Effects of TCDD on thyroid hormone homeostasis in the rat

Effect of TCDD exposure in adult female and male mice on the expression of miRNA in the ovaries and testes and associated reproductive functions

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant found widely across the world. While animal and human studies have shown that exposure to TCDD may cause significant alterations in the reproductive tract, the effect of TCDD on the expression of miRNA in the reproductive organs has not been previously tested. In the current study, we exposed adult female or male mice to TCDD or vehicle and bred them to study the impact on reproduction. The data showed that while TCDD treatment of females caused no significant change in litter size, it did alter the survival of the pups. Also, TCDD exposure of either the male or female mice led to an increase in the gestational period. While TCDD did not alter the gross morphology of the ovaries and testes, it induced significant alterations in the miRNA expression. The ovaries showed the differential expression of 426 miRNAs, of which 315 miRNAs were upregulated and 111 miRNA that were downregulated after TCDD exposure when compared to the vehicle controls. In the testes, TCDD caused the differential expression of 433 miRNAs, with 247 miRNAs upregulated and 186 miRNAs downregulated. Pathway analysis showed that several of these dysregulated miRNAs targeted reproductive functions. The current study suggests that the reproductive toxicity of TCDD may result from alterations in the miRNA expression in the reproductive organs. Because miRNAs also represent one of the epigenetic pathways of gene expression, our studies suggest that the transgenerational toxicity of TCDD may also result from dysregulation in the miRNAs.

Sexual Dimorphism in Adipose-Hypothalamic Crosstalk and the Contribution of Aryl Hydrocarbon Receptor to Regulate Energy Homeostasis

There are fundamental sex differences in the regulation of energy homeostasis. Better understanding of the underlying mechanisms of energy balance that account for this asymmetry will assist in developing sex-specific therapies for sexually dimorphic diseases such as obesity. Multiple organs, including the hypothalamus and adipose tissue, play vital roles in the regulation of energy homeostasis, which are regulated differently in males and females. Various neuronal populations, particularly within the hypothalamus, such as arcuate nucleus (ARC), can sense nutrient content of the body by the help of peripheral hormones such leptin, derived from adipocytes, to regulate energy homeostasis. This review summarizes how adipose tissue crosstalk with homeostatic network control systems in the brain, which includes energy regulatory regions and the hypothalamic–pituitary axis, contribute to energy regulation in a sex-specific manner. Moreover, development of obesity is contingent upon diet and environmental factors. Substances from diet and environmental contaminants can exert insidious effects on energy metabolism, acting peripherally through the aryl hydrocarbon receptor (AhR). Developmental AhR activation can impart permanent alterations of neuronal development that can manifest a number of sex-specific physiological changes, which sometimes become evident only in adulthood. AhR is currently being investigated as a potential target for treating obesity. The consensus is that impaired function of the receptor protects from obesity in mice. AhR also modulates sex steroid receptors, and hence, one of the objectives of this review is to explain why investigating sex differences while examining this receptor is crucial. Overall, this review summarizes sex differences in the regulation of energy homeostasis imparted by the adipose–hypothalamic axis and examines how this axis can be affected by xenobiotics that signal through AhR.

The Complex Biology of the Aryl Hydrocarbon Receptor and Its Role in the Pituitary Gland

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor best known for its ability to mediate the effects of environmental toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin), polycyclic aromatic hydrocarbons (PAHs), benzene, and polychlorinated biphenyls (PCBs) through the initiation of transcription of a number of metabolically active enzymes. Therefore, the AHR has been studied mostly in the context of xenobiotic signaling. However, several studies have shown that the AHR is constitutively active and plays an important role in general cell physiology, independently of its activity as a xenobiotic receptor and in the absence of exogenous ligands. Within the pituitary, activation of the AHR by environmental toxins has been implicated in disruption of gonadal development and fertility. Studies carried out predominantly in mouse models have revealed the detrimental influence of several environmental toxins on specific cell lineages of the pituitary tissue mediated by activation of AHR and its downstream effectors. Activation of AHR during fetal development adversely affected pituitary development while adult models exposed to AHR ligands demonstrated varying degrees of pituitary dysfunction. Such dysfunction may arise as a result of direct effects on pituitary cells or indirect effects on the hypothalamic-pituitary-gonadal axis. This review offers in-depth analysis of all aspects of AHR biology, with a particular focus on its role and activity within the adenohypophysis and specifically in pituitary tumorigenesis. A novel mechanism by which the AHR may play a direct role in pituitary cell proliferation and tumor formation is postulated. This review therefore attempts to cover all aspects of the AHR's role in the pituitary tissue, from fetal development to adult physiology and the pathophysiology underlying endocrine disruption and pituitary tumorigenesis.

