Abstract
Two experiments investigated the effects of amount of exposure to morphine on naloxone (0.4 mg/kg)-induced changes in hamster running-wheel activity. Naloxone injections were given on the first, fifth, or ninth test day of morphine administration. In Experiment 1, naloxone was given 2 h after a 15-mg/kg dose of morphine. The results indicated that naloxone-precipitated hypoactivity increased across test days. In Experiment 2, naloxone was given 10 min after a 2.5-mg/kg dose of morphine. The results indicated that naloxone antagonized morphine-elicited hyperactivity and that naloxone’s effects increased across test days. These findings are discussed in terms of several hypotheses of naloxone-induced changes in morphine-treated hamsters.
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