Thyroid hormone regulation of N-methyl-D-aspartic acid receptor subunit mRNA expression in adult 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.

Thyroid dysfunction and Alzheimer's disease, a vicious circle

Recently, research into the link between thyroid dysfunction and Alzheimer's disease (AD) remains a current topic of interest. Previous research has primarily concentrated on examining the impact of thyroid dysfunction on the risk of developing AD, or solely explored the mechanisms of interaction between hypothyroidism and AD, a comprehensive analysis of the mechanisms linking thyroid dysfunction, including hyperthyroidism and hypothyroidism, to Alzheimer's disease (AD) still require further elucidation. Therefore, the aim of this review is to offer a thorough and comprehensive explanation of the potential mechanisms underlying the causal relationship between thyroid dysfunction and AD, highlighting the existence of a vicious circle. The effect of thyroid dysfunction on AD includes neuron death, impaired synaptic plasticity and memory, misfolded protein deposition, oxidative stress, and diffuse and global neurochemical disturbances. Conversely, AD can also contribute to thyroid dysfunction by affecting the stress repair response and disrupting pathways involved in thyroid hormone (TH) production, transport, and activation. Furthermore, this review briefly discusses the role and significance of utilizing the thyroid as a therapeutic target for cognitive recovery in AD. By exploring potential mechanisms and therapeutic avenues, this research contributes to our understanding and management of this devastating neurodegenerative disease.

Hyperthyroidism leads learning and memory impairment possibly via GRIN2B expression alterations

The hippocampus as an important structure for learning and memory functions contains a high level of thyroid hormone receptors. Although there are numerous studies investigating the effects of thyroid hormones on cognitive dysfunction and psychiatric symptoms, the underlying molecular processes of these disorders have not yet been fully elucidated. In the present study, 24 male adult rats (4 months) were divided into 3 groups: control group, sham group and hyperthyroid group. Hyperthyroid group and sham group were treated with l-thyroxine or saline for 21 days. Each group was exposed to Morris water maze testing (MWMT), measuring their performance in a hidden-platform spatial task. After learning and memory tests, intracardiac blood was taken from the rats for serum thyroxine levels. Following blood collection, the rats were decapitated to isolate hippocampal tissue. GRIN2A, GRIN2B, BDNF, cFOS, Cdk5, cdk5r1 (p35), and cdk5r2 (p39) gene expression were evaluated using quantitative reverse transcriptase-PCR. Serum thyroxine level was found to be higher in hyperthyroid rats than in the control and sham groups. According to our MWMT findings, the memory performance of the hyperthyroid group was significantly impaired compared to the control and sham groups (p < 0.05). In the hippocampus, the GRIN2A gene expression level was decreased in the sham group, and the GRIN2B gene expression level was decreased in the sham and hyperthyroid groups compared to the control group (p < 0.05). There was no significant difference in other genes (p > 0.05). Hyperthyroidism impaired hippocampus-dependent spatial memory. Hyperthyroidism caused decreased level of GRIN2B gene expression in the hippocampus.

Impaired learning and memory generated by hyperthyroidism is rescued by restoration of AMPA and NMDA receptors function

Hyperthyroidism has been identified as a risk factor for cognitive disorders. The hippocampus is a key brain region associated with cognitive function, among which excitatory synapse transmission plays an important role in the process of learning and memory. However, the mechanism by which hyperthyroidism leads to cognitive dysfunction through a synaptic mechanism remains unknown. We investigated the synaptic mechanisms in the effects of hyperthyroidism in an animal model that involved repeated injection of triiodothyronine (T3). These mice displayed impaired learning and memory in the Novel object recognition test, Y-maze test, and Morris Water Maze test, as well as elevated anxiety in the elevated plus maze. Mature dendritic spines in the hippocampal CA1 region of hyperthyroid mice were significantly decreased, accompanied by decreased level of AMPA- and NMDA-type glutamate receptors in the hippocampus. In primary cultured hippocampal neurons, levels of AMPA- and NMDA-type glutamate receptors also decreased and whole-cell patch-clamp recording revealed that excitatory synaptic function was obviously attenuated after T3 treatment. Notably, pharmacological activation of AMPAR or NMDAR by intraperitoneal injection of CX546, an AMPAR agonist, or NMDA, an NMDAR agonist can restore excitatory synaptic function and corrected impaired learning and memory deficit in hyperthyroid mice. Together, our findings uncovered a previously unrecognized AMPAR and NMDAR-dependent mechanism involved in regulating hippocampal excitatory synaptic transmission and learning and memory disorders in hyperthyroidism.

Prediction of the cognitive impairment development in patients with autoimmune thyroiditis and hypothyroidism

Objective. The aim of the present work is to define the risk factors that can affect the presence of a cognitive impairment and analyze the associations of the brain-derived neurotrophic factor (BDNF) gene polymorphism (rs6265), vitamin D receptor (VDR) gene polymorphism (rs2228570), and N-methyl-D-aspartate (NMDA) receptor gene polymorphism (rs4880213) with the cognitive impairment in patients with autoimmune thyroiditis and hypothyroidism in the Western Ukraine population and predict the development of cognitive disorders in these patients.

Methods. The study involved 153 patients with various forms of thyroid pathology (hypothyroidism, autoimmune thyroiditis, elevated serum antibodies anti-thyroglobulin and anti-thyroid peroxidase). Cognitive impairment in the examined patients was evaluated based on the results of the Mini-Mental State Examination (MMSE) test. BDNF, GRIN2B, and 25-OH Vitamin D levels in the serum of the patients and healthy individuals were quantified using highly sensitive commercial enzyme-linked immunosorbent assay kits. Genotyping of the VDR (rs2228570), BDNF (rs6265), and NMDA receptor (rs4880213) gene polymorphism was performed using TaqMan probes and Taq-Man Genotyping Master Mix (4371355) on CFX96™Real-Time PCR Detection System. Polymerase chain reaction (PCR) for TaqMan genotyping was carried out according to the kit instructions.

