Dysfunctional dopaminergic neurotransmission in asocial BTBR mice.
Bottom Line: Recent psychosocial and neuroimaging studies have highlighted reward-processing deficits and reduced dopamine (DA) mesolimbic circuit reactivity in ASD patients.However, the neurobiological and molecular determinants of these deficits remain undetermined.DA D1 receptor-dependent behavioural and signalling responses were found to be unaltered in BTBR mice, whereas dramatic reductions in pre- and postsynaptic DA D2 and adenosine A2A receptor function was observed in these animals.
Affiliation: Ceinge Biotecnologie Avanzate, Naples, Italy.
Autism spectrum disorders (ASD) are neurodevelopmental conditions characterized by pronounced social and communication deficits and stereotyped behaviours. Recent psychosocial and neuroimaging studies have highlighted reward-processing deficits and reduced dopamine (DA) mesolimbic circuit reactivity in ASD patients. However, the neurobiological and molecular determinants of these deficits remain undetermined. Mouse models recapitulating ASD-like phenotypes could help generate hypotheses about the origin and neurophysiological underpinnings of clinically relevant traits. Here we used functional magnetic resonance imaging (fMRI), behavioural and molecular readouts to probe dopamine neurotransmission responsivity in BTBR T(+) Itpr3(tf)/J mice (BTBR), an inbred mouse line widely used to model ASD-like symptoms owing to its robust social and communication deficits, and high level of repetitive stereotyped behaviours. C57BL/6J (B6) mice were used as normosocial reference comparators. DA reuptake inhibition with GBR 12909 produced significant striatal DA release in both strains, but failed to elicit fMRI activation in widespread forebrain areas of BTBR mice, including mesolimbic reward and striatal terminals. In addition, BTBR mice exhibited no appreciable motor responses to GBR 12909. DA D1 receptor-dependent behavioural and signalling responses were found to be unaltered in BTBR mice, whereas dramatic reductions in pre- and postsynaptic DA D2 and adenosine A2A receptor function was observed in these animals. Overall these results document profoundly compromised DA D2-mediated neurotransmission in BTBR mice, a finding that is likely to have a role in the distinctive social and behavioural deficits exhibited by these mice. Our results call for a deeper investigation of the role of dopaminergic dysfunction in mouse lines exhibiting ASD-like phenotypes, and possibly in ASD patient populations.
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Mentions: DA D2 receptors (Drd2) have a crucial role in mediating the stimulating effects of DA,48 as well as in reward processing and stimulus discrimination,52 a contribution that is believed to result from the opposing action of presynaptic and postsynaptic Drd2 pools.53 Based on these considerations, we investigated the state of Drd2 mRNA expression and functional responsivity at presynaptic sites in BTBR and B6 mice. mRNA expression levels of Drd2 in the substantia nigra was measured in B6 and BTBR mice using in situ hybridization (Figure 4). This measure reflects the mRNA distribution and occurrence of presynaptically localized Drd2 autoreceptors, a receptor pool that provides negative feedback mechanism, which adjusts neuronal firing rate, synthesis and DA release.48,49 No difference in the intensity of autoradiographic signals (Figure 4a, b and c) was observed between strains (P=0.60, Student's t-test). We next probed the functional state of presynaptic Drd2 receptors in controlling DA release.49,54 This assay was performed by using constant potential amperometry measurement in the presence of the selective Drd2-like agonist, quinpirole (Figure 4d). In line with previous observations, quinpirole robustly depressed striatal DA release in B6 mice (P<0.0001, paired t-test, Figure 4d), an effect mediated by activity at presynaptic Drd2 receptors.49,54 However, quinpirole-induced DA release inhibition in BTRB mice was only marginal, and greatly attenuated with respect to the one observed in B6 mice (90.5±2.96% vs 60.3±3.56%, n=9, respectively; P=0.00002, Student's t-test). In line with the dramatic Drd2-related differences found between strains, quinpirole administration robustly inhibited locomotor activity in B6 mice, but failed to elicit significant motor responses in BTBR mice (one-way ANOVA: B6: F(1,12)=14.725, P=0.0024; BTBR: F(1,10)=1.992, P=0.1885; Figure 4e).