Differential effects of Lactobacillus casei in probiotic and paraprobiotic forms on behavioral performance, oxidative stress, neuroinflammation, and cholinergic dysfunction in a streptozotocin-induced model of sporadic Alzheimer's disease.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by cognitive decline, oxidative stress, neuroinflammation, and cholinergic dysfunction. Increasing evidence suggests that gut microbiota disturbances contribute to AD progression, encouraging the investigation of microbiota-modulating approaches such as probiotics and paraprobiotics. This study evaluated the effects of Lactobacillus casei probiotic and its thermally inactivated paraprobiotic on cognitive, behavioral, oxidative, cholinergic, and inflammatory alterations in a sporadic AD model induced by intracerebroventricular streptozotocin (STZ-ICV; 3 mg/3 μL/site) in female rats. From day 4, animals received daily oral treatment with probiotic L. casei (1 × 109 CFU), paraprobiotic (100 mg/kg), or saline for 14 days. Behavioral assessments of memory and exploratory activity were performed alongside biochemical analyses of oxidative stress markers, antioxidant defenses, acetylcholinesterase (AChE) activity, and neuroinflammatory parameters in central and peripheral tissues. STZ-ICV administration induced impairments in working and long-term memory, increased oxidative stress and neuroinflammation, elevated AChE activity, and promoted intestinal and behavioral alterations. Both probiotic and paraprobiotic treatments attenuated memory deficits, reduced lipid peroxidation, inhibited AChE activity, and decreased inflammatory markers. However, their effects differed in magnitude and tissue specificity. The probiotic mainly improved peripheral antioxidant defenses, whereas the paraprobiotic exerted broader neuroprotective effects, reducing cerebral oxidative stress, restoring non-enzymatic antioxidant levels in the hippocampus and colon, improving jejunal catalase activity, and attenuating hippocampal astrogliosis. These findings demonstrate that modulation of the gut-brain axis through L. casei-based interventions mitigates key pathological features of sporadic AD, with paraprobiotics emerging as a promising and stable alternative with enhanced neuroprotective potential independent of bacterial viability.