Follicular Epithelial Cell Hypertrophy Induced by Chronic Oral Administration of 2,3,7,8-Tetrachlorodibenzo-p-Dioxin in Female Harlan Sprague—Dawley Rats

2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD) affects the thyroid morphologically and/or functionally in adult animals. Recently, the National Toxicology Program conducted a 2-year gavage study of TCDD in female Harlan Sprague—Dawley rats. The only treatment-related alterations found in thyroid follicles were decreased luminal size and increased height of the follicular epithelial cells, without prominent protrusion into the lumen. The present study elucidated the nature of these follicular lesions. Thyroid glands of 10 rats each from the control, high (100 ng/kg/day)-dose, and stop-study (100 ng/kg/day, 30 weeks; vehicle to study termination) groups in the 2-year study were evaluated microscopically. Twenty randomly selected follicles were measured morphometrically in each animal. TCDD treatment significantly decreased the mean ratio of luminal/epithelial areas and increased the mean sectional epithelial height of the high-dose group compared to controls. Thyroid sections were immunostained with antibody against minichromosome maintenance (MCM) proteins, a novel cell-cycle biomarker. The MCM labeling index of the high-dose group was significantly higher than that of the control; however, the TUNEL labeling index was also higher in the high-dose group than the control. All data from the stop group were comparable to those from controls. These results indicate that the follicular cell response was hypertrophic and reversible. This information should contribute to diagnosis of nonneoplastic thyroid follicular lesions in rats.

Developmental triclosan exposure decreases maternal, fetal, and early neonatal thyroxine: a dynamic and kinetic evaluation of a putative mode-of-action

This work tests the mode-of-action (MOA) hypothesis that maternal and developmental triclosan (TCS) exposure decreases circulating thyroxine (T4) concentrations via up-regulation of hepatic catabolism and elimination of T4. Time-pregnant Long-Evans rats received TCS po (0-300mg/kg/day) from gestational day (GD) 6 through postnatal day (PND) 21. Serum and liver were collected from dams (GD20, PND22) and offspring (GD20, PND4, PND14, PND21). Serum T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) concentrations were measured by radioimmunoassay. Ethoxy-O-deethylase (EROD), pentoxyresorufin-O-depentylase (PROD) and uridine diphosphate glucuronyltransferase (UGT) enzyme activities were measured in liver microsomes. Custom Taqman(®) qPCR arrays were employed to measure hepatic mRNA expression of select cytochrome P450s, UGTs, sulfotransferases, transporters, and thyroid hormone-responsive genes. TCS was quantified by LC/MS/MS in serum and liver. Serum T4 decreased approximately 30% in GD20 dams and fetuses, PND4 pups and PND22 dams (300mg/kg/day). Hepatic PROD activity increased 2-3 fold in PND4 pups and PND22 dams, and UGT activity was 1.5 fold higher in PND22 dams only (300mg/kg/day). Minor up-regulation of Cyp2b and Cyp3a expression in dams was consistent with hypothesized activation of the constitutive androstane and/or pregnane X receptor. T4 reductions of 30% for dams and GD20 and PND4 offspring with concomitant increases in PROD (PND4 neonates and PND22 dams) and UGT activity (PND22 dams) suggest that up-regulated hepatic catabolism may contribute to TCS-induced hypothyroxinemia during development. Serum and liver TCS concentrations demonstrated greater fetal than postnatal internal exposure, consistent with the lack of T4 changes in PND14 and PND21 offspring. These data support the MOA hypothesis that TCS exposure leads to hypothyroxinemia via increased hepatic catabolism; however, the minor effects on thyroid hormone metabolism may reflect the low efficacy of TCS as thyroid hormone disruptor or highlight the possibility that other MOAs may also contribute to the observed maternal and early neonatal hypothyroxinemia.

