Background

Aging and Parkinson’s disease (PD) produce distributed changes across basal ganglia‑thalamo‑cortical (BTC) and cerebello‑thalamo‑cortical (CTC) networks without obvious regional atrophy1. Iron builds up preferentially in thalamic nuclei and cerebellar lobules with age, altering neuronal excitability and synaptic plasticity5. In early PD, dopamine loss shifts signaling from direct (D1) to indirect (D2) striatal pathways, increasing enkephalin and decreasing substance P, which pushes behavior toward rigid habits23. Chronic L‑DOPA treatment can cause D1‑ERK1/2 activation to spread dorsally, producing dyskinesias2.

Hypothesis

We hypothesize that age‑dependent iron accumulation in the pulvinar thalamus weakens its ability to sustain CTC‑mediated compensatory activity. This weakening removes a braking influence on dorsal striatal D1 neurons, allowing ERK1/2 signaling to spill over into motor territories earlier than in young brains. The premature D1‑ERK1/2 spread amplifies habit formation circuits and lowers the threshold for L‑DOPA‑induced dyskinesia, creating a synergistic aging‑PD effect that predicts faster motor decline and earlier complication onset.

Predictions

• Older mice with intracerebral iron overload in the pulvinar will show reduced CTC‑BTC functional connectivity measured by resting‑state fMRI, despite preserved thalamic volume1.
• In these animals, dopaminergic depletion will produce a larger increase in phospho‑ERK1/2 immunoreactivity in dorsal striatum compared with young controls after equivalent dopamine loss2.
• Behavioral assays will reveal an accelerated shift from goal‑directed to habitual lever pressing in aged‑iron mice after mild dopaminergic challenge, detectable before overt motor signs appear3.
• Chronic L‑DOPA exposure will induce dyskinetic abnormal involuntary movements at lower cumulative doses in aged‑iron mice, correlating with the extent of dorsal ERK1/2 spread2.
• Chelating iron locally in the pulvinar with deferiprone‑loaded nanoparticles will restore CTC‑BTC coupling, normalize D1‑ERK1/2 localization, and delay habit rigidity and dyskinesia onset.

Experimental Approach

1. Animal groups: Young (3 mo) and aged (18 mo) C57BL/6 mice receiving bilateral pulvinar injections of ferric ammonium citrate to model age‑related iron load; matched controls receive saline.
2. Imaging: Resting‑state functional MRI at 7 T to assess CTC‑BTC connectivity; quantitative susceptibility mapping to verify iron deposition.
3. Neurochemistry: After unilateral 6‑OHDA lesion of the medial forebrain bundle, collect striatal tissue for Western blot of phospho‑ERK1/2, D1 receptor, and c‑Fos; perform immunohistochemistry to map spatial extent of ERK activation.
4. Behavior: Operant conditioning with outcome devaluation to index goal‑directed versus habitual performance; abnormal involuntary movement scoring during L‑DOPA challenge.
5. Intervention: Separate cohort receives pulvinar‑targeted deferiprone nanoparticles before 6‑OHDA lesion; repeat imaging, neurochemistry, and behavior.

If iron‑induced CTC disruption drives early D1‑ERK1/2 spillover, we expect the predicted connectivity loss, biochemical spread, and behavioral shifts only in aged‑iron mice, reversible by chelation. Failure to observe these links would falsify the hypothesis and suggest that age‑related risk operates via alternative mechanisms.