Results. Strong direct relationship between the “Level GRIN2B” and cognitive impairments (p=0.006) was established after evaluating the complex model based on the values of the regression coefficient. An increase in the likelihood of cognitive impairment by 24.898-fold (p=0.012) was seen after assessing the effect of the CT rs6265 genotype. In addition, direct relationship between the influence of the TT rs6265 genotype and an increase in the likelihood of cognitive impairment by a factor of 21.734 (p=0.024) was also established.

Conclusion. The present data indicate that the BDNF, TSH, fT4, and vitamin D levels prognostically belong to the significant indicators of the cognitive impairment development.

Association between NMDA gene polymorphism (rs4880213) and GRIN2B blood serum levels in thyroid pathology patients

The article discusses a new hypothesis that autoimmune diseases of the thyroid gland can lead to depression and neurological complications. It is believed that the neuronal N-methyl-D-aspartate receptor plays a significant role in depression pathophysiology and neurological and mental diseases, respectively. The study involved 153 patients with various forms of thyroid pathology. GRIN2B levels in the sera of the patients and healthy individuals were quantified using enzyme-linked immunosorbent assay with highly sensitive Human GRIN2B (Glutamate Receptor, Ionotropic, N-Methyl-D-Aspartate 2B) ELISA Kit. Genotyping of the glutamate ionotropic receptor NMDA type subunit 1, GRIN1 (rs4880213) gene polymorphism. The CT genotype of the NMDA gene (rs4880213) was predominant in the surveyed population. The C allele of the NMDA gene was more frequent than the T allele among patients with thyroid disease. GRIN2B levels were significantly decreased in patients with postoperative hypothyroidism 3.45 times, and in patients with AIT-induced hypothyroidism, there was a probable increase in GRIN2B levels by 1.58 times compared with controls. GRIN2B levels were significantly different in patients of different groups depending on thyroid pathology. Our study showed direct close correlation (r=0.635) between GRIN2B and anti-TPO levels (p<0.001), a significant direct close correlation (r=0.527) between GRIN2B and anti-TG levels in the blood (p<0.001). Our results allow us to consider the GRIN2B level as an important prognostic minimally invasive marker of neurological complications in endocrine pathology.

The Role of Thyroid Function in Alzheimer's Disease

The thyroid gland is crucial for the regulation of metabolism, growth, and development of various tissues, organs, systems, including the central nervous system. Recent studies have implicated the role of thyroid dysfunction in the etiology of Alzheimer's disease (AD), while AD leads to a significant increase in the prevalence of thyroid dysfunction. In this review, we have analyzed the role of thyroid function in the pathophysiology of AD as well as its biomarkers. The present review aims to provide encouraging targets for early screening of AD risk factors and intervention strategies.

Gallic Acid Prevents the Oxidative and Endoplasmic Reticulum Stresses in the Hippocampus of Adult-Onset Hypothyroid Rats

Thyroid hormone is essential for hippocampal redox environment and neuronal viability in adulthood, where its deficiency causes hypothyroidism related to oxidative and endoplasmic reticulum stresses in the hippocampus, resulting in neuronal death. One option of treatment is antioxidants; however, they must be transported across the blood-brain barrier. Gallic acid is a polyphenol that meets these criteria. Thus, this study aimed to prove that the neuroprotective mechanism of GA is associated with the prevention of oxidative and endoplasmic reticulum stresses in the hippocampus of adult-onset hypothyroid rats. Male Wistar rats were divided into euthyroid (n = 20) and hypothyroid groups (n = 20). Thyroidectomy with parathyroid gland reimplementation caused hypothyroidism. Each group was subdivided into two: vehicle and 50 mg/kg/d of gallic acid. 3 weeks after thyroidectomy, six animals of each group were euthanized, and the hippocampus was dissected to evaluate oxidative and endoplasmic reticulum stress markers. The rest of the animals were euthanized after 4 weeks of treatment for histological analysis of the hippocampus. The results showed that hypothyroidism increased lipid peroxidation, reactive oxygen species, and nitrites; it also increased endoplasmic reticulum stress by activating the inositol-requiring enzyme-1α (IRE1α) pathway, the protein kinase RNA-like endoplasmic reticulum kinase (PERK) and activated transcription factor 6α (ATF6α) pathways associated with a proapoptotic state that culminates in hippocampal neuronal damage. Meanwhile, the hypothyroid rat treated with gallic acid reduced oxidative stress and increased endoplasmic reticulum-associated degradation (ERAD) through IRE1α and ATF6. Also, the gallic acid treatment prevented the Bax/BCl2 ratio from increasing and the overexpression of p53 and caspase 12. This treatment in hypothyroid animals was associated with the neuronal protection observed in the hippocampus. In conclusion, gallic acid prevents hypothyroidism-induced hippocampal damage associated with oxidative and endoplasmic reticulum stresses.

Association of altered thyroid hormones and neurometabolism to cognitive dysfunction in unmedicated bipolar II depression

BACKGROUND

The underlying mechanism of cognitive impairment in bipolar II depression (BD II) remains unclear. Studies show disturbances of the hypothalamus-pituitary-thyroid (HPT) axis are suspected of correlating to brain neurometabolic alterations and cognitive deficits in psychiatric disorders. While, the nature of their inter-relationships in BD II depression remain enigmatic.

