Atrazine disrupts the hypothalamic control of pituitary-ovarian function

The chloro-S-triazine herbicides (i.e., atrazine, simazine, cyanazine) constitute the largest group of herbicides sold in the United States. Despite their extensive usage, relatively little is known about the possible human-health effects and mechanism(s) of action of these compounds. Previous studies in our laboratory have shown that the chlorotriazines disrupt the hormonal control of ovarian cycles. Results from these studies led us to hypothesize that these herbicides disrupt endocrine function primarily through their action on the central nervous system. To evaluate this hypothesis, we examined the estrogen-induced surges of luteinizing hormone (LH) and prolactin in ovariectomized Sprague-Dawley (SD) and Long-Evans hooded (LE) rats treated with atrazine (50-300 mg/kg/day, by gavage) for 1, 3, or 21 days. One dose of atrazine (300 mg/kg) suppressed the LH and prolactin surge in ovariectomized LE, but not SD female rats. Atrazine (300 mg/kg) administered to intact LE females on the day of vaginal proestrus was without effect on ovulation but did induce a pseudopregnancy in 7 of 9 females. Three daily doses of atrazine suppressed the estrogen-induced LH and prolactin surges in ovariectomized LE females in a dose-dependent manner, but this same treatment was without effect on serum LH and prolactin in SD females. The estrogen-induced surges of both pituitary hormones were suppressed by atrazine (75-300 mg/kg/day) in a dose-dependent manner in females of both strains evaluated after 21 days of treatment. Three experiments were then performed to determine whether the brain, pituitary, or both organs were the target sites for the chlorotriazines. These included examination of the ability of (1) the pituitary lactotrophs to secrete prolactin, using hypophyosectomized females bearing pituitary autotransplants (ectopic pituitaries); (2) the synthetic gonadotropin-releasing hormone (GnRH) to induce LH secretion in females treated with high concentrations of atrazine for 3 days; and (3) atrazine (administered in vivo or in vitro) to suppress LH and prolactin secretion from pituitaries, using a flow-through perifusion procedure. In conclusion, the results of these studies demonstrate that atrazine alters LH and prolactin serum levels in the LE and SD female rats by altering the hypothalamic control of these hormones. In this regard, the LE female appeared to be more sensitive to the hormone suppressive effects of atrazine, as indicated by the decreases observed on treatment-day 3. These experiments support the hypothesis that the effect of atrazine on LH and prolactin secretion is mediated via a hypothalamic site of action.

The effects of triclosan on puberty and thyroid hormones in male Wistar rats

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is a potent antibacterial and antifungal compound that is widely used in personal care products, plastics, and fabrics. Recently triclosan has been shown to alter endocrine function in a variety of species. The purpose of this study was to determine effects of triclosan on pubertal development and thyroid hormone concentrations in the male rat. Weanling rats were exposed to 0, 3, 30, 100, 200, or 300 mg/kg of triclosan by oral gavage from postnatal day (PND) 23 to 53. Preputial separation (PPS) was examined beginning on PND 33. Rats were killed on PND 53, organ weights were recorded and serum was collected for subsequent analysis. Triclosan did not affect growth or the onset of PPS. Serum testosterone was significantly decreased at 200 mg/kg, however no effects were observed on androgen-dependent reproductive tissue weights. Triclosan significantly decreased total serum thyroxine (T4) in a dose-dependent manner at 30 mg/kg and higher (no observed effect level of 3 mg/kg). Triiodothyronine (T3) was significantly decreased only at 200 mg/kg, but thyroid stimulating hormone was not statistically different at any dose. Liver weights were significantly increased at 100 mg/kg triclosan and above suggesting that the induction of hepatic enzymes may have contributed to the altered T4 and T3 concentrations, but it does not appear to correlate with the T4 dose-response. This study demonstrates that triclosan exposure does not alter androgen-dependent tissue weights or onset of PPS; however, triclosan exposure significantly impacts thyroid hormone concentrations in the male juvenile rat.

Assessment of DE-71, a Commercial Polybrominated Diphenyl Ether (PBDE) Mixture, in the EDSP Male and Female Pubertal Protocols

