Histamine (HA) is a pleiotropic biogenic amine synthesized exclusively by histidine decarboxylase (HDC) in most mammalian tissues. The literature on the role of HA within the male gonad has expanded over the last years, attracting attention to potential unexpected side-effects of anti-histamines on testicular function. In this regard, HA receptors (HRH1, HRH2 and HRH4) have been described in Leydig cells of different species, including human. Via these receptors, HA has been reported to trigger positive or negative interactions with the LH/hCG signaling pathway depending upon its concentration, thereby contributing to the local control of testicular androgen levels. It should then be considered that anti-histamines may affect testicular homeostasis by increasing or decreasing steroid production. Additionally, HRH1 and HRH2 receptors are present in peritubular and germ cells, and HRH2 antagonists have been found to negatively affect peritubular cells and reduce sperm viability. The potential negative impact of anti-histamines on male reproduction becomes even more dramatic if we consider that HA has also been associated with human sexual behavior and penile erection. What is more, although testicular mast cells are the major source of locally produced HA, recent studies have described HDC expression in macrophages, Leydig cells and germ cells, revealing the existence of multiple sources of HA within the testis. Undoubtedly, the more we learn about the testicular histaminergic system, the more opportunities there will be for rational design of drugs aimed at treating HA-related pathologies, with minimum or nule negative impact on fertility.
Undeniably, histamine (HA) is a biogenic amine with great significance in medicine and biology. It is produced by α-decarboxylation of l-histidine, and this reaction is catalyzed exclusively by histidine decarboxylase (HDC) in most mammalian tissues (Ohtsu 2010). Since its discovery in 1910, HA has been shown to mediate a plethora of physiological and pathological actions via its four G protein-coupled receptors, named H1, H2, H3 and H4 (histamine receptor H1 (HRH1)–HRH4), which are differentially expressed in numerous cell types (Akdis & Simons 2006, Parsons & Ganellin 2006). In particular, the literature on the relation between HA and male reproduction has greatly expanded over the past years, resulting in the identification of novel functions for HA within the male reproductive system of several species, including human. This review summarizes what is presently known regarding the effects of HA on the different cell types of the testis, with special focus on Leydig cells. Potential local sources of HA other than testicular mast cells are also described, as well as adverse effects of anti-histamines on normal testis function. The rising prevalence of allergic diseases in the industrialized world underscores the significance of this topic.
Histamine and Leydig cell steroidogenesis
Evidence accumulated so far indicates the presence of functional HRH1 and HRH2 in Leydig cells of mouse, rat, human and the ectothermic vertebrate Hemidactylus flaviviridis (Mayerhofer et al. 1989, Albrecht et al. 2005, Mondillo et al. 2005, 2007, 2009, Khan & Rai 2007). In line with these findings, HA has been reported to exert a biphasic effect on steroidogenesis in the MA-10 mouse Leydig tumor cell line and in rat Leydig cells in primary culture, depending upon its concentration: while nanomolar HA stimulates basal steroid production and enhances the steroidogenic response to luteinizing homone (LH)/human chorionic gonadotropin (hCG), micromolar concentrations display a potent anti-steroidogenic effect (Mondillo et al. 2005). Accordingly, in the wall lizard H. flaviviridis, although HA does not affect testosterone production from unstimulated Leydig cells, it has dual concentration-related effects on follicle-stimulating hormone (FSH)-induced testosterone production: stimulatory at a low concentration of 10−10 M while inhibitory at a high concentration of 10−5 M (Khan & Rai 2007). Based on these and subsequent publications (Mondillo et al. 2005, 2007, 2009, Khan & Rai 2007), HA-induced stimulation of steroidogenesis would primarily be mediated via HRH2 activation, which signals by Gs protein coupling and increased cAMP production (Fig. 1), whereas reduction of steroid levels by micromolar HA would involve HRH1 activation, Gq protein coupling, stimulation of the phospholipase C (PLC)/inositol 1,4,5-triphosphate (IP3) pathway and increased nitric oxide synthase (NOS) activity (Fig. 2) (Mondillo et al. 2009). Very recently, HRH4 has been identified in mouse and rat Leydig cells, in agreement with a previous report by O’Reilly et al. (2002), which indicated HRH4 expression in the human testis (Abiuso et al. 2014). Considering that activation of HRH4 on Leydig cells with specific agonists leads to a decreased steroidogenic response to LH/hCG, it is tempting to speculate that HRH4 may also participate in the aforesaid inhibitory effect of HA on steroid synthesis (Fig. 2). Indeed, synergistic effects between HRH1 and HRH4 have been described in other cell types, in which HRH1 colocalizes with HRH4 (Thurmond et al. 2008, Deiteren et al. 2014). Having opposing effects on steroidogenesis by triggering positive or negative interactions with the LH/hCG signaling pathway, HA can control androgen levels, thereby contributing to testicular homeostasis. This new role for HA in the physiological regulation of Leydig cell function is further supported by studies showing that, in HA-deprived HDC-knockout (KO) mice, the steroidogenic efficiency of Leydig cells is significantly lower in comparison with wild-type (WT) mice (Mondillo et al. 2005). In this regard, it has been shown that HDC KO Leydig cells do not respond to LH/hCG as effectively as WT Leydig cells (Mondillo et al. 2007) and that LH/hCG stimulus induces Hrh1 and Hrh2 expression in WT Leydig cells, while this does not occur in KO mice (Mondillo et al. 2005).
