Role for estradiol in female-typical brain and behavioral sexual differentiation

Abstract

The importance of estrogens in controlling brain and behavioral sexual differentiation in female rodents is an unresolved issue in the field of behavioral neuroendocrinology. Whereas, the current dogma states that the female brain develops independently of estradiol, many studies have hinted at possible roles of estrogen in female sexual differentiation. Accordingly, it has been proposed that alpha-fetoprotein, a fetal plasma protein that binds estrogens with high affinity, has more than a neuroprotective role and specifically delivers estrogens to target brain cells to ensure female differentiation. Here, we review new results obtained in aromatase and alpha-fetoprotein knockout mice showing that estrogens can have both feminizing and defeminizing effects on the developing neural mechanisms that control sexual behavior. We propose that the defeminizing action of estradiol normally occurs prenatally in males and is avoided in fetal females because of the protective actions of alpha-fetoprotein, whereas the feminizing action of estradiol normally occurs postnatally in genetic females.

Fig. 1

Sexual differentiation of the brain. In male rodents, testosterone (T) secreted by the testes enters the brain where it is aromatized to estradiol (E), which subsequently binds to the estradiol receptor to promote gene expression that masculinize and defeminize the neural mechanisms controlling sexual behavior.

Fig. 2

Lordosis quotients of female wild-type (WT), heterozygous (HET), and aromatase knockout (ArKO) mice. All females were ovariectomized in adulthood and subsequently treated with estradiol and progesterone prior to each behavioral test with a sexually active male. *p < 0.05 compare to WT and HET females. Data shown are means (±SEM) of a total of five tests.

Fig. 3

Mounting behavior of female wild-type (WT), heterozygous (HET), and aromatase knockout (ArKO) mice. All females were ovariectomized in adulthood and tested once for mounting behavior with an estrous female when treated with testosterone and then once more when treated with both testosterone and estradiol (5 μg/mouse/day). *p < 0.05 compared to WT and HET females, ^#p < 0.05 compared to WT females. Data shown are means ± SEM.

Fig. 4

Total time spent investigating volatile odors by female wild-type (WT), heterozygous (HET), and aromatase knockout (ArKO) mice when given the choice between volatile body odors from an intact male versus those from an estrous female in a Y-maze. All female subjects were ovariectomized in adulthood and first tested for their odor preferences when receiving testosterone and then when receiving estradiol. *p < 0.05 compared to WT and HET females. Data shown are means ± SEM of two successive tests.

Fig. 5

Time spent by ovariectomized female wild-type (WT), heterozygous (HET), and aromatase knockout (ArKO) mice investigating deionized water or volatile urinary stimuli. *p < 0.05 between the time spent investigating the third presentation of a particular stimulus and the first presentation of the subsequent stimulus.

Fig. 6

Ability of wild-type (WT) and aromatase knockout (ArKO) male and female mice to learn to discriminate between two female urine stimuli in an olfactometer test. S+ = rewarded stimulus; S− = non-rewarded stimulus. Data expressed as means ± SEM. *p < 0.05 compared to WT males and females, and ArKO males.

Fig. 7

Two competing hypotheses on the role of α-fetoprotein (AFP), a fetal plasma protein that binds estrogens with high affinity, in female sexual differentiation: (a) AFP may serve to keep estrogens from entering the brain (hypothesis proposed by [64]) and (b) AFP may deliver estrogens to specific brain regions to promote feminization of the neural mechanisms controlling female sexual behavior (hypothesis proposed by [99]).

Fig. 8

Behavioral and neurochemical phenotype of female α-fetoprotein knockout (AFP-KO) mice. (a) Lordosis quotients of female wild-type (WT), heterozygous (HET), and AFP-KO mice. All female subjects were ovariectomized in adulthood and subsequently tested with estradiol and progesterone prior to each behavioral test. *p < 0.05 compared to WT and HET females. (b) Mounting behavior of female WT, HET, and AFP-KO females when paired with an estrous female. Female subjects were ovariectomized in adulthood and subsequently treated with estradiol. *p < 0.05 compared to WT females. (c) Numbers of tyrosine hydroxylase-immunoreactive (THIR) neurons in the anteroventral preoptic area (AVPv). Female subjects were ovariectomized in adulthood and treated with estradiol. *p < 0.05 compared to WT males, and HET and AFP-KO females, ^#p < 0.05 compared to WT males and females and AFP-KO females. (d) Brain vasopressin expression assessed by the fractional areas covered by vasopressin-IR structures in the posterior lateral septum. Female subjects were ovariectomized in adulthood and treated with estradiol. *p < 0.05 compared to all female groups.

Fig. 9

Female phenotype of AFP-KO females is rescued by prenatal treatment with the aromatase inhibitor ATD. (a) Lordosis quotients of wild-type (WT), AFP-KO, and heterozygous (HET-ATD) and AFP-KO females treated prenatally with ATD (AFP-KO-ATD). All female subjects were ovariectomized in adulthood and treated subsequently with estradiol and progesterone. *p < 0.05 compared to WT, HET-ATD, and AFP-KO-ATD females. (b) Numbers of tyrosine hydroxylase-immunoreactive (THIR) neurons in the anteroventral preoptic region (AVPv). Female subjects were ovariectomized in adulthood and treated with estradiol. *p < 0.05 compared to WT males and AFP-KO females.