Analgesic efficacy of small-molecule angiotensin II type 2 receptor antagonists in a rat model of antiretroviral toxic polyneuropathy.

Individuals infected with the HIV and taking certain antiretroviral drugs to suppress viral replication have a high prevalence of neuropathic pain that is not alleviated by analgesic/adjuvant drugs that are often efficacious for the relief of other types of neuropathic pain. There is therefore a great need for new analgesics to alleviate the pain of antiretroviral toxic neuropathy (ATN). Small-molecule angiotensin II type 2 receptor (AT2R) antagonists, with ≥1000-fold selectivity over the angiotensin II type 1 receptor, produced analgesia in the chronic constriction injury of the sciatic nerve rat model of peripheral nerve trauma. Hence, the present study was designed to assess their analgesic efficacy in a rat model of ATN. The analgesic efficacy of small-molecule AT2R antagonists (EMA200 and EMA300) was assessed in a rat model of dideoxycytidine (ddC)-induced ATN. Single intraperitoneal bolus doses of EMA200 (0.3-10 mg/kg) induced dose-dependent analgesia in ddC-rats; the mean ED50 was 3.2 mg/kg. Twice-daily intraperitoneal administration of EMA300 at 30 mg/kg to ddC-rats for 3 days produced significant analgesia on days 2 and 3 of the treatment period. Therefore, small-molecule AT2R antagonists should be investigated further as novel analgesics for the relief of ATN.

Pathogenesis of HIV-associated sensory neuropathy: evidence from in vivo and in vitro experimental models.

HIV-associated sensory neuropathy (HIV-SN) is a frequent neurological complication of HIV infection and its treatment with some antiretroviral drugs. We review the pathogenesis of the viral- and drug-induced causes of the neuropathy, and its primary symptom, pain, based on evidence from in vivo and in vitro models of HIV-SN. Viral coat proteins mediate nerve fibre damage and hypernociception through direct and indirect mechanisms. Direct interactions between viral proteins and nerve fibres dominate axonal pathology, while somal pathology is dominated by indirect mechanisms that occur secondary to virus-mediated activation of glia and macrophage infiltration into the dorsal root ganglia. The treatment-induced neuropathy and resulting hypernociception arise primarily from drug-induced mitochondrial dysfunction, but the sequence of events initiated by the mitochondrial dysfunction that leads to the nerve fibre damage and dysfunction are still unclear. Overall, the models that have been developed to study the pathogenesis of HIV-SN, and hypernociception associated with the neuropathy, are reasonable models and have provided useful insights into the pathogenesis of HIV-SN. As new models are developed they may ultimately lead to identification of therapeutic targets for the prevention or treatment of this common neurological complication of HIV infection.

Anxiety-like behaviour is attenuated by gabapentin, morphine and diazepam in a rodent model of HIV anti-retroviral-associated neuropathic pain.

Neuropathic pain is commonly associated with affective disorders such as anxiety and depression. We have previously characterised a rodent model of HIV, anti-retroviral-associated neuropathy in which rats develop hypersensitivity to a punctate mechanical stimulus and display anxiety-like behaviour in the open field paradigm. To assess the potential of this behavioural paradigm for the assessment of pain related co-morbidities in rodent models of pain, here we test the sensitivity of this anxiety-like behaviour to the analgesic agents gabapentin and morphine in comparison to the known anxiolytic drug diazepam. We found that gabapentin (30 mg/kg, i.p.) and morphine (2.5 mg/kg, i.p.), which reduce mechanical hypersensitivity in these rats, significantly reduces measures of thigmotaxis in the open field. The effect of gabapentin and morphine did not differ significantly from diazepam (1 mg/kg, i.p.). This study highlights the potential use of this rodent model and behavioural paradigm in the validation of the affective component of novel analgesic pharmacological targets and elucidation of underlying pathophysiological mechanisms.

Characterization of rodent models of HIV-gp120 and anti-retroviral-associated neuropathic pain.

