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

Abstract

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.

From seeing to believing: labelling strategies for in vivo cell-tracking experiments

“…The excitation and emission peaks are indicated, together with references to some examples taken from available literature. For photoswitchable/photoconvertible proteins, excitation and emission peaks both before and after conversion are indicated. excitation peak (nm)emission peak (nm) in vivo imagingmousezebrafish fluorescent proteins  Cerulean433475[33] GFP (EGFP)396 (488)508[12–14,34][10,17–19,28,35] Emerald487509[36][37] Azami-Green492505[38] YFP514527[39][40] Venus515528[41][42] mKO548559[43] Kusabira-Orange548561[38] mOrange1548562[44] tdTomato554581[45]…”

Development of the Terminal Nerve System in the Shark <b><i>Scyliorhinus canicula</i></b>

“…A placodal origin of terminal nerve ganglion neurons has been proposed in mice using GnRH immunohistochemistry as an early marker of these populations [Schwanzel-Fukuda and Pfaff, 1989], and similar placodal origin of terminal nerve ganglion cells has also been suggested in other vertebrates [Valverde et al, 1993;Parhar et al, 1995;Quanbeck et al, 1997;Malz and Kuhn, 2002]. However, studies in developing zebrafish indicate that olfactory and terminal nerve neuronal populations originate from different precursor territories, namely the placode and the cranial neural crest [Whitlock et al, 2003;Whitlock, 2004;Whitlock et al, 2006;Ramakrishnan et al, 2010;Harden et al, 2012]. We have evidence that Pax3 (a marker of neural crest derivatives) is expressed in some cells of the early olfactory epithelium and adjacent mesenchyme [unpubl.…”

“…The presence of gonadotropin-releasing hormone (GnRH)/luteinizing hormone-releasing hormone immunoreactivity in the terminal nerve ganglion cells and fibers of several species (with the exception of lampreys) provided additional evidence of its implication in reproductive functions [for a revision, see Wray, 2010], although it has been also related to neuromodulatory nonreproductive functions during embryogenesis [Abraham et al, 2008;Ramakrishnan et al, 2010;Zhao et al, 2013]. Besides GnRH, a variety of neuroactive peptides such as Phe-Met-Arg-Phe-NH2 peptide (FMRF-amide peptide or FMRF-amide) or neuropeptide tyrosine (NPY) have been reported consistently in different subpopulations of neurons belonging to the terminal nerve system of different vertebrate species [von Bartheld, 2004], including elasmobranchs Chiba et al, 1991;Chiba, 2000], thus representing reliable markers of this system.…”

“…The developmental origin of the terminal nerve neurons is under discussion, and either the olfactory placode [Murakami et al, 1992;Northcutt and Muske, 1994;von Bartheld and Baker, 2004] or, alternatively, neural crestderived cells related to the early placodal region [Whitlock et al, 2003;von Bartheld and Baker, 2004;Whitlock, 2004;Barraud et al, 2010;Ramakrishnan et al, 2010;Forni et al, 2011;Katoh et al, 2011;Harden et al, 2012] have been proposed as their embryonic sources. In most developing vertebrates studied hitherto, cells of the terminal nerve system migrate from the region of the olfactory placode and enter the brain along a migration route associated with olfactory fibers [Schwanzel-Fukuda and Pfaff, 1989;Wray et al, 1989a, b;Tobet et al, 1997;Drapkin and Silverman, 1999;Schwanzel-Fukuda, 1999].…”

Characterization of voltage-activated ionic currents in the GnRH-containing terminalis nerve in transgenic zebrafish

“…Recently, several TN-specific transgenic zebrafish lines have been described (Abraham et al, 2008; Wang et al, 2010; Ramakrishnan et al, 2010). The transgenic fish provide an excellent model for further studies of TN cell development (Abraham et al, 2008), axonal projection (Wang et al, 2010) and physiology (Wang et al, 2010; and the current study).…”