Scientists are now uncovering increasing evidence that the brain not only responds to hormones produced by the reproductive system, but that these hormones-the so-called "female hormones," estrogen and progestin, and the "male" androgens, such as testosterone-play an important role in the development of differences between male and female brains.
"Understanding the impact of hormones on sex differences in the brain is important for understanding human health and disease," says University of Michigan biopsychologist Jill Becker, PhD. "Some conditions-persistent pain syndromes, such as fibromyalgia and TMJ (temporomandibular joint syndrome), for example-are more frequently diagnosed in women than in men. More women than men also suffer from mood disorders, such as major depression, and anxiety disorders. On the other hand, more men than women develop alcoholism and abuse drugs."
The study of hormone-related differences between male and female brains is not as simple as it may seem at first. Straightforward comparisons of males and females are not possible because of the cyclical nature of reproductive hormone production in females, Becker points out. The menstrual cycle in humans and other primates and the estrous cycle in rats and mice involve constantly changing levels of reproductive hormones in the blood and in the brain. Furthermore, although brain development begins before birth, it continues well into young adulthood, and there is increasing evidence that parts of the brain continue to grow, die back, and change throughout the life span.
"Reproductive hormones have effects on all of these stages of brain growth and development," says Becker. "For these and other reasons, the study of sex differences in the brain is both complicated and fascinating."
At the University of British Columbia , Liisa Galea, PhD, has been investigating the contribution of one form of estrogen, estradiol, to learning and memory. In recent animal studies, Galea and her colleagues examined the effect of low and high levels of estradiol on working and reference memory. Working memory, or short-term memory, manipulates and retrieves information that is needed for a temporary task; it decays rapidly. Reference memory involves the long-term storage of information, and is stable over time. "When you remember where your car is in a shopping center parking lot on any given day, you're using working memory," explains Galea. "When you find the shopping center parking lot day after day, you're using reference memory."
In their study, Galea and her colleagues removed the ovaries from adult female rats (to eliminate naturally produced estrogens) and then gave the rats various levels of estradiol. "We found that low levels of estradiol improved the animals' working memory, but high levels impaired both their working and their reference memory," says Galea. In addition to modulating forms of learning and memory, estradiol influences cell growth in many areas of the brain-and, as Galea and her colleagues have found-estradiol's effects on that growth are different in the brains of males and females. In recent animal studies, Galea and her colleagues discovered that high levels of estradiol in females initially increased, then subsequently suppressed, the production of new brain cells in the dentate gyrus of the hippocampus, an area of the brain that is involved in learning and memory and that produces new neurons throughout life. This same pattern does not appear to be similar in the male brain. Once the new brain cells were formed, however, estradiol enhanced their survival differently in males and females. "In male rats, estradiol enhanced the survival of new cells only during a discrete period of time," says Galea, "but in females, the estradiol-induced enhancement of new neurons occurred during all the time periods tested."
As these and other studies show, estradiol has complex interactions with learning and memory and with brain cell growth-and these interactions are different in the brains of males and females. This work promises to lead to a more complete understanding of the effects of reproductive hormones in the brain. In particular, it may bring new insight into the confusing and seemingly contradictory effects of the use of estrogen by post-menopausal women on learning and memory. It may also help explain the mechanisms underlying gender differences in cognition and susceptibility to mental diseases.
At the University of Michigan , Jon-Kar Zubieta, MD, PhD, and his colleagues have been studying the effect of sex hormones on pain and stress-response systems in the brain. They have found that differences between men and women in the perception and experience of sustained pain-by itself, a physical and psychological stressor-may be due, in part, to the influence of reproductive hormones on the brain. These findings promise to lead to a greater understanding of how and why certain diseases characterized by chronic pain, such as fibromyalgia and TMJ, occur more often in women than in men.
Zubieta's research has focused on a particular neurochemical pathway in the brain-the mu-opioid neurotransmitter system-which uses endogenous chemicals called opioids (commonly known as endorphins and enkephalins) to send signals between brain cells to suppress the sensation of pain. When a stressful stimulus, such as sustained physical pain or severe emotional stress, threatens the well-being of the body, the brain releases these opioid peptides, which then bind to receptors (known as mu-opioid receptors) located in various regions throughout the brain. "The rapid activation of this system suppresses an individual's perception of a stressful event and the emotions that accompany it," explains Zubieta, "thus making the pain and stress more tolerable."
To study the mu-opioid neurotransmitter system, Zubieta and his colleagues use positron-emission tomography (PET) brain imaging, which allows them to observe the system in action. Volunteers are scanned over a period of 20 minutes as they receive a moderately pain-causing but harmless injection of salt water in their jaw muscle. (The pain dissipates within minutes of completion of the experiment and causes no residual damage.) The studies are double-blind and placebo-controlled.
In a study involving 14 men and 14 women, Zubieta found significant differences in how the brains of men and women respond to pain. The men experienced an increase in the amount of endorphins released in certain regions of their brains during the painful state of the experiment, while most of the women showed a decrease. The participants were asked to rate the intensity and unpleasantness of the pain. Women consistently gave higher ratings for both.
