Effects of psychological stress on male fertility.

REVIEWS Effects of psychological stress on male fertility Vinod H. Nargund Abstract | Psychological stress can be defined as any uncomfortable ‘emotional experience’ accompanied by predictable biochemical, physiological and behavioural changes or responses. Many clinical studies looking at the effects of psychological stress on male fertility have shown that stress is associated with reduced paternity and abnormal semen parameters. Enough scientific evidence exists to suggest that psychological stress could severely affect spermatogenesis, mainly as a result of varying testosterone secretion. The hypothalamic–pituitary–adrenal axis has a direct inhibitory action on the hypothalamic–pituitary–gonadal (HPG) axis and Leydig cells in the testes. The newly discovered hormone, gonadotropin-inhibitory hormone (GnIH), also has an inhibitory effect on the HPG axis. Inhibition of the HPG axis results in a fall in testosterone levels, which causes changes in Sertoli cells and the blood–testis barrier, leading to the arrest of spermatogenesis. Germ cells also become vulnerable to gonadotoxins and oxidation. However, the extent and severity of the effects of psychological stress on human testes is difficult to study and data mostly come from animal models. Despite this limitation, stress as a causative factor in male infertility cannot be ignored and patients should be made aware of its effects on testicular function and fertility and helped to manage them. Nargund, V. H. Nat. Rev. Urol. advance online publication 9 June 2015; doi:10.1038/nrurol.2015.112

Introduction

Department of Urology, Homerton University Hospital, Homerton Row, London E9 6SR, UK. [email protected]

Stress is defined as a real or perceived threat from internal or external adverse events (or stressors) to the homeostasis or well-being of an organism.1,2 The organism responds by changing its behaviour and physiological processes in an attempt to re-establish homeostasis.2 Psychological stress in humans can be defined as any uncomfortable ‘emotional experience’ accompanied by predictable biochemical, physiological and behavioural changes or responses.3 In physiological terms, psychological stress is a chain of events leading to the disruption of homoeostasis as a result of the direct effect of stress on the mind.4 Stress is a byword in modern life, which is a mishmash of personal ambitions, hassles, deadlines, demands and frustrations. In small doses, stress can be a big motivator and at times can make people perform at their best. The stress response also helps to maintain health, mood, productivity, relationships and quality of life. Short-term stress has a typical ‘flightor-fight’ response and all physiological changes revert to prestress levels once the stress is over.5 Appropriate responsiveness of the stress system to stressors is a crucial pre requisite for a sense of well-being, adequate performance of tasks and positive social interactions. A short period of stress, however, could become chronic as a result of negative socioeconomic factors such as job insecurity, high pressure at work, redundancy, isolation, loneliness, debts, financial problems or bereavement. All of these factors could cause feelings of distress and/or frustration and consequently trigger a prolonged stress response mechanism. The triggering factor for stress, if it is prolonged or repetitive, makes the stress response a Competing interests The author declares no competing interests.

pathological one, leading to adjustments in homeostasis, which include pathological effects on metabolism, vascular function, tissue repair, immune function and the nervous system.6 Thus, psychological stress and emotional responses to it can profoundly influence the body’s ability to remain healthy and resist or overcome disease or a pathological process.7 Psychological stress has been perceived clinically as a potential risk factor affecting male fertility. However, its potential effect on spermatogenesis and testicular function has received little attention compared with the role of testicular oxidative-stress mechanisms. Spermatogenesis is affected by genetic, testicular and extratesticular factors. The basic testicular prerequisites necessary for spermatogenesis are endogenous production of testosterone and oestrogens (secreted by Leydig cells) a normal functioning germinal epithelium and an adequate number of Sertoli cells. In men, oestrogens are produced by aromatization of testosterone in Leydig cells, the testicular interstitium and adipose tissue. Oestrogens modulate pituitary gonadotropin secretion in response to gonadotropin-releasing hormone (GnRH),8 and also have a regulatory role in spermatogenesis.9 However, the mechanism is complex, as other neuroendocrine mechanisms are involved in influencing testosterone secretion and spermatogenesis. Psychological stress could affect these factors directly or indirectly at various levels. The relationship between male infertility and psycho logical stress is controversial as reports are conflict ing. Some clinical studies clearly demonstrate an inverse relationship between psychological stress and semen parameters whereas others do not show such a relationship.

NATURE REVIEWS | UROLOGY

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REVIEWS Key points ■■ Psychological stress has been perceived clinically as a potential risk factor for male infertility, although to what extent it affects human male fertility is difficult to study and evaluate ■■ Clinical studies demonstrate an inverse relationship between psychological stress and semen parameters ■■ The paraventricular nucleus (PVN) in the hypothalamus regulates stress responses and activates the sympathetic–adrenal system (SAS), and the hypothalamic–pituitary–gonadal (HPG) and hypothalamic–pituitary–adrenal (HPA) axes ■■ Activation of HPG and HPA axes leads to a fall in testosterone levels in the testes, affecting Sertoli cells and the blood—testis barrier

Stressors

Stress

SAS

HPA

Decreased prolactin

HPG

Reduced testosterone

GnIH

Acute stress

Chronic stress

Increased growth hormone

Decreased growth hormone

Arrest of spermatogenesis

Figure 1 | The mechanisms of the stress response. Stressors can activate three Nature Reviews | Urology different cascades: the HPA axis, the SAS and the HPG axis. Stress can also increase levels of GnIH, which has an inhibitory effect on the HPG axis. These signalling cascades have a direct effect on testosterone secretion on spermatogenesis. Abbreviations: GnIH gonadotropin-inhibiting hormone; HPA, hypothalamic–pituitary–adrenal; HPG, hypothalamic–pituitary–gonadal; SAS, sympathetic–adrenal system.

