Hypothesis

Age-related cholinergic decline stems not from neuronal loss but from excessive presynaptic Ca²⁺ buffering that suppresses spontaneous release, locking basal forebrain circuits into a high‑confidence, low‑entropy state. Introducing modest, stochastic increases in presynaptic Ca²⁺ transient amplitude—or decreasing the buffering capacity—should re‑introduce uncertainty, weaken over‑consolidated synaptic weights, and restore NGF‑TrkA responsiveness without adding exogenous trophic factor.

Mechanistic Basis

Synaptic rigidity in aged cholinergic terminals is marked by enhanced presynaptic Ca²⁺ buffering and reduced spontaneous postsynaptic current frequency 2. This buffering raises the threshold for vesicle release, favoring only strong, predictable inputs and attenuating the stochastic "noise" that drives synaptic plasticity. Concurrently, reduced GABAergic tone diminishes shunting inhibition, further biasing the network toward deterministic firing patterns. Over time, the system over‑optimizes its internal model, treating unexpected signals as errors to be ignored rather than opportunities to update.

Failed NGF‑TrkA retrograde signaling 3 likely follows this synaptic state: TrkA phosphorylation depends on membrane depolarization and Ca²⁺‑dependent kinases; when presynaptic Ca²⁺ spikes are dampened, TrkA remains under‑phosphorylated even if ligand is present. Gene expression shifts toward pre‑adult profiles 5 reflect a loss of activity‑dependent maintenance of mature cholinergic identity, not a degenerative cascade.

Thus, the primary lesion is a biophysical one—excessive Ca²⁺ buffering—that creates a self‑reinforcing loop of low activity, reduced trophic signaling, and phenotypic suppression.

Testable Predictions

1. Pharmacologically lowering presynaptic Ca²⁺ buffering (e.g., with low‑dose ruthenium red to inhibit mitochondrial Ca²⁺ uptake) will increase spontaneous mEPSC frequency in cholinergic terminals from aged rats.
2. This increase will correlate with restored TrkA autophosphorylation and ERK activation after hippocampal NGF application.
3. Behavioral assays of pattern separation (e.g., contextual fear discrimination) will improve in treated aged animals, whereas global cholinergic agonism will not.
4. Conversely, artificially boosting buffering (via overexpression of presynaptic calbindin) in young animals will replicate the aged phenotype: reduced mEPSC frequency, blunted TrkA signaling, and impaired discrimination learning.

Experimental Approach

• Slice electrophysiology: Whole‑cell recordings from identified basal forebrain cholinergic neurons in young (3 mo) and aged (24 mo) rats. Measure mEPSC frequency and amplitude before and after application of RuRu (10 µM) or BAPTA‑AM (5 µM) to manipulate buffering.
• Biochemical readouts: Western blot for p‑TrkA and p‑ERK after exogenous NGF (50 ng ml⁻¹) exposure in the same slices.
• In vivo validation: Stereotactic delivery of AAV‑shRNA targeting presynaptic mitochondrial calcium uniporter (MCU) to the nucleus basalis of aged rats. After 3 weeks, assess ChAT immunoreactivity (to confirm no cell loss), conduct in vivo microdialysis for ACh release variability, and run discrimination learning tasks.
• Control groups: Vehicle, scrambled shRNA, and systemic donepezil to test whether cholinergic enhancement alone fails to rescue plasticity when buffering remains high.

Falsifiability: If lowering presynaptic Ca²⁺ buffering does not increase mEPSC frequency, restore TrkA signaling, or improve discrimination learning, the hypothesis that synaptic over‑consolidation via Ca²⁺ buffering drives cholinergic rigidity would be refuted. Conversely, a positive outcome would support the claim that re‑introducing controlled uncertainty at the synapse—not merely supplementing NGF—can reverse age‑related cholinergic decline.