Quantifying the psychomotor activating effects of cocaine in the rat

Studies that involve analysis of the psychomotor activating effects of drugs often use locomotor activity as the sole measure of psychomotor activation. At low doses, psychostimulant drugs typically produce primarily locomotor hyperactivity. As dose is increased, behavior, however, changes in complex ways, in part because of a transition to behavior progressively dominated by more and more stereotyped actions, such as repetitive head movements. Thus, at some doses an increase in a drug effect is reflected by an increase in locomotion and at others by a decrease, making the interpretation of changes in locomotor activity difficult. Using an automated video analysis system (Clever Sys., Inc. Reston, Virginia, USA), we quantified various components of the psychomotor response to cocaine in the rat, including locomotor activity and lateral head movements, as well as the velocity and/or frequency of these behaviors. We report that the combination of these measures provides an especially sensitive measure of the psychomotor activating effects of cocaine, and how behavior changes as a consequence of repeated drug treatment.