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

Life-time exposure to decabromodiphenyl ethane (DBDPE) caused transgenerational epigenetic alterations of thyroid endocrine system in zebrafish

Because of its ubiquitous occurrence in the environment, decabromodiphenyl ethane (DBDPE), a novel brominated flame retardant, has been widely concerned. However, its transgenerational thyroid disrupting potential and intricate mechanism are barely explored. Therefore, zebrafish embryos were exposed to environmentally relevant concentrations of DBDPE (0, 0.1, 1 and 10 nM) until sexual maturity. The results indicated that life-time exposure to DBDPE caused anxiety-like behavior in unexposed offspring. Furthermore, the changing of thyroid hormones as well as transcriptional and DNA methylation level in the promoter region of related genes were evaluated. The thyroid disruptions observed in F1 larvae were primarily attributed to excessive transfer of thyroid hormone from F0 adults to F1 eggs. Conversely, the disruptions in F2 larvae were likely due to inherited epigenetic changes, specifically hypomethylation of crh and hypermethylation of ugt1ab, passed down from the F1 generation. Additionally, our results revealed sex-specific responses of the hypothalamic-pituitary-thyroid (HPT) axis in adult zebrafish. Furthermore, thyroid disruptions observed in unexposed offspring were more likely inherited from their mothers. The current results prompted our in-depth understanding of the multi- and transgenerational toxicity by DBDPE, and also highlighted the need to consider their adverse effects on persistent and inheritable epigenetic changes in future research on emerging pollutants.

Vulnerability of the Hippocampus to Insults: Links to Blood-Brain Barrier Dysfunction

The hippocampus is a critical brain substrate for learning and memory; events that harm the hippocampus can seriously impair mental and behavioral functioning. Hippocampal pathophysiologies have been identified as potential causes and effects of a remarkably diverse array of medical diseases, psychological disorders, and environmental sources of damage. It may be that the hippocampus is more vulnerable than other brain areas to insults that are related to these conditions. One purpose of this review is to assess the vulnerability of the hippocampus to the most prevalent types of insults in multiple biomedical domains (i.e., neuroactive pathogens, neurotoxins, neurological conditions, trauma, aging, neurodegenerative disease, acquired brain injury, mental health conditions, endocrine disorders, developmental disabilities, nutrition) and to evaluate whether these insults affect the hippocampus first and more prominently compared to other brain loci. A second purpose is to consider the role of hippocampal blood-brain barrier (BBB) breakdown in either causing or worsening the harmful effects of each insult. Recent research suggests that the hippocampal BBB is more fragile compared to other brain areas and may also be more prone to the disruption of the transport mechanisms that act to maintain the internal milieu. Moreover, a compromised BBB could be a factor that is common to many different types of insults. Our analysis indicates that the hippocampus is more vulnerable to insults compared to other parts of the brain, and that developing interventions that protect the hippocampal BBB may help to prevent or ameliorate the harmful effects of many insults on memory and cognition.

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.

Epigenetic and Long-Term Effects of Nicotine on Biology, Behavior, and Health

Tobacco and nicotine use are associated with disease susceptibility and progression. Health challenges associated with nicotine and smoking include developmental delays, addiction, mental health and behavioral changes, lung disease, cardiovascular disease, endocrine disorders, diabetes, immune system changes, and cancer. Increasing evidence suggests that nicotine-associated epigenetic changes may mediate or moderate the development and progression of a myriad of negative health outcomes. In addition, nicotine exposure may confer increased lifelong susceptibility to disease and mental health challenges through alteration of epigenetic signaling. This review examines the relationship between nicotine exposure (and smoking), epigenetic changes, and maladaptive outcomes that include developmental disorders, addiction, mental health challenges, pulmonary disease, cardiovascular disease, endocrine disorders, diabetes, immune system changes, and cancer. Overall, findings support the contention that nicotine (or smoking) associated altered epigenetic signaling is a contributing factor to disease and health challenges.

Association of exposure to ambient particulate matter with maternal thyroid function in early pregnancy

BACKGROUND

It is known that maternal thyroid dysfunction during early pregnancy can cause adverse pregnancy complications and birth outcomes. This study was designed to examine the association between ambient particulate matter with aerodynamic diameters ≤2.5 μm (PM2.5) and particulate matter with aerodynamic diameters ≤10 μm (PM10) exposure and maternal thyroid function during early pregnancy.

METHODS

This study was based on data from a birth cohort study of 921 pregnant women in China. We estimated associations between ambient PM2.5 and PM10 exposure during the first trimester of pregnancy (estimated with land-use regression models) and maternal thyroid hormone concentrations (free thyroxine (FT4), free tri-iodothyronine (FT3), and thyroid-stimulating hormone (TSH)) collected between weeks 10 and 17 of gestation using linear regression models adjusting for potential confounders. Ambient PM2.5 and PM10 concentrations were modeled per interquartile range (IQR) increment and as tertiles based on the distribution of the exposure levels.