METHODS

106 patients with unmedicated BD II depression and 100 healthy controls underwent cognitive function assessment using Trail Making Test, Part-A (TMT-A), Digit Symbol Substitution Test (DSST), and Semantic Verbal Fluency testing (SVF). Of those, 69 patients and 53 healthy controls had serum thyroid hormone levels measured including free tri-iodothyronine (FT3), total tri-iodothyronine (TT3), free thyroxin (FT4), total thyroxin (TT4) and thyroid-stimulating hormone (TSH). Additionally, 79 of the patients and 76 of the healthy controls underwent proton magnetic resonance spectroscopy (H-MRS) to obtain ratios of N-acetyl aspartate to creatine (NAA/Cr) and choline-containing compounds to creatine (Cho/Cr) in the prefrontal cortex (PFC), anterior cingulate cortex (ACC) and thalamus. Finally, association and multiple regression analysis were conducted to investigate their inter-relationships.

RESULTS

Patients with BD II depression showed significantly lower DSST and verbal fluency scores and longer completion time of TMT-A than did healthy controls. The FT3, TT3, and TSH levels of the BD cohort significantly decreased, while their FT4 levels increased. We also found significantly lower NAA/Cr ratios in the PFC and higher NAA/Cr ratios in the left thalamus of patients with BD II depression than in healthy controls. Furthermore, association analysis showed that increased FT4 negatively correlated to DSST and SVF, while increased FT4 correlation significantly with increasing TSH and DSST. Multiple regression analyses revealed relationships between TSH and NAA in the left PFC and the left thalamus, while correlating to SVF testing within the BD II depression cohort.

CONCLUSIONS

Our results demonstrate coinciding thyroid hormone abnormalities, cognitive dysfunction, and neurometabolic alterations of the PFC-thalamic circuitry occur in an early course of BD II depression. Further understanding of the interaction between thyroid-stimulating hormone and NAA/Cr of PFC-thalamic circuitry may shed light on the etiology of associated cognitive impairment.

Experimental hypothyroidism raises brain kynurenic acid - Novel aspect of thyroid dysfunction

Hypothyroidism frequently manifests with altered mood and disturbed cognition. Kynurenic acid may influence cognition through antagonism of N-methyl-d-aspartate receptors (NMDA) and α7 nicotinic receptors. In here, thyroid hormones effects on kynurenic acid synthesis in rat cortical slices and on kynurenine aminotransferases (KATs) activity in semi-purified cortical homogenates were studied. Furthermore, brain kynurenic acid levels and KATs activities were evaluated in experimental model of hypothyroidism, induced by chronic administration of 0.05% propylthiouracil in drinking water. In vitro, L-thyroxine (T₄) and 3,3,5-triiodothyronine (T₃), reduced kynurenic acid synthesis and KATs activities (IC₅₀ ~ 50-150 μM). In vivo, propylthiouracil increased cortical, hippocampal and striatal, but not cerebellar kynurenic acid content (192%, 142% and 124% of control, respectively), despite uniformly decreased KAT II activity and lower cortical and striatal KAT I activity. T₄ application to hypothyroid animals restored kynurenic acid levels to control values and reversed enzymatic changes. T₄ alone did not change brain kynurenic acid levels, despite increased activities of brain KATs. Hence, thyroid hormones modulate kynurenic acid levels by two opposing mechanisms, stimulation of KATs activity, most probably transcriptional, and direct, post-translational inhibition of KATs. Lack of correlation between KATs activity and kynurenic acid level may reflect the influence of T₄ on organic anion transporter and result from impaired removal of kynurenic acid from the brain during hypothyroidism. Our data reveal novel mechanism linked with thyroid hormones deficiency and imply the potential involvement of increased brain kynurenic acid in the hypothyroidism-related cognitive disturbance.

Modulation of glutamate levels and Na+,K+-ATPase activity contributes to the chrysin memory recovery in hypothyroidism mice

Abnormalities in the thyroid hormones, like in hypothyroidism, are closely related to dementia and Alzheimer's disease demonstrating the main symptom of these disorders: memory deficit. In this study we evaluated the effect of chrysin on deficit spatial and aversive memories and the contribution of glutamatergic, cholinergic pathways and Na⁺, K⁺-ATPase activity on hippocampus and prefrontal cortex in hypothyroid adult female mice C57BL/6. Hypothyroidism was induced by the continuous exposure to 0.1% methimazole (MTZ) in drinking water for 31 days. The exposure to MTZ was associated to low plasma levels of thyroid hormones (TH) compared to the control group on the 32nd. Subsequently, euthyroid and MTZ-induced hypothyroid mice received (intragastrically) either vehicle or chrysin (20 mg/kg) once a day for 28 consecutive days. After treatments mice performed the following behavioral assessments: open-field test (OFT), morris water maze (MWM) and passive avoidance test. Additionally, plasma TH levels were measured again, as well as glutamate levels, Na⁺,K⁺-ATPase and acetylcholinesterase (AChE) activities were analyzed in the hippocampus and prefrontal cortex of mice. Mice with hypothyroidism showed a deficit of spatial and aversive memory and chrysin treatment reversed these deficits. It also reduced the levels of glutamate and decreased Na⁺,K⁺-ATPase activity in both cerebral structures in the hypothyroid mice compared with the euthyroid ones, with the exception of glutamate in the hippocampus, which was a partial reversal. AChE activity was not altered by treatments. Together, our results demonstrate that chrysin normalized hippocampal glutamate levels and Na⁺,K⁺-ATPase activity, which could be involved in the reversal of memory deficit.

