Study the mechanisms of U-50,488 to prevent the development of morphine tolerance in guinea pigs.
Previously we have shown that low dose of [trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide hydrochloride] (U-50,488) could prevent the development of morphine tolerance in guinea pigs. In the present study we tried to investigate the role of glutamate and nitric oxide in this process. Male Hartley guinea pigs (200-300 g) were chronically treated s.c. with either saline or morphine (15 mg/kg) or morphine + U-50,488 (0.003 mg/kg) twice a day for 7 days. Antinociceptive activity was assessed by hot-plate test on the first, fourth and seventh day. Spinal cord slices (450 microm) were prepared 30 min after drug treatment on eighth day and [3H] glutamate and nitric oxide (NO) released were determined. We found that coadministration of U-50,488 (0.003 mg/kg) suppressed the development of morphine tolerance to antinociceptive effect as we reported before. The percentage of in vitro spinal release of [3H] glutamate by 100 microM morphine was significantly higher in the chronic morphine group than the control group. On the other hand, coadministration of U-50,488 with morphine for 7 days blocked this effect significantly. The basal NO level released from the spinal cord slices was significantly higher in chronic morphine group but not in chronic (morphine + U-50,488) group. In vitro morphine (100 microM) increased the NO level in control group and chronic (morphine + U-50,488) group and also further increased NO in chronic morphine group. From the NMDA-displaced [3H] glutamate binding in guinea pig spinal cord, we found that the Bmax decreased in chronic morphine group but not in the chronic (morphine + U-50,488) group. In conclusion, chronic morphine treatment may activate the NMDA receptors by increasing the release of glutamate which causes the increase of synthesis and release of NO and following uncertain mechanisms to induce the development of morphine tolerance. And the mechanisms of U-50,488 to prevent the development of morphine tolerance may involve the inhibition of glutamate released by chronic morphine and also the decrease of NO induced by chronic morphine.
Tao
P L
PL
Department of Pharmacology, National Defense Medical Center, Taipei, Taiwan, ROC. pltao@ndmctsgh.edu.tw
Wu
S C
SC
Yang
C H
CH
Wu
C C
CC
D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology*; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology*; D000818:Animals; D004361:Drug Tolerance; D018690:Excitatory Amino Acid Agonists / Q000378:metabolism / Q000494:pharmacology; D018698:Glutamic Acid / Q000378:metabolism / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D016202:N-Methylaspartate / Q000378:metabolism / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D009569:Nitric Oxide / Q000378:metabolism; D017288:Pain Threshold / Q000187:drug effects; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D013116:Spinal Cord / Q000187:drug effects / Q000378:metabolism; D014316:Tritium
D018712:Analgesics, Non-Narcotic; D018690:Excitatory Amino Acid Agonists; D009294:Narcotics; D016194:Receptors, N-Methyl-D-Aspartate; D014316:Tritium; D009569:Nitric Oxide; D018698:Glutamic Acid; D016202:N-Methylaspartate; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D009020:Morphine