Dioxin-induced changes in epididymal sperm count and spermatogenesis

A single in utero exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on gestation day 15 decreased epididymal sperm count in adult rats and thus was used to establish a tolerable daily intake for TCDD. However, several laboratories have been unable to replicate these findings. Moreover, conflicting reports of TCDD effects on daily sperm production suggest that spermatogenesis may not be as sensitive to the adverse effects of TCDD as previously thought. We performed a PubMed search using relevant search terms linking dioxin exposure with adverse effects on reproduction and spermatogenesis. Developmental exposure to TCDD is consistently linked with decreased cauda epididymal sperm counts in animal studies, although at higher dose levels than those used in some earlier studies. However, the evidence linking in utero TCDD exposure and spermatogenesis is not convincing. Animal studies provide clear evidence of an adverse effect of in utero TCDD exposure on epididymal sperm count but do not support the conclusion that spermatogenesis is adversely affected. The mechanisms underlying decreased epididymal sperm count are unknown; however, we postulate that epididymal function is the key target for the adverse effects of TCDD.

Quercetin prevents 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced testicular damage in rats

The protective effect of quercetin on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced testicular damage in rats was investigated. Twenty-two rats were equally divided into four groups; first group was kept as control and given corn oil as carrier. In second group, TCDD was orally administered at the dose of 2 μ (kg week)(-1) for 60 days. In third group, quercetin was orally administered at the dose of 20 mg (kg day)(-1) by gavages, and in fourth group TCDD and quercetin were given together at the same doses. Although TCDD increased the formation of thiobarbituric acid reactive substances (TBARS) significantly, it caused a significant decline in the levels of glutathione (GSH), catalase (CAT), GSH-Px and CuZn-Superoxide Dismutase (CuZn-SOD) in rats. In contrast, quercetin significantly increased the GSH, CAT, GSH-Px and CuZn-SOD levels but decreased the formation of TBARS. In addition, sperm motility, sperm concentration and serum testosterone levels were significantly decreased but abnormal sperm rate and testicular damage were increased with TCDD treatment. However, these effects of TCDD on sperm parameters, histological changes and hormone levels were eliminated by quercetin treatment. Our results show that administration of TCDD induces testicular damage (oxidative stress, testes tissue damage, serum hormone level and sperm parameters), and quercetin prevents TCDD-induced testicular damage in rats. Thus, quercetin may be useful for the prevention and treatment of TCDD-induced testicular damage.

Aryl hydrocarbon receptor activation in lactotropes and gonadotropes interferes with estradiol-dependent and -independent preprolactin, glycoprotein alpha and luteinizing hormone beta gene expression

Arylhydrocarbon receptor (Ahr) activation by 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) interferes with female reproductive functions, but there is little information on the specific targets of TCDD in the hypothalamic-pituitary-gonadal (HPG) axis. In these studies, we found that TCDD upregulated known AhR target genes, cytochrome p450 1a1 (Cyp1a1), Cyp1a2 and Cyp1b1 in the rat pituitary gland. Moreover, 75% of pituitary lactotropes and 45% of gonadotropes contained Ahr mRNA, and most Ahr-containing cells were estrogen receptor 1 (Esr1)-positive. TCDD abrogated estradiol (E(2))-induced prolactin (Prl) expression in vivo and in vitro; conversely, E(2) blocked TCDD upregulation of luteinizing hormone beta (Lhb) and glycoprotein hormone alpha polypeptide (Cga) expression. TCDD had no effect on levels of Ahr mRNA, but upregulated Esr1 mRNA. E(2) independently repressed Ahr and Esr1 expression and blocked TCDD upregulation of Esr1. Thus, complex interactions between Ahr and Esr alter Prl and luteinizing hormone (LH) synthesis by direct actions in lactotropes and gonadotropes. These findings provide important insights into how TCDD disrupts female reproductive functions.