Neuroprotective and long term potentiation improving effects of vitamin E in juvenile hypothyroid rats

Abstract. Protective effects of vitamin E (Vit E) on long term potentiation (LTP) impairment, neuronal apoptosis and increase of nitric oxide (NO) metabolites in the hippocampus of juvenile rats were examined. The rats were grouped (n=13) as: (1) control; (2) hypothyroid (Hypo) and (3) Hypo-Vit E. Propylthiouracil (PTU) was given in drinking water (0.05%) during 6 weeks. Vit E (20 mg/ kg) was daily injected (IP). To evaluate synaptic plasticity, LTP from the CA1 area of the hippocampus followed by high frequency stimulation to the ipsilateral Schafer collateral pathway was carried out. The cortical and hippocampal tissues were then removed to measure NO metabolites. The brains of 5 animals in each group were removed for apoptosis study. The hypothyroidism status decreased the slope, 10–90% slope and amplitude of field excitatory post synaptic potential (fEPSP) compared to the control group (P<0.01–P<0.001). Injection of Vit E increased the slope, 10–90% slope and amplitude of the fEPSP in the Hypo-Vit E group in comparison to the Hypo group (P<0.05–P<0.01). TUNEL positive neurons and NO metabolites were higher in the hippocampus of the Hypo rats, as compared to those in the hippocampus of the control ones (P<0.001). Treatment of the Hypo rats by Vit E decreased apoptotic neurons (P<0.01–P<0.001) and NO metabolites (P<0.001) in the hippocampus compared to the Hypo rats. The results of the present study showed that Vit E prevented the LTP impairment and neuronal apoptosis in the hippocampus of juvenile hypothyroid rats.

Memantine, an NMDA Receptor Antagonist, Prevents Thyroxin-induced Hypertension, but Not Cardiac Remodeling

Stimulation of glutamatergic tone has been causally linked to myocardial pathogenesis and amplified systemic blood pressure (BP). Memantine, a noncompetitive N-methyl-D-aspartate glutamatergic receptor (NMDA-R) antagonist, has been proposed to be an active cardioprotective drug. However, the efficacy of memantine and subsequently the possible involvement of the NMDA-R in the thyroxin (T4)-induced cardiovascular complications have never been investigated. We examined the effect of memantine (30 mg·kg·d) on the T4 (500 μg·kg·d)-provoked increase in mouse BP, cardiac hypertrophy indicated by enlarged overall myocardial mass, and reformed reactions of the contractile myocardium both in vivo and ex vivo after 2 weeks of treatment. Memantine alone did not result in any cardiovascular pathology in mice. Instead, memantine significantly prevented the T4-triggered systemic hypertension. But, it did not reverse cardiac hypertrophy, coupled in vivo left ventricular dysfunction (LV) or ex vivo right ventricular (RV) papillary muscle contractile alterations of the T4-treated mice. Our results openly direct the cardiovascular safety and tolerability of memantine therapy. Yet, extra research is necessary to endorse these prospective advantageous outcomes. Also, we believe that this is the first study to inspect the possible role of NMDA-R in the T4-stimulated cardiovascular disorders and concluded that NMDA-R could play a key role in the T4-induced hypertension.

Peripheral and central administration of T3 improved the histological changes, memory and the dentate gyrus electrophysiological activity in an animal model of Alzheimer's disease

The amyloid beta (Aβ) induced Alzheimer's disease (AD) is associated with formation the amyloid plaques, cognitive impairments and decline in spontaneous discharge of neurons. In the current study, we evaluated the effect of subcutaneous (S. C) and intrahippocampal (I. H) administrations of triiodothyronine (T3) on the histological changes, memory and the dentate gyrus (DG) electrophysiological activity in an animal model of AD. Eighty adult male Wistar rats (250-300 g) were divided randomly into five groups: Sham-Operated (Sh-O), AD + Vehicle (S. C), AD + Vehicle (I. H), AD+ T3 (S. C) and AD + T3 (I. H). In order to induce animal model of AD, Aβ (10 ng/μl, bilaterally) were injected intrahippocampally. Rats were treated with T3 and/or normal saline for 10 days. Passive avoidance and spatial memory were evaluated in shuttle box apparatus and Morris water maze, respectively. Neuronal single unit recording was assessed from hippocampal DG. The percent of total time that animals spent in target quarter, the mean latency time (sec), the step through latency and the average number of spikes/bin were decreased significantly in AD rats compared with the Sh-O group (p < 0.001) and were increased significantly in AD groups that have received T3 (S. C and I. H) in compared with AD group (p < 0.01, p < 0.001). Also, formation of amyloid plaques was decreased in AD rats treated with T3.The results showed that T3 injection (S. C and I. H), by reduction of neural damage and increment of neuronal spontaneous activity improved the memory deficits in Aβ-induced AD rats.

Effects of Dendropanax morbifera Léveille extract on hypothyroidism-induced oxidative stress in the rat hippocampus

In this experiment, we verified the effects of Dendropanax morbifera Léveille stem extract (DMS) on hypothyroidism-induced oxidative stress in the hippocampus of rats. Hypothyroidism was induced in rats by treating them with 0.03% 2-mercapto-1-methyl-imidazole dissolved in drinking water for 5 weeks. DMS (100 mg/kg) was also orally administered to the rats during the same period and the animals were sacrificed at 12 weeks of age. DMS administration tended to ameliorate these hypothyroidism-induced changes in serum triiodothyronine (T₃), thyroxine (T₄), and thyroid-stimulating hormone levels. DMS administration significantly reduced the hypothyroidism-induced increases in reactive oxygen species production as well as in lipid peroxidation in the hippocampus. In addition, DMS administration increased hippocampal Cu, Zn-superoxide dismutase (SOD1), catalase (CAT), and glutathione peroxidase (GPx) levels. These results suggest that DMS potentially ameliorates hypothyroidism-induced neuroendocrine phenotypes and oxidative stress in the hippocampus via the induction of antioxidant enzymes.

