Hypothesis

Chronic disruption of the suprachiasmatic nucleus (SCN) circadian rhythm alters autonomic outflow to the intestine, which reshapes gut microbial composition and barrier integrity, thereby setting the baseline for neuroinflammation and cognitive decline in brain aging.

Mechanistic Rationale

The SCN coordinates sympathetic and parasympathetic tone via hypothalamic autonomic centers. Sympathetic norepinephrine release reduces intestinal tight‑junction protein expression (e.g., claudin‑1, occludin) while parasympathetic acetylcholine enhances mucin secretion and antimicrobial peptide release. When SCN rhythms are frayed—by irregular light exposure or shift‑work‑like schedules—the balance tilts toward sympathetic dominance, increasing permeability and favoring taxa that thrive in low‑oxygen, inflammatory milieus (e.g., Enterobacteriaceae). These microbes produce lipopolysaccharide (LPS) and tryptophan catabolites that cross a compromised barrier, activating hepatic acute‑phase responses and vagal afferents that signal to the nucleus tractus solitarius and subsequently to microglia, priming a pro‑inflammatory phenotype. Conversely, restored parasympathetic tone reinforces barrier function, promotes short‑chain‑fatty‑acid (SCFA) producers, and dampens microglial activation via GPR43‑mediated anti‑inflammatory signaling.

This view flips the conventional gut‑to‑brain causality: the brain’s circadian‑autonomic state is the primary driver, with gut changes acting as mediators rather than initiators.

Experimental Design

1. Animal model – Aged (18‑month) C57BL/6J mice housed under either a normal 12 h light/12 h dark cycle (control) or a chronic circadian‑disruption schedule (8 h light/16 h dark, rotating every 3 days) for 8 weeks.
2. Autonomic manipulation – Sub‑cohorts receive chemogenetic inhibition (hM4Di DREADDs) of SCN‑projecting parvocellular paraventricular nucleus (PVN) neurons to specifically blunt sympathetic output, or optogenetic activation of the dorsal motor nucleus of the vagus (DMV) to boost parasympathetic tone.
3. Readouts –
   • Fecal 16S rRNA sequencing to quantify shifts in Enterobacteriaceae vs. SCFA‑producing taxa (e.g., Lachnospiraceae).
   • Intestinal permeability measured by FITC‑dextran serum levels and immunobludding of zonulin‑1/claudin‑1.
   • Portal blood LPS and SCFA concentrations via LC‑MS.
   • Vagal afferent firing recorded in vivo using extracellular electrodes.
   • Neuroinflammation assessed by Iba1‑positive microglial morphology and cytokine panel (IL‑1β, TNF‑α, IL‑6) in hippocampus and cortex.
   • Cognitive performance evaluated with Morris water maze and novel object recognition.
4. Control groups – Sham‑treated, antibiotic‑treated (to deplete microbiota), and fecal microbiota transplantation (FMT) from disrupted‑rhythm donors into germ‑free recipients to test sufficiency.

Predictions and Falsifiability

• Prediction 1 – Circadian‑disrupted mice will show increased sympathetic markers (plasma norepinephrine), heightened gut permeability, a bloom of LPS‑rich taxa, and elevated microglial activation compared with controls.
• Prediction 2 – Chemogenetic silencing of SCN‑PVN sympathetic neurons will rescue barrier integrity, normalize microbial composition, reduce vagal LPS signaling, and attenuate neuroinflammatory and cognitive deficits despite persistent light‑schedule disruption.
• Prediction 3 – Optogenetic DMV activation will mimic the protective effects of sympathetic silencing, increasing SCFA levels and improving cognition.
• Prediction 4 – FMT from disrupted donors into germ‑free recipients will transmit the inflammatory phenotype only when recipients retain an intact sympathetic tone; blocking sympathetic signaling in recipients will abolish the transferred phenotype.

Falsification – If chronic circadian disruption fails to alter gut permeability or microbial composition, or if manipulating autonomic outflow does not correspondingly shift neuroinflammatory outcomes and cognition, the hypothesis that brain‑driven circadian autonomic signals set the gut‑brain axis direction in aging would be refuted. Conversely, confirmation of these links would establish a testable, bottom‑up framework for longevity interventions that target circadian autonomic health upstream of the gut microbiome.