Dr. Laura C. Klein, assistant professor of biobehavioral health, says the study showed a direct relationship between adolescent nicotine exposure and elevations in the adult rats' stress hormones. In humans, these same hormones are associated with increased risk for anxiety, depression and cognitive impairments.
The results of the present study are consistent, she adds, with a recent investigation involving human subjects conducted by other scientists, which showed that adolescent smoking may cause depression. The rat studies may offer a biochemical explanation for the effects observed in the human study.
Klein reported her results in the August issue of the journal, Experimental and Clinical Psychopharmacology. The study is the first to involve the use of adolescent rats to investigate the effects of adolescent nicotine exposure on the likely effects of consuming addictive drugs in adulthood.
In the study, 30 male and 30 female rats were exposed to nicotine throughout adolescence and then given an opportunity to consume opiates when they became adults. The adolescent rats were fitted with minipumps that administered nicotine directly to their bloodstreams at either a low or high nicotine dosage, similar to those that yield biochemical and behavioral effects in adult humans who smoke. After 19 days, the minipumps were removed and the rats, now all adults, went for 7 days without nicotine exposure. They were then offered an opiate solution that they could access whenever they chose. Finally, after a five-day opiate withdrawal period, the stress hormone levels in the rats' blood plasma were measured.
The finding that adolescent nicotine exposure increases levels of stress hormones when the rats become adults was unexpected and intriguing because of the long period of time - 44 days - that passed between the adolescent nicotine exposure and the time that the stress hormone levels were measured in adulthood, says Klein.
"Low doses of nicotine produced a doubling of stress hormones when the rats became adults," the Penn State researcher says. "High doses of nicotine quadrupled stress hormones for both adult male and female rats."
"These stress hormones are similar to the stress hormones that are related to depression, anxiety and attention problems in humans. Thus, the study suggests that stress hormones induced by adolescent nicotine exposure may be the route by which nicotine leads to depression in adult human smokers," adds Klein.
In addition, the research suggested a positive relationship between adolescent nicotine exposure and opioid consumption among male rats. The male rats exposed to low amounts of nicotine during adolescence consumed significantly more opioids when they became adults. The findings were also consistent with Klein's previous work, which showed that female rats consume more opioids than do males.
Overall, Klein writes, "the results reveal that there are complicated biobehavioral effects of adolescent nicotine exposure in rats and that the nicotine minipump paradigm can be used to evaluate biobehavioral effects of nicotine in adolescent rats."
"The present experiment is a first step in developing an animal model of nicotine exposure that could be used to evaluate biobehavioral mechanisms of nicotine addiction as well as factors that may contribute to drug use by adolescents and young adults," she adds.