Distribution of glycinergic neurons in the brain of glycine transporter‐2 transgenic Tg(glyt2:Gfp) adult zebrafish: Relationship to brain–spinal descending systems

Abstract

We used a Tg(glyt2:gfp) transgenic zebrafish expressing the green fluorescent protein (GFP) under control of the glycine transporter 2 (GLYT2) regulatory sequences to study for the first time the glycinergic neurons in the brain of an adult teleost. We also performed in situ hybridization using a GLYT2 probe and glycine immunohistochemistry. This study was combined with biocytin tract tracing from the spinal cord to reveal descending glycinergic pathways. A few groups of GFP(+) /GLYT2(-) cells were observed in the midbrain and forebrain, including numerous pinealocytes. Conversely, a small nucleus of the midbrain tegmentum was GLYT2(+) but GFP(-) . Most of the GFP(+) and GLYT2(+) neurons were observed in the rhombencephalon and spinal cord, and a portion of these cells showed double GLYT2/GFP labeling. In the hindbrain, GFP/GLYT2(+) populations were observed in the medial octavolateral nucleus; the secondary, magnocellular, and descending octaval nuclei; the viscerosensory lobes; and reticular populations distributed from trigeminal to vagal levels. No glycinergic cells were observed in the cerebellum. Tract tracing revealed three conspicuous pairs of GFP/GLYT2(+) reticular neurons projecting to the spinal cord. In the spinal cord, GFP/GLYT2(+) cells were observed in the dorsal and ventral horns. GFP(+) fibers were observed from the olfactory bulbs to the spinal cord, although their density varied among regions. The Mauthner neurons received very rich GFP(+) innervation, mainly around the axon cap. Comparison of the zebrafish glycinergic system with the glycinergic systems of other adult vertebrates reveals shared patterns but also divergent traits in the evolution of this system.

Gustatory and general visceral centers and their connections in the brain of adult zebrafish: a carbocyanine dye tract‐tracing study

“…With regard to the adult zebrafish, the organization of primary taste nuclei is primarily known from comparisons with carp and goldfish, consisting of a large medial facial lobe and two large paired vagal lobes, whereas the small glossopharyngeal lobes are closely associated with the vagal lobes and the facial lobe (Wullimann et al, ; Rupp et al, ). Some immunohistochemical and in situ hybridization studies have characterized different cell types in the zebrafish taste lobes based on their neurochemical signatures (Díaz et al, ; Castro et al, ; Coppola et al, ; Barreiro‐Iglesias et al, ).…”

Parvalbumin-Neurons of the Ventrolateral Hypothalamic Parvafox Nucleus Receive a Glycinergic Input: A Gene-Microarray Study

“…However, glycine-immunoreactive fibers and terminals have been observed in various parts of the brain by different authors (Zeilhofer et al, 2005; Barreiro-Iglesias et al, 2013) and glycine-induced chloride-currents were discovered already in 1989 in dissociated hypothalamic neurons (Akaike and Kaneda, 1989). Functional glycine receptors have been detected in higher brain centers, such as the hippocampus (Brackmann et al, 2004; Keck and White, 2009; Xu and Gong, 2010), and the glycine transporter 2 has been demonstrated in the whole brain as well as in the hypothalamus (Zeilhofer et al, 2005).…”

Neuronal organization of the brain in the adult amphioxus (Branchiostoma lanceolatum): A study with acetylated tubulin immunohistochemistry

“…With the exception of the rostralmost Rohde cell, whose axon courses in the ventral midline, the giant axons of Rohde cross the ventral midline near the perikaryon and run along the spinal cord contralaterally to the side of the axon exit, regardless of their location in rostral or caudal CNS regions (Rohde, 1887(Rohde, , 1888. This crossed axonal organization is reminiscent of the giant nerve system described for the collar of balanoglossids by Bullock (1944), but it does not correspond well to the large reticulospinal axons relating the rhombencephalon and spinal cord in fishes, which are largely ipsilateral (lampreys: Davis and McClellan, 1994;zebrafish: Becker et al, 1997;Barreiro-Iglesias et al, 2013). Rohde cells appear to receive inputs from fibers on both sides of the cord, which is also unlike the case for vertebrate reticulospinal neurons.…”

“…Cells with this morphology were not labeled in tracing experiments from the spinal cord (Ekhart et al, 2003), which suggests that they have mostly short projections. Inhibitory (GABAergic and/or glycinergic) brain neurons with spinal projections are very scarce in the sea lamprey and zebrafish (Valle-Maroto et al, 2011;Barreiro-Iglesias et al, 2013), which suggests shared patterns between vertebrates and amphioxus. With regard to excitatory neurotransmitters, high concentrations of glutamate and aspartate have been reported for the neural cord of adult amphioxus (D'Aniello and Garcia-Fern andez, 2007), although their neuronal distribution has not been studied.…”