined by using double-immunofluorescence labeling. Induction of tissue inflammation significantly increased the mRNA and protein levels of CB2Rs in the skin tissue. Also, both 2 Hz and 100 Hz EA, applied to GB30 and GB34, significantly increased the mRNA and protein levels of CB2Rs in inflamed tissues compared to the sham EA group. CB2Rimmunoreactivities were mainly distributed in keratinocytes, macrophages, and T-lymphocytes in the epidermis and dermis of the inflamed skin tissue. Inflammation caused a significant increase in the number of CB2R-immunoreactive keratinocytes, macrophages, and T-lymphocytes. Furthermore, compared to the sham EA group, EA at 2 or 100 Hz significantly increased the number of keratinocytes, macrophages, and T-lymphocytes with CB2R-immunoreactivity in the inflamed skin tissue. Therefore, our findings suggest that EA is associated with upregulation of local CB2Rs in the inflamed skin tissue. EA primarily potentiates the expression of CB2Rs on keratinocytes and infiltrating inflammatory cells at the site of inflammation.

PERSPECTIVE

This study shows that electroacupuncture increases the CB2 receptor expression on keratinocytes and infiltrating inflammatory cells in inflammatory skin tissues. This finding provides new evidence showing the potential role of CB2 receptors in the analgesic effect of acupuncture on inflammatory pain.

Spinal and peripheral analgesic effects of the CB2 cannabinoid receptor agonist AM1241 in two models of bone cancer-induced pain

BACKGROUND AND PURPOSE

The activation of CB(2) receptors induces analgesia in experimental models of chronic pain. The present experiments were designed to study whether the activation of peripheral or spinal CB(2) receptors relieves thermal hyperalgesia and mechanical allodynia in two models of bone cancer pain.

EXPERIMENTAL APPROACH

NCTC 2472 osteosarcoma or B16-F10 melanoma cells were intratibially inoculated to C3H/He and C57BL/6 mice. Thermal hyperalgesia was assessed by the unilateral hot plate test and mechanical allodynia by the von Frey test. AM1241 (CB(2) receptor agonist), AM251 (CB(1) receptor antagonist), SR144528 (CB(2) receptor antagonist) and naloxone were used. CB(2) receptor expression was measured by Western blot.

KEY RESULTS

AM1241 (0.3-10 mg.kg(-1)) abolished thermal hyperalgesia and mechanical allodynia in both tumour models. The antihyperalgesic effect was antagonized by subcutaneous, intrathecal or peri-tumour administration of SR144528. In contrast, the antiallodynic effect was inhibited by systemic or intrathecal, but not peri-tumour, injection of SR144528. The effects of AM1241 were unchanged by AM251 but were prevented by naloxone. No change in CB(2) receptor expression was found in spinal cord or dorsal root ganglia.

CONCLUSIONS AND IMPLICATIONS

Spinal CB(2) receptors are involved in the antiallodynic effect induced by AM1241 in two neoplastic models while peripheral and spinal receptors participate in the antihyperalgesic effects. Both effects were mediated by endogenous opiates. The use of drugs that activate CB(2) receptors could be a useful strategy to counteract bone cancer-induced pain symptoms.

Antidepressant treatment of neuropathic pain: looking for the mechanism

Neuropathic pain arises as a direct consequence of a lesion or disease affecting the somatosensory system. Among the recommended first-line treatments are antidepressant drugs – that is, molecules that were initially developed to treat other disorders of the nervous system. While their clinical efficacy against neuropathic pain was established more than 30 years ago, there is little information on the mechanism underlying their antidepressant action. However, understanding the therapeutic mechanism of these treatments could help to improve them, or even lead to new therapeutic approaches. In this article, we discuss the difficulties in conducting relevant preclinical research on neuropathic pain treatment with antidepressant drugs and we present the most recent findings on the putative mechanism, which highlight the role of β2-adrenoceptors and δ-opioid receptors.

Intact cutaneous C fibre afferent properties in mechanical and cold neuropathic allodynia

Patients with neuropathy, report changes in sensory perception, particularly mechanical and thermal allodynia, and spontaneous pain. Similar sensory changes are seen in experimental neuropathies, in which alteration in primary afferent properties can also be determined. The neural correlate of spontaneous pain is ongoing activity in sensory afferents. Mechanical and heat allodynia are thought to result from lowered activation thresholds in primary afferent and/or central neurones, but the mechanisms underlying cold allodynia are very poorly understood.

We investigated nociceptive behaviours and the properties of C and A fibre intact afferents running adjacent to damaged afferents following a partial ligation injury of the saphenous nerve (PSNI). Animals developed mechanical and cold allodynia by 3 days after PSNI. Intact mechanosensitive C fibre afferents developed ongoing activity, and had slower conduction velocities 3 and 7 days following nerve injury, with no change in mechanical threshold. There was a large increase (∼46-fold) in calculated afferent input 3 days after nerve injury, as a result of the ongoing activity in these fibres. Mechano-cooling-sensitive C fibre afferents showed both enhanced cooling-evoked firing, and increased ongoing activity. The afferent barrage associated with mechano-cooling-sensitive afferents was increased 26-fold 7 days after nerve injury. We observed no differences in the properties of intact A fibre mechanosensitive afferents.

These studies demonstrate for the first time that the altered nociception seen after PSNI is associated with ongoing activity and enhanced cooling-evoked activity in intact C fibre afferents in the saphenous nerve, with no concurrent alteration in A fibre afferents.