Analyses of the semen of medical students during examination periods10,11 and of men who have experienced times of war,12—14 periods of stress at work15 and a recent bereavement 16 have shown reduced sperm concentration and alterations in other sperm parameters. However, other studies contradict these investigations.10–16 According to Hjolland and colleagues,17 the effect of a man’s everyday psychological stress on semen quality is small or nonexistent. They observed, for the first time, that there was no association between job strain and any semen characteristic or sexual hormones in 399 Danish couples trying to conceive. The treatment of infertility itself can become stressful for men. A couple failing to achieve pregnancy might experience feelings of frustration, disappointment and emotional upheaval,18 owing to prolonged efforts in terms of taking medication, treatment cost and treatment time. Fertility problems in men can also lead to stress-related conditions such as erectile dysfunction and premature and retrograde ejaculation, which can indirectly affect fertility status. This Review briefly describes the physiology of acute and chronic stress mechanisms and assesses our current understanding of the effects of stress in general.

The neuroendocrine mechanisms related to psychological stress, with particular reference to its effects on the male reproductive system and testicular function, are discussed in detail.

Central stress mechanism

The key to understanding the various endocrine responses to stress is the limbic–hypothalamic–pituitary– adrenal axis, which is essential in coordinating both rapid and long-term behavioural, physiological and molecular responses to psychological stressors. 19,20 Higher cognitive centres, which have an important role in memory, anxiety and decision-making processes, are the targets of stress-related hormones such as adrenaline and noradrenaline.21 A general consensus exists that the main centre that regulates responses to stress is the paraventricular nucleus (PVN) of the hypothalamus.22 The PVN has extensive connections in the hypothalamus and the brainstem and is recognized for its role in autonomic control, with its neurons having vital roles in metabolism, growth, reproduction, immune and other autonomic functions (such as gastrointestinal, renal and cardiovascular functions).23 The PVN is composed of two types of neurons, the first type are the magnocellular neurosecretory cells, whose axons enter the posterior pituitary. These cells synthesize and release arginine vasopressin (AVP) and oxytocin. The second type are the smaller parvocellular neurons, which secrete corticotropin releasing hormone (CRH) and AVP and project into different sites, including the brainstem, the median eminence and the arcuate nucleus of the hypothalamus. CRH and the PVN together stimulate adrenocorticotropic hormone (ACTH) secretion from the anterior pituitary.24 The areas of the brain that organize and control the stress response include the prefrontal cortex, the amygdala, the hippocampus and the nucleus accumbens.24 Changes in extracellular concentrations of various neuro transmitters such as dopamine, acetylcholine, glutamate and γ‑aminobutyric acid (GABA) are responsible for the activation and modulation of behavioural processes for coping with the stressor effects and interactions in these brain areas.25

Peripheral stress mechanisms

For descriptive purposes and simplification, the cascade of peripheral responses to stress in relation to male reproductive function can be categorized into three different pathways: the sympathetic–adrenal system (SAS) pathway, the hypothalamic–pituitary–adrenal (HPA) axis pathway and the HPG axis pathway (Figure 1).26

The SAS and the HPA axis The sympathetic–adrenal system The SAS is one of the main components of the stressor system and includes a central locus coeruleus and noradrenaline system (and other catecholaminergic cell groups of the medulla and pons) and a peripheral system consisting of the sympathetic nervous system (SNS) and the adrenal medulla.27 Noradrenaline is the

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REVIEWS Stressors

PVN (hypothalamus)

Cortisol ? Autonomic norepinephrine centres ■ Locus coeruleus ■ Cerebral cortex ■ Hippocampus ■ Spinal cord

−

Prolactin Nerve growth factor

CRH and AVP

ACTH

Glucocorticoids

POMCs ■ α-melanocyte-stimulating hormones ■ β-endorphine ■ Lipotropins

− Leydig cells

Decreased spermatogenesis

BTB damage

Decrease in testosterone

Sertoli cells

Figure 2 | The HPA axis pathway. The HPA axis is activated by a variety of stressors which stimulate the PVN in the hypothalamus and the release of CRH and AVP . Nature Reviews | Urology These neuropeptides stimulate corticotrophs, leading to the release of ACTH. The adrenal cortex is the main target of ACTH, which in turn releases glucocorticoids, causing Leydig cell apoptosis and a decrease in testosterone levels, which has a knock-on effect on Sertoli cells and the BTB, decreasing spermatogenesis. CRH also triggers the autonomic norepinephrine centres in the brainstem in addition to releasing POMC peptides such as α-melanocytestimulating hormone, β-endorphin and lipotropins and the release of prolactin and nerve growth factor. Abbreviations: ACTH, adrenocorticotropic hormone; AVP, arginine vasopressin; BTB, blood–testis barrier; CRH, corticotropin-releasing hormone; HPA, hypothalamic–pituitary–adrenal; POMC, pro-opiomelanocortin; PVN, paraventricular nucleus.

