Brain temperature change and movement activation induced by intravenous cocaine delivered at various injection speeds in rats

Speed of intravenous (i.v.) injection presumably affects the rewarding effects of cocaine in humans. Work with animals has shown alterations in the behavioral and neurochemical effects of cocaine based on delivery speed. We studied the effects of cocaine (1 mg/kg) as both a single i.v. injection and a series of five repeated injections (8-min intervals) delivered at different speeds (4, 16, and 64 s) on brain, muscle, and skin temperatures, and locomotion in awake, unrestrained rats. Since cocaine has a distinctive action on temperature, any enhancement of cocaine's properties by injection speed should readily be seen. When given as a single injection, cocaine increased brain temperature and locomotion, but transiently decreased skin and muscle temperatures; these effects were augmented by a high injection speed. Regardless of injection speed, however, changes in brain temperature and locomotion were strongly correlated with basal brain temperatures; higher temperatures were associated with less change after cocaine injection. When given as a series of five injections, cocaine increased brain temperature and locomotion. Although skin temperature initially decreased, it increased after successive cocaine injections. With each successive cocaine injection in the series, measures of temperature and movement parameters increased to a plateau and brain temperature change became biphasic. While confirming the results of others that rapid injection speed enhances cocaine-induced locomotor activation, our study suggests that delivery rate also affects the basic physiological actions of cocaine.