Acquisition of spontaneous electrical activity during embryonic development of gonadotropin-releasing hormone-3 neurons located in the terminal nerve of transgenic zebrafish (Danio rerio).

There are multiple populations of gonadotropin releasing hormone (GnRH) neurons that have distinct physiological and behavioral functions. Teleost fish have a population of GnRH3 neurons located in the terminal nerve (TN) associated with the olfactory bulb that is thought to play a neuromodulatory role in multiple physiological systems, including olfactory, visual, and reproductive. We used transgenic zebrafish in which the GnRH3 promoter drives expression of a green fluorescent protein to identify GnRH3 neurons during development in live embryos. Unlike with hypophysiotropic GnRH neurons of zebrafish, TN-GnRH3 neurons are of neural crest origin and are one of the first populations of GnRH neurons to develop in the early embryo. Using a combination of optical imaging and electrophysiology, we showed that during the first three days post-fertilization, TN-GnRH3 neurons increase in number, extend neural projections, move in association with tissue expansion, and acquire an adult-pattern of spontaneous action potential firing. Early during development, about half of the neurons were quiescent/non-firing. Later, at three days post-fertilization, there was an increase in the proportion of neurons showing action potential firing and an increase in the number of neurons that showed an adult-like tonic or beating pattern of action potential firing with a firing frequency similar to that seen in adult TN-GnRH3 neurons. This study represents the first neurophysiological investigation of developing GnRH neurons in live embryos -- an important advance in understanding their potential non-reproductive roles during embryogenesis.

促性腺激素释放激素（GnRH）神经元存在多个群体，它们具有不同的生理和行为功能。硬骨鱼在与嗅球相关的终神经（TN）中有一群GnRH3神经元，其被认为在包括嗅觉、视觉和生殖在内的多个生理系统中发挥神经调节作用。我们使用转基因斑马鱼，其中GnRH3启动子驱动绿色荧光蛋白的表达，以便在活胚胎发育过程中识别GnRH3神经元。与斑马鱼的促垂体GnRH神经元不同，TN - GnRH3神经元起源于神经嵴，是早期胚胎中最早发育的GnRH神经元群体之一。通过结合光学成像和电生理学方法，我们发现，在受精后的前三天，TN - GnRH3神经元数量增加，延伸神经突起，随着组织扩张而移动，并获得一种自发动作电位发放的成年模式。在发育早期，大约一半的神经元处于静止/不发放状态。后来，在受精后三天，发放动作电位的神经元比例增加，并且表现出类似成年的持续性或节律性动作电位发放模式且发放频率与成年TN - GnRH3神经元相似的神经元数量也增加。这项研究是对活胚胎中发育的GnRH神经元的首次神经生理学研究——这是在理解它们在胚胎发生过程中潜在的非生殖作用方面的一个重要进展。