DE-71, a commercial mixture, was used to test the sensitivity of the female and male pubertal protocol to detect thyroid active chemicals. These protocols are being evaluated for the U.S. EPA's Endocrine Disruptor Screening Program as part of a Tier I Screening Battery. To examine the ability of these protocols to screen for chemicals that induce the clearance of thyroid hormone, we examined male and female Wistar rats following DE-71 exposure. Rats were gavaged daily with 0, 3, 30, or 60 mg/kg DE in corn oil from postnatal day (PND) 23-53 in the male or PND 22-41 in the female. The temporal effects of DE-71 on liver enzymes and thyroid hormones were measured in another group of males and females following only 5 days of dosing (PND 21 to 26 in females and PND 23 to 28 in males). Serum T4 was significantly decreased at 30 and 60 mg/kg following the 5-day exposures and in the 21-day exposed females. Doses of 3, 30, and 60 mg/kg decreased T4 in 31-day exposed males. Serum T3 was decreased and TSH elevated by 30 and 60 mg/kg in the 31-day exposed males only. Decreased colloid area and increased follicular cell heights (indicative of the hypothyroid state) were observed in thyroids of the 60 mg/kg groups of 20- and 31-day exposed female and males. Increased liver-to-body weight ratios coincided with a significant induction of uridinediphosphate-glucuronosyltransferase (UDGPT; two to four-fold), and ethoxy- and pentoxy-resorufin-O-deethylase (EROD and PROD) at the two highest doses in all exposures. Of the androgen dependent tissues in the 31-day exposed males, seminal vesicle (SV) and ventral prostate (VP) weights were reduced at 60 mg/kg, while testes and epididymal weights were not affected. Preputial separation (PPS) was also significantly delayed by doses of 30 and 60 mg/kg. In the female, the 60 mg/kg dose also caused a significant delay in the age of vaginal opening. Based upon the thyroid hormone response data, this study provides evidence that the 31-day alternative Tier 1 male protocol is a more sensitive test protocol than the 5-day or female pubertal protocol for thyrotoxic agents that act via up-regulation of hepatic metabolism. This apparent greater sensitivity may be due a greater body burden attained following the longer dosing regimen as compared with that of the female protocol, or to gender specific differences in thyroid hormone metabolism. Also, the delay in PPS and reduction in SV and VP weights may indicate a modification or inhibition of endogenous androgenic stimulation directly by DE-71 or a secondary effect that occurs in response to a DE-induced change in thyroid hormones.

The effect of atrazine on puberty in male wistar rats: an evaluation in the protocol for the assessment of pubertal development and thyroid function

Since atrazine (ATR), a chlorotriazine herbicide, has been shown previously to alter the secretion of luteinizing hormone (LH) and prolactin (PRL) through a direct effect on the central nervous system (CNS), we hypothesized that exposure to ATR in the EDSTAC male pubertal protocol (juvenile to peripubertal) would alter the development of the male rat reproductive system. We dosed intact male Wistar rats from postnatal day (PND) 23 to 53 and examined several reproductive endpoints. ATR (0, 12.5, 25, 50, 100, 150, or 200 mg/kg) was administered by gavage and an additional pair-fed group was added to compare the effects of any decreased food consumption in the high dose group. Preputial separation (PPS) was significantly delayed in the 12.5, 50, 100, 150, and 200 mg/kg ATR dose groups. PPS was also delayed in the pair-fed group, although significantly less than in the high dose-ATR group. The males were killed on PND 53 or 54, and pituitary, thyroid, testes, epididymides, seminal vesicles, and ventral and lateral prostates were removed. ATR (50 to 200 mg/kg) treatment resulted in a significant reduction in ventral prostate weights, as did the reduced food consumption of the pair-fed group. Testes weights were unaffected by atrazine treatment. Seminal vesicle and epididymal weights were decreased in the high dose-ATR group and the control pair-fed group. However, the difference in epididymal weights was no longer significantly different when body weight was entered as a covariable. Intratesticular testosterone was significantly decreased in the high dose-ATR group on PND 45, but apparent decreases in serum testosterone were not statistically significantly on PND 53. There was a trend for a decrease in luteinizing hormone (LH) as the dose of ATR increased; however, dose group mean LH was not different from controls. Due to the variability of serum prolactin concentrations on PND 53, no significant difference was identified. Although prolactin is involved in the maintenance of LH receptors prior to puberty, we observed no difference in LH receptor number at PND 45 or 53. Serum estrone and estradiol showed dose-related increases that were significant only in the 200 mg/kg-ATR group. No differences were observed in thyroid stimulating hormone (TSH) and thyroxine (T4) between the ATR groups and the control; however triiodothyronine (T3) was elevated in the high dose-ATR group. No differences in hormone levels were observed in the pair-fed animals. These results indicate that ATR delays puberty in the male rat and its mode of action appears to be altering the secretion of steroids and having subsequent effects on the development of the reproductive tract, which appear to be due to ATR's effects on the CNS. Thus, ATR tested positive in the pubertal male screen that the Endocrine-Disrupter Screening and Testing Advisory Committee (EDSTAC) is considering as an optional screen for endocrine disrupters.

In vivo and in vitro anti-androgenic effects of DE-71, a commercial polybrominated diphenyl ether (PBDE) mixture

PBDEs have been synthesized in large quantities as flame retardants for commercial products, such as electronic equipment and textiles. The rising in levels of PBDEs in tissues in wildlife species and in human milk and plasma samples over the past several years have raised concerns about possible health effects. Recently, we showed that the PBDE mixture, DE-71, delayed puberty and suppressed the growth of androgen-dependent tissues in male Wistar rat following a peri-pubertal exposure. These effects suggested that DE-71 may be either inducing steroid hormone metabolism or acting as an androgen receptor (AR) antagonist. To elucidate the potential anti-androgenic effects of this mixture, we evaluated DE-71 in several in vivo assays, which are responsive to alterations in androgen activity. In a pubertal exposure study designed to further evaluate the delay in preputial separation (PPS), we observed a dose-dependent delay in PPS with 60 and 120 mg/kg/day of DE-71 (4 and 5 days) and a corresponding suppression of ventral prostate (VP) and seminal vesicle growth at both doses. Adult males exposed to 60 mg/kg DE-71 for 3 days resulted in a significant increase in luteinizing hormone and a non-significant increase in testosterone, androstenedione and estrone. DE-71 also tested positive for anti-androgenic activity in an immature rat Hershberger assay, with decreases in mean VP and seminal vesicle weight following doses of 30-240 mg/kg. DE-71 and the individual BDE congeners which comprise the mixture (BDE-47, -99, -100, -153, -154) were also evaluated in vitro. First, AR binding was evaluated in a competitive binding assay using rat VP cytosol. In addition, we evaluated gene activation in a transcriptional activation assay using the MDA-kb2 cell line which contains an endogenous human AR and a transfected luciferase reporter. DE-71 and BDE-100 (2, 4, 6-pentaBDE) both inhibited AR binding, with IC50s of approximately 5 microM. In addition, DE-71 and two of the congeners (BDE-100 and BDE-47) inhibited DHT-induced transcriptional activation. The pattern of inhibition shown in the double-reciprocal plot for BDE-100 and the linear slope replot confirmed that the in vitro mechanism is pure competitive inhibition, with a inhibition constant (Ki) of 1 microM. The delay in puberty in the male rat and decreased growth of androgen-dependent tissues observed previously following exposure to DE-71 were likely due to this inhibition of AR binding by several of the congeners which make up this mixture.

