Hormones

Hormones are chemical substances that are produced, stored, and released into the bloodstream by secretory structures (glands) of the endocrine system. The endocrine system, in turn, is designed to modulate various body functions, including digestion, metabolism, growth and development, reproduction, and response to stress and injury. Hormones are released by specific glands, move through the bloodstream to target locations, and act there upon cellular “receptors” designed specifically to receive and be activated by particular hormones. Hormones are capable of altering the structure and function of many organs, including the brain. There are more than 50 distinct hormones that play diverse and pivotal roles in development and homeostasis and that may also affect behavior. Among the most well-known are the sex or steroid hormones, including androgen (e.g., testosterone, produced in the testes, ovaries, and adrenals) and estrogen (produced in the ovaries, testes, and adrenals). Alone, these two groups of substances exert widespread effects on the developmental process, as well as throughout life, and will be the primary exemplars of hormones in the following discussion.

How Do Steroid Hormones Determine Sex?

All mammals secrete sex hormones. Beginning in the prenatal period, the process of sexual differentiation begins with genotypic sex (as determined by the presence of sex-specific chromosomes, e.g., XX for female and XY for male), which in turn codes for the development of either testes or ovaries. At this early prenatal stage, both genotypic males and females possess dual sets of duct systems—the müllerian ducts and wolffian ducts. Hence, early in development, the fetus is considered “bipotential” with regard to sex. As the testes or  ovaries  develop,  however,  and  begin  to  secrete hormones, further sexual differentiation of the body occurs. Specifically, the production of testosterone and other male-specific hormones leads to the regression of the müllerian duct system and the formation of the primal male internal reproductive system (epididymis, vas deferens, and seminal vesicle). Conversely, the absence of these male substances coupled with later ovarian output results in a female phenotype (including the development of fallopian tubes, the uterus, and the inner part of the vagina). Thus, structural or “physical” sexual characteristics define phenotypic sex. In most all cases, genotypic sex and phenotypic sex are identical, but occasional hormone anomalies may lead to an outward sexual appearance and behavioral pattern that is inconsistent with genotypic sex.

Hormones  also  lead  to  sexual  differentiation  of the external genitalia and modulate the emergence of sex-specific behaviors. Thus, testosterone directs the masculinization of external genitalia and the organization of male-specific behavior, whereas a lack of testosterone coupled with the later presence of ovarian activity results in female external genitalia (and the accompanying establishment of female-typical behaviors). Following the development of internal reproductive organs, external genitalia, and the organizational effects of hormones on brain and behavior, secretion of sex steroid hormones drops to quiescent levels until the approach of puberty. However, steroid hormones go on to play an important role in the activation of sex-specific behaviors in puberty and adulthood.

Hormones In Adulthood

Puberty is marked by the beginning of gametogenesis (the development of an egg or sperm by the gonads), a rise in secretion of sex hormones by the gonads, the accompanying development of secondary sexual characteristics, and the activation of reproductive functions that have been “organized” by early gonadal hormone secretions. These behaviors may reflect permanent alterations to brain structure and function, which nevertheless require the presence of circulating hormones to trigger their activation. This explains why reproductive behaviors are not seen (or occur at much lower levels) in prepubertal mammals, even though the groundwork for the display of these behaviors appears to be laid down in the brain early in development, during the process of sexual differentiation.

Mammalian females, for example, are characterized by an ovarian cycle. After puberty, females show cyclic production and release of ovarian hormones that are in turn responsible for activating female-typical sexual behaviors. Such behaviors have been studied extensively  in  nonhuman  mammals,  particularly  rodents (i.e., sexual receptivity and mating behaviors). Moreover, these sex-typical female behaviors have been shown to be conditional on the presence of estrogen and another ovarian hormone, progesterone.

Sexual behaviors in male mammals are also dependent on the presence of hormones. Testosterone is secreted by the testes and activates sexual behaviors such as penile erection and mating behavior. In the absence of testosterone, sexual behavior is severely compromised. However, testosterone is converted into two different metabolites, dihydrotestosterone (DHT) and estrogen, and these two by-products appear to have different effects on behavior. For example, it has been shown in rodents that one behavior (penile erection) is induced by the conversion of testosterone into DHT, whereas other behaviors (mating behaviors) are activated when testosterone is converted intracellularly into estrogen. Moreover, these androgen metabolites act in different areas of the brain (estrogen) and body (DHT) to mediate these different sexual functions.

