Opioidergic modulation of stress-induced hyperalgesia in adult zebrafish

Depression and pain share overlapping central neurobiological pathways that represent key pharmacological targets in neuropsychiatric and pain research. However, the neuropharmacological mechanisms by which unpredictable chronic stress (UCS) modulates nociception in translational vertebrate models remains poorly understood. Here, we characterized a zebrafish model of stress-induced sensitization to a visceral chemical challenge by exposing adult fish to a 7- or 14-day UCS protocol (UCS7/UCS14) followed by intraperitoneal acetic acid (AA) injection (1.0-5.0% v/v). In unstressed fish, AA at 1.0% remained subthreshold for inducing the characteristic writhing-like body curvature endpoint, whereas at 5.0% produced a robust response, hence validating the assay. In contrast, UCS exposure revealed a marked nociception-like response to AA 1.0%, reflected by increased body curvature index, indicating a lowered response threshold after chronic stress. Locomotor endpoints exhibited stress-dependent cross-over effects: AA at 1.0% decreased distance traveled in unstressed fish but increased locomotion and reduced immobility in UCS-exposed fish. Pharmacological validation (in UCS7) showed that morphine attenuated the UCS-sensitized body curvature response, whereas diclofenac did not, consistent with an opioidergic contribution to the nociception-like endpoint. Whole-body cortisol levels were elevated by UCS exposure (with or without AA 1.0%) and reduced by morphine in the UCS + AA condition, whereas diclofenac was inactive. Together, these findings establish a pharmacologically tractable adult zebrafish model of stress-induced sensitization with translational relevance for CNS-targeted analgesic discovery and mechanistic studies of stress-pain comorbidity.