Adeno-associated virus-mediated gene transfer of arginine decarboxylase to the central nervous system prevents opioid analgesic tolerance.

Agmatine, a decarboxylated form of L-arginine, prevents opioid analgesic tolerance, dependence, and self-administration when given by both central and systemic routes of administration. Endogenous agmatine has been previously detected in the central nervous system. The presence of a biochemical pathway for agmatine synthesis offers the opportunity for site-specific overexpression of the presumptive synthetic enzyme for local therapeutic effects. In the present study, we evaluated the development of opioid analgesic tolerance in ICR-CD1 mice pre-treated with either vehicle control or intrathecally delivered adeno-associated viral vectors (AAV) carrying the gene for human arginine decarboxylase (hADC). Vehicle-treated or AAV-hADC-treated mice were each further divided into two groups which received repeated delivery over three days of either saline or systemically-delivered morphine intended to induce opioid analgesic tolerance. Morphine analgesic dose-response curves were constructed in all subjects on day four using the warm water tail flick assay as the dependent measure. We observed that pre-treatment with AAV-hADC prevented the development of analgesic tolerance to morphine. Peripheral and central nervous system tissues were collected and analyzed for presence of hADC mRNA. In a similar experiment, AAV-hADC pre-treatment prevented the development of analgesic tolerance to a high dose of the opioid neuropeptide endomorphin-2. Intrathecal delivery of anti-agmatine IgG (but not normal IgG) reversed the inhibition of endomorphin-2 analgesic tolerance in AAV-hADC-treated mice. To summarize, we report here the effects of AAV-mediated gene transfer of human ADC (hADC) in models of opioid-induced analgesic tolerance. This study suggests that gene therapy may contribute to reducing opioid analgesic tolerance.

Interleukin-8 as a therapeutic target for chronic low back pain: Upregulation in human cerebrospinal fluid and pre-clinical validation with chronic reparixin in the SPARC-null mouse model.

BACKGROUND: Low back pain (LBP) is the leading global cause of disability and is associated with intervertebral disc degeneration (DD) in some individuals. However, many adults have DD without LBP. Understanding why DD is painful in some and not others may unmask novel therapies for chronic LBP. The objectives of this study were to a) identify factors in human cerebrospinal fluid (CSF) associated with chronic LBP and b) examine their therapeutic utility in a proof-of-concept pre-clinical study. METHODS: Pain-free human subjects without DD, pain-free human subjects with DD, and patients with chronic LBP linked to DD were recruited and lumbar MRIs, pain and disability levels were obtained. CSF was collected and analyzed by multiplex cytokine assay. Interleukin-8 (IL-8) expression was confirmed by ELISA in CSF and in intervertebral discs. The SPARC-null mouse model of progressive, age-dependent DD and chronic LBP was used for pre-clinical validation. Male SPARC-null and control mice received systemic Reparixin, a CXCR1/2 (receptors for IL-8 and murine analogues) inhibitor, for 8 weeks. Behavioral signs of axial discomfort and radiating pain were assessed. Following completion of the study, discs were excised and cultured, and conditioned media was evaluated with a protein array. FINDINGS: IL-8 was elevated in CSF of chronic LBP patients with DD compared to pain-free subjects with or without DD. Chronic inhibition with reparixin alleviated low back pain behaviors and attenuated disc inflammation in SPARC-null mice. INTERPRETATION: These studies suggest that the IL-8 signaling pathway is a viable therapy for chronic LBP. FUND: Supported by NIH, MMF, CIHR and FRQS.

The Subjective Experience of Pain: An FMRI Study of Percept-Related Models and Functional Connectivity.

OBJECTIVE: Previous work suggests that the perception of pain is subjective and dependent on individual differences in physiological, emotional, and cognitive states. Functional magnetic resonance imaging (FMRI) studies have used both stimulus-related (nociceptive properties) and percept-related (subjective experience of pain) models to identify the brain networks associated with pain. Our objective was to identify the network involved in processing subjective pain during cold stimuli. METHODS: The current FMRI study directly contrasted a stimulus-related model with a percept-related model during blocks of cold pain stimuli in healthy adults. Specifically, neuronal activation was modelled as a function of changes in stimulus intensity vs as a function of increasing/decreasing levels of subjective pain corresponding to changes in pain ratings. In addition, functional connectivity analyses were conducted to examine intrinsic correlations between three proposed subnetworks (sensory/discriminative, affective/motivational, and cognitive/evaluative) involved in pain processing. RESULTS: The percept-related model captured more extensive activation than the stimulus-related model and demonstrated an association between higher subjective pain and activation in expected cortical (dorsolateral prefrontal cortex, ventrolateral prefrontal cortex, insula, dorsal anterior cingulate cortex [dACC] extending into pre-supplementary motor area) and subcortical (thalamus, striatum) areas. Moreover, connectivity results supported the posited roles of dACC and insula as key relay sites during neural processing of subjective pain. In particular, anterior insula appeared to link sensory/discriminative regions with regions in the other subnetworks, and dACC appeared to serve as a hub for affective/motivational, cognitive/evaluative, and motor subnetworks. CONCLUSIONS: Using a percept-related model, brain regions involved in the processing of subjective pain during the application of cold stimuli were identified. Connectivity analyses identified linkages between key subnetworks involved in processing subjective pain.

Analgesic effects of a soy-containing diet in three murine bone cancer pain models.

UNLABELLED: Bone is a common metastatic site for prostate and breast cancer, and bone cancer is usually associated with severe pain. Traditional treatments for cancer pain can sometimes be ineffective or associated with side effects. Thus an increasing number of patients seek alternative therapies. In this study we investigated the analgesic effects of a soy diet on 3 experimental models of bone cancer pain. Mice were fed a diet in which the protein source was either soy or casein. After 1 week on the diet, sarcoma cells (NCTC 2472) were injected into the medullary cavity of the humeri, femur, or calcaneus. Experimenters blinded to diet of the animal assessed the pain behavior in these animals, forelimb grip force in the humerus model and paw withdrawal frequency to mechanical stimuli in the calcaneus and femur models. The effect of morphine on cancer-induced pain behavior was investigated in calcaneus and femur models. In addition, in the femur model, the effects of soy on tumor size and bone destruction were studied. The soy diet reduced secondary mechanical hyperalgesia in the femur model but had no effect on primary mechanical hyperalgesia in the calcaneus model or on movement-related hyperalgesia in the humerus model. No dietary impact was discerned in measurements of tumor size, bone destruction, and body weight in the femur model, suggesting that the soy diet had no effect on cancer growth. Morphine dose-dependently reduced hyperalgesia with no diet-based difference. These results suggest that a soy diet might provide analgesia in certain forms of hyperalgesia associated with bone cancer. PERSPECTIVE: The study raises the possibility of dietary supplements influencing aspects of cancer pain. Further research will help determine if use of nutritional supplements, such as soy proteins, can reduce opioid analgesic use in chronic pain states and help minimize the side effects associated with long term use of opioids.