Cannabinoid-opioid interactions during neuropathic pain and analgesia

Opiates and exogenous cannabinoids, both potent analgesics used for the treatment of patients with neuropathic pain, bind to and activate class A G-protein-coupled receptors (GPCRs). Several lines of evidence have recently suggested that opioid and cannabinoid receptors can functionally interact in the central nervous system (CNS). These interactions may be direct, such as through receptor heteromerization, or indirect, such as through signaling cross-talk that includes agonist-mediated release and/or synthesis of endogenous ligands that can activate downstream receptors. Interactions between opioid and cannabinoid receptors may mediate many of the behavioral phenomena associated with the use of these drugs, including the production of acute antinociception and the development of tolerance and cross-tolerance to the antinociceptive effects of opioid and cannabinoid-specific ligands. This review summarizes behavioral, anatomical, and molecular data characterizing these interactions during the development of neuropathic pain and during antinociceptive treatment with these drugs alone or in combination. These studies are critical for understanding how the receptor systems involved in pain relief are altered during acute or chronic pain, and for designing better antinociceptive drug therapies, such as the combined use of opioid and cannabinoid receptor agonists or selective activation of receptor heteromers, that directly target the altered neurophysiology of patients experiencing pain.

The endocannabinoid system and pain

The therapeutic potential of cannabinoids has been the topic of extensive investigation following the discovery of cannabinoid receptors and their endogenous ligands. Cannabinoid receptors and their endogenous ligands are present at supraspinal, spinal and peripheral levels. Cannabinoids suppress behavioral responses to noxious stimulation and suppress nociceptive processing through activation of cannabinoid CB(1) and CB(2) receptor subtypes. Endocannabinoids, the brain's own cannabis-like substances, share the same molecular target as Delta(9)-tetrahydrocannabinol, the main psychoactive component in cannabis. Endocannabinoids serve as synaptic circuit breakers and regulate multiple physiological and pathological conditions, e.g. regulation of food intake, immunomodulation, inflammation, analgesia, cancer, addictive behavior, epilepsy and others. This review will focus on uncovering the roles of anandamide and 2-arachidonoylglycerol, the two best characterized endocannabinoids identified to date, in controlling nociceptive responding. The roles of anandamide and 2-arachidonoylglycerol, released under physiological conditions, in modulating nociceptive responding at different levels of the neuraxis will be emphasized in this review. Effects of modulation of endocannabinoid levels through inhibition of endocannabinoid hydrolysis and uptake is also compared with effects of exogenous administration of synthetic endocannabinoids in acute, inflammatory and neuropathic pain models. Finally, the therapeutic potential of the endocannabinoid signaling system is discussed in the context of identifying novel pharmacotherapies for the treatment of pain.

2,3-Dihydro-1-benzofuran derivatives as a series of potent selective cannabinoid receptor 2 agonists: design, synthesis, and binding mode prediction through ligand-steered modeling

We recently discovered and reported a series of N-alkyl-isatin acylhydrazone derivatives that are potent cannabinoid receptor 2 (CB(2)) agonists. In an effort to improve the druglike properties of these compounds and to better understand and improve the treatment of neuropathic pain, we designed and synthesized a new series of 2,3-dihydro-1-benzofuran derivatives bearing an asymmetric carbon atom that behave as potent selective CB(2) agonists. We used a multidisciplinary medicinal chemistry approach with binding mode prediction through ligand-steered modeling. Enantiomer separation and configuration assignment were carried out for the racemic mixture for the most selective compound, MDA7 (compound 18). It appeared that the S enantiomer, compound MDA104 (compound 33), was the active enantiomer. Compounds MDA42 (compound 19) and MDA39 (compound 30) were the most potent at CB(2). MDA42 was tested in a model of neuropathic pain and exhibited activity in the same range as that of MDA7. Preliminary ADMET studies for MDA7 were performed and did not reveal any problems.

Behavioral models of pain states evoked by physical injury to the peripheral nerve

Physical injury or compression of the root, dorsal root ganglion, or peripheral sensory axon leads to well-defined changes in biology and function. Behaviorally, humans report ongoing painful dysesthesias and aberrations in function, such that an otherwise innocuous stimulus will yield a pain report. These behavioral reports are believed to reflect the underlying changes in nerve function after injury, wherein increased spontaneous activity arises from the neuroma and dorsal root ganglion and spinal changes increase the response of spinal projection neurons. These pain states are distinct from those associated with tissue injury and pose particular problems in management. To provide for developing an understanding of the underlying mechanisms of these pain states and to promote development of therapeutic agents, preclinical models involving section, compression, and constriction of the peripheral nerve or compression of the dorsal root ganglion have been developed. These models give rise to behaviors, which parallel those observed in the human after nerve injury. The present review considers these models and their application.

Cannabinoids as pharmacotherapies for neuropathic pain: from the bench to the bedside

