naloxone and Mononeuropathies

For optimal use of antinociceptive gene therapy, it may be important to have extrinsic control of the expression of the transfected gene. To achieve this goal, we used a tetracycline-inducible system (Tet-On) composed of three plasmids coding for beta-endorphin, the tetracycline transcriptional activator rtTA, and the silencer tTS. The regulation of beta-endorphin expression was first assessed in cultures of dorsal root ganglion neurons. The three plasmids were then electrotransfected into the spinal cord of mononeuropathic rats and the analgesic potential of this therapy in vivo was evaluated by thermal-withdrawal latency and the mechanical-withdrawal threshold. Intraperitoneal injections of doxycycline were made to evaluate the possibility of exogenous upregulation of transfected beta-endorphin gene expression in vivo. The levels of beta-endorphin were analyzed by intrathecal microdialysis and radioimmunoassay. We found that, after doxycycline administration, the expression of beta-endorphin was rapid, stable, and tightly regulated (low background and high induction level) both in vitro and in vivo. The beta-endorphin protein was secreted into cerebrospinal fluid at a peak level of 53 pmol/L in dialysate, which was sufficient to inhibit neuropathic pain. In conclusion, tightly controlled expression of beta-endorphin can be obtained following intrathecal electrotransfer of a tetracycline-inducible, three-plasmid-based system, and doxycycline-dependent beta-endorphin protein expression in this system alleviates sciatic nerve constriction-induced limb pain.

Topics: Animals; beta-Endorphin; Cells, Cultured; Doxycycline; Electroporation; Ganglia, Spinal; Genetic Therapy; Humans; Immunohistochemistry; Injections, Spinal; Male; Mononeuropathies; Naloxone; Neurons; Nociceptors; Pain Management; Plasmids; Rats; Rats, Sprague-Dawley; Tetracycline

Recent imaging reports demonstrate the activation of the orbitofrontal cortical (OFC) area during acute and chronic pain. The aim of this study was to compare the effects of chronic perfusion of this area with morphine on nociception in control rats and in rats subjected to mononeuropathy. Chronic perfusion of morphine, using miniosmotic pumps, produced significant and naloxone-reversible depression of tactile and cold allodynias and thermal hyperalgesia, observed in neuropathic rats, while it produced significant elevation and naloxone insensitive increase of acute nociceptive thresholds in control rats. The observed results support the idea that this area is a component of a flexible cerebral network involved in pain processing and perception.

Topics: Analgesics, Opioid; Animals; Behavior, Animal; Disease Models, Animal; Drug Administration Routes; Drug Administration Schedule; Frontal Lobe; Hyperalgesia; Mononeuropathies; Morphine; Naloxone; Narcotic Antagonists; Pain; Pain Measurement; Pain Threshold; Rats; Rats, Sprague-Dawley; Reaction Time; Time Factors

Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor-like1), has been shown to be anti- or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 microg/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw-pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 microg/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunities in the treatment of neuropathic pain.

Topics: Animals; Behavior, Animal; Body Weight; Diabetes Mellitus, Experimental; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Interactions; Hyperalgesia; Male; Mononeuropathies; Morphine; Naloxone; Narcotic Antagonists; Nociceptin; Nociceptin Receptor; Opioid Peptides; Pain; Pain Measurement; Rats; Rats, Sprague-Dawley; Receptors, Opioid; Sciatic Nerve; Streptozocin; Time Factors; Vocalization, Animal

Topics: alpha-Methyltyrosine; Analgesics; Animals; Antidepressive Agents; Cyclohexanols; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Interactions; Enzyme Inhibitors; Fenclonine; Male; Mononeuropathies; Motor Activity; Naloxone; Narcotic Antagonists; Pain; Pain Threshold; Rats; Rats, Sprague-Dawley; Sciatic Nerve; Venlafaxine Hydrochloride; Vocalization, Animal

The mechanisms of involvement of the opioidergic system in the antinociceptive effect of antidepressants remain to be elucidated. The present study was designed to determine what type of opioid receptors may be involved at the spinal and supraspinal levels in the antihyperalgesic effect of clomipramine, a tricyclic antidepressant commonly prescribed in the treatment of neuropathic pain. Its antihyperalgesic effect on mechanical hyperalgesia (paw pressure test) in rats induced by chronic constriction injury of the sciatic nerve was assessed after repeated administrations (five injections every half-life, a regimen close to clinical use). Naloxone administered at a dose of 1 mg/kg i.v., which blocks all opioid receptors, or at a low dose of 1 microg/kg i.v., which selectively blocks the mu-opioid receptor, inhibited the anti-hyperalgesic effect of clomipramine and hence indicated that mu-opioid receptor is involved. Depending on whether they are administered by the intracerebroventricular or intrathecal route, specific antagonists of the various opioid receptor subtypes [D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-ThrNH2 (CTOP), mu; naltrindole (NTI), delta; and nor-binaltorphimine (nor-BNI), kappa] differently modify the antihyperalgesic effect of chronically injected clomipramine. The effect was inhibited by intrathecal administration of CTOP and intracerebroventricular administration of naltrindole, whereas nor-BNI was ineffective whatever the route of injection. These results demonstrate a differential involvement of opioid receptors according to the level of the central nervous system: delta-receptors at the supraspinal level and mu-receptors at the spinal level. Clomipramine could act via a neuronal pathway in which these two receptors are needed.

Topics: Animals; Clomipramine; Dose-Response Relationship, Drug; Drug Interactions; Hyperalgesia; Male; Mononeuropathies; Naloxone; Narcotic Antagonists; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Receptors, Opioid, mu; Selective Serotonin Reuptake Inhibitors; Spinal Cord

The present study was performed in rats with experimentally induced mononeuropathy after common sciatic nerve ligation. The hind-paw withdrawal latencies to thermal and mechanical stimulation were increased significantly after intrathecal injection of 3 nmol of galanin. The increased hind-paw response latencies induced by galanin were attenuated by following intrathecal injection of 22 nmol, but not 11 or 2.75 nmol of the opioid receptor antagonist naloxone. Further, the increased hind-paw response latencies induced by galanin were prevented by following intrathecal injection of 10 nmol of mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), but not by 10 nmol of delta-opioid receptor antagonist, natrindole or 10 nmol of kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI). Intrathecal 10 nmol of beta-FNA alone had no significant effects on the hind-paw withdrawal responses. These results demonstrate the existence of a specific interaction between galanin and opioids in the transmission of presumed nociceptive information in the spinal cord of mononeuropathic rats. This interaction involves the activation of mu-opioid receptor.

Topics: Animals; Galanin; Hindlimb; Injections, Spinal; Male; Mononeuropathies; Naloxone; Naltrexone; Narcotic Antagonists; Narcotics; Pain Measurement; Rats; Rats, Sprague-Dawley; Sciatic Nerve; Spinal Cord