One obvious question arises as to how estrogen can exert both feminizing effects in females and simultaneous masculinizing effects in males. One possibility that could account for these differing effects of estrogen involves differences in the relative amounts of estrogen between the sexes. Specifically, in males, estrogen appears mainly within the cell, where it has been actively converted from circulating testosterone into estrogen, and intracellular levels of estrogen derived through this metabolic pathway are much higher in males than intracellular levels of estrogen in females. In addition, estrogen may act on different structures and at different time points in male and female development.

Hormones And Cognition

Hormones also play a role in behaviors other than sexual behavior. For example, men generally have been shown to outperform women on tests of visuospatial ability, whereas women perform better on tests of verbal fluency and perceptual speed. It should be noted that these differences are typically small, particularly as compared with population and individual variability, and are largely irrelevant to issues of gender, ability, and career choice. Nevertheless, studies have shown that performance on some cognitive tasks may be influenced by circulating levels of hormones (in fact, hormone levels may be as important as gender in mediating some of the observed sex differences in cognition).

In support of this view, animal research shows that neonatal testosterone affects a number of measures of cognitive performance, shifting it in a male-like direction. However, female hormones also influence aspects of nonsexual behavior. As one example of the complexity of sex differences in nonreproductive behavior, spatial navigation behavior has been shown to differ in male and female rats. Moreover, it has been shown that males and females utilize different strategies to complete the same task. Finally, some aspects of cognitive performance, which involve the use of differing strategies, appear to change over the ovarian cycle, specifically changing with fluctuating hormonal levels. Thus, it appears that “sex” differences on cognitive tasks may be not just a function of gender but also of circulating hormones.

Hormonal Manipulations

Throughout life, humans also manipulate their hormonal levels to achieve different outcomes. For example, anabolic steroids (which represent synthetic forms of testosterone) are used by athletes for their effects on the musculoskeletal system (i.e., increased lean body mass and increased muscle strength). Oral contraceptives also represent hormonal manipulation, in this case used to prevent pregnancy in females by altering estrogen and progesterone. These two hormones, which direct many of the processes surrounding the menstrual cycle, work by preventing an egg from being released from the ovaries most of the time and also make the uterus a hostile environment for an embryo by causing a thinning of the uterine lining. Evidence suggests that both anabolic steroids and oral contraceptives may exert some effects on cognition and emotions, although the specific nature and pattern of these effects remains unclear.

Hormones And Aging

As we age, our hormonal levels also decrease. In females, ovulation ceases after middle age, and in males, testosterone levels decrease with age. In postmenopausal women, there is a decline in the ovarian production of estrogen that results in estrogen deficiency. This deficiency is associated with a number of health problems as well as cognitive and behavioral changes. A way to compensate for these changes involves the use of hormone replacement therapy (HRT). This therapy supplements the age-related decline in naturally produced estrogen and progesterone with an external source, although controversy indicates that the negative side effects of HRT may outweigh benefits for some women. In males, testosterone levels decrease with age, and they experience similar cognitive and mood changes, as well as a loss of sexual drive. One way to deal with the latter problem is with drugs designed to combat the decline in testosterone and its effect on sexual performance. For example, certain drugs can be used to exert a direct influence on male erectile mechanism, specifically by working on the brain and spinal cord pathways that control penile erection. The male counterpart to dealing with declining estrogen and progesterone levels in women is to take testosterone. However, there are few studies to date addressing the effects of HRT in men, and preliminary evidence suggests both beneficial and detrimental effects on health and cognition. Some studies addressing the influence of HRT on cognition in aging males show that testosterone enhances spatial memory and possibly verbal and working memory, whereas other studies show no influence on cognition. A similar discrepancy exists for aging women and HRT. Although some studies have shown that HRT has a positive effect on verbal memory and mood, others have not found significant differences between users and nonusers of HRT on any cognitive measure.

Summary

The data discussed here represent only a small portion of the many effects that hormones exert on human physiology. In this entry, sex steroids are addressed as an exemplar, given the highly significant effects these substances exert across the human life span. However, many other hormonal systems affect metabolism, sleep, stress, and other systems, and the reader is referred  to  one  of  the  excellent resources below for more information on these diverse effects.

References:

1. Challem, (1999). ABC’s of hormones. New York: McGrawHill.

2. Greenspan, F. S, & Gardner, D. G. (2003). Basic and clinical endocrinology. Norwalk, CT: Appleton &
3. Becker, J. B., Breedlove, S. M., Crews, D., & McCarthy, M. (2002). Behavioral endocrinology. Cambridge: MIT
4. The Hormone Foundation, http://www.hormone.org
5. Williams, H., Larsen, P. R., Kronenberg, H. M., Melmed, S., Polonsky, K. S., Wilson, J. D., et al. (2002). Williams textbook of endocrinology. Philadelphia: WB Saunders.