Discriminative effects of morphine in the squirrel monkey

Area of Science:

• Pharmacology
• Neuroscience
• Behavioral Science

Background:

• Morphine is a potent opioid analgesic.
• Understanding drug discrimination is crucial for pharmacology.
• Previous studies established morphine's discriminative properties in rats.

Purpose of the Study:

• To investigate if squirrel monkeys can discriminate between morphine and saline.
• To characterize the discriminative effects of morphine and other opioids in squirrel monkeys.
• To confirm morphine's specific narcotic effects in a new species.

Main Methods:

• Squirrel monkeys were trained in a two-choice discrete trial avoidance task.
• Intramuscular injections of saline and varying doses of morphine were administered.
• The discriminative stimulus effects of morphine were tested with other narcotic analgesics and antagonists.

Main Results:

• Morphine produced a dose-related increase in lever choices associated with the drug.
• Other tested narcotic analgesics (fentanyl, oxymorphone, levorphanol, methadone, meperidine) produced similar stimulus control.
• Naloxone blocked morphine's stimulus control, and stereospecificity was observed with levorphanol but not dextrorphan.
• The duration of stimulus control varied, with morphine effects lasting up to 14 hours and fentanyl up to 0.5 hours.

Conclusions:

• Squirrel monkeys exhibit stimulus control by morphine consistent with a specific narcotic effect.
• These findings demonstrate that morphine's discriminative properties are similar across species (rats and squirrel monkeys).
• This study validates the squirrel monkey as a model for studying opioid drug discrimination.