Neuropathic pain is a debilitating form of chronic pain resulting from nerve injury, disease states, or toxic insults. Neuropathic pain is often refractory to conventional pharmacotherapies, necessitating validation of novel analgesics. Cannabinoids, drugs that share the same target as Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the psychoactive ingredient in cannabis, have the potential to address this unmet need. Here, we review studies evaluating cannabinoids for neuropathic pain management in the clinical and preclinical literature. Neuropathic pain associated with nerve injury, diabetes, chemotherapeutic treatment, human immunodeficiency virus, multiple sclerosis, and herpes zoster infection is considered. In animals, cannabinoids attenuate neuropathic nociception produced by traumatic nerve injury, disease, and toxic insults. Effects of mixed cannabinoid CB(1)/CB(2) agonists, CB(2) selective agonists, and modulators of the endocannabinoid system (i.e., inhibitors of transport or degradation) are compared. Effects of genetic disruption of cannabinoid receptors or enzymes controlling endocannabinoid degradation on neuropathic nociception are described. Specific forms of allodynia and hyperalgesia modulated by cannabinoids are also considered. In humans, effects of smoked marijuana, synthetic Delta(9)-THC analogs (e.g., Marinol, Cesamet) and medicinal cannabis preparations containing both Delta(9)-THC and cannabidiol (e.g., Sativex, Cannador) in neuropathic pain states are reviewed. Clinical studies largely affirm that neuropathic pain patients derive benefits from cannabinoid treatment. Subjective (i.e., rating scales) and objective (i.e., stimulus-evoked) measures of pain and quality of life are considered. Finally, limitations of cannabinoid pharmacotherapies are discussed together with directions for future research.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Animal models of pain: progress and challenges

Many are frustrated with the lack of translational progress in the pain field, in which huge gains in basic science knowledge obtained using animal models have not led to the development of many new clinically effective compounds. A careful re-examination of animal models of pain is therefore warranted. Pain researchers now have at their disposal a much wider range of mutant animals to study, assays that more closely resemble clinical pain states, and dependent measures beyond simple reflexive withdrawal. However, the complexity of the phenomenon of pain has made it difficult to assess the true value of these advances. In addition, pain studies are importantly affected by a wide range of modulatory factors, including sex, genotype and social communication, all of which must be taken into account when using an animal model.

Emerging strategies for exploiting cannabinoid receptor agonists as medicines

Medicines that activate cannabinoid CB(1) and CB(2) receptor are already in the clinic. These are Cesamet (nabilone), Marinol (dronabinol; Delta(9)-tetrahydrocannabinol) and Sativex (Delta(9)-tetrahydrocannabinol with cannabidiol). The first two of these medicines can be prescribed to reduce chemotherapy-induced nausea and vomiting. Marinol can also be prescribed to stimulate appetite, while Sativex is prescribed for the symptomatic relief of neuropathic pain in adults with multiple sclerosis and as an adjunctive analgesic treatment for adult patients with advanced cancer. One challenge now is to identify additional therapeutic targets for cannabinoid receptor agonists, and a number of potential clinical applications for such agonists are mentioned in this review. A second challenge is to develop strategies that will improve the efficacy and/or the benefit-to-risk ratio of a cannabinoid receptor agonist. This review focuses on five strategies that have the potential to meet either or both of these objectives. These are strategies that involve: (i) targeting cannabinoid receptors located outside the blood-brain barrier; (ii) targeting cannabinoid receptors expressed by a particular tissue; (iii) targeting up-regulated cannabinoid receptors; (iv) targeting cannabinoid CB(2) receptors; or (v) 'multi-targeting'. Preclinical data that justify additional research directed at evaluating the clinical importance of each of these strategies are also discussed.

Opioids and sensory nerves

This chapter reviews the expression and regulation of opioid receptors in sensory neurons and the interactions of these receptors with endogenous and exogenous opioid ligands. Inflammation of peripheral tissues leads to increased synthesis and axonal transport of opioid receptors in dorsal root ganglion neurons. This results in opioid receptor upregulation and enhanced G protein coupling at peripheral sensory nerve terminals. These events are dependent on neuronal electrical activity, and on production of proinflammatory cytokines and nerve growth factor within the inflamed tissue. Together with the disruption of the perineurial barrier, these factors lead to an enhanced analgesic efficacy of peripherally active opioids. The major local source of endogenous opioid ligands (e.g. beta-endorphin) is leukocytes. These cells contain and upregulate signal-sequence-encoding messenger RNA of the beta-endorphin precursor proopiomelanocortin and the entire enzymatic machinery necessary for its processing into the functionally active peptide. Opioid-containing immune cells extravasate using adhesion molecules and chemokines to accumulate in inflamed tissues. Upon stressful stimuli or in response to releasing agents such as corticotropin-releasing factor, cytokines, chemokines, and catecholamines, leukocytes secrete opioids. Depending on the cell type, this release is contingent on extracellular Ca(2+) or on inositol triphosphate receptor triggered release of Ca(2+) from endoplasmic reticulum. Once secreted, opioid peptides activate peripheral opioid receptors and produce analgesia by inhibiting the excitability of sensory nerves and/or the release of proinflammatory neuropeptides. These effects occur without central untoward side effects such as depression of breathing, clouding of consciousness, or addiction. Future aims include the development of peripherally restricted opioid agonists, selective targeting of opioid-containing leukocytes to sites of painful injury, and the augmentation of peripheral opioid peptide and receptor synthesis.

Selective activation of cannabinoid CB2 receptors suppresses neuropathic nociception induced by treatment with the chemotherapeutic agent paclitaxel in rats

