The adolescent brain appears to be more responsive to nicotine’s rewarding effects than the adult brain, a UC Irvine Transdisciplinary Tobacco Use Research Center (TTURC) animal study has found.
In addition, the researchers found that the first exposure to nicotine during adolescence changes subsequent behavioral responses to the drug. These findings may help explain why teen smokers are prone to continue the habit into adulthood.
The study is the first of its kind to show the rapid changes in the brain and behavior of adolescents after just a single administration of nicotine. Study results appeared in the online version of Psychopharmacology.
“These results suggest that the first exposure to nicotine is rewarding and increases sensitivity in adolescents in a way that might contribute to the increased risk for smoking in this age group,” said lead researcher James Belluzzi, adjunct professor of pharmacology and UCI TTURC investigator.
According to the Centers for Disease Control, 80 percent of adult smokers start smoking as adolescents. While there are multiple reasons why teens try smoking, this study supports earlier findings identifying biological reasons for cigarette addiction. These earlier studies have shown that teens develop symptoms of dependence after minimal tobacco exposure and that those who smoke daily as teens will more likely have difficulty quitting than those who start as adults.
In order to examine the period of the greatest reinforcing effects of nicotine, Belluzzi and colleague Frances Leslie, UCI TTURC director and professor of pharmacology, tested adolescent and adult rats for conditioned place preference. In these tests, drug injections are matched with distinctive environmental cues. Animals are allowed to explore both sides of a test chamber, each with unique sensory cues. Whereas animals initially show no preference for either of two distinct compartments, they will later choose to spend more time in an environment in which they received a drug that has rewarding effects.
In this test, Belluzzi and Leslie monitored behavioral responses and found that nicotine reinforcement did not occur in adult and late adolescent rats. In contrast, rats tested during the earliest adolescent stage showed a significant preference following one brief exposure to nicotine.
Belluzzi and Leslie also found a profound motor excitation, showing a considerable increase in physical activity, in late adolescent rats as compared to adult rats after a single exposure to nicotine. “Late adolescents also appear vulnerable to nicotine,” Belluzzi said.
“These findings provide a strong argument that access to tobacco products should be restricted during adolescence,” Leslie said. “This study adds to a growing body of human and animal data that suggest adolescence may be a developmental time during which the rewarding pathways in the brain are highly responsive to nicotine.”
Improved understanding of the biological mechanisms underlying youth tobacco use and addiction, derived from animal studies such as those of the UCI TTURC, offers promise for improvements in future treatment strategies.
Alex G. Lee and Heather S. Oliff of the UCI Department of Pharmacology assisted with the study, which was funded by the National Cancer Institute (NCI) and the National Institute of Drug Abuse (NIDA). http://www.uci.edu