Effect of thyroid hormone deficiency on RC3/neurogranin mRNA expression in the prenatal and adult caprine brain

Synaptic Plasticity and Cognitive Ability in Experimental Adult-Onset Hypothyroidism

Adult-onset hypothyroidism impairs normal brain function. Research on animal models of hypothyroidism has revealed critical information on how deficiency of thyroid hormones impacts the electrophysiological and molecular functions of the brain, which leads to the well known cognitive impairment in untreated hypothyroid patients. Currently, such information can only be obtained from experiments on animal models of hypothyroidism. This review summarizes important research findings that pertain to understanding the clinical cognitive consequences of hypothyroidism, which will provide a better guiding path for therapy of hypothyroidism. SIGNIFICANCE STATEMENT: Cognitive impairment occurs during adult-onset hypothyroidism in both humans and animal models. Findings from animal studies validate clinical findings showing impaired long-term potentiation, decreased CaMKII, and increased calcineurin. Such findings can only be gleaned from animal experiments to show how hypothyroidism produces clinical symptoms.

A Fine Regulation of the Hippocampal Thyroid Signalling Pro-Tects Hypothyroid Mice against Glial Cell Activation

Adult-onset hypothyroidism is associated with learning and cognitive dysfunctions, which may be related to alterations in synaptic plasticity. Local reduced levels of thyroid hormones (THs) may impair glia morphology and activity, and promote the increase of pro-inflammatory cytokine levels mainly in the hippocampus. Given that neuroinflammation induces memory impairments, hypothyroidism-related glia dysfunction may participate in brain disorders. Thus, we investigated the mechanisms linking hypothyroidism and neuroinflammation, from a protective perspective. We induced hypothyroidism in adult C57BL/6J and wild-derived WSB/EiJ male mice by a seven-week propylthiouracil (PTU) treatment. We previously showed that WSB/EiJ mice were resistant to high-fat diet (HFD)-induced obesity, showing no neuroinflammatory response through adaptive abilities, unlike C57BL/6J. As PTU and HFD treatments are known to induce comparable inflammatory responses, we hypothesized that WSB/EiJ mice might also be protected against hypothyroidism-induced neuroinflammation. We showed that hypothyroid WSB/EiJ mice depicted no hippocampal neuroinflammatory response and were able to maintain their hippocampal thyroid signalling despite low circulatisng TH levels. In contrast, C57BL/6J mice exhibited disturbed hippocampal TH signalling, accompanied by neuroinflammation and memory impairment. Our results reinforce the preponderance of the hippocampal TH regulatory system over TH circulating levels in the hippocampal glial reactivity.

Serotonin 2A receptor function and depression-like behavior in rats model of hypothyroidism

Hypothyroidism causes somatic, psychosocial and affective psychosis, including depression-like behaviors. In this study, (hypothyroidism group; HP group) adult male Sprague Dawley (SD) rats were induced to hypothyroidism after 5 weeks of exposure to 0.05% propylthiouracil (PTU) in potable water, control animals (CON group) were given the same amount of water. The following behavioral experiments were conducted, respectively: open-field test (OFT), forced swimming test (FST), tail suspension test (TST). TT[Formula: see text] and TT[Formula: see text] levels were measured after the behavior tests and the expression levels of 5-HT[Formula: see text] receptor and 5-HT[Formula: see text] receptor proteins were analyzed in the hippocampus and prefrontal cortex. The level of TT[Formula: see text] and TT[Formula: see text] in the HP group rats was much lower than that in the CON group. The hypothyroid rats also showed weight loss, much longer immobility time in tail suspension test and forced swimming test. Besides, 5 weeks of PTU administration was associated with significantly decreased expression levels of 5-HT[Formula: see text] receptor and 5-HT[Formula: see text] receptor proteins compared with control group, which were significantly negatively correlated with immobility time in FST and TST. In conclusion, our results suggest that hypothyroidism induces depressive behaviors through the influence of the serotonin system, and the decreased expression of the 5-HT[Formula: see text] receptor is an important cause of the depressive behaviors in hypothyroidism.

