Naloxone enhances the expression of morphine-induced conditioned place preference

J. L. Neisewander, R. C. Pierce, M. T. Bardo

Research output: Contribution to journalArticlepeer-review

48 Scopus citations


The present study examined the effects of naloxone on acquisition and expression of morphine-induced conditioned place preference (CPP). Three groups of rats were given morphine (5 mg/kg, SC), both morphine and naloxone (1 mg/kg, SC), or saline paired with a distinctive environment. On alternating days they were given saline paired with another distinctive environment. After four exposures to each environment, the animals were given a preference test in which they had access to both environments simultaneously while under the influence of either naloxone (1 mg/kg, SC) or saline. Morphine-conditioned animals showed CPP evident as an increased amount of time spent in the drug-associated environment relative to saline controls. Rats given both naloxone and morphine during conditioning, and saline on the test day, did not show CPP. In contrast, morphine-conditioned animals given naloxone on the test day showed stronger CPP than morphine-conditioned animals given saline. These findings indicate that naloxone blocks the acquisition, but enhances the expression of morphine-induced CPP. In a separate experiment, the effects of naloxone on locomotor activity were determined during the CPP test. The results indicated that naloxone decreased locomotor activity. In morphine-conditioned animals only, naloxone also produced an increase in the amount of time per entry in the drug-associated environment. The results suggest that naloxone may enhance morphine-induced CPP by decreasing locomotor activity that may otherwise compete with expression of CPP.

Original languageEnglish (US)
Pages (from-to)201-205
Number of pages5
Issue number2
StatePublished - Feb 1990
Externally publishedYes


  • Conditioned place preference
  • Locomotor activity
  • Morphine
  • Naloxone

ASJC Scopus subject areas

  • Pharmacology


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