Thyroid follicular lesions induced by oral treatment for 2 years with 2,3,7,8-tetrachlorodibenzo-p-dioxin and dioxin-like compounds in female Harlan Sprague-Dawley rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and structurally-similar dioxin-like compounds affect thyroid function and morphology and thyroid hormone metabolism in animals and humans. The National Toxicology Program conducted eight 2-year gavage studies in female Harlan Sprague-Dawley rats to determine the relative potency of chronic toxicity and carcinogenicity of TCDD, 3,3',4,4',5-pentachlorobiphenyl (PCB126), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 2,3',4,4',5-pentachlorobiphenyl (PCB118), 2,2',4,4',5,5'-hexachloro-biphenyl (PCB153), a tertiary mixture of TCDD/PCB126/PeCDF, and two binary mixtures (PCB126/PCB153 and PCB126/PCB118). Administration of these compounds was associated with increased incidences of thyroid follicular cell hypertrophy, variably observed in the 14-, 31-, and 53-week interim and 2-year sacrifice groups. In all studies, the incidences of follicular cell adenoma and carcinoma were not increased. Decreased levels of serum thyroxine were primarily noted in the 14-or-later -week interim groups of all chemicals. Serum triiodothyronine (T3) levels were increased in the TCDD, PCB126, PeCDF, TCDD/PCB126/PeCDF, and PCB126/PCB153 studies, while decreased levels were noted in the PCB153 and PCB126/PCB118 studies. TCDD, PCB126, PCB126/PCB153, and PCB126/PCB118 increased levels of serum thyroid-stimulating hormone almost in a dose-dependent manner in the 14-week groups. These data suggest that although dioxin-like compounds alter thyroid hormones and increase follicular cell hyperplasia, there is not an increase in thyroid adenoma or carcinoma in female Sprague-Dawley rats.

Differential effects of polychlorinated biphenyl congeners on serum thyroid hormone levels in rats

Polychlorinated biphenyls (PCBs) are known to reduce serum thyroxine (T(4)) in rats, but the relative effects of individual PCB congeners on thyroid hormones are not known. Thus, male Sprague-Dawley rats were administered Aroclor 1254, Aroclor 1242 (4, 8, 16, or 32 mg/kg/day), PCB 95 (2,2',3,5',6-pentachlorobiphenyl), PCB 99 (2,2',4,4',5-pentachlorobiphenyl), PCB 118 (2,3',4,4',5-pentachlorobiphenyl) (2, 4, 8, or 16 mg/kg/day), PCB 126 (3,3'4,4',5-pentachlorobiphenyl) (2.5, 5, 10, 20, or 40 microg/kg/day), TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) (0.14, 0.43, 1.3, or 3.9 microg/kg/day), or corn oil via oral gavage for 7 days. Rats were necropsied 24 h after the last dose. Serum thyroid hormone levels were evaluated by radioimmunoassay, and induction of hepatic Cyp1a (a TCDD-inducible protein) and Cyp2b (a phenobarbital [PB]-inducible protein) activity was determined by ethoxyresorufin-O-deethylase and pentoxyresorufin-O-deethylase assays, respectively. Significant increases in Cyp1a activity occurred in response to PCBs, except PCB 95 and PCB 99. Aroclor 1254, PCB 99, and PCB 118 significantly induced Cyp2b activity. Serum total T(4) and free T(4) were dramatically reduced in response to each of the seven treatments in a dose-dependent manner. The marked T(4) reductions occurred in response to Aroclor 1254, PCB 99 (a PB-type congener), and PCB 118 (a mixed-type congener). In contrast, reductions in serum triiodothyronine (total and free) were variable and mild, and serum thyroid-stimulating hormone was not significantly affected by any of the compounds. These data indicate that the PB and mixed-type PCB congeners are more effective than the TCDD-type PCB congeners at reducing serum T(4).

Dioxin-induced changes in epididymal sperm count and spermatogenesis

BACKGROUND

A single in utero exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on gestation day 15 decreased epididymal sperm count in adult rats and thus was used to establish a tolerable daily intake for TCDD. However, several laboratories have been unable to replicate these findings. Moreover, conflicting reports of TCDD effects on daily sperm production suggest that spermatogenesis may not be as sensitive to the adverse effects of TCDD as previously thought.

DATA SOURCES

We performed a PubMed search using relevant search terms linking dioxin exposure with adverse effects on reproduction and spermatogenesis.

DATA SYNTHESIS

Developmental exposure to TCDD is consistently linked with decreased cauda epididymal sperm counts in animal studies, although at higher dose levels than those used in some earlier studies. However, the evidence linking in utero TCDD exposure and spermatogenesis is not convincing.