Thyroid hormone modulates neuroglobin and cytoglobin in rat brain

Thyroid hormones (THs) are essential and crucial for brain development, playing a role in growth and differentiation. Two globins named neuroglobin (Ngb) and cytoglobin (Cygb) are located in the brain, and each one has different distribution and function: They seem to have similar action by providing O(2) for respiratory chain, and detoxification of reactive oxygen species (ROS) and nitric oxide (NO) protecting tissues against irreversible lesions. We aimed to investigate the influence of thyroid state in Ngb and Cygb metabolism in different brain regions and evaluate their responses in cerebellum, hippocampus and cerebral cortex (hereafter called as cortex) after supraphysiological doses at different time points of TH administration. Experiments were carried out in rats, divided in eight experimental groups Control (C), thyroidectomy (Tx), and thyroidectomy treated with jugular intravenous injection (i.v). T3 (100 μl/100 g) injection and sacrificed after 30, 60, 120 min and 6, 12 and 24 h. In cortex, we found increase in Ngb gene and protein expression in different time points compared to C group, however Cygb gene and protein expression were decreased. In hippocampus, Ngb and Cygb protein expression increased 24 h after i.v. T3 injection in comparison to Tx. In cerebellum, we found increased Ngb gene expression after 120 min, 6, 12 and 24 h after T3 administration compared to Tx, and in contrast, protein expression was found to be significantly increased only 12 and 24 h compared to Tx. Ngb and Cygb expression in brain is influenced by thyroid hormone state both by its lack or excess.

Perchlorate exposure induces hypothyroidism and affects thyroid-responsive genes in liver but not brain of quail chicks

Ground-dwelling birds in perchlorate-contaminated areas are exposed to perchlorate ion, a known thyroid disruptor, and might be vulnerable to the developmental effects of perchlorate-induced hypothyroidism. We hypothesized that perchlorate-induced hypothyroidism would alter the expression of thyroid-responsive genes involved in thyroid hormone (TH) regulation and in the development of target organ function. Japanese quail chicks were exposed to 2000 mg/L ammonium perchlorate in drinking water for 7.5 weeks beginning on day 5 posthatch. Hypothyroidism was evident after 2 weeks of exposure as lower plasma THs and lower TH content in exposed chicks than in controls. The degree of hypothyroidism was increased at 7.5 weeks, as indicated by significant thyroid gland hypertrophy and sustained changes in thyroid function. After 2 weeks of exposure, hypothyroidism increased type 2 5'-deiodinase (D2) mRNA level and decreased Spot 14 (SP14) mRNA level in the liver, whereas D2 mRNA and RC3 mRNA levels in brain were not affected. After 7.5 weeks of exposure, mRNA levels in the exposed group did not differ from those in controls in either the liver or brain, suggesting the responsiveness of these genes to THs decreased during development. These results suggest that the brain, but not the liver, was protected from the effects of hypothyroidism, probably by changes in D2 activity at the protein level and/or regulation of TH entry and exit from the brain. We concluded that perchlorate exposure caused hypothyroidism in young Japanese quail and affected the expression of thyroid-responsive genes during early posthatch development.

Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain

Persistent polyhalogenated organic pollutants are present worldwide and accumulate along the food chain. They interfere with human and animal health and are particularly harmful for pre- and perinatal neurodevelopment. The mechanisms behind the observed effects vary depending on the specific compound investigated. Co-planar polychlorinated biphenyls (PCBs) can act via the arylhydrocarbon receptor while many ortho-substituted PCBs disrupt intracellular Ca(2+) homeostasis. A common mechanism for a wide variety of PCBs is interference with thyroid hormone (TH) signalling in developing brain, by changing intracellular TH availability or by interacting directly at the level of the TH receptors. Studies on gene expression in cortex and cerebellum revealed both hypothyroid- and hyperthyroid-like effects. However, since THdependent gene expression plays a crucial role in the coordination of neuronal proliferation, migration, synaptogenesis, myelination, etc., both reduced/delayed and increased/premature expression may result in permanent structural changes in neuronal communication networks, leading to lifelong deficits in cognitive performance, motor functions, and psychobehavior. In a similar way, PCBs are able to interfere with estrogen- and androgen-dependent brain development and in some studies neurobehavioral outcome was shown to be gender-specific. Other persistent organohalogens like polychlorinated dibenzo-p-dioxins (PCDDs) and polybrominated diphenyl ethers (PBDEs) also act as endocrine disrupters in the developing brain. Several of the mechanisms involved are similar to those of PCBs, but each group also works via own specific pathways. The fact that persistent organohalogens can amplify the neurotoxic effects of other environmental pollutants, such as heavy metals, further increases their risk for human and animal neurodevelopment.

