Maternal hypothyroxinemia impairs spatial learning and synaptic nature and function in the offspring

Relationship between maternal biological features, environmental factors, and newborn neuromotor development associated with visual fixation abilities

Newborn visual fixation abilities predict future cognitive, perceptive, and motor skills. However, little is known about the factors associated with the newborn visual fixation, which is an indicator of neurocognitive abilities. We analyzed maternal biological and environmental characteristics associated with fine motor skills (visual tracking) in 1 month old infants. Fifty-one infants were tested on visual tracking tasks (Infant Visuomotor Behavior Assessment Scale/ Guide for the Assessment of Visual Ability in Infants) and classified according to visual conducts scores. Differences between groups were compared considering motor development (Alberta Infant Motor Scale) maternal mental health (Edinburgh Postnatal Depression Scale and Hamilton Anxiety Scale); home environment (Affordances in the Home Environment for Development Scale); maternal care (Coding Interactive Behavior); breastmilk composition (total fatty acids, proteins, and cortisol); and maternal metabolic profile (serum hormones and interleukins). Mothers of infants with lower visual fixation scores had higher levels of protein in breastmilk at 3 months. Mothers of infants with better visual conduct scores had higher serum levels of T4 (at 1 month) and prolactin (at 3 months). There were no associations between visual ability and motor development, home environment, or maternal care. Early newborn neuromotor development, especially visual and fine motor skills, is associated with maternal biological characteristics (metabolic factors and breastmilk composition), highlighting the importance of early detection of maternal metabolic changes for the healthy neurodevelopment of newborns.

Development of an animal model of hypothyroxinemia during pregnancy in Wistar rats

BACKGROUND

Hypothyroxinemia is a subclinical thyroid hormone deficiency in which the mother has inadequate levels of T4 during pregnancy. The fetus relies entirely on the mother's T4 hormone level for early neurodevelopment. Isolated maternal hypothyroxinemia (IMH) in the first trimester of pregnancy can lead to lower intelligence, lower motor scores, and a higher risk of mental illness in descendants. Here, we focus on the autism-like behavior of IMH offspring.

METHODS

The animals were administered 1 ppm of propylthiouracil (PTU) for 9 weeks. Then, the concentrations of T3, T4, and thyroid-stimulating hormone (TSH) were detected using enzyme-linked immunosorbent assay (ELISA) to verify the developed animal model of IMH. We performed four behavioral experiments, including the marble burying test, open-field test, three-chamber sociability test, and Morris water maze, to explore the autistic-like behavior of 40-day-old offspring rats.

RESULTS

The ELISA test showed that the serum T3 and TSH concentrations in the model group were normal compared with the negative control group, whereas the T4 concentration decreased. In the behavioral experiments, the number of hidden marbles in the offspring of IMH increased significantly, the frequency of entering the central compartment decreased, and the social ratio decreased significantly.

CONCLUSION

The animal model of IMH was developed by the administration of 1 ppm of PTU for 9 weeks, and there were autistic-like behavior changes such as anxiety, weakened social ability, and repeated stereotyping in the IMH offspring by 40 days.

Gestational hypothyroxinemia induces ASD-like phenotypes in behavior, proinflammatory markers, and glutamatergic protein expression in mouse offspring of both sexes

Background

The prevalence of autism spectrum disorder (ASD) has significantly risen in the past three decades, prompting researchers to explore the potential contributions of environmental factors during pregnancy to ASD development. One such factor of interest is gestational hypothyroxinemia (HTX), a frequent condition in pregnancy associated with cognitive impairments in the offspring. While retrospective human studies have linked gestational HTX to autistic traits, the cellular and molecular mechanisms underlying the development of ASD-like phenotypes remain poorly understood. This study used a mouse model of gestational HTX to evaluate ASD-like phenotypes in the offspring.

Methods

To induce gestational HTX, pregnant mice were treated with 2-mercapto-1-methylimidazole (MMI), a thyroid hormones synthesis inhibitor, in the tap-drinking water from embryonic days (E) 10 to E14. A separate group received MMI along with a daily subcutaneous injection of T4, while the control group received regular tap water during the entire pregnancy. Female and male offspring underwent assessments for repetitive, anxious, and social behaviors from postnatal day (P) 55 to P64. On P65, mice were euthanized for the evaluation of ASD-related inflammatory markers in blood, spleen, and specific brain regions. Additionally, the expression of glutamatergic proteins (NLGN3 and HOMER1) was analyzed in the prefrontal cortex and hippocampus.

Results

The HTX-offspring exhibited anxious-like behavior, a subordinate state, and impaired social interactions. Subsequently, both female and male HTX-offspring displayed elevated proinflammatory cytokines in blood, including IL-1β, IL-6, IL-17A, and TNF-α, while only males showed reduced levels of IL-10. The spleen of HTX-offspring of both sexes showed increased Th17/Treg ratio and M1-like macrophages. In the prefrontal cortex and hippocampus of male HTX-offspring, elevated levels of IL-17A and reduced IL-10 were observed, accompanied by increased expression of hippocampal NLGN3 and HOMER1. All these observations were compared to those observed in the Control-offspring. Notably, the supplementation with T4 during the MMI treatment prevents the development of the observed phenotypes. Correlation analysis revealed an association between maternal T4 levels and specific ASD-like outcomes.

Discussion

This study validates human observations, demonstrating for the first time that gestational HTX induces ASD-like phenotypes in the offspring, highlighting the need of monitoring thyroid function during pregnancy.

Emerging research themes in maternal hypothyroidism: a bibliometric exploration

Background

Hypothyroidism, a prevalent endocrine disorder, carries significant implications for maternal and infant health, especially in the context of maternal hypothyroidism. Despite a gradual surge in recent research, achieving a comprehensive understanding of the current state, focal points, and developmental trends in this field remains challenging. Clarifying these aspects and advancing research could notably enhance maternal-infant health outcomes. Therefore, this study employs bibliometric methods to systematically scrutinize maternal hypothyroidism research, serving as a reference for further investigations.

Objective

Through bibliometric analysis, this study seeks to unveil key research focus areas, developmental trends, and primary contributors in Maternal Hypothyroidism. The findings offer insights and recommendations to inform future research endeavors in this domain.

Methods

Literature metrics analysis was performed on data retrieved and extracted from the Web of Science Core Collection database. The analysis examined the evolution and thematic trends of literature related to Maternal Hypothyroidism. Data were collected on October 28, 2023, and bibliometric analysis was performed using VOSviewer, CiteSpace, and the Bibliometrix software package, considering specific characteristics such as publication year, country/region, institution, authorship, journals, references, and keywords.

Results

Retrieved from 1,078 journals, 4,184 articles were authored by 18,037 contributors in 4,580 institutions across 113 countries/regions on six continents. Maternal Hypothyroidism research publications surged from 44 to 310 annually, a 604.54% growth from 1991 to 2022. The USA (940 articles, 45,233 citations), China Medical University (82 articles, 2,176 citations), and Teng, Weiping (52 articles, 1,347 citations) emerged as the most productive country, institution, and author, respectively. "Thyroid" topped with 233 publications, followed by "Journal of Clinical Endocrinology & Metabolism" (202) with the most citations (18,513). "Pregnancy" was the most cited keyword, with recent high-frequency keywords such as "outcome," "gestational diabetes," "iodine intake," "preterm birth," "guideline," and "diagnosis" signaling emerging themes in Maternal Hypothyroidism.

Conclusions

This study unveils developmental trends, global collaboration patterns, foundational knowledge, and emerging frontiers in Maternal Hypothyroidism. Over 30 years, research has predominantly focused on aspects like diagnosis, treatment guidelines, thyroid function during pregnancy, and postpartum outcomes, with a central emphasis on the correlation between maternal and fetal health.

Transient gestational hypothyroxinemia accelerates and enhances ulcerative colitis-like disorder in the male offspring

Introduction

Gestational hypothyroxinemia (HTX) is a condition that occurs frequently at the beginning of pregnancy, and it correlates with cognitive impairment, autism, and attentional deficit in the offspring. Evidence in animal models suggests that gestational HTX can increase the susceptibility of the offspring to develop strong inflammation in immune-mediated inflammatory diseases. Ulcerative colitis (UC) is a frequent inflammatory bowel disease with unknown causes. Therefore, the intensity of ulcerative colitis-like disorder (UCLD) and the cellular and molecular factors involved in proinflammatory or anti-inflammatory responses were analyzed in the offspring gestated in HTX (HTX-offspring) and compared with the offspring gestated in euthyroidism (Control-offspring).

