Tolerance to one stimulant response does not establish tolerance to another stimulant, even when both act at the dopamine transporter. A 2011 rat study found reduced cocaine-induced dopamine effects after cocaine self-administration, while amphetamine retained activity on the measured endpoints.
The experiment separates responses to two drugs under specific exposure conditions. It does not establish that a person tolerant to cocaine is safe to take amphetamine, or that an unchanged laboratory response means unchanged impairment.
What exposure histories were compared?
Ferris and colleagues used male Sprague-Dawley rats weighing 375–400 g. Cocaine was self-administered intravenously at 1.5 mg/kg per injection under a fixed-ratio-one schedule, meaning one lever press produced an injection. Sessions ended after 40 injections, with five consecutive daily sessions used for the principal comparisons.
As animals completed sessions more quickly, their rate of intake increased. The imposed limit of 40 injections remained distinct from the time required to obtain them; a faster session did not mean an unlimited increase in daily injection count.
A separate group self-administered amphetamine at 0.56 mg/kg per intravenous infusion for five hours per day over five days. Investigators selected that dose to produce dopamine elevations approximately comparable to, or greater than, those associated with cocaine. This was an attempt to compare substantial dopaminergic exposure, not proof that every aspect of the regimens was matched.
Which measurements were made?
Microdialysis measured extracellular dopamine in the nucleus accumbens core of freely moving rats. Separate fast-scan cyclic-voltammetry experiments used isolated brain slices to examine electrically evoked dopamine signals and model release and uptake.
The slice experiments applied cocaine or amphetamine in the bathing solution. The methods give cocaine concentrations of 0.03–30 micromolar and amphetamine concentrations of 0.1–10 micromolar. These are experimental bath concentrations, not blood concentrations, and they cannot supply a human forensic cutoff.
Different analyses had different animal denominators. For example, baseline microdialysis comparisons reported five cocaine-exposed animals and six naive controls; the early slice time-course comparisons reported five animals after one cocaine session and four after three sessions. Repeated samples and concentration steps did not increase the number of independent animals.
What changed after cocaine self-administration?
A cocaine challenge produced a much smaller extracellular dopamine response in cocaine-experienced animals than in naive controls. In slices, cocaine was also less effective at inhibiting dopamine uptake.
The model’s apparent Km parameter summarized uptake inhibition under the assay conditions. Following five cocaine sessions, the maximal reported apparent Km was 7.37 ± 1.38 micromolar, compared with 28.75 ± 3.14 micromolar in naive animals. These were model-derived dopamine-uptake parameters, not measured cocaine concentrations in the rats.
A reduced cocaine effect appeared after one session and remained after the longer exposure histories. The study also found that changes in maximal baseline uptake rate followed a different time course. Reduced sensitivity to cocaine was therefore not simply interchangeable with faster or slower ordinary dopamine clearance.
Was amphetamine similarly blunted?
No corresponding reduction was detected on the studied amphetamine endpoints. After cocaine self-administration, amphetamine retained its ability to affect uptake in slices, and a 0.56 mg/kg intravenous amphetamine challenge still increased extracellular dopamine.
Conversely, after amphetamine self-administration, both cocaine- and amphetamine-related transporter responses remained present without the cocaine-specific reduction produced by the cocaine regimen. This comparison argued against attributing the adaptation solely to a history of elevated extracellular dopamine.
The appropriate wording is that the selected amphetamine responses were preserved under these conditions. A nonsignificant group difference does not independently prove exact equivalence, and the study did not assess every amphetamine effect, dose or exposure pattern.
Did the cocaine-related change persist indefinitely?
No. Slice experiments after five cocaine sessions found reduced cocaine uptake inhibition at one and three days of withdrawal, but a return toward control values by 14 days.
That is an animal assay time course, not an individual human recovery schedule. The return of one transporter-related measure does not establish recovery of mood, cognition, cardiovascular function or all other consequences of repeated exposure.
Why does the paper’s timing require caution?
The microdialysis methods describe testing approximately 48 hours after the final self-administration session, whereas the initial results paragraph describes a comparison less than 24 hours afterward. Those descriptions should not be silently reconciled into a precise single sampling interval.
The slice methods describe testing around 18 hours after the final session, apart from the planned recovery experiments. Microdialysis and slice measurements thus should not be treated as simultaneous observations from one uniform post-dose window.
Does this prove a structural change in the transporter?
No. The authors proposed possible changes in transporter conformation, chemical modification or interactions among transporter proteins. The experiments demonstrated functional differences but did not identify one molecular mechanism as the cause.
They also did not measure subjective euphoria or establish that transporter adaptation alone caused faster cocaine intake. The paper explicitly considered the difficulty of linking the rapid laboratory change to later behavioral escalation.
The practical lesson is specificity: identify which drug, exposure history, specimen or preparation, timing and endpoint support a tolerance claim. A history of tolerance to one stimulant cannot replace evidence about an individual’s response to another.
Scientific source
Ferris MJ, Mateo Y, Roberts DCS, Jones SR. Cocaine-insensitive dopamine transporters with intact substrate transport produced by self-administration. Biological Psychiatry. 2011;69(3):201–207. doi:10.1016/j.biopsych.2010.06.026. Full-text author manuscript. Historical original rat experiments using in vivo microdialysis and ex vivo brain slices.
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