Effects of maternal exposure to ammonium perchlorate on thyroid function and the expression of thyroid-responsive genes in Japanese quail embryos

Perchlorate, a known thyroid disruptor, is deposited in eggs of exposed female birds, raising concerns that the embryos from these eggs may become hypothyroid, which may in turn affect the development and function of thyroid-dependent organs. We hypothesized that exposure to ammonium perchlorate (AP) would decrease hen and embryonic thyroid function and affect the expression of thyroid-responsive genes in embryonic brain and liver. Laying Japanese quail hens were treated with 2000 mg/l or 4000 mg/l AP in drinking water. Thyroid status and expression of thyroid-responsive genes were examined in the embryos from eggs of exposed hens. Perchlorate exposure led to hypothyroidism in hens from both treatment groups; egg production was decreased in the high dosage group only. Embryos from eggs of perchlorate-exposed hens had hypertrophied thyroid glands and significantly lower thyroidal hormone storage, indicating hypothyroidism in these embryos. The embryonic hypothyroidism was associated with decreased embryonic growth, delayed hatching and greater mortality during hatching. The mRNA level of type 2 deiodinase (D2) in the liver of embryos from eggs of perchlorate-exposed hens was increased compared to the control embryos, a compensatory response that increases the production of metabolically active T(3). However, the mRNA levels of D2 and RC3 in the brain were not affected. These results suggest that the embryonic brain is protected from hypothyroidism by other mechanisms known to influence hormone entry into and exit from the brain. Our study shows that maternal perchlorate exposure led to embryonic hypothyroidism and may have interfered with embryonic development.

T3 administration in adult hypothyroid mice modulates expression of proteins involved in striatal synaptic plasticity and improves motor behavior

Adult-onset hypothyroidism is associated with neurological changes such as cognitive dysfunction and impaired learning, which may be related to alterations of synaptic plasticity. We investigate the consequence of adult-onset hypothyroidism on thyroid-mediated transcription events in striatal synaptic plasticity, and the effect of triiodothyronine (T3) replacement. We used hypothyroid mice, treated with propylthiouracil (PTU) and methimazole (MMI), with or without subsequent administration of T3. We evaluated the amount of T3 nuclear receptors (TRalpha1, TRbeta) and striatal plasticity indicators: neurogranin (RC3), Ras homolog enriched in striatum (Rhes), Ca2+/calmodulin-dependent protein kinase (CaMKII), and dopamine- and cAMP-regulated phosphoprotein (DARPP-32). In addition, we assessed hypothyroid mice motor behavior as related to striatum synaptic functions. Hypothyroid mice exhibited significantly reduced TRbeta, RC3 and Rhes expression. T3 administration reversed the expression of TRbeta, RC3, and up-regulated CaMKII levels as well as motor behavior, and decreased DARPP-32 protein phosphorylation. We suggest that thyroid hormone modulation had a major impact on striatal synaptic plasticity of adult mice which produced in turn motor behavior modifications.

Levothyroxin restores hypothyroidism-induced impairment of LTP of hippocampal CA1: Electrophysiological and molecular studies

Hypothyroidism impairs synaptic plasticity as well as learning and memory. Clinical reports are conflicting about the ability of thyroid hormone replacement therapy to fully restore the hypothyroidism-induced learning and memory impairment. Recently, we have shown that hypothyroidism impairs LTP and cognition in adult rats. We have studied the effect of thyroxin replacement therapy on hypothyroidism-induced LTP impairment using electrophysiological and molecular approaches. Recording from CA1 region of the hippocampus in anesthetized adult rat indicated that 6 weeks of thyroxin replacement therapy (20 microg/kg/day) fully restored LTP impaired by hypothyroidism. Western blotting showed reduction in phosphorylated (P)-CAMKII, total-CaMKII, neurogranin, and calmodulin basal levels in the CA1 region of the hippocampus of hypothyroid rats. The levels of these molecules were normalized by thyroxin replacement therapy. The hypothyroid-induced elevation of basal calcineurin levels and activity was also normalized by thyroxin treatment. However, thyroxin replacement therapy did not restore hypothyroidism-induced reduction in PKCgamma basal protein levels. Additionally, real-time PCR, showed a reduction in basal neurogranin mRNA level that was normalized by thyroxin replacement therapy. In the sham (control) rats, induction of LTP by high-frequency stimulation increases P-CaMKII, and total CaMKII levels as well as CaMKII phosphotransferase activity. However, in hypothyroid rats, the same stimulation protocol induced an increase only in total-CaMKII. Thyroxin treatment normalized the levels and activity of these molecules. The results demonstrated that thyroxin therapy normalized the electrophysiological and molecular effects of hypothyroidism on the CA1 region and emphasized the critical role P-CaMKII plays in hypothyroidism-induced LTP impairment.

