Morphine discrimination in the pigeon using a color tracking procedure

Area of Science:

• Pharmacology
• Behavioral Neuroscience
• Drug Discrimination Studies

Background:

• Opioid drugs, like morphine, produce distinct subjective effects.
• Understanding the specific receptor interactions and stereoselectivity of these effects is crucial for developing targeted therapies.

Purpose of the Study:

• To investigate the pharmacologically specific and stereoselective discriminative stimulus effects of morphine in pigeons.
• To determine which other opioid and non-opioid drugs generalize to the discriminative stimulus produced by morphine.

Main Methods:

• Pigeons were trained to discriminate 5.0 mg/kg morphine from saline, associating red keys with morphine and green keys with saline.
• Generalization tests were conducted with various drugs, including dl-methadone, naloxone, dl-cyclazocine, meperidine, diazepam, cocaine, d-amphetamine, phencyclidine, and pentobarbital.

Main Results:

• dl-Methadone produced morphine-like responding, primarily due to its l-isomer.
• Naloxone (a mu-opioid antagonist) shifted the generalization curve 10-fold to the right, indicating mu-opioid receptor involvement.
• dl-Cyclazocine generalized to morphine's effects, while phencyclidine did not, suggesting the discriminative cue is not solely mediated by sigma opiate receptors.

Conclusions:

• The discriminative stimulus effects of morphine in pigeons are stereoselective and pharmacologically specific.
• Generalization patterns suggest that the morphine cue involves interactions with opioid receptors, but not exclusively sigma opiate receptors.