Reduced presynaptic dopamine activity in adolescent dorsal striatum

Abstract

Adolescence coincides with symptomatic onset of several psychiatric illnesses including schizophrenia and addiction. Excess limbic dopamine activity has been implicated in these vulnerabilities. We combined molecular and dynamic indices of dopamine neurotransmission to assess dopamine function in adolescent rats in two functionally distinct striatal subregions: nucleus accumbens (NAc) and dorsal striatum (DS). In adolescents, we find an overall reduction in dopamine availability selective to the DS. Dopamine release in the DS, but not in the NAc, was less responsive to amphetamine in adolescents compared to adults. The dopamine transporter (DAT) inhibitor, nomifensine, similarly inhibited basal and amphetamine-induced dopamine release in either regions of both the age groups, suggesting that the reduced effectiveness of amphetamine is not due to differences in DAT function. Furthermore, DAT and vesicular monoamine transporter-2 expressions were similar in the DS and NAc of adolescent rats. In contrast, expression of tyrosine hydroxylase (TH) was reduced in the DS, but not in the NAc, of adolescents compared to adults. Behaviorally, adolescents were less sensitive to amphetamine but more sensitive to a TH inhibitor. These data indicate that, in contrast to the general notion that dopamine is hyperactive in adolescents, there is diminished presynaptic dopamine activity in adolescents that is selective to the DS and may result from attenuated TH activity. Given recent reports of altered dopamine activity in associative/dorsal striatum of individuals at a clinically high risk of psychosis, our data further support the idea that dorsal, as opposed to ventral, regions of the striatum are a locus of vulnerability for psychosis.

Figure 1

Histology. Schematic representations of microdialysis probe placements in the DS and NAc.

Figure 2

Amphetamine causes lower increases in DS dopamine and motor behavior in adolescent rats. Adolescent (orange) and adult (blue) rats were given an i.p. injection of 1.0 mg/kg amphetamine (squares) or 0.9% saline vehicle (circles). Dopamine release was measured in the (a) NAc (adolescent amphetamine group, n=6; adult amphetamine group, n=6; adolescent vehicle group, n=5; adult vehicle group, n=5), and (b) DS (adolescent amphetamine group, n=6; adult amphetamine group, n=5; adolescent vehicle group, n=6; adult vehicle group, n=5). Motor behavior was also measured for each animal, expressed as (c) ambulations (adolescent amphetamine group, n=8; adult amphetamine group, n=6; adolescent vehicle group, n=9; adult vehicle group, n=9), and (d) fine movements (adolescent amphetamine group, n=8; adult amphetamine group, n=6; adolescent vehicle group, n=9; adult vehicle group, n=9). In each graph, baseline dopamine release and motor behavior were measured prior to injection, represented by the first three time points displayed. All data are expressed as mean±SEM. Black arrows denote time of injection. Asterisk(s) denotes significant difference between amphetamine-treated age groups at a given time point (*p<0.05, **p<0.01, ***p<0.001).

Figure 3

Adolescent rats have decreased levels of TH and DAT in the DS. Diagrams in the upper right corner of each panel depict general regions of dissection of tissue-free hand dissected bilaterally from both the NAc (outlined in green) and DS (outlined in red) of adolescent and adult rats. Protein expression levels of DAT (adolescents, n=4; adults, n=3), VMAT2 (n=6 for each group), and TH (n=6 for each group) were measured using western blot analysis in the (a) NAc, and (b) DS of adolescent (orange bars) and adult (blue bars) rats. All protein optical densities were normalized to tubulin, shown along the bottom of the figure. All data are expressed as mean±SEM. Asterisk(s) denotes significant differences between age groups (*p<0.05, **p<0.01, ***p<0.001).

Figure 4

Blockade of DAT produced similar regional effects on dopamine release in both adolescents and adults. The DAT inhibitor nomifensine was infused directly into the NAc and DS of adolescent (orange) and adult (blue) rats. Dopamine release was measured in the (a) NAc (adolescent group, n=11; adult group, n=10), and (b) DS (adolescent group, n=14; adult group, n=11). When maximum dopamine release was reached for at least three consistent time points during nomifensine infusion, animals were given an i.p. injection of 1.0 mg/kg amphetamine (squares) or 0.9% saline vehicle (circles). Dopamine release continued to be measured in the (c) NAc (adolescent amphetamine group, n=6; adult amphetamine group, n=6; adolescent vehicle group, n=5; adult vehicle group, n=5), and (d) DS (adolescent amphetamine group, n=6; adult amphetamine group, n=5; adolescent vehicle group, n=8; adult vehicle group, n=6) following injection. In graphs (a) and (b), baseline dopamine release was measured prior to infusion, represented by the first three time points displayed. In graphs (c) and (d), baseline dopamine release was measured after nomifensine infusion caused a plateau in dopamine release, represented by the first three time points displayed. All data are expressed as mean±SEM. Black bar denotes nomifensine infusion. Black arrow denotes time of amphetamine injection.

Figure 5

TH inhibition causes lower increases in fine movements in adolescent rats after amphetamine administration. Adolescent (orange) and adult (blue) rats were pretreated with alpha-methyl-DL-tyrosine before given an i.p. injection of 1.0 mg/kg amphetamine (squares) or 0.9% saline vehicle (circles). Motor behavior was measured for each animal (n=10 for each group), expressed as (a) ambulation, and (b) fine movements. In each graph, baseline motor behavior during pretreatment was measured prior to amphetamine injection, represented by the first six time points displayed. All data are expressed as mean±SEM. Black arrow denotes time of injection.