Chronic hyperactivation of midbrain dopamine neurons causes preferential dopamine neuron degeneration
Parkinson's disease (PD) is characterized by the death of substantia nigra (SNc) dopamine (DA) neurons, but the pathophysiological mechanisms that precede and drive their death remain unknown. The activity of DA neurons is likely altered in PD, but we understand little about if or how chronic changes in activity may contribute to degeneration. To address this question, we developed a chemogenetic (DREADD) mouse model to chronically increase DA neuron activity, and confirmed this increase using ex vivo electrophysiology. Chronic hyperactivation of DA neurons resulted in prolonged increases in locomotor activity during the light cycle and decreases during the dark cycle, consistent with chronic changes in DA release and circadian disturbances. We also observed early, preferential degeneration of SNc projections, recapitulating the PD hallmarks of selective vulnerability of SNc axons and the comparative resilience of ventral tegmental area axons. This was followed by eventual loss of midbrain DA neurons. Continuous DREADD activation resulted in a sustained increase in baseline calcium levels, supporting a role for increased calcium in the neurodegeneration process. Finally, spatial transcriptomics from DREADD mice examining midbrain DA neurons and striatal targets, and cross-validation with human patient samples, provided insights into potential mechanisms of hyperactivity-induced toxicity and PD. Our results thus reveal the preferential vulnerability of SNc DA neurons to increased neural activity, and support a potential role for increased neural activity in driving degeneration in PD. Overall design: Homozygous 3-4mo male DATiCre mice (Jax 006660) were split into 3 groups. The experimental group (GqCNO, N=3) was bilaterally injected in the midbrain with a conditional AAV to express excitatory hM3Dq(Gq)-DREADDs specifically in dopamine neurons. These mice were administered the DREADD ligand clozapine-N-oxide (CNO) via their drinking water for one week (300mg/L, with 2% sucrose) to chronically activate dopamine neurons. Two control groups were included: one where mice were bilaterally injected with the DREADD and given vehicle (sucrose) water (GqVeh, N=3), and another where mice were not injected but were given CNO water (CNOAlone, N=2). Midbrain and striatal slices from each mouse (16 slices total) were analyzed using spatial transcriptomics.