RESULTS

An IQR increment (68 μg/m3) in PM2.5 exposure was associated with a significant decrease in maternal FT4 levels (β = -0.60, 95% CI: -1.07, -0.12); and a significant decrease in FT4/FT3 ratio (β = -0.13, 95% CI: -0.25, -0.02). Further analyses showed that, relative to the lowest tertile, women in both the middle and highest tertiles of PM2.5 had significantly lower concentrations of maternal FT4 and FT4/FT3 ratio. No significant associations were found between PM2.5 and FT3 or TSH levels. PM10 exposure was not significantly associated with maternal thyroid function.

CONCLUSIONS

Our study suggested that higher ambient PM2.5, not PM10, exposed during the first trimester of pregnancy were associated with a significant decrease in maternal serum FT4 concentrations and FT4/FT3 ratio. Studies in populations with different exposure levels are needed to replicate our study results.

Implication of thyroid hormone receptors in methamphetamine neurocognitive effects

Methamphetamine (MA) induces neurocognitive effects via several mechanisms. In the present study, we investigated the alteration of thyroid hormone receptor's expression in the context of MA-induced memory impairment and explored the protective effects of exogenous thyroid hormones (THs). Male wistar rats, received increasing regime of MA (1-10mg/kg, intraperitoneal, twice a day for 10 days), were treated with T3 (40μg/rat/day; intranasal, 2.5μl/nostril) or T4 (20µg/kg/day; intraperitoneal) for 7 days after MA cessation. All rats were subjected to novel object recognition memory test and then the mRNA levels of TH nuclear receptors (TRα1 and TRβ1) and seladin-1, an anti-apoptotic factor, and the protein level of TH cell surface receptor (integrin αvβ3) were measured in the hippocampus of rats. Our results showed that MA-induced memory impairment is concomitant with decreased level of TRα1 mRNA. T3 or T4 treatment significantly alleviated MA-induced memory impairment, but had no significant effect on the mRNA levels of TH nuclear receptors. However, T4 treatment significantly increased the protein level of cell surface receptor (av subunit) in MA-treated rats. These findings suggest that MA neurocognitive effects can be associated with impaired TH signaling in the brain and introduce this pathway as a promising therapeutic approach against MA-induced memory impairment.

Triiodothyronine attenuates neurocognitive dysfunction induced by sevoflurane in the developing brain of neonatal rats

BACKGROUND

Whilst concerns have been raised about the detrimental effects of general anaesthetics on the brain's development and function in the young, reports have indicated that thyroid hormones are able to promote neurogenesis in the developing brain. This present study aimed to investigate the effects of triiodothyronine (T3) on the neonatal rat brain, following sevoflurane exposure.

METHODS

Postnatal day 7 (P7) ratpups were treated with Triiodothyronine (T3) (1 µg/100 g body weight, i.p. injection, once/day for 3 days) after 2% sevoflurane exposure for 6 h. They were sacrificed at either P7 (immediately), P15 or P30 and their brains were harvested to assess cell death, proliferation in the hippocampus, N-methyl-D-aspartate (NMDA) receptor subunit A and B, and a post-synaptic protein (PSD-95 in the hippocampus,). Neuro-behavioral changes in other cohorts between P27 and P30 were evaluated with Morris water maze and open field tests.

RESULTS

Sevoflurane exposure caused cell death and suppressed the proliferation of astrocytes and neurons, as well as the dendritic growth of neurons in the hippocampus which were all reversed by the administration of T3. Moreover, cognitive function, including learning, memory, and adaptability to a new environment, were impaired by sevoflurane exposure, which was also negated by T3 treatment. Furthermore, sevoflurane decreased the expression of NMDA receptor subunits NR2A and NR2B, as well as PSD-95 in the hippocampus at P15 and those effects of sevoflurane were abolished by T3 administration.

CONCLUSIONS

A potential therapeutic role of T3 in protecting general anesthetic induced neuronal injury in the developing brain is likely to occur through enhancing expression of PSD-95 and the NMDA NR2A and NR2B expression.

MorbiNet Study: Hypothyroidism Comorbidity Networks in the Adult General Population

PURPOSE

Multimorbidity impacts quality of life. We constructed hypothyroidism comorbidity networks to identify positive and negative associations with other prevalent diseases.

METHODS

We analyzed data of 285 342 patients with hypothyroidism from 3 135 948 adults with multimorbidity in a population-based study in Catalonia, Spain, (period: 2006-2017). We constructed hypothyroidism comorbidity networks using logistic regression models, adjusted by age and sex, and for men and women separately. We considered relevant associations those with odds ratios (OR) >1.2 or <0.8 and P value < 1e-5 to identify coexistence greater (or smaller) than the expected by the prevalence of diseases. Multivariate models considering comorbidities were used to further adjust OR values.

RESULTS

The conditions associated included larynx cancer (adjusted OR: 2.48), congenital anomalies (2.26), thyroid cancer (2.13), hyperthyroidism (1.66), vitamin B12/folate deficiency anemia (1.57), and goiter (1.56). The network restricted to men had more connections (mental, cardiovascular, and neurological) and stronger associations with thyroid cancer (7.26 vs 2.55), congenital anomalies (5.11 vs 2.13), hyperthyroidism (4.46 vs 1.69), larynx cancer (3.55 vs 1.67), and goiter (3.94 vs 1.64). After adjustment for comorbidities, OR values were more similar in men and women. The strongest negative associations after adjusting for comorbidities were with HIV/AIDS (OR: 0.71) and tobacco abuse (0.77).