main neurotransmitter of the SAS and is initially secreted by the locus coeruleus (the major contributor), and the tractus solitarius. Noradrenaline, along with adrenaline, has a major role in PVN activation.28 Reciprocal connections exist between CRH and noradrenaline neurons in the brain and they can also stimulate each other through CRH type I and α1-adrenergic receptors.29,30 The peripheral component of the SAS initiates a wide range of changes in systemic functions including cardiovascular, respiratory, gastrointestinal, renal and endocrine organs controlled by the SNS.31 Activation of the parasympathetic nervous system by stressors also occurs, indicating that the whole autonomic nervous system is activated.31 Although the major neurotransmitter of the SNS is noradrenaline, there are subpopulations of neurons that express other neuropeptides, such as adenosine triphosphate (ATP), nitric oxide (NO) and lipid mediators of inflammation. CRH, neuropeptide Y (NPY), growth-hormone-inhibitory hormone (GHIH) and galanin are also found in postganglionic noradrenergic neurons.31,32 NPY and galanin are mainly secreted by the hypothalamus. NPY is a powerful vasoconstrictor,33 it stimulates CRH secretion and suppresses the locus coeruleus and noradrenaline system activity,31 but the exact role of galanin is unclear. Acute stress In acute stress, the response to the stressor is executed by the ‘stress systems’ that are located both in the central

and peripheral nervous systems but are mainly coordinated and directed by the hypothalamus. The hypothalamus receives information from all parts of the brain and exerts its action through the SAS and the HPA axis (Figures 1 and 2).34 Activation of the HPA axis is associated with the release of CRH and AVP from the neurons in the PVN. These neuropeptides are secreted into the hypothalamic–hypophyseal portal system and reach the anterior pituitary, where they stimulate cortico trophs, leading to the release of ACTH. The adrenal cortex is the main target of ACTH, which in turn releases glucocorticoids and adrenal androgens; glucocorticoids from the adrenal cortex are the final effectors of the HPA axis and are also responsible for regulating the HPA axis and terminating the stress response (Figure 2).26 CRH also triggers the locus coeruleus and other autonomic noradrenaline centres in the brainstem in addition to releasing pro-opiomelanocortin (POMC) peptides such as α‑melanocyte-stimulating hormone, β‑endorphin27,31,35 and lipotropins.36 Release of prolactin and nerve growth factor also occurs.37 Chronic stress Chronic stress means that the stressor exerts its activity for an extended period of time and a marked reorganization of the HPA axis occurs. When animals are exposed to the same stressful stimulus for several weeks, adapt ive changes are observed in the SAS, including increased synthesis, storage and basal levels of catecholamines.38 A number of factors could influence this response, such as predictability of stressor, its intensity and duration, the interval between each episode of stress and number of presentations of the stressor.39 If chronically stressed animals are exposed to a novel stressful stimulus, an exaggerated SAS response (compared with the response in animals exposed to the same stimulus, for the first time) occurs.38 Initial response to the stressor in chronic stress is same as in acute stress but adaptive by virtue of interactive processes that include behaviour and the autonomic, endocrine and immune systems. Sustained and prolonged stressor activity can cause profound changes in physio logy, with long-term systemic implications.1 A consensus exists that all types of stressors activate the HPA axis, and that this activation leads to glucocorticoid activity. Unlike acute stress, which generates a predictable HPA axis response, chronic stress elicits a much wider variety of HPA axis responses that depend on the type of stressor and factors relating to the individual experiencing stress.40 HPA axis activity is, therefore, likely to fluctuate. Adaptation to chronic stress is mediated through behavioural and biological responses; the role of gluco corticoids in this process cannot be underestimated, owing to their widespread actions at various levels in the stressor system. In fact, the research linking chronic stress and HPA axis activity is contradictory, with some studies indicating that chronic stress is associated with increased HPA axis activity while others report that it is associated with reduced activity. 37 Most clinical work on



REVIEWS Hypothalamus

Kisspeptins and GPR54?

