Morphine tolerance as a function of ratio schedule: response requirement or unit price?
More pecks, less tolerance—raw response count beats price formulas in morphine self-administration.
01Research in Context
What this study did
Scientists gave pigeons morphine every day. The birds had to peck a key to get each dose.
Some birds needed 30 pecks. Others needed 90. The team watched how fast drug tolerance grew.
They asked: does the plain number of pecks matter, or does "unit price" rule?
What they found
The 90-peck group stayed sensitive to morphine longer. Tolerance grew slowest there.
Unit price could not explain the pattern. Only the raw response count made the difference.
How this fits with other research
English et al. (1995) saw the same limits of unit price with monkeys. Their animals paid higher prices as the dose got smaller, yet the simple price formula still failed.
Wade-Galuska et al. (2011) later showed that withdrawal flips choice and demand. Together the three studies say: ratio size, not price math, sets the ceiling; withdrawal then moves demand inside that ceiling.
All three papers knock down tidy economic rules and put plain response requirements back in the driver’s seat.
Why it matters
If you study or treat substance use, remember that schedule mechanics live inside the skin. Higher response costs can physically slow tolerance, while withdrawal can override preference. When you shape alternative behavior, push the response requirement for problem behavior up and keep the new task easy at first.
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02At a glance
03Original abstract
Key pecking by 3 pigeons was maintained by a multiple fixed-ratio 10, fixed-ratio 30, fixed-ratio 90 schedule of food presentation. Components differed with respect to amount of reinforcement, such that the unit price was 10 responses per 1-s access to food. Acute administration of morphine, l-methadone, and cocaine dose-dependently decreased overall response rates in each of the components. When a rate decreasing dose of morphine was administered daily, tolerance, as measured by an increase in the dose that reduced response rates to 50% of control (i.e., the ED50 value), developed in each of the components; however, the degree of tolerance was smallest in the fixed-ratio 90 component (i.e., the ED50 value increased the least). When the l-methadone dose-effect curve was redetermined during the chronic morphine phase, the degree of cross-tolerance conferred to l-methadone was similar across components, suggesting that behavioral variables may not influence the degree of cross-tolerance between opioids. During the chronic phase, the cocaine dose-effect curve shifted to the right for 2 pigeons and to the left for 1 pigeon, which is consistent with predictions based on the lack of pharmacological similarity between morphine and cocaine. When the morphine, l-methadone, and cocaine dose-effect curves were redetermined after chronic morphine administration ended, the morphine and l-methadone ED50s replicated those obtained prior to chronic morphine administration. The morphine data suggest that the fixed-ratio value (i.e., the absolute output) determines the degree of tolerance and not the unit price.
Journal of the experimental analysis of behavior, 2005 · doi:10.1901/jeab.2005.35-04