The discriminative stimulus effects of morphine in pigeons responding under a progressive ratio schedule of food presentation

M. R. Brandt, R. Galici, Charles P. France

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


The purpose of this study was to determine whether progressive ratio (PR) schedules might provide additional information, as compared with fixed ratio or fixed interval shedules, on pharmacologic features of discriminative stimuli (e.g. stimulus intensity). Five pigeons discriminated between 5.6 mg/kg morphine and saline under an arithmetic PR5 schedule of food presentation. The final ratio before pigeons either stopped responding for 5 min or switched responding from the selected to the non-selected key was designated as the last completed ratio (LCR). Pigeons responded 6.8% on the drug key following saline and 96.4% on the drug key following 5.6 mg/kg morphine. The average LCR value for saline was not significantly different from the average LCR value for morphine. A larger dose of morphine (10.0 mg/kg) increased the LCR value and significantly decreased rates of responding. Smaller doses of morphine (0.32 and 1.0 mg/kg) occasioned primarily saline-appropriate responding and decreased LCR values. Buprenorphine substituted for morphine and significantly increased LCR values, whereas nalbuphine produced only partial (20-80%) morphine-key responding and significantly decreased LCR. Cocaine did not substitute for morphine or modify LCR compared with saline control. Together, these results suggest that the stimulus effects of μ-opioids vary on a dimension (e.g. intensity) that can be quantified using PR schedules. (C) 2000 Lippincott Williams and Wilkins.

Original languageEnglish (US)
Pages (from-to)15-28
Number of pages14
JournalBehavioural pharmacology
Issue number1
StatePublished - 2000
Externally publishedYes


  • Buprenorphine
  • Cocaine
  • Drug discrimination
  • Morphine
  • Nalbuphine
  • Pigeon
  • Progressive ratio
  • Stimulus intensity

ASJC Scopus subject areas

  • Psychiatry and Mental health
  • Pharmacology


Dive into the research topics of 'The discriminative stimulus effects of morphine in pigeons responding under a progressive ratio schedule of food presentation'. Together they form a unique fingerprint.

Cite this