The discriminative stimulus effects of morphine in pigeons responding under a progressive ratio schedule of food presentation.
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 schedules, 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 micro-opioids vary on a dimension (e.g. intensity) that can be quantified using PR schedules.
Brandt
M R
MR
Department of Pharmacology and Experimental Therapeutics, Louisiana State University Health Sciences Center, New Orleans 70112-1393, USA.
Galici
R
R
France
C P
CP
D000818:Animals; D002047:Buprenorphine / Q000494:pharmacology; D003042:Cocaine / Q000494:pharmacology; D010856:Columbidae; D003216:Conditioning, Operant / Q000187:drug effects; D004193:Discrimination Learning / Q000187:drug effects; D004192:Discrimination, Psychological / Q000187:drug effects*; D018765:Dopamine Uptake Inhibitors / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D005502:Food; D009020:Morphine / Q000494:pharmacology*; D009266:Nalbuphine / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D012055:Reinforcement Schedule; D013997:Time Factors
D018765:Dopamine Uptake Inhibitors; D009292:Narcotic Antagonists; D009294:Narcotics; D002047:Buprenorphine; D009020:Morphine; D003042:Cocaine; D009266:Nalbuphine