Developmental exposure to domoic acid disrupts startle response behavior and circuitry

Harmful algal blooms produce potent neurotoxins that accumulate in seafood and are hazardous to human health. Developmental exposure to the harmful algal bloom toxin, domoic acid (DomA), has behavioral consequences well into adulthood, but the cellular and molecular mechanisms are largely unknown. To assess these, we exposed zebrafish embryos to DomA during the previously identified window of susceptibility (2 days post-fertilization) and used the well-known startle response circuit as a tool to identify specific neuronal components that are targeted by exposure to DomA. Exposure to DomA reduced the probability of eliciting a startle after auditory/vibrational or electrical stimuli and led to the dramatic reduction of one type of startle, short latency c-start (SLC) responses. Furthermore, DomA-exposed larvae had altered kinematics of both SLC and long latency c-start (LLC) startle responses, exhibiting shallower bend angles and slower maximal angular velocities. Using vital dye staining, immunolabelling, and live imaging of transgenic lines, we determined that while the sensory inputs were intact, the reticulospinal neurons required for SLC responses were absent in most DomA-exposed larvae. Furthermore, axon tracing revealed that DomA-treated larvae also showed significantly reduced primary motor neuron axon collaterals. Overall, these results show that developmental exposure to DomA leads to startle deficits by targeting specific subsets of neurons. These findings provide mechanistic insights into the neurodevelopmental effects of excess glutamatergic signaling caused by exposure to DomA. It further provides a model for using the startle response circuit to identify neuronal populations targeted by toxin or toxicant exposures.