Novel male exposure reduces uterine e-cadherin, increases uterine luminal area, and diminishes progesterone levels while disrupting blastocyst implantation in inseminated mice

Data quality and the comparative method: the case of pregnancy failure in rodents

In mammalian species where infanticide by males is likely, females exhibit counterstrategies to prevent or mitigate the costs of infanticide. One putative mitigation strategy is the “Bruce effect,” in which pregnant or inseminated females exposed to an unfamiliar male experience pregnancy block or failure. Females then mate with the new male, thus shifting investment from a “doomed” pregnancy to a more fruitful one. However, the Bruce effect may be an adaptive response to other factors besides infanticide. For example, if paternal care is necessary for offspring survival, and an unfamiliar male replacing the original mate is unlikely to provide such care to offspring of a litter it did not sire, then a female may terminate a pregnancy to initiate a new one. The infanticide and paternal care hypotheses have not been rigorously tested because comparative data on the Bruce effect across mammals are scarce. We compiled data on the Bruce effect, infanticide, and paternal care from one particularly rich source of information, rodents, but found the data set to be less rich than expected. The Bruce effect, infanticide, and paternal care were common among rodent species, but we found no clear relationship among the traits. However, this was likely due to 1) a bias toward positive results, 2) missing data, and 3) a reliance on studies of captive animals. These are common problems in comparative research, and we outline standards that should be implemented to successfully answer questions of importance in the field.

Bivariate genome-wide association analyses identified genetic pleiotropic effects for bone mineral density and alcohol drinking in Caucasians

Several studies indicated bone mineral density (BMD) and alcohol intake might share common genetic factors. The study aimed to explore potential SNPs/genes related to both phenotypes in US Caucasians at the genome-wide level. A bivariate genome-wide association study (GWAS) was performed in 2069 unrelated participants. Regular drinking was graded as 1, 2, 3, 4, 5, or 6, representing drinking alcohol never, less than once, once or twice, three to six times, seven to ten times, or more than ten times per week respectively. Hip, spine, and whole body BMDs were measured. The bivariate GWAS was conducted on the basis of a bivariate linear regression model. Sex-stratified association analyses were performed in the male and female subgroups. In males, the most significant association signal was detected in SNP rs685395 in DYNC2H1 with bivariate spine BMD and alcohol drinking (P = 1.94 × 10^-8). SNP rs685395 and five other SNPs, rs657752, rs614902, rs682851, rs626330, and rs689295, located in the same haplotype block in DYNC2H1 were the top ten most significant SNPs in the bivariate GWAS in males. Additionally, two SNPs in GRIK4 in males and three SNPs in OPRM1 in females were suggestively associated with BMDs (of the hip, spine, and whole body) and alcohol drinking. Nine SNPs in IL1RN were only suggestively associated with female whole body BMD and alcohol drinking. Our study indicated that DYNC2H1 may contribute to the genetic mechanisms of both spine BMD and alcohol drinking in male Caucasians. Moreover, our study suggested potential pleiotropic roles of OPRM1 and IL1RN in females and GRIK4 in males underlying variation of both BMD and alcohol drinking.

Concurrent administration of diethylhexyl phthalate reduces the threshold dose at which bisphenol A disrupts blastocyst implantation and cadherins in mice

Many people are repeatedly exposed to both bisphenol A (BPA) and diethylhexyl phthalate (DEHP), but there has been little research concerning their effects in combination. Both can disrupt blastocyst implantation in inseminated females, albeit at high doses. We exposed mice on gestation days (GD) 1-4 to combinations of BPA and DEHP in doses below the threshold necessary to disrupt implantation on their own. On GD 6, there were fewer normally-developed implantation sites and more underdeveloped implantation sites in females given the combined subthreshold doses. Uterine epithelial cadherin (e-cadherin), a protein that assists in blastocyst adhesion to the uterine epithelium, was significantly reduced by these combined doses, but not by the individual doses. A similar trend was seen in integrin αvβ3, another uterine adhesion molecule. Cadherin-11 was disrupted by BPA but not DEHP. These data are consistent with competition of BPA and DEHP for conjugating enzymes.

Absorption and distribution of estradiol from male seminal emissions during mating