CONCLUSIONS

Networks show direct and indirect hypothyroidism multimorbidity associations. The strongest connections were thyroid and larynx cancer, congenital anomalies, hyperthyroidism, anemia, and goiter. Negative associations included HIV/AIDS and tobacco abuse. The network restricted to men had more and stronger associations, but not after adjusting for comorbidities, suggesting important indirect interactions.

Thyroid redox imbalance in adult Wistar rats that were exposed to nicotine during breastfeeding

Maternal nicotine exposure causes several consequences in offspring phenotype, such as obesity and thyroid dysfunctions. Nicotine exposure can increase oxidative stress levels, which could lead to thyroid dysfunction. However, the mechanism by which nicotine exposure during breastfeeding leads to thyroid gland dysfunction remains elusive. We aimed to investigate the long-term effects of maternal nicotine exposure on redox homeostasis in thyroid gland, besides other essential steps for thyroid hormone synthesis in rats from both sexes. Lactating Wistar rats were implanted with osmotic minipumps releasing nicotine (NIC, 6 mg/kg/day) or saline (control) from postnatal day 2 to 16. Offspring were analyzed at 180-day-old. NIC males showed lower plasma TSH, T₃ and T₄ while NIC females had higher T₃ and T₄. In thyroid, NIC males had higher sodium-iodide symporter protein expression, whereas NIC females had higher thyroid-stimulating hormone receptor (TSHr) and thyroperoxidase (TPO) protein expression. TPO activity was lower in NIC males. Hydrogen peroxide generation was decreased in NIC males. Activities of superoxide dismutase, catalase and glutathione peroxidase were compromised in NIC animals from both sexes. 4-Hydroxynonenal was higher only in NIC females, while thiol was not affected in NIC animals from both sexes. NIC offspring also had altered expression of sex steroid receptors in thyroid gland. Both sexes showed similar thyroid morphology, with lower follicle and colloid size. Thyroid from female offspring exposed to nicotine during breastfeeding developed oxidative stress, while the male gland seemed to be protected from redox damage. Thyroid dysfunctions seem to be associated with redox imbalance in a sex-dependent manner.

The effects of lettuce extract on the level of T4, memory and nerve conduction velocity in male rats

OBJECTIVE

According to the traditional medicine, lettuce can affect nerve conduction velocity and memory. So, to investigate the effect of lettuce seeds extract on body activities, lettuce seeds were used.

MATERIALS AND METHODS

In the present study, the effects of lettuce (Lactuca sativa) seeds extract consumption (in drinking water) on T4 level, animals' weight, water and food consumption, nerve conduction velocity (NCV), and memory in Wistar rats, were investigated. In this study, 24 Wistar rats were used, and divided into three groups: control, L 200 mg/kg, and L 400 mg/kg.

RESULTS

The results showed that, the T4 level, food and water intake, time spent and distance travelled in Q1, delay time to enter and the number of entrance into the dark room in both treated groups were not significantly different from the control group. Animal weight and NCV, in 400 mg/kg group were not significantly different from the control group, but in 200 mg/kg group, they were significantly decreased (p<0.05). The duration spent in the dark room (48 hr after shock) in L 400 mg/kg increased compared to the control group (p<0.05), but in L 200 mg/kg group at all time points, and in L 400 mg/kg treated group 3 and 24 hr after shock, it was not significantly different from the control group.

CONCLUSION

Based on these findings, the T4 level, memory, food and water intake were not changed by lettuce extract, while NCV and animal weight were decreased following treatment with lettuce extract.

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.

Advances in TRH signaling

The activity of the hypothalamus-pituitary-thyroid axis (HPT) is coordinated by hypophysiotropic thyrotropin releasing hormone (TRH) neurons present in the paraventricular nucleus of the hypothalamus. Hypophysiotropic TRH neurons act as energy sensors. TRH controls the synthesis and release of thyrotropin, which activates the synthesis and secretion of thyroid hormones; in target tissues, transporters and deiodinases control their local availability. Thyroid hormones regulate many functions, including energy homeostasis. This review discusses recent evidence that covers several aspects of TRH role in HPT axis regulation. Knowledge about the mechanisms of TRH signaling has steadily increased. New transcription factors engaged in TRH gene expression have been identified, and advances made on how they interact with signaling pathways and define the dynamics of TRH neurons response to acute and/or long-term influences. Albeit yet incomplete, the relationship of TRH neurons activity with positive energy balance has emerged. The importance of tanycytes as a central relay for the feedback control of the axis, as well as for HPT responses to alterations in energy balance, and other stimuli has been reinforced. Finally, some studies have started to shed light on the interference of prenatal and postnatal stress and nutrition on HPT axis programing, which have confirmed the axis susceptibility to early insults.