Hedgehog and Fgf signaling pathways regulate the development of tphR‐expressing serotonergic raphe neurons in zebrafish embryos

Abstract

Serotonin (5HT) plays major roles in the physiological regulation of many behavioral processes, including sleep, feeding, and mood, but the genetic mechanisms by which serotonergic neurons arise during development are poorly understood. In the present study, we have investigated the development of serotonergic neurons in the zebrafish. Neurons exhibiting 5HT-immunoreactivity (5HT-IR) are detected from 45 h postfertilization (hpf) in the ventral hindbrain raphe, the hypothalamus, pineal organ, and pretectal area. Tryptophan hydroxylases encode rate-limiting enzymes that function in the synthesis of 5HT. As part of this study, we cloned and analyzed a novel zebrafish tph gene named tphR. Unlike two other zebrafish tph genes (tphD1 and tphD2), tphR is expressed in serotonergic raphe neurons, similar to tph genes in mammalian species. tphR is also expressed in the pineal organ where it is likely to be involved in the pathway leading to synthesis

The serotonergic phenotype is acquired by converging genetic mechanisms within the zebrafish central nervous system

“…In agreement with our finding, Teraoka et al (2004) described expression of tph2 in the anterior cluster located just posterior to the trochlear nucleus (nIV) and anterior to the trigeminal motor nucleus (nV). We do not as yet have a mechanistic interpretation for this surprising interspecies difference.…”

“…We analyzed its temporal and spatial expression pattern during development and adulthood, thereby also verifying pet1 as a specific marker for anterior raphe serotonergic neurons in zebrafish. Three Tph enzymes with largely nonoverlapping expression domains share 5HT synthesis in this species (Bellipanni et al, 2002;Teraoka et al, 2004), and pet1 transcription precedes expression of the tph ortholog specific for the raphe, tph2. However, other tph2-positive 5HT clusters, for example, in the pretectum, appear not to rely on Pet1 expression for their development.…”

“…In mammals and chicken, the generation of rostral hindbrain 5HT neurons depends on both the midline signal Sonic hedgehog (Shh), the isthmic organizer signal fibroblast growth factor 8 (Fgf8), and possibly also Fgf2 and/or Fgf4 from the primitive streak at early stages (Ye et al, 1998;Cordes, 2005). Recent results indicate that similar mechanisms control the induction of serotonergic raphe neurons in the zebrafish embryo (Teraoka et al, 2004). Studies in mammals and chicken identified several distinct transcription factors involved in induction or maturation of 5HT raphe neurons.…”

Characterization and Expression of Serotonin Transporter Genes in Zebrafish

“…Tryptophan hydroxylase catalyzes the first step of serotonin and melatonin biosynthesis. The molecular cloning and expression of three tryptophan hydroxylases, tphD1, tphD2 and tphR in zebrafish have been reported (Bellipanni et al 2002;Teraoka et al 2004). An immunohistochemical approach was applied to study the development of a zebrafish serotonergic system Fetcho 2004a, 2004b).…”

“…Sense and anti-sense RNA probes labeled with Digoxigenin-labeled UTP (Roche diagnostics, Basel Switzerland) for serta, sertb and tphR (Teraoka et al 2004) were generated by in vitro transcription from respective cDNAs. In situ hybridization and immunostaining to whole-mount embryos were carried out as we previously described (Li et al 2004(Li et al , 2005Nüsslein-Volhard and Dahm 2002).…”