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 analgesic research. The relationship between these two conditions is bidirectional, with chronic pain contributing to the development or exacerbation of depressive symptoms, and depression intensifying the perception and tolerance of pain. However, the neuropharmacological mechanisms by which unpredictable chronic stress (UCS) modulates nociception remain poorly understood in translational vertebrate models. Here we pharmacologically characterized stress-induced nociceptive responses using a 7–14-day UCS protocol in zebrafish, followed by intraperitoneal administration of 1–5% (v/v) acetic acid to induce nociceptive responses. Behavioral assays were performed immediately after the injection, testing abdominal constriction (writhing-like behavior) as a pain-related endpoint, and locomotor activity levels as an additional behavioral measure related to nociception and stress. The UCS exposure elevated whole-body cortisol levels, which were attenuated by morphine but not by diclofenac, supporting the involvement of central stress–pain neuropharmacological pathways. Together, these findings establish a pharmacologically tractable zebrafish model of stress-induced hyperalgesia with translational relevance for CNS-targeted analgesic discovery, highlighting the overlap between stress-related and nociceptive pathways and supporting this species as a model to investigate stress–pain comorbidity.