Activation of cannabinoid CB(2) receptors suppresses neuropathic pain induced by traumatic nerve injury. The present studies were conducted to evaluate the efficacy of cannabinoid CB(2) receptor activation in suppressing painful peripheral neuropathy evoked by chemotherapeutic treatment with the antitumor agent paclitaxel. Rats received paclitaxel (2 mg/kg i.p./day) on 4 alternate days to induce mechanical hypersensitivity (mechanical allodynia). Mechanical allodynia was defined as a lowering of the threshold for paw withdrawal to stimulation of the plantar hind paw surface with an electronic von Frey stimulator. Mechanical allodynia developed in paclitaxel-treated animals relative to groups receiving the Cremophor EL/ethanol/saline vehicle at the same times. Two structurally distinct cannabinoid CB(2) agonists, the aminoalkylindole (R,S)-AM1241 [(R,S)-(2-iodo-5-nitrophenyl)-[1-((1-methyl-piperidin-2-yl)methyl)-1H-indol-3-yl]-methanone] and the cannabilactone AM1714 (1,9-dihydroxy-3-(1',1'-dimethylheptyl)-6H-benzo[c]chromene-6-one), produced a dose-related suppression of established paclitaxel-evoked mechanical allodynia after systemic administration. Pretreatment with the CB(2) antagonist SR144528 [5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-N-(1,3,3-trimethylbicyclo[2.2.1]heptan-2-yl)-1H-pyrazole-3-carboxamide], but not the CB(1) antagonist SR141716 [5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-N-(piperidin-1-yl)-1H-pyrazole-3-carboxamide], blocked the antiallodynic effects of both (R,S)-AM1241 and AM1714. Moreover, (R)-AM1241, but not (S)-AM1241, suppressed paclitaxel-evoked mechanical allodynia relative to either vehicle treatment or preinjection thresholds, consistent with mediation by CB(2). Administration of either the CB(1) or CB(2) antagonist alone failed to alter paclitaxel-evoked mechanical allodynia. Moreover, (R,S)-AM1241 did not alter paw withdrawal thresholds in rats that received the Cremophor EL vehicle in lieu of paclitaxel, whereas AM1714 induced a modest antinociceptive effect. Our data suggest that cannabinoid CB(2) receptors may be important therapeutic targets for the treatment of chemotherapy-evoked neuropathy.

MDA7: a novel selective agonist for CB2 receptors that prevents allodynia in rat neuropathic pain models

BACKGROUND AND PURPOSE

There is growing interest in using cannabinoid type 2 (CB(2)) receptor agonists for the treatment of neuropathic pain. In this report, we describe the pharmacological characteristics of MDA7 (1-[(3-benzyl-3-methyl-2,3-dihydro-1-benzofuran-6-yl)carbonyl]piperidine), a novel CB(2) receptor agonist.

EXPERIMENTAL APPROACH

We characterized the pharmacological profile of MDA7 by using radioligand-binding assays and in vitro functional assays at human cannabinoid type 1 (CB(1)) and CB(2) receptors. In vitro functional assays were performed at rat CB(1) and CB(2) receptors. The effects of MDA7 in reversing neuropathic pain were assessed in spinal nerve ligation and paclitaxel-induced neuropathy models in rats.

KEY RESULTS

MDA7 exhibited selectivity and agonist affinity at human and rat CB(2) receptors. MDA7 treatment attenuated tactile allodynia produced by spinal nerve ligation or by paclitaxel in a dose-related manner. These effects were selectively antagonized by a CB(2) receptor antagonist but not by CB(1) or opioid receptor antagonists. MDA7 did not affect rat locomotor activity.

CONCLUSION AND IMPLICATIONS

MDA7, a novel selective CB(2) agonist, was effective in suppressing neuropathic nociception in two rat models without affecting locomotor behaviour. These results confirm the potential for CB(2) agonists in the treatment of neuropathic pain.

Modulation of the anti-nociceptive effects of 2-arachidonoyl glycerol by peripherally administered FAAH and MGL inhibitors in a neuropathic pain model

BACKGROUND AND PURPOSE

There are limited options for the treatment of neuropathic pain. Endocannabinoids, such as anandamide and 2-arachidonoyl glycerol (2-AG), are promising pain modulators and there is recent evidence of interactions between anandamide and 2-AG biosynthesis and metabolism. It has been clearly demonstrated that 2-AG degradation is mainly catalysed not only by monoacylglycerol lipase (MGL) but also by a fatty acid amide hydrolase (FAAH). Inhibitors specifically targeting these two enzymes have also been described: URB602 and URB597, respectively. However, the anti-nociceptive effects of the combination of peripherally injected 2-AG, URB602 and URB597 in a neuropathic pain model have not yet been determined. This was performed in the presence or absence of cannabinoid CB(1) (AM251) and CB(2) (AM630) receptor antagonists.

EXPERIMENTAL APPROACH

Mechanical allodynia and thermal hyperalgesia were evaluated in 213 male Wistar rats allocated to 32 different groups. Drugs were injected subcutaneously in the dorsal surface of the hind paw (50 muL) 15 min before pain tests.

KEY RESULTS

2-AG, URB602 and URB597 significantly decreased mechanical allodynia and thermal hyperalgesia with ED50 of 1.6+/-1.5 and 127+/-83 mug for 2-AG and URB602, respectively. These effects were mediated locally and were mostly inhibited by the two cannabinoid antagonists.

CONCLUSIONS AND IMPLICATIONS

The combination of the three compounds did not produce any greater anti-allodynic or anti-hyperalgesic effects, suggesting that FAAH inhibition could reduce or limit the anti-nociceptive effects of 2-AG. Peripheral administration of endocannabinoids or MGL/FAAH inhibitors is a promising analgesic approach requiring further investigation.

