Discriminative stimulus effects of acute morphine followed by naltrexone in the squirrel monkey.
BACKGROUND
The discriminative stimulus effects of a combination of acute morphine followed by naltrexone have been described in rats. OBJECTIVE. The purpose of this study was to extend observations to a non-human primate.


METHODS
Eight squirrel monkeys were trained in a discrete-trial avoidance/escape procedure to discriminate morphine (1.7 mg/kg, IM, 4 h) followed by naltrexone (0.1 mg/kg, IM, 0.25 h) (MOR-->NTX) versus saline (1.0 ml/kg, IM, 4 h) followed by naltrexone (0.1 mg/kg, IM, 0.25 h) (SAL-->NTX).


RESULTS
Seven subjects acquired the discrimination in an average of 108+/-14 sessions. MOR-->NTX-appropriate responding increased as an orderly function of increasing dose of morphine (0.56-1.7 mg/kg) and of naltrexone (0.01-10 mg/kg). The discrimination was also dependent upon interval between morphine and naltrexone administration. The MOR-->NTX cue was fully generalized to the combination of levorphanol (0.3 mg/kg) followed by naltrexone, but not to the non-opioid stereoisomer of levorphanol, dextrorphan (0.3 and 3.0 mg/kg) or the kappa-opioid-receptor-selective agonist U69,593 (0.3 mg/kg) followed by naltrexone. Naltrexone administered 15 min before morphine dose-dependently blocked MOR-->NTX-appropriate responding.


CONCLUSIONS
This is the first non-rodent study of the discriminative effects of MOR-->NTX. MOR-->NTX produces a unique interoceptive stimulus that is pharmacologically selective, requires occupation of opioid receptors, presumably mu, for some minimum period of time, and is reversible. This discrimination procedure might provide new insights into the early drug-receptor interactions that underlie the development of physical dependence upon morphine-like drugs.
White
David A
DA
Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road NE, Suite 5074, Atlanta, GA 30322, USA. dwhite4@emory.edu
Holtzman
Stephen G
SG
D000818:Animals; D044482:Benzeneacetamides; D003216:Conditioning, Operant / Q000187:drug effects; D003917:Dextrorphan / Q000494:pharmacology; D004193:Discrimination Learning / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005793:Generalization, Psychological; D007981:Levorphanol / Q000494:pharmacology; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009271:Naltrexone / Q000008:administration & dosage / Q000494:pharmacology*; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009294:Narcotics / Q000494:pharmacology*; D011759:Pyrrolidines / Q000494:pharmacology; D011957:Receptors, Opioid / Q000819:agonists; D012453:Saimiri; D013375:Substance Withdrawal Syndrome / Q000523:psychology*
D044482:Benzeneacetamides; D009292:Narcotic Antagonists; D009294:Narcotics; D011759:Pyrrolidines; D011957:Receptors, Opioid; D003917:Dextrorphan; D007981:Levorphanol; D009270:Naloxone; D009271:Naltrexone; D009020:Morphine; C045444:U 69593