Abstract

There has been controversy over whether early exposure to stimulant drugs, including methylphenidate (MPH), used in the treatment of attention deficit hyperactivity disorder (ADHD) increases the risk for drug abuse in adult life. This concern is based on research in adult animals showing that repeated exposure to stimulants results in sensitization to their rewarding effects. Yet, relatively little is known about the effects of developmental exposure to stimulants. Whether such effects vary between individuals with ADHD and others without the disorder is another question. In this thesis, I used place conditioning to examine whether MPH pretreatment alters the rewarding effects of cocaine later in life, first, in a typical strain of rats (Sprague Dawley) and, second, in an animal model of ADHD, the Spontaneously Hypertensive rat (SHR). Groups of male peripubertal rats were pretreated with MPH or saline for ten consecutive days. Twenty-five days later, the reward value of cocaine was assessed using place conditioning. Rats learned to associate cocaine with one of two dissimilar compartments. During the preconditioning phase, both MPH- and saline-pretreated rats spent equivalent amounts of time in each compartment. After conditioning, saline-pretreated rats given moderate and high doses of cocaine spent more time in cocaine-associated compartments than they did before conditioning. However, in MPH-pretreated rats only the highest dose of cocaine established place preferences. These results suggest that peripubertal MPH administration decreases rather than increases the sensitivity to the rewarding effects of cocaine in both a typical strain of rats and an animal model of ADHD.