Estradiol-17β (E₂) plays critical roles in female maturation, sexual receptivity, ovulation and fertility. In many mammals, contact with males can similarly affect these female parameters, whereas male excretions contain significant quantities of E₂ We administered radiolabeled estradiol ([³H]E₂) to male mice in doses representing a small fraction of their endogenous E₂ These males were paired with sexually receptive females, and radioactivity was traced into the females' systems. In Experiment 1, males were given [³H]E₂ at 24 and 1 h before mating. Male-to-female [³H]E₂ transfer intensified with increasing numbers of intromissions and spiked in the uterus after insemination. In Experiment 2, sexually experienced young males received [³H]E₂ at 72 and 24 h before mating, and all mated to ejaculation. The copulatory plug deposited in the female reproductive tract contained substantial levels of radioactivity. The uteri, other tissues and blood serum of females displayed radioactivity indicative of E₂ transfer. In Experiment 3, radioactivity was observed 3 and 18 h after insemination in the females' uteri and other tissues, including parts of the brain. In Experiment 4, we observed substantial levels of radioactivity in semen as well as the copulatory plugs retrieved from the females after mating. Transferred E₂ could directly affect abundant estrogen receptors in the female reproductive tract without potential metabolism by the liver. Sexually transferred E₂ may facilitate uterine preparation for blastocyst implantation. These data converge with several lines of evidence indicating that male-sourced E₂ can transfer to proximate females in bioactive form, contributing to various mammalian pheromonal effects.

Stress lowers the threshold dose at which bisphenol A disrupts blastocyst implantation, in conjunction with decreased uterine closure and e-cadherin

Exposure to stress can disrupt blastocyst implantation in inseminated female mice, and evidence implicates elevation of the female's estrogen:progesterone ratio. Exposure to the xenoestrogen, bisphenol A (BPA) can also disrupt implantation. Undisturbed control female CF-1 mice were compared to other females that were exposed to predators (rats) across a wire-mesh grid during gestation days (GD) 1-4, a procedure that elevates corticosterone but does not on its own disrupt implantation in this genetic strain. They were concurrently exposed to varied doses of BPA that on their own were below the threshold dose sufficient to disrupt implantation. On GD 6, we measured the number of intrauterine implantation sites and extracted their uteri, which subsequently were stained and analyzed for uterine luminal area and epithelial cadherin (e-cadherin), a molecule that causes uterine closure and adhesion of blastocysts to the uterine epithelium. The combination of rat-exposure stress and BPA significantly disrupted implantation and increased uterine luminal area, whereas either manipulation on its own did not. E-cadherin was significantly reduced by exposure to BPA, positively correlated with the number of implantation sites, and inversely correlated with luminal area. BPA exposure was also associated with nonmonotonic perturbation of urinary corticosterone concentrations and increased urinary estradiol concentrations on GD 6. These data are consistent with a potential summation of stress-induced estrogen and xenoestrogen activity.

Sex steroids as pheromones in mammals: the exceptional role of estradiol

This article is part of a Special Issue (Chemosignals and Reproduction). Whether from endogenous or exogenous sources, 17β-estradiol (E2) has very powerful influences over mammalian female reproductive physiology and behavior. Given its highly lipophilic nature and low molecular mass, E2 readily enters excretions and can be absorbed from exogenous sources via nasal, cutaneous, and other modes of exposure. Indeed, systemic injection of tritiated estradiol ((3)H-E2) into a male mouse or bat has been shown to produce significant levels of radioactivity in the reproductive tissues and brain of cohabiting female conspecifics. Bioactive E2 and other steroids are naturally found in male mouse urine and other excretions, and males actively direct their urine at proximate females. Very low doses of E2 can mimic the Bruce effect (disruption of peri-implantation pregnancy by novel males), the Vandenbergh effect (early reproductive maturation induced by novel males), and male-induced estrus and ovulation. Males' capacities to induce the Bruce and Vandenbergh effects can both be diminished by manipulations that reduce their urinary E2. Uterine dynamics during the Bruce and Vandenbergh effects are consistent with the actions of E2. Collectively, these data demonstrate a critical role of male-sourced E2 in these major mammalian pheromonal effects.

Estradiol transfer from male big brown bats (Eptesicus fuscus) to the reproductive and brain tissues of cohabiting females, and its action as a pheromone

The powerful estrogen, 17β-estradiol, has been found to pass from male excretions to the reproductive organs, brain, and other tissues of cohabiting females in laboratory mice. The current studies were designed to examine whether this phenomenon also occurs in big brown bats (Eptesicus fuscus), a mammal appropriate for testing cross-species generality because of its phylogenetic distance from mice. When tritiated estradiol ((3)H-E2) was administered directly on the nasal area of adult female bats, radioactivity was reliably observed in the uterus and ovaries, and also in the brain and other tissues. When (3)H-E2 was applied to the skin, radioactivity was observed in reproductive and other peripheral tissues. We injected male bats with minute quantities of (3)H-E2 and housed each of them directly with groups of adult females for 48h. We then measured radioactivity in male and female bat tissues. In each of several replications of one male housed with three females, radioactivity was reliably observed in the uterus of all females, and in many other tissues in almost every female. Measurement in the organs of males directly exposed to (3)H-E2 showed high levels of radioactivity in the testes and especially the epididymides. These data indicate that estradiol is transferred from males to females, likely via absorptions from males' excretions and potentially also via intravaginal exposure during mating. Given the potency of estradiol in regulating female reproductive physiology and behavior, our data strongly suggest the potential for pheromonal action whereby male mammals induce sexual receptivity and ovulation in females.