Endocrine-disrupting chemicals: prepubertal exposures and effects on sexual maturation and thyroid function in the male rat. A focus on the EDSTAC recommendations. Endocrine Disrupter Screening and Testing Advisory Committee

Puberty in mammalian species is a period of rapid interactive endocrine and morphological changes. Therefore, it is not surprising that exposure to a variety of pharmaceutical and environmental compounds has been shown to dramatically alter pubertal development. This concern was recognized by the Endocrine Disrupter Screening and Testing Advisory Committee (EDSTAC) that acknowledged the need for the development and standardization of a protocol for the assessment of the impact of endocrine-disrupting compounds (EDC) in the pubertal male and recommended inclusion of an assay of this type as an alternative test in the EDSTAC tier one screen (EPA, 98). The pubertal male protocol was designed to detect alterations of pubertal development, thyroid function, and hypothalamic-pituitary-gonadal (HPG) system peripubertal maturation. In this protocol, intact 23-day-old weanling male rats are exposed to the test substance for 30 days during which pubertal indices are measured. After necropsy, reproductive and thyroid tissues are weighed and evaluated histologically and serum taken for hormone analysis. The purpose of this review was to examine the available literature on pubertal development in the male rat and evaluate the efficacy of the proposed protocol for identifying endocrine-disrupting chemicals. The existing data indicate that this assessment of puberty in the male rat is a simple and effective method to detect the EDC activity of pesticides and toxic substances.

Developmental exposure to a commercial PBDE mixture, DE-71: neurobehavioral, hormonal, and reproductive effects

Developmental effects of polybrominated diphenyl ethers (PBDEs) have been suspected due to their structural similarities to polychlorinated biphenyls (PCBs). This study evaluated neurobehavioral, hormonal, and reproductive effects in rat offspring perinatally exposed to a widely used pentabrominated commercial mixture, DE-71. Pregnant Long-Evans rats were exposed to 0, 1.7, 10.2, or 30.6 mg/kg/day DE-71 in corn oil by oral gavage from gestational day 6 to weaning. DE-71 did not alter maternal or male offspring body weights. However, female offspring were smaller compared with controls from postnatal days (PNDs) 35-60. Although several neurobehavioral endpoints were assessed, the only statistically significant behavioral finding was a dose-by-age interaction in the number of rears in an open-field test. Developmental exposure to DE-71 caused severe hypothyroxinemia in the dams and early postnatal offspring. DE-71 also affected anogenital distance and preputial separation in male pups. Body weight gain over time, reproductive tissue weights, and serum testosterone concentrations at PND 60 were not altered. Mammary gland development of female offspring was significantly affected at PND 21. Congener-specific analysis of PBDEs indicated accumulation in all tissues examined. Highest PBDE concentrations were found in fat including milk, whereas blood had the lowest concentrations on a wet weight basis. PBDE concentrations were comparable among various brain regions. Thus, perinatal exposure to DE-71 leads to accumulation of PBDE congeners in various tissues crossing blood-placenta and blood-brain barriers, causing subtle changes in some parameters of neurobehavior and dramatic changes in circulating thyroid hormone levels, as well as changes in both male and female reproductive endpoints. Some of these effects are similar to those seen with PCBs, and the persistence of these changes requires further investigation.

Evaluating Chemicals for Thyroid Disruption: Opportunities and Challenges with in Vitro Testing and Adverse Outcome Pathway Approaches

BACKGROUND

Extensive clinical and experimental research documents the potential for chemical disruption of thyroid hormone (TH) signaling through multiple molecular targets. Perturbation of TH signaling can lead to abnormal brain development, cognitive impairments, and other adverse outcomes in humans and wildlife. To increase chemical safety screening efficiency and reduce vertebrate animal testing, in vitro assays that identify chemical interactions with molecular targets of the thyroid system have been developed and implemented.

OBJECTIVES

We present an adverse outcome pathway (AOP) network to link data derived from in vitro assays that measure chemical interactions with thyroid molecular targets to downstream events and adverse outcomes traditionally derived from in vivo testing. We examine the role of new in vitro technologies, in the context of the AOP network, in facilitating consideration of several important regulatory and biological challenges in characterizing chemicals that exert effects through a thyroid mechanism.

