HEATING RATE DEPENDENT PLASTICITY SHAPES NEURONAL RESILIENCE TO ACUTE HEAT STRESS IN LARVAL ZEBRAFISH

Acute heat stress disrupts brain function and can trigger spreading depolarization (SD), a pathological wave of near-complete neuronal depolarization implicated in heat stroke, ischemia, and epilepsy. While temperature influences neuronal excitability, the role of heating rate and time-dependent cellular plasticity in shaping SD vulnerability remains poorly understood. Here, we investigated how heating rate and protein-dependent adaptive mechanisms regulate neuronal activity during acute heat stress in 5-day-old zebrafish larvae.
Using epifluorescence calcium imaging in zebrafish larvae, we applied controlled heat ramps and quantified calcium event frequency and SD onset temperature. Intermediate ramps (0.98-0.31 °C/min), reaching SD within ~15-40 min, triggered SD at lower temperatures. In contrast, slow ramps (0.16 °C/min), lasting ~80 min, shifted SD onset to higher temperatures and were accompanied by a distinct pre-SD depression phase. Together, these features suggest engagement of time-dependent protective mechanisms during gradual heating.
To test the cellular basis of this resilience, we pharmacologically disrupted candidate processes during slow ramps. Inhibition of dynamin-dependent endocytosis or ER-Golgi trafficking did not alter SD onset. In contrast, blocking protein synthesis with cycloheximide or inhibiting heat shock protein induction with KNK437, lowered SD onset temperature, and abolished the pre-SD depression phase.
These findings demonstrate that neuronal vulnerability to heat is not determined by temperature alone but depends on heating rate and translation-dependent stress responses, providing mechanistic insight into how acute thermal insults promote SD in neurological disease.