Indolaminergic System in Adult Rat Testes: Evidence for a Local Serotonin System

Francisco Jiménez-Trejo,Julio Rojas-Castañeda,Margarita Chávez-Saldaña,Cristian Arriaga-Canon,Bladimir Roque-Ramírez,Marco Cerbón,Rosa M Vigueras-Villaseñor,Isabel Coronado-Mares,Luis A Herrera

doi:10.3389/fnana.2020.570058

Abstract

Serotonin (5-HT) is member of a family of indolamine molecules that participate in a wide variety of biological processes. Despite its important role in the regulation of local blood systems, little is known about the physiological function of 5-HT in reproductive organs, its functional implications, and its role in the reproduction of mammals. In the present work, we evaluated the localization and distribution of 5-HT (using histochemical analysis of indolamines) and different components of the serotoninergic system in rat testes. We detected local synthesis and degradation through immunofluorescence and western blot analyses against the TPH1, MAOA, 5-HTT, and VMAT1 serotonin transporters. We also identified the localization and distribution of the 5-HT1B, 5-HT2A, and 5-HT3A receptors. RT-PCR results showed the presence of the Tph1, Maoa, Slc6a4, and Htr3a genes in testes and in the brain stem (Tph1 was used as a negative control). High-performance liquid chromatography was used to determine the presence of 5-HT and the activity of tryptophan hydroxylase in testes homogenates in vitro. Our observations suggest that TPH1 activity and local 5-HT synthesis befall in rat testes. We propose that 5-HT could participate in the regulation of testosterone synthesis and in the spermatogenesis process via local serotoninergic system. However, more studies are needed before concluding that rat testes, or those of other mammals, contain an active form of tryptophan hydroxylase and produce 5-HT.

Highlights

Indolamines are a family of neurotransmitters that share a common molecular structure and are biologically synthesized from the essential amino acid tryptophan
In a first attempt to decipher its role, we have described the distribution of several elements of the serotoninergic system in the testes during spermatogenesis, the mating period, fertilization, and the sperm maturation processes occurring in the caput epididymis (Jiménez-Trejo et al, 2007, 2012, 2018)
Maoa (MAOA), Slc6a4 (5-HTT), and Htr3a (5HT3A) transcripts were present in both adult rat testes and brain stem (Figure 1)

Summary

Introduction

Indolamines are a family of neurotransmitters that share a common molecular structure and are biologically synthesized from the essential amino acid tryptophan. They function as neurotransmitters and neuromodulators in the central nervous system (CNS), whereas in the periphery they serve as neurohormones such as catecholamine, serotonin, and melatonin (Rapport et al, 1948; Young, 2007). L-tryptophan (L-Trp) is an essential amino acid and precursor for the biosynthesis of 5-HT in the brain and in peripheral tissues. TPH1 is expressed predominantly in peripheral tissues These enzymes are encoded by two different genes and are the rate limiting enzymes for 5HT biosynthesis in central and peripheral organs (Walther and Bader, 2003; Walther et al, 2003). These enzymes are encoded by two different genes and are the rate limiting enzymes for 5HT biosynthesis in central and peripheral organs (Walther and Bader, 2003; Walther et al, 2003). 5-hydroxytryptophan is decarboxylated to 5-HT (Gaspar et al, 2003)

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