of abuse.9,12,13 Animal studies utilizing the intravenous cocaine selfadministration procedure have shown that periods of extended daily access to cocaine and other addictive drugs can trigger escalating cocaine intake in rats similar to that observed in human drug users.7,11,14,15 Rodents also show escalating levels of intake during extended daily access to self-administered heroin,16 nicotine,17 or methamphetamine.18 Prolonged access to cocaine also results in the development of drug-seeking responses (lever presses) that become progressively less sensitive to the suppressant effects of a noxious stimulus (cue that predicts the onset of aversive footshocks).14 This compulsive-like responding for cocaine in rats is similar to the drug seeking observed in human addicts that persists even in the face of negative social, economic, and/or health consequences associated with their drug habit.2 Prolonged cocaine intake (>3 months) can trigger cocaine responding that persists even when drug delivery is directly paired with punishing electric shock.15 Thus, a history of extended drug access can induce an addiction-like state in rats, characterized by a loss of control over the amounts of drug consumed (reflected in escalating daily intake),7,11,14,15 and drug seeking that is impervious to negative outcome.5,8,19 Understanding the underlying neurobiological mechanisms of these processes in rats may reveal
