“Appetite suppression” can combine effects that occur at different times and adapt differently. A 2010 rat experiment found that amphetamine’s immediate effect on feeding persisted across repeated tests, while a later reduction in intake became smaller under one dosing schedule. Alternating the same dose with a higher dose produced a different pattern.
The study concerns food earned during scheduled feeding opportunities in rats. It does not establish a human weight-loss regimen, diagnose withdrawal or show when a person is functionally unimpaired.
How were feeding and exposure controlled?
White, Hundley and White used 24 adult male Wistar rats, divided among three successive experimental conditions of eight animals each. Rats learned to obtain 94-mg food pellets by lever pressing. During testing, one-hour meal opportunities began every three hours under a 12-hour light/dark cycle.
Each animal underwent eight five-day tests. Saline was administered subcutaneously at light onset on day one; saline or d-amphetamine was administered subcutaneously at light onset on day three. D-amphetamine sulfate was prepared with concentrations expressed as amphetamine base.
In the low-dose condition, day-three treatment alternated between saline and 0.5 mg/kg amphetamine. In the higher-dose condition, it alternated between 1.0 and 2.0 mg/kg. A third group always received 1.0 mg/kg on day three. Alternation order was counterbalanced within the alternating groups.
The one-hour feeding windows are important. The experiment measured intake within that schedule rather than unrestricted moment-to-moment eating, and treatment occurred at the beginning of the rats’ normally less active light phase.
What happened immediately after dosing?
All three active doses—0.5, 1.0 and 2.0 mg/kg—abolished intake during the first post-treatment meal opportunity, approximately the second hour after administration. That immediate effect persisted across testing; the investigators did not observe tolerance to it under this protocol.
A dose sufficient to suppress the first meal was therefore not necessarily sufficient to produce the later feeding pattern. The 0.5 mg/kg dose reduced early intake but did not produce the progressive longer-term reduction seen with the two higher doses.
How was the later effect identified?
The researchers compared meal-by-meal intake after amphetamine with intake after the saline baseline treatment, then accumulated those differences over time. With 1.0 and 2.0 mg/kg, the cumulative deficit deepened later in the first post-treatment day, reaching its greatest extent around 20–23 hours.
This approach preserved timing that a single 24-hour food total could obscure. It also distinguished an initial missed meal from additional reductions at later meal opportunities.
The experiment did not measure plasma amphetamine. The authors discussed rat pharmacokinetic literature when considering possible explanations, but those external estimates are not direct concentration measurements from these subjects. A late feeding effect cannot be converted into a human detection window or impairment duration.
Did repeated 1.0 mg/kg exposure produce tolerance?
When 1.0 mg/kg was administered alone across tests, the later cumulative intake deficit was smaller in tests seven and eight than in earlier test pairs. The authors described this as a tolerance-like effect. The immediate abolition of intake at the first meal remained.
When 1.0 mg/kg alternated with 2.0 mg/kg, comparable tolerance to the longer-term effect was not observed. The authors suggested that occasional higher-dose exposure might maintain or augment the later response to the lower dose.
These are comparisons between particular exposure histories. They do not establish that adding a higher dose causes the same result in humans or that failure to detect tolerance proves an effect will persist indefinitely.
Why did baseline intake complicate the interpretation?
Baseline daily intake declined across tests in the 1.0-mg/kg-only group. A smaller drug-versus-baseline difference could therefore partly reflect a lower comparison baseline, rather than solely a reduced capacity of amphetamine to suppress feeding.
The authors explicitly acknowledged this alternative and called for further study. They also reported continuous weight gain in all groups, which prevents reduced intake at selected meals from being equated with demonstrated progressive weight loss.
A power failure lost the first amphetamine-treatment day’s data in the 1.0-mg/kg-only condition. Its earliest paired summary consequently relied on the second amphetamine administration rather than two complete drug days. That missing observation belongs in the interpretation of the early-to-late comparison.
What conclusions remain well supported?
The study separated short- and longer-term feeding effects by their dose thresholds and patterns across repeated exposure. It also showed why the same nominal dose may behave differently after different prior regimens.
Its mechanistic discussion—including possible changes in hunger regulation, conditioned responses and sensitization—was interpretive. These processes were not independently demonstrated by the feeding measurements. The eight-animal groups, sequential conditions, scheduled food access and changing baselines limit broader claims.
For toxicology interpretation, the practical lesson is to specify the outcome, observation period and comparison baseline before declaring tolerance. Adaptation in a late feeding measure does not establish adaptation in immediate feeding, cognition, cardiovascular function or driving performance.
Scientific source
White W, Hundley MB, White IM. The effects of dose and repeated administration on the longer-term hypophagia produced by amphetamine in rats. Pharmacology Biochemistry and Behavior. 2010;97(2):384–391. doi:10.1016/j.pbb.2010.09.008. Full-text author manuscript. Historical original rat experiment.
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