Developmental hypothyroidism disrupts visual signal detection performance in rats

Early postnatal hypothyroidism reduces juvenile play behavior, but prenatal hypothyroidism compensates for these effects

Perinatal hypothyroidism causes long-lasting effects on behavior, including hyperactivity, cognitive delays/deficits, and a reduction in anxiety. Although there is some evidence that hypothyroidism during fetal development in humans has been associated with later autism spectrum disorder diagnosis or autism-like traits, the relationships between early thyroid hormones and social behaviors are largely unknown. Previously, we found that a moderate dose of the hypothyroid-inducing drug methimazole during embryonic and postnatal development dramatically increased juvenile play in male and female rats. The goal of the current study was to determine the extent to which thyroid hormones act in prenatal or postnatal development to organize later social behaviors. Subjects were exposed to methimazole in the drinking water during prenatal (embryonic day 12 to birth), postnatal (birth to postnatal day 23), or pre- and postnatal development; control animals received regular drinking water throughout the experiment. They were tested for play behavior as juveniles (P30-32). We found an interaction between pre- and postnatal methimazole administration such that postnatal hypothyroidism decreased some play behaviors, whereas sustained pre- and postnatal hypothyroidism restored play to control levels. The effects were similar in males and females. To our knowledge, this is the first report of an interaction between pre- and postnatal hypothyroidism on later behavior. The complexity of the timing of these effects may help explain why epidemiological studies have not consistently found a relationship between gestational hypothyroidism and later behavior.

Neuroprotection elicited by resveratrol in a rat model of hypothyroidism: Possible involvement of cholinergic signaling and redox status

Both the cholinergic pathway and oxidative stress are important mechanisms involved in the pathogenesis of hypothyroidism, a condition characterized by low levels of thyroid hormone that predispose the patient to brain dysfunction. Phenolic compounds have numerous health benefits, including antioxidant activity. This study evaluates the preventive effects of resveratrol in the cholinergic system and redox status in rats with methimazole-induced hypothyroidism. Hypothyroidism increases acetylcholinesterase (AChE) activity and density in the cerebral cortex and hippocampus and decreases the α7 and M1 receptor densities in the hippocampus. Hypothyroidism also increases cellular levels of reactive oxygen species (ROS) and thiobarbituric acid reactive substances (TBARS), but reduces total thiol content, and catalase and superoxide dismutase activities in the serum. In the cerebral cortex and hippocampus, hypothyroidism increases the levels of ROS and nitrites. In this study, resveratrol (50 mg/kg) treatment prevents the observed increase in AChE in the cerebral cortex, and increases the protein levels of NeuN, a marker of mature neurons. Resveratrol also prevents changes in serum ROS levels and brain structure, as well as the levels of TBARS, total thiol content, and serum catalase enzyme activity. These collective findings suggest that resveratrol has a high antioxidant capacity and can restore hypothyroidism-triggered alterations related to neurotransmission. Thus, it is a promising agent for the prevention of brain damage resulting from hypothyroidism.

Childhood polybrominated diphenyl ether (PBDE) serum concentration and reading ability at ages 5 and 8 years: The HOME Study

BACKGROUND

Polybrominated diphenyl ethers (PBDEs) exist extensively in the environment and human beings. PBDE concentrations are higher in children than adults. A previous study found that prenatal PBDE exposure was associated with decreased reading skills in children; however, evidence is limited on the potential impact of childhood exposure to PBDEs. The study examined the association between childhood PBDE exposures and reading ability in children at ages 5 and 8 years.

METHODS

The study included 230 children from an ongoing prospective pregnancy and birth cohort study, the Health Outcomes and Measures of Environment (HOME) Study, conducted in Cincinnati, Ohio. Children's serum concentrations of eleven PBDE congeners were measured at 1, 2, 3, 5, and 8 years. The Woodcock-Johnson Tests of Achievement - III and the Wide Range Achievement Test - 4 were administered to assess children's reading skills at ages 5 and 8 years, respectively. We used multiple informant models to examine the associations between repeated measures of PBDEs and reading scores at ages 5 and 8 years. We also estimated the βs and 95% CIs of the association of PBDE measure at each age by including interaction terms between PBDE concentrations and child age in the models.

