Domoic acid induces developmental stage-specific effects on microglia in zebrafish.

Harmful algal blooms (HABs), driven by warming oceans and increased eutrophication, are negatively affecting aquatic ecosystems as well as human health. Blooms of diatom Pseudo-nitzschia spp. produce a neurotoxin, domoic acid (DA), which can be concentrated by filter feeders, including shellfish. Consumption of DA-contaminated seafood causes amnesic shellfish poisoning. DA causes well-established effects on neurons, inducing neurotoxicity by binding to ionotropic glutamate receptors. However, its effects on non-neuronal cells including microglia, the resident immune cells in the brain, are less well understood. Microglia play critical roles in brain health, and disruptions in microglial activity during development can have long-term impacts on brain function and disease risk. The objective of this study was to examine the effects of developmental DA exposure on microglia using zebrafish (Danio rerio). We characterized effects of DA exposure on microglial abundance and morphology at two developmental stages (2 and 4 days post-fertilization (dpf)). We additionally assessed impacts on cell death, cytokine expression, and startle response behavior. In older larvae (4 dpf), microglial effects occurred only at the highest dose (0.3 ng DA per larvae) and coincided with severe morphological defects. In contrast, 2 dpf exposure to lower doses (0.1 ng DA per embryo) caused transient tremors without gross abnormalities but exposure to either 0.1 or 0.3 ng DA per larvae increased microglial abundance and altered microglial morphology. In contrast, 4 dpf exposure to 0.3 ng DA per larvae reduced microglial numbers. Increases in mRNA levels of il1b, il4, and tgfb were observed after 4 dpf exposure, but no significant cytokine changes were detected at 2 dpf. Overall, the effects of DA are highly developmental stage-specific, with microglial reactivity occurring at doses that do not cause gross morphological changes. These findings suggest that neuroinflammation may arise at lower DA doses, indicating a potential mechanism for subtle toxic effects and emphasizing the need for further research to better understand the consequences of low-dose DA exposure on microglial function.