CONCLUSIONS

Animal studies provide clear evidence of an adverse effect of in utero TCDD exposure on epididymal sperm count but do not support the conclusion that spermatogenesis is adversely affected. The mechanisms underlying decreased epididymal sperm count are unknown; however, we postulate that epididymal function is the key target for the adverse effects of TCDD.

A critical comparison of murine pathology and epidemiological data of TCDD, PCB126, and PeCDF

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, or dioxin) and dioxin-like compounds (DLCs) induce numerous toxicities, including developmental, endocrine, immunological, and multi-organ carcinogenic, in animals and/or humans. Multiple studies completed by the National Toxicology Program (NTP) focused on the effects caused in Harlan Sprague-Dawley rats by specific DLCs, among them the prototypical dioxin, TCDD. Because humans are exposed daily to a combination of DLCs, primarily via ingestion of food, the Toxic Equivalency Factor (TEF) was developed in order to evaluate health hazards caused by these mixtures. Herein we review the pathological effects reported in humans exposed to TCDD; 3,3',4,4',5-pentachlorobiphenyl (PCB 126); and 2,3,4,7,8,-pentachlorodibenzofuran (PeCDF) and compare them to similar changes seen in NTP murine studies performed with the same compounds. While there were differences in specific pathologies observed, clear consistency in the target organs affected (liver, oral cavity, cardiovascular system, immune system, thyroid, pancreas, and lung) could be seen in both human studies and rodent toxicity and carcinogenicity investigations.

Panax ginseng effects on DNA damage, CYP1A1 expression and histopathological changes in testes of rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effects of Panax ginseng extracts on DNA damage, expression of cytochrome P450 (CYP) 1A1 and reproductive toxicity were evaluated in the testis of rats exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxinthe (TCDD). Fifty rats were divided into five groups according to treatment with 2,3,7,8-TCDD and P. ginseng extracts. Single cell gel electrophoresis assays were performed to evaluate DNA damage that occurred in the lymphocytes of rats. Histological changes in the seminiferous tubules of the testis were determined using Johnsen's scoring system and Real Time-PCR was performed to evaluate the mRNA expression of CYP1A1. Significant pathological effects were observed in the 2,3,7,8-TCDD treated rats including a reduced seminiferous tubular diameter, an increased number of damaged tubules (maturation arrest, eosinophilic degeneration and spermatid giant cells) and increased Johnsen's score. DNA damage and the expression of CYP1A1 mRNA were significantly increased in rat testes. There were no significant differences between the control and animals treated with P. ginseng extracts. However, a significantly decreased level of DNA damage, decreased CYP1A1 expression and reduced pathological effects were observed in the 2,3,7,8-TCDD with P. ginseng extracts treated groups when compared with the TCDD treated group. In summary, our study demonstrates that 2,3,7,8-TCDD induces the pathological and genotoxical damage in rat testes, while P. ginseng extract treatment exhibits a therapeutic capacity to reduce these effects via reduction of CYP1A1 mRNA.

The Effects of Methimazole on Development of the Fathead Minnow, Pimephales promelas, from Embryo to Adult

The importance of thyroid hormones in regulating early developmental processes of many amphibian and fish species is well known, but the impacts of exposure to disrupters of thyroid homeostasis during the embryo-larval-juvenile transitions are unclear. To investigate these impacts, fathead minnows, Pimephales promelas, were exposed to a model thyroid axis disrupter, methimazole, an inhibitor of thyroid hormone synthesis, at control (0), 32, 100, and 320 mug/l, starting at <24-h postfertilization, for 28, 56, and 83/84 days postfertilization (dpf). Thyroid disruption was evident at 28 dpf, when survival was significantly reduced by 32 or 100 mug/l methimazole concomitant with a reduced thyroxine (T(4)) content. However, the T(3) content of these fish was similar to that of control fish, and body mass was unaffected (as in all groups), suggesting compensatory mechanisms overcame reduced T(4) synthesis. At the highest concentration of methimazole (320 mug/l), activation of feedback mechanisms on the hypothalamic-pituitary-thyroid axis was suggested by the normal T(4) content after 28 dpf exposure to methimazole, although triiodothyronine (T(3)) content of these fish was significantly reduced. The generally less pronounced disruption of thyroid hormone homeostasis after 56 days exposure to methimazole also suggests compensatory mechanisms in juvenile/adult fish that may regulate T(4) content, despite exposure to methimazole at 32 or 100 mug/l (in fish held in 320 mug/l methimazole, the T(4) content was significantly higher than in controls). Whole body T(3) content at 56 dpf was significantly depressed only in fish held in 100 mug/l methimazole. By 83/84 dpf, length, body mass, and thyroid hormone concentrations were similar in all experimental groups and controls, indicating that adult fish may achieve regulation of their thyroid axis despite prolonged exposures to thyroid disruptors throughout early development.

