Synaptic crossroads: navigating the circuits of movement.

The anterior lateral motor area (ALM) is crucial in preparing and executing voluntary movements through its diverse neuronal subpopulations that target different subcortical areas. A recent study by Xu et al. utilized an elaborate viral tracing strategy in mice to provide comprehensive whole-brain maps of monosynaptic inputs to the major descending pathways of ALM.

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-MSNs or D2-MSNs receive afferent inputs from different brain regions or whether the extrastriatal afferents express distinct dopamine receptors. To assess whether these afferents also contained D1Rs or D2Rs, we generated double transgenic mice, in which D1R-expressing and D2R-expressing neurons were fluorescently labeled. We used 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 (CeA) 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 extrastriatal neurons that projected to D1-MSNs or D2-MSNs, we found that cortical structures preferentially contained D1R-expressing neurons that projected to D1-MSNs or D2-MSNs, while the thalamus, substantia nigra pars compacta (SNc), 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.

Retrograde monosynaptic tracing through an engineered human embryonic stem cell line reveals synaptic inputs from host neurons to grafted cells.

Retrograde monosynaptic tracing with EnvA-pseudotyped rabies virus has been employed to identify the afferent and efferent connectivity of transplanted human embryonic stem (hES) cell-derived neurons in animal models. Due to the protracted development of transplanted human neurons in host animals, it is important that those transplanted cells express avian leukosis and sarcoma virus subgroup A receptor (TVA) and rabies glycoprotein G (Rgp) for a period of up to several months to enable identification of the synaptic inputs from host neurons to grafted neurons through this rabies virus-based method. Here, we report the generation of an engineered hES cell line through CRISPR/Cas9-mediated targeting to the AAVS1 locus of an EnvA-pseudotyped rabies virus-based tool for retrograde monosynaptic tracing. This engineered hES cell line, named H1-CAG-GTRgp, expresses GFP, TVA and Rgp. Upon transplantation of H1-CAG-GTRgp-derived neural progenitor cells (NPCs) into the rat brain after traumatic injury, the grafted neurons derived from H1-CAG-GTRgp cells expressed GFP, TVA, and Rgp stably for up to 6 months post-transplantation and received robust synaptic inputs from host neurons in the target regions of the orthotopic neural circuitry. The retrograde monosynaptic tracing hES cell line provides an efficient approach to analyze transplant connectivity for the comprehensive assessment of host-donor cell innervation.