Studies on action mechanisms of morphine and 1-methadone in mice.

Chronically administered morphine and 1-methadone has previously been shown to yield different cross-tolerance patterns in mice. It was therefore tested if the two drugs differ in selectivities towards opioid binding sites in mouse brain membranes. Morphine and 1-methadone were found to show almost identical binding selectivities in vitro. It is suggested that the two drugs act by the same receptor(s), and that the observed asymmetry in cross-tolerance patterns could be due to differences in the receptor reserve for morphine- and 1-methadone-induced antinociception in mice.