Methods

Gestational HTX was induced by the administration of 2-mercapto-1-methylimidazole in drinking water to pregnant mice during E10-E14. The HTX-offspring were induced with UCLD by the acute administration of dextran sodium sulfate (DSS). The score of UCLD symptomatology was registered every day, and colon histopathology, immune cells, and molecular factors involved in the inflammatory or anti-inflammatory response were analyzed on day 6 of DSS treatment.

Results

The HTX-offspring displayed earlier UCLD pathological symptoms compared with the Control-offspring. After 6 days of DSS treatment, the HTX-offspring almost doubled the score of the Control-offspring. The histopathological analyses of the colon samples showed signs of inflammation at the distal and medial colon for both the HTX-offspring and Control-offspring. However, significantly more inflammatory features were detected in the proximal colon of the HTX-offspring induced with UCLD compared with the Control-offspring induced with UCLD. Significantly reduced mRNA contents encoding for protective molecules like glutamate-cysteine ligase catalytic subunit (GCLC) and mucin-2 (MUC-2) were found in the colon of the HTX-offspring as compared with the Control-offspring. Higher percentages of Th17 lymphocytes were detected in the colon tissues of the HTX-offspring induced or not with UCLD as compared with the Control-offspring.

Discussion

Gestational HTX accelerates the onset and increases the intensity of UCLD in the offspring. The low expression of MUC-2 and GCLC together with high levels of Th17 Lymphocytes in the colon tissue suggests that the HTX-offspring has molecular and cellular features that favor inflammation and tissue damage. These results are important evidence to be aware of the impact of gestational HTX as a risk factor for UCLD development in offspring.

Nicotine's effect on cognition, a friend or foe?

Tobacco smoking is a preventable cause of morbidity and mortality throughout the world. Smoking comes in form of absorption of many compounds, among which nicotine is the main psychoactive component of tobacco and its positive and negative reinforcement effects are proposed to be the key mechanism for the initiation and maintenance of smoking. Growing evidence suggests that the cognitive enhancement effects of nicotine may also contribute to the difficulty of quitting smoking, especially in individuals with psychiatric disorders. In this review, we first introduce the beneficial effect of nicotine on cognition including attention, short-term memory and long-term memory. We next summarize the beneficial effect of nicotine on cognition under pathological conditions, including Alzheimer's disease, Parkinson's disease, Schizophrenia, Stress-induced Anxiety, Depression, and drug-induced memory impairment. The possible mechanism underlying nicotine's effect is also explored. Finally, nicotine's detrimental effect on cognition is discussed, including in the prenatal and adolescent periods, and high-dose nicotine- and withdrawal-induced memory impairment is emphasized. Therefore, nicotine serves as both a friend and foe. Nicotine-derived compounds could be a promising strategy to alleviate neurological disease-associated cognitive deficit, however, due to nicotine's detrimental effect, continued educational programs and public awareness campaigns are needed to reduce tobacco use among pregnant women and smoking should be quitted even if it is e-cigarette, especially for the adolescents.

Interaction between isolated maternal hypothyroxinemia and pregnancy-related anxiety on preschooler's internalizing and externalizing problems: A birth cohort study

BACKGROUND

Isolated maternal hypothyroxinemia (IMH) and pregnancy-related anxiety may increase the risk of offspring's emotional and behavioral problems, but little is known about their potential interactive effect on preschoolers' internalizing and externalizing problems.

METHODS

We conducted a large prospective cohort study in Ma'anshan Maternal and Child Health Hospital between May 2013 and September 2014. There were a total of 1372 mother-child pairs from the Ma'anshan birth cohort (MABC) included in this study. IMH was defined as the thyroid-stimulating hormone (TSH) level within the normal reference range (2.5-97.5th percentile) and the free thyroxine (FT₄) level below the 2.5th percentile, and negative TPOAb. The pregnancy-related anxiety questionnaire (PRAQ) was used to assess women's pregnancy-related anxiety status in the first (1-13 weeks), second (14-27 weeks) and third (after 28 weeks) trimesters of pregnancy. The Achenbach Child Behavior Checklist (CBCL/1.5-5) was used to assess preschoolers' internalizing and externalizing problems.

RESULTS

Preschoolers born of mothers with IMH and anxiety had an increased risk of anxious/depressed (OR = 6.40, 95% CI 1.89-21.68), somatic complaints (OR = 2.69, 95% CI 1.01-7.20), attention problems (OR = 2.95, 95% CI 1.00-8.69) and total problems (OR = 3.40, 95% CI 1.60-7.21). Particularly, mothers with IMH and anxiety was associated with an increased risk of preschool girls' anxious/depressed (OR = 8.14, 95% CI 1.74-38.08), withdrawn (OR = 7.03, 95% CI 2.25-21.92), internalizing problems (OR = 2.66, 95% CI 1.00-7.08), and total problems (OR = 5.50, 95% CI 2.00-15.10).

CONCLUSIONS

IMH and pregnancy-related anxiety during pregnancy may synergistically increase the risk of internalizing and externalizing problems in preschooler children. This interaction is distinct in internalizing problems of preschool girls.

CaMKIV mediates spine growth deficiency of hippocampal neurons by regulation of EGR3/BDNF signal axis in congenital hypothyroidism

Congenital hypothyroidism (CH) will cause cognitive impairment in the condition of delayed treatment. The hippocampus is one of the most affected tissues by CH, in which the functional structures of hippocampal neurons manifest deficiency due to aberrant expression of effector molecules. The Ca²⁺/Calmodulin-dependent protein kinase, CaMKIV, is downregulated in the hippocampal neurons, influencing the growth of dendritic spines in response to CH. However, the underlying mechanism is not fully elucidated. In the present study, the early growth response factor 3 (EGR3) was regulated by CaMKIV in the hippocampal neurons of CH rat pups, as was analyzed by transcriptome sequencing and in vitro cell experiments. EGR3 localized within hippocampal neurons in CA1, CA3, and dentate gyrus regions. Deficient EGR3 in the primary hippocampal neurons significantly reduced the density of dendritic spines by downregulating the expression of BDNF, and such effects could be rescued by supplementing recombinant BDNF protein. Taken together, CH mediates cognitive impairment of pups through the inactivation of CaMKIV in the hippocampal neurons, which decreases the expression of EGR3 and further reduces the production of BDNF, thereby impairing the growth of dendritic spines. Identifying CaMKIV/EGR3/BDNF pathway in the hippocampal neurons in the context of CH will benefit the drug development of intellectual disability caused by CH.

Female offspring gestated in hypothyroxinemia and infected with human Metapneumovirus (hMPV) suffer a more severe infection and have a higher number of activated CD8+ T lymphocytes

Maternal thyroid hormones (THs) are essential for the appropriate development of the fetus and especially for the brain. Recently, some studies have shown that THs deficiency can also alter the immune system development of the progeny and their ability to mount an appropriate response against infectious agents. In this study, we evaluated whether adult mice gestated under hypothyroxinemia (Hpx) showed an altered immune response against infection with human metapneumovirus (hMPV). We observed that female mice gestated under Hpx showed higher clinical scores after seven days of hMPV infection. Besides, males gestated under Hpx have higher lung viral loads at day seven post-infection. Furthermore, the female offspring gestated in Hpx have already reduced the viral load at day seven and accordingly showed an increased proportion of activated (CD71⁺ and FasL⁺) CD8⁺ T cells in the lungs, which correlated with a trend for a higher histopathological clinical score. These results support that T₄ deficiency during gestation might condition the offspring differently in males and females, enhancing their ability to respond to hMPV.

Multisensory Stimulation Improves Cognition and Behavior in Adult Male Rats Born to LT4-treated Thyroidectomized Dams

Gestational hypothyroidism can impair development, cognition, and mood. Here, we tested whether multisensory stimulation (MS) improves the phenotype of rats born to surgically thyroidectomized (Tx) dams suboptimally treated with LT4. 8-week-old female Tx Wistar rats were kept on daily LT4 (0.7 µg/100 g body weight) dosed by gavage (serum TSH and T4 levels indicated moderate hypothyroidism) and 3 weeks later placed for breeding. MS of the litter started at age 60 days and lasted for 8 weeks. It consisted of twice per week of physical, cognitive, sensorial, and food stimuli. The offspring were assessed before and after MS for standardized tests of locomotor activity, cognition, and mood. Gestational hypothyroidism resulted in reduced litter size and increased offspring mortality. The pups exhibited delayed physical development, impairment of short- and long-term memory, and anxiety- and depressive-like behaviors. Nonetheless, ambulatory activity, social memory, and social preference were not affected by gestational hypothyroidism. MS restored short-term memory and anxiety while improving depressive like-behaviors. MS did not improve long-term memory. MS also did not modify the performance of control litter born to intact dams. We conclude that cognition and mood impairments caused by moderate gestational hypothyroidism were reversed or minimized in rats through MS. Further studies should define the molecular mechanisms involved.