DISCUSSION

There is a substantial body of knowledge describing chemical effects on molecular and physiological regulation of TH signaling and associated adverse outcomes. Until recently, few alternative nonanimal assays were available to interrogate chemical effects on TH signaling. With the development of these new tools, screening large libraries of chemicals for interactions with molecular targets of the thyroid is now possible. Measuring early chemical interactions with targets in the thyroid pathway provides a means of linking adverse outcomes, which may be influenced by many biological processes, to a thyroid mechanism. However, the use of in vitro assays beyond chemical screening is complicated by continuing limits in our knowledge of TH signaling in important life stages and tissues, such as during fetal brain development. Nonetheless, the thyroid AOP network provides an ideal tool for defining causal linkages of a chemical exerting thyroid-dependent effects and identifying research needs to quantify these effects in support of regulatory decision making. https://doi.org/10.1289/EHP5297.

Atrazine and reproductive function: mode and mechanism of action studies

Atrazine, a chlorotriazine herbicide, is used to control annual grasses and broadleaf weeds. In this review, we summarize our laboratory's work evaluating the neuroendocrine toxicity of atrazine (and related chlorotriazines) from an historic perspective. We provide the rationale for our work as we have endeavored to determine: 1) the underlying reproductive changes leading to the development of mammary gland tumors in the atrazine-exposed female rat; 2) the cascade of physiological events that are responsible for these changes (i.e., the mode of action for mammary tumors); 3) the potential cellular mechanisms involving adverse effects of atrazine; and 4) the range of reproductive alterations associated with this pesticide.

Effect of atrazine on ovarian function in the rat

The effect of the chlorotriazine herbicide, atrazine, on ovarian function was studied in Long-Evans hooded (LE-hooded) and Sprague-Dawley (SD) rats. Atrazine was administered by gavage for 21 d to females displaying regular 4-d estrous cycles. In both strains, 75 mg/kg/d disrupted the 4-d ovarian cycle; however, no distinct alteration (i.e., irregular cycles but not persistent estrus or diestrus) was apparent at this dose. At 150 mg/kg/d, atrazine induced repetitive pseudopregnancies in females of both strains. The highest dose tested (300 mg/kg/d) also induced repetitive pseudopregnancies in the SD females, while the ovaries of the LE-hooded female appeared regressed and the smear cytology was indicative of the anestrous condition. Although a NOAEL was not established, the doses employed in this experiment were in excess of those used in chronic feeding studies in which an early onset of mammary gland tumors was noted. These data demonstrate that atrazine can disrupt ovarian function and bring about major changes in the endocrine profile of the female.

The effects of atrazine on female wistar rats: an evaluation of the protocol for assessing pubertal development and thyroid function

The effects of atrazine (ATR), a chlorotriazine herbicide, on the onset of puberty were evaluated in Wistar rats. Female rats were dosed by oral gavage from postnatal day(s) (PND) 22 through PND 41 with 0, 12.5, 25, 50, 100, or 200 mg ATR/kg. Vaginal opening (VO) was significantly delayed 3.4, 4.5, or greater than 6.8 days by 50, 100, and 200 mg/kg, respectively. VO had not occurred in 4 of 15 females in the 200 mg/kg group by the time of necropsies (PND 41). Body weight (bw) at necropsy was reduced in the 200 mg/kg group by 11.6%, but was not different from the control (0) in the 50 and 100 mg/kg groups. To examine the influence of reduced bw on pubertal development, a group of pair-fed controls was included whose daily food intake was dependent upon the amount consumed by their counterpart in the 200 mg/kg group. Although necropsy bw was reduced to the same extent as the ATR females, VO in the pair-fed controls was not significantly delayed. Adrenal, kidney, pituitary, ovary, and uterine weights were reduced by 200 mg/kg ATR. Serum T(3), T(4), and TSH were unaltered by ATR, which was consistent with no histopathologic/morphologic changes in the thyroid. Estrous cyclicity was monitored in a second group of females from VO to PND 149. The number of females displaying regular 4- or 5-day estrous cycles during the first 15-day interval after VO was lower in the 100 and 200 mg/kg ATR and pair-fed controls. Irregular cycles were characterized by extended periods of diestrus. By the end of the second 15-day interval (PND 57-71), no effects on estrous cyclicity were observed. These data show that ATR can delay the onset of puberty and alter estrous cyclicity in the female Wistar rat ( NOAEL of 25 mg/kg). Reduced food consumption and bw did not account for the delay in VO, because this effect was not observed in the pair-fed controls. In addition, the effect on estrous cyclicity was observed in the 100 mg/kg ATR group where no significant reduction in bw was observed.

