Reductive stress refers to a pathological shift toward a more reduced state in one or more defined redox couples, including NAD+/NADH, NADP+/NADPH, and GSSG/GSH, within specific cellular or subcellular compartments. Low-molecular-weight (LMW) thiols, including cysteine, cysteamine, glutathione, homocysteine, and hydrogen sulfide, contribute to cellular redox buffering, thiol-disulfide exchange, and redox-dependent metabolic regulation. Under chronic activation of antioxidant enzymatic systems, expansion of the reduced thiol pool may shift redox homeostasis toward a hyperreduced state, suppress physiological ROS-dependent signaling, and impair protein thiol-disulfide regulation. This review examines the cellular and molecular mechanisms of reductive stress within the neurovascular unit, the functional complex comprising microcapillary endothelial cells, pericytes, astrocytes, and neurons that maintains blood-brain barrier integrity and metabolic coupling. A reductive imbalance disrupts physiological redox signaling, impairs mitochondrial function, induces endoplasmic reticulum stress and the unfolded protein response (UPR), dysregulates ion channels and calcium homeostasis, promotes DNA damage, and activates cellular senescence programs and inflammasomes. Critically, reductive stress is not isolated but forms an integrated pathological network with oxidative, nitrosative, and glycation stresses, creating a self-sustaining cycle of mitochondrial dysfunction, macromolecular damage, and chronic neuroinflammation. Particular attention is given to compartment-specific redox regulation by mitochondrial glutathione and the peroxiredoxin/thioredoxin system, the dual role of ROS as physiological second messengers and mediators of secondary oxidative injury, and the links between hyperreduction, cellular senescence, AGE/RAGE signaling, and BBB dysfunction. We emphasize that reliable identification of reductive stress requires the simultaneous, compartment-resolved assessment of several redox couples and their functional consequences. This review also summarizes experimental approaches for modeling and detecting reductive stress and discusses emerging therapeutic strategies, including xenotopic enzymes, genetically encoded metabolic tools, and NAD+ modulation, aimed at restoring redox homeostasis and slowing neurodegeneration and aging.
