A new study has founded that adolescents who are frequently exposed to high doses of amphetamine, a stimulant that is used to treat attention deficit hyperactivity disorder (ADHD) and narcolepsy, are at-risk of developing long-term changes in their brain function.

For this study, the researchers examined how amphetamine can affect the dopamine system in younger rats that were comparable to human adolescents. They found that repeated exposure in rats causes changes in dopamine signaling. Since dopamine can affect memory, learning, attention span and reward-motivated behaviors, any changes can negatively impact these functions

"The dopamine system, which continues to develop throughout adolescence and young adulthood, is a primary target of psychostimulant drugs like amphetamine," said Joshua Gulley, a psychology professor at the University of Illinois and the lead investigator of the study. "Changes in dopamine function in response to repeated drug exposure are likely to contribute to the behavioral consequences - addiction and relapse, for example - that abusers experience."

The researchers could not pinpoint how the stimulant was affecting brain function, but they reasoned that since the adolescent brain is still developing, early exposure can impact the brain's development process. To understand this link further, the researchers analyzed the effects that amphetamine had on the prefrontal cortex, which is one of the last regions in the brain to fully develop in adolescence. The researchers found that repeated exposure affected how cells responded to dopamine.

"You need cells that are firing and communicating with one another, but you also need cells to stop communicating with one another at certain times and become quiet," Gulley explained. "Our research suggests that a subtype of dopamine receptor, the D1 receptor, is altered following amphetamine exposure. It's either not responding to dopamine or there are not as many of these receptors after exposure as there used to be."

The research team noted that the changes in dopamine signaling lasted for 14 weeks. At this point, the rats would represent people in their 30s.

"This shows pretty clear evidence that drug use during adolescence, a time when the brain is still developing, has extremely long-lasting consequences that go far beyond the last drug exposure," Gulley said.

The researchers acknowledged that since their study was done in rats, the findings might not apply to human children. However, Gulley noted that "rats [can] exhibit many of the characteristics that human adolescents do." This is not the first time that Gulley has studied the effects of amphetamine abuse. In a 2013 study, Gulley found that frequent amphetamine exposure affected the working memory in rats.

The findings were published recently in the journal Neuroscience.