Spatial pattern analysis reveals similarities between amphetamine conditioned and unconditioned locomotion

The repeated injection of drugs which increase locomotion, paired with a distinctive testing environment, has been shown to result in conditioned hyperactivity upon subsequent exposure to the environment alone. Unknown, however, are the form and characteristics of the behavioral response that makes up the conditioned hyperactivity. Conditioned and unconditioned locomotor responses in rats were compared with a computerized Behavioral Pattern Monitor (BPM), a system which provides detailed information regarding the amount and qualitative patterning of locomotor activity and investigatory responses. After an initial 2h exposure to the BPM chambers (baseline day), rats were randomly assigned into one of two groups and injected each day with saline or 0.75mg/kg amphetamine (AMPH) prior to placement in the BPM chambers for a 2h experimental session. Rats were then returned to their home cages and injected with the alternate solution. This procedure was repeated for 5 days, and on the sixth day all rats were injected with saline prior to placement in the BPM. Rats that had received AMPH/BPM pairings exhibited significant increases in horizontal locomotion, rearing and investigatory holepokes on the test day compared to rats that received saline/BPM pairings. The AMPH group also exhibited an increased sensitivity to a physical movement of the test chambers. Analyses of the daily locomotor paths for each rat using transition matrices revealed that the conditioned activity measured on the test day resembled the last AMPH day more than either the first AMPH day or the predrug baseline day. These descriptive measures of each animal's spatial patterns of locomotion suggest that the particular pattern induced by AMPH in each animal stabilized over time and became conditioned to environmental cues. In this case, the increased motor activity observed on the test day was qualitatively similar to the unconditioned response produced by AMPH.