GnRH through the hypothalamic–hypophyseal portal system

GnRH binds to pituitary gonadotrophs FSH

LH

Sertoli cells

Leydig cells

Negative feedback

Negative feedback Inhibin B

Testosterone

Oestradiol

Nature Reviews | Urology Figure 3 | The GnRH pathway. GnRH binds to the gonadotrophs of the anterior pituitary to stimulate the secretion of LH and FSH. FSH acts on Sertoli cells to aid spermatogenesis while Leydig cells secrete testosterone in pulses, in response to LH stimulation. An increase in serum testosterone levels inhibits secretion of LH via negative feedback to the hypothalamus and pituitary gland. GnRH and pituitary gonadotropin regulation is modulated by oestradiol, which is derived from aromatization of testosterone. Negative feedback on FSH secretion occurs through inhibin B, produced by Sertoli cells, and oestradiol. Although Sertoli cells are affected by stress, the FSH loop seems to be less affected by the stress mechanism than the LH loop. Kisspeptins and their receptor G-protein-coupled receptor—GPR54—have an important role in the secretion of GnRH and negative feedback effects on testosterone and oestradiol. Kisspeptin signalling might have a role in the mammalian stress axis. Abbreviations: FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone.

chronic stress has been based on human subjects in stressful environments, such as soldiers in the midst of a battlef ield, refugees displaced by war, victims of sexual assaults, and carers.37,40 Other stressful conditions that have been looked into include unemployment, bereavement, divorce, redundancy, and domestic violence.37,40 Evidence exists of both elevated and reduced glucocorticoids in these conditions.37 The most common method of assessing stress-related HPA axis activity is by measuri ng salivary, blood and urinary glucocorticoid levels, specifically cortisol.41 Normal HPA axis activity has a diurnal variation and ACTH and cortisol are secreted in short pulsatile episodes; their levels are lowest from 2000 h to 0200 h then start rising rapidly through the late night and predawn hours, reaching maximum between 0800 h and 1000 h. Levels decline throughout the course of the day into the evening.42 The nature and intensity of the stress affects cortisol secretions, for example, Miller et al.37 observed that stress posing as a threat to the ‘social self ’ is associated with significantly elevated cortisol levels in the morning, afternoon and evening. A possible explanation for this increase in cortisol is the importance of social aspects of life for humans and the human need to be part of a social group.43 Notably, psychological stress studies related to male fertility have not made a distinction between acute and chronic forms of stress, and generally presume that psychological stress is chronic stress.

The HPG axis The HPG axis drives and orchestrates reproductive function in both sexes (Figure 3). The hypothalamus secretes the key hypothalamic hormone in the male reproductive system in relation to spermatogenesis, GnRH—a decapeptide secreted from the arcuate nucleus of the hypothalamus. GnRH stimulates the gonadotrophs of the anterior pituitary to secrete follicular stimulating hormone (FSH) and luteinizing hormone (LH), which regulate testicular function. The testicular steroids, testosterone and oestrogen, along with other testicular hormones, such as inhibin and activin, control GnRH and gonadotropin secretion.44 Other factors also influence and modulate the HPG axis and gonadotrophins. These factors include glucocortocoids, leptin and opioid peptides. Increased HPA axis activity has a direct inhibitory effect on GnRH secretion mediated through glucocorticoids.45 Opioid peptides are found in the central nervous system and peripheral tissues and fall into three categories—enkephalins, endorphins and dynorphins. 46 Opioid peptides are released in stress and help in heightening pain control. Opioid peptide systems aid the mediation, modulation and regulation of stress responses including the HPA axis, the autonomic nervous system and behavioural responses,47,48 they also inhibit the HPG axis. Long term usage of exogenous opioids (such as morphine) leads to hypogonadism and low serum testosterone concentrations.48 Leptin is a protein that is synthesized and secreted by adipocytes in white adipose tissue in a pulsatile fashion with increased levels in the evening and early morning hours.49 Leptin binds to specific receptors in the brain and peripheral tissues seems to have a role in relaying metabolic information to the reproductive axis.50 Circulating leptin levels decrease in fasting in humans, indicating that leptin levels can also reflect acute changes in metabolic status.51 Several animal models have demon strated the importance of leptin in the regulation of the HPG axis but the exact mechanism of this regulation is not known. That reduced leptin signalling leads to reduced GnRH neuronal activity is well known.52 This phenomenon could be caused by decreased hypothalamic KISS1 expression (a potent regulator of GnRH, LH and FSH release) as kisspeptin neurons express leptin receptors. Increased HPA axis activity stimulates leptin synthesis and leptin exerts negative feedback on CRH.53 GnRH is transported as storage granules by axonal transport to the hypothalamic median eminence and is then secreted episodically into the capillaries of the hypothalamic–hypophyseal portal system.54 The amino acid neurotransmitter GABA seems to regulate GnRH neurons by both depolarizing and hyperpolarizing them, but details of the mechanism need further elucidation.55 GnRH binds to the gonadotrophs of the anterior pituitary to stimulate the secretion of LH and FSH.56 FSH acts on Sertoli cells to aid spermatogenesis while LH stimulates Leydig cells to secrete testosterone. Most testosterone in men is secreted in pulses by Leydig cells in response to LH stimulation. An increase in serum testosterone levels inhibits secretion of LH via a negative



REVIEWS feedback effect on the hypothalamus and pituitary gland. GnRH and pituitary gonadotropin regulation is modulated by testosterone, dihydrotestosterone (DHT) and oestradiol acting on the hypothalamus and pituitary gonadotrophs.57,58 Negative testicular feedback on FSH secretion occurs through inhibin B produced by Sertoli cells. The major negative feedback on FSH comes from oestradiol, derived from aromatization of testosterone.59 Although Sertoli cells are affected by stress, which causes low levels of testosterone, the FSH loop seems to be less affected by the stress mechanism (Figure 3).