A distal symmetrical sensory peripheral neuropathy is frequently observed in people living with Human Immunodeficiency Virus Type 1 (HIV-1). This neuropathy can be associated with viral infection alone, probably involving a role for the envelope glycoprotein gp120; or a drug-induced toxic neuropathy associated with the use of nucleoside analogue reverse transcriptase inhibitors as a component of highly active anti-retroviral therapy. In order to elucidate the mechanisms underlying drug-induced neuropathy in the context of HIV infection, we have characterized pathological events in the peripheral and central nervous system following systemic treatment with the anti-retroviral agent, ddC (Zalcitabine) with or without the concomitant delivery of HIV-gp120 to the rat sciatic nerve (gp120+ddC). Systemic ddC treatment alone is associated with a persistent mechanical hypersensitivity (33% decrease in limb withdrawal threshold) that when combined with perineural HIV-gp120 is exacerbated (48% decrease in threshold) and both treatments result in thigmotactic (anxiety-like) behaviour. Immunohistochemical studies revealed little ddC-associated alteration in DRG phenotype, as compared with known changes following perineural HIV-gp120. However, the chemokine CCL2 is significantly expressed in the DRG of rats treated with perineural HIV-gp120 and/or ddC and there is a reduction in intraepidermal nerve fibre density, comparable to that seen in herpes zoster infection. Moreover, a spinal gliosis is apparent at times of peak behavioural sensitivity that is exacerbated in gp120+ddC as compared to either treatment alone. Treatment with the microglial inhibitor, minocycline, is associated with delayed onset of hypersensitivity to mechanical stimuli in the gp120+ddC model and reversal of some measures of thigmotaxis. Finally, the hypersensitivity to mechanical stimuli was sensitive to systemic treatment with gabapentin, morphine and the cannabinoid WIN 55,212-2, but not with amitriptyline. These data suggests that both neuropathic pain models display many features of HIV- and anti-retroviral-related peripheral neuropathy. They therefore merit further investigation for the elucidation of underlying mechanisms and may prove useful for preclinical assessment of drugs for the treatment of HIV-related peripheral neuropathic pain.

Focal lysolecithin-induced demyelination of peripheral afferents results in neuropathic pain behavior that is attenuated by cannabinoids.

Demyelinating diseases can be associated with painful sensory phenomena such as tactile allodynia and hyperalgesia. To study the mechanisms underlying demyelination-induced pain, we have characterized a novel model of demyelination of the sciatic or saphenous nerve. Topical lysolecithin application causes focal demyelination of afferent nerve A-fibers without axonal loss, as assessed either by electron and light microscopy or by immunohistochemical analysis of dorsal root ganglia (DRG) for a neuronal injury marker, activating transcription factor 3. Focal demyelination is accompanied by spontaneous action potentials in afferents and increased expression of neuropeptide Y and Na(v)1.3 sodium channels specifically in DRG neurons that coexpress a specific marker of myelinated afferents. In contrast, expression of tetrodotoxin-resistant, Na(v)1.8 sodium channels is specifically decreased in the same subgroup of DRG cells. Central sensitization of somatosensory processing is also induced, with increased behavioral reflex responsiveness to thermal and mechanical stimuli. These changes are reversed by intrathecal administration of an NMDA receptor antagonist or cannabinoid (CB) receptor agonist, but not by a mu-opioid receptor agonist. Recovery of behavioral reflexes occurred approximately 3 weeks after lysolecithin treatment. This is the first time that demyelination of afferent A-fibers has been shown to specifically induce neuropathic pain and indicates that axonal damage is not a prerequisite for development of the pain state. The profile of phenotypic changes in DRG is distinct from other pain models and displays a sensitivity to NMDA and CB receptor agents that may be exploitable therapeutically.

Ultrasound vocalisation by rodents does not correlate with behavioural measures of persistent pain.

Three well-established rodent models of somatic, visceral and neuropathic pain were used to test the hypothesis that a stress and anxiety evoked behaviour, namely ultrasound vocalisation, correlates with other well-characterised indices of pain behaviour, such as limb withdrawal and stereotypical behaviour. Persistent pain presents a significant clinical problem for which there remains relatively ineffective clinical management and animal models of pain are commonly employed to investigate the underlying pathophysiology and for pre-clinical evaluation of novel therapies. At present, the assessment of such animal models largely relies on the observation of simple reflex responses which may not entirely represent the full range of rodent pain behaviour. Therefore, additional integrated behavioural indices for the quantification of pain could improve the veracity of animal models. In stressful or harmful situations, it is thought that rodents produce ultrasound vocalisations to communicate within the social group. In this study, the number of ultrasound vocalisations (22 kHz) was measured during both evoked and ongoing pain. Ultrasound vocalisation was not associated with other pain behaviour in any of the inflammatory, visceral or neuropathic pain models examined and is therefore not a useful integrated correlate of pain behaviour.

Unexplained lower limb pain in Parkinson's disease: a phenotypic variant of "painful Parkinson's disease".

UNLABELLED: Unexplained pain is a key non-motor symptom (NMS) of Parkinson's disease (PD) that is prevalent throughout the condition and often unrecognized and under treated. We have identified a cluster of patients with unexplained refractory and intrusive lower limb pain. METHODS: Patients with 'unexplained' pain were identified from a survey using a non-motor symptom questionnaire and scale (NMSQuest, NMSS). Patients reporting unexplained severe proximal lower limb pain were then assessed using a newly devised PD pain scale. Co-morbid conditions were excluded and all had lower spine imaging. RESULTS: From a cohort of 225 patients we have identified 22 with unexplained lower limb pain described as persistent leg pain ranging from unilateral to bilateral sometimes associated with whole body pain. None had significant spinal pathology or evidence of large fiber neuropathy. 14 were on regular analgesics with poor effect while 9 took morphine based remedies. Severe unremitting pain led to accidental overdose and death in one patient. All took a combination of levodopa and dopamine agonist treatment including 3 on intrajejunal levodopa infusion. All had high NMSQuest (mean = 17/30) and NMSS (mean = 95 (normal range 30-60)) scores. CONCLUSIONS: We believe that unexplained lower limb pain is a specific non-motor phenotype in Parkinson's disease that is problematic and often undisclosed. This may be a variant of central pain and merits recognition.

