Early-life seizures (ELS) can lead to the development of epilepsy in humans (Elger et al., 2004). Dynamic animal models are essential to study the effects of ELS in the developing brain. Zebrafish have emerged as a valuable model for neuroscience research due to their rapid development, the potential for high-throughput screening, and their genetic similarity with mammals (Stewart et al., 2014). While rodent ELS studies show spontaneous seizures later in life (Stafstrom & Sasaki-Adams, 2003), the effects of ELS on later-life seizures in zebrafish has yet to be explored.

Based on human and rodent studies of ELS, we hypothesized that larval zebrafish would become more susceptible to seizures within two weeks after experiencing ELS. To test this, we induced ELS in larval zebrafish using the chemoconvulsant pentylenetetrazol (PTZ), a drug commonly used to induce seizures in zebrafish (Baraban et al., 2005). Zebrafish were exposed to 5mM PTZ for 40 minutes per day from 5-7 days post-fertilization (dpf). Clutchmate controls remained in a beaker inside an incubator for the duration of this time and were not exposed to PTZ. All fish were placed in a tank on an automated rack system at 8 dpf where they remained unperturbed until 21 dpf. At 21 dpf, zebrafish from both groups were pipetted into 12-well plates where they were exposed to either 1, 2.5, or 5 mM PTZ for 20 minutes. For each condition and PTZ concentration, the number of stage two (whirlpool) seizures were measured.

In this pilot study, we found in 0%, 36.4%, and 50% of control fish demonstrated stage two seizures when administered 1, 2.5, and 5 mM PTZ at 21 dpf, respectively (n=8,11, and 12). However, 12.5%, 66.7%, and 66.7% of 21 dpf zebrafish that had previously experienced ELS had stage two seizures at those same concentrations, respectively (n = 8, 12, and 12). The number of stage two seizures demonstrated per fish were similar between conditions for 1mM and 5mM PTZ, but at 2.5 mM PTZ, zebrafish that experienced prior ELS had three times as many stage two seizures as controls (mean = 1.6 ± 0.5 SE for ELS fish, and 0.5 ± 0.2 SE for controls).

These observations provide evidence that larval zebrafish that experienced PTZ-induced ELS as very young larvae (5-7 dpf) may develop increased seizure susceptibility later in life. These results support the use of zebrafish in research aiming to identify the mechanisms by which ELS might lead to the development of epilepsy.