The effects of atrazine metabolites on puberty and thyroid function in the male Wistar rat

Recently we reported that atrazine (ATR), a chlorotriazine herbicide, alters the onset of puberty in male Wistar rats. In this study, we examined the same reproductive parameters in the developing male rat following a similar exposure to the primary, chlorinated metabolites of atrazine. Intact male Wistar rats were gavaged from postnatal day (PND) 23 through PND 53 and several reproductive endpoints were examined. The doses selected were the molar equivalents to atrazine in our previous work. Deethylatrazine (DEA), deisopropyl-atrazine (DIA), and diaminochlorotriazine (DACT) were administered by gavage at doses equivalent to the atrazine equimolar doses (AED) of 6.25, 12.5, 25, 50, 100, or 200 mg/kg. Preputial separation (PPS) was significantly delayed by DEA at 25, 100, and 200 AED, by DIA at 25, 100, and 200 AED, and by DACT at 12.5 through 200 AED. When the males were killed on PND 53, DEA (100 and 200 AED), DIA (50 through 200 AED), and DACT (200 AED) treatments caused a significant reduction in ventral prostate weight, while only the highest doses of DIA and DEA resulted in a significant decrease in lateral prostate weight. Seminal vesicle weight was reduced by DEA (25, 100, and 200 AED), DIA (100 and 200 AED), and 100 and 200 AED of DACT. Epididymal weights were reduced in the DEA (200 AED), DIA (200 AED), and DACT (100 and 200 AED) treatment groups. Serum testosterone was reduced only in the males receiving the 2 highest doses of DIA. Serum estrone was increased in the 2 highest doses of the DACT group, while serum estradiol was not different in any group. No differences were observed in any of the thyroid measures. In summary, the metabolites of ATR delay puberty in a manner similar to that observed in the previous study testing atrazine. These data also suggest that the 3 chlorinated metabolites are similar to ATR, by affecting the CNS control of the pituitary/gonadal axis and subsequent development of the reproductive tract.

Maternal exposure to atrazine during lactation suppresses suckling-induced prolactin release and results in prostatitis in the adult offspring

The availability of prolactin (PRL) to the neonatal brain is known to affect the development of the tuberoinfundibular (TIDA) neurons and, as a consequence, lead to alterations in subsequent PRL regulation. Without early lactational exposure to PRL (derived from the dam's milk), TIDA neuronal growth is impaired and elevated PRL levels are present in the prepubertal male. These observations, combined with the finding that alterations in PRL secretion (i.e., hyperprolactinemia) in the adult male rat have been implicated in the development of prostatitis, led us to hypothesize that early lactational exposure to agents that suppress suckling-induced PRL release would lead to a disruption in TIDA development, altered PRL regulation, and subsequent prostatitis in the male offspring. To test this hypothesis, suckling-induced PRL release was measured in Wistar dams treated twice daily with the herbicide atrazine (ATR, by gavage, on PND 1-4 at 0, 6.25, 12.5, 25, and 50 mg/kg body weight), or twice daily with the dopamine receptor agonist bromocriptine (BROM, sc, at 0.052, 0.104, 0.208, and 0.417 mg/kg); BROM is known to suppress PRL release. Similarly, atrazine has also been reported to suppress PRL in adult females. Serum PRL was measured on PND 3 using a serial sampling technique and indwelling cardiac catheters. A significant rise in serum PRL release was noted in all control females within 10 min of the initiation of suckling. Fifty-mg/kg ATR inhibited suckling-induced PRL release in all females, whereas 25 and 12.5 mg/kg ATR inhibited this measure in some dams and had no discernible effect in others. The 6.25 mg/kg dose of ATR was without effect. BROM, used here as a positive control, also inhibited suckling-induced PRL release at doses of 0.104 to 0.417 mg/kg, with no effect at 0.052 mg/kg. To examine the effect of postnatal ATR and BROM on the incidence and severity of inflammation (INF) of the lateral prostate of the offspring, adult males were examined at 90 and 120 days. While no effect was noted at 90 days of age, at 120 days, both the incidence and severity of prostate inflammation was increased in those offspring of ATR-treated dams (25 and 50 mg/kg). The 12.5 mg/kg ATR and the two highest doses of BROM increased the incidence, but not the severity, of prostatitis. Combined treatment of ovine prolactin (oPRL) and 25 or 50 mg/kg ATR on PND 1-4 reduced the incidence of inflammation observed at 120 days, indicating that this increase in INF, seen after ATR alone, resulted from the suppression of PRL in the dam. To determine whether or not there is a critical period for these effects, dams were dosed with 25 and 50 mg/kg on PND 6-9 and PND 11-14. Inflammation was increased in those offspring from dams treated on PND 6-9, but this increase was not significant. Dosing on PND 11-14 was without effect. These data demonstrate that ATR suppresses suckling-induced PRL release and that this suppression results in lateral prostate inflammation in the offspring. The critical period for this effect is PND 1-9.

Pubertal development in female Wistar rats following exposure to propazine and atrazine biotransformation by-products, diamino-S-chlorotriazine and hydroxyatrazine

