Neuroendocrine mechanism of selective serotonin reuptake inhibitor-induced reproductive failure in male zebrafish

Abstract

Selective serotonin reuptake inhibitors (SSRIs) are widely used antidepressants in the treatment of depression. However, it is well known that SSRIs can cause sexual side effects, which is a common reason for non-adherence to and discontinuation of pharmacotherapy, which may lead to relapse of depressive symptoms. Serotonin (5-HT) is a neurotransmitter involved in the regulation of a wide variety of functions in the brain, including memory, mood and reproduction. Vertebrate reproduction is regulated by two major neuro peptides, gonadotropin-releasing hormone (GnRH) and kisspeptin. However, the role of 5-HT in the regulation of neuroendocrine systems and their involvement in SSRI-induced sexual dysfunction is unknown. Using sexually mature male zebrafish, the study aims to understand the functional role of 5-HT in the control of reproduction. The thesis includes chapter 1 (literature review), chapter 2, chapter 3 and chapter 4 (three experimental chapters) and chapter 5 (general discussion). Chapter 2: In the present study, we examined the functional association between 5-HT-GnRH (GnRH2 and salmon GnRH3) and 5-HT-kisspeptin (Kiss2) systems in the zebrafish. Confocal laser microscopy followed by 3D-construction analysis showed close associations between 5-HT fibers with GnRH3 and Kiss2 fibers, but very few 5-HT fibers were seen in close association with GnRH3 or Kiss2 cell soma suggesting the potential role of 5-HT in GnRH3 and Kiss2. Chapter 3: In the present study, we examined the effect of citalopram on GnRH and kisspeptin neuronal systems and identification of mechanism of action of citalopram on GnRH-kisspeptin system. Short term (two weeks) citalopram-treatment at (low, medium and high doses), significantly decreased the density of 5-HT, GnRH3 and Kiss2 fibers as well as the numbers of close associations between 5-HT with GnRH3 or Kiss2 fibers in the brain when compared to controls. Short term treatment with citalopram at low and medium doses, decreased the expression of gnrh3, kiss2 and 5-HT related genes (tph2 and sert) in the brain suggesting that 5-HT plays an inhibitory role on GnRH3 and Kiss2 synthesis. While treatment with citalopram at high dose increased gnrh2, gnrh3 and lhβ mRNA expression levels and decreased fshβ mRNA expression could be due to the androgen actions. However, there was no effect of short term citalopram treatment on sperm production at any doses suggesting that short term citalopram treatment had no effect on spermatogenesis. Long term (one month) citalopram treatment at medium and high doses significantly decreased gnrh3, kiss2, lhβ and fshβ mRNA levels and inhibition of spermatogenesis suggests that the suppression at the brain, pituitary mRNA levels and sperm production could be mediated through the 5-HT system. Chapter 4: Administration of GnRH3 and kisspeptin effect on citalopram induced reproductive failure. In the present study, I examined the effect of intraperitoneal injection of GnRH3 and Kiss2 in citalopram-treated fish. Furthermore, kiss2, gpr54, gnrh3, lhβ and fshβ mRNA levels were decreased after Kiss2 and GnRH3 could be due to suppression of the HPG functions due to chronic administration of GnRH3 and Kiss2 administration. However, Kiss2 injection increased sperm density area about (40%) in comparison to GnRH3 injected or control fish suggesting the potential potency of Kiss2 on SSRI induced reproductive failure. Chapter 5: In conclusion citalopram affects the reproductive neuroendocrine neurons such as Kiss2 and GnRH3 through the 5-HT system. In addition, the results suggest that Kiss2 could be utilized as a possible treatment for SSRI induced reproductive failure.