Evidence for a novel functional role of cannabinoid CB(2) receptors in the thalamus of neuropathic rats

Cannabinoid CB₁ receptors have analgesic effects in models of neuropathic pain, but can also produce psychoactive side-effects. A supraspinal location of CB₂ receptors has recently been described. CB₂ agonists are also antinociceptive, although the functional role of supraspinal CB₂ receptors in the control of nociception is unknown. Herein, we provide evidence that CB₂ receptors in the thalamus play a functional role in the modulation of responses of neurons in the ventral posterior nucleus (VPL) of the thalamus in neuropathic, but not sham-operated, rats. Spontaneous and mechanically evoked activity of VPL neurons was recorded with a multichannel electrode array in anaesthetized spinal nerve-ligated (SNL) rats and compared to sham-operated rats. Intra-VPL administration of the CB₂ agonist JWH-133 (30 ng in 500 nL) significantly reduced spontaneous (P < 0.05), non-noxious (P < 0.001) and noxious (P < 0.01) mechanically evoked responses of VPL neurons in SNL rats, but not in sham-operated rats. Inhibitory effects of JWH-133 on spontaneous (P < 0.01) and noxious-evoked (P < 0.001) responses of neurons were blocked by the CB₂ antagonist SR144528. Local administration of SR144528 alone did not alter spontaneous or evoked responses of VPL neurons, but increased burst activity of VPL neurons in SNL rats. There were, however, no differences in levels of the endocannabinoids anandamide and 2AG in the thalamus of SNL and sham-operated rats. These data suggest that supraspinal CB₂ receptors in the thalamus may contribute to the modulation of neuropathic pain responses.

Characterization of a model of persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR)

Various surgical procedures, e.g. thoracotomy and inguinal hernia repair, frequently evoke persistent pain lasting for many months following the initial surgery. The essential prolonged tissue retraction required during such surgeries may account for the persistence and high incidence of postoperative pain in these patient populations. This study describes a new rat model of persistent postoperative pain evoked by skin/muscle incision and retraction (SMIR), akin to a clinical procedure. Under anaesthesia, skin and superficial muscle of the medial thigh were incised and a small pair of retractors inserted. This tissue was retracted for 1h causing potential stretch of the saphenous nerve. SMIR surgery evoked persistent significant mechanical hypersensitivity to von Frey stimulation of the plantar ipsilateral hindpaw, compared to either pre-surgery responses or concurrent responses of sham-operated rats. SMIR-evoked mechanical hypersensitivity was observed by postoperative day 3, most prominent between postoperative days 10 and 13, persisted until at least postoperative day 22 and had dissipated by postoperative day 32. Overall, mechanical sensitivity of the SMIR contralateral paw and the sham ipsilateral paw did not significantly change from pre-surgery responses. SMIR did not evoke significant heat hyperalgesia or cold allodynia. Light microscopy of saphenous nerve sections did not show degeneration or oedema in the saphenous nerve at, or proximally or distally to, the surgical site. In addition, very little to no degeneration was detected with ATF3 staining in DRG from SMIR-operated rats. These data suggest that prolonged retraction of superficial tissue evokes a persistent pain syndrome that is not driven by neuronal damage.

Involvement of central cannabinoid CB2 receptor in reducing mechanical allodynia in a mouse model of neuropathic pain

We sought to examine the involvement of central cannabinoid CB2 receptor activation in modulating mechanical allodynia in a mouse model of neuropathic pain. JWH133 was demonstrated to be a selective cannabinoid CB2 receptor agonist in mice, reducing forskolin-stimulated cAMP production in CHO cells expressing mouse cannabinoid CB2 and cannabinoid CB1 receptors with EC50 values of 63 nM and 2500 nM, respectively. Intrathecal administration of JWH133 (50 and 100 nmol/mouse) significantly reversed partial sciatic nerve ligation-induced mechanical allodynia in mice at 0.5 h after administration. In contrast, systemic (intraperitoneal) or local (injected to the dorsal surface of the hindpaw) administration of JWH133 (100 nmol/mouse) was ineffective. Furthermore, the analgesic effects of intrathecal JWH133 (100 nmol/mouse) were absent in cannabinoid CB2 receptor knockout mice. These results suggest that the activation of central, but not peripheral, cannabinoid CB2 receptors play an important role in reducing mechanical allodynia in a mouse model of neuropathic pain.

Cannabinoid CB2 receptors: a therapeutic target for the treatment of inflammatory and neuropathic pain

Cannabinoids suppress behavioural responses to noxious stimulation and suppress nociceptive transmission through activation of CB1 and CB2 receptor subtypes. CB1 receptors are expressed at high levels in the central nervous system (CNS), whereas CB2 receptors are found predominantly, but not exclusively, outside the CNS. CB2 receptors are also upregulated in the CNS and dorsal root ganglia by pathological pain states. Here, we review behavioural, neurochemical and electrophysiological data, which identify cannabinoid CB2 receptors as a therapeutic target for treating pathological pain states with limited centrally, mediated side effects. The development of CB2-selective agonists (with minimal affinity for CB1) as well as mutant mice lacking CB2 receptors has provided pharmacological and genetic tools required to evaluate the effectiveness of CB2 agonists in suppressing persistent pain states. This review will examine the efficacy of cannabinoid CB2-selective agonists in suppressing acute, inflammatory and neuropathic nociception following systemic and local routes of administration. Data derived from behavioural, neurochemical and neurophysiological approaches are discussed to better understand the relationship between antinociceptive effects induced by CB2-selective agonists in behavioural studies and neural mechanisms of pain suppression. Finally, the therapeutic potential and possible limitations of CB2-based pharmacotherapies for pathological pain states induced by tissue and nerve injury are discussed.