We showed previously that the chlorotriazine herbicide, atrazine (ATR), delays the onset of pubertal development in female rats. ATR and its biotransformation by-products are present in soil and groundwater. Since current maximum contaminant levels are set only for ATR, it is important to determine whether these by-products can also alter pubertal development and possibly pose a cumulative exposure hazard. We evaluated the effects of two ATR by-products, diamino-s-chlorotriazine (DACT) and hydroxyatrazine (OH-ATR), and a structurally similar chlorotriazine, propazine (PRO), on female pubertal development. Rats were gavaged from postnatal days (PNDs) 22 through PND 41 with DACT (16.7, 33.8, 67.5, 135 mg/kg), OH-ATR (22.8, 45.7, 91.5, 183 mg/kg), or PRO (13, 26.7, 53, 106.7, 213 mg/kg). The dose range for each chemical was selected as the molar equivalent of ATR (12.5, 25, 50, 100, 200 mg/kg). The females were monitored daily for vaginal opening (VO) and killed on PND 41. DACT, a by-product of ATR that occurs in the environment and is also the primary chlorinated metabolite of ATR in animal tissue, delayed VO by 3.2, 4.8, and 7.6 days compared to the controls (33.1 +/- 0.4 (SE) days of age) following exposure to 33.8, 67.5, and 135 mg/kg, respectively. The no effect level (NOEL) for DACT (16.7 mg/kg) was identical to the equimolar NOEL for ATR (25 mg/kg). Although the body weight (BW) on PND 41 was reduced by the high dose of DACT (8.4% reduction), this reduction did not exceed the criteria for selecting the maximum tolerated dose (e.g., a dose that causes >10% decrease in BW at necropsy). None of the lower doses of DACT caused a significant difference in BW gain. Additionally, 33.8, 67.5, and 135 mg/kg of DACT significantly increased the BW on the day of VO. PRO (107 or 213 mg/kg) delayed VO by 4 days but did not alter the BW on PND 41. While no significant delays in pubertal development were observed in two separate dose-response studies with doses ranging up to 183 mg/kg (OH-ATR), a minor but statistically significant delay in the onset of puberty in a pilot study using OH-ATR raises the possibility that an effect might occur following exposure to higher doses. However, it is clear from these data that OH-ATR has a much lower potency when compared with equimolar doses of DACT and PRO. Together, these data demonstrate that PRO and DACT can delay the onset of puberty in the female rat at doses equimolar to ATR and provide the scientific basis for the use of additivity in the upcoming risk assessments.

Use of the Laboratory Rat as a Model in Endocrine Disruptor Screening and Testing

The screening and testing program the US Environmental Protection Agency (EPA) is currently developing to detect endocrine-disrupting chemicals (EDCs) is described. EDCs have been shown to alter the following activities: hypothalamic-pituitary-gonadal (HPG) function; estrogen, androgen, and thyroid hormone synthesis; and androgen and estrogen receptor-mediated effects in mammals and other animals. The value and limitations of mammalian in vivo assays are described that involve the use of the laboratory rat, the EPA Endocrine Disruptor Screening and Testing Advisory Committee species of choice. The discussion includes the evaluation of high-priority chemicals positive in the Tier 1 Screening (T1S) battery, and of subsequent testing in the Tier 2 (T2) battery, with additional short-term screening assays proposed for use in T1.5 to eliminate any uncertainty about T1S results. Descriptions include the in vivo uterotropic assay, which detects estrogens and antiestrogens; the pubertal female assay, which assesses steroidogenesis, antithyroid activity, antiestrogenicity, and HPG function; and the Hershberger assay, which detects the weight of androgen-dependent tissues in castrate-immature-male rats (antiandrogens). Of the several alternative mammalian in vivo assays proposed, a short-term pubertal male rat assay appears most promising for inclusion in T1 or T1.5. An additional in utero-lactational screening protocol is being evaluated, but appears to be better suited for T1.5 or T2 due to the size, complexity, and duration of the assay. The adult intact male assay, also proposed as an alternative for T1, attempts to identify EDCs in a hormonal battery, but has limited value as a screen due to lack of sensitivity and specificity. For Tier 2 testing, the number of endocrine-sensitive endpoints and offspring (F1) examined in multigenerational tests must be thoughtfully expanded for EDCs on a mode-of-action-specific basis, with consideration given to tailoring T2 based on the results of T1S.

Perinatal exposure to estrogenic compounds and the subsequent effects on the prostate of the adult rat: evaluation of inflammation in the ventral and lateral lobes

Although the effects of estrogenic compounds administered during the perinatal period on the size and morphology of the prostate have been well documented, the effects of such exposures on inflammatory changes in the prostate have not been well characterized. Since neonatal estradiol exposure has been shown to cause periods of hyperprolactinemia later in life and a relationship exists between high prolactin levels and rat lateral prostate inflammation, we hypothesized that an exposure to environmental compounds with estrogenic activity could result in an increase in lateral prostate inflammation in adulthood. To investigate this possibility and compare differences between estrogen agonists and antagonists, we examined the effect of a perinatal exposure to 17beta-estradiol, the insecticide methoxychlor, the partial estrogen agonist tamoxifen, and the pure antiestrogen ICI 182,780. Dams were dosed from gestation day (GD)18 to parturition and then the pups were dosed from postnatal day (PND) 1 to 5 with 0.1 mL of a solution of 0.355 mM and .0178 mM by sc injection, respectively, of all compounds in sesame oil, except for methoxychlor, which was administered only to the dam by gavage from GD 18 through PND 5 at a dose of 50 mg/kg in sesame oil. At 90 d of age, the weight of the lateral and ventral prostate in the estradiol group was significantly decreased. Tamoxifen caused a decrease in the weight of the lateral prostate, whereas the ventral lobe was not affected. ICI 182,780 did not alter prostate weight. The methoxychlor exposure increased the lateral lobe weight, but the ventral lobe weight was not affected. In the estradiol and tamoxifen groups, an inflammatory infiltrate was observed in the ventral prostates in 45.0 and 27.8% of the animals, respectively. There was a significant increase in the percent and severity of inflammation in the lateral prostate (as determined by a myeloperoxidase or neutrophil quantification assay) in the estradiol, tamoxifen, and methoxychlor groups as compared to controls. The ICI group was comparable to the controls in both ventral and lateral lobes. This study demonstrates that perinatal exposure to estrogenic compounds can result in alterations in the size of the adult prostate and increase the incidence of prostatitis.

