naloxone and Sciatica

Glutamate homeostasis and microglia activation play an important role in the development and maintenance of neuropathic pain. We designed this investigation to examine whether ultra-low dose naloxone administered alone or in combination with morphine could alter the concentration of the excitatory amino acids (EAAs) glutamate and aspartate, as well as the expression of tumor necrosis factor-α (TNF-α) and its receptors (TNFR1 and TNFR2) in the spinal cord dorsal horn of rats with partial sciatic nerve transection (PST).. Male Wistar rats underwent intrathecal catheter implantation for drug delivery and were divided in 7 groups: sham-operated + saline (sham), PST + saline (S), PST + 15 ng naloxone (n), PST + 15 µg naloxone (N), PST + 10 µg morphine (M), PST + 15 ng naloxone + 10 µg morphine (Mn), PST + 15 µg naloxone + 10 µg morphine (MN). Thermal withdrawal latency and mechanical withdrawal threshold, TNF-α and TNFR expression in the spinal cord and dorsal root ganglia, and EAAs glutamate and aspartate concentration in cerebrospinal fluid dialysates were measured.. Ten days after PST, rats developed hyperalgesia (P < 0.0001) and allodynia (P < 0.0001), and increased TNF-α (P < 0.0001) and TNFR1 expression (P = 0.0009) were measured in the ipsilateral spinal cord dorsal horn. The antihyperalgesic and antiallodynic effects of morphine (10 μg) were abolished by high-dose naloxone (15 μg; P = 0.0031) but enhanced by ultra-low dose naloxone (15 ng; P = 0.0015), and this was associated with a reduction of TNF-α (P < 0.0001) and TNFR1 (P = 0.0009) expression in the spinal cord dorsal horn and EAAs concentration (glutamate: P = 0.0001; aspartate: P = 0.004) in cerebrospinal fluid dialysate. Analysis of variance (ANOVA) or Student t test with Bonferroni correction were used for statistical analysis.. Ultra-low dose naloxone enhances the antihyperalgesia and antiallodynia effects of morphine in PST rats, possibly by reducing TNF-α and TNFR1 expression, and EAAs concentrations in the spinal dorsal horn. Ultra-low dose naloxone may be a useful adjuvant for increasing the analgesic effect of morphine in neuropathic pain conditions.

Topics: Analgesics, Opioid; Animals; Aspartic Acid; Behavior, Animal; Disease Models, Animal; Down-Regulation; Drug Synergism; Glutamic Acid; Hyperalgesia; Injections, Spinal; Male; Morphine; Naloxone; Narcotic Antagonists; Pain Threshold; Posterior Horn Cells; Rats; Rats, Wistar; Reaction Time; Receptors, Tumor Necrosis Factor, Type I; Receptors, Tumor Necrosis Factor, Type II; Sciatic Nerve; Sciatica; Time Factors; Tumor Necrosis Factor-alpha

Neuropathic pain is a debilitating condition that is often difficult to treat using conventional pharmacological interventions and the exact mechanisms involved in the establishment and maintenance of this type of chronic pain have yet to be fully elucidated. The present studies examined the effect of chronic nerve injury on μ-opioid receptors and receptor-mediated G-protein activity within the supraspinal brain regions involved in pain processing of mice. Chronic constriction injury (CCI) reduced paw withdrawal latency, which was maximal at 10 days post-injury. [d-Ala2,(N-Me)Phe4,Gly5-OH] enkephalin (DAMGO)-stimulated [(35)S]GTPγS binding was then conducted at this time point in membranes prepared from the rostral ACC (rACC), thalamus and periaqueductal grey (PAG) of CCI and sham-operated mice. Results showed reduced DAMGO-stimulated [(35)S]GTPγS binding in the thalamus and PAG of CCI mice, with no change in the rACC. In thalamus, this reduction was due to decreased maximal stimulation by DAMGO, with no difference in EC(50) values. In PAG, however, DAMGO E(max) values did not significantly differ between groups, possibly due to the small magnitude of the main effect. [(3)H]Naloxone binding in membranes of the thalamus showed no significant differences in B(max) values between CCI and sham-operated mice, indicating that the difference in G-protein activation did not result from differences in μ-opioid receptor levels. These results suggest that CCI induced a region-specific adaptation of μ-opioid receptor-mediated G-protein activity, with apparent desensitization of the μ-opioid receptor in the thalamus and PAG and could have implications for treatment of neuropathic pain.

Topics: Analgesics, Opioid; Animals; Constriction; Disease Models, Animal; Dose-Response Relationship, Drug; Drug Interactions; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; GTP-Binding Proteins; Guanosine 5'-O-(3-Thiotriphosphate); Hyperalgesia; Male; Mice; Naloxone; Narcotic Antagonists; Pain Threshold; Protein Binding; Receptors, Opioid, mu; Sciatica; Sulfur Isotopes; Thalamus; Time Factors; Tritium

Phosphorylation of specific sites in the second intracellular loop and in the C-terminal domain have previously been suggested to cause desensitization and internalization of the mu-opioid receptor (MOP-R). To assess sites of MOP-R phosphorylation in vivo, affinity-purified, phosphoselective antibodies were raised against either phosphothreonine-180 in the second intracellular loop (MOR-P1) or the C-terminal domain of MOP-R containing phosphothreonine-370 and phosphoserine-375 (MOR-P2). We found that MOR-P2-immunoreactivity (IR) was significantly increased within the striatum of wild-type C57BL/6 mice after injection of the agonist fentanyl. Pretreatment with the antagonist naloxone blocked the fentanyl-induced increase. Furthermore, mutant mice lacking MOP-R showed only non-specific nuclear MOR-P2-IR before or after fentanyl treatment, confirming the specificity of the MOR-P2 antibodies. To assess whether MOP-R phosphorylation occurs following endogenous opioid release, we induced chronic neuropathic pain by partial sciatic nerve ligation (pSNL), which caused a significant increase in MOR-P2-IR in the striatum. pSNL also induced signs of mu opioid receptor tolerance demonstrated by a rightward shift in the morphine dose response in the tail withdrawal assay and by a reduction in morphine conditioned place preference (CPP). Mutant mice selectively lacking all forms of the beta-endorphin peptides derived from the proopiomelanocortin (Pomc) gene did not show increased MOR-P2-IR, decreased morphine antinociception, or reduced morphine CPP following pSNL. In contrast gene deletion of either proenkephalin or prodynorphin opioids did not block the effects of pSNL. These results suggest that neuropathic pain caused by pSNL in wild-type mice activates the release of the endogenous opioid beta-endorphin, which subsequently induces MOP-R phosphorylation and opiate tolerance.

Topics: Analgesics, Opioid; Analysis of Variance; Animals; Behavior, Animal; beta-Endorphin; Cell Line, Transformed; Conditioning, Operant; Corpus Striatum; Drug Interactions; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Green Fluorescent Proteins; Humans; Hyperalgesia; Mice; Mice, Knockout; Mutagenesis; Naloxone; Narcotic Antagonists; Phosphorylation; Phosphothreonine; Receptors, Opioid, mu; Sciatica; Transfection