Central and peripheral administrations of levothyroxine improved memory performance and amplified brain electrical activity in the rat model of Alzheimer's disease

Alzheimer's disease (AD) is associated with cognitive impairments and a decline in the spontaneous neuronal discharge. In the current study, we evaluated the effect of subcutaneous (S.C.) and intrahippocampal (I.H.) administrations of levothyroxine (LT-4) on the passive avoidance and spatial memory, as well as electrophysiological activity in an animal model of AD. One hundred-sixty male Wistar rats were divided into two main groups. The S.C. group included two Sham and four AD (vehicle or L-T4 25, 50 & 100μg/kg); and the I.H. had consisted of two Sham and two AD (vehicle or L-T4 10μg/kg) subgroups. To make an animal model of AD, amyloid beta (Aβ) plus ibotenic acid (Ibo) were injected I.H. Rats were treated with L-T4 and/or normal saline for ten days. Passive avoidance and spatial memory were evaluated in shuttle box and Morris water maze, respectively. Neuronal single unit recording was assessed from hippocampal dentate gyrus (DG). Results showed that the mean latency time (s) increased significantly (p<0.001) in AD animals and decreased significantly in both S.C. and I.H. L-T4 injected AD animals, compared with the AD group (p<0.001). The percentage of total time that animals spent in goal quarter and the step through latency decreased significantly in AD rats (p<0.001) and increased significantly in both S.C. and I.H. L-T4 injected AD animals in comparison with the AD group (p<0.01, p<0.001). Data showed that the average number of spikes/bin significantly decreased in the AD group (p<0.001). The S.C. and I.H. L-T4 injections in AD rats significantly increased the spike rate in comparison to the AD group (p<0.001). In conclusion, both S.C. and I.H. injections of L-T4 alleviated memory deficits and spontaneous neuronal activity in Aβ-induced AD rats. Also, I.H. microinjection of L-T4 had more beneficial effects on memory and neuronal electrophysiological activity in comparison to S.C. administration.

Influence of maternal care on the developing brain: Mechanisms, temporal dynamics and sensitive periods

Variation in maternal care can lead to divergent developmental trajectories in offspring with implications for neuroendocrine function and behavioral phenotypes. Study of the long-term outcomes associated with mother-infant interactions suggests complex mechanisms linking the experience of variation in maternal care and these neurobiological consequences. Through integration of genetic, molecular, cellular, neuroanatomical, and neuroendocrine approaches, significant advances in our understanding of these complex pathways have been achieved. In this review, we will consider the impact of maternal care on male and female offspring development with a particular focus on the issues of timing and mechanism. Identifying the period of sensitivity to maternal care and the temporal dynamics of the molecular and neuroendocrine changes that are a consequence of maternal care represents a critical step in the study of mechanism.

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.

Adult onset-hypothyroidism: alterations in hippocampal field potentials in the dentate gyrus are largely associated with anaesthesia-induced hypothermia

Thyroid hormone (TH) is essential for a number of physiological processes and is particularly critical during nervous system development. The hippocampus is strongly implicated in cognition and is sensitive to developmental hypothyroidism. The impact of TH insufficiency in the foetus and neonate on hippocampal synaptic function has been fairly well characterised. Although adult onset hypothyroidism has also been associated with impairments in cognitive function, studies of hippocampal synaptic function with late onset hypothyroidism have yielded inconsistent results. In the present study, we report hypothyroidism induced by the synthesis inhibitor propylthiouracil (10 p.p.m., 0.001%, minimum of 4 weeks), resulted in marginal alterations in excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude in the dentate gyrus measured in vivo. No effects were seen in tests of short-term plasticity, and a minor enhancement of long-term potentiation of the EPSP slope was observed. The most robust synaptic alteration evident in hypothyroid animals was an increase in synaptic response latency, which was paralleled by a failure to maintain normal body temperature under anaesthesia, despite warming on a heating pad. Latency shifts could be reversed in hypothyroid animals by increasing the external heat source and, conversely, synaptic delays could be induced in control animals by removing the heat source, with a consequent drop in body and brain temperature. Thermoregulation is TH- dependent, and anaesthesia necessary for surgical procedures posed a thermoregulatory challenge that was differentially met in control and hypothyroid animals. Minor increases in field potential EPSP slope, decreases in PS amplitudes and increased latencies are consistent with previous reports of hypothermia in naive control rats. We conclude that failures in thyroid-dependent temperature regulation rather than direct action of TH in synaptic physiology are responsible for the observed effects. These findings stand in contrast to the synaptic impairments observed in adult offspring following developmental TH insufficiency, and emphasise the need to control for the potential unintended consequences of hypothermia in the interpretation of hypothyroid-induced changes in physiological systems, most notably synaptic transmission.

The effect of omega-3 on cognition in hypothyroid adult male rats

Thyroid hormones and omega-3 are essential for normal brain functions. Recent studies have suggested that omega-3 may protect against the risk of dementia. The aim of this study was to investigate the effect of hypothyroidism on spatial learning and memory in adult male rats, the underlying mechanisms and the possible therapeutic value of omega-3 supplementation. Thirty male rats were divided into three groups; control, hypothyroid and omega-3 treated. Hypothyroidism induced significant deficits in working and reference memories in radial arm maze, retention deficits in passive avoidance test and impaired intermediate and long-term memories in novel object recognition test. Serum total antioxidant capacity (TAC) and hippocampal serotonin and γ-aminobutyric acid (GABA) levels were decreased in the hypothyroid group as compared to the control group. Moreover, the hippocampus of hypothyroid rats showed marked structural changes as diffuse vacuolar degeneration and distortion of the pyramidal cells. Immunohistochemistry showed that the expression of Cav1.2 (the voltage dependent LTCC alpha 1c subunit) protein was increased in the hypothyroid group as compared to the control group. Omega-3 supplementation ameliorated memory deficits, increased TAC, decreased the structural changes and decreased the expression of Cav1.2 protein. In conclusion omega-3 could be useful as a neuroprotective agent against hypothyroidism-induced cognitive impairment.

Decreased pain threshold and enhanced synaptic transmission in the anterior cingulate cortex of experimental hypothyroidism mice

Background

Thyroid hormones are essential for the maturation and functions of the central nervous system. Pain sensitivity is related to the thyroid status. However, information on how thyroid hormones affect pain processing and synaptic transmission in the anterior cingulate cortex (ACC) is limited. Nociceptive threshold and synaptic transmission in the ACC were detected in the experimental hypothyroidism (HT) mice.

