The BDNF/TrkB Neurotrophin System in the Sensory Organs of Zebrafish

Abstract

The brain-derived neurotrophic factor (BDNF) was discovered in the last century, and identified as a member of the neurotrophin family. BDNF shares approximately 50% of its amino acid with other neurotrophins such as NGF, NT-3 and NT-4/5, and its linear amino acid sequences in zebrafish (Danio rerio) and human are 91% identical. BDNF functions can be mediated by two categories of receptors: p75NTR and Trk. Intriguingly, BDNF receptors were highly conserved in the process of evolution, as were the other NTs' receptors. In this review, we update current knowledge about the distribution and functions of the BDNF-TrkB system in the sensory organs of zebrafish. In fish, particularly in zebrafish, the distribution and functions of BDNF and TrkB in the brain have been widely studied. Both components of the system, associated or segregated, are also present outside the central nervous system, especially in sensory organs including the inner ear, lateral line system, retina, taste buds and olfactory epithelium.

Keywords: BDNF; TrkB receptor; neurotrophins; sensory systems; zebrafish.

Figure 1

Stereomicrograph of zebrafish head with lateral line system canal pore (a). Graphical representation of the canal of the lateral line system; pore (arrow), neuromast (nt) and connettive tissue (ct); (b). Immunohistochemical detection of BDNF; (c) and TrkB; (d) in the sensory hair cells (white arrows) of supra-orbital neuromasts. Magnification 40× (c,d).

Figure 2

Stereomicrograph of zebrafish inner ear (a). Light micrograph of Masson Trichrome with aniline blue-stained dorso-ventral section of an adult zebrafish head; semicircular anterior canal (ac) and semicircular horizontal canal (hc) of the inner ear with its ampulla (ha) containing the crista ampullaris (dashed insert); (b). Utricular macula, sensory hair cells (white arrows) with numerous stereocilia (arrowheads) and less numerous but longer kinocilia (asterisk) are visible; (c). BDNF and TrkB immunoreactive sensory hair cells (arrows) in the crista ampullaris (d,f) and in the macula (e,g). Magnification 20× (b); 40× (c–g).

Figure 3

Stereomicrograph of zebrafish eye (a). Longitudinal section of zebrafish retina stained with Masson Trichrome with aniline blue, ganglion cell layer (GCL); inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), photoreceptor cell segment (phot); (b). Immunohistochemical localization of BDNF; (c) and TrkB; (d) in zebrafish retina layer. BDNF immunoreactivity was observed in the outer nuclear layer (ONL), outer plexiform layer (OPL) and inner plexiform layer (IPL), whereas TrkB immunoreactivity (b,d) was restricted to the inner plexiform layer (IPL). Magnification 20× (b); 40× (c,d). Magnification 20× (d), 40× (c,d).

Figure 4

Stereomicrograph of zebrafish nose after removing nostril cartilage (a, insert); olfactory epithelium is visible (a, dashed area); Scanning electron micrograph of zebrafish olfactory epithelium organized in lamellae (b); Sensory cells (insert) of the olfactory epithelium of the sensory part of the rosette lamella: cryptic neuron (arrowhead), neuron with microvilli (star), ciliate neurons (asterisks), immature neuron (arrow) (c); Immunohistochemical localization of BDNF (d); and TrkB (e) in crypt cells (asterisks) and sensory cells (arrows) in basal and sensory segment of olfactory lamellae. Scale bar: 1 mm (a), 20 µm (b). Magnification 40× (c–e).

Figure 5

Scanning electron micrograph of zebrafish taste bud (a); Schematic drawing of a taste bud (b) with light cells (cl), dark cells (cd), light cells with microvilli (cmv), marginal cells (cm), basal cells (cb), nerve (n), and basal layer (bl). Light micrograph of a Masson Trichrome with aniline, blue-stained sagittal section of a taste bud from the oral cavity (c); BDNF immunoreactive dark cells (asterisks) in zebrafish cutaneous (d); and oral (e) taste buds. TrkB immunoreactive dark cells (arrows) and light cells (asterisks) in zebrafish cutaneous (f); and oral (g) taste buds. Scale bar: 20 µm (a). Magnification 40× (c–g).