Amphetamine accelerates and attenuates ultradian activity rhythms in preweanling rats

Developing rats, studied in environmental isolation, display prominent fluctuations in locomotor activity with a periodicity of about 1-3 hr. This ultradian rhythmic pattern is most marked at 2 weeks of age, and appears to be endogenously mediated. (+)Amphetamine (1 mg/kg) was administered to 2 week old rat pups, and their locomotor activity levels were recorded continuously and stored in 5 min intervals using a sensitive computer-interfaced vibrational activity monitor. Activity was recorded for 12 hr after treatment and resulting time-series data were analyzed by harmonic spectral techniques. During the first 6 hr of testing, amphetamine induced a prominent low frequency perturbation in baseline activity levels corresponding to the expected period of acute drug action. During this time, normally prominent ultradian activity rhythms in the range of 8-12 cycles per day (cpd) were diminished in amplitude, even following low frequency smoothing to remove the changes in baseline. Correspondingly, there was also an increase in ultradian rhythm amplitude in amphetamine-treated pups at higher frequencies (32-40 cpd). During the final 6 hr of testing there was a marked suppression of typical ultradian rhythms in amphetamine-treated pups but not in controls. These results suggest that amphetamine treatment both accelerates and attenuates ultradian activity rhythms in developing rats during the acute period of drug action, and produces a prominent diminution in these rhythms during subsequent rebound and recovery periods.