Developmental Hypothyroidism Influences the Development of the Entorhinal-Dentate Gyrus Pathway of Rat Offspring

BACKGROUND

Developmental hypothyroidism impairs learning and memory in offspring, which depend on extensive neuronal circuits in the entorhinal cortex, together with the hippocampus and neocortex. The entorhinal-dentate gyrus pathway is the main entrance of memory circuits. We investigated whether developmental hypothyroidism impaired the morphological development of the entorhinal-dentate gyrus pathway.

METHODS

We examined the structure and function of the entorhinal-dentate gyrus pathway in response to developmental hypothyroidism induced using 2-mercapto-1-methylimidazole.

RESULTS

1,1´-Dioctadecyl-3,3,3´,3´-tetramethylindocarbocyanine perchlorate tract tracing indicated that entorhinal axons showed delayed growth in reaching the outer molecular layer of the dentate gyrus at postnatal days 2 and 4 in hypothyroid conditions. The proportion of fibers in the outer molecular layer was significantly smaller in the hypothyroid group than in the euthyroid group at postnatal day 4. At postnatal day 10, the pathway showed a layer-specific distribution in the outer molecular layer, similar to the euthyroid group. However, the projected area of entorhinal axons was smaller in the hypothyroid group than in the euthyroid group. An electrophysiological examination showed that hypothyroidism impaired the long-term potentiation of the perforant and the cornu ammonis 3-cornu ammonis 1 pathways. Many repulsive axon guidance molecules were involved in the formation of the entorhinaldentate gyrus pathway. The hypothyroid group had higher levels of erythropoietin-producing hepatocyte ligand A3 and semaphorin 3A than the euthyroid group.

CONCLUSION

We demonstrated that developmental hypothyroidism might influence the development of the entorhinal-dentate gyrus pathway, contributing to impaired long-term potentiation. These findings improve our understanding of neural mechanisms for memory function.

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.

Maternal Prenatal Stress, Thyroid Function and Neurodevelopment of the Offspring: A Mini Review of the Literature

Fetal brain is extremely plastic and vulnerable to environmental influences that may have long-term impact on health and development of the offspring. Both the Hypothalamic-Pituitary-Adrenal (HPA) and the Hypothalamic-Pituitary-Thyroid (HPT) axes are involved in stress responses, whereas, their final effectors, the Glucocorticoids (GCs) and the Thyroid Hormones (TH s), mediate several fundamental processes involved in neurodevelopment. The effects of these hormones on brain development are found to be time and dose-dependent. Regarding THs, the developing fetus depends on maternal supply of hormones, especially in the first half of pregnancy. It is acknowledged that inadequate or excess concentrations of both GCs and THs can separately cause abnormalities in the neuronal and glial structures and functions, with subsequent detrimental effects on postnatal neurocognitive function. Studies are focused on the direct impact of maternal stress and GC excess on growth and neurodevelopment of the offspring. Of particular interest, as results from recent literature data, is building understanding on how chronic stress and alterations of the HPA axis interacts and influences HPT axis and TH production. Animal studies have shown that increased GC concentrations related to maternal stress, most likely reduce maternal and thus fetal circulating THs, either directly or through modifications in the expression of placental enzymes responsible for regulating hormone levels in fetal microenvironment. The purpose of this review is to provide an update on data regarding maternal stress and its impact on fetal neurodevelopment, giving particular emphasis in the interaction of two axes and the subsequent thyroid dysfunction resulting from such circumstances.

Regulation of Thyroid-disrupting Chemicals to Protect the Developing Brain

Synthetic chemicals with endocrine disrupting properties are pervasive in the environment and are present in the bodies of humans and wildlife. As thyroid hormones (THs) control normal brain development, and maternal hypothyroxinemia is associated with neurological impairments in children, chemicals that interfere with TH signaling are of considerable concern for children's health. However, identifying thyroid-disrupting chemicals (TDCs) in vivo is largely based on measuring serum tetraiodothyronine in rats, which may be inadequate to assess TDCs with disparate mechanisms of action and insufficient to evaluate the potential neurotoxicity of TDCs. In this review 2 neurodevelopmental processes that are dependent on TH action are highlighted, neuronal migration and maturation of gamma amino butyric acid-ergic interneurons. We discuss how interruption of these processes by TDCs may contribute to abnormal brain circuitry following developmental TH insufficiency. Finally, we identify issues in evaluating the developmental neurotoxicity of TDCs and the strengths and limitations of current approaches designed to regulate them. It is clear that an enhanced understanding of how THs affect brain development will lead to refined toxicity testing, reducing uncertainty and improving our ability to protect children's health.

Transient Hypothyroidism During Lactation Alters the Development of the Corpus Callosum in Rats. An in vivo Magnetic Resonance Image and Electron Microscopy Study

Magnetic resonance imaging (MRI) data of children with late diagnosed congenital hypothyroidism and cognitive alterations such as abnormal verbal memory processing suggest altered telencephalic commissural connections. The corpus callosum (CC) is the major inter-hemispheric commissure that contra-laterally connects neocortical areas. However, in late diagnosed neonates with congenital hypothyroidism, the possible effect of early transient and chronic postnatal hypothyroidism still remains unknown. We have studied the development of the anterior, middle and posterior CC, using in vivo MRI and electron microscopy in hypothyroid and control male rats. Four groups of methimazole (MMI) treated rats were studied. One group, as a model for early transient hypothyroidism, was MMI-treated from postnatal day (P) 0 to P21; some of these rats were also treated with L-thyroxine (T4) from P15 to 21. Another group modeling chronic hypothyroid, were treated with MMI from P0 to 150 and from embryonic day 10 to P170. The results obtained from these groups were compared with same age control rats. The normalized T2 signal obtained using MRI was higher in MMI-treated rats and correlated with a low number and percentage of myelinated axons. The number and density of myelinated axons decreased in transient and chronic hypothyroid rats at P150. The g-ratio (inner to outer diameter ratio) and the estimated conduction velocity of myelinated axons were similar between MMI-treated and controls, but the conduction delay decreased in the posterior CC of MMI-treated rats compared to controls. These data show that early postnatal transient and chronic hypothyroidism alters CC maturation in a way that may affect the callosal transfer of information. These alterations cannot be reversed after delayed T4-treatment. Our data support the findings of neurocognitive delay in late T4-treated children with congenital hypothyroidism.

Adult-Onset Hypothyroidism Alters the Metaplastic Properties of Dentate Granule Cells by Decreasing Akt Phosphorylation

The expression of homosynaptic long-term depression (LTD) governs the subsequent induction of long-term potentiation (LTP) at hippocampal synapses. This process, called metaplasticity, is associated with a transient increase in the levels of several kinases, such as extracellular signal-regulated protein kinases 1/2 (ERK1/2), c-Jun N-terminal kinase (JNK), and Akt kinase. It has been increasingly realized that the chemical changes in the hippocampus caused by hypothyroidism may be the key underlying causes of the learning deficits, memory loss, and impaired LTP associated with this disease. However, the functional role of thyroid hormones in the "plasticity of synaptic plasticity" has only begun to be elucidated. To address this issue, we sought to determine whether the administration of 6-n-propyl-2-thiouracil (PTU) alters the relationship between priming and the induction of subsequent LTP and related signaling molecules. The activation of ERK1/2, JNK, and Akt was measured in the hippocampus at least 95 min after priming onset. We found that priming stimulation at 5 Hz for 3 s negatively impacted the induction of LTP by subsequent tetanic stimulation in hypothyroid animals, as manifested by a more rapid decrease in the fEPSP slope and population spike amplitude. This phenomenon was accompanied by lower levels of phosphorylated Akt in the surgically removed hippocampus of the hypothyroid rats compared to the euthyroid rats. The metaplastic response and the expression of these proteins in the 1-Hz-primed hippocampus were not different between the two groups. These observations suggest that decreased PI3K/Akt signaling may be involved in the compromised metaplastic regulation of LTP observed in hypothyroidism, which may account for the learning difficulties/cognitive impairments associated with this condition.

