Impact of Grin2b/GluN2B disruption on dopaminergic function and related behavior

PROJECT SUMMARY The gene that encodes the NMDA-GluN2B subunit (GRIN2B) is one of the top 10 genes implicated by de novo, gene-disrupting single nucleotide variants (SNVs) in autism spectrum disorder (ASD). Multiple mouse models of ASD risk genes show abnormalities in the dopamine system, suggesting this as one potential point of convergence. Some neuroimaging and clinical data also point to the dopamine system in ASD, including the use of medications targeting the dopamine system and triggering of repetitive behaviors following dopamine agonists. In prior work, we had found that increased or decreased expression of the neuronal glutamate transporter EAAT3, which modulates NMDA-GluN2B receptor function, reciprocally impacts dopaminergic neuron firing, striatal dopamine release, and resulting repetitive behavior. We now have evidence that pharmacological inhibition of NMDA-GluN2B attenuates dopaminergic neuron firing and downstream behavior. Despite published and preliminary data suggesting that NMDA receptors containing GluN2B play a critical role in modulating dopaminergic neuron function, this has not been assessed using genetic mouse models of GluN2B disruption. We therefore propose to use a conditional Cre/lox strategy to assess the impact of GluN2B loss in dopaminergic neurons. We hypothesize that diminished signaling at NMDA-GluN2B receptors will decrease dopaminergic neuron firing, resulting in diminished baseline and evoked dopamine release, and downstream alterations in impulsive behavior and habitual learning. We will test this hypothesis by comparing mice with heterozygous loss of GluN2B in dopaminergic neurons with littermate controls. Importantly, we will primarily focus on heterozygous loss to parallel the human de novo variant findings; however, we will also study homozygous Grin2b conditional knockout animals to better reveal the biological impact of GluN2B-containing NMDA receptors in dopaminergic neurons, as well as the potential for developmental compensation. We will pursue our hypothesis using in vivo extracellular single-unit recordings from optogenetically identified dopaminergic neurons, fiber photometry monitoring of dopamine release in the striatum, and operant-based behavioral tasks assessing impulsive responding and habitual learning. Our results will illuminate NMDA-GluN2B mechanisms of dopaminergic neuron regulation, as well as the relationship with downstream behavior. They may also reveal opportunities for precision medicine interventions, both for GRIN2B-related developmental disorders and for other subgroups with altered dopaminergic function.