The evaluation of pain in amyotrophic lateral sclerosis: a case controlled observational study.

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder leading to progressive paralysis. ALS is complicated by a number of non-motor symptoms including pain. Pain in ALS has been poorly studied and poorly managed. This study aimed to collate information regarding pain in ALS using standardized pain questionnaires. Forty-two patients with ALS participated in the study. Control subjects included 41 age-matched healthy volunteers and 42 age-matched patients with neurological conditions other than ALS. Data on pain were collected using the The Brief Pain Inventory and The painDetect Questionnaire. Eighty-five percent of subjects with ALS reported pain versus 50% of neurology clinic controls and 35% of healthy controls (p < 0.01). Pain in ALS included cramping, aching, tiring, sharp and tender, and was non-neuropathic. Pain impacted significantly on mood, general activity, relationships and general enjoyment of life. Fifty-four percent of people with painful ALS used regular analgesia and 29% regular opioids. Other non-motor symptoms suffered included tiredness, constipation, urinary problems, itching and drowsiness. In conclusion, these data support the fact that pain is a significant symptom in ALS which impacts on quality of life. These data can be used to educate clinicians and patients to promote better multidisciplinary management of ALS symptoms and a better quality of life.

Comparison of dorsal root ganglion gene expression in rat models of traumatic and HIV-associated neuropathic pain.

To elucidate the mechanisms underlying peripheral neuropathic pain in the context of HIV infection and antiretroviral therapy, we measured gene expression in dorsal root ganglia (DRG) of rats subjected to systemic treatment with the anti-retroviral agent, ddC (Zalcitabine) and concomitant delivery of HIV-gp120 to the rat sciatic nerve. L4 and L5 DRGs were collected at day 14 (time of peak behavioural change) and changes in gene expression were measured using Affymetrix whole genome rat arrays. Conventional analysis of this data set and Gene Set Enrichment Analysis (GSEA) was performed to discover biological processes altered in this model. Transcripts associated with G protein coupled receptor signalling and cell adhesion were enriched in the treated animals, while ribosomal proteins and proteasome pathways were associated with gene down-regulation. To identify genes that are directly relevant to neuropathic mechanical hypersensitivity, as opposed to epiphenomena associated with other aspects of the response to a sciatic nerve lesion, we compared the gp120+ddC-evoked gene expression with that observed in a model of traumatic neuropathic pain (L5 spinal nerve transection), where hypersensitivity to a static mechanical stimulus is also observed. We identified 39 genes/expressed sequence tags that are differentially expressed in the same direction in both models. Most of these have not previously been implicated in mechanical hypersensitivity and may represent novel targets for therapeutic intervention. As an external control, the RNA expression of three genes was examined by RT-PCR, while the protein levels of two were studied using western blot analysis.

Pharmacological, behavioural and mechanistic analysis of HIV-1 gp120 induced painful neuropathy.

A painful neuropathy is frequently observed in people living with human immunodeficiency virus type 1 (HIV-1). The HIV coat protein, glycoprotein 120 (gp120), implicated in the pathogenesis of neurological disorders associated with HIV, is capable of initiating neurotoxic cascades via an interaction with the CXCR4 and/or CCR5 chemokine receptors, which may underlie the pathogenesis of HIV-associated peripheral neuropathic pain. In order to elucidate the mechanisms underlying HIV-induced painful peripheral neuropathy, we have characterised pathological events in the peripheral and central nervous system following application of HIV-1 gp120 to the rat sciatic nerve. Perineural HIV-1 gp120 treatment induced a persistent mechanical hypersensitivity (44% decrease from baseline), but no alterations in sensitivity to thermal or cold stimuli, and thigmotactic (anxiety-like) behaviour in the open field. The mechanical hypersensitivity was sensitive to systemic treatment with gabapentin, morphine and the cannabinoid WIN 55,212-2, but not with amitriptyline. Immunohistochemical studies reveal: decreased intraepidermal nerve fibre density, macrophage infiltration into the peripheral nerve at the site of perineural HIV-1 gp120; changes in sensory neuron phenotype including expression of activating transcription factor 3 (ATF3) in 27% of cells, caspase-3 in 25% of cells, neuropeptide Y (NPY) in 12% of cells and galanin in 13% of cells and a spinal gliosis. These novel findings suggest that this model is not only useful for the elucidation of mechanisms underlying HIV-1-related peripheral neuropathy but may prove useful for preclinical assessment of drugs for the treatment of HIV-1 related peripheral neuropathic pain.