Rats prefer cocaine over nicotine in a two-lever self-administration choice test

Smoking is considered the leading cause of preventable death in the United States, but studies in animals suggest that nicotine is only weakly reinforcing. The maintenance of a dangerous habit by a weakly reinforcing agent hats been the topic of some dispute. Using a two-lever 'choice' self-administration procedure developed in our laboratory, we evaluated drug preferences as an index of relative reward strength for nicotine versus cocaine in nicotine-trained rats. Rats were initially exposed to each drug separately in single-lever self-administration sessions and then allowed to choose between them in a two-lever choice test session offering both drugs. When offered choices between different nicotine doses [8, 25, and 75 mug/kg/injection (inj), free base], rats responded approximately equally for any dose, regardless of which doses were compared. Rats clearly preferred 267 or 800 mug/kg/inj cocaine hydrochloride to any of the nicotine doses. These results indicate that cocaine has greater reward strength than nicotine and supports previous findings that self-administering rats seek to maximize reward magnitude regardless of the self-administered drug or training history. It is possible that dependence elevates nicotine's reward magnitude or nicotine addiction may rely more importantly upon negative rather than pure positive reinforcement. (C) 2002 Elsevier Science B.V. All rights reserved.