Testosterone in spermatogenesis

The pivotal role of testosterone in spermatogenesis has been known for decades; however, the molecular mechanisms by which testosterone supports spermatogenesis are only now being investigated. The presence of endogenous testosterone and its functional receptor (androgen receptor [AR]) in the testes are crucial for spermatogenesis and male fertility. The AR is present in Sertoli cells, peritubular cells and Leydig cells.60 The presence of ARs in human germ cells is debated as and has not yet been observed; however, some animal studies have identified a few scattered ARs in germ cells.61 The most important target cells for testosterone are Sertoli cells, which are involved in spermatogenesis. The biochemical relationship between Leydig cells and Sertoli cells through testosterone underpins the mechanism of spermatogenesis. Testosterone and ARs aid spermatogenesis at a number of levels. The absence of either or both testosterone and ARs leads to a defective blood– testis barrier (BTB) and means that germ cells do not progress beyond meiosis, and stress causes testosterone depletion.60,62 In these circumstances, immature germ cells are prematurely displaced from Sertoli cells, while mature germ cells cannot be released.62 A defective BTB also exposes postmeiotic cells to autoimmune attack and gonadotoxic agents such as alcohol, tobacco, illicit drugs, chemotherapeutic agents, antihypertensive agents, hormonal agents, psychotherapeutic drugs and antibiotics.63 The fact that low testosterone levels compromise Sertoli cells and the BTB also means that gonadotoxins can easily gain access into the seminiferous tubules, where sperm production takes place. At its origin, testosterone also has a local effect on the interstitium and seminiferous tubules indirectly, facilitating spermatogenesis and sperm maturation by virtue of its action on Sertoli cells.62 Intratesticular testosterone levels could be 25-fold–125-fold higher than levels in the serum.62 In men, some 20% of testosterone is converted to oestrogens by aromatase that is present in the testes, while 80% of conversion to oestrogen takes place in peripheral tissues, mainly in adipose tissue and striated muscle cells.64 The presence of oestrogen receptors and aromatase in various parts of the testes indicates their important role in spermatogenesis—especially in meiotic and postmeiotic stages—and, more generally, in male reproduction; however, the effect of stress or fluctuating testosterone levels on oestrogen and aromatase levels is not yet known.65

Most testosterone binds to sex-hormone-binding globulin (SHBG) and serum albumin. SHBG is a glyco protein that is synthesized in the liver and has a high binding affinity for testosterone and DHT but a lower affinity for oestradiol. 66 Plasma levels of SHBG are important in the regulation of free and albumin-bound androgens and oestrogens.66 Free testosterone and testosterone bound to albumin in the serum are readily available for biological actions. SHBG levels are inhibited by insulin secretion and are stimulated by cortisol secretion. Psychological stress is associated with elevated levels of cortisol, which in turn leads to increased SHBG levels and a consequent decrease in free testosterone levels in the plasma, 66 which could have a knock-on effect on spermatogenesis. Other androgens in circulation include DHT, androstenedione, dehydroepiandrosterone (DHEA) and DHEA sulphate (DHEAS). The conversion of testosterone to more potent DHT by α‑reductase isozymes type 1 and type 2 takes place in certain peripheral tissues such as the liver, the scalp, genital skin and the prostate.67 Both type 1 and type 2 α‑reductase isozymes are expressed in the liver; in addition, type 1 is expressed in nongenital skin, and testes and type 2 is expressed in the male urogenital tract and genital skin. The action of DHT is limited to its target tissues (including the prostate, the scalp and genital skin). Only a small amount of DHT is available in circulation and most of it is bound to SHBG—0.8% is free in the serum.64 The exact role of DHT in spermatogenesis is not known. Finasteride, an α‑reductase-inhibitor, which blocks the conversion of testosterone to DHT and is used for the treatment of alopecia and benign prostatic enlargement, has been found to cause reduced sperm motility.68 Sporadic case reports exist on recovery of sperm function following discontinuation of this drug. DHT has inhibitory effect on HPA axis activity and evidence exists to suggest that inhibition occurs in the PVN itself, as a high number of ARs and relevant steroid hormone metabolizing enzymes have been observed here.69 The main androgen, testosterone, is converted to oestrogen or DHT in brain cells by aromatase or 5α-reductase (5α‑R) enzymes, respectively.70 In animal studies, 5α-Rs seem to be important in the control of the stress-responsive neuroendocrine function by testosterone. Administration of finasteride prevents the inhibitory effects of testosterone on the HPA axis.70 This finding needs further clinical evaluation in relation to the role of 5α‑R activity in the brain controlling the activity of the HPA axis. Further clinical investigations to eluci date whether this pathway is one of the main control pathways or a mitigating mechanism (which acts in conjunction with other systems), and also its role in chronic stress, are needed. The adrenal cortical sex steroids—androstenedione, DHEA and DHEAS—are weaker androgens than testosterone, with much lower affinity for the AR. 70 DHEA and DHEAS are also synthesized in the brain, independent of their production in the adrenal cortex. Their exact function is unclear but they are involved in neuroprotection, catecholamine synthesis and have



