The aromatase knock-out mouse provides new evidence that estradiol is required during development in the female for the expression of sociosexual behaviors in adulthood

Abstract

We used estrogen-deficient aromatase knock-out (ArKO) mice to determine whether estrogens contribute to the development of the brain and behavior in females. Female mice of three different genotypes [i.e., wild type (WT), heterozygous (HET), and homozygous (ArKO)] were ovariectomized in adulthood and subsequently tested for odor preferences (choice: intact male vs estrous female) in a Y-maze. When treated with testosterone, ArKO females spent significantly less time sniffing odors (both volatile and nonvolatile) from either male or female stimuli compared with WT and HET females. When given direct access to anesthetized stimulus animals or when given a choice between odor and visual cues from both stimulus animals, ArKO females continued to spend less time investigating the stimuli compared with WT and HET females. These defects in olfactory investigation of ArKO females were partially corrected with estradiol treatment in adulthood. Estradiol-treated ArKO females no longer differed from WT and HET females in the time spent investigating either nonvolatile odors or the anogenital region of anesthetized animals. However, ArKO females still investigated volatile odors and/or visual cues less than WT and HET females. Sexual receptivity was severely impaired in ArKO females after treatments with estradiol and progesterone that successfully induced receptivity in WT and HET females. Furthermore, ArKO females showed diminished levels of male sexual behaviors, whereas WT and HET females readily mounted an estrous female. Together, these findings demonstrate that estrogen is required for normal female development. The concept that the female brain develops in the absence of any hormonal stimulation should therefore be reconsidered.

Fig. 1.

A, C, Mean amount of time that female WT, HET, and ArKO mice spent investigating volatile olfactory cues when given a choice between intact male odors and estrous female odors in a Y-maze. B, D, Total amount of time spent sniffing volatile odors. A, B, Results when subjects were treated with testosterone. C, D, Results when the same subjects were treated with estradiol. Means with differentletters above them are significantly different from each other by post hoc comparisons. Data shown are the mean ± SEM of two successive behavior tests.

Fig. 2.

A, C, Mean amount of time that female WT, HET, and ArKO mice spent sniffing nonvolatile olfactory cues when given a choice between soiled bedding from intact males and that from estrous females in a Y-maze. B, D, Total amount of time spent sniffing nonvolatile odors. A, B, Results when subjects were treated with testosterone. C, D, Results when the same subjects were treated with estradiol. Means with different letters above them are significantly different from each other by post hoc comparisons.

Fig. 3.

A, C, Mean amount of time that female WT, HET, and ArKO mice spent sniffing the anogenital region and flanks when given a choice between an anesthetized intact male and an anesthetized estrous female in a Y-maze. B, D, Total amount of time spent sniffing the anogenital region and flanks.A, B, Results when subjects were treated with testosterone. C, D, Results when the same subjects were treated with estradiol. Means with different lettersabove them are significantly different from each other by post hoc comparisons.

Fig. 4.

A, C, Mean amount of time that female WT, HET, and ArKO mice spent investigating odor and visual cues when given a choice between an awake intact male and an awake estrous female in a Y-maze. B, D, Total amount of time investigating odor and visual cues. A, B, Results when subjects were treated with testosterone. C, D, Results when the same subjects were treated with estradiol. Means with different letters above them are significantly different from each other by post hoc comparisons.

Fig. 5.

Female sexual behavior with a stud male of female WT, HET, and ArKO mice. A, Lordosis quotients;B, receptivity index. *p < 0.05 compared with WT and HET females.

Fig. 6.

A, C, Mean number of mounts and intromission-like behaviors displayed by female WT, HET, and ArKO mice when paired with an estrous female. B, D, Total number of mounts and intromissions together. A, B, Results when subjects were treated with testosterone. C, D, Results when the same subjects also received daily injections with estradiol while remaining on testosterone. Means with differentletters above them are significantly different from each other by post hoc comparisons.