Although the dangers of smoking are well-known, nicotine's effects on the brain are less clearly understood. A study published online September 28 in Neuropsychopharmacology shows that nicotine makes the brain's reward pathways more sensitive. In addition, the number and frequency of cigarettes smoked may allow each smoker's brain to derive maximum satisfaction from nicotine and other stimuli.
Paul Kenny and Athina Markou of the Scripps Research Institute trained rats to press levers for a burst of intravenous nicotine. In sessions lasting either one or 12 hours for 20 days, the rats could inject themselves whenever they liked. Before and after each session, the rats were tested on a separate apparatus to evaluate their "reward thresholds." In this phase of the investigation, each rat turned a wheel to stimulate an electrode implanted in the brain's reward circuitry. The threshold was defined as the intensity of current needed to ensure that the rat would try again.
Both the one-hour and 12-hour groups quickly stabilized their daily consumption of nicotine. Patterns of intake also stabilized; the one-hour rats took more in the first half-hour and less in the second, while the 12hour rats "loaded up" with nicotine in the first and last 30 minutes but during the rest of the session limited themselves to one or two infusions 30 minutes apart.
After each session, the reward thresholds proved significantly lower in all rats, suggesting that the animals had consumed enough nicotine to reset the brain's sensitivity to reward. The heightened sensitivity lasted for at least 36 days after the nicotine sessions were stopped.
According to the authors, the data may account for some of the unique properties of nicotine compared to other drugs of abuse-cocaine and heroin, for example-that lead to compulsive increases in intake while delivering diminishing rewards. The study also shows how nicotine can enhance related pleasures, such as alcohol, caffeine, and food.