Effects of cocaine on fixed-interval responding reinforced by the opportunity to run.
Cocaine’s rate-dependent effect works even when the pay-off is exercise, not food.
01Research in Context
What this study did
Scientists gave rats cocaine before a session. The rats pressed a lever on a fixed-interval schedule. Each press earned time to run in a wheel.
They tested four doses: 0, 1, 8, and 16 mg/kg. The goal was to see if cocaine changes how fast the animals work for wheel access.
What they found
Only the highest dose, 16 mg/kg, did anything. It made slow pressers press faster and fast pressers press slower.
Lower doses acted like saline—no change. The effect followed the old rule: baseline rate predicts direction of drug change.
How this fits with other research
Lucki et al. (1983) proved the rule with amphetamine and food pellets. Lejuez et al. (2001) now show the rule also holds when the reinforcer is wheel running, not food.
Smith et al. (1975) saw that amphetamine changes lever pressing predictably but wheel spinning only in some rats. The new study flips the roles: wheel access is now the prize, and cocaine still follows the rate rule.
Simpson et al. (2001) warn that wheel running drops within a session because of habituation, not fatigue. Clinicians should watch for this natural decline before blaming a drug effect.
Why it matters
You probably won’t give cocaine, but the rate-dependency rule pops up everywhere. If a client’s Ritalin seems to help one task and hurt another, check baseline rates. Fast behaviors may slow, slow behaviors may speed. Measure first, then decide if the change is drug or just physics.
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02At a glance
03Original abstract
Rate-dependent drug effects have been observed for operant responding maintained by food, water, heat, light onset, electrical brain stimulation, shock-stimulus termination, and shock presentation. The present study sought to determine if the effects of cocaine on lever pressing maintained by the opportunity to run could also be described as rate dependent. Seven male Wistar rats were trained to respond on levers for the opportunity to run in a wheel. The schedule of reinforcement was fixed-interval 60 s, and the reinforcing consequence was the opportunity to run for 60 s. On this schedule, overall rates of responding were low, usually below six presses per minute, and pauses frequently exceeded the 60-s interval. Despite these differences, an overall scalloped pattern of lever pressing was evident for each rat. Doses of 1, 2, 4, 8, and 16 mg/kg cocaine were administered 10 min prior to a session. Only at the 16 mg/kg dose did the responding of the majority of rats change in a manner suggestive of a rate-dependent drug effect. Specifically, lower response rates at the beginning of the intervals increased and higher rates at the end of the intervals decreased, as indicated by the fact that slopes from the regression of drug rates on control rates decreased. These data provide tentative support for the generalization of rate-dependent effects to operant responding maintained by wheel running. Differences in the baseline performance maintained by wheel running compared to those for food and water point to the need for further experimentation before this effect can be firmly established.
Journal of the experimental analysis of behavior, 2001 · doi:10.1901/jeab.2001.75-77