Dioxin-like activity and maternal thyroid hormone levels in second trimester maternal serum

OBJECTIVE

Developmental exposure to dioxin-like compounds has been associated with cognitive and motor impairments in children. These toxicants have been shown to be thyroid toxicants in animal studies. Therefore, the objective of this study was to quantify the overall dioxin-like activity in maternal serum and determine the association between dioxin-like activity and thyroid hormone levels.

STUDY DESIGN

Cross-sectional examination of serum from pregnant women (n = 150) attending a prenatal diagnosis clinic between January 2002 and December 2003.

RESULTS

Serum dioxin-like activity was measured in 145 of 150 (96.7%) maternal serum samples. The mean (+/- SEM) serum lipid-adjusted dioxin-like activity was 0.34 +/- 0.01 pg/g. Multiple regression analysis failed to demonstrate a relationship between maternal serum dioxin-like activity and serum thyroid hormone levels.

CONCLUSION

Dioxin-like activity is quantifiable in an overwhelming majority of second-trimester maternal serum samples but there was no relationship between dioxin-like activity and thyroid hormone levels in our study population.

Altered thyroid status in lake trout (Salvelinus namaycush) exposed to co-planar 3,3',4,4',5-pentachlorobiphenyl

Recent studies indicate that co-planar 3,3',4,4',5-pentachlorobiphenyl (PCB) congeners or their metabolites may disrupt thyroid function in fishes. Although co-planar PCB have been detected at microgram per kilogram levels in fish from contaminated areas, few studies have examined mechanisms whereby, co-planar PCBs may alter thyroid function in fish. We treated immature lake trout by intraperitoneal (i.p.)-injection or dietary gavage with vehicle containing 0, 0.7, 1.2, 25 or 40 microg 3,3',4,4',5-pentachlorobiphenyl (PCB 126) per kgBW. Blood and tissue samples were collected at various times up to 61 weeks following exposure. The treatments produced sustained dose-dependent elevations of tissue (PCB 126) concentrations. Thyroid epithelial cell height (TECH), plasma thyroxine (T4) and 3,3',5-triiodo-l-thyronine (T3) concentrations, hepatic 5'-monodeiodinase, hepatic glucuronidation of T4 and T3, as well as plasma T4 kinetics and fish growth were analyzed. Exposure to the highest doses of PCB 126 caused increased TECH, plasma T4 dynamics and T4-glucuronidation (T4-G). PCB 126 did not affect 5'-monodeiodinase and T3-glucuronidation (T3-G) and there were no effects on fish growth or condition. Because T3 status and growth were unaffected, the thyroid system was able to compensate for the alterations caused by the PCB 126 exposure. It is clear that concentrations of co-planar PCBs similar to those found in predatory fish from contaminated areas in the Great Lakes are capable of enhancing metabolism of T4. These changes may be of significance when T4 requirements are high for other reasons (e.g. periods of rapid growth, warm temperatures, metamorphosis, and parr-smolt transformation).

Effects of acute postnatal exposure to 3,3',4,4'-tetrachlorobiphenyl on sperm function and hormone levels in adult rats

Polychlorinated biphenyls (PCBs) are considered potential endocrine disruptors due to their ability to act as estrogens, antiestrogens and goitrogens. The aim of this study is to ascertain whether acute postnatal treatment with 3,3',4,4'-tetrachlorobiphenyl (CB 77) affects sperm function and hormone levels in adult rats. Male Sprague-Dawley rats received CB 77 by ip injection of 2 or 20 mg/kg at day 21 and sacrificed at day 112. At day 112, right and left testis weights were significantly increased, whereas sperm count, motility, total motile sperm count, curvilinear velocity, average path velocity, straight-line velocity, and beat-cross frequency for motile sperm were significantly decreased in rats treated with 20 mg/kg CB 77. Sperm-oocyte penetration rate was significantly reduced in rats treated with either 2 or 20 mg/kg CB 77. There was high sperm acrosome reaction rate (ARR) in the 20 mg/kg CB 77-treated rats. There was a significant increase in thyroid-stimulating hormone level in the 20 mg/kg CB 77 group. However, no changes were seen in serum testosterone, thyroid hormones, or prolactin concentrations at day 112. In summary, this study showed that postnatal exposure to CB 77 might affect spermatogenesis, motility, ARR, and ability of fertilizing oocytes in mature rats. These results suggest that the sperm functions may be more susceptible or adapt less readily than the thyroid functions to endocrine disruption caused by dioxin-like PCB congeners.