High-Throughput Screening and Quantitative Chemical Ranking for Sodium-Iodide Symporter Inhibitors in ToxCast Phase I Chemical Library

Thyroid uptake of iodide via the sodium-iodide symporter (NIS) is the first step in the biosynthesis of thyroid hormones that are critical for health and development in humans and wildlife. Despite having long been a known target of endocrine disrupting chemicals such as perchlorate, information regarding NIS inhibition activity is still unavailable for the vast majority of environmental chemicals. This study applied a previously validated high-throughput approach to screen for NIS inhibitors in the ToxCast phase I library, representing 293 important environmental chemicals. Here 310 blinded samples were screened in a tiered-approach using an initial single-concentration (100 μM) radioactive-iodide uptake (RAIU) assay, followed by 169 samples further evaluated in multi-concentration (0.001 μM-100 μM) testing in parallel RAIU and cell viability assays. A novel chemical ranking system that incorporates multi-concentration RAIU and cytotoxicity responses was also developed as a standardized method for chemical prioritization in current and future screenings. Representative chemical responses and thyroid effects of high-ranking chemicals are further discussed. This study significantly expands current knowledge of NIS inhibition potential in environmental chemicals and provides critical support to U.S. EPA's Endocrine Disruptor Screening Program (EDSP) initiative to expand coverage of thyroid molecular targets, as well as the development of thyroid adverse outcome pathways (AOPs).

Xenoendocrine disrupters-tiered screening and testing: filling key data gaps

The US Environmental Protection Agency (EPA) is developing a screening and testing program for endocrine disrupting chemicals (EDCs) to detect alterations of hypothalamic-pituitary-gonadal (HPG) function, estrogen (ER), androgen (AR) and thyroid hormone synthesis and AR and ER receptor-mediated effects in mammals and other animals. High priority chemicals would be evaluated in the Tier 1 Screening (T1S) battery and chemicals positive in T1S would then be tested (Tier 2). T1S includes in vitro ER and AR receptor binding and/or gene expression, an assessment of steroidogenesis and mammalian (rat) and nonmammalian in vivo assays (Table 1). In vivo, the uterotropic assay detects estrogens and antiestrogens, while steroidogenesis, antithyroid activity, (anti)estrogenicity and HPG function are assessed in a 'Pubertal Female Assay'. (Anti-) androgens are detected in the Hershberger Assay (weight of AR-dependent tissues in castrate-immature-male rats). Fish and amphibian assays also are being developed. The fathead minnow assay can identify EDCs displaying several mechanisms of concern, including AR and ER receptor agonists and antagonists and inhibitors of steroid hormone synthesis. An amphibian metamorphosis assay is being developed to detect thyroid-active substances. Several alternative mammalian in vivo assays have been proposed. Of these, a short-term pubertal male rat assay appears most promising. An in utero-lactational screening protocol also is being evaluated. For Tier 2, the numbers of endocrine sensitive endpoints and offspring (F1) examined in multigenerational tests need to be expanded for EDCs. Consideration should be given to tailoring T2, based on the results of T1S. Tier 1 and 2 also should examine relevant mixtures of EDCs. Toxicants that induce malformations in AR-dependent tissues produce cumulative effects even when two chemicals act via different mechanisms of action.

High-throughput screening and chemotype-enrichment analysis of ToxCast phase II chemicals evaluated for human sodium-iodide symporter (NIS) inhibition

In support of the Endocrine Disruptor Screening Program (EDSP), the U.S.EPA's Office of Research and Development (ORD) is developing high-throughput screening (HTS) approaches to identify chemicals that alter target sites in the thyroid hormone (TH) pathway. The sodium iodide symporter (NIS) is a transmembrane glycoprotein that mediates iodide uptake into the thyroid as the initial step of TH biosynthesis. Previously, we screened 293 ToxCast chemicals (ph1v2) using a HEK293T cell line expressing human NIS in parallel radioactive iodide uptake (RAIU) and cell viability assays to identify potential environmental NIS inhibitors. Here, we expanded NIS inhibitor screening for a set of 768 ToxCast Phase II (ph2) chemicals, and applied a novel computational toxicology approach based on the ToxPrint chemotype to identify chemical substructures associated with NIS inhibition. Following single-concentration screening (at 1 × 10-4 M with a 20% inhibition cutoff), 235 samples (228 chemicals) were further tested in multiple-concentration (1 × 10-9 - 1 × 10-4 M) format in both RAIU and cell viability assays. The 167 chemicals that exhibited significant RAIU inhibition were then prioritized using combined RAIU and cell viability responses that were normalized relative to the known NIS inhibitor sodium perchlorate. Some of the highest ranked chemicals, such as PFOS, tributyltin chloride, and triclocarban, have been previously reported to be thyroid disruptors. In addition, several novel chemicals were identified as potent NIS inhibitors. The present results were combined with the previous ph1v2 screening results to produce two sets of binary hit-calls for 1028 unique chemicals, consisting of 273 positives exhibiting significant RAIU inhibition, and 63 positives following application of a cell viability filter. A ToxPrint chemotype-enrichment analysis identified >20 distinct chemical substructural features, represented in >60% of the active chemicals, as significantly enriched in each NIS inhibition hit-call space. A shared set of 9 chemotypes enriched in both hit-call sets indicates stable chemotype signals (insensitive to cytotoxicity filters) that can help guide structure-activity relationship (SAR) investigations and inform future research.

