Beyond the brain: early autonomic dysfunction in Alzheimer's disease.

Abstract

Alzheimer's disease (AD) is classically defined by central hallmarks such as amyloid-beta plaques, tau hyperphosphorylation, and synaptic failure. However, mounting evidence suggests that dysfunction outside the brain, particularly in the peripheral nervous system, may also play a significant role in disease progression. The adrenal medulla-a key regulator of systemic neurotransmission and stress response-has received little attention in this context. In this study, we investigated whether chromaffin cells (CCs) from the triple transgenic AD mouse model (3xTg) exhibit functional alterations that could contribute to peripheral neurochemical imbalance. Using electrophysiology, high-resolution amperometry, and neurotransmitter quantification, we identified early and progressive defects in CC function. Remarkably, even at two months of age-prior to cognitive decline-3xTg CCs showed impaired exocytosis, reduced vesicle release, and slower fusion pore kinetics. These changes were accompanied by diminished sodium (I_Na), calcium (I_Ca), and nicotinic (I_ACh) currents, compromising CC excitability. With age, a shift toward increased potassium (I_K) currents and enhanced catecholamine secretion may reflect compensatory adaptations aimed at preserving output. These functional deficits were paralleled by structural remodeling of the actin cytoskeleton and systemic neurotransmitter disturbances. Noradrenaline levels increased in both plasma and brain, while dopamine decreased peripherally but paradoxically increased in the prefrontal cortex and hippocampus. Serotonin levels consistently declined across compartments. These imbalances correlated with altered behavior: 3xTg mice displayed increased exploration of exposed areas and heightened behavioral despair, pointing to anxiety- and depression-like phenotypes. Together, our findings identify the adrenal medulla as a previously underrecognized site of early catecholaminergic dysregulation in AD. The observed associations between peripheral CC dysfunction, systemic neurotransmitter imbalance, and behavioral changes point to a potential link between peripheral neuroendocrine alterations and central disease features. These results broaden the current understanding of AD pathophysiology and support the adrenal medulla as a promising candidate for further investigation as a therapeutic target and source of peripheral biomarkers.