To study the behavioral effects of cocaine, Drs. Stephanie Collins and Sari Izenwasser administered either cocaine or a placebo to 12 adolescent rats and 16 adult rats daily for seven days. Each day, the rats were placed in an enclosure with a photobeam array that recorded their activity. After 10 days during which the rats did not receive any substance, all of the rats were given cocaine and their activity was recorded.
Adult rats receiving cocaine for seven days showed a significantly higher daily increase in activity than the adult group receiving the placebo. Ten days after cocaine administration stopped, the activity level of cocaine-exposed adult rats was greater when re-exposed to the drug than when they were initially exposed, indicating that they had developed a sensitivity to the behavioral effects of the drug.
Like cocaine-exposed adult rats, adolescent rats receiving cocaine had higher activity levels than the placebo group. However, in contrast to the activity level of adult rats, the increase in activity level of adolescent rats receiving cocaine remained stable during the seven days of cocaine treatment. Further, the increase in activity after cocaine exposure 10 days later was no different than after their first exposure to the drug.
In a separate experiment to investigate the neurochemical effects of cocaine, either cocaine or placebo was administered to eight adolescent and eight adult rats daily for seven days. Ten days later, their brains were examined for changes in dopamine and serotonin receptors and transporters in the caudate putamen and the nucleus accumbens regions of the striatum. Compared with adult rats in the placebo group, adult rats treated with cocaine showed significant increases in dopamine transporter density in the caudate putamen. Serotonin transporter densities also were increased in the caudate putamen and the nucleus accumbens. Adolescent rats showed no changes in dopamine or serotonin transporter densities in either region.
WHAT IT MEANS: Different behavioral and neurochemical adaptations to repeated cocaine administration may be related to development factors and have implications for future directions in prevention and treatment interventions.
This study, funded by the National Institute on Drug Abuse, is published in the September 20, 2002 issue of Developmental Brain Research.