Naloxone reversal of morphine elicited hyperactivity

When naloxone is administered during morphine elicited hyperactivity, hyperactivity is reversed and hypoactivity occurs in its place. The present experiment tested the hypothesis that this effect is the result of morphine induced supersensitivity to naloxone. Two groups of hamsters received equivalent pretreatment with 15 mg/kg morphine (Groups M/M and M/S) for three days while a third group received saline (Group S/S). During subsequent testing one group received a morphine injection (Group M/M) while the others received saline (Groups M/S and S/S) before being placed in running wheels for a three hour session. Two hours later half the animals in each group received an injection of 0.4 mg/kg naloxone and half received saline. Naloxone produced hypoactivity in animals running under the influence of morphine (Group M/M), but neither in those with an equivalent history of morphine pre-treatment (Group M/S), nor in saline controls (Group S/S). These results are inconsistent with the hypothesis under test, but congruent with a modified dual-action hypothesis.