Modulation of pain transmission by G-protein-coupled receptors

The heterotrimeric G-protein-coupled receptors (GPCR) represent the largest and most diverse family of cell surface receptors and proteins. GPCR are widely distributed in the peripheral and central nervous systems and are one of the most important therapeutic targets in pain medicine. GPCR are present on the plasma membrane of neurons and their terminals along the nociceptive pathways and are closely associated with the modulation of pain transmission. GPCR that can produce analgesia upon activation include opioid, cannabinoid, alpha2-adrenergic, muscarinic acetylcholine, gamma-aminobutyric acidB (GABAB), groups II and III metabotropic glutamate, and somatostatin receptors. Recent studies have led to a better understanding of the role of these GPCR in the regulation of pain transmission. Here, we review the current knowledge about the cellular and molecular mechanisms that underlie the analgesic actions of GPCR agonists, with a focus on their effects on ion channels expressed on nociceptive sensory neurons and on synaptic transmission at the spinal cord level.

Activation of cannabinoid CB1 and CB2 receptors suppresses neuropathic nociception evoked by the chemotherapeutic agent vincristine in rats

BACKGROUND AND PURPOSE

The ability of cannabinoids to suppress mechanical hypersensitivity (mechanical allodynia) induced by treatment with the chemotherapeutic agent vincristine was evaluated in rats. Sites of action were subsequently identified.

EXPERIMENTAL APPROACH

Mechanical hypersensitivity developed over the course of ten daily injections of vincristine relative to groups receiving saline at the same times. Effects of the CB1/CB2 receptor agonist WIN55,212-2, the receptor-inactive enantiomer WIN55,212-3, the CB2-selective agonist (R,S)-AM1241, the opiate agonist morphine and vehicle on chemotherapy-induced neuropathy were evaluated. WIN55,212-2 was administered intrathecally (i.t.) or locally in the hindpaw to identify sites of action. Pharmacological specificity was established using competitive antagonists for CB1 (SR141716) or CB2 receptors (SR144528).

KEY RESULTS

Systemic administration of WIN55,212-2, but not WIN55,212-3, suppressed vincristine-evoked mechanical allodynia. A leftward shift in the dose-response curve was observed following WIN55,212-2 relative to morphine treatment. The CB1 (SR141716) and CB2 (SR144528) antagonists blocked the anti-allodynic effects of WIN55,212-2. (R,S)-AM1241 suppressed vincristine-induced mechanical hypersensitivity through a CB2 mechanism. Both cannabinoid agonists suppressed vincristine-induced mechanical hypersensitivity without inducing catalepsy. Spinal sites of action are implicated in cannabinoid modulation of chemotherapy-induced neuropathy. WIN55,212-2, but not WIN55,212-3, administered i.t. suppressed vincristine-evoked mechanical hypersensitivity at doses that were inactive following local hindpaw administration. Spinal coadministration of both the CB1 and CB2 antagonists blocked the anti-allodynic effects of WIN55,212-2.

CONCLUSIONS AND IMPLICATIONS

Cannabinoids suppress the maintenance of vincristine-induced mechanical allodynia through activation of CB1 and CB2 receptors. These anti-allodynic effects are mediated, at least in part, at the level of the spinal cord.

Cannabinoid CB2 receptor-mediated anti-nociception in models of acute and chronic pain

The endocannabinoid system consists of cannabinoid CB(1) and CB(2) receptors, endogenous ligands and their synthesising/metabolising enzymes. Cannabinoid receptors are present at key sites involved in the relay and modulation of nociceptive information. The analgesic effects of cannabinoids have been well documented. The usefulness of nonselective cannabinoid agonists can, however, be limited by psychoactive side effects associated with activation of CB(1) receptors. Following the recent evidence for CB(2) receptors existing in the nervous system and reports of their up-regulation in chronic pain states and neurodegenerative diseases, much research is now aimed at shedding light on the role of the CB(2) receptor in human disease. Recent studies have demonstrated anti-nociceptive effects of selective CB(2) receptor agonists in animal models of pain in the absence of CNS side effects. This review focuses on the analgesic potential of CB(2) receptor agonists for inflammatory, post-operative and neuropathic pain states and discusses their possible sites and mechanisms of action.

Levels of neuropeptides nocistatin, nociceptin/orphanin FQ and their precursor protein in a rat neuropathic pain model

Neuropeptides nociceptin/orphanin FQ (N/OFQ) and nocistatin (NST) are related to pain modulation. The amounts of these peptides and their precursor protein, prepronociceptin (ppN/OFQ) in the brain, spinal cord and serum samples of rats with partial sciatic nerve ligation (PSNL) were compared with those in naïve rats using radioimmunoassay (RIA). There was a significant rise in the levels of ppN/OFQ, N/OFQ and NST in the brains of PSNL rats. Their spinal cords showed significantly increased ppN/OFQ and NST levels but no change in N/OFQ levels. The PSNL rats also had increased serum NST (statistically significant) and N/OFQ (statistically insignificant) with decreased ppN/OFQ suggesting important roles of these peptides in neuropathic pain mechanism.

The Antinociceptive Effects of Local Injections of Propofol in Rats Are Mediated in Part by Cannabinoid CB1 and CB2 Receptors

BACKGROUND

Propofol can inhibit fatty acid amidohydrolase, the enzyme responsible for the metabolism of anandamide (an endocannabinoid). To study the potential antinociceptive effect of propofol, we administered different doses (0.005, 0.05, 0.5, 5, and 500 microg) of the anesthetic in the hind paw of animals to determine an ED50. To further investigate the mechanisms by which propofol produced its antinociceptive effect, we used specific antagonists for the cannabinoid CB1 (AM251) and CB2 (AM630) receptors and measured fatty-acid amide/endocannabinoid (anandamide, 2-arachidonylglycerol, and palmitoylethanolamide) concentrations in skin paw tissues.

