Hypothesis

Chronic mTOR inhibition impairs habit learning by destabilizing D1/D2 receptor signaling in the basal ganglia, whereas pulsatile mTOR activity sustains both autophagic clearance and synaptic maintenance, preserving behavioral flexibility.

Mechanistic Rationale

mTOR functions as a dynamic dial that toggles between cellular 'civilization' (protein synthesis, synaptic plasticity) and 'survival' (autophagy, stress resistance) modes. In dopaminergic neurons of the VTA and substantia nigra, transient mTOR activation drives translation of synaptic proteins such as GluA1 and DARPP-32, which are essential for D1- and D2‑mediated signaling cascades. Simultaneously, brief mTOR suppression permits TFEB‑dependent lysosomal biogenesis and α‑synuclein clearance. When mTOR is constitutively inhibited, the synthesis of receptors and scaffolding proteins falls below the threshold needed to maintain excitatory‑inhibitory balance, leading to increased GABAergic tone and degraded D1/D2 signaling. This shift biases the striatum toward habitual, inflexible output, mirroring the early motor‑cognitive deficits seen in Parkinson’s models.

Testable Predictions

1. Aged mice receiving intermittent rapamycin (e.g., 3 days on/4 days off) will show higher D1 and D2 receptor protein levels in the dorsal striatum compared with mice on continuous rapamycin or vehicle.
2. The same intermittent regimen will rescue performance on a habit‑formation task (e.g., lever‑press after outcome devaluation) without compromising autophagy flux, as measured by LC3‑II/p62 ratios.
3. Chemogenetic activation of mTORC1 (via Rheb‑DREADDs) in VTA dopaminergic neurons for brief windows (5 min every hour) will mimic the beneficial effects of intermittent rapamycin on receptor expression and habit learning.
4. Conversely, optogenetic inhibition of mTORC1 during active phases will reproduce the D1/D2 downregulation and habit deficits seen with chronic suppression.

Experimental Approach

• Use 18‑month‑old C57BL/6 mice; administer rapamycin via chow to achieve intermittent or continuous dosing.
• Quantify striatal D1/D2 receptor levels by Western blot and immunofluorescence; assess autophagic flux with lysosomal inhibitors.
• Conduct a two‑stage habit‑learning assay: initial goal‑directed training followed by devaluation via lithium chloride-induced malaise; measure persistence of lever pressing.
• Apply Cre‑dependent Rheb‑DREADDs or Raptor‑shRNA in DAT‑Cre neurons to manipulate mTORC1 temporally; validate with p‑S6 immunostaining.
• Analyze circuit dynamics using in‑vivo electrophysiology in the dorsolateral striatum to evaluate excitation‑inhibition ratios.

If intermittent mTOR activation preserves receptor balance and habit flexibility while chronic inhibition does not, the hypothesis will be supported, positioning mTOR pulsatility—not static suppression—as the optimal strategy for protecting basal ganglia function during aging.