Whole-Brain Mapping of Direct Inputs to Dopamine D1 and D2 Receptor-Expressing Medium Spiny Neurons in the Posterior Dorsomedial Striatum.

The posterior dorsomedial striatum (pDMS) is mainly composed of medium spiny neurons (MSNs) expressing either dopamine D1 receptors (D1Rs) or D2Rs. Activation of these two MSN types produces opposing effects on addictive behaviors. However, it remains unclear whether pDMS D1- or D2-MSNs receive afferent inputs from different brain regions or whether the extra-striatal afferents express distinct dopamine receptors. To assess whether these afferents also contained D1Rs or D2Rs, we generated double transgenic mice, in which D1R- and D2R-expressing neurons were fluorescently labeled. We utilized rabies virus-mediated retrograde tracing in these mice to perform whole-brain mapping of direct inputs to D1-MSNs or D2-MSNs in the pDMS. We found that D1-MSNs preferentially received inputs from the secondary motor, secondary visual, and cingulate cortices, whereas D2-MSNs received inputs from the primary motor and primary sensory cortices, and the thalamus. We also discovered that the bed nucleus of the stria terminalis (BNST) and the central nucleus of the amygdala contained abundant D2R-expressing, but few D1R-expressing, neurons in a triple transgenic mouse model. Remarkably, although limited D1R or D2R expression was observed in extra-striatal neurons that projected to D1- or D2-MSNs, we found that cortical structures preferentially contained D1R-expressing neurons that projected to D1- or D2-MSNs, while the thalamus, substantia nigra pars compacta, and BNST had more D2R-expressing cells that projected to D2-MSNs. Taken together, these findings provide a foundation for future understanding of the pDMS circuit and its role in action selection and reward-based behaviors. Significance Statement The dorsomedial striatum (DMS) is a brain region that has critical roles in drug addiction. The DMS receives and integrates information from multiple other regions, such as the cortex and thalamus. These extra-striatal inputs onto two DMS cell types, D1-neurons and D2-neurons, provide a substrate for how the brain segregates information process, as D1-neurons positively and D2-neurons negatively control addiction-associated behaviors. The present study found that the cortex and other brain regions that are involved in motivational and decision-making behaviors preferentially targeted D1-neurons, whereas the thalamus, which plays essential roles in behavioral flexibility, preferentially projected to D2-neurons. These findings of distinct innervations of D1- and D2-neurons provide a foundation for future studies of DMS function.