All the women in this early study were in the early follicular phase of their menstrual cycle (shortly after menses) when blood levels of both estrogen and progestin are low. None were taking hormonal birth control and all had ovulated the month before. For their latest study, Zubieta and his colleagues decided to examine whether varying blood levels of estrogen would modify the response of the mu-opioid system in women. Using the same jaw pain model, the scientists studied a group of women during the follicular phase of their menstrual cycle and again during the same phase in another month-but after the women had been wearing an estrogen-releasing skin patch for a week. The results were striking. When estrogen levels were high, the women showed marked increases in their ability to release endorphins and activate the mu-opioid receptors-increases that rivaled and even surpassed those of men. The women also rated the intensity and unpleasantness of the pain lower than they had when their estrogen levels were low.
"Our studies demonstrate that some of the sex differences in an individual's response to pain, or, in more general terms, to a substantial stressor are mediated through specific chemical systems in the human brain and that these responses are modulated by blood levels of sex hormones," says Zubieta.
The linking of pain sensitivity and regulation to reproductive hormones-particularly estrogen-makes some "evolutionary" sense, Zubieta says. Women require more flexible, adaptive mechanisms to protect themselves from injury during their reproductive years to preserve the reproductive function of the species, he says. At the same time, women also have to adapt to the body changes and pain that takes place during pregnancy and childbirth-a time when reproductive hormones are at an all-time high. "So they have to develop mechanisms, like the mu-opioid neurotransmitter system, that promote that type of flexibility," Zubieta says. "Interestingly, this neurotransmitter system is also involved in maternal-offspring attachment behavior, another area where estrogens may play a role."
At McMaster University , Meir Steiner, MD, PhD, and his colleagues have been studying the role that estrogen and other reproductive hormones may play in gender differences in depression. Women are two to three times more likely than men to experience a major depressive episode during their lifetime.
"The underlying cause of the gender difference in depression and other mood disorders is not entirely clear," says Steiner, "but the differences, which begin rather dramatically at puberty and become less marked after menopause, strongly suggest a link to fluctuating estrogen and progesterone levels." Although hormones fluctuate in both men and women, the fluctuations are much more pronounced in women, particularly around their menstrual cycles, during the weeks immediately after pregnancy (postpartum), and in the period leading up to menopause (perimenopause). These fluctuations, Steiner has proposed, cause disturbances along the hypothalamic-pituitary-adrenal axis, a part of the neuroendocrine system that is believed to play a primary role in the body's reaction to stress.
Because hormonal fluctuations occur in all women, yet not all women experience clinical depression, it's likely that women who develop hormone-related depression and mood disorders have a genetic predisposition to them, Steiner adds. Further research is needed, he says, to identify the specific genetic markers that might lead to a better understanding of how the balance between estrogen, progesterone, testosterone, and other reproductive hormones affects brain function and increases women's susceptibility to depression.
In recent animal studies, Rebecca Craft, PhD, and her colleagues at Washington State University have found that differences in sensitivity to pain and opiate analgesia between males and females may be due primarily to testosterone exposure during early development. Using a rat model that exhibits particularly dramatic sex differences in response to opiate analgesia, the researchers manipulated hormones in rats when the animals were very young and then again when the rats were adults. The animals' pain thresholds and their response to morphine-induced analgesia were then evaluated using two different tests.
Craft and her colleagues found that early testosterone deprivation increased pain sensitivity in males, making them more like "normal" females. Surprisingly, this effect could be reversed with only two weeks of testosterone treatment during adulthood. Females given testosterone as neonates showed different pain thresholds than untreated females-in other words, they became more like "normal" males. Unlike with the male rats, however, giving the female rats testosterone supplements in adulthood had no further effect.
"These results suggest that differential pain sensitivity in adult male rats compared to females is primarily due to testosterone exposure during development," says Craft.
Early testosterone exposure also influenced the rats' sensitivity to morphine-induced analgesia when they were adults. The males deprived of testosterone as neonates were less sensitive to morphine than normal males (i.e., more like "normal" females), and the females given testosterone as neonates were more sensitive to morphine than control females (i.e., more like "normal" males). Interestingly, the effects of neonatal testosterone deprivation in males could be reversed in some tests simply by exposing the males to testosterone for two weeks during adulthood. In contrast, females exposed to testosterone soon after birth and then again in adulthood showed no further effect from the later exposure.
"Thus, it can be concluded that testosterone exposure before birth or on the first day after birth is sufficient to enable males to regain their 'male phenotype' as long as they are exposed to testosterone in adulthood," says Craft, "whereas exposing females to testosterone soon after birth is insufficient to completely change them into the male phenotype in terms of their sensitivity to pain and to morphine-induced analgesia."
Craft and her colleagues are now attempting to replicate their findings in other strains of rat. "Overall, our current results suggest that adult females' greater sensitivity to pain and lesser sensitivity to the pain-killing effects of morphine will be difficult to ameliorate by simple hormone treatment-with testosterone, for example-in adulthood," says Craft. "Instead, these sex differences would appear to be determined very early in development."