Gestational Hypothyroxinemia Affects Its Offspring With a Reduced Suppressive Capacity Impairing the Outcome of the Experimental Autoimmune Encephalomyelitis

Hypothyroxinemia (Hpx) is a thyroid hormone deficiency (THD) condition highly frequent during pregnancy, which although asymptomatic for the mother, it can impair the cognitive function of the offspring. Previous studies have shown that maternal hypothyroidism increases the severity of experimental autoimmune encephalomyelitis (EAE), an autoimmune disease model for multiple sclerosis (MS). Here, we analyzed the immune response after EAE induction in the adult offspring gestated in Hpx. Mice gestated in Hpx showed an early appearance of EAE symptoms and the increase of all parameters of the disease such as: the pathological score, spinal cord demyelination, and immune cell infiltration in comparison to the adult offspring gestated in euthyroidism. Isolated CD4⁺CD25⁺ T cells from spleen of the offspring gestated in Hpx that suffer EAE showed reduced capacity to suppress proliferation of effector T cells (T_Eff) after being stimulated with anti-CD3 and anti-CD28 antibodies. Moreover, adoptive transfer experiments of CD4⁺CD25⁺ T cells from the offspring gestated in Hpx suffering EAE to mice that were induced with EAE showed that the receptor mice suffer more intense EAE pathological score. Even though, no significant differences were detected in the frequency of T_reg cells and IL-10 content in the blood, spleen, and brain between mice gestated in Hpx or euthyroidism, T cells CD4⁺CD25⁺ from spleen have reduced capacity to differentiate in vitro to T_reg and to produce IL-10. Thus, our data support the notion that maternal Hpx can imprint the immune response of the offspring suffering EAE probably due to a reduced capacity to trigger suppression. Such “imprints” on the immune system could contribute to explaining as to why adult offspring gestated in Hpx suffer earlier and more intense EAE.

Effects of Mild Perinatal Hypothyroidism on Cognitive Function of Adult Male Offspring

Mild perinatal hypothyroidism may result from inadequate iodine intake, insufficient treatment of congenital hypothyroidism, or exposure to endocrine-disrupting chemicals. Because thyroid hormones are critical for brain development, severe hypothyroidism that is untreated in infancy causes irreversible cretinism. Milder hypothyroidism may also affect cognitive development; however, the effects of mild and/or moderate hypothyroidism on brain development are not fully understood. In this study, we examined the behavior of adult male mice rendered mildly hypothyroid during the perinatal period using low-dose propylthiouracil (PTU). PTU was administered through drinking water (5 or 50 ppm) from gestational day 14 to postnatal day 21. Cognitive performance, studied by an object in-location test (OLT), was impaired in PTU-treated mice at postnatal week 8. These results suggest that, although the hypothyroidism was mild, it partially impaired cognitive function. We next measured the concentration of neurotransmitters (glutamate, γ-aminobutyric acid, and glycine) in the hippocampus using in vivo microdialysis during OLT. The concentrations of neurotransmitters, particularly glutamate and glycine, decreased in PTU-treated mice. The expression levels of N-methyl-d-aspartate receptor subunits, which are profound regulators of glutamate neurotransmission and memory function, also were decreased in PTU-treated mice. These data indicate that mild perinatal hypothyroidism causes cognitive disorders in adult offspring. Such disorders may be partially induced secondary to decreased concentrations of neurotransmitters and receptor expression.

Iodine as Essential Nutrient during the First 1000 Days of Life

Iodine is an essential micronutrient incorporated into thyroid hormones. Although iodine deficiency can lead to a broad spectrum of disorders throughout life, it is most critical in the early stages of development, as the foetal brain is extremely dependent on iodine supply. During the last two decades, our understanding of thyroid physiology during gestation has substantially improved. Furthermore, thyroid hormone receptors have been identified and characterised in placental and embryonic tissues, allowing us to elucidate the maternal-foetal transfer of thyroid hormones. Experimental studies have demonstrated that the cyto-architecture of the cerebral cortex can be irreversibly disturbed in iodine deficiency causing abnormal neuron migratory patterns which are associated with cognitive impairment in children. In this context, the role of iodine as key factor in the programming of foetal and infant neurodevelopment, needs to be revisited with a special focus on areas of mild to moderate iodine deficiency. The objective of this review is to summarize the available evidence from both animals and human studies, for the effect of iodine deficiency (particularly, of maternal hypothyroxinemia) on brain development and neurological or behavioural disorders, such as lower intelligence quotient (IQ) or attention deficit hyperactivity disorder (ADHD).

Mild Maternal Hypothyroxinemia During Pregnancy Induces Persistent DNA Hypermethylation in the Hippocampal Brain-Derived Neurotrophic Factor Gene in Mouse Offspring

BACKGROUND

Thyroid hormones are essential for normal development of the central nervous system (CNS). Experimental rodents have shown that even a subtle thyroid hormone insufficiency in circulating maternal thyroid hormones during pregnancy may adversely affect neurodevelopment in offspring, resulting in irreversible cognitive deficits. This may be due to the persistent reduced expression of the hippocampal brain-derived neurotrophic factor gene Bdnf, which plays a crucial role in CNS development. However, the underlying molecular mechanisms remain unclear.

METHODS

Thiamazole (MMI; 0.025% [w/v]) was administered to dams from two weeks prior to conception until delivery, which succeeded in inducing mild maternal hypothyroxinemia during pregnancy. Serum thyroid hormone and thyrotropin levels of the offspring derived from dams with mild maternal hypothyroxinemia (M offspring) and the control offspring (C offspring) were measured. At 70 days after birth, several behavior tests were performed on the offspring. Gene expression and DNA methylation status were also evaluated in the promoter region of Bdnf exon IV, which is largely responsible for neural activity-dependent Bdnf gene expression, in the hippocampus of the offspring at day 28 and day 70.

RESULTS

No significant differences in serum thyroid hormone or thyrotropin levels were found between M and C offspring at day 28 and day 70. M offspring showed an impaired learning capacity in the behavior tests. Hippocampal steady-state Bdnf exon IV expression was significantly weaker in M offspring than it was in C offspring at day 28. At day 70, hippocampal Bdnf exon IV expression at the basal level was comparable between M and C offspring. However, it was significantly weaker in M offspring than in C offspring after the behavior tests. Persistent DNA hypermethylation was also found in the promoter region of Bdnf exon IV in the hippocampus of M offspring compared to that of C offspring, which may cause the attenuation of Bdnf exon IV expression in M offspring.

CONCLUSIONS

Mild maternal hypothyroxinemia induces persistent DNA hypermethylation in Bdnf exon IV in offspring as epigenetic memory, which may result in long-term cognitive disorders.

Maternal hormonal milieu influence on fetal brain development

An adverse maternal hormonal environment during pregnancy can be associated with abnormal brain growth. Subtle changes in fetal brain development have been observed even for maternal hormone levels within the currently accepted physiologic ranges. In this review, we provide an update of the research data on maternal hormonal impact on fetal neurodevelopment, giving particular emphasis to thyroid hormones and glucocorticoids. Thyroid hormones are required for normal brain development. Despite serum TSH appearing to be the most accurate indicator of thyroid function in pregnancy, maternal serum free T4 levels in the first trimester of pregnancy are the major determinant of postnatal psychomotor development. Even a transient period of maternal hypothyroxinemia at the beginning of neurogenesis can confer a higher risk of expressive language and nonverbal cognitive delays in offspring. Nevertheless, most recent clinical guidelines advocate for targeted high-risk case finding during first trimester of pregnancy despite universal thyroid function screening. Corticosteroids are determinant in suppressing cell proliferation and stimulating terminal differentiation, a fundamental switch for the maturation of fetal organs. Not surprisingly, intrauterine exposure to stress or high levels of glucocorticoids, endogenous or synthetic, has a molecular and structural impact on brain development and appears to impair cognition and increase anxiety and reactivity to stress. Limbic regions, such as hippocampus and amygdala, are particularly sensitive. Repeated doses of prenatal corticosteroids seem to have short-term benefits of less respiratory distress and fewer serious health problems in offspring. Nevertheless, neurodevelopmental growth in later childhood and adulthood needs further clarification. Future studies should address the relevance of monitoring the level of thyroid hormones and corticosteroids during pregnancy in the risk stratification for impaired postnatal neurodevelopment.

c-Fos downregulation positively regulates EphA5 expression in a congenital hypothyroidism rat model

The EphA5 receptor is well established as an axon guidance molecule during neural system development and plays an important role in dendritic spine formation and synaptogenesis. Our previous study has showed that EphA5 is decreased in the developing brain of congenital hypothyroidism (CH) and the EphA5 promoter methylation modification participates in its decrease. c-Fos, a well-kown transcription factor, has been considered in association with brain development. Bioinformatics analysis showed that the EphA5 promoter region contained five putative c-fos binding sites. The chromatin immunoprecipitation (ChIP) assays were used to assess the direct binding of c-fos to the EphA5 promoter. Furthermore, dual-luciferase assays showed that these three c-fos protein binding sites were positive regulatory elements for EphA5 expression in PC12 cells. Moreover, We verified c-fos positively regulation for EphA5 expression in CH model. Q-PCR and Western blot showed that c-fos overexpression could upregulate EphA5 expression in hippocampal neurons of rats with CH. Our results suggest that c-fos positively regulates EphA5 expression in CH rat model.

