Hypothesis

Progressive aging triggers a senescence‑associated secretory phenotype (SASP) in striatal microglia that epigenetically represses Drd1 and upregulates Drd2 expression in medium spiny neurons, shifting the D1/D2 ratio toward indirect pathway dominance prior to dopaminergic loss. This receptor imbalance degrades habit formation in the caudal sensorimotor striatum and lowers the threshold for Parkinson’s disease (PD) onset.

Mechanistic Rationale

Aging microglia accumulate DNA damage and secrete IL‑1β, TNF‑α, and ROS, which activate HDAC2 in neighboring neurons. HDAC2 deacetylates histone H3 at the Drd1 promoter, reducing transcription, while simultaneously increasing histone acetylation at the Drd2 locus via reduced HDAC2 recruitment to repressive complexes. Consequently, D1‑expressing direct pathway neurons lose excitability, whereas D2‑expressing indirect pathway neurons become hyperactive, reproducing the pathway imbalance observed in symptomatic PD but occurring before substantial dopamine depletion. The resulting bias impairs stimulus‑response learning, measurable as declining Daily Habit Scale scores, and creates a permissive environment for alpha‑synuclein pathology to disrupt motor circuits.

Testable Predictions

1. In middle‑aged humans (45‑60 years) without motor signs, PET radioligands for D1 (e.g., [(11)C]NNC112) and D2 (e.g., [(11)C]raclopride) will reveal a progressive decline in the D1/D2 binding ratio that correlates with SASP biomarkers (CSF IL‑1β, sTREM2) and poorer performance on habit‑learning tasks.
2. Longitudinally, individuals exhibiting the steepest D1/D2 ratio decline will show a higher incidence of clinically diagnosed PD or prodromal markers (REM sleep behavior disorder, hyposmia) within five years.
3. In aged mouse models, selective senolytic clearance of microglia (using navitoclax) will restore Drd1 expression, normalize D1/D2 ratios, rescue habit learning on lever‑press automation tasks, and delay motor deficits induced by low‑dose MPTP.

Experimental Design

• Human cohort: Recruit 200 participants aged 45‑60, baseline PET/MRI, CSF collection, and habit learning (sequential reaction time task). Follow annually for five years for motor/clinical endpoints. Use mixed‑effects modeling to test whether baseline D1/D2 ratio predicts change in habit scores and PD risk, controlling for age, sex, and APOE status.
• Mouse study: Aged (18‑month) C57BL/6J mice receive intracerebral striatal delivery of a microglia‑targeted senolytic nanoparticle or vehicle. After four weeks, quantify Drd1/D2 mRNA via RNAscope, conduct autoradiographic binding assays, and assess habit learning in a dual‑task operant chamber. A parallel group receives low‑dose MPTP to evaluate neuroprotection.

If the D1/D2 ratio shift precedes habit impairment and predicts PD risk, the hypothesis is supported; if no relationship is found or senolytics fail to alter receptor expression or behavior, the hypothesis is falsified.