Hypothesis

Pulsed (intermittent) mTOR inhibition reproduces the longevity‑extending hormetic response of continuous rapamycin while allowing recovery periods that prevent the chronic‑stress molecular signature in amygdala‑dependent fear circuits.

Background

Continuous mTOR inhibition with rapamycin extends lifespan via hormesis, up‑regulating oxidative‑stress defenses (SOD1, glutathione reductase) and activating SKN‑1/NRF2 and DAF‑16/FOXO [1]. However, the same treatment suppresses phospho‑mTOR, p70S6K, S6, ERK1/2, Akt1 and GluR1 in the amygdala, mirroring the molecular profile of chronic unpredictable stress [3] and coincides with age‑related fear‑extinction deficits [4]. mTORC1 also mediates stress‑induced hippocampal synapse loss [5] and sympathetic output [6].

Mechanistic Insight

We propose that the longevity benefit stems from a transient activation of cellular stress‑response pathways (e.g., NRF2‑mediated transcription, autophagy initiation) that does not require sustained suppression of mTORC1 in neurons. Brief inhibition spikes autophagic flux and ROS‑scavenging enzyme expression, then the inhibitor clears, permitting mTORC1‑dependent synaptic plasticity mechanisms (GluR1 trafficking, ERK‑mediated LTP) to reset during drug‑free intervals. This intermittent pattern mimics natural cycles of feast and famine that our ancestors experienced, thereby engaging conserved survival programs without locking emotional‑learning circuits into a low‑resource state.

Testable Predictions

1. Molecular – Mice receiving rapamycin on a 2‑day‑on/5‑day‑off schedule will show comparable increases in hepatic SOD1 and glutathione reductase levels as daily dosing [7], but phospho‑S6 and phospho‑GluR1 in the basolateral amygdala will return to baseline during off‑days.
2. Behavioral – Intermittent rapamycin‑treated aged mice will perform fear‑extinction retention tests indistinguishable from vehicle controls, whereas continuously treated mice will show impaired extinction.
3. Cellular – Electrophysiological recordings from amygdala slices will reveal preserved LTP magnitude in the intermittent group, matching vehicle, while the continuous group exhibits reduced LTP.
4. Transcriptomic – RNA‑seq of amygdala tissue will show NRF2 target genes upregulated in both regimens, but only continuous treatment will sustain chronic‑stress signatures (e.g., elevated FKBP5, reduced BDNF).

Potential Confounds

• Pharmacokinetic variability could produce uneven brain exposure; measuring rapamycin and metabolite levels in plasma and brain homogenates will be essential.
• Off‑day intervals might allow rebound mTORC1 hyperactivity; monitoring p‑S6 kinetics will clarify the optimal window.

Implications

If validated, this hypothesis reframes rapamycin‑based gerotherapeutics: dosing schedules can be tuned to uncouple systemic stress‑resistance gains from region‑specific neurobehavioral costs, offering a path to extend healthspan without compromising adaptive emotional learning.