The endocrine control of sexual behavior in the human female has been the subject of a substantial amount of research. Undoubtedly, among the primates there is no species that has attracted as much attention as Homo sapiens. There are several good reasons for this. One is that disorders of the reproductive system are rather common, and one treatment for some of these disorders is ovariectomy. The considerable number of women who have been subjected to ovariectomy, from the early fertile age until well into menopause, has allowed researchers to evaluate the behavioral consequences in a rather large number of individuals. Another reason is that ovarian function ceases at a relatively young age in the human. Many women can expect to live 30 years or more after the ovaries have stopped producing the typical ovarian hormones, estrogens and progesterone. In many respects, the endocrine changes associated with menopause are similar to those occurring after ovariec-tomy. In fact, it has become customary to call the state related to ovariectomy in premenopausal women 'surgical menopause'. The fact that most women will spend a large part of their life without the cyclic variations in ovarian hormones typical of most other mammalian species poses some challenges for any hypothesis trying to relate sexual activity to blood hormone concentrations.
It seems convenient to begin the discussion of the human female with a summary of the hormonal changes occurring after puberty. We all know that women have a menstrual cycle, and any introductory physiology and even psychology textbook will provide us with an excellent description of the hormonal changes during the cycle. Suffice to mention that estradiol, the main follicular estrogen, is mainly secreted during the first half of the cycle with a peak around ovulation. Progesterone, produced by the corpus luteum, is elevated from shortly after ovulation until the beginning of menstruation, when it rapidly drops. This is the picture of the menstrual cycle generally divulged in textbooks and on Internet sites. However, in addition to producing estrogens and progesterone, the ovaries also produce androgens. In fact, human female blood contains several androgens, like dehydroepiandrosterone sulphate, dehydroepiandrosterone, androstenedione, testosterone and dihydrotestosterone. The physiologically most important of these is certainly testosterone. The blood concentration of this hormone varies according to the menstrual cycle, with a peak around ovulation and a minimum during the early follicular phase (Abraham, 1974; Longcope, 1986). About 25% of the testosterone is produced in the ovaries, while another 25% stems from the adrenals. The remaining 50% is produced in peripheral tissues by conversion of other andro-gens, mainly androstenedione (Burger, 2002).
The blood concentration of testosterone starts to decline in early adulthood, and it has been reported that the testosterone concentration at age 40-50 is about half of that found at age 20 (Zumoff et al., 1995). Thereafter, the decline is slower. Of particular importance here is the fact that menopause fails to affect blood androgen concentrations (Davison et al., 2005). A similar observation had been made a few years earlier (Burger et al., 2000). In contrast to the lack of effect of menopause on blood androgen concentrations, it was found that the concentrations of sex hormone binding globulin was reduced at menopause. The reason for this was suggested to be the significant decline in estradiol occurring at menopause. Since part of the circulating androgens are bound to the sex hormone binding globulin, and thus not immediately bioavailable, a reduction of this globulin may in fact enhance androgen availability. Another study revealed that blood testosterone concentration increased from ages 50 to 89 when the values were adjusted for body mass index (Laughlin et al., 2000). All these data convincingly show that women produce important amounts of androgens from the beginning of the fertile age until the end of life.
There are data suggesting that the ovarian stroma is one source of menopausal androgens. Indeed, ovariectomy in women between 55 and 75 years of age leads to a marked reduction of blood androgen concentrations (Laughlin et al., 2000; Davison et al., 2005). The impact of this reduction on sexual functions is, as we will see, not great but it is large enough to justify mentioning.
We can now place our discussion of the hormonal control of sexual behaviors in women within an appropriate endocrine context. From puberty until the climacteric, there is a large cyclic variation in the blood concentrations of estrogens and progesterone. During menopause, the concentration of these hormones declines sharply. From puberty until the end of life, androgens are also present in the circulation. In premenopausal women, there is a modest cyclic variation in the concentration of these hormones superimposed on a steady decline from age 20 to 45-50. Thereafter, there either is no change or a small increase. Any hypothesis concerning the endocrine control of women's sexual behavior needs to take the hormonal facts into consideration. If we propose that women are like rodents or sheep with estradiol + progesterone as the critical hormones, then we should expect large cyclic variations in sexual activity and a significant reduction during menopause, independently of whether it is surgical or natural. On the contrary, if we believe that women are like other primates with androgens as the crucial hormones, then we must expect a maximum of sexual activity during early fertile age followed by a decline until menopause. After that, sexual activity should remain at a stable level. We should expect ovariectomy, but not menopause, to produce a diminished sexual activity, since the ovaries contribute to the maintenance of blood androgen concentration both before and during menopause. Some slight variation during the menstrual cycle, with a maximum around ovulation when testosterone concentrations are at their highest may also be expected. All these predictions can be confronted to empirical data. Let us begin with the estradiol + progesterone hypothesis and the prediction that sexual activity varies according to the phase of the menstrual cycle, with a maximum around the estrogen peak associated with ovulation.
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