Can zebrafish scuba dive? The effect of hyperbaric oxygen on the brain

Cellular respiration requires oxygen (O2) to produce energy and is vital to the survival of most organisms. However, exposure to high partial pressures of oxygen (PO2) may lead to the development of a potentially fatal neurological condition - central nervous system oxygen toxicity (CNS-OT). CNS-OT is characterized by loss of consciousness, convulsions, and even death. Despite the risk of CNS-OT, breathing hyperbaric oxygen (HBO) has several medical and diving applications. We used a costum-made mini pressure chamber and live imaging in zebrafish to understand the effect of HBO on brain function. The findings showed that HBO alter the expression levels of genetic markers for hypoxia and oxidative stress. In addition, similar to mammals, the heart rate decreased under HBO conditions. Using video tracking of behavior and 2-photon microscope imaging of genetically encoded calcium indicators (GCaMP6s), we characterized region-specific brain activity and behavior under HBO compared to normal PO2. Notably, using double labeling of vascular structures and blood plasma in live transgenic zebrafish, we imaged changes in the diffusion rates of the blood through the blood-brain barrier (BBB), and suggest that BBB leakage is one of the causes to CNS-OT under HBO conditions.