Histamine and Leydig cell proliferation
Pap and coworkers (2002) have reported serious alterations in Leydig cell ultrastructure in adult HDC KO mice, and a significant reduction in testicular weight in litters as early as 7 days of age (Pap et al. 2002). Notably, the distribution and morphology of gonadotropin-releasing hormone (GnRH) neurons is normal in these mice, as well as hypothalamic Gnrh1 mRNA expression (Pap et al. 2002). Hence, even when the major role of LH as a modulator of Leydig cell morphology and function is undisputable (Lejeune et al. 1996), the lack of endogenous HA production in HDC KO mice may presumably result in changes during the course of embryonic testicular ontogenesis, leading to deficient adult Leydig cell function. This theory is reinforced by former scientific evidence showing that testicular HDC expression and HA concentration are significantly elevated in the neonatal testis compared to the adult gonad (Zieher et al. 1971, Pagotto et al. 2012), implying that important HA-dependent events may occur during the development of the testis. Consistently, Gaytan et al. (1992) have observed that proliferation and differentiation of mast cells and Leydig cells happen at the same time in the rat testis, suggesting the existence of a dynamic relationship between the two cell types via their secretory products (Gaytan et al. 1992). It is tempting to speculate, in view of the aforementioned observations in HDC KO mice and the fact that HA is a well-known regulator of cell proliferation (Falus et al. 2010), that HA could be one of the mast cell-derived local factors influencing Leydig cell number in the neonatal testis. This would not be the case in the adult gonad. In this regard, HA fails to affect FSH-stimulated adult Leydig cell proliferation in the wall lizard (H. flaviviridis) (Khan & Rai 2007). Also, it has been demonstrated that immature and progenitor Leydig cells, isolated from 21- and 35-day-old rats, respectively, do not exhibit a proliferative response upon stimulation with HA, whereas they proliferate when stimulated with insulin-like growth factor type 1 (IGF1) (Pagotto et al. 2012). Interestingly, HA can stimulate the proliferation of MA-10 Leydig tumor cells via activation of HRH2, transient elevation of cAMP production and increased extracellular signal–regulated kinase (ERK) phosphorylation (Pagotto et al. 2012). Indeed, MA-10 cells show significantly heightened HDC expression compared to normal immature Leydig cells or whole testicular lysate, and inhibition of HDC activity decreases MA-10 cell proliferation. These results suggest that autocrine overproduction of HA might be somewhat linked to abnormally increased proliferation in Leydig cells, as occurs in numerous cell types (Adams et al. 1994, Ai et al. 2006, Falus et al. 2010). Thus, although the possibility that HA may influence Leydig cell numbers in the normal fetal and/or neonatal testis certainly deserves further research, a putative role for HA as an autocrine/paracrine modulator of Leydig cell proliferation in testicular pathological states should also be considered.
Testicular histamine targets other than Leydig cells
Aside from Leydig cells, germ cells, peritubular cells and macrophages of different species express HRH1 and/or HRH2 receptors and are therefore potential targets for locally produced HA (Albrecht et al. 2005, Khan & Rai 2007). In this respect, as observed in Leydig cells, in the wall lizard (H. flaviviridis), HA-regulated testicular macrophage immune responses in a dual concentration-dependent manner (Khan & Rai 2007). It inhibited phagocytosis and superoxide production at a high concentration (10−5 M), while stimulated superoxide production and could not affect phagocytosis at a low concentration (10−10 M) (Khan & Rai 2007). With regard to peritubular cells, it was not until very recently that they were found to be regulated by mast cell and macrophage products, and in response, produce factors that can fuel inflammatory changes. Also, recent studies have revealed that peritubular cells transport immotile sperm – a function important for male fertility, produce extracellular matrix components and contribute to the spermatogonial stem cell niche via secreted factors (Mayerhofer 2013). The fact that these cells bare HRH1 receptors opens the question whether HA may directly influence the aforementioned functions. This possibility has not been explored yet. No studies have been done on isolated Sertoli cells or endothelial cells, but it is tempting to speculate, in view of indirect evidence in cells derived from the testicular germinal region and the fact that vascular cells are known to respond to HA, that such cells are also targets of HA. With regard to the latter point, the action of HA on most of the endothelial cells and tissue barriers has been recently demonstrated (Adderley et al. 2015, Ashina et al. 2015). Additionally, a very old publication has documented blood–testis barrier impairment by administration of the HA releaser 48/80 to guinea pigs (Nemetallah et al. 1985).
