Connections from PFC to the NAc continue to rise during adolescence as well, with developmental increases in the number of PFC pyramidal cells that project to NAc, along with a transient increase in the proportion of these projection neurons that express DA D1 receptors (D1-Rs). As a result, the percentage of accumbal-projecting pyramidal cells containing D1-Rs peaks at levels notably higher late in adolescence (>40%) than at younger or older ages (<4–5%) (Brenhouse et al., 2008). These findings are intriguing, given evidence for the importance of PFC projections to NAc in drug seeking (e.g., Kalivas et al., 2005) and for a potential role of PFC D1-Rs in increasing the reinforcing efficacy of drugs (see Brenhouse et al., 2008). The pruning of these PFC DA receptors does not occur until early adulthood, with substantial declines in D1- and D2-R density evident between P60 and P80 (Andersen et al., 2000).