Control of thyroid hormone action in the developing rat brain

Thyroid hormones play important roles in brain development. The physiologic function of thyroid hormones in the developing brain is to provide a timing signal that leads to the induction of differentiation and maturation programs during precise stages of development. Inappropriate initiation of these timing events leads to asynchrony in developmental processes and a deleterious outcome. The developing brain is protected from premature thyroid hormone signaling through a variety of measures. Firstly, local brain levels of both thyroxine and triiodothyronine are controlled by ontogenically regulated patterns of production and metabolism. Secondly, developmentally regulated expression of nuclear proteins involved with the nuclear TH response apparatus control the temporal response of brain genes to thyroid hormone. Finally, developmental regulation of TH action modulating transcription factor expression also controls TH action in the developing brain. Together these molecular mechanisms cooperatively act to temporally control TH action during brain development. A description of these controlling mechanisms is the subject of this review.

Action of thyroid hormone in brain

Among the most critical actions of thyroid hormone in man and other mammals are those exerted on brain development. Severe hypothyroidism during the neonatal period leads to structural alterations, including hypomyelination and defects of cell migration and differentiation, with long-lasting, irreversible effects on behavior and performance. A complex regulatory mechanism operates in brain involving regulation of the concentration of the active hormone, T3, and the control of gene expression. Most brain T3 is formed locally from its precursor, T4, by the action of type II deiodinase which is expressed in glial cells, tanycytes, and astrocytes. Type III deiodinase (DIII) is also involved in the regulation of T3 concentrations, especially during the embryonic and early post-natal periods. DIII is expressed in neurons and degrades T4 and T3 to inactive metabolites. The action of T3 is mediated through nuclear receptors, which are expressed mainly in neurons. The receptors are ligand-modulated transcription factors, and a number of genes have been identified as regulated by thyroid hormone in brain. The regulated genes encode proteins of myelin, mitochondria, neurotrophins and their receptors, cytoskeleton, transcription factors, splicing regulators, cell matrix proteins, adhesion molecules, and proteins involved in intracellular signaling pathways. The role of thyroid hormone is to accelerate changes of gene expression that take place during development. Surprisingly, null-mutant mice for the T3 receptors show almost no signs of central nervous system involvement, in contrast with the severe effects of hypothyroidism. The resolution of this paradox is essential to understand the role of thyroid hormone and its receptors in brain development and function.

Congenital hypothyroidism impairs response alternation discrimination behavior

The behavior of six congenitally hypothyroid and six normal control rats was assessed under forced alternation fixed-ratio, alternating lever cyclic-ratio (ALCR) and progressive-ratio schedules of reinforcement. Hypothyroidism was produced by adding methimazole (MMI) to the drinking water of pregnant dams from embryonic day 16 to postnatal day 25. There were no differences in behavioral performance between MMI-treated and control animals under the fixed-ratio and progressive ratio schedules. There were also no differences in circulating triiodothyronine levels between groups at the end of the study. Under the ALCR schedule, when alternation of responding was forced during the first three cycles but both levers (choice) were presented during the last three cycles (correct lever active), the entire control group reached a competency criteria in nine sessions. In contrast, only two MMI-treated animals reached criteria after 17 sessions, and the remaining four MMI-treated animals did not reach criteria by 30 sessions of training. These results suggest that congenital hypothyroidism impairs learning when a discrimination between correct and incorrect operanda is made available.