Results

HT was induced by methimazole and potassium perchlorate in distilled drinking water for 4 weeks. The threshold of pain perception to hot insults, but not mechanical ones, decreased in hypothyroid mice. After treatment with tri-iodothyronine (T3) or thyroxine (T4) for 2 weeks, thermal pain threshold recovered. Electrophysiological recordings revealed enhanced glutamatergic synaptic transmission and reduced GABAergic synaptic transmission in the ACC. Supplementation with T3 or T4 significantly rescued this synaptic transmission imbalance. In the same model, HT caused the up-regulation of the GluR1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor and NR2B-containing N-methyl-D-aspartate receptors, but it down-regulated γ-aminobutyric acid A receptors in the ACC. Supplementation with T3 or T4 notably recovered the levels of above proteins.

Conclusions

These results suggest that HT promotes hypersensitivity to noxious thermal, and that supplementation with T3 or T4 rescues the imbalance between excitatory and inhibitory transmission in the ACC.

Low-dose thyroxine attenuates autism-associated adverse effects of fetal alcohol in male offspring's social behavior and hippocampal gene expression

BACKGROUND

Fetal alcohol spectrum disorder (FASD) is characterized by neurodevelopmental anomalies manifesting in cognitive and behavioral deficits in the offspring with diverse severities. Social behavior is affected in FASD, and these deficits overlap with those of autism spectrum disorder (ASD). Identifying some of the molecular characteristics related to ASD in an animal model of FASD could ultimately provide details on the underlying molecular mechanisms of both disorders that could lead to novel treatments.

METHODS

Pregnant Sprague-Dawley rats received the following diets: control (C; ad libitum standard laboratory chow), nutritional control pair-fed (PF), ethanol (EtOH), or an EtOH diet supplemented with 0.3, 1.5, or 7.5 mg thyroxine (T4)/l in the diet. Social behavior and memory were tested in the adult offspring. Plasma total T4, free T3 (fT3), and thyroid-stimulating hormone (TSH) levels were measured. Hippocampal expression of Gabrb3, Ube3a, Nr2b, Rasgrf1, and Dio3 were measured by RT-qPCR and protein levels of Mecp2 and Slc25a12 by Western blotting.

RESULTS

Adult male offspring of EtOH dams showed elevated fT3 and low TSH levels. Adult male, but not female, offspring of EtOH dams exhibited social behavior and memory deficits. Expression of autism candidates, Gabrb3, Ube3a, Mecp2, and Slc25a12, was significantly increased in the hippocampus of male offspring of EtOH dams. Hippocampal Nr2b and Dio3 were also increased, while Rasgrf1 was decreased in the same population. Peripheral thyroid function, social behavioral deficits, and altered expression of the above genes were normalized by simultaneous administration of 0.3 mg/l T4 in the EtOH diet.

CONCLUSIONS

Our data suggest that social interaction deficits of FASD share molecular mechanism with ASD by showing altered hippocampal expression of several ASD candidate genes. Social interaction deficits as well as the gene expression changes in the offspring of EtOH-consuming dams can be reversed by low dose of thyroid hormone supplementation to the mothers.

New approaches to thyroid hormones and purinergic signaling

It is known that thyroid hormones influence a wide variety of events at the molecular, cellular, and functional levels. Thyroid hormones (TH) play pivotal roles in growth, cell proliferation, differentiation, apoptosis, development, and metabolic homeostasis via thyroid hormone receptors (TRs) by controlling the expression of TR target genes. Most of these effects result in pathological and physiological events and are already well described in the literature. Even so, many recent studies have been devoted to bringing new information on problems in controlling the synthesis and release of these hormones and to elucidating mechanisms of the action of these hormones unconventionally. The purinergic system was recently linked to thyroid diseases, including enzymes, receptors, and enzyme products related to neurotransmitter release, nociception, behavior, and other vascular systems. Thus, throughout this text we intend to relate the relationship between the TH in physiological and pathological situations with the purinergic signaling.

Enhanced sensitivity to learning fearful associations during adolescence

The majority of anxiety disorders emerge during adolescence, yet there is a paucity of research examining factors that contribute to the "storm and stress" of this period. Understanding how juvenile (P23), adolescent (P35), and adult (P90) rats differ on basic fear conditioning tasks may shed light on this issue. In Experiment 1, P23, P35, and P90 rats were given 6 CS-US presentations. There were four training conditions: Delay (i.e., CS co-terminating with the US), Trace 20 and Trace 40 (i.e., an interval of 20s and 40s between the CS and US, respectively), and Unpaired (i.e., explicitly Unpaired presentations of the CS and US). Twenty-four hours after conditioning, freezing was measured to assess fear of the CS in a novel context. At test, there were no age differences in CS-elicited freezing in group Delay, and this condition exhibited significantly higher levels of freezing compared to group Unpaired. However, the adolescent rats were the only age group to exhibit higher levels of freezing following training with the 20s and 40s trace intervals, compared to Unpaired controls. Experiment 2 replicated the finding that adolescent but not adult rats exhibit fear following conditioning with a 20s trace interval, while also demonstrating that both age groups display learning with a shorter trace interval of 5s. Experiment 3 showed that exposure to corticosterone (200 μg/ml) in the drinking water for 1 week prior to conditioning selectively disrupts Trace 20 but not Delay conditioning during adolescence. Lastly, in Experiment 4 the test procedures were changed such that freezing was measured both during the CS and during a stimulus free trace interval. Once again, P35 but not P90 rats exhibited fear following training with a 20s trace interval. Taken together, these findings demonstrate that adolescent rats show a heightened propensity to learn fearful associations, and that this is disrupted following exposure to corticosterone.

