Effects of morphine and naltrexone on impulsive decision making in rats

Artur K. Kieres, Kathryn A. Hausknecht, Andrew M. Farrar, Ashley Acheson, Harriet De Wit, Jerry B. Richards

Research output: Contribution to journalArticlepeer-review

76 Scopus citations


Rationale: It has been reported that human opiate addicts discount delayed rewards more than non-addicts, indicating that they are more impulsive. However, it is not clear whether this difference reflects preexisting traits, or the effects of exposure to the opiates. Objectives: This study was designed to investigate the effects of an opioid agonist and antagonist on delay discounting in rats. The study had three objectives: to determine (1) the acute effects of the opioid agonist morphine (MOR) on delay discounting, (2) the acute effects of the opioid antagonist naltrexone (NAL) on delay discounting, and (3) whether NAL reverses the effects of MOR on delay discounting. Methods: An adjusting amount procedure (AdjAmt) was used to determine how much animals discounted the value of delayed rewards. Acute doses of MOR (0.3, 1.0, and 1.8 mg/kg SC), NAL (0.01, 0.1, 1.0, and 10 mg/kg SC) and NAL (0.1 mg/kg SC) prior to MOR (1.8 mg/kg SC) were tested in 15 rats. Results: MOR dose dependently increased the rate of delay discounting (i.e., made the animals more impulsive). NAL alone had no effect on the value of delayed rewards, but NAL blocked the effects of MOR. Conclusions: These results suggested that the direct effects of MOR may contribute to the high level of impulsive behavior seen among opiate users.

Original languageEnglish (US)
Pages (from-to)167-174
Number of pages8
Issue number1
StatePublished - Apr 2004


  • Choice
  • Conditioning
  • Delay discounting
  • Impulsivity
  • Operant
  • Opiates

ASJC Scopus subject areas

  • Pharmacology


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