A gas-chromatographic method has been developed for the simultaneous determination of naltrexone, alpha-naltrexol, and beta-naltrexol as trimethylsiyl derivatives. Analysis of urine from rabbit, monkey, and rat demonstrated that, like man, these species reduce naltrexone primarily to beta-naltrexol. In naltrexone maintenance patients receiving 125 mg po three times per week, an average of 37% of the dose was recovered in 48-hr urine as free naltrexone (0.8%), conjugated naltrexone (7.6%), free beta-naltrexol (16.8%), and conjugated beta-naltrexol (11.8%). Thirty-four percent of the dose appeared in 0-24 hr and 3% during 24-48 hr. The ratio of beta-naltrexol to naltrexone rose from 2 at 0-4 hr to 34-48 hr. Monkeys receiving a daily dose of 12 mg/kg po, chronically, excreted very little free beta-naltrexol and exhibited an apparent sex-related difference in excretion patterns, with females excreting more than twice as much total base as males. Rabbits given a dose of 30 mg/kg ip for 4 days excreted conjugated naltrexone as the predominant urinary metabolite, accounting for 80% of total base recovered in 24 hr. In rats receiving 100 mg/kg po, less than 1% of the administered dose could be accounted for in the 24-hr urine, indicating that although the beta-naltrexol is produced as a urinary metabolite, other means of disposition of the drug must exist. Thus, in man and the monkey, beta-naltrexol is the predominant and persistent urinary metabolite. Urinary excretion profiles of naltrexone differ greatly between species commonly examined for chronic toxicity studies.

Administration, Oral↗

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The role of N-methyl-D-aspartate (NMDA) receptors in pain and morphine tolerance.

To determine the importance of the N-methyl-D-aspartate (NMDA) receptor in pain hypersensitivity following injury, the NMDAR1 subunit was selectively deleted in the lumbar spinal cord of adult mice by the localized injection of an adeno-associated virus expressing the Cre recombinase into floxed NR1 mice. This procedure resulted in more than an 80% reduction in the expression of both NR1 mRNA and protein and a corresponding loss of NMDA, but not AMPA currents, in the lamina II neurons in the injected area. This spatially and temporally restricted knockout dramatically reduced the response to pain hypersensitivity resulting from the intraplantar injection of formalin but did not alter heat or cold paw withdrawal latencies, mechanical threshold, or motor function. Thus, the NMDA receptor in the spinal cord dorsal horn is essential for central sensitization, the central facilitation of pain transmission produced by peripheral injury. Agents that act on these spinal receptors may provide a therapeutic approach to ameliorate injury-induced pain.

Analgesics, Opioid↗

PubMedDate not supplied

Pharmacology of methadone and its isomers.

UNLABELLED: Methadone is a synthetic opioid analgesic that is used as an alternate to morphine and hydromorphone for patients with severe pain. It is increasingly being used in opioid rotation schedules. Methadone has an asymmetric carbon atom resulting in 2 enantiomeric forms, the d and l isomers. The racemic mixture (dl-methadone) is the form commonly used clinically. Recent studies have revealed the pharmacological activity of the d-methadone isomer. We found that the d isomer of methadone has N-methyl-D-aspartate (NMDA) receptor antagonist activity both in vitro and in vivo. Studies were designed to examine the ability of d-methadone to attenuate the development of morphine tolerance and to modify NMDA-induced hyperalgesia in rats. Repeated dosing with intrathecal morphine produced a 38-fold increase in the morphine ED50 value. This decrease in the potency of morphine was completely prevented by the coadministration of intrathecal d-methadone at 160 microg/rat. In addition, the decrease in thermal paw withdrawal latency induced by the intrathecal administration of 1.64 microg/rat NMDA was completely blocked by pretreatment with 160 microg/rat d-methadone. Thus, the same dose of intrathecal d-methadone that attenuates the development of spinal morphine tolerance blocks NMDA-induced hyperalgesia in rats. These results support the CONCLUSIONS: that d-metha-done affects the development of morphine tolerance and NMDA-induced hyperalgesia by virtue of its NMDA receptor antagonist activity.

Analgesics, Opioid↗

PubMedDate not supplied

Isolation and stereochemical identification of a metabolite of naltrexone from human urine.

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Animals↗

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Amniotic fluid methadone in women maintained on methadone.

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Adult↗