Neuroimmune Cross-Talk and Multilevel Cascades in Fentanyl Toxicity: Interplay of Hypoxic Stress, Glial Activation, and Synaptic Dysregulation in Systems-Level Neurodegeneration.

Fentanyl, an ultra-potent synthetic opioid, has traditionally been characterized by its acute toxic effects, particularly respiratory depression. However, accumulating research indicates that its neurobiological influence extends far beyond its short pharmacological window, intersecting with several core mechanisms implicated in major neurodegenerative disorders. This review integrates multiscale evidence to propose a unified conceptual framework in which fentanyl may function not only as an acute neurotoxin but also as a putative accelerator of long-term neurodegenerative vulnerability. Drawing from molecular signaling, cellular stress pathways, glial-neuronal cross-talk, neurovascular regulation, synaptic architecture, and large-scale neural networks, we highlight fentanyl's capacity to trigger a convergent cascade encompassing hypoxic-metabolic reprogramming, mitochondrial fragmentation, TLR4-NF-κB-driven inflammation, NLRP3 inflammasome activation, complement-mediated synaptic pruning, astrocytic EAAT2 downregulation, and blood-brain barrier compromise. These alterations propagate through recursive cross-talk loops that progressively diminish neuronal resilience, destabilize oscillatory coherence, and weaken circuit-level adaptability. Importantly, mechanistic overlaps with Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis suggest that fentanyl exposure may be mechanistically associated with processes capable of accelerating disease onset, exacerbating progression, or unmasking latent vulnerabilities, particularly in genetically or metabolically predisposed individuals. By reframing fentanyl as a systems-level destabilizer capable of imprinting persistent neurobiological changes, this model underscores the need for comprehensive biomarker development, longitudinal risk assessment, and targeted neuroprotective interventions. The integrative framework presented herein offers a foundation for predicting the long-term neurological consequences of fentanyl exposure and calls for urgent reconsideration of its role in population-level neurodegenerative risk.