Neurobiological Pathways Underlying Opioid Addiction and Overdose: Mechanisms, Therapeutic Targets, and Insights from Zebrafish Model

The worldwide opioid epidemic continues to claim thousands of lives each year and remains one of the most urgent global public health challenges, driven by the high abuse potential and overdose risk of potent, fully synthetic μ-opioid receptor (MOR) agonists such as fentanyl. While fentanyl is highly effective in pain management, it also carries severe risks including respiratory depression, addiction, and fatal overdose. Understanding the neurobiological pathways underlying these risks is essential for developing safer analgesics and effective interventions. The MOR, a G-protein-coupled receptor (GPCR) that modulates neuronal excitability and synaptic transmission in important brain circuits like the ventral tegmental area (VTA), nucleus accumbens (NAc), and prefrontal cortex (PFC), is the primary receptor through which opioids exert their therapeutic effects. Dopaminergic dysregulation, transcriptional remodeling, and altered synaptic plasticity are neuroadaptations brought on by long-term opioid exposure that contribute to dependence and relapse. Life-threatening opioid toxicity is primarily driven by MOR-mediated suppression of respiratory networks, with risk shaped by dose, pharmacokinetics, receptor efficacy, and circuit sensitivity. Zebrafish (Danio rerio) provide a powerful complementary system for dissecting opioid-related neurocircuitry and screening for potential therapeutics. Zebrafish share conserved opioid-signaling components with mammals, and larval transparency facilitates in vivo imaging of neuronal activity at cellular resolution. Behavioral paradigms capture reward-related and drug-seeking phenotypes, while CRISPR-based approaches offer a means to investigate receptor-specific functions in validated models. In addition, zebrafish larvae provide an efficient platform for high-throughput screening of compounds that modulate opioid signaling and respiratory-related phenotypes. Together, insights from human, rodent, and zebrafish studies provide a more integrated understanding of opioid overdose and addiction neurobiology. This review incorporates current knowledge on the molecular and circuit-level mechanisms driving these processes and discusses how zebrafish models accelerate the discovery of safer analgesics and novel treatments for opioid use disorder and overdose.