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Background: Homeobox transcription factor encoding genes, genomic screen homeobox 1 and 2 (gsx1 and gsx2), are expressed during neurodevelopment in multiple vertebrates. However, we have limited knowledge of the dynamic expression of these genes through developmental time and the gene networks that they regulate in zebrafish. Results: We confirmed that gsx1 is expressed initially in the hindbrain and diencephalon and later in the optic tectum, pretectum, and cerebellar plate. gsx2 is expressed in the early telencephalon and later in the pallium and olfactory bulb. gsx1 and gsx2 are co‐expressed in the hypothalamus, preoptic area, and hindbrain, however, rarely co‐localize in the same cells. gsx1 and gsx2 mutant zebrafish were made with TALENs. gsx1 mutants exhibit stunted growth, however, they survive to adulthood and are fertile. gsx2 mutants experience swim bladder inflation failure that prevents survival. We also observed significantly reduced expression of multiple forebrain patterning distal‐less homeobox genes in mutants, and expression of foxp2 was not significantly affected. Conclusions: This work provides novel tools with which other target genes and functions of Gsx1 and Gsx2 can be characterized across the central nervous system to better understand the unique and overlapping roles of these highly conserved transcription factors. Key Findings: gsx1 and gsx2 expression is dynamic during embryonic and larval stages of neurodevelopment in zebrafish, and gsx1 expression appears to be in differentiating neurons based on their morphology and position.gsx1 and gsx2 are expressed together in several developing brain regions, however, they rarely colocalize in the same cells.Early forebrain patterning in gsx1 mutants appears normal, and Gsx1 might partially compensate for loss of Gsx2 function as more severe forebrain gene expression loss is observed in gsx1 and gsx2 mutants compared to gsx2 mutants in some instances.Complementation testing between different gsx1 or gsx2 mutant alleles shows that mutant phenotypes arise from altered gsx1 or gsx2 function. [ABSTRACT FROM AUTHOR]

The increased abuse of novel drugs has created a critical need for cheap and rapid in vivo models to understand whole organism drug‐induced toxicity and metabolic impacts. One such model is zebrafish, which share many similarities to human. Assays have been developed for behavioral, toxicity, and metabolism elucidation following chemical exposure. The zebrafish model provides the advantage of assessing these parameters within a single study. Previous zebrafish studies have evaluated the behavioral effects of fentanyl, but not developmental toxicity and its relation to metabolism. In this study, we evaluate the effects of fentanyl on the development of wild‐type (TL strain) zebrafish and its metabolism over 4 days. Fertilized eggs were exposed to six concentrations of fentanyl (0.01, 0.1, 1, 10, 50, and 100 μM) through embryo media incubated at 28–29°C. Observations included egg coagulation, somite formation, heartbeat, tail and yolk morphology, pericardial formation, and swim bladder inflation. The incubation media was analyzed for the presence of metabolites using a targeted metabolomics approach. Fentanyl concentration caused significant effects on survival and development, with notable defects to the tail, yolk, and pericardium at 50 and 100 μM. Despropionyl fentanyl (4‐ANPP), β‐hydroxy fentanyl, and norfentanyl were detected in zebrafish larvae. We present a single in vivo model to assess toxicity and metabolism of fentanyl exposure in a vertebrate model system. Our findings provide a foundation for further investigations into fentanyl's mechanism of action and translation to human drug exposure. The increased abuse of novel drugs has created a need for cheap and rapid in vivo models to understand whole organism drug‐induced toxicity and metabolic impacts. In this study, we evaluate the effects of fentanyl on the development of zebrafish and its metabolism. We report metabolites and significant developmental defects in zebrafish larvae upon exposure to fentanyl. Our findings provide a foundation for further investigations into fentanyl's mechanism of action and translation to human drug exposure. [ABSTRACT FROM AUTHOR]