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Influence of Stress on Male Reproductive Physiology
Environmental, physiological, psychological, and managerial stressors have been implicated as causes of reproductive disorders and decreased fertility in animals and humans. Herd reproductive efficiency and the stud industry depend upon the sperm producing capabilities and libido of bulls and stallions. With respect to male reproductive physiology, the steroidogenic and spermatogenic functions of the testis can be negatively impacted by stress-induced secretion of endogenous glucocorticosteroids (GC) as well as by treatment with exogenous GC agonists. The testes’ gametogenic function, a primary component of male fertility, is dependent upon appropriate transmission, receipt, and processing of specific endocrine signals. The deleterious effects of stress upon reproductive performance are presumably signaled by GC activation of the glucocorticoid receptor (NR3C1). Questions related to molecular mechanisms whereby stress affects specific components of the hypothalamic-pituitary-testicular (HPT) axis of male rodents, primates, cattle, sheep, pigs, horses and other species have been pursued by the use of in vitro and in vivo methods. This paper will provide a targeted overview of potential impacts of stressors upon the endocrine aspects of the HPT axis, with particular focus on direct testicular effects. Specific data to be presented are derived from studies of the influences of endogenous and exogenous GC on androgen biosynthesis and gene expression in testes of bulls and stallions. Chronic administration of a synthetic GC has been reported to increase the incidence of abnormal spermatozoa by direct action or perhaps by disruption of the endocrine or genetic mechanisms that support sperm production in bulls and stallions. Acute elevation of the systemic concentration of GC by pharmacologic methods or by mimicry of physiologic stress have inhibited testicular steroidogenesis and transiently decreased the systemic concentration of testosterone. The inhibitory action of endogenous GC concentrations on testicular steroidogenesis under stressed and non-stressed conditions indicates that activity of the hypothalamic-pituitary-adrenal axis may be of critical importance in establishment or maintenance of a functional HPT axis during prenatal, prepubertal, and postpubertal life. Homeostatic regulation of reproductive processes involves a physiological integration of the adrenal and testicular axes. The biologic and economic importance of deleterious influences of stress upon male reproductive processes dictate a thorough evaluation of adrenal–testicular interrelationships in domestic livestock species.
Keywords: Stress; Steroidogenesis; Spermatogenesis