Four groups of rats differing in the number of periods of prior exposure to morphine sulphate in the i.v. self-administration paradigm were studied under conditions of narcotic blockade. Three groups of subjects also differing in the amount of prior exposure to morphine sulphate were studied under saline conditions. At effective blocking doses of naloxone, opioid-seeking behavior was eliminated in relatively drug naive animals, whereas the persistence of secondary reinforcers in rats with longer addiction histories served to maintain opioid consumption in the presence of adequate pharmacological blockade. Data from saline-treated animals were very similar to data obtained in naloxone-treated animals. The authors conclude that at adequate blocking doses of narcotic antagonist the length of addiction appears to be the best predictor of opioid consumption.
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