RESULTS

All childhood BDE-153 concentrations were inversely associated with reading scores at 5 and 8 years, but associations were not statistically significant after covariate adjustment. For example, a 10-fold increase in BDE-153 concentrations at ages 3 and 5 years was associated with a -5.0 (95% confidence interval (CI): -11.0, 1.0) and -5.5 (95% CI: -12.5, 1.4) point change in Basic Reading score at age 5 years, respectively. Similarly, the estimates for Brief Reading score at age 5 years were -4.5 (95% CI: -10.5, 1.5) and -5.2 (95% CI: -12.2, 1.7) point changes, respectively. Serum concentration of BDE-47, -99, -100, and Sum₄PBDEs (sum of BDE-47, 99, 100, and 153) at every age were inversely associated with reading scores at ages 5 and 8 years in unadjusted analyses. While the adjusted estimates were much attenuated and became non-significant, the direction of most of the associations was not altered.

CONCLUSION

Our study has shown a suggestive but non-significant trend of inverse associations between childhood PBDE serum concentrations, particularly BDE-153, and children's reading skills. Future studies with a larger sample size are needed to examine these associations.

Thyroid Hormone-Dependent Epigenetic Regulation of Melanocortin 4 Receptor Levels in Female Offspring of Obese Rats

Maternal obesity is a risk factor for offspring obesity. The melanocortin 4 receptor (Mc4r) is one of the mediators of food intake and energy balance. The present study examined the epigenetic mechanisms underlying altered Mc4r levels in the hypothalamic paraventricular nucleus in the offspring of high-fat diet (HFD)-induced obese dams and sought to elucidate the role of thyroid hormones in epigenetic regulation and tagging of their nucleosome at the Mc4r promoter. Female Wistar rats were fed an HFD or standard chow from weaning through gestation and lactation. Epigenetic alterations were analyzed in the offspring on postnatal day 21 at the Mc4r promoter using chromatin immunoprecipitation and bisulfite sequencing. To study the role of triiodothyronine (T3) in Mc4r downregulation, dams received methimazole (MMI), an inhibitor of thyroid hormone production. Offspring of HFD-fed dams had a greater body weight, elevated plasma T3 concentrations, and lower Mc4r messenger RNA levels than controls. At the Mc4r promoter, offspring of HFD-fed mothers demonstrated increased histone 3 lysine 27 acetylation (H3K27ac) with a greater association to thyroid hormone receptor-β (TRβ), an inhibitor of Mc4r transcription. Moreover, TRβ coimmunoprecipitated with H3K27ac, supporting their presence in the same complex. Maternal MMI administration prevented the HFD reduction in Mc4r levels, the increase in TRβ, and the increase in the TRβ-H3K27ac association, providing further support for the role of T3 in downregulating Mc4r levels. These findings demonstrate that a perinatal HFD environment affects Mc4r regulation through a T3 metabolic pathway involving histone acetylation of its promoter.

Influence of maternal thyroid hormones during gestation on fetal brain development

Thyroid hormones (THs) play an obligatory role in many fundamental processes underlying brain development and maturation. The developing embryo/fetus is dependent on maternal supply of TH. The fetal thyroid gland does not commence TH synthesis until mid gestation, and the adverse consequences of severe maternal TH deficiency on offspring neurodevelopment are well established. Recent evidence suggests that even more moderate forms of maternal thyroid dysfunction, particularly during early gestation, may have a long-lasting influence on child cognitive development and risk of neurodevelopmental disorders. Moreover, these observed alterations appear to be largely irreversible after birth. It is, therefore, important to gain a better understanding of the role of maternal thyroid dysfunction on offspring neurodevelopment in terms of the nature, magnitude, time-specificity, and context-specificity of its effects. With respect to the issue of context specificity, it is possible that maternal stress and stress-related biological processes during pregnancy may modulate maternal thyroid function. The possibility of an interaction between the thyroid and stress systems in the context of fetal brain development has, however, not been addressed to date. We begin this review with a brief overview of TH biology during pregnancy and a summary of the literature on its effect on the developing brain. Next, we consider and discuss whether and how processes related to maternal stress and stress biology may interact with and modify the effects of maternal thyroid function on offspring brain development. We synthesize several research areas and identify important knowledge gaps that may warrant further study. The scientific and public health relevance of this review relates to achieving a better understanding of the timing, mechanisms and contexts of thyroid programing of brain development, with implications for early identification of risk, primary prevention and intervention.