Hypothyroidism in the adult rat causes incremental changes in brain-derived neurotrophic factor, neuronal and astrocyte apoptosis, gliosis, and deterioration of postsynaptic density

BACKGROUND

Adult hypothyroidism is a highly prevalent condition that impairs processes, such as learning and memory. Even though tetra-iodothyronine (T(4)) treatment can overcome the hypothyroidism in the majority of cases, it cannot fully recover the patient's learning capacity and memory. In this work, we analyzed the cellular and molecular changes in the adult brain occurring with the development of experimental hypothyroidism.

METHODS

Adult male Sprague-Dawley rats were treated with 6-propyl-2-thiouracil (PTU) for 20 days to induce hypothyroidism. Neuronal and astrocyte apoptosis were analyzed in the hippocampus of control and hypothyroid adult rats by confocal microscopy. The content of brain-derived neurotrophic factor (BDNF) was analyzed using enzyme-linked immunosorbent assay (ELISA) and in situ hybridization. The glutamatergic synapse and the postsynaptic density (PSD) were analyzed by electron microscopy. The content of PSD proteins like tyrosine receptor kinase B (TrkB), p75, and N-methyl-D-aspartate receptor (NMDAr) were analyzed by immunoblot.

RESULTS

We observed that the hippocampus of hypothyroid adult rats displayed increased apoptosis levels in neurons and astrocyte and reactive gliosis compared with controls. Moreover, we found that the amount of BDNF mRNA was higher in the hippocampus of hypothyroid rats and the content of TrkB, the receptor for BDNF, was reduced at the PSD of the CA3 region of hypothyroid rats, compared with controls. We also observed that the glutamatergic synapses from the stratum radiatum of CA3 from hypothyroid rats, contained thinner PSDs than control rats. This observation was in agreement with a reduced content of NMDAr subunits at the PSD in hypothyroid animals.

CONCLUSIONS

Our data suggest that adult hypothyroidism affects the hippocampus by a mechanism that alters the composition of PSD, reduces neuronal and astrocyte survival, and alters the content of the signaling neurotrophic factors, such as BDNF.

Oxytocin regulates changes of extracellular glutamate and GABA levels induced by methamphetamine in the mouse brain

Oxytocin (OT), a neurohypophyseal neuropeptide, affects adaptive processes of the central nervous system. In the present study, we investigated the effects of OT on extracellular levels of glutamate (Glu) and γ-aminobutyric acid (GABA) induced by methamphetamine (MAP) in the medial prefrontal cortex (mPFC) and dorsal hippocampus (DHC) of freely moving mice, using in vivo microdialysis coupled to high-performance liquid chromatography and fluorescence detection. The results showed that OT had no effect on basal Glu levels, but attenuated MAP-induced Glu increase in the mPFC and decrease in the DHC. OT increased the basal levels of extracellular GABA in mPFC and DHC of mice, and inhibited the MAP-induced GABA decrease in DHC. Western blot results indicated that OT significantly inhibited the increased glutamatergic receptor (NR1 subunit) levels in the PFC after acute MAP administration, whereas OT further enhanced the elevated levels of glutamatergic transporter (GLT1) induced by MAP in the hippocampus of mice. Atosiban, a selective inhibitor of OT receptor, antagonized the effects of OT. The results provided the first neurochemical evidence that OT, which exerted its action via its receptor, decreased Glu release induced by MAP, and attenuated the changes in glutamatergic neurotransmission partially via regulation of NR1 and GLT1 expression. OT-induced extracellular GABA increase also suggests that OT acts potentially as an inhibitory neuromodulator in mPFC and DHC of mice.

The effects of olibanum administered to methimazole-treated dams during lactation on learning and memory of offspring rats

The effect of olibanum administered during lactation to methimazole-treated dams on the learning of offspring was evaluated. The animals were treated for 60 days from the first day of lactation period: group 1--tap water, group 2-0.03% methimazole and groups 3 and 4-0.03% methimazole with 0.25% or 0.125% olibanum, respectively. The serum thyroxin level in the offspring of group 2 was significantly lower than that of group 1. However, there was no difference compared to groups 3 and 4. In a Morris water maze, the distance and time latency to reach the platform in offspring of group 2 was significantly higher than groups 1, 3 and 4. In probe trial, the time spent in target quadrant (Q₁) by offspring of group 2 was lower than groups 1, 3 and 4. It is suggested that impaired learning and memory in the offspring of hypothyroid rats may be prevented by olibanum.

Peripheral glutamate signaling in head and neck areas

The major excitatory neurotransmitter glutamate is also found in the periphery in an increasing number of nonexcitable cells. In line with this it became apparent that glutamate can regulate a broad array of peripheral biological responses, as well. Of particular interest is the discovery that glutamate receptor reactive reagents can influence tumor biology. However, the knowledge of glutamate signaling in peripheral tissues is still incomplete and, in the case of head and neck areas, is almost lacking. The roles of glutamate signaling pathways in these regions are manifold and include orofacial pain, periodontal bone production, skin and airway inflammation, as well as salivation. Furthermore, the interrelations between glutamate and cancers in the oral cavity, thyroid gland, and other regions are discussed. In summary, this review shall strengthen the view that glutamate receptor reagents may also be promising targets for novel therapeutic concepts suitable for a number of diseases in peripheral tissues. The contents of this review cover the following sections: Introduction; The "Glutamate System"; The Taste of Glutamate; Glutamate Signaling in Dental Regions; Glutamate Signaling in Head and Neck Areas; Glutamate Signaling in Head and Neck Cancer; A Brief Overview of Glutamate Signaling in Other Cancers; and Conclusion.

Experimentally induced hyperthyroidism disrupts hippocampal long-term potentiation in adult rats

BACKGROUND

Manipulating thyroid hormones has been shown to influence learning and memory. Although a large body of literature is available on the effects of thyroid hormone deficiency on learning and memory functions during developmental or adult-onset hypothyroidism, electrophysiological findings are limited. This limitation is especially notable with respect to thyroxine administration in adult, normothyroid animals.

