Enhanced affinity of opiate receptors for naloxone in striatal slices of morphine-dependent mice.

Slices of corpus striatum were allowed to accumulate 3H-morphine and then they were placed in a small chamber and superfused with Krebs-Ringer bicarbonate solution. 100 to 1 pM of naloxone caused an immediate increase in the release of morphine from the slices when it was placed in the superfusion fluid at 0, 5 or 15 min after the the start of the superfusion. Slices from morphine-dependent mice were more sensitive to the naloxone-induced release of morphine than those of control mice. The data suggest that the affinity of the opiate receptors in the corpus straitum for naloxone is increased in morphine-dependent mice.