The same drugs can produce different later behavior depending on what an animal does while exposed. Two experiments examining continuous d-amphetamine and cocaine reinforcement found that timing and active self-administration mattered. Their findings challenge an explanation based only on both drugs being present.
These were animal studies of operant responding. They did not establish a human treatment regimen, demonstrate clinical abstinence or justify advising someone to continue cocaine use during treatment.
What did the rhesus-monkey experiment test?
Czoty, Martelle and Nader studied four adult male rhesus monkeys with extensive cocaine self-administration and prior research-drug histories. Cocaine was available intravenously under a progressive-ratio schedule: successive injections required increasing numbers of lever presses.
After establishing baseline responding for 0.045 mg/kg cocaine per injection, investigators suspended cocaine sessions and infused d-amphetamine continuously for five days at 0.01, 0.03 or 0.1 mg/kg/hour intravenously. Each animal received all three rates in randomized order, separated by at least 14 days of baseline cocaine access.
The units are infusion rates, not single injections or daily doses. Cocaine sessions resumed on the afternoon when amphetamine infusion stopped. Food-reinforced responding was also monitored under a different schedule.
Did reduced cocaine responding persist after infusion ended?
After the 0.03 mg/kg/hour treatment, cocaine injections earned were significantly reduced for four days; after 0.1 mg/kg/hour, they were reduced for five. The lowest infusion rate did not significantly alter responding.
Food responding was also disrupted at the higher rates during treatment, and remained significantly reduced for four days after the highest rate. After the intermediate rate, post-treatment food responding remained numerically below baseline without reaching significance. The study therefore did not show an effect entirely confined to cocaine reinforcement.
Food and cocaine were available under different reinforcement schedules and were not explicitly matched for reinforcing strength. That limits claims of selective effects.
What happened when cocaine access was delayed?
In three of the monkeys, the investigators repeated the intermediate-rate treatment but inserted a planned six-day drug-free interval before restoring cocaine access. The subsequent number of cocaine injections did not differ significantly from baseline.
The contrast suggested that early experience with cocaine around the end of amphetamine treatment contributed to the longer-lasting reduction in responding. The authors proposed a learned or behavioral adaptation. They did not directly measure a neural learning mechanism or individual plasma amphetamine concentrations in this experiment.
Only one cocaine dose was used for the principal post-treatment comparison. Fewer injections at that dose cannot automatically be generalized to every dose, access condition or measure of cocaine use.
How did the rat experiment distinguish drug exposure from active responding?
Zimmer, Chiodo and Roberts studied male Sprague-Dawley rats in three groups. Eight received continuous subcutaneous d-amphetamine sulfate at 5 mg/kg/day for seven days while actively self-administering intravenous cocaine at 0.19 mg/kg per infusion. Eight received the same amphetamine treatment with passive cocaine infusions. Seven saline-pump controls actively self-administered cocaine.
The passive-infusion pattern used average timing and injection counts derived from the active amphetamine group. It approximated that group’s drug exposure rather than yoking every passive animal to one individual’s exact sequence.
The primary measure was a progressive-ratio breakpoint, operationalized as the number of cocaine infusions earned before a one-hour period passed without completing the next response requirement. Breakpoints measure behavior under an escalating work requirement, not a direct report of pleasure or a diagnosis of dependence.
What changed in the active and passive groups?
In the active amphetamine group, breakpoints fell approximately 35% from the last baseline day to the last treatment day. Group comparisons were significant on the final three treatment days. After pump removal, responding remained lower for several days before recovering.
Rats that had received passive cocaine during amphetamine treatment did not show the corresponding post-treatment reduction relative to saline controls. Thus, concurrent pharmacological exposure alone was insufficient to reproduce the observed result under this protocol.
The overall post-treatment group effect had p=.08, while the group-by-day interaction was significant and supported specific day-level comparisons. Reporting only a global “amphetamine worked” conclusion would obscure that time-dependent analysis.
Do the studies prove that learning was the sole mechanism?
No. Together, the delayed-access and passive-infusion comparisons support an associative contribution: the experience of seeking and obtaining cocaine under the changed conditions appeared important. They do not exclude pharmacological mechanisms operating alongside learning.
The studies differed in species, routes, infusion rates, treatment duration and access schedules. Their results should be compared conceptually rather than pooled into a single numerical effect. Both involved only males and relatively small groups, with no human clinical endpoint.
The practical research lesson is that exposure history includes behavior and timing, not only a list of drugs and doses. The clinical implications remained hypotheses for human testing. These experiments do not establish current treatment recommendations or the safety of amphetamine–cocaine co-use.
Scientific sources
Czoty PW, Martelle JL, Nader MA. Effects of chronic d-amphetamine administration on the reinforcing strength of cocaine in rhesus monkeys. Psychopharmacology. 2010;209(4):375–382. doi:10.1007/s00213-010-1807-x. Full-text author manuscript.
Zimmer BA, Chiodo KA, Roberts DCS. Reduction of the reinforcing effectiveness of cocaine by continuous d-amphetamine treatment in rats: importance of active self-administration during treatment period. Psychopharmacology. 2014;231(5):949–954. doi:10.1007/s00213-013-3305-4. Full-text author manuscript. Both are historical original animal experiments.
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