REVIEWS antioxidant, anti-inflammatory and antiglucocorticoid effects. They also vary diurnally, similar to cortisol71,72 and are also metabolized to more potent testosterone by 17β-hydroxysteroid dehydrogenase.64 Small amounts of DHEA and DHEAS are produced in the testes and serve as precursors to androgens and oestradiol.73 Although acute psychological stress increases the levels of DHEA and DHEAS temporarily, long-term stress is associated with a reduced capacity to produce DHEAS and to lower basal DHEAS levels,74,75 which could indirectly affect testosterone levels; therefore, the role of DHEA and DHEAS in spermatogenesis warrants further investigation. Kisspeptins are a family of neuropeptides encoded by KISS1 and are mainly produced by neuronal clusters of discrete hypothalamic nuclei.76 Kisspeptins and their G‑protein-coupled receptor—GPR54—have an important role in the production and secretion of GnRH and in the negative feedback effects of testosterone and oestradiol on the hypothalamus (Figure 3).76 Kisspeptin signalling might have a role in the mammalian stress axis as there seems to be an interaction between the HPG and stress axes where kisspeptins are involved. Populations of kisspeptin-expressing neurons have been identified in the arcuate nucleus, the hypothalamic preoptic area and the PVN. 77 Kisspeptin fibres are abundant in other areas where perikarya of corticotropin-releasing factor, AVP, and oxytocin secretory neurons are located. 78 Additionally, kisspeptin neurons project into the limbic structures and amygdala, which innervate the PVN through GABAergic inputs. 79 This intricate connection indicates a very intimate ‘cross-talk’ between main the HPA and HPG axes. Kisspeptins might be involved in a gatekeeping mechanism but their exact role is not clear. A study of human volunteers has shown that plasma LH, FSH and testosterone levels increase following acute intravenous administration of kisspeptin, suggesting an important role for the kisspeptin–GPR54 system in stimulating reproductive hormones by stimulating testicular function and spermatogenesis.80 Experimental evidence indicates that increased levels of glucocorticoids suppress testosterone levels and KISS1 expression;81 however, these observations need further corroboration in humans.

The HPA axis and male fertility

Stimulation of the HPA axis is associated with inhibition of the HPG axis by glucocorticoids via the inhibition of GnRH at the hypothalamic level.82 The main regulators of the HPA axis include CRH, glucocorticoids, AVP and ACTH. The ‘stress system’ seems to have many negative effects on male reproductive function. The HPA axis might also have a direct effect on the testes, as CRH receptors have been identified in male reproductive tissues.24

The effects of glucocorticoids Glucocorticoids affect testicular function at multiple levels of the HPG axis through their receptors in hypothalamic neurons, the pituitary gland and the testes.

At the hypothalamic level, glucocorticoid receptors contribute to glucocorticoid-mediated downregulation of the HPG axis.83 The downregulation of GnRH by glucocorticoids leads to impairment of pulsatile release of LH and FSH from the pituitary gland.84 At the pituitary level, glucocorticoids regulate the GnRH receptor gene (GNRHR),60 thereby modulating the effect of GnRH on the release of gonadotropins. Glucocorticoids also reduce the testicular response to LH and concentrations of LH receptors in both animals and humans, which leads to reduced testosterone secretion.61,84 In the testes, glucocorticoid receptors are localized to various cells, including Leydig cells, macrophages, fibroblasts, smooth muscle cells and endothelial cells of blood vessels.61 They are also present in zygotene and early pachytene primary spermatocytes during stages I–III and XIII–XIV of the spermatogenic cycle. In addition to their presence in Leydig cells, glucocorticoid receptors have been observed in in other male reproductive accessory tissues, such as the epididymis, the vas deferens, and the prostate,85 although their functional role in these organs is not known. Experimental and clinical evidence indicates that glucocorticoids directly inhibit testosterone production from Leydig cells and that this inhibition is probably caused by direct activation of glucocorticoid receptors in Leydig cells 61 through genomic and nongenomic mechanisms. 84 Clinically, signs of testosterone suppression have been observed in men with Cushing syndrome, a clinical condition associated with excessive free cortisol secretion.86 Experimental studies suggest that the Leydig cell response to glucocorticoids is caused by direct inhibition of the transcription of genes that encode testosterone biosynthetic enzymes, such as cytochrome P450-dependent cholesterol side chain cleavage enzyme and cytochrome P450-dependent 17α-hydroxylase/C17– C20 lyase.61,87 Moreover,excessive glucocorticoid exposure not only inhibits androgen synthesis but might also decrease serum testosterone levels by inducing apoptosis of Leydig cells, thereby reducing the numbers of Leydig cells in the testes, which could have knock-on effect on Sertoli cells and spermatogenesis.61

