Hormonal determinants during infancy of adult sexual behavior in the female rat

Sexual incentive motivation and male and female copulatory behavior in female rats given androgen from postnatal day 20

Masculinization and feminization of rat sexual behavior has been supposed to occur during a short postnatal period. However, much data have made it evident that these processes may continue until adolescence. In the present study, we evaluated whether androgen treatment of females from postnatal day 20 and onwards could alter sexual motivation and behavior in a male direction. Juveniles were ovariectomized on day 20 and concurrently implanted with Silastic capsules containing either testosterone or dihydrotestosterone. Controls were implanted with an empty capsule. Tests for sexual incentive motivation and male sexual behavior were performed every fifth day when the females were between 50 and 75 days of age. At day 80, a test for female sexual behavior was performed. Females treated with testosterone approached a female sexual incentive far more than a male incentive, showing that sexual motivation had been changed in a male-like direction. Dihydrotestosterone had a similar, albeit smaller, effect. Females implanted with an empty capsule approached both incentives equally. Testosterone produced a high level of mounting behavior, whereas intromission-like behavioral patterns were rare and ejaculation-like behavior was absent. In the test for female sexual behavior, the testosterone-treated animals displayed a relatively high lordosis quotient, far above that displayed in females implanted with dihydrotestosterone or an empty capsule. It is concluded that treatment with an aromatizable androgen during the peripubertal-adolescent period masculinizes sexual motivation and partly sexual behavior. A non-aromatizable androgen weakly masculinize sexual motivation without enhancing male sexual behavior. It appears that simultaneous actions on androgen and estrogen receptors are needed for significant masculinization during the period studied here. Since the testosterone-treated females displayed lordosis, sexual behavior was not defeminized. In sum, these results suggest that sexual differentiation continues well into the peripubertal and adolescent periods.

Nonneural Androgen Receptors Affect Sexual Differentiation of Brain and Behavior

Testosterone, acting via estrogenic and androgenic pathways, is the major endocrine mechanism promoting sexual differentiation of the mammalian nervous system and behavior, but we have an incomplete knowledge of which cells and tissues mediate these effects. To distinguish between neural and nonneural actions of androgens in sexual differentiation of brain and behavior, we generated a loxP-based transgenic mouse, which overexpresses androgen receptors (ARs) when activated by Cre. We used this transgene to overexpress AR globally in all tissues using a cytomegalovirus (CMV)-Cre driver (CMV-AR), and we used a Nestin-Cre driver to overexpress AR only in neural tissue (Nes-AR). We then examined whether neural or global AR overexpression can affect socio-sexual behaviors using a resident-intruder paradigm. We found that both neural and global AR overexpression resulted in decreased aggressive behaviors and increased thrusting during mounting of intruders, consistent with a neural site of action. Global, but not neural, AR overexpression in males led to an increase in same-sex anogenital investigation. Together, these results suggest novel roles for nonneural AR in sexual differentiation of mice, and indicate that excess AR can lead to a paradoxical reduction of male-typical behavior.

Microarray analysis of perinatal-estrogen-induced changes in gene expression related to brain sexual differentiation in mice

Sexual dimorphism of the behaviors or physiological functions in mammals is mainly due to the sex difference of the brain. A number of studies have suggested that the brain is masculinized or defeminized by estradiol converted from testicular androgens in perinatal period in rodents. However, the mechanisms of estrogen action resulting in masculinization/defeminization of the brain have not been clarified yet. The large-scale analysis with microarray in the present study is an attempt to obtain the candidate gene(s) mediating the perinatal estrogen effect causing the brain sexual differentiation. Female mice were injected with estradiol benzoate (EB) or vehicle on the day of birth, and the hypothalamus was collected at either 1, 3, 6, 12, or 24 h after the EB injection. More than one hundred genes down-regulated by the EB treatment in a biphasic manner peaked at 3 h and 12-24 h after the EB treatment, while forty to seventy genes were constantly up-regulated after it. Twelve genes, including Ptgds, Hcrt, Tmed2, Klc1, and Nedd4, whose mRNA expressions were down-regulated by the neonatal EB treatment, were chosen for further examination by semiquantitative RT-PCR in the hypothalamus of perinatal intact male and female mice. We selected the genes based on the known profiles of their potential roles in brain development. mRNA expression levels of Ptgds, Hcrt, Tmed2, and Nedd4 were significantly lower in male mice than females at the day of birth, suggesting that the genes are down-regulated by estrogen converted from testicular androgen in perinatal male mice. Some genes, such as Ptgds encoding prostaglandin D2 production enzyme and Hcrt encording orexin, have been reported to have a role in neuroprotection. Thus, Ptgds and Hcrt could be possible candidate genes, which may mediate the effect of perinatal estrogen responsible for brain sexual differentiation.

