Effects of morphine and naltrexone on impulsive decision making in rats.
BACKGROUND
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 pre-existing traits, or the effects of exposure to the opiates.


OBJECTIVE
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.
Kieres
Artur K
AK
Division of Behavioral Medicine, Farber Hall, Room G56, Building no. 26, University at Buffalo, The State University of New York, 3435 Main Street, Buffalo, NY 14214-3000, USA.
Hausknecht
Kathryn A
KA
Farrar
Andrew M
AM
Acheson
Ashley
A
de Wit
Harriet
H
Richards
Jerry B
JB
D000701:Analgesics, Opioid / Q000494:pharmacology; D000704:Analysis of Variance; D000818:Animals; D001522:Behavior, Animal; D003657:Decision Making / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D007175:Impulsive Behavior / Q000523:psychology*; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009271:Naltrexone / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley
D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D009271:Naltrexone; D009020:Morphine