Tolerance to methadone lethality and microsomal enzyme induction in mice tolerant to and dependent on morphine.

In an attempt to explain a loss of cross-tolerance between morphine and methadone and an increased tolerance to methadone lethality in morphine-dependent mice administered methadone orally for six days, the possibility that methadone was stimulating its own metabolism was investigated. It was found that methadone did enhance its own metabolism two-fold. This increase in activity correlated with the development of tolerance to the lethal effects of methadone as measured by an elevation of the oral methadone LD50. Furthermore, SKF-525A, a potent microsomal inhibitor, abolished this tolerance. The intracerbroventricular methadone LD50 was not altered by six days administration of oral methadone, suggesting that the tolerance observed was dispositional in nature.

Naloxone-reversible analgesic action of SKF 525-A in mice.

SKF 525-A, given i.p. to mice at doses from 50 to 100 mg/kg, had analgesic activity approximately 40% the analgesic potency of d-propoxyphene HCl, a chemically similar narcotic. While the analgesia produced by propoxyphene was totally antagonized by naloxone, however, that produced by SKF 525-A was only partly reversed. We suspect that SKF 525-A may exert its antinociceptive actions partly by an interaction with CNS narcotic receptors and partly by a nonspecific sedating action.

Myelination in chronically-alcoholic mice.

Chronically-administered ethanol, not impaired nutrition, exerts a specific effect to decrease myelination as measured by the activity of the marker enzyme for myelin, 2',3'-cyclic nucleotide 3'-phosphohydrolase.

Model to study long-term effects of propoxyphene napsylate (darvon-N) on narcotic-dependent mice.

Five different strains of mice were investigated for their suitability to serve as models for the study of long-term effects of propoxyphene napsylate (PN) on narcotic-dependent mice. The NIH/Swiss outbred strain proved to be most suitable because of its high liability to manifest physical dependence on morphine. Following removal of subcutaneously-implanted morphine pellets from the mice, the incidence of spontaneous withdrawal jumping was used as a quantifiable criterion of physical dependence. An orally administered suspension of PN suppressed the withdrawal jumping in a dose-dependent manner. However, chronic (daily) administration of PN at doses high enough to prevent withdrawal jumping was characterized by a high degree of toxicity.