Hypothesis: In the aging substantia nigra pars compacta (SNc), a subset of astrocytes enters a senescent state that secretes a neuroprotective SASP dominated by TGF‑β1. This cytokine keeps neighboring microglia in a low‑activation phenotype by inhibiting NADPH oxidase‑2 (NOX2)–derived superoxide production, thereby limiting oxidative damage to residual dopaminergic terminals. When senolytics eliminate these astrocytes, TGF‑β1 signaling drops, microglia shift to a NOX2‑high, pro‑inflammatory state, and accelerated loss of dopaminergic terminals and striatum‑dependent motor learning occurs, even in the absence of overt proteinopathy.

Rationale: Normal aging leads to gradual dopaminergic neuron loss in the SNc, with Parkinson’s disease representing an accelerated trajectory Frontiers in Aging Neuroscience. Age‑related changes in basal ganglia connectivity (e.g., increased putamen‑globus pallidus synchrony) and dopamine‑dependent D1/D2 receptor imbalance are well documented Frontiers in Aging Neuroscience; PMC. Yet no study has examined whether senescent glia contribute to these changes. The prevailing senolytic paradigm assumes senescent cells are purely deleterious, but the SASP can include anti‑inflammatory factors such as TGF‑β1, activin A, and IGF‑1 that support neuronal survival in other contexts.

Mechanistic insight: Senescent astrocytes upregulate SMAD2/3 signaling, releasing TGF‑β1 that binds microglial TGF‑βRII, leading to SMAD‑dependent transcriptional repression of NOX2 (Cybb) and reduced ROS. This creates a redox buffer that preserves dopamine uptake terminals and prevents excessive microglial phagocytosis of stressed axons. Loss of this brake would increase microglial oxidative burst, exacerbate dopamine terminal fragmentation, and heighten indirect pathway overactivity, manifesting as accelerated habit‑formation deficits.

Testable predictions:

1. In aged (18‑month) wild‑type mice, immunofluorescence will show co‑localization of senescence markers (p16^Ink4a, SA‑β‑gal) with astrocytic marker GFAP in the SNc, accompanied by elevated TGF‑β1 in the extracellular space.
2. Genetic ablation of p16^+ astrocytes (via INK‑ATTAC) or pharmacological senolytic treatment (e.g., dasatinib+quercetin) will reduce SNc TGF‑β1 levels by >50% and increase microglial NOX2‑derived superoxide (measured by dihydroethidium staining).
3. Consequently, dopamine transporter (DAT) staining in the dorsal striatum will decline ~30% faster over 8 weeks compared to vehicle‑treated controls, and rotarod and lever‑press habit learning tasks will show significantly worse performance.
4. Exogenous TGF‑β1 supplementation (via intranasal delivery) in senolytic‑treated aged mice will rescue microglial NOX2 activity, preserve DAT density, and restore motor learning to control levels.

Falsifiability: If senescent astrocyte removal does not alter microglial NOX2 activity, TGF‑β1 levels, or dopaminergic terminal integrity, or if motor performance improves after senolysis, the hypothesis is refuted. Conversely, confirmation of the predicted cascade would support a protective, chaperone‑like role for senescent glia in nigrostriatal aging, challenging the blanket assumption that senescent cells are uniformly harmful and indicating that timing and cell‑type specificity are critical for senolytic interventions.