Different critical perinatal periods and hypothalamic sites of oestradiol action in the defeminisation of luteinising hormone surge and lordosis capacity in the rat

Female rats show a gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) surge in the presence of a preovulatory level of oestrogen, whereas males do not because of brain defeminisation during the developmental period by perinatal oestrogen converted from androgen. The present study aimed to identify the site(s) of oestrogen action and the critical period for defeminising the mechanism regulating the GnRH/LH surge. Animals given perinatal treatments, such as steroidal manipulations, brain local implantation of oestradiol (E(2) ) or administration of an NMDA antagonist, were examined for their ability to show an E(2) -induced LH surge at adulthood. Lordosis behaviour was examined to compare the mechanisms defeminising the GnRH/LH surge and sexual behaviour. A single s.c. oestradiol-benzoate administration on either the day before birth (E21), the day of birth (D0) or day 5 (D5) postpartum completely abolished the E(2) -induced LH surge at adulthood in female rats, although the same treatment did not inhibit lordosis. Perinatal castration on E21 or D0 partially rescued the E2-induced LH surge in genetically male rats, whereas castration from E21 to D5 totally rescued lordosis. Neonatal E(2) implantation in the anterior hypothalamus including the anteroventral periventricular nucleus (AVPV)/preoptic area (POA) abolished the E(2) -induced LH surge in female rats, whereas E(2) implantation in the mid and posterior hypothalamic regions had no inhibitory effect on the LH surge. Lordosis was not affected by neonatal E(2) implantation in any hypothalamic regions. In male rats, neonatal NMDA antagonist treatment rescued lordosis but not the LH surge. Taken together, these results suggest that an anterior hypothalamic region such as the AVPV/POA region is a perinatal site of oestrogen action where the GnRH/LH regulating system is defeminised to abolish the oestrogen-induced surge. The mechanism for defeminisation of the GnRH/LH surge system might be different from that of sexual behaviour, in terms of the site(s) of oestrogen action and critical period, as well as the neurotransmitter system involved.

Exogenous androgen during development alters adult partner preference and mating behavior in gonadally intact male rats

In the rat, neonatal administration of testosterone propionate to a castrated male causes masculinization of behavior. However, if an intact male is treated neonatally with testosterone (hyper-androgen condition), male sexual behavior in adulthood is disrupted. There is a possibility that the hyper-androgen treatment is suppressing male sexual behavior by altering the male's partner preference and thereby reducing his motivation to approach the female. If so, this would suggest that exposure to supra-physiological levels of androgen during development may result in the development of male-oriented partner preference in the male. To test this idea, male rats were treated either postnatally or prenatally with testosterone, and partner preference and sexual behavior were examined in adulthood. The principal finding of this study was that increased levels of testosterone during early postnatal life, but not prenatal, decreased male sexual behavior and increased the amount of time a male spent with a stimulus male, without affecting the amount of time spent with a stimulus female during partner preference tests. Thus, the reduction in male sexual behavior produced by early exposure to high levels of testosterone is not likely due to a reduction in the male's motivation to approach a receptive female.

Estrogen treatment during development alters adult partner preference and reproductive behavior in female laboratory rats

There is broad acceptance for the idea that during development estradiol 'organizes' many aspects of reproductive behavior including partner preferences in the laboratory rat. With respect to partner preference, this idea is drawn from studies where estrogen action was in someway blocked, either through aromatase or estrogen receptor inhibition, during development in male rats. The lack of estrogens neonatally results in a decrease in the male rat's preference for females. In this study, the effect of early postnatal estradiol treatment on the partner preferences of female rats was examined as a further test of the hypothesis that male-typical partner preference is dependent upon early exposure to estrogens. Our principal finding was that increased postnatal estradiol exposure during development affected partner preference in the expected direction, and this effect was seen under several adult hormonal and behavioral testing conditions. Female rats that received exogenous estradiol during development spent more time with an estrous female and less time with a sexually active male than did cholesterol treated females. The estradiol treatment also disrupted normal female sexual behavior, receptivity, and proceptivity.

