Making It So

From “The Neurobiology of Addiction and Recovery in Adolescence: A Guide for Parents,” an internal document of the Treatment Research Institute, Philadelphia, PA.

During adolescence, judgment and decision-making slowly improve, becoming more adultlike. These functions are associated with the brain’s prefrontal cortex, the frontmost area of the brain. This area, and its connections to other areas of the brain, do not mature until early adulthood (Chambers, Taylor, & Potenza, 2003). Two major kinds of changes take place in the maturing adolescent prefrontal cortex. These are myelination and pruning. Myelination means that the connecting parts of brain cells are being wrapped in insulation. Once insulated, these brain cells will send signals faster and more efficiently. Pruning means that unused brain connections are disappearing, which increases the efficiency of neural circuits (Schepis, Adinoff, & Rao, 2008). Overall, during adolescence, the number of brain cells decreases and the number of brain connections increases (Giedd, Blumenthal, Jeffries, Castellanos, Liu, Zijdenbos, Paus, Evans, & Rapoport, 1999). Because of these processes, the adult brain is fast and highly efficient, and has strong pathways for different parts of the brain to communicate with each other. But these processes do not finish until at least age 20 (Schepis, Adinoff, & Rao, 2008).

Myelination and pruning are changes to the structure of the brain. Because these structures change during adolescence, some aspects of the brain’s function change as well. The systems of chemical neurotransmitters that the brain uses to communicate between nerve cells are maturing. In particular, the system for the neurotransmitter dopamine is altered.

When one nerve cell signals to another nerve cell, it does so by releasing a certain amount of neurotransmitter. That neurotransmitter crosses the space between the cells and binds to another chemical, a receptor, on the surface of the receiving cell, like a baseball into a glove. The young brain has many more of these receptors for dopamine than it actually needs, and studies in rats show that adolescents may have higher levels of dopamine in their brains than adults (Song, Wang, Zhao, Wang, Zhai, & Lu, 2007). Dopamine receptor pruning does not occur until late in adolescence (Schepis, Adinoff, & Rao, 2008).

Dopamine is used in many parts of the brain for different purposes. Dopamine in the region of the brain called the nucleus accumbens promotes motivated action and learning (Schepis, Adinoff, & Rao, 2008). The nucleus accumbens acts with the prefrontal cortex and a region called the striatum to create a circuit for appetitive behaviors – that is, behaviors that regulate or control other behaviors and cognitive processes so that a person takes steps toward attaining a reward such as food or drugs. These regions take input about emotion and drive states, such as hunger, and translate them into behavior (Schepis, Adinoff, & Rao, 2008). Because of their immature prefrontal cortices, adolescents are less able than adults to use long-term information about consequences to adjust their motivational behavior (Schepis, Adinoff, & Rao, 2008) and are more prone to relying on immediate emotion.

The neurotransmitter system for the neurotransmitter serotonin is also slow to mature. Serotonin appears to act against dopamine. Dopamine promotes appetitive behavior, whereas serotonin inhibits appetitive behavior (Schepis, Adinoff, & Rao, 2008). The extra dopamine receptors in the adolescent brain mean that serotonin cannot keep up, and appetitive behaviors – behaviors that seek out anything rewarding or pleasant – are stronger than behaviors and motivations to limit reward-seeking.

Dopamine neuron firing is associated with the brief motivational components of learning and cognitive behavior (Schultz, Apicella, & Ljungberg, 1993). The dopamine neurons respond to stimuli that predict rewards (Schultz, Apicella, & Ljungberg, 1993), and these responses are particularly important during the learning of a behavior (Schultz, Apicella, & Ljungberg, 1993). Learning about rewards is mediated by dopamine connections to the frontal lobe and striatum (Schultz, 1998). The dopamine response depends on how unpredictable the reward is (Schultz, 1998). This dopamine signal changes during learning (Schultz, 1998). Early in learning, dopamine neurons respond to the presence of a reward. After learning, however, these responses are due to the earliest stimuli that predict the presence of a reward (Schultz, 1998; Schultz, Apicella, & Ljungberg, 1993). Dopamine neurons also respond to novel stimuli that are positively motivating and to stimuli similar to reward predictors (Schultz, 1998). This means that there are two kinds of inputs to dopamine neurons: one for the presence of a reward, and one for the expectation of a reward (Schultz, Apicella, & Ljungberg, 1993). These two kinds of responses are important because they indicate the effect of the drug or alcohol, and the craving for the drug or alcohol, respectively.