Distribution of two isoforms of tryptophan hydroxylase in the brain of rainbow trout (Oncorhynchus mykiss). An in situ hybridization study

Mauro Chivite,Jesús M Míguez,Esther Leal,Jose Miguel Cerdá-Reverter

doi:10.1007/s00429-021-02322-8

Abstract

Serotonin (5-HT) is one of the principal neurotransmitters in the nervous system of vertebrates. It is initially synthesized by hydroxylation of tryptophan (Trp) by means of tryptophan hydroxylase or TPH which is the rate-limiting enzyme in the production of 5-HT. In most vertebrates, there are two isoforms of TPH present, TPH1 and TPH2, which exhibit different catalytic or substrate specificity as well as different expression domains. Studies carried out in mammals show that only tph2 is expressed in the brain whereas tph1-mRNA is primarily localized in the enterochromaffin cells and pineal gland. A large number of neurons are also considered to be serotonergic or “pseudo-serotonergic” as they accumulate and release 5-HT yet do not produce it as no amine-synthetic enzymes are expressed, yet a combination of 5-HT transporters is observed. Therefore, tph expression is considered to be the only specific marker of 5-HT-producing neurons that can discriminate true 5-HT from pseudo-serotonergic neurons. This work examined in situ hybridization to study the mRNA distribution of one paralogue for tph1 and tph2 in the central nervous system of rainbow trout. Results show a segregated expression for both paralogues that predominantly match previous immunocytochemical studies. This study thus adds valuable information to the scarce analyses focusing on the central distribution of the expression of serotonergic markers, particularly tphs, in the vertebrate brain thus characterizing the true serotonergic brain territories.

Highlights

The monoamine serotonin or 5-hydroxytryptamine (5-HT) is one of the major neurotransmitters of the central nervous system (CNS)
The genome of the chondrichthyes (Callorhinchus milii) displays both tph1 and tph2 suggesting that the duplication of tph genes took place in the gnathostomata following the divergence of the cyclostomata species
Teleost fish have undergone an extra genome duplication commonly known as teleost-specific genome duplication (TSGD) resulting in an extra-duplication of all their genes (Amores et al 1998)

Summary

Introduction

The monoamine serotonin or 5-hydroxytryptamine (5-HT) is one of the major neurotransmitters of the central nervous system (CNS). 5-HTP is subsequently decarboxylated to become 5-HT by the aromatic L-amino acid decarboxylase (AADH) enzyme. Tph expression is specific for the 5-HT producing cells/neurons and the only specific marker of 5-HT-producing neurons as aadh and mao are expressed by other monoaminergic neurons, many cells use SERT/SLC6A4 for 5-HT reuptake without undergoing synthesis (Norton et al 2008; Lillesaar 2011). Peripheral organs such as the enterochromaffin cells, mammary and pineal glands, placenta and pancreatic beta cells predominantly use TPH1 for 5-HT synthesis. TPH2 is basically a central isoform, yet some cells in the periphery use it for 5-HT synthesis, such as the serotonergic myenteric cells (Panula et al 2010; Gaspar and Lillesaar 2012)

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Conclusion