Serum 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) levels and thyroid function in Air Force veterans of the Vietnam War

PURPOSE

We assessed potential health effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentration in serum on thyroid function in US Air Force veterans involved in Operation Ranch Hand, the unit responsible for the aerial spraying of herbicides, including TCDD-contaminated Agent Orange, during the Vietnam War from 1962 to 1971. Other Air Force veterans who were not involved with spraying herbicides were included as Comparisons.

METHODS

We analyzed thyroxine (total T4), thyroid stimulating hormone (TSH), triiodothyronin percent uptake (T3% uptake), the free thyroxine index (FTI), and thyroid diseases against serum TCDD levels. Data was available for 1,009 Ranch Hand and 1,429 Comparison veterans compliant to any of five examinations in 1982, 1985, 1987, 1992, and 1997. Each veteran was assigned to one of four exposure categories based on serum TCDD levels, named Comparison, Ranch Hand Background, Ranch Hand Low Elevated, and Ranch Hand High Elevated.

RESULTS

Cross-sectional analyses found statistically significantly increased TSH means at the 1985 and 1987 examinations in the High category and a significant increasing trend across the three Ranch Hand TCDD categories in 1982, 1985, 1987 and 1992. A repeated-measures analysis found significantly increased TSH means in the High TCDD category. We found no significant relation between the occurrence of thyroid disease and TCDD category.

CONCLUSIONS

These findings suggest that TCDD affects thyroid hormone metabolism and function in Ranch Hand veterans. Further follow-up will be necessary to understand the relation, if any, between thyroid disease and TCDD levels.

Dietary accumulation and biochemical responses of juvenile rainbow trout (Oncorhynchus mykiss) to 3,3',4,4',5-pentachlorobiphenyl (PCB 126)

Juvenile rainbow trout (Oncorhynchus mykiss) (initial weights 2-5 g) were exposed to three dietary concentrations (0, 12.4 and 126 ng g(-1), wet weight) of a 14C-labelled 3,3',4,4',5-pentachlorobiphenyl (PCB 126) for 30 days followed by 160 days of clean food. We assessed bioaccumulation, histology (liver and thyroid) and biochemical responses (liver ethoxyresorufin-O-deethylase (EROD), liver vitamins (retinoids and tocopherol) and muscle thyroid hormone levels) along with growth and survival. The half-life of PCB 126 in the rainbow trout ranged from 82 to 180 days while biomagnification factors (BMF) ranged from 2.5 to 4.1 providing further evidence that PCB 126 is among the most bioaccumulative PCB congeners. Toluene extractable 14C declined with time in the trout suggesting the possibility of some biotransformation and/or covalent bonding with biological macromolecules. The threshold for liver EROD induction by PCB 126 was approximately 0.1 ng g(-1) (wet weight). EROD activities in the low- and high treatments were 9 and 44 times greater than control, respectively, and remained elevated throughout the experiment. EROD activity was correlated with whole body concentrations of PCB 126 although there was evidence of EROD activity suppression in the highly exposed fish. Liver didehydroretinoids and tocopherol concentrations were depressed by the high PCB 126 dose after 30 days exposure. Initially, muscle concentrations of thyroxine (T4) and triiodo-L-thyronine (T3) declined as the fish grew during the experiment, and exposure to PCB 126 accelerated the growth related decline. More information is needed to assess the functional significance of the reduced muscular stores of thyroid hormones. Despite the changes in liver EROD, liver vitamins and muscle thyroid hormones, liver and thyroid histology in trout examined after 30 days exposure and growth parameters were unaffected by PCB 126. This indicates that the functional competences of the physiological factors associated with growth were maintained under the experimental conditions.