In their study, the researchers compared the amount of nicotine self-administered by adolescent rats to the amount used by animals first exposed during adulthood. Young rats showed nearly double the rate of nicotine use compared with those initially exposed as adults, the study found.
The adolescents' heavier nicotine use persisted into adulthood, the team reports in the September 2003 issue of the journal Psychopharmacology.
"The results indicate that early nicotine exposure can leave a lasting imprint on the brain," said Edward Levin, Ph.D., professor in the psychiatry and behavioral sciences department at Duke University Medical Center and a researcher at Duke's Nicotine Research Center. The study was supported by grants from the National Institute on Drug Abuse and the National Institute of Mental Health.
Most tobacco use begins during adolescence, Levin pointed out. Among smokers in the United States, 88 percent smoked their first cigarette before the age of 18 and 60 percent before age 14. Adolescence is also a crucial period for the brain, he said, in which the final phase of neuron development occurs.
"The great majority of tobacco addiction begins during adolescence, yet little is known about differential effects of nicotine in adolescents versus adults," Levin said.
Other studies have suggested that smokers who take up the habit at a young age are more likely to continue to smoke, Levin said. However, researchers find it difficult to assess the underlying cause of an association between age and addiction in humans, he added, because the same factors that make people prone to nicotine addiction may also encourage them to begin using the drug at a young age.
To clarify the basis of early nicotine addiction, Levin and colleagues tested for a link between the age of initial nicotine use and addiction in female rats in the laboratory. The researchers provided some rats with nicotine at 40 to 46 days of age, while others were provided nicotine only after 70 to 76 days, once they had reached adulthood. Rats could self-administer a dose of nicotine by pressing a lever.
The adolescent rats self-administered significantly more nicotine than did adults, the researchers found. In a test for chronic nicotine use in the rats during a period of four weeks, animals that began using nicotine during adolescence continued to use more of the drug even after they became adults.
The results suggest that people who begin using nicotine during adolescence may be at greater risk for long-lasting addiction, the team reports.
"The brain continues to develop throughout the teenage years," Levin said. "Early nicotine use may cause the wiring of the brain to proceed inappropriately. In essence, the brains of adolescents who use tobacco may be sculpted around an addiction to nicotine."
Other Duke participants in the study included Amir Rezvani, Ph.D., Daniel Montoya, Jed Rose, Ph.D., and H. Scott Swartzwelder, Ph.D., also of the Durham Veterans Administration Hospital.