Gestational Hypothyroxinemia Imprints a Switch in the Capacity of Astrocytes and Microglial Cells of the Offspring to React in Inflammation

Hypothyroxinemia (Hpx) is a highly frequent condition characterized by low thyroxine (T₄) and normal 3,3',5'-triiodothyronine (T₃) and thyroid stimulating hormone (TSH) levels in the blood. Gestational Hpx is closely related to cognitive impairment in the human offspring. In animal models gestational Hpx causes impairment at glutamatergic synapsis, spatial learning, and the susceptibility to suffer strong autoimmune diseases like experimental autoimmune encephalomyelitis (EAE). However, the mechanisms underlying these phenotypes are unknown. On the other hand, it has been shown that astrocytes and microglia affect the outcome of EAE. In fact, the activation of astrocytes and microglia in the central nervous system (CNS) contributes to EAE progression. Thus, in this work, the reactivity of astrocytes and microglia from rats gestated in Hpx was evaluated aiming to understand whether these cells are targets of gestational Hpx. Interestingly, microglia derived from the offspring gestated in Hpx were less reactive compared to microglia derived from offspring gestated in euthyroidism. Instead, astrocytes derived from the offspring gestated in Hpx were significantly more reactive than the astrocytes from the offspring gestated in euthyroidism. This work contributes with novel information regarding the effects of gestational Hpx over astrocytes and microglia in the offspring. It suggests that astrocyte could react strongly to an inflammatory insult inducing neuronal death in the CNS.

Thyroid hormones and learning-associated neuroplasticity

Thyroid hormones (THs) are crucial for brain development and maturation in all vertebrates. Especially during pre- and perinatal development, disruption of TH signaling leads to a multitude of neurological deficits. Many animal models provided insight in the role of THs in brain development, but specific data on how they affect the brain's ability to learn and adapt depending on environmental stimuli are rather limited. In this review, we focus on a number of learning processes like spatial learning, fear conditioning, vocal learning and imprinting behavior and on how abnormal TH signaling during development shapes subsequent performance. It is clear from multiple studies that TH deprivation leads to defects in learning on all fronts, and interestingly, changes in local expression of the TH activator deiodinase type 2 seem to have an important role. Taking into account that THs are regulated in a very space-specific manner, there is thus increasing pressure to investigate more local TH regulators as potential factors involved in neuroplasticity. As these learning processes are also important for proper adult human functioning, further elucidating the role of THs in developmental neuroplasticity in various animal models is an important field for advancing both fundamental and applied knowledge on human brain function.

Iodine Deficiency and the Brain: Effects and Mechanisms

Iodine is an essential micronutrient needed in human diets. As iodine is an integral component of thyroid hormone, it mediates the effects of thyroid hormone on brain development. Iodine deficiency is the most prevalent and preventable cause of mental impairment in the world. The exact mechanism through which iodine influences the brain is unclear, but is generally thought to begin with genetic expression. Many brain structures and systems appear to be affected with iodine deficiency, including areas such as the hippocampus, microstructures such as myelin, and neurotransmitters. The clearest evidence comes from the studies examining cognition in the cases of iodine deprivation or interventions involving iodine supplementation. Nevertheless, there are many inconsistencies and gaps in the literature of iodine deficiency, especially over the lifespan. This paper summarizes the literature on this topic, suggests a causal mechanism for iodine's effect on the brain, and indicates areas for the future research (e.g., using magnetic resonance imaging (MRI) and functional MRI to examine how iodine supplementation facilitates cognitive functioning).

Adult hippocampal neurogenesis is impaired by transient and moderate developmental thyroid hormone disruption

The hippocampus maintains a capacity for neurogenesis throughout life, a capacity that is reduced in models of adult onset hypothyroidism. The effects of developmental thyroid hormone (TH) insufficiency on neurogenesis in the adult hippocampus, however, has not been examined. Graded degrees of TH insufficiency were induced in pregnant rat dams by administration of 0, 3 or 10ppm of 6-propylthiouracil (PTU) in drinking water from gestational day (GD) 6 until weaning. Body, brain, and hippocampal weight were reduced on postnatal day (PN) 14, 21, 78 and hippocampal volume was smaller at the 10 but not 3ppm dose level. A second experiment examined adult hippocampal neurogenesis following developmental or adult onset hypothyroidism. Two male offspring from 0 and 3ppm exposed dams were either maintained on control water or exposed to 3ppm PTU to create 4 distinct treatment conditions (Control-Control; Control-PTU, PTU-Control, PTU-PTU) based on developmental and adult exposures. Beginning on the 28th day of adult exposure to 0 or 3ppm PTU, bromodeoxyuridine (BrdU, 50mg/kg, ip) was administered twice daily for 5days, and one male from each treatment was sacrificed 24h and 28days after the last BrdU dose and brains processed for immunohistochemistry. Although no volume changes were seen in the hippocampus of the neonate at 3ppm, thinning of the granule cell layer emerged in adulthood. Developmental TH insufficiency produced a reduction in newly born cells, reducing BrdU+ve cells at 1 with no further reduction at 28-days post-BrdU. Similar findings were obtained using the proliferative cell marker Ki67. Neuronal differentiations was also altered with fewer doublecortin (Dcx) expressing cells and a higher proportion of immature Dcx phenotypes seen after developmental but not adult TH insufficiency. An impaired capacity for neurogenesis may contribute to impairments in synaptic plasticity and cognitive deficits previously reported by our laboratory and others following moderate degrees of developmental TH insufficiency induced by this PTU model.

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.

MANAGEMENT OF ENDOCRINE DISEASE: Isolated maternal hypothyroxinemia during pregnancy: knowns and unknowns

Isolated maternal hypothyroxinemia (IMH) during pregnancy is defined as a low maternal T₄ in the absence of TSH elevation. As IMH is common, with a prevalence of 1-2% in iodine-sufficient populations, and early research has suggested adverse effects on fetal neurodevelopment, it has been the focus of many studies in the last decade. In the current review, we first discuss the significance of IMH based on data from animal models and recent discoveries regarding the role of thyroid hormone on neurodevelopment. We address issues surrounding the definition and prevalence of this entity and discuss new insights into the etiologies, clinical consequences and management of IMH. A number of large cohort studies have investigated the effects of IMH on the risk of various pregnancy complications and child neurodevelopment. We review these studies in detail and describe their limitations. We discuss the available research on management of IMH, including two recent randomized controlled trials (RCTs). Finally, we delineate the remaining uncertainties in this field and emphasize the need for a sufficiently powered, placebo-controlled RCT on the treatment of IMH early in the first trimester of pregnancy.

Thyroid hormone disruption and cognitive impairment in rats exposed to PBDE during postnatal development

Polybrominated diphenyl ether flame-retardants (PBDEs) are thyroid-disrupting environmental chemicals. We investigated the effects of postnatal exposure to DE-71 (a mixture of tetra- and penta-brominated congeners), n-propylthiouracil (PTU) and thyroxine (T4) replacement on open-field (OF) and radial maze (RAM) tests. Wistar rats (5 males/5 females per litter, 32 litters) were treated orally (PND 5-22) with PTU (4mg/kg bw/d), DE-71 (30mg/kg bw/d), with and without co-administration of T4 (15μg/kg bw/d, sc). PTU depressed T4 serum levels and body weight gain and enlarged thyroid gland. Although decreasing T4 levels, DE-71 did not change thyroid and body weights. PTU-treated rats showed hyperactivity (PND 42 and 70), and working and reference memory learning deficits (RAM, PND 100). Although not altering motor activity and working memory, DE-71 caused a reference memory deficit (females only). T4 co-administration averted hypothyroxinemia and long-term cognitive deficits caused by PTU and DE-71.

