Developmental Exposure to Domoic Acid Disrupts Startle Response Behavior and Circuitry in Zebrafish

Abstract

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 of DomA developmental neurotoxicity are largely unknown. To assess these, we exposed zebrafish embryos to DomA during the previously identified window of susceptibility 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 startle responsiveness to both auditory/vibrational and electrical stimuli, and even at the highest stimulus intensities tested, led to a dramatic reduction of one type of startle (short latency c-starts). Furthermore, DomA-exposed larvae had altered kinematics for both types of startle responses tested, 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 short latency c-starts 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 targets large reticulospinal neurons and motor neuron axon collaterals, resulting in measurable deficits in startle behavior. They further provide a framework for using the startle response circuit to identify specific neural populations disrupted by toxins or toxicants and to link these disruptions to functional consequences for neural circuit function and behavior.

Developmental exposure to domoic acid targets reticulospinal neurons and leads to aberrant myelination in the spinal cord

“…Previous studies showed that exposure at 2 dpf (0.14 ng, injected at 48-52 hpf) led to startle response deficits and myelin defects, whereas exposures at 1 dpf (injected at 28-32 hpf) did not lead to any of these effects. 27,28 We sought to determine whether, as seen for the behavioral and myelin phenotypes, the loss of Mauthner cells does not occur after exposure at 1 dpf. Even though the Mauthner cells were present at the time of exposure (Fig.…”

“…DomA-induced disruptions to myelin sheath formation in the central nervous system 27,28 led us to look into the effects of DomA on cells of the oligodendrocyte lineage, which are responsible for myelinating the central nervous system. 29,30 Successful myelination requires oligodendrocyte precursor cells (OPCs) to form, migrate, differentiate into oligodendrocytes, and then initiate the process of myelination in different regions within the nervous system.…”

“…Our previous work has shown that DomA targets both myelin in the spinal cord and reticulospinal neurons – hindbrain neurons with axons that extend to the spinal cord. 27,28 However, it is unclear whether DomA initially targets axons, leading to deficits in myelination, or whether DomA initially targets myelinating oligodendrocytes, leading to later axonal degeneration. To resolve this, we assessed the effects of DomA on these potential cellular targets close to the time of exposure to determine which cell types are the first to be perturbed by DomA.…”

“…The objective of this study was to identify the initial cellular targets of DomA. To accomplish this, we exposed fish to DomA at 2 dpf as before 27,28 and then assessed the effects on candidate cell targets between 4 and 8 hours post-exposure, prior to myelination. Candidate cell targets included OPCs, reticulospinal neurons, sensory neurons, and motor neurons.…”

Orphan cytochrome P450 20a1 CRISPR/Cas9 mutants and neurobehavioral phenotypes in zebrafish

“…Vibroacoustic startle latency was assessed as described previously 37 , 38 and the same set-up was used to test for startle habituation as a form of non-associative learning in 6 dpf larval zebrafish. For each trial, 16 larvae with inflated swim bladders were distributed in a 4 × 4 acrylic well-plate which was mounted on a minishaker (Brüel & Kjaer, Vibration Exciter 4810) connected to an amplifier (Brüel & Kjaer, Power Amplifier Type 2718).…”