Striatal Projection Neuron Changes in Dopamine Depletion and Parkinsons Disease

National Institute On Aging

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Abstract

Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by hallmark motor symptoms—resting tremor, bradykinesia, rigidity, and postural instability—as well as a broad spectrum of non-motor manifestations including depression, cognitive decline, and autonomic dysfunction. The motor features of PD primarily result from the degeneration of dopaminergic neurons (DANs) in the substantia nigra pars compacta (SNc), with the ventral tier aldehyde dehydrogenase 1A1–positive (ALDH1A1+) subpopulation being particularly vulnerable. These neurons play a critical role in maintaining striatal dopamine tone and shaping basal ganglia output for movement initiation and execution. The naturally occurring Pitx3-deficient aphakia (Pitx3^ak/ak^) mouse offers a unique opportunity to examine the long-term consequences of selective ALDH1A1+ DAN depletion. In these mice, SNc DANs are lost during development, while ventral tegmental area (VTA) DANs remain largely intact. Surprisingly, despite the absence of this key dopaminergic population, Pitx3^ak/ak^ mice exhibit preserved or even elevated locomotor activity under certain conditions. This suggests that compensatory adaptations occur within downstream striatal circuits, particularly among striatal projection neurons (SPNs) in the dorsal striatum—the principal targets of midbrain DANs. SPNs fall into two major functional pathways: direct-pathway SPNs (dSPNs), expressing dopamine receptor D1 (DRD1) and projecting to the internal globus pallidus (GPi) and substantia nigra pars reticulata (SNr), and indirect-pathway SPNs (iSPNs), expressing dopamine receptor D2 (DRD2) and projecting to the external globus pallidus (GPe). Both populations are further organized into patch (striosome) and matrix compartments, which differ in molecular signatures, connectivity, and modulatory input. Patchy dSPNs are known to provide strong inhibitory input to ALDH1A1+ DANs, suppressing dopamine release and locomotion, whereas patchy iSPNs may enhance movement—underscoring the complexity of compartment- and cell type–specific regulation of motor behavior. In this study, we investigate how the developmental absence of ALDH1A1+ DANs reshapes SPN organization, connectivity, and function, with a particular emphasis on patchy subpopulations. Using a combination of genetic labeling, quantitative histological mapping, and projection tracing, we assess the distribution and axonal targeting of dSPNs and iSPNs within patch and matrix compartments in Pitx3^ak/ak^ mice. We further employ optogenetic activation of molecularly identified patchy SPNs to directly test their influence on locomotor output. Finally, we analyze how these circuit modifications might alter dopamine release dynamics, providing a mechanistic link between structural reorganization and functional compensation. By defining the adaptive plasticity of basal ganglia circuits following selective dopaminergic loss, this work will deepen our understanding of how motor networks compensate in PD. The findings are expected to reveal cell type– and compartment-specific mechanisms that could be exploited for targeted neuromodulation strategies, such as deep brain stimulation (DBS) or circuit-specific pharmacological interventions, aimed at restoring balanced motor control in PD patients.