HYPOTHYROXINAEMIA AND BRAIN DEVELOPMENT

The aim of this review is to indicate the current position on the role of thyroxine (T4) and fetal brain development with particular relevance to the human situation. Adequate maternal iodine nutrition and maternal circulating thyroxine (T4) concentrations are essential to ensure optimum T4 placental passage which in turn will ensure transport of T4 into fetal brain cells. These processes are discussed and the role of thyroid hormone transporters is considered. The emphasis on isolated maternal hypothyroxinaemia (IH) as an important factor affecting brain development is discussed from the animal experimental point of view as well as in the clinical setting. There is evidence of neurocognitive impairment as assessed by different modalities in children up to the age of 8 years and some suggestion of increased psychiatric disorder in older persons whose mothers had IH during gestation. Although international guidelines have not in general recommended thyroxine therapy for IH the recent demonstration of adverse obstetric outcomes in women with isolated maternal hypothyroxinaemia may warrant a revision of this strategy.

Gestational Hypothyroxinemia Affects Glutamatergic Synaptic Protein Distribution and Neuronal Plasticity Through Neuron-Astrocyte Interplay

Gestational hypothyroxinemia, characterized by low levels of maternal thyroxine (T₄) during gestation, is closely associated with cognitive impairment in offspring. Studies in animal models have shown that this condition alters neuronal glutamatergic synapses in the hippocampus. Given that astrocytes critically contribute to the establishment and functioning of synapses, the aim of this study was to determine the effects of gestational hypothyroxinemia on the capacity of astrocytes to regulate glutamatergic synapses. In an in vitro co-culture model of astrocytes and hippocampal neurons, gestational hypothyroxinemia profoundly affected the synaptic patterns of GluN1 and CD3ζ in an astrocyte-dependent manner. These effects were associated with impaired plasticity that was dependent on both neuronal and astrocyte contributions. These results highlight the importance of neuron-astrocyte interplay in the deleterious effects of gestational hypothyroxinemia and the timely diagnosis and treatment of this condition during gestation to ensure proper central nervous system development in offspring.

Thyroid hormone signaling: Contribution to neural function, cognition, and relationship to nicotine

Cigarette smoking is common despite its adverse effects on health, such as cardiovascular disease and stroke. Understanding the mechanisms that contribute to the addictive properties of nicotine makes it possible to target them to prevent the initiation of smoking behavior and/or increase the chance of successful quit attempts. While highly addictive, nicotine is not generally considered to be as reinforcing as other drugs of abuse. There are likely other mechanisms at work that contribute to the addictive liability of nicotine. Nicotine modulates aspects of the endocrine system, including the thyroid, which is critical for normal cognitive functioning. It is possible that nicotine's effects on thyroid function may alter learning and memory, and this may underlie some of its addictive potential. Here, we review the literature on thyroid function and cognition, with a focus on how nicotine alters thyroid hormone signaling and the potential impact on cognition. Changes in cognition are a major symptom of nicotine addiction. Current anti-smoking therapies have modest success at best. If some of the cognitive effects of nicotine are mediated through the thyroid hormone system, then thyroid hormone agonists may be novel treatments for smoking cessation therapies. The content of this review is important because it clarifies the relationship between smoking and thyroid function, which has been ill-defined in the past. This review is timely because the reduction in smoking rates we have seen in recent decades, due to public awareness campaigns and public smoking bans, has leveled off in recent years. Therefore, novel treatment approaches are needed to help reduce smoking rates further.

Thyroid receptor β involvement in the effects of acute nicotine on hippocampus-dependent memory

Cigarette smoking is common despite adverse health effects. Nicotine's effects on learning may contribute to addiction by enhancing drug-context associations. Effects of nicotine on learning could be direct or could occur by altering systems that modulate cognition. Because thyroid signaling can alter cognition and nicotine/smoking may change thyroid function, nicotine could affect learning through changes in thyroid signaling. These studies investigate the functional contributions of thyroid receptor (TR) subtypes β and α1 to nicotine-enhanced learning and characterize the effects of acute nicotine and learning on thyroid hormone levels. We conducted a high throughput screen of transcription factor activity to identify novel targets that may contribute to the effects of nicotine on learning. Based on these results, which showed that combined nicotine and learning uniquely acted to increase TR activation, we identified TRs as potential targets of nicotine. Further analyses were conducted to determine the individual and combined effects of nicotine and learning on thyroid hormone levels, but no changes were seen. Next, to determine the role of TRβ and TRα1 in the effects of nicotine on learning, mice lacking the TRβ or TRα1 gene and wildtype littermates were administered acute nicotine prior to fear conditioning. Nicotine enhanced contextual fear conditioning in TRα1 knockout mice and wildtypes from both lines but TRβ knockout mice did not show nicotine-enhanced learning. This finding supports involvement of TRβ signaling in the effect of acute nicotine on hippocampus-dependent memory. Acute nicotine enhances learning and these effects may involve processes regulated by the transcription factor TRβ.

Gestational and early postnatal hypothyroidism alters VGluT1 and VGAT bouton distribution in the neocortex and hippocampus, and behavior in rats

Thyroid hormones are fundamental for the expression of genes involved in the development of the CNS and their deficiency is associated with a wide spectrum of neurological diseases including mental retardation, attention deficit-hyperactivity disorder and autism spectrum disorders. We examined in rat whether developmental and early postnatal hypothyroidism affects the distribution of vesicular glutamate transporter-1 (VGluT1; glutamatergic) and vesicular inhibitory amino acid transporter (VGAT; GABAergic) immunoreactive (ir) boutons in the hippocampus and somatosensory cortex, and the behavior of the pups. Hypothyroidism was induced by adding 0.02% methimazole (MMI) and 1% KClO4 to the drinking water starting at embryonic day 10 (E10; developmental hypothyroidism) and E21 (early postnatal hypothyroidism) until day of sacrifice at postnatal day 50. Behavior was studied using the acoustic prepulse inhibition (somatosensory attention) and the elevated plus-maze (anxiety-like assessment) tests. The distribution, density and size of VGluT1-ir and VGAT-ir boutons in the hippocampus and somatosensory cortex was abnormal in MMI pups and these changes correlate with behavioral changes, as prepulse inhibition of the startle response amplitude was reduced, and the percentage of time spent in open arms increased. In conclusion, both developmental and early postnatal hypothyroidism significantly decreases the ratio of GABAergic to glutamatergic boutons in dentate gyrus leading to an abnormal flow of information to the hippocampus and infragranular layers of the somatosensory cortex, and alter behavior in rats. Our data show cytoarchitectonic alterations in the basic excitatory hippocampal loop, and in local inhibitory circuits of the somatosensory cortex and hippocampus that might contribute to the delayed neurocognitive outcome observed in thyroid hormone deficient children born in iodine deficient areas, or suffering from congenital hypothyroidism.

Maternal Hypothyroxinemia-Induced Neurodevelopmental Impairments in the Progeny

Maternal hypothyroxinemia can induce neurodevelopmental impairments in the developing fetus. We here review recent studies on the epidemiology and molecular mechanisms associated with this important public health issue. In 2011, the American Thyroid Association defined maternal hypothyroxinemia as low serum free thyroxine (FT4) levels (<5th or <10th percentile) existing in conjunction with normal serum free triiodothyronine (FT3) or thyroid stimulating hormone (TSH) levels during pregnancy. Compared to clinical or subclinical hypothyroidism, hypothyroxinemia is more commonly found in pregnant women. Hypothyroxinemia usually ensues in response to several factors, such as mild iodine deficiency, environmental endocrine disrupters, or certain thyroid diseases. Unequivocal evidence demonstrates that maternal hypothyroxinemia leads to negative effects on fetal brain development, increasing the risks for cognitive deficits and poor psychomotor development in resulting progeny. In support of this, rodent models provide direct evidence of neurodevelopmental damage induced by maternal hypothyroxinemia, including dendritic and axonal growth limitation, neural abnormal location, and synaptic function alteration. The neurodevelopmental impairments induced by hypothyroxinemia suggest an independent role of T4. Increasing evidence indicates that adequate thyroxine is required for the mothers in order to protect against the abnormal brain development in their progeny.

