Distribution of thyrotropin‐releasing hormone (TRH) immunoreactivity in the brain of the zebrafish (Danio rerio)

Abstract

The distribution of thyrotropin-releasing hormone (TRH) in the brain of the adult zebrafish was studied with immunohistochemical techniques. In the telencephalon, abundant TRH-immunoreactive (TRHir) neurons were observed in the central, ventral, and supra- and postcommissural regions of the ventral telencephalic area. In the diencephalon, TRHir neurons were observed in the anterior parvocellular preoptic nucleus, the suprachiasmatic nucleus, the lateral hypothalamic nucleus, the rostral parts of the anterior tuberal nucleus and torus lateralis, and the posterior tuberal nucleus. Some TRHir neurons were also observed in the central posterior thalamic nucleus and in the habenula. The mesencephalon contained TRHir cells in the rostrodorsal tegmentum, the Edinger-Westphal nucleus, the torus semicircularis, and the nucleus of the lateral lemniscus. Further TRHir neurons were observed in the interpeduncular nucleus. In the rhombencephalon, TRHir cells were observed in the nucleus isthmi and the locus coeruleus, rostrally, and in the vagal lobe and vagal motor nucleus, caudally. In the forebrain, TRHir fibers were abundant in several regions, including the medial and caudodorsal parts of the dorsal telencephalic area, the ventral and commissural parts of the ventral telencephalic area, the preoptic area, the posterior tubercle, the anterior tuberal nucleus, and the posterior hypothalamic lobe. The dorsal thalamus exhibited moderate TRHir innervation. In the mesencephalon, the optic tectum received a rich TRHir innervation between the periventricular gray zone and the stratum griseum centrale. A conspicuous TRHir longitudinal tract traversed the tegmentum and extended to the rhombencephalon. The medial and lateral mesencephalic reticular areas and the interpeduncular nucleus were richly innervated by TRHir fibers. In the rhombencephalon, the secondary gustatory nucleus received abundant TRHir fibers. TRHir fibers moderately innervated the ventrolateral and ventromedial reticular area and richly innervated the vagal lobe and Cajal's commissural nucleus. Some TRHir fibers coursed in the lateral funiculus of the spinal cord. Some TRHir amacrine cells were observed in the retina. The wide distribution of TRHir neurons and fibers observed in the zebrafish brain suggests that TRH plays different roles. These results in the adult zebrafish reveal a number of differences with respect to the TRHir systems reported in other adult teleosts but were similar to those found during late developmental stages of trout (Díaz et al., 2001).

Distribution of Thyrotropin-Releasing Hormone (TRH) Immunoreactivity in the Brain of Urodele Amphibians

“…Thus, in untreated animals TRHir material was detectable in nerve fibers and terminals, but the cell bodies were only weakly stained or did not stain at all [Tsuruo et al, 1987[Tsuruo et al, , 1988aJózsa et al, 1988]. However, in recent studies in different anamniotes wide distribution patterns for TRH were described that included nicely stained cell bodies and in none of these studies colchicines was used [Lamacz et al, 1989;Miranda and Affanni, 2000;Díaz et al, 2001Díaz et al, , 2002De Andrés et al, 2002;Teijido et al, 2002]. In our study, we have used one animal of each species with colchicine pretreatment as controls and have observed that, at least in urodeles, the same immunoreactive cell groups are observed for each species with only a slight improvement of the intensity of the reaction; a result that contrasts with the situation found in amniotes.…”

“…In general, TRH was primarily detected in neurons of the preoptic area and hypothalamus, although the presence of TRH in neurons of other brain regions has also been reported [Matz and Takahashi, 1994;Díaz et al, 2001Díaz et al, , 2002Teijido et al, 2002].…”

“…The structural similarity of TRH across vertebrates led to studies of the brain distribution of TRHir cells and fibers in numerous non-mammalian species from different vertebrate classes [agnathans: De Andrés et al, 2002;elasmobranchs: Teijido et al, 2002;teleosts: Batten et al, 1990a, b;Hamano et al, 1990;Matz and Takahashi, 1994;Díaz et al, 2001Díaz et al, , 2002birds: Józsa et al, 1988birds: Józsa et al, , 1989Péc-zely and Kiss, 1988;Kawano et al, 1991, Geris et al, 1999]. In general, TRH was primarily detected in neurons of the preoptic area and hypothalamus, although the presence of TRH in neurons of other brain regions has also been reported [Matz and Takahashi, 1994;Díaz et al, 2001Díaz et al, , 2002Teijido et al, 2002].…”

Zebrafish diencephalic A11-related dopaminergic neurons share a conserved transcriptional network with neuroendocrine cell lineages

“…4A-C, arrowheads). The expression of trh in zebrafish has only been analyzed in adult brain (Diaz et al, 2002). In 76 hpf forebrain, trh is expressed in the preoptic region in a domain close to the anterior commissure.…”

Embryonic exposure to excess thyroid hormone causes thyrotrope cell death

“…Thyroid follicles (the analog of the human thyroid gland) first produce T4 by 3 days post fertilization (dpf) (16), and substantial changes in tissue levels of T4 first occur by 5 dpf (17). TRH-producing cells are widely distributed in the zebrafish brain, including in the diencephalon, where thyrotropin-releasing hormone-containing (TRH-containing) fibers project to the adenohypophysis (18). Importantly, hormone levels can be easily manipulated in the zebrafish embryo, and thyrotropes can be tracked in real time, facilitating the analysis of HPT axis development under hyperthyroid conditions.…”