METHODS

Formalin tests were performed on 65 Wistar rats allocated to six different groups: 1) control (Intralipidtrade mark 10%); 2) propofol (ED50 dose); 3) AM251; 4) AM251 + propofol; 5) AM630; 6) AM630 + propofol. Drugs were injected subcutaneously in the dorsal surface of the hind paw (50 microL) 15 min before 2.5% formalin injection into the same paw. Fatty-acid amide/endocannabinoid levels were measured by high performance liquid chromatography/mass spectrometry analysis.

RESULTS

Propofol produced a dose-dependent antinociceptive effect for the early and late phases of the formalin test with an ED50 of 0.08 +/- 0.061 microg for the latter phase. This effect was antagonized by AM251 and AM630. It was locally mediated, since a higher dose of propofol given in the contralateral paw was not antinociceptive. Finally, only paw concentrations of palmitoylethanolamide were significantly increased.

CONCLUSION

In a test of inflammatory pain, locally injected propofol decreased pain behavior in a dose-dependent manner. This antinociceptive effect was mediated, in part, by CB1 and CB2 receptors.

Pre-emptive antinociceptive effects of a synthetic cannabinoid in a model of neuropathic pain

The antinociceptive effects of WIN55,212-2, a synthetic cannabinoid, were evaluated in the model of partial sciatic nerve ligation after daily subcutaneous administration of 0.1 mg/kg a week before and two weeks after surgery. Mechanical allodynia and thermal hyperalgesia were evaluated in 46 rats allocated to receive: (1) Vehicle (before surgery)-Vehicle (after surgery); (2) Vehicle-WIN55,212-2; (3) WIN55,212-2-Vehicle; (4) WIN55,212-2-WIN55,212-2; (5) AM251+vehicle; (6) AM251+WIN55,212-2; (7) AM630+vehicle; (8) AM630+WIN55,212-2; (9) Sham receiving vehicle; and (10) Sham receiving WIN55,212-2. The decreased in mechanical allodynia and thermal hyperalgesia by WIN55,212-2 was significantly greater when it was administered during one week before surgery. In conclusion, pre-emptive use of cannabinoids produced greater antinociceptive effects in a model of neuropathic pain and this effect is mediated by cannabinoid CB(1) and CB(2) receptors.

Refinement of Symptoms of Neuropathic Pain Measurements After Various Transections of the Nerve Endings of the Sciatic and Femoral Nerve in Rats: An Exploratory Behavioral Analysis

BACKGROUND

Many animal models can be used to study the underlying pathophysiological mechanisms of neuropathic pain. Most of these models rely on a partial denervation of the limb of the animal by ligating a selected nerve. In this study, we performed nerve lesions on three peripheral nerves supplying the plantar side of the rat hindpaw by differentially transecting the saphenous, the tibial, and the sural nerves alone or in paired combinations.

METHODS

The development of neuropathic pain symptoms at three different anatomical areas (medial, central, and lateral) of the glabrous skin of the hindpaw was evaluated by sensory testing over a 12-wk period. Mechanical hyperalgesia (pinprick), cold allodynia (acetone), and abnormalities of hindpaw posture were continuously present in animals with tibial and tibial and saphenous nerve transection.

RESULTS

Transection of the tibial and sural nerves induced cold allodynia and moderate mechanical hyperalgesia. Transection of the sural, the saphenous, or both nerves simultaneously induced no signs of specific neuropathic pain behavior and no abnormalities in posture of the affected hindpaw were noted after adequate stimulation.

CONCLUSIONS

The overlapping innervation of nerve distribution can complicate the interpretation of nerve ligation studies of peripheral neuropathies.

Characterization of tests of functional recovery after median and ulnar nerve injury and repair in the rat forelimb

The majority of human peripheral nerve injuries occur in the upper limb but the majority of studies in the rat are performed in the hindlimb. The upper and lower limbs differ in dexterity and control by supraspinal systems, so an upper limb model is a better representation of the common form of human injury. The purpose of this study was to further develop a rat model involving lesions of the median and ulnar nerves. To produce different degrees of misdirection of axons following nerve repair, we studied nerve crush, cut and repair of the two nerves, and cut and repair with crossover. Assessment of functional recovery was performed using a battery of motor and sensory tests: the staircase test, which assesses skilled forepaw reaching; grip strength meter, which assesses grip strength; pawprint analysis, which assesses toe spread and print length; horizontal ladder, which assesses forepaw placement during skilled locomotion; modified Randall-Selitto device and electronic von Frey probes, which assess fine touch; and cold probes, which assess temperature sensation. All tests revealed deficits in forepaw function after nerve injury except the print length and modified Randall-Selitto device. The time course of functional recovery was observed over 15 weeks. The final degree of functional recovery achieved was related to the misdirection of axon regeneration. The tests that most clearly revealed the effects of axon misdirection on function were the skilled paw reaching and grip strength tests. The lesion model and functional tests that we have developed will be useful in testing therapeutic strategies for treating the consequences of inaccurate axon regeneration following peripheral nerve injury in humans.

Blocking sodium channels to treat neuropathic pain

Neuropathic pain remains a large unmet medical need. A number of therapeutic options exist, but efficacy and tolerability are less than satisfactory. Based on animal models and limited data from human patients, the pain and hypersensitivity that characterize neuropathic pain are associated with spontaneous discharges of normally quiescent nociceptors. Sodium channel blockers inhibit this spontaneous activity, reverse nerve injury-induced pain behavior in animals and alleviate neuropathic pain in humans. Several sodium channel subtypes are expressed primarily in sensory neurons and may contribute to the efficacy of sodium channel blockers. In this report, the authors review the current understanding of the role of sodium channels and of specific sodium channel subtypes in neuropathic pain signaling.

