Adolescence can be an awkward time. For humans, the teenage years can be marked by behaviors that are not seen in people of other ages. These behaviors can be relatively benign, such as dressing in all black and going to all-night dance parties, and they can be more dangerous and risky, such as indiscriminate sexual behavior and drug use. Teens usually outgrow most of these behaviors (how many “emo” adults do you see?), but drug use is particularly problematic; research has shown that someone who first uses drugs as an adolescent is far more likely to become chronically addicted than someone who first uses as an adult. Why?
Many scientists have suggested that addiction is the consequence of an over-active “reward” circuit in the brain. Under this view, drug addiction originates as a drive to achieve an outcome that is pleasurable. In support of this view, drugs like cocaine increase the activation of dopamine, a neurotransmitter that is associated with pleasure. But this doesn’t fit every case; for example, why do addicts continue to use drugs even when the outcomes—losing a marriage, a family, a career—are so clearly unpleasant? In fact, drug addicts often report a strong desire to quit using drugs, but find that their behavior is almost automatic, outside their control.
My lab studies the neural circuitry responsible for learning, especially the kind of learning that helps us make connections between an environmental cue and its meaning. Think about that long drive up to the cabin, when you see a big, yellow “M” up ahead. You barely have time to think, “fries and a chocolate shake” before your hands jerk the steering wheel to the right and your car swerves as you make the exit. This link between the sign and its meaning is certainly something that is learned (that is, no one is born knowing McDonalds has fries), but once it is learned the link is automatic; there is no need to think about how the Golden Arches is a sign that stands for McDonalds, that McDonalds is a place to get food, and that the menu includes hamburgers, French fries and chocolate shakes. This form of learning that connects a sign to its meaning is called “associative learning.”
An alternative view of drug addiction is that it is a form of associative learning, where the connections between environmental cues and drug use grow so strong that drug use becomes a compulsive, almost automatic behavior. A smoker lights a cigarette in a particular environment (for instance, in the car) and soon, the environment becomes the reason for the behavior; without thinking, she lights up when she gets in a car. This may be one reason that relapse among former addicts more commonly occurs in familiar environments than in new environments. If this viewpoint holds, then by studying how the brain circuitry responsible for learning changes during adolescence, we should be able to get clues about adolescent drug use, too.
In a paper we published recently in the journal PLoS ONE, we used an odor-guided associative learning task to test whether associative learning changes during adolescence, and we examined gene and protein expression to see whether the dopamine system (which is found in brain areas important for associative learning) changes during this time. We used rats to explore these changes, so that we could look very closely at what was happening in the brain. We used odor associations rather than visual cues because rats get around primarily by using their sense of smell, and they pay careful attention to odors in their environment.
Three undergraduate students were co-authors on the publication: Anna Garske (now pursuing a PhD in neuroscience at the University of Colorado), Chloe Lawyer (a Goldwater Scholar and still working in the lab), and Brittni Peterson (now pursuing a PhD in neuroscience at the University of Minnesota). Six more undergraduates provided help with the two-year project (Kate Hanson, Netsanet Negussie, Rolf Skyberg, Betsy Smith, Anthony Spano and Kim Uy).
We trained rats to dig in a cup of sand to obtain a tasty treat: Froot Loops (rats love Froot Loops). After the rats learned to dig, we put odors in the sand and gave rats a choice between two cups: one cup (e.g., minty sand) contained a Froot Loop, while the other cup (e.g., cinnamon sand) contained nothing. We observed how many trials it would take for the rats to learn the meaning of an odor, which we defined as a rat digging in the Froot Loop cup on eight out of 10 consecutive trials. We tested rats at three different ages: juvenile (21-28 days old), adolescent (34-49 days old) and adult (50-150 days old).
We found that juvenile and adult rats learned the task very quickly; on average, they learned the odor association in about 12 trials. Interestingly, adolescent rats took almost twice as many trials to learn the association (on average, about 22 trials). In addition, we found that adolescent rats had a higher number of trials where they were distracted during the task, either grooming or exploring their cages rather than digging in the cups of sand. Juvenile rats almost never became distracted, and adult rats did so only occasionally. So adolescent rats were not only slower at learning the task, they were also less focused on the task and more distractible. Sound like any adolescent humans you know?
Next, we wanted to find out whether experience with the task would help adolescent animals become faster learners, or at least help them become less distractible. To explore this, we first trained juvenile rats on an odor association task, and then tested them on a new odor association task when they became adolescents. We found that these animals learned the task in about 12 trials, and they also displayed very low levels of distractibility. So previous experience with an associative learning task helped adolescent rats learn faster, and helped them stay focused during the task.
Could the developing dopamine system be playing a role?
Previous research by others has shown that the dopamine pathways that interact with the learning circuitry are starting to mature during adolescence. We were interested whether the cells within the learning circuit were also undergoing changes in how they process dopamine. First, we used real-time quantitative PCR to explore mRNA levels for the dopamine receptor. We found that compared to adult and juvenile brains, there was significantly more mRNA for dopamine receptors in adolescent rats in the areas of the brain that are responsible for associative learning. Second, we used immunohistochemical methods to determine how the dopamine receptor protein was distributed within these areas. We found that the receptors were expressed in about twice as many cells in these brain areas during adolescence compared to juvenile or adult rats. Taking both methods into consideration, we concluded that there are more dopamine receptors in the learning circuitry in adolescent brains than in adult or juvenile brains.
These findings prompted us to test whether targeting the dopamine system with drugs during adolescence would change how well rats could learn the odor association task. We used two drugs that targeted specific dopamine receptor subtypes. When we administered low doses of these drugs to adolescent rats, we found that they learned the associations as fast as adult and juvenile animals, taking about 10 trials to reach criterion. Further, these rats were highly focused during the task, displaying very low levels of distractibility.
These results suggest that dopaminergic modulation of cortical function may be important for learning the meaning of environmental stimuli, and that developmental changes in cortical dopaminergic circuitry may underlie age-related differences in associative learning. These results also suggest that early experience with learning tasks may improve adolescent associative learning.
Our study shows that associations are learned more rapidly, and that behavior becomes more focused, when the dopamine system is more highly activated. Such activation could occur either by a highly rewarding situation or through drug use. What does this mean for drug addiction? It might mean that during adolescence, a drug that is able to activate the dopamine system also activates learning circuitry, which learns behavior of taking drugs, and voilà: an addiction is born.
What could our results be telling us about adolescent behavior in general? What is the advantage of increased distractibility and slower learning during adolescence? Perhaps the system ensures that learning is specific to associations that are repeatedly confirmed, or that are highly rewarding and lead to greater dopamine release. Such a system would be expected to increase behavioral variety and flexibility, including exploratory and risk-taking behaviors. In this way, behaviors that are important for survival such as foraging, colonizing new territory, and engaging in social behaviors are promoted. While these behaviors would increase the survival risk for individuals, the expanded territory and increased social interactions that would result from a population biased towards these behaviors would benefit the survival of the species.