The dithiocarbamate fungicide thiram disrupts the hormonal control of ovulation in the female rat

Thiram has been reported to inhibit dopamine-beta-hydroxylase (D beta H), thereby affecting norepinephrine (NE) synthesis. Because NE is a neurotransmitter that is known to play an important role in the hypothalamic regulation of pituitary function, the acute effects of the thiram on the hormonal control of ovulation in the rat were investigated. Ovariectomized, estrogen-primed female rats were given a single injection of thiram (0, 6, 12, 25, 50, and 100 mg/kg, i.p.) at 1100 h and serum LH was measured in serial bleeds. Thiram at 100 and 50 mg/kg completely blocked the LH surge in all rats tested, while 12 and 25 mg/kg blocked the surge in 40 and 75% of the treated animals, respectively. Six mg/kg had no effect. Ovulation was then assessed in intact, proestrous females in response to thiram administration (0, 12, 25, or 50 mg/kg) at 0900, 1100, 1300, or 1800 h. Ovulation was blocked by 25 and 50 mg/kg at 1300 h in all rats, but when injected at 1100 h only the 50 mg/kg dose was effective. No such blockade was found with 50 mg/kg injected at 0900 and 1800 h. To assess the influence of thiram on the LH surge in intact rats, additional females were dosed at 1300 h on the day of proestrus and blood collected over that same day. Thiram at 50 mg/kg blocked the LH surge in all rats, while 25 mg/kg blocked the surge in 60% of the females tested. No effect occurred with 12 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Prepubertal exposure to compounds that increase prolactin secretion in the male rat: effects on the adult prostate

To test the hypothesis that a transient increase in prolactin (PRL) secretion prior to puberty can result in an alteration of the adult prostate, male rats were exposed from postnatal Days (PND) 22 to 32 to compounds that increase PRL secretion. These compounds included pimozide (a dopamine antagonist), estradiol-17beta, and bisphenol A (a monomer of polycarbonate plastics reported to have weak estrogenic activity). During dosing, pimozide (PIM), bisphenol A (BPA), and estradiol-17beta (E(2)) stimulated an increased secretion of PRL. At 120 days of age, the lateral prostate weight was increased in the PIM and BPA groups as compared to the vehicle-injected controls. Examination of the prostates revealed inflammation in the lateral lobes of all treated groups. Results of a myeloperoxidase assay, a quantitative assay to assess acute inflammation, indicated an increase in the percentage of males with neutrophil infiltrate in the lateral prostates of the PIM and E(2) treatment groups compared to their respective controls. The histological evaluations of these tissues confirmed an increase in luminal polymorphonuclear cells and interstitial mononuclear cells of the lateral prostates in all treatment groups. Administration of the dopamine agonist, bromocriptine, to the estradiol-implanted males from PND 22 to 32 reversed the induction of lateral prostate inflammation by estradiol, suggesting that PRL was necessary for the inflammatory effect. This study demonstrates that prepubertal exposures to compounds that increase PRL secretion, albeit through different mechanisms, can increase the incidence of lateral prostate inflammation in the adult.

Blockade of ovulation in the rat by the fungicide sodium N-methyldithiocarbamate: relationship between effects on the luteinizing hormone surge and alterations in hypothalamic catecholamines

Sodium N-methyldithiocarbamate (SMD), also known as metam sodium, is a commonly employed soil fungicide and nematocide. Structurally related dithiocarbamates have been found to decrease norepinephrine (NE) synthesis by suppressing the activity of dopamine-beta-hydroxylase. Because brain hypothalamic catecholamine (CA) activity is involved in generating the proestrus afternoon surge in blood luteinizing hormone (LH) which stimulates the final stages of ovulation, this study explored the effect of SMD on this hormonal trigger and its relationship to changes in hypothalamic CAs. Ovariectomized, steroid-primed Long-Evans rats showed a dose-related (25-100 mg/kg, IP) suppression of the surge and a drop in NE when SMD was given at 1100 h, a few h prior to the expected LH rise. The surge effect was reversed by the alpha-adrenergic agonist clonidine. With cycling rats, a decline with dose (50-300 mg/kg, 1300 h, proestrus) was seen in the percentage of ovulating females, with earlier injections (0900 h) being less effective at the highest dose. At all doses, low circulating levels of LH and prolactin at 1600 h suggested either a blockade in the proestrus surges of each hormone or a displacement in their time of occurrence. Anterior and posterior hypothalamic NE fell by 3 h postinjection and was accompanied by a rise in dopamine, while serotonin was unchanged. Although there was a distinct parallel between the alterations in regional CAs and the incidence of ovulation at the high doses of SMD, the relationship did not hold as the dose decreased. A similar dissociation between ovulation and CAs was seen when equimolar doses of SMD or methylisothiocyanate, a principal metabolite, were given by gavage. At the regional level of analysis employed, the data indicate that while IP injections of SMD are able to block the LH surge and ovulation in these rats, the dose-response relationship suggests that, along with induced alterations in CA metabolism, an additional factor may be involved in the observed effects.