Raloxifene effects upon the neuronal system controlling sexual receptivity in female rats

Selective estrogen receptor modulators (SERMs) constitute a new family of drugs with growing interest in the management of estrogen-associated pathology. Raloxifene is a SERM that mimics estrogen action on bone and blood lipid concentration but whether it acts as estrogen in the central nervous system remains to be fully established. In the present communication, we aimed at evaluating the estrogenic/antiestrogenic effects of raloxifene upon organization and activation of sexual receptivity, an estrogen-dependent event, in female rats. To this end, the effects of raloxifene, administered during the neonatal period, were compared with those of estrogen in terms of disruption of sexual receptivity in estrogen-progesterone-primed ovariectomized (OVX) female rats. In addition, the ability of raloxifene to induce sexual receptivity in progesterone-primed OVX females was analyzed. Similarly, the effects of the combined administration of estrogen and raloxifene were studied. Our results suggest that raloxifene does not act as estrogen upon the organization of the neuronal system involved in the control of sexual receptivity in female rats and exerted an antiestrogenic action in adult OVX estrogen-primed female rats.

Prepubertal testosterone treatment of female rats: defeminization of behavioral and endocrine function in adulthood

This study assessed the capacity of testosterone (T) administered well after the neonatal "critical" period to permanently sexually differentiate reproductive function. Females received T filled or empty Silastic capsules during days 15-30 of age and vaginal cyclicity, ovarian weight and appearance, lordosis and proceptive behaviors, mounting behavior, and the gonadotropin response to estrogen and progesterone were measured in adulthood. T-treated females (plasma levels of 0.66 ng T/ml) showed constant vaginal estrus from the day of vaginal opening and small, polyfollicular ovaries. Proceptive behaviors were dramatically reduced whether or not the ovaries were present after day 15 of age, but lordosis behavior was not affected. Exposure to T for 5-6 h was ineffective. Compared to controls, T-treated females had dramatically reduced plasma FSH and LH surges. No effects were observed on mounting behavior, phallus size, or body weights. These results suggest that androgen at approximately male levels can act on neural substrates well beyond the neonatal period to permanently defeminize endocrine and behavioral function in the female rat.

Estrogens before birth and development of sex-related reproductive traits in the female guinea pig

Influences of estrogens on the differentiation of psychosexual traits in the female guinea pig were studied. Pregnant animals were injected intramuscularly with either 1, 2, or 3.3 micrograms estradiol benzoate (EB) or with 1 or 3 micrograms diethylstilbestrol dipropionate (DESDP). Injections were started on the 29th day of pregnancy, given daily for 6 days, and continued every other day until parturition. Female offspring were evaluated for onset of puberty, ovarian function, and lordosis and mounting behavior in adulthood. Prenatal treatment with 3 micrograms DESDP caused delayed puberty, impaired ovarian function, reduced responsiveness of lordosis to EB and P in adulthood (defeminization), augmented mounting in the absence of hormones (masculinization), and reduced responsiveness of mounting to exogenous EB and P in adulthood (defeminization). Prenatal treatment with 1 microgram DESDP produced similar but less pronounced effects. Prenatal treatment with 3.3 micrograms EB also caused a delay in puberty. However, responsiveness of lordosis to EB and P in adulthood was enhanced by treatment with either 1 or 3.3 micrograms EB prenatally. Further, neither mounting in the absence of hormones nor mounting in response to EB and P in adulthood were affected in any measurable way by any prenatal treatment with EB. These results show that estrogens can have masculinizing and defeminizing effects on sexually dimorphic reproductive traits in guinea pigs. The failure of EB to duplicate or parallel the effects of DESDP is not completely understood at this time, but it may indicate that less of the active substance reaches the target tissues following maternal and placental metabolism of EB than of DESDP.

Angiotensin-estrogen central interaction: localization and mechanism

Intracerebroventricular (ICVT) administration of estradiol benzoate (EB) to ovariectomized female rats decreased drinking and pressor responses to central injections of angiotensin II (AII). Estrogen treatment does not have this effect in male rats. As EB given ICVT reaches many brain areas, the site of action of EB was localized using crystalline implants of EB in the medial preoptic area or the ventromedial nucleus of the hypothalamus. These areas were chosen as they have a high density of estrogen receptors. Only medial preoptic area application of estrogen decreased angiotensin II-induced drinking. Angiotensin receptor binding was examined in homogenates from different brain regions to determine if the mechanism through which estrogen decreases central responses to AII involves altered receptor function. Systemic EB did not affect AII receptor binding in several brain regions but binding was decreased in homogenates from the preoptic area and septum-thalamus blocks which encompassed structures (median preoptic nucleus, organum vasculosum, and subfornical organ) implicated in central actions of AII. The sex specificity of the effect of estrogen was dependent on sexual differentiation of the brain. Manipulation of the neonatal hormone environment, which alters this brain differentiation, also altered the characteristic responses of the two sexes to estrogen. Neonatal androgenization of females, which causes masculinization and defeminization, resulted in animals which as adults no longer responded to EB with decreased drinking. On the other hand, preventing the development of a male brain by neonatal castration produced animals which as adults tended to decrease their drinking following estrogen. In summary, this study found that EB acts in the preoptic area to depress AII-induced responses by a site specific modulation of central AII receptors. Alteration of early brain development changed the responses of the two sexes to estrogen, perhaps by altering sexual differentiation of the preoptic area.

