Restoring central dopaminergic tone rescues age‑related colonic dysmotility by reestablishing brain‑to‑gut inhibitory dopamine signaling

Central dopaminergic decline with age parallels a rise in colonic dopamine and a fall in D2 receptor expression, suggesting a loss of tonic inhibitory brain‑to‑gut signaling. We hypothesize that the aged colon’s hyperdopaminergic state is a compensatory response to absent central drive, and that restoring midbrain dopaminergic flux will normalize peripheral dopamine levels, reestablish D2‑mediated inhibition of colonic motility, and improve gastrointestinal transit. Mechanistically, we propose that descending dopaminergic fibers from the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) release dopamine onto colonic sympathetic preganglionic neurons, activating D2 receptors on these neurons to suppress norepinephrine release and thereby reduce α‑adrenergic‑mediated contraction. With aging, reduced VTA/SNc firing diminishes this brake, leading to unchecked sympathetic tone, elevated colonic dopamine synthesis via tyrosine hydroxylase (TH) upregulation, and downstream D2 desensitization. Consequently, the gut exhibits dysmotility despite high luminal dopamine. Testing this hypothesis requires three interlinked experiments: (1) chemogenetic activation of VTA‑SNc dopaminergic projections in aged mice to measure changes in colonic TH, DAT, and D2 expression via qPCR and immunofluorescence; (2) simultaneous in vivo colonic manometry to assess motility improvements correlated with restored central dopamine tone measured by fast‑scan cyclic voltammetry in the striatum; (3) pharmacological blockade of colonic D2 receptors (e.g., with eticlopride) to determine whether motility gains depend on peripheral D2 signaling. A successful outcome—normalized colonic TH/DAT, rescued D2 levels, and improved transit only when central dopaminergic drive is intact—would falsify the afferent‑centric view of the gut‑brain axis and reorient longevity strategies toward upstream dopaminergic rescue, such as exercise‑induced neurogenesis, GLP‑1 agonists that enhance VTA firing, or targeted vagal‑dopaminergic neuromodulation. Conversely, if colonic dopamine remains high and motility unchanged despite central activation, the hypothesis would be refuted, supporting alternative afferent‑driven mechanisms.

Additionally, we predict that microbiota‑derived short‑chain fatty acids (SCFAs) will not account for the observed colonic dopamine elevation, because germ‑free aged mice receiving VTA‑SNc activation will still show TH normalization and motility improvement, whereas antibiotic‑treated conventional mice will not. This manipulation would dissociate microbial influence from central dopaminergic control, addressing a major confound in the field. Finally, longitudinal behavioral assays measuring anxiety‑like behavior and reward learning will determine whether peripheral gut rescue translates to central cognitive benefits, providing a holistic read‑out of the brain‑gut feedback loop.