Targeted gonadotropin-releasing hormone-3 neuron ablation in zebrafish: effects on neurogenesis, neuronal migration, and reproduction.

Hypophysiotropic GnRH neurons are located in the preoptic area and ventral hypothalamus of sexually mature vertebrates. In several species, the embryonic origin of hypophysiotropic GnRH neurons remains unclear. Using the Tg(GnRH3:EGFP) zebrafish line, in which GnRH3 neurons express EGFP, GnRH3 neurons in the olfactory region were specifically and individually ablated during early development using laser pulses. After ablation, the olfactory region maintained the capacity to regenerate GnRH3 neurons. However, this capacity was time-limited. When ablation of GnRH3 cells was conducted at 2 d after fertilization, high regeneration rates were observed, but regeneration capacity significantly decreased when ablation was performed at 4 or 6 d after fertilization. Unilateral GnRH3 neuron ablation results in unilateral soma presence. These unilateral somata are capable of projecting fiber extensions bilaterally. Successful bilateral GnRH3 soma ablation during development resulted in complete lack of olfactory, terminal nerve, preoptic area, and hypothalamic GnRH3 neurons and fibers in 12-wk-old animals. Mature animals lacking GnRH3 neurons exhibited arrested oocyte development and reduced average oocyte diameter. Animals in which GnRH3 neurons were partially ablated exhibited normal oocyte development; however, their fecundity was significantly reduced. These findings demonstrate that the hypophysiotropic GnRH3 populations in zebrafish consist of neurons that originate in the olfactory region during early development. The presence of GnRH3 neurons of olfactory region origin in reproductively mature zebrafish is a prerequisite for normal oocyte development and reproduction.