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Adolescent-Typical Behavior Patterns 17in striatum, with declines of to ✉ of the striatal D1 and D2 receptors seen inhuman adolescents, as well as adolescents of other species (Seeman et al., 1987;Tarazi & Baldessarini, 2000; Teicher et al., 1995). Developmental declines instriatum may be sex dependent, with effects in rats generally limited to males(Andersen et al., 1997), although the presence or absence of gonadal steroids hadno effect on the pruning process (Andersen et al., 2002; nor was this decline affectedby blockade of NMDA receptors; Teicher et al., 2003).Data are more mixed with regard to the ontogenetic course of DA binding in thenucleus accumbens (n.Acc), with work reporting early adolescent peaks in D1, D2,and D4 receptors that are about greater than those seen in early adulthood (Taraziet al., 1998, 1999) contrasting with other data reporting no notable overproductionand pruning in this region (Teicher et al., 1995). D1 and D2 receptor overproductionand pruning is also seen in the PFC, although the timing of the decline is relativelydelayed, not occurring until postadolescence (Andersen et al., 2000).Studies in laboratory animals have revealed complementary alterations in the ratesof DA synthesis and turnover in mesolimbic/striatal (e.g., n.Acc and striatal)and mesocortical (e.g., PFC) brain regions during adolescence (Andersenet al., 1997; Teicher et al., 1993). Early in adolescence, estimates of basal rates ofDA synthesis and turnover in PFC are high, declining to lower levels by late adolescenceand adulthood, whereas DA synthesis and turnover estimates in n.Acc andDA turnover estimates in striatum conversely are lower early than late in adolescence(Anderson et al., 1997; Teicher et al., 1993; but see also Leslie et al., 1991).These data are consistent with typical reciprocal relationships often seen amongforebrain DA terminal regions, with levels of DA activity in PFC generally beinginversely related to DA release in subcortical regions in studies conducted in bothrats (Deutsch, 1992) and nonhuman primates (Wilkinson, 1997).These developmental alterations in DA synthesis and turnover have led to thesuggestion that there is a developmental shift in the balance between mesolimbic/striatal and mesocortical DA systems during adolescence (Andersen, 2003; Spear,2000), with mesocortical DA influences peaking early in adolescence, followedlater by a gradual shift toward enhanced activity in mesolimbic/striatal DA terminalregions (see Spear, 2000, for review). This relative shift toward mesocorticalDA predominance early in adolescence likely would be even further exacerbatedby stressors, given the greater sensitivity of mesocortical DA projections thanmesolimbic or striatal terminal regions to activation by stressors (Dunn, 1988).Such developmental alterations in DA balance across these brain regions may beof functional significance for the adolescent, given that DA projections to n.Accand other mesolimbic regions form part of the circuitry critical for labeling incentivestimuli with motivational relevance (Robinson & Berridge, 2003). Indeed,as discussed later, relatively low levels of mesolimbic DA activity earlyin adolescence may contribute to the emergence of adolescent risk taking, given