Defeminization of brain functions by a single injection of estrogen receptor α or β agonist in neonatal female rats

Neonatal exposure to chlordecone alters female social behaviors and central estrogen alpha receptor expression in socially monogamous mandarin voles

Chlordecone (CD) is one of the common persistent organic pollutants in nature and has a profound impact on the environment and on public health. Accumulating evidence has demonstrated that neonatal exposure of CD influences adult physiology and behavior due to its estrogenic properties. Using socially monogamous mandarin voles as an experimental animal model, the present study aimed to evaluate the impact of neonatal exposure to CD on female social behaviors and central estrogen receptor alpha (ERα) expression in adulthood. After receiving a single subcutaneous injection with sesame seed oil (female control group), 17 beta-estradiol (E₂ group), or CD group on postnatal Day 1, the social behaviors of adult animals and ERα expression in specific brain regions were assessed. The data indicated that CD or E₂-treated female animals displayed increased affiliative behaviors and decreased aggressive behaviors with regard to the unfamiliar females in the social interaction test. In addition, CD or E₂-treated female voles exhibited significant preferences to females over males in the sexual preference test. Moreover, CD-treated female animals exhibited higher levels of ERα expression in the bed nucleus of the stria terminalis, the central amygdala, the medial amygdala and the medial preoptic area compared with those of the control voles. The results suggested that neonatal exposure to CD may masculinize female social behaviors, possibly via CD-induced changes in the ERα expression of relevant brain regions.

Estrogenic action by tris(2,6-dimethylphenyl) phosphate impairs the development of female reproductive functions

Developmental exposure to environmental chemicals with estrogen-like activity is suspected to permanently impair women's health. In this study, a mouse model was used to evaluate whether tris(2,6-dimethylphenyl) phosphate (TDMPP), a chemical with a putative estrogen-like action, impairs sexual differentiation of the brain. Either TDMPP and 17β-estradiol (E₂) as positive controls or sesame oil as a negative control were administered subcutaneously to dams from gestational day (GD) 14 to parturition, and to pups from postnatal day (PND) 0 to 9. Precocious puberty, irregular estrous cycles, and a lowered lordosis response were found in the TDMPP- and E₂-treated groups. A certain amount of TDMPP and its metabolites in the perinatal brain and the masculinization of sexual dimorphic nuclei in the hypothalamus of female mice after treatment were also detected. The experimental evidence demonstrates that TDMPP directly enters the fetal and neonatal brain, thereby inducing changes of sex-related brain structures and impairing female reproductive functions.

Neonatal septal lesions prevent behavioral defeminization caused by neonatal treatment with estradiol in female rats

Male rats rarely show lordosis, a female sexual behavior, because of strong inhibition of the behavior in the lateral septum. Because neonatal treatment with estradiol (E₂) in female rats decreases lordosis, it is believed that the lateral septum is a target of E₂ action to defeminize or masculinize the lordosis-inhibiting system. Here, we tested the hypothesis that disruption of the lateral septum before E₂ treatment prevents the effect of neonatal E₂ on lordosis. Female rats that underwent radiofrequency-induced septal lesions or sham operation on postnatal day 4 (PD4, day of birth = PD1) were subcutaneously injected with E₂ or sesame oil vehicle alone on PD5. Vaginal opening and smears were checked. After sexual maturation, lordosis tests were performed. The effects of neonatal septal lesions on lordosis in male rats were also observed. Sham-operated and E₂-treated female rats showed a reduction in lordosis and irregular estrous cycles. Conversely, septal lesioned and E₂-treated females exhibited higher levels of lordosis, although their estrous cycles were irregular. These results suggest that neonatal septal lesions prevent females from being behaviorally defeminized by neonatal E₂. Additionally, neonatally septal lesioned males displayed higher levels of lordosis than sham-operated males. These results suggest that E₂, which is produced by the aromatization of testicular testosterone in the neonatal period, acts on the lateral septum to organize the lordosis-inhibiting system.

The role of estrogen receptor subtypes for induction of delayed effects on the estrous cycle and female reproductive organs in rats

It has been reported that neonatal exposure to estrogens at relatively low doses can induce early onset anovulation as a delayed effect in female rats. Dysfunction of kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) was proposed to be a trigger for this effect. To determine the roles of estrogen receptor (ER) subtypes in the induction of delayed effects, we conducted a series of experiments using Donryu rats to examine whether neonatal injection of an ERα agonist (PPT), an ERβ agonist (DPN) or an ERα antagonist (ICI) could induce delayed effects. Also, involvement of the kisspeptin neurons in the AVPV for induction of delayed effect by PPT and DPN was investigated. We observed that neonatal exposure to PPT, DPN and ICI induced the early onset of abnormal estrous cyclicity after sexual maturation, suggesting that the compounds capable of inducing delayed effects are not limited to ERα agonists. On the other hand, the data suggested the possibility that DPN and ICI functioned partially as ERα agonists in the neonatal brain. Regardless of the agents used, there is a possibility that dysfunction of kisspeptin neurons in the AVPV might contribute to induction of early onset anovulation.

Neuroanatomy and sex differences of the lordosis-inhibiting system in the lateral septum

Female sexual behavior in rodents, termed lordosis, is controlled by facilitatory and inhibitory systems in the brain. It has been well demonstrated that a neural pathway from the ventromedial hypothalamic nucleus (VMN) to the midbrain central gray (MCG) is essential for facilitatory regulation of lordosis. The neural pathway from the arcuate nucleus to the VMN, via the medial preoptic nucleus, in female rats mediates transient suppression of lordosis, until female sexual receptivity is induced. In addition to this pathway, other regions are involved in inhibitory regulation of lordosis in female rats. The lordosis-inhibiting systems exist not only in the female brain but also in the male brain. The systems contribute to suppression of heterotypical sexual behavior in male rats, although they have the potential ability to display lordosis. The lateral septum (LS) exerts an inhibitory influence on lordosis in both female and male rats. This review focuses on the neuroanatomy and sex differences of the lordosis-inhibiting system in the LS. The LS functionally and anatomically links to the MCG to exert suppression of lordosis. Neurons of the intermediate part of the LS (LSi) serve as lordosis-inhibiting neurons and project axons to the MCG. The LSi-MCG neural connection is sexually dimorphic, and formation of the male-like LSi-MCG neural connection is affected by aromatized testosterone originating from the testes in the postnatal period. The sexually dimorphic LSi-MCG neural connection may reflect the morphological basis of sex differences in the inhibitory regulation of lordosis in rats.