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-elicited fMRI responses have been demonstrated to serve as a plausible surrogate for mesolimbic dopaminergic reactivity in rodent species.38, 39, 40 As human fMRI studies reported reduced reactivity of ventro-striatal DA terminals in individuals with ASD,11, 12, 13 we employed pharmacological fMRI24,41 to map the functional response elicited by selective DAT inhibitor GBR 12909 in BTBR and B6 strain. Consistent with previous fMRI studies with DA-releasing agents,24,39,42 GBR 12909 elicited widespread and robust activation of cortical and subcortical regions in B6 mice with respect to vehicle (Figure 1a, b and c; Z>1.6, P<0.01, corrected vs vehicle), with a clear and robust involvement of mesolimbic DA terminals such as the nucleus accumbens and medial prefrontal cortex (Figure 1a, b and c). Interestingly, in BTBR mice the drug did not produce detectable fMRI responses in the aforementioned striatal and mesolimbic dopaminergic areas, but elicited only a focal pattern of activation that comprised the thalamus, motor and somatosensory regions and the dorsal (but not medial) prefrontal cortex. The magnitude of GBR 12909-induced fMRI response in these areas was comparable to that observed in B6 mice (Supplementary Figure S1), thus arguing against the presence of generalized vascular or metabolic disturbance in these animals. Lack of appreciable GBR 12909-induced fMRI activation was also found in hippocampal regions, as well as in postero-parietal and latero-cortical regions of BTBR mice. We detected no significant confounding effect of genotype on anaesthesia during fMRI as assessed by the magnitude of mean arterial blood pressure, a sensitive indicator of anaesthesia depth in rodents43 (P=0.17). GBR administration produced a short-lived (~8 min) blood pressure decrease (−6.0±6.3 mm Hg and −22.50±5.1 mm Hg in BTBR and B6, respectively) that was, however, well within the autoregulation window under halothane anaesthesia44,45 and temporally uncorrelated with the fMRI responses (the latter lasting >25 min), thus arguing against a peripheral origin of the discrepant fMRI responses mapped in the two strains.