<p>A ‘gut-brain axis’ is an intricate bidirectional connection between the gut and the central nervous system, serving as a key pathway for signal exchange. However, current in vitro models do not fully capture these dynamic interactions, limiting mechanistic insight and therapeutic testing. Here, we show a 3D human gut-brain-vascular microphysiological platform that integrates lumenized villus-like intestinal barrier, blood vascular-astrocyte interactions, and brain tissue to model circulation-mediated crosstalk between the gut and brain. Using this system, we demonstrate gut-to-brain signaling by delivering bacterial-derived toxins to the gut compartment, which traverse the gut and neurovascular barriers and trigger neuroinflammatory responses and tau-associated pathology in the brain tissue. Conversely, we show that Alzheimer’s- and Parkinson’s-relevant stimuli applied to the brain compartment elicit neuroinflammation and disrupt both vascular and intestinal barrier integrity, indicating brain-to-gut feedback. Together, our platform provides a human-relevant tool to dissect mechanisms of bidirectional gut-brain communication and to evaluate therapeutic strategies for neurogastrointestinal disease.</p>