The brain changes that occur during adolescence may mean teenagers smokers are more likely to become seriously hooked than those who take up the habit as adults, suggests a new rat study.
Previous studies have suggested that the earlier someone starts smoking, the more likely they are to be life-long smokers and the more trouble they have in kicking their addiction, says Edward Levin, who led the study at the Duke University Medical Center in Durham, North Carolina, US.
Experiments to distinguish between the various possible causes could not ethically be conducted in humans. But when Levin and his team let teenage rats help themselves to nicotine, they found that female teen rats - at an age equivalent to 14-year-old girls - take twice as much as rats only exposed to nicotine in adulthood. And this insatiable use carries on when they grow up.
"The results indicate that early nicotine exposure can leave a lasting imprint on the brain," says Levin. "The brain continues to develop throughout the teenage years. Early nicotine use may cause the wiring of the brain to [develop] inappropriately."
The majority of smokers take up their habit during their teenage years, says Levin. In the US, 88 per cent of smokers had started before they were 18, despite it being illegal to sell cigarettes to anyone under that age.
The team provided teen rats aged about 40 days with nicotine. Others were given nicotine only after they had passed into adulthood, at about 70 days. The rats could help themselves to the drug using a lever system, which when pressed gave them an intravenous shot of nicotine.
"The adolescents took twice as much on a per kilogram basis as adults," he told New Scientist. "Furthermore they continued to administer twice as much when they became adults."
Levin says that during adolescence, areas involved in cognition such as the frontal cortex and hippocampus are fine-tuned, as are the reward centres of the brain.
The infant development of the brain involves making more synapses, he explains, but adolescent development involves paring down the nerve junctions. "If you sculpt the brain around addiction, you could make it much more permanent," he says.
It is this trimming away of extra synapses that may also make teenagers better learners than adults. "Previous experiments [in rats] found adolescents quite a bit superior to adults in learning new things - whether learning where to go in a maze or learning addiction," he says.
Levin thinks the team's findings could have implications for treatment. "There may be different strategies for smoking cessation for people starting earlier or later," he says.
Journal reference: Psychopharmacology (vol 169, p 141)
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