Multiple sources of histamine within the testis
Although testicular mast cells are the major source of local HA, recent studies have revealed the existence of multiple potential sources of HA within the testis. In this regard, HDC expression has been detected in macrophages, Leydig cells (as indicated in a previous section of this manuscript) and germ cells (Safina et al. 2002, Albrecht et al. 2005, Pagotto et al. 2012). Interestingly, a similar scenario has been described in the female uterus and mammary gland. In both organs, mast cell-derived HA and epithelial cell-derived HA regulate physiological functions (Paria et al. 1998, Wagner et al. 2003). Whether non-mast cell-related HA has a crucial role in testicular physiology is yet unclear. However, the presence of multiple HA-synthesizing cells in the different compartments of the testis highlights the significance of local HA production and should definitely arouse interest in the potential deleterious effects of long-term anti-histamine therapy on both the endocrine and reproductive functions of the male gonad. Figure 3 and Table 1 show the different cellular targets of HA within the testis, as well as potential non-mast cell HA sources.
Potential sources of HA and HA targets within the testis.
|Cell type||Potential HA source||Targets for locally produced HA|
|Leydig cell (LC)||+||+||+||+|
|LC-derived MA-10 cell line||+||+||+||+|
ND, not determined.
Negative impact of anti-histamines on male reproduction
Considering the diversity of syndromes associated with HA, in recent years, a wide variety of anti-histamine compounds have been introduced in the clinics for the treatment of these diseases, particularly HRH1 and HRH2 antagonists such as loratadine, desloratadine, cimetidine, ranitidine, nizatidine and famotidine, among others. Moreover, the discovery of HRH4 revitalized the interest in HA receptor pharmacology, leading to a considerable increase in HRH4-related publications and patents (Leurs et al. 2009, Schreeb et al. 2013). Clinicians and patients should be aware, though, that the overuse of anti-histamines could have long-term side effects on the various target organs of HA, at best only reversible. More particularly, concerning the male gonad, the new role for HA as a modulator of Leydig cell function clearly suggests that anti-histamine drugs may affect testicular homeostasis by enhancing or decreasing androgen production (Mondillo et al. 2005, 2007, 2009, Khan & Rai 2007). In this respect, it has been reported that ingesting around 1200 mg/day cimetidine can decrease testosterone levels (Elliott 1979, Fuentes & Dolinsky 1979, Babb 1980). Additionally, cimetidine was found to lower the sperm count in humans, especially at doses greater than 1000 mg/day (Elliott 1979, Fuentes & Dolinsky 1979, Babb 1980, Wang et al. 1982, Buchanan & Davis 1984, Van Thiel et al. 1987), while it exerted moderate effects on sperm morphology and motility (Wang et al. 1982, Thomas & Turner 1983, Buchanan & Davis 1984, Bianchi Porro et al. 1985). The deleterious effect of cimetidine on the number of sperm cells was later confirmed by Aprioku et al. (2014) in Wistar rats. Additionally, a previous study carried out by Sinha et al. (2006) in albino rats demonstrated a reduction in sperm count as well as a decrease in sperm morphology and motility after a 15-day cimetidine treatment. Also, Nayeri and Kazerouni (2002) reported that the seminal vesicles of male rats treated with this drug at 100 mg/kg for 5 weeks had lower weights compared with the seminal vesicles of the untreated group. A putative explanation for the deleterious impact of cimetidine on sperm quality is that it increases intrasperm Ca2+ levels, and Ca2+ elevation has also been shown to reduce sperm viability (Gupta et al. 2004). Moreover, cimetidine has been found to decrease the height of the germinal epithelium and the diameter of seminiferous tubules in the testes of mice when applied at 10–100 mg/day for 15 days (Gill et al. 1991). Consistent results were observed in male albino Wistar rats, which were injected cimetidine intraperitoneally at a dose of 50 mg/kg for 52 days (Sasso-Cerri & Miraglia 2002). A subsequent study conducted by Sasso-Cerri & Cerri (2008) indicated that cimetidine may lead to Sertoli cell detachment and apoptosis, hence negatively affecting sperm quality. In 2009, Sasso-Cerri reinforced his previous results by demonstrating that cimetidine enhances oestrogen receptor beta expression and apoptosis in germ cells of adult male rats (Sasso-Cerri 2009). To date, the effect of ranitidine on semen quality is a matter of debate (Banihani 2016). Famotidine is the only HRH2 antagonist that has been reported to benefit sperm physiology; nonetheless, non-clinical experiments