Injection of diethylstilbestrol on the first day of incubation affects morphology of sex glands and reproductive behavior of Japanese quail

Japanese quail eggs were injected with DES (0.9-1,000 micrograms) dissolved in 50-microliter of corn oil on day 1 of incubation. Higher doses of DES (250-1,000 micrograms) reduced hatchability to 37-33% compared to 61% for corn oil-injected controls. Lower doses of DES (0.9-125 micrograms) had no effects on hatchability. In a second study, eggs were injected with 0.9 or 1.9 micrograms of DES and the survivors were assessed up to 12 weeks posthatch. DES did not affect hatchability, but did increase mortality during the first 4 weeks posthatch. Females were affected more than males. At 10 days of age, open-field activity of some birds was examined. The acquisition and reversal of a visual discrimination task was studied at 6 weeks of age. DES had no effect on these measurements. Ten females from each group were chosen randomly to determine egg production over a single 28-day period beginning at 6 weeks of age. Exposure to DES blocked egg production in these birds. The oviduct weights of 12-week-old females were decreased by 50%, but ovarian weights were not affected. Testicular weights were not affected. In a third study reproductive behaviors and social-dominance behaviors of males were markedly attenuated in birds exposed to 0.48 or 1.9 micrograms DES in ovo.

Effects of neonatal estrogen treatment of female guinea pigs on mounting behavior in adulthood

Female guinea pigs were treated with 200 micrograms estradiol dipropionate (ED) daily from Days 1 to 15 of life. As adults they all ovulated. After gonadectomy and estrogen-progesterone treatment in adulthood, lordosis behavior was evident and showed no decrements (compared with neonatally oil-treated controls) except for a marginal decrease in maximum duration scores. However, testosterone propionate treatment in adulthood significantly increased mounting behavior in neonatally ED-treated females compared to the controls. On the other hand, estrogen-progesterone treatment in adulthood stimulated mounting in the neonatally oil-treated group, but not in the neonatally estrogenized group. The results suggest that at least some aspects of steroid-dependent behaviors can be permanently influenced by estrogen treatment after the presumed prenatal critical period for sexual differentiation has been completed in guinea pigs.

Female sexual behavior in male rats: effect of hour of castration at birth

In the male rat, a dramatic increase in serum testosterone occurs during the first four hours of postnatal life. The experiments sought to determine whether such an increase would participate directly on the defeminization process. Newborn male rats were castrated either at 0 hr in utero (literally at the moment of birth) or at 6 or 12 hrs after birth. Some males were castrated at 0 hr in utero and injected at the time of surgery with 1 or 5 micrograms testosterone propionate (TP). At about 90 days of age, each animal was injected with estrogen and progesterone and tested for female sex behavior. Males castrated at 0 hr in utero displayed typical female sex behavior. Males castrated at 6 or 12 hrs after birth were less receptive than males castrated at 0 hr. Males castrated at 0 hr and injected with testosterone at this time almost never showed lordosis as adults after treatment with ovarian hormones. These results are consistent with the idea that the rapid elevation in serum testosterone which occurs shortly after birth suppresses the development of sexual behavior sensitivity to ovarian hormonal stimulation.

Differentiation of coital behavior in mammals: a comparative analysis

The evidence reviewed suggests that in all mammalian species the adult male's ability to display masculine coital behavior depends in part on exposure of the developing brain to testicular testosterone or its metabolites. In many mammals, particularly rodents, ruminants, and some carnivores, perinatal exposure to androgen also causes behavioral defeminization, i.e., reduced capacity to display typically feminine coital behavior in response to gonadal hormones in adulthood. The data reviewed suggest that no such process occurs in certain other mammalian species, including ferret, rhesus monkey, marmoset, and man. Testicular androgen may cause behavioral defeminization only in those species in which expression of feminine sexual behavior normally depends on the neural action of progesterone, acting synergistically with estradiol; new data support this claim in the ferret. The possible contribution of estrogenic and 5 alpha-reduced androgenic metabolites of testosterone to the occurrence of behavioral masculinization and defeminization is considered in those mammalian species for which data are available.