In vivo binding of naloxone to opioid receptors in morphine-dependent mice.

In vivo binding assay of opioid receptors for naloxone was tested in morphine-dependent mice. The content of naloxone in the brain (whole brain minus cerebellum) and cerebellum was determined as the total binding and non-specific binding, respectively, 20 min after the intravenous injection of [3H]-naloxone. Acute treatment with 8 mg/kg of morphine sulfate markedly enhanced the non-specific binding of naloxone. In contrast, withdrawal for 6 hr after 72 hr of implantation of morphine decreased the non-specific binding of naloxone. The Scatchard analysis of the specific binding revealed two binding sites. The apparent low affinity Kd values of both morphine-tolerant and morphine-withdrawn mice were significantly decreased when compared to their respective controls. The apparent high affinity Kd and the high and low Bmax values were not altered by the morphine dependent-tolerant state.