At a finer scale, rat and primate studies have demonstrated numerous differences in adolescent neurotransmitter systems. Adolescents tend to over-express dopaminergic, adrenergic, serotonergic and endocannabinoid receptors across many regions followed by pruning to adult levels (Lidow and Rakic, 1992; Rodriguez de Fonseca et al., 1993). They express D1 and D2 dopamine receptors at higher levels in subcortical targets such as the dorsal striatum and nucleus accumbens, although some have not found reduced adult expression in this latter region (Gelbard et al., 1989; Tarazi and Baldessarini, 2000; Tarazi et al., 1999; Teicher et al., 1995). During adolescence, there are also changes in dopamine production and turnover, as well as evidence for changes in downstream effects of receptor-ligand binding (Badanich et al., 2006; Cao et al., 2007; Coulter et al., 1996; Laviola et al., 2001; Tarazi et al., 1998). Functionally, there is evidence from anesthetized rats that the spontaneous activity of midbrain dopamine neurons peaks during adolescence and then decreases (McCutcheon and Marinelli, 2009). Developmental changes in mesocorticolimbic dopamine circuitry and activity may underlie some differences in motivated behavior generally, as well