Gonadotropin-inhibiting hormone Gonadotropin-inhibiting hormone (GnIH)—a relatively new discovery in terms of the hormonal control of reproductive function—is a decapeptide belonging to the family of RFamide peptides and was first isolated from the quail hypothalamus.88 Mammalian GnIH is also known as RFamide-related peptide (RFRP).89 Two forms of GnIH (RFRP‑1 and RFRP‑3) have been identified in the human hypothalamus and are able to regulate the HPG axis in men.90 Hinuma et al.91 identified a specific receptor for mammalian GnIH that was identical to the previously-described receptor, GPR147.92 The molecular details of GnIH are eloquently described elsewhere.89,90 GnIH neurons are situated in the dorsomedial hypo thalamic area in mammals and the PVN in birds.90 GnIH axon terminals project into the preoptic area where they



REVIEWS come in contact with GnRH neurons, and GPR147 has been identified in GnRH neurons, suggesting that direct inhibition occurs.90 GnIH is also released into the hypothalamic–hypophyseal portal system where it inhibits secretion of LH and FSH by gonadotrophs.93 GnIH and its receptors are expressed in mammalian testes, indicating the potential for a direct effect on testosterone and spermatogenesis.90 GnIH expression is regulated by melatonin, glucocorticoids and the social environment in both birds and mammals, which stimulate its release 94 (GnIH has inhibitory effects on entire the HPG axis [hypothalamus to testes]);90 by virtue of this action at multi ple levels, GnIH can suppress testosterone secretion, reducing spermatogenesis.

Nongonadotropic hormones

The endocrine effects of HPA axis activation are not just confined to the HPA or HPG axes; they also affect the activities of other hormones. The two main pituitary hormones implicated in sp ermatogenesis— prolactin and growth hormone (GH)—are affected by glucocorticoid activity.

Prolactin Prolactin is a peptide hormone produced by pituitary lactotrophs and also by the prostate gland.95 Although prolactin has no specific target organ in men, it seems to have a number of nonspecific actions, including control of testosterone secretion96 and control of sexual behaviour and activity.97 Expression of prolactin receptors on the choroid plexuses and the hypothalamus suggests a latent role for prolactin in the regulation of male fertility.71 Prolactin secretion is regulated by dopamine released from the neurons of hypothalamus directly through the hypothalamic–hypophyseal portal system; consequently, any interference with dopamine release leads to hyperprolactinaemia.98 Hyperprolactinaemia (for example, in prolactinoma, a benign secretory tumour of pituitary gland) suppresses testosterone synthesis and male fertility through prolactin-induced hypersecretion of adrenal cortico steroids, by inhibition of GnRH or secretion of LH and FSH.99 Increased levels of prolactin cause spermatogenic arrest, impaired sperm motility and altered sperm quality.100 Prolactin levels are well known to be affected by psychological stress, with long-term psychological stress leading to significantly increased serum prolactin levels.101 Increased prolactin levels can also affect testosterone secretion and sexual behaviour and activity. The magnitude of the prolactin response seems to be related to the magnitude of the response of the HPA axis and, to some extent, cardiovascular responses to stress.102 The role of fluctuating levels of prolactin in spermatogenesis needs further clinical study. Growth hormone GH is secreted in a pulsatile fashion by somatotrophs of the pituitary gland103 and has anabolic properties. Its secretion is triggered by hypothalamic secretion

of growth-hormone-releasing hormone (GHRH or somatoc rinin) and inhibited by GHIH (also known as somatostatin) from the hypothalamus, in response to increased GH levels. GH increases secretion of insulin- like growth factor‑1 (IGF1), which is a member of insulin/IGF/relaxin family of proteins.104 GH instigates production of IGF1 through the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling pathway in target organs. 105 IGF1 has a critical role in male reproductive function, in particular with relation to the development and function of the testes.106 Immunohistology of human testes has shown increased expression of IGF1 in Sertoli cells and reduced expression in primary spermatocytes and Leydig cells. IGF1 receptors are highly expressed in secondary spermatocytes and early spermatids with reduced expression in both Sertoli cells and Leydig cells.107 Activation of the HPA axis affects GH secretion in a complex manner. CRH has an important role in inhibiting GH via GHIH; it also inhibits secretion of thyrotropin-releasing hormone and thyroid-stimulating hormone, thereby suppressing reproductive function, growth, and thyroid function.108,109 GHIH not only inhibits GHRH secretion from the arcuate nucleus but also has a direct inhibitory effect on pituitary somatotrophs. Acute stress increases GH levels, mainly owing to the direct effects of glucocorticoids on somatotrophs. 110 However, prolonged stimulation of the HPA axis leads to a reduction in circulating GH caused by CRH secretion. In addition, high levels of glucocorticoids have a direct inhibitory effect on GH secretion. This suppressive effect is likely to affect IGF1 production in Sertoli cells, and, therefore, affect early spermatids and secondary spermatocytes in spermatogenesis.