The antinociceptive effects of intraplantar injections of 2-arachidonoyl glycerol are mediated by cannabinoid CB2 receptors

BACKGROUND AND PURPOSE

2-arachidonoyl glycerol (2-AG) is an endogenous cannabinoid with central antinociceptive properties. Its degradation is catalysed by monoacylglycerol lipase (MGL) whose activity is inhibited by URB602, a new synthetic compound. The peripheral antinociceptive effects of 2-AG and URB602 in an inflammatory model of pain are not yet determined. We have evaluated these effects with and without the cannabinoid CB(1) (AM251) and CB(2) (AM630) receptor antagonists.

EXPERIMENTAL APPROACH

Inflammation was induced in rat hind paws by intraplantar injection of formalin. Nociception was assessed behaviourally over the next 60 min, in 19 experimental groups: (1) control; (2-6) 2-AG (0.01-100 microg); (7) AM251 (80 microg); (8) AM251+2-AG (10 microg); (9) AM630 (25 microg); (10) AM630+2-AG (10 microg); (11-16) URB602 (0.1-500 microg); (17) 2-AG+URB602 (ED(50)); (18) AM251+URB602 (ED(50)); (19) AM630+URB602 (ED(50)). Drugs were injected s.c. in the dorsal surface of the hind paw (50 microl), 15 min before formalin injection into the same paw.

KEY RESULTS

2-AG and URB602 produced dose-dependent antinociceptive effects for the late phases of the formalin test with ED(50) of 0.65+/-0.455 mug and 68+/-14.3 microg, respectively. Their combination at ED(50) doses produced an additive antinociceptive effect. These effects were inhibited by AM630 but not by AM251 for 2-AG and by the two cannabinoid antagonists for URB602.

CONCLUSIONS AND IMPLICATIONS

Locally injected 2-AG and URB602 decreased pain behaviour in a dose-dependent manner in an inflammatory model of pain. The antinociceptive effect of 2-AG was mediated by the CB(2) receptor.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Characterization of chronic constriction of the saphenous nerve, a model of neuropathic pain in mice showing rapid molecular and electrophysiological changes

Neuropathic pain is one of the most inextricable problems encountered in clinics, because few facts are known about its etiology. Nerve injury often leads to allodynia and hyperalgesia, which are symptoms of neuropathic pain. The aim of this study was to understand some molecular and electrophysiological mechanisms of neuropathic pain after chronic constriction of the saphenous nerve (CCS) in mice. After surgery, CCS mice displayed significant allodynia and hyperalgesia, which were sensitive to acute systemic injection of morphine (4 mg/kg), gabapentin (50 mg/kg), amitriptyline (10 mg/kg), and the cannabinoid agonist WIN 55,212-2 (5 mg/kg). These behavioral changes were accompanied after surgery by an increase of c-Fos expression and by an overexpression of mu-opioid and cannabinoid CB1 and CB2 receptors in the spinal cord and the dorsal hind paw skin. In combination with the skin-nerve preparation, this model showed a decrease in functional receptive fields downstream to the injury and the apparition of A-fiber ectopic discharges. In conclusion, CCS injury induced behavioral, molecular, and electrophysiological rearrangements that might help us in better understanding the peripheral mechanisms of neuropathic pain. This model takes advantage of the possible use in the future of genetically modified mice and of an exclusively sensory nerve for a comprehensive study of peripheral mechanisms of neuropathic pain.

Antihyperalgesic effects of local injections of anandamide, ibuprofen, rofecoxib and their combinations in a model of neuropathic pain

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit fatty acid amidohydrolase (FAAH), the enzyme responsible for the metabolism of anandamide, an endocannabinoid. The analgesic interactions between anandamide (0.01 microg), ibuprofen (0.1 microg) and rofecoxib (0.1 microg) or their combinations administered locally in the hind paw of neuropathic rats were investigated together with the effects of specific antagonists for the cannabinoid CB(1) (AM251; 80 microg) and CB(2) (AM630; 25 microg) receptors. Mechanical allodynia and thermal hyperalgesia were evaluated in 108 Wistar rats allocated to: (1-4) NaCl 0.9%; anandamide; ibuprofen; rofecoxib; (5-6) anandamide+ibuprofen or rofecoxib; (7-8) AM251 or AM630; (9-10) anandamide+AM251 or AM630; (11-12) ibuprofen+AM251 or AM630; (13-14) rofecoxib+AM251 or AM630; (15-16) anandamide+ibuprofen+AM251 or AM630; (17-18) anandamide+rofecoxib+AM251 or AM630. Drugs were given subcutaneously in the hind paw 15min before pain tests. Anandamide, ibuprofen, rofecoxib and their combinations significantly decreased mechanical allodynia and thermal hyperalgesia with an ED(50) of 1.6+/-0.68ng and 1.1+/-1.09 ng for anandamide, respectively. The effects of NSAIDs were not antagonized by AM251 or AM630 but those of anandamide were inhibited by AM251 but not by AM630. In conclusion, locally injected anandamide, ibuprofen, rofecoxib and their combinations decreased pain behavior in neuropathic animals. Local use of endocannabinoids to treat neuropathic pain may be an interesting way to treat this condition without having the deleterious central effects of systemic cannabinoids.