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

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.

Downregulation of Serotonergic System Components in an Experimentally Induced Cryptorchidism in Rabbits

“…The presence of 5-HT3 A in the sperm acrosome in control rabbits is in agreement with the finding of these receptors in other mammal species including humans [ 32 , 51 , 52 ] and reinforces the hypothesis that these cation-selective 5-HT-gated ion channels could be important for the sperm movement when they are in the female tract. The presence of these receptors in the interstitial zone was unexpected, and in a previous work of the description of indoleaminergic elements in normal rat testes 5-HT3 A , it was found in the seminiferous tubules but not interstitial space [ 51 ], but in an earlier work, Dufau et al (1993) found that 5-HT could modulate the corticotropin-releasing factor (CRF) through 5HT 2 receptors in Leydig cells [ 30 ]. Although 5HT3 agonists were not able to induce the release of CRF, it is possible that 5-HT3 A receptors could participate in other cellular processes, but further experiments are required to test this statement.…”

“…In sharp contrast, the presence of immature gonocytes and the absence of sperm in COI rabbits are consistent with a failure in spermatogenesis. It has been previously reported that TPH1 is expressed in mature sperm, at least in horses, rats, and humans [ 32 , 51 , 52 ]. On the other hand, the catabolic monoamine oxidase A (MAO A ) enzyme was found in the Leydig cells, spermatogonia, and spermatocytes of control rabbits and across all cell layers of seminiferous tubules but not in Leydig cells in COI rabbits.…”

“…On the other hand, the catabolic monoamine oxidase A (MAO A ) enzyme was found in the Leydig cells, spermatogonia, and spermatocytes of control rabbits and across all cell layers of seminiferous tubules but not in Leydig cells in COI rabbits. MAO A enzyme degrades 5-HT, dopamine, norepinephrine, and tyramine through oxidative deamination, and its activity has been related to the modulation of 5-HT concentration when the anabolic TPH enzyme is present too [ 30 , 51 , 53 ]. If Leydig cells from COI rabbits lack this enzyme, it is probably that an imbalance in the concentration of this indoleamine could affect steroidogenesis, but this condition could be “rescued” by another serotonergic element, such as 5HT T (see below).…”

“…Finally, we quantified 5-HT concentration levels through ELISA assays, and we found that they were kept relatively high when related to the decreased levels of TPH1 enzyme present in COI testes. This could be attributable to the “peripheral” affluence of 5-HT from several sources like the bloodstream, mast cells located in the testicular capsule [ 66 , 67 ], nerve fibers that run through the interstitial zone [ 51 ], and importantly, the presence of VMAT1 in the Leydig cells of COI rabbits. Although 5-HT has been linked to testicular pathologies such as varicocele [ 37 ], which induces an increase in reactive oxygen species (ROS), it seems unlikely that this indoleamine could be responsible for the main morphological changes that are present in COI testes; instead, the disruption of the local serotonergic system could be affecting late phases of spermatogenesis both directly and indirectly [ 31 , 33 ].…”

Hindlimb unloading induces time-dependent disruption of testicular histology in mice

“…Raman spectra indicated a robust decline in the concentration of tryptophan and hydroxyproline in a time-dependent manner. Tryptophan is implicated in testosterone production and spermatogenesis in testes 27 . Thus, a reduced expression of tryptophan is consistent with histopathological disruption, including the reduced density of spermatozoa in the HU testes.…”