Estrogen Receptor 2b Is Involved in Regulating Gonadotropin-Inhibitory Hormone Expression During Early Development in Zebrafish

Gonadotropin-inhibitory hormone (<i>GnIH</i>) is a neuropeptide involved in the regulation of reproductive function in vertebrates. It is able to inhibit the synthesis and secretion of <i>GnRH</i> and Gth in the HPG axis. However, the regulatory role and mechanism by which current gonadal steroid hormones regulate <i>GnIH</i> are still unclear. In this study, transcription factor binding site analysis was performed on the promoter sequence of zebrafish <i>GnIH</i>. Whereafter, transgenic zebrafish (<i>GnIH</i>: mCherry) accurately labeled <i>GnIH</i> and <i>esr2b</i> knockout zebrafish, which were constructed previously, were used to explore the regulation between estrogen and <i>GnIH</i>. In vitro exposure of wild-type zebrafish embryos and transgenic zebrafish embryos to estradiol showed that 10 μM estradiol significantly increased the transcription level of <i>GnIH</i>. However, both 10 μM and 50 μM estradiol significantly increased the transcription level of <i>GnIH</i> in <i>esr2b</i> knockout zebrafish. We compared the expression levels of <i>GnIH</i> in <i>esr2b</i> knockout zebrafish and wild-type zebrafish at different developmental stages (48 hpf-120 hpf). The results showed that from 96 hpf, the expression level of <i>GnIH</i> in <i>esr2b</i> knockout zebrafish was significantly higher than that in wild-type zebrafish, indicating that <i>esr2b</i> is involved in the negative regulation of <i>GnIH</i>, and this regulatory relationship is established on the fourth day of zebrafish development.