Hippocampal Functioning and Verbal Associative Memory in Adolescents with Congenital Hypothyroidism

Thyroid hormone (TH) is essential for normal development of the hippocampus, which is critical for memory and particularly for learning and recalling associations between visual and verbal stimuli. Adolescents with congenital hypothyroidism (CH), who lack TH in late gestation and early life, demonstrate weak verbal recall abilities, reduced hippocampal volumes, and abnormal hippocampal functioning for visually associated material. However, it is not known if their hippocampus functions abnormally when remembering verbal associations. Our objective was to assess hippocampal functioning in CH using functional magnetic resonance imaging (fMRI). Fourteen adolescents with CH and 14 typically developing controls (TDC) were studied. Participants studied pairs of words and then, during fMRI acquisition, made two types of recognition decisions: in one they judged whether the pairs were the same as when seen originally and in the other, whether individual words were seen before regardless of pairing. Hippocampal activation was greater for pairs than items in both groups, but this difference was only significant in TDC. When we directly compared the groups, the right anterior hippocampus was the primary region in which the TDC and CH groups differed for this pair memory effect. Results signify that adolescents with CH show abnormal hippocampal functioning during verbal memory processing.

Unusual ratio between free thyroxine and free triiodothyronine in a long-lived mole-rat species with bimodal ageing

Ansell's mole-rats (Fukomys anselli) are subterranean, long-lived rodents, which live in eusocial families, where the maximum lifespan of breeders is twice as long as that of non-breeders. Their metabolic rate is significantly lower than expected based on allometry, and their retinae show a high density of S-cone opsins. Both features may indicate naturally low thyroid hormone levels. In the present study, we sequenced several major components of the thyroid hormone pathways and analyzed free and total thyroxine and triiodothyronine in serum samples of breeding and non-breeding F. anselli to examine whether a) their thyroid hormone system shows any peculiarities on the genetic level, b) these animals have lower hormone levels compared to euthyroid rodents (rats and guinea pigs), and c) reproductive status, lifespan and free hormone levels are correlated. Genetic analyses confirmed that Ansell's mole-rats have a conserved thyroid hormone system as known from other mammalian species. Interspecific comparisons revealed that free thyroxine levels of F. anselli were about ten times lower than of guinea pigs and rats, whereas the free triiodothyronine levels, the main biologically active form, did not differ significantly amongst species. The resulting fT4:fT3 ratio is unusual for a mammal and potentially represents a case of natural hypothyroxinemia. Comparisons with total thyroxine levels suggest that mole-rats seem to possess two distinct mechanisms that work hand in hand to downregulate fT4 levels reliably. We could not find any correlation between free hormone levels and reproductive status, gender or weight. Free thyroxine may slightly increase with age, based on sub-significant evidence. Hence, thyroid hormones do not seem to explain the different ageing rates of breeders and non-breeders. Further research is required to investigate the regulatory mechanisms responsible for the unusual proportion of free thyroxine and free triiodothyronine.

An evo-devo approach to thyroid hormones in cerebral and cerebellar cortical development: etiological implications for autism

The morphological alterations of cortical lamination observed in mouse models of developmental hypothyroidism prompted the recognition that these experimental changes resembled the brain lesions of children with autism; this led to recent studies showing that maternal thyroid hormone deficiency increases fourfold the risk of autism spectrum disorders (ASD), offering for the first time the possibility of prevention of some forms of ASD. For ethical reasons, the role of thyroid hormones on brain development is currently studied using animal models, usually mice and rats. Although mammals have in common many basic developmental principles regulating brain development, as well as fundamental basic mechanisms that are controlled by similar metabolic pathway activated genes, there are also important differences. For instance, the rodent cerebral cortex is basically a primary cortex, whereas the primary sensory areas in humans account for a very small surface in the cerebral cortex when compared to the associative and frontal areas that are more extensive. Associative and frontal areas in humans are involved in many neurological disorders, including ASD, attention deficit-hyperactive disorder, and dyslexia, among others. Therefore, an evo-devo approach to neocortical evolution among species is fundamental to understand not only the role of thyroid hormones and environmental thyroid disruptors on evolution, development, and organization of the cerebral cortex in mammals but also their role in neurological diseases associated to thyroid dysfunction.

Maternal Subclinical Hypothyroidism Impairs Neurodevelopment in Rat Offspring by Inhibiting the CREB Signaling Pathway

Thyroid hormone is indispensable for fetal brain development, and maternal thyroid hormone deficiency is thought to result in severe and irreversible brain impairments in learning and memory. Epidemiological and animal studies by our group had shown that maternal subclinical hypothyroidism had significant negative impact on neurodevelopment. But, the underlying mechanisms responsible for these neurological alterations remain unclear. In the present study, we performed thyroidectomy and injected L-T4 daily in Wistar rats to induce maternal subclinical hypothyroidism. Our data indicated that the pups from subclinical group showed prolonged latencies during the learning process in the Morris water maze as compared to the control group. Transcription factor cAMP response element-binding protein (CREB) signaling pathway is closely associated with synaptic plasticity, learning, and memory. Consistent with behavioral results, Western blotting also showed decreased activation of three important upstream modulators of CREB signaling pathway: phospho-mitogen-activated protein kinases (P-ERK1/2), phospho-calcium-dependent-calmodulin kinase IV (P-CaMKIV), phospho-serine/threonine protein kinase AKT(P-AKT), as well as total CREB and phospho-CREB as compared to the control at postnatal day 7 (PND 7) in hippocampus. Our findings suggested that decreased activation of the CREB signaling pathway in pups was related to impairments of cognitive function caused by maternal subclinical hypothyroidism.

Subcortical band heterotopia in rat offspring following maternal hypothyroxinaemia: structural and functional characteristics

Thyroid hormones (TH) play crucial roles in brain maturation and are important for neuronal migration and neocortical lamination. Subcortical band heterotopia (SBH) represent a class of neuronal migration errors in humans that are often associated with childhood epilepsy. We have previously reported the presence of SBH in a rodent model of low level hypothyroidism induced by maternal exposure to the goitrogen, propylthiouracil (PTU). In the present study, we report the dose-response characteristics of this developmental malformation and the connectivity of heterotopic neurones with other brain regions, as well as their functionality. Pregnant rats were exposed to varying concentrations of PTU through the drinking water (0-10 p.p.m.) beginning on gestational day 6 to produce graded levels of TH insufficiency. Dose-dependent increases in the volume of the SBH present in the corpus callosum were documented in the adult offspring, with a clear presence at concentrations of PTU that resulted in minor (< 15%) reductions in maternal serum thyroxine as measured when pups were weaned. SBH contain neurones, oligodendrocytes, astrocytes and microglia. Monoaminergic and cholinergic processes were prevalent and many of the axons were myelinated. Anatomical connectivity of SBH neurones to cortical neurones and the synaptic functionality of these anatomical connections was verified by ex vivo field potential recordings. SBH persisted in adult offspring despite a return to euthyroid status on termination of exposure and these offspring displayed an increased sensitivity to seizures. Features of this model are attractive with respect to the investigation of the molecular mechanisms of cortical development, the effectiveness of therapeutic intervention in hypothyroxinaemia during pregnancy and the impact of the very modest TH imbalance that accompanies exposure to environmental contaminants.

Fetal and neonatal iron deficiency exacerbates mild thyroid hormone insufficiency effects on male thyroid hormone levels and brain thyroid hormone-responsive gene expression

Fetal/neonatal iron (Fe) and iodine/TH deficiencies lead to similar brain developmental abnormalities and often coexist in developing countries. We recently demonstrated that fetal/neonatal Fe deficiency results in a mild neonatal thyroidal impairment, suggesting that TH insufficiency contributes to the neurodevelopmental abnormalities associated with Fe deficiency. We hypothesized that combining Fe deficiency with an additional mild thyroidal perturbation (6-propyl-2-thiouracil [PTU]) during development would more severely impair neonatal thyroidal status and brain TH-responsive gene expression than either deficiency alone. Early gestation pregnant rats were assigned to 7 different treatment groups: control, Fe deficient (FeD), mild TH deficient (1 ppm PTU), moderate TH deficient (3 ppm PTU), severe TH deficient (10 ppm PTU), FeD/1 ppm PTU, or FeD/3 ppm PTU. FeD or 1 ppm PTU treatment alone reduced postnatal day 15 serum total T4 concentrations by 64% and 74%, respectively, without significantly altering serum total T3 concentrations. Neither treatment alone significantly altered postnatal day 16 cortical or hippocampal T3 concentrations. FeD combined with 1 ppm PTU treatment produced a more severe effect, reducing serum total T4 by 95%, and lowering hippocampal and cortical T3 concentrations by 24% and 31%, respectively. Combined FeD/PTU had a more severe effect on brain TH-responsive gene expression than either treatment alone, significantly altering Pvalb, Dio2, Mbp, and Hairless hippocampal and/or cortical mRNA levels. FeD/PTU treatment more severely impacted cortical and hippocampal parvalbumin protein expression compared with either individual treatment. These data suggest that combining 2 mild thyroidal insults during development significantly disrupts thyroid function and impairs TH-regulated brain gene expression.

