The medial septal-medial habenula cholinergic circuit: A new mechanism of exercise improving cognitive function in AD mice

Background: Alzheimer's disease (AD) is a progressively and incurably neurodegenerative disorder with cognitive dysfunction (e.g., memory decline). Exercise intervention, such as aerobic and resistance training, has increasingly been accepted as a feasible strategy to improve cognitive function. However, the potential neurobiological mechanism of exercise-induced cognitive benefits requires further investigation, especially the combined regime (i.e., aerobic + resistance training) recommended by the World Health Organization guidelines on physical activity. Against this background, we aimed to investigate whether the medial septum (MS)-medial habenula (MHb) cholinergic circuit mediated the effects of combined (aerobic and resistance) exercise intervention on cognitive function in AD mice.

Methods: Six-week-old male C57BL/6 J wild-type (WT) mice were randomly divided into the sham-operated control group (Sham), the AD model group (AD), the early exercise with pre-AD group (Ex + AD), the post-AD with exercise group (AD + Ex), and the exercise with both pre-AD and post-AD group (Ex + AD + Ex), with 10 mice in each group. Five-month-old male C57BL/6 J background 5 × FAD transgenic AD mice were randomly divided into six groups: WT control group (WT), 5 × FAD group (FAD), 5 × FAD + exercise intervention group (FAD + Ex), chemical inhibition of the MS + exercise intervention group (FAD + MS (i) + Ex), chemical inhibition of the MHb group (FAD + MHb (i) + Ex), and chemical activation of MS + chemical inhibition of MHb + exercise intervention group (FAD + MS (q) + MHb (i) + Ex), with eight mice in each group. The mice in the exercise intervention groups were subjected to aerobic treadmill training combined with resistance ladder climbing. The projection relationship between MS and MHb cholinergic neurons was examined using neuroanatomical tracing experiments. Cognitive function was assessed using the Morris water maze, novel object recognition test, and Y-maze test. Neuronal damage was evaluated by hematoxylin and eosin staining and Nissl staining in the MS and MHb regions, as well as immunofluorescence staining for Amyloid-β (Aβ), choline acetyltransferase (ChAT), hyperphosphorylated microtubule-associated protein tau (p-Tau), neurofilament light chain (NFL), α-synuclein (α-Syn), retinoic acid-related Orphan Receptor β (RORβ), neuronal nuclei (NeuN), and microtubule-associated protein 2 (MAP2), and Western blotting for Aβ, ChAT, p-Tau, NFL, α-Syn, and RORβ. Mitochondrial structure and function in MS and MHb neurons were assessed by transmission electron microscopy and Western blotting for mitochondrial transcription regulator A (TFAM), nuclear respiratory factor 1 (NFR1), and adenosine triphosphate (ATP) synthase subunit β (ATPB) protein expression levels.

Results: Exercise training significantly alleviated cognitive dysfunction and neuronal damage in the MS region of Aβ mice, especially exercise preconditioning. Cholinergic neurons in the MS region of mice projected to the MHb region. Moreover, the MS-MHb cholinergic circuit mediated the beneficial effects of exercise on ameliorating neuronal damage and improving cognitive function in 5 × FAD mice.

Conclusion: Exercise, mediated by the MS-MHb cholinergic circuit, improved neuronal damage and cognitive function in AD mice. This study provides scientific evidence for the potential application of exercise in the prevention and control of AD and offers new insights for future clinical intervention strategies from the perspective of neuroscience.

AD; Cholinergic neurons; Exercise; MS–MHb; Mitochondria.