Rewiring the future: drugs abused in adolescence may predispose to mental illness in adult life by altering dopamine axon growth.

Adolescence is a period of increased exploration and novelty-seeking, which includes new social behaviors, as well as drug experimentation, often spurred on by peer pressure. This is unfortunate, as the immature state of the adolescent brain makes it particularly susceptible to the negative developmental impact of drug use. During adolescence, dopamine terminals, which have migrated from the ventral tegmental area, pause in the nucleus accumbens, before segregating by either forming local connections or growing towards the prefrontal cortex (PFC). This developmentally late and lengthy process renders adolescent dopamine axon pathfinding vulnerable to disruption by substance use. Indeed, exposure to stimulant drugs in adolescent male mice, but not females, triggers dopamine axons to mistarget the nucleus accumbens and to grow ectopically to the PFC. Some evidence suggests that at this novel site, the functional organization of the ectopic dopamine axons mirrors that of the intended target. The structural rewiring dysregulates local synaptic connectivity, leading to poor impulse control ability, deficits of which are a core symptom of substance-use disorders. In the present commentary, we argue that different substances of abuse induce dopamine mistargeting events with the off-target trajectory prescribed by the type of drug, leading to psychiatric outcomes later in life.