Effects of maternal hypothyroidism on offspring hippocampus and memory

BACKGROUND

Rodents with gestational thyroid-hormone (TH) deficiencies and children with congenital hypothyroidism show abnormal hippocampal development. Given that the human hippocampus starts to develop early in gestation, we asked if children born to women with hypothyroidism during pregnancy also show hippocampal abnormalities and if this is related to the severity of maternal TH insufficiency and current memory functioning. We additionally sought to determine whether effects were more prominent in anterior or posterior hippocampal subsections given these support different memory functions and have different developmental trajectories. We hypothesized that these children would have smaller than normal hippocampal volumes than controls and show memory deficits on both standardized tests and indices of "everyday" memory functioning.

METHODS

We studied 54 children aged 9 to 12 years: 30 controls and 24 HYPO cases-offspring from women diagnosed with hypothyroidism prior to or during pregnancy and treated with l-thyroxine. All children received a thorough assessment of memory functions and an MRI scan. For each child, right and left hippocampi were manually traced, and volumes of right and left hippocampi and anterior and posterior segments were determined.

RESULTS

HYPO cases showed significantly smaller right and left hippocampal volumes than controls, particularly in right posterior and left anterior segments. In HYPO children, hippocampal volumes were negatively correlated with maternal third-trimester TSH levels and positively correlated with third-trimester fT4. HYPO cases scored significantly below controls on one objective and several subjective memory indices, and these were correlated with hippocampal volumes.

CONCLUSION

Early TH insufficiency from maternal hypothyroidism affects offspring hippocampal development and memory.

The Na+/I- symporter (NIS): mechanism and medical impact

The Na(+)/I(-) symporter (NIS) is the plasma membrane glycoprotein that mediates active I(-) transport in the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. In the thyroid, NIS-mediated I(-) uptake plays a key role as the first step in the biosynthesis of the thyroid hormones, of which iodine is an essential constituent. These hormones are crucial for the development of the central nervous system and the lungs in the fetus and the newborn and for intermediary metabolism at all ages. Since the cloning of NIS in 1996, NIS research has become a major field of inquiry, with considerable impact on many basic and translational areas. In this article, we review the most recent findings on NIS, I(-) homeostasis, and related topics and place them in historical context. Among many other issues, we discuss the current outlook on iodide deficiency disorders, the present stage of understanding of the structure/function properties of NIS, information gleaned from the characterization of I(-) transport deficiency-causing NIS mutations, insights derived from the newly reported crystal structures of prokaryotic transporters and 3-dimensional homology modeling, and the novel discovery that NIS transports different substrates with different stoichiometries. A review of NIS regulatory mechanisms is provided, including a newly discovered one involving a K(+) channel that is required for NIS function in the thyroid. We also cover current and potential clinical applications of NIS, such as its central role in the treatment of thyroid cancer, its promising use as a reporter gene in imaging and diagnostic procedures, and the latest studies on NIS gene transfer aimed at extending radioiodide treatment to extrathyroidal cancers, including those involving specially engineered NIS molecules.

Gestational hypothyroidism increases the severity of experimental autoimmune encephalomyelitis in adult offspring

BACKGROUND

Maternal thyroid hormones play a fundamental role in appropriate fetal development during gestation. Offspring that have been gestated under maternal hypothyroidism suffer cognitive impairment. Thyroid hormone deficiency during gestation can significantly impact the central nervous system by altering the migration, differentiation, and function of neurons, oligodendrocytes, and astrocytes. Given that gestational hypothyroidism alters the immune cell ratio in offspring, it is possible that this condition could result in higher sensitivity for the development of autoimmune diseases.

METHODS

Adult mice gestated under hypothyroidism were induced with experimental autoimmune encephalomyelitis (EAE). Twenty-one days after EAE induction, the disease score, myelin content, immune cell infiltration, and oligodendrocyte death were evaluated.

RESULTS

We observed that mice gestated under hypothyroidism showed higher EAE scores after disease induction during adulthood compared to mice gestated in euthyroidism. In addition, spinal cord sections of mice gestated under hypothyroidism that suffered EAE in adulthood showed higher demyelination, CD4(+) and CD8(+) infiltration, and increased oligodendrocyte death.

CONCLUSIONS

These results show for the first time that a deficiency in maternal thyroid hormones during gestation can influence the outcome of a central nervous system inflammatory disease, such as EAE, in their offspring. These data strongly support evaluating thyroid hormones in pregnant women and treating hypothyroidism during pregnancy to prevent increased susceptibility to inflammatory diseases in the central nervous system of offspring.

Evidence for triclosan-induced activation of human and rodent xenobiotic nuclear receptors

The bacteriostat triclosan (2,4,4'-trichloro-2'-hydroxydiphenylether) (TCS) decreases rat serum thyroxine via putative nuclear receptor (NR) interaction(s) and subsequent transcriptional up-regulation of hepatic catabolism and clearance. However, due to the evolutionary divergence of the constitutive androstane and pregnane-X receptors (CAR, PXR), TCS-mediated downstream effects may be species-dependent. To test the hypothesis that TCS activates xenobiotic NRs across species, cell-based NR reporter assays were employed to assess potential activation of rat, mouse, and human PXR, and rat, mouse, and three splice variants of human CAR. TCS activated hPXR, acted as an inverse agonist of hCAR1, and as a weak agonist of hCAR3. TCS failed to activate rPXR in full-length receptor reporter assays, and instead acted as a modest inverse agonist of rCAR. Consistent with the rat data, TCS also failed to activate mPXR and was a modest inverse agonist of mCAR. These data suggest that TCS may interact with multiple NRs, including hPXR, hCAR1, hCAR3, and rCAR in order to potentially affect hepatic catabolism. Overall these data support the conclusion that TCS may interact with NRs to regulate hepatic catabolism and downstream thyroid hormone homeostasis in both rat and human models, though perhaps by divergent mechanisms.

Triclosan exposure reduces thyroxine levels in pregnant and lactating rat dams and in directly exposed offspring

Thyroid disrupting chemicals can potentially disrupt brain development. Two studies investigating the effect of the antibacterial compound triclosan on thyroxine (T₄) levels in rats are reported. In the first, Wistar rat dams were gavaged with 75, 150 or 300 mg triclosan/kg bw/day throughout gestation and lactation. Total T₄ serum levels were measured in dams and offspring, and all doses of triclosan significantly lowered T₄ in dams, but no significant effects on T₄ levels were seen in the offspring at the end of the lactation period. Since this lack of effect could be due to minimal exposure through maternal milk, a second study using direct per oral pup exposure from postnatal day 3-16 to 50 or 150 mg triclosan/kg bw/day was performed. This exposure pointed to significant T₄ reductions in 16 day old offspring in both dose groups. These results corroborate previous studies showing that in rats lactational transfer of triclosan seems limited. Since an optimal study design for testing potential developmental neurotoxicants in rats, should include exposure during both the pre- and postnatal periods of brain development, we suggest that in the case of triclosan, direct dosing of pups may be the best way to obtain that goal.

Late maternal hypothyroidism alters the expression of Camk4 in neocortical subplate neurons: a comparison with Nurr1 labeling

Maternal thyroid hormones (THs) are essential for normal offspring's neurodevelopment even after onset of fetal thyroid function. This is particularly relevant for preterm children who are deprived of maternal THs following birth, are at risk of suffering hypothyroxinemia, and develop attention-deficit/hyperactivity disorder. Expression of neocortical Ca(2+)/calmodulin kinase IV (Camk4), a genomic target of thyroid hormone, and nuclear receptor-related 1 protein (Nurr1), a postnatal marker of cortical subplate (SP) cells, was studied in euthyroid fetuses and in pups born to dams thyroidectomized in late gestation (LMH group, a model of prematurity), and compared with control and developmentally hypothyroid pups (C and MMI groups, respectively). In LMH pups, the extinction of heavy Camk4 expression in an SP was 1-2 days delayed postnatally compared with C pups. The heavy Camk4 and Nurr1 expression in the SP was prolonged in MMI pups, whereas heavy Camk4 and Nurr1 expression in layer VIb remains at P60. The abnormal expression of Camk4 in the cortical SP and in layer VIb might cause altered cortical connectivity affecting neocortical function.