indicate that its use may have negative effects on some sperm parameters (Banihani 2016). Further clinical studies will be of great significance to more precisely evaluate the effects of famotidine on semen quality. With regard to desloratadine and loratadine Kuzminov et al. (2014) have reported a cytotoxic action in bull spermatozoa. The potential negative impact of anti-histamines on male reproduction becomes even more significant if we consider that HA has been implicated in penile erection and sexual behavior as well as steroidogenesis and spermatogenesis (Cara et al. 1995, Par et al. 2003, Sinha et al. 2006). In this regard, it has been reported that HA can exert a dose-dependent relaxation of isolated human corpus cavernosum smooth muscle, which is inhibited by cimetidine and potentiated in the presence of the HA HRH1 receptor antagonist mepyramine (Penttilae & Vartiainen 1964, Cará et al. 1995, Teixeira et al. 1998). Aside from the erectile function, some case studies have also suggested an impact of HA on the ejaculatory response of the adult man (Raja 1999, Holtmann et al. 2003, Labbate 2008). In apparent contradiction, a more recent report indicates a role for HA in the physiological mechanism modifying human sexual desire following orgasm/ejaculation, rather than an involvement in the maintenance of penile flaccidity or rigidity or the termination of erection (Ükert et al. 2011). These results then reinforce the idea that HA would be a mediator in a neuroendocrine feedback system modulating male sexual function.
State of the art
An issue of considerable scientific and clinical interest, which certainly deserves further research, is the potential role of HA and/or HA-receptor transport via extracellular vesicles (EV) secreted by testicular cells. Intercellular communication via EV has drawn much attention recently, as EV have been shown to carry a number of bioactive molecules, surface receptors and genetic information that modulate the activities of recipient cells both in normal physiology and pathological conditions (Pitt et al. 2016, Simon et al. 2018). Most, if not all, cell types release EV, which then enter the body fluids (Robbins & Morelli 2014, Robbins et al. 2016). Within the male reproductive system, this modality of cell–cell crosstalk has solely been described in the epididymis. In this regard, it has been extensively demonstrated that small membranous vesicles named epididymosomes, which are secreted along the epididymal intraluminal compartment, transfer fertility-modulating proteins to the sperm surface (Martin-DeLeon et al. 2015, Sullivan 2016, Simon et al. 2018). Recently, mast cells have been shown to be rich sources of secreted EV (Shefler et al. 2011). These mast cell-derived EV could then influence the development of the male gonad during embryogenesis or its function later in adulthood. Also, histaminergic effects could be provided constantly by EV secreted by other testicular cells expressing HA receptors. Clearly, this hypothesis deserves further research.
The increasing list of processes relevant to human (patho) physiology in which HA has a preponderant role has prompted researchers in the discipline to strive for the complete elucidation of its functions and the properties of its receptors. The data compiled in this review collectively indicate the crucial involvement of HA in the orchestration of testicular functions during development and adulthood. Even so, there is still much to learn regarding the complex interplay between HA-synthesizing cells and targets for HA within the male reproductive system. In view of the evidence reviewed herein, the evaluation of potential HA-drug-induced effects on reproductive and sexual health should definitely be made part of clinical studies, in order to develop novel drugs aimed at treating HA-related pathologies with minimum or nule negative impact on fertility.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this review.
Studies mentioned in this review were supported by grants from Agencia Nacional de Promoción Científica y Tecnológica (ANPCYT, PICT 2012 N° 2776 and PICT 2015 N° 2650), Alberto J Roemmers Foundation, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET, PIP 392) and University of Buenos Aires (UBA, UBACYT 2013-20020120100205).
The authors are very grateful to Dr Omar Pedro Pignataro, Head of the Laboratory of Molecular Endocrinology and Signal Transduction. The authors would like to thank former lab members, namely Drs Zoraida Patrignani, Cecilia Reche, Mariela Marazita, Bárbara Piotrkowski, Romina Pagotto and Casandra Monzón, as well as current members, especially Marcos Besio Moreno, whose valuable contributions are cited in this review.
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