Effects of stress on seminal plasma

Eskiocak et al.10 investigated the effects of psychological stress on the L‑arginine–NO pathway in semen samples collected from medical students just before examinations (the stressful period) and 3 months after examinations (the stress-free period). NO is synthesized from L‑arginine by a family of isoenzymes known as the NO synthases (NOSs). NO synthesis occurs via at least two physiological pathways: NOS-dependent synthesis, where L‑arginine is the main precursor, and NOS-independent synthesis. Additionally, L‑citrulline might be a secondary NO donor in the NOS-dependent pathway, as it can be converted to L‑arginine. Nitrate and nitrite are the main substrates that produce NO via the NOS-independent pathway.111 Excessive NO has been shown to damage sperm ATP. High levels of ATP maintain the motility of spermatozoa.10 Apart from ATP derived from sugars, sperm also are likely to obtain their ATP from fatty acids that are metabolized in the mitochondrial and peroxi somal pathways.112 Excessive NO leads to the formation of peroxynitrite (ONOO–), which is highly toxic for sperm as it rapidly reacts with its proteins, lipids and DNA. The consequent membrane damage compromises various sperm functions, including motility.113



REVIEWS Eskiocak and colleagues observed that, during the stressful period, sperm concentration, progressive motility and seminal plasma arginase activity were significantly lower than in the stress-free situation, and concentrations of NO in the seminal plasma were signifi cantly higher. The results showed a negative correlation between NO concentration and sperm param eters. However, the exact mechanism of the effects of L‑arginine is unclear.

Other effects of psychological stress

Psychological stress has a number of other effects on the male reproductive system, including effects on sexual function, and it is implicated in chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS).

Sexual dysfunction The effect of psychological stress on erectile function is a well-known clinical entity. Both acute and chronic psychological stress can lead to erectile dysfunction.114 Couples experiencing fertility problems often engage in timed intercourse during the ovulation period. This strict timing could be quite stressful for men and might lead to anxiety and frustration, potentially causing erectile and ejaculatory dysfunction.115 Similarly, posttraumatic stress disorder (a long-standing condition that occurs after exposure to a life-threatening event) can result in sexual dysfunction (including erectile dysfunction and ejaculatory problems) amongst other numerous symptoms including intrusive memories, a state of hyperarousal, and avoidance of stimuli.116 CP/CPPS Psychological stress has long been implicated as a possible contributory factor in the aetiology of CP/CPPS and also in its exacerbation.117,118 However, the relationship between chronic prostatitis and male infertility remains unknown and is controversial. Prostatic fluid is a major contributor to seminal fluid and contains high levels 1.

2.

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7.

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of zinc, citric acid, calcium and phosphate.119 Clinical studies have shown that prostatitis has a negative effect on sperm motility and morphology.119–121 Other studies have not shown any difference in sperm parameters (including concentration, motility and morphology) between men with prostatitis and controls.122,123 CP/ CPPS is also known to cause sexual dysfunction including loss of libido, premature ejaculation and erectile dysfunction. The exact cause of sexual dysfunction in CP/CPPS is not known.

Conclusions

Evidence clearly indicates the presence of neuro endocrine signalling in the male reproductive system in response to psychological stress, both acute and chronic; however, the clinical relevance of this signalling and the changes that occur as a result are still not clear. A close relationship seems to exist between the testes and the stress system, as demonstrated by neuroendocrine nexus between the HPA and HPG axes, which could easily be affected by psychological stress. The HPA axis also seems to influence many other hormones outside the HPA and HPG axes. The newly identified hormone GnIH is emerging as a neuroendocrine factor that potentially disrupts testosterone secretion at all levels; however, further research in humans is needed to understand its effects. The effects of the HPA axis on Leydig cells and Sertoli cells make them vulnerable to gonadotoxins, which could cause permanent damage. As far as testosterone secretion is concerned, it is clear that the HPA axis exerts an inhibitory effect; however, the extent to which male fertility is affected in real terms is not known. The extent and severity of the effects of psychological stress on human testes is difficult to investigate and data mostly come from animal studies. However, stress as a causative factor in male infertility cannot be ignored and patients should be made aware of its effects on testicular function and fertility, and should be helped to address their issues.

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Effect of psychological stress on fertility hormones and seminal quality in male partners of infertile couples.

The Effects of Psychological Stress on Depression.

The effects of cigarette smoking on male fertility.

The effects of liraglutide on male fertility: a case report.

The effects of smoking-related sensory cues on psychological stress.

Male fertility.

Effect of vitamin C on male fertility in rats subjected to forced swimming stress.

Effects of lactation on fertility.

Hypertension and Male Fertility.

TSPY and Male Fertility.

Effects of chemotherapy on fertility.

Varicocele and male fertility.

Effects of new multi-site hormone blockers on the fertility of male rats.

The long-term effects of superovulation on fertility and sexual behavior of male offspring in mice.

The effects of diabetes on male fertility and epigenetic regulation during spermatogenesis.

Effects of a GnRH agonist on fertility following administration to prepubertal male and female rats.

The effect of ketoconazole on fertility of male rats.

The effect of micronutrient supplements on male fertility.

Fertility regulation in the male.

Effects of electromagnetic radiation exposure on stress-related behaviors and stress hormones in male wistar rats.

Effects of work stress and home stress on autonomic nervous function in Japanese male workers.

The effects of fascioliasis on ewe fertility.

Development of male-fertility-regulating agents.

Recent scenario of obesity and male fertility.