Some neurotransmitter systems appear to show refinement during the teenage years. Brain regions with input from the neurotransmitter dopamine that comprise the reward system (i.e., prefrontal cortex, striatum, and nucleus accumbens) undergo pronounced developmental changes during adolescence (Spear 2009). In particular, the density of dopaminergic connections to the prefrontal cortex increases in this phase of life (Lambe et al. 2000; Rosenberg and Lewis 1994; Tunbridge et al. 2007). Activity of the dopamine degrading enzyme catechol-O-methyltransferase (COMT) appears to increase in mid-adulthood (after age 30) based on post mortem studies (Tunbridge et al. 2007), but does not differ between infants, adolescents, and young adults. This post-adolescent maturation may facilitate improved dopaminergic transmission to and within the prefrontal cortex. Dopamine synthesis and turnover in portions of the prefrontal cortex that project to subcortical regions (e.g., striatum) increase from adolescence to adulthood (Andersen et al. 1997; Teicher et al. 1993), and this dopamine balance shift between prefrontal and subcortical structures may be due to pruning in the neocortex (Bourgeois et al. 1994; Woo et al. 1997).
