During early zebrafish embryogenesis, spontaneous tail contractions represent the first sign of locomotion and
result from innervation of primary motoneuron axons to target axial muscles. Based on a high-content screen,
we previously demonstrated that exposure of zebrafish embryos to abamectin – an avermectin insecticide –
from 5–25 hours post-fertilization (hpf) abolished spontaneous activity in the absence of effects on survival
and gross morphology. Therefore, the objective of this study was to begin investigating the mechanism of
abamectin-induced hypoactivity in zebrafish. Similar to 384-well plates, static exposure of embryos to abamectin
from 5–25 hpf in glass beakers resulted in elimination of activity at low micromolar concentrations. However,
abamectin did not affect neurite outgrowth from spinal motoneurons and, compared with exposure from
5–25 hpf, embryos were equally susceptible to abamectin-induced hypoactivity when exposures were initiated
at 10 and 23 hpf. Moreover, immersion of abamectin-exposed embryos in clean water resulted in complete
recovery of spontaneous activity relative to vehicle controls, suggesting that abamectin reversibly activated
ligand-gated chloride channels and inhibited neurotransmission. To test this hypothesis, we pretreated embryos
to vehicle or non-toxic concentrations of fipronil or endosulfan – two insecticides that antagonize the
γ-aminobutyric acid (GABA) receptor – from 5–23 hpf, and then exposed embryos to vehicle or abamectin
from 23–25 hpf. Interestingly, activity levels within abamectin-exposed embryos pretreated with either
antagonist were similar to embryos exposed to vehicle alone. Using quantitative PCR and phylogenetic analyses,
wethen confirmed the presence of GABA receptorα1andβ2 subunits at 5, 10, and 23 hpf, and demonstrated that
zebrafish GABA receptor subunits are homologous to mammalian GABA receptor subunits. Overall, our data
collectively suggest that abamectin induces rapid and reversible hypoactivity within early zebrafish embryos,
an effect that may be mediated through the GABA receptor.