METHODS

Experiments were carried out on 12 adult male Wistar rats, each 9-10 months of age. Rats were randomly divided into hyperthyroid (0.2 mg/kg/day intraperitoneal thyroxine injection, for 21 days) and control groups (n = 6 animals in each group). Following spatial learning performance tests on hyperthyroid and control groups, rats were anesthetized with urethane and placed in a stereotaxic frame. A bipolar, tungsten electrode was used to stimulate the medial perforant path. A glass micropipette was inserted within the granule cell layer of the ipsilateral dentate gyrus to record field excitatory postsynaptic potentials (fEPSP). Following a 15-min baseline recording of fEPSPs, long-term potentiation (LTP) was induced by four sets of tetanic pulse trains.

RESULTS

Thyroxine-treated rats showed significantly worse performance in the spatial memory task and attenuated input-output relationships in the electrophysiological analyses. Treated rats also showed a lower efficacy of LTP induction when compared with controls.

CONCLUSION

The present study provides clear in vivo evidence for the action of L-thyroxine in the impairment of synaptic plasticity and in inducing spatial memory task deficits in adult rats. These findings may explain the complaints of cognitive function reductions in hyperthyroid patients.

Effects of hypothyroidism on cell proliferation and neuroblasts in the hippocampal dentate gyrus in a rat model of type 2 diabetes

We observed how the hypothyroid state affects diabetic states and modifies cell proliferation and neuroblast differentiation in the hippocampal dentate gyrus (DG). For this, 0.03% methimazole, an anti-thyroid drug, was administered to 7-week-old, pre-diabetic Zucker diabetic fatty (ZDF) rats by drinking water for 5 weeks, and the animals were sacrificed at 12 weeks of age. At this age, corticosterone levels were significantly increased in the ZDF rats compared to those in the control (Zucker lean control, ZLC) rats. Methimazole (methi) treatment in the ZDF rats (ZDF-methi rats) significantly decreased corticosterone levels and diabetes-induced hypertrophy of adrenal glands. In the DG, Ki67 (a marker for cell proliferation)- and doublecortin (DCX, a marker for neuronal progenitors)-immunoreactive cells were much lower in the ZDF rats than those in the ZLC rats. However, in ZDF-methi rats, numbers of Ki67- and DCX-immunoreactive cells were similar to those in the ZLC rats. These suggest that methi significantly reduces diabetes-induced hypertrophy of the adrenal gland and alleviates the diabetes-induced reduction of cell proliferation and neuronal progenitors in the DG.

The beneficial effects of olibanum on memory deficit induced by hypothyroidism in adult rats tested in Morris water maze

Functional consequences of hypothyroidism include impaired learning and memory and inability to produce long-term potentiation (LTP) in hippocampus. Olibanum has been used for variety of therapeutic purposes. In traditional medicine, oilbanum is used to enhance learning and memory. In the present study the effect of olibanum on memory deficit in hypothyroid rats was investigated. Male wistar rats were divided into four groups and treated for 180 days. Group 1 received tap drinking water while in group 2, 0.03% methimazol was added to drinking water. Group 3 and 4 were treated with 0.03% methimazole as well as 100 and 500 mg/kg olibanum respectively. The animals were tested in Morris water maze. The swimming speed was significantly lower and the distance and time latency were higher in group 2 compared with group 1. In groups 3 and 4 the swimming speed was significantly higher while, the length of the swim path and time latency were significantly lower in comparison with group 2. It is concluded that methimazole-induced hypothyroidism impairs learning and memory in adult rats which could be prevented by using olibanum.

Nitric oxide contributes to learning and memory deficits observed in hypothyroid rats during neonatal and juvenile growth

INTRODUCTION

Severe cognitive impairment follows thyroid hormone deficiency during the neonatal period. The role of nitric oxide (NO) in learning and memory has been widely investigated.

METHODS

This study aimed to investigate the effect of hypothyroidism during neonatal and juvenile periods on NO metabolites in the hippocampi of rats and on learning and memory. Animals were divided into two groups and treated for 60 days from the first day of lactation. The control group received regular water, whereas animals in a separate group were given water supplemented with 0.03% methimazole to induce hypothyroidism. Male offspring were selected and tested in the Morris water maze. Samples of blood were collected to measure the metabolites of NO, NO2, NO3 and thyroxine. The animals were then sacrificed, and their hippocampi were removed to measure the tissue concentrations of NO2 and NO3.

DISCUSSION

Compared to the control group's offspring, serum thyroxine levels in the methimazole group's offspring were significantly lower (P<0.01). In addition, the swim distance and time latency were significantly higher in the methimazole group (P<0.001), and the time spent by this group in the target quadrant (Q1) during the probe trial was significantly lower (P<0.001). There was no significant difference in the plasma levels of NO metabolites between the two groups; however, significantly higher NO metabolite levels in the hippocampi of the methimazole group were observed compared to controls (P<0.05).

CONCLUSION

These results suggest that the increased NO level in the hippocampus may play a role in the learning and memory deficits observed in childhood hypothyroidism; however, the precise underlying mechanism(s) remains to be elucidated.

Effects of thyroid hormone replacement on associative learning and hippocampal synaptic plasticity in adult hypothyroid rats

Activity-dependent changes taking place at the hippocampal perforant pathway-dentate gyrus synapse during classical eyeblink conditioning were recorded in adult thyroidectomized (hypothyroid) and control (euthyroid) rats, and in animals treated with thyroid hormones 20 days after thyroidectomy (recovery rats). The aim was to determine the contribution of thyroid hormones and the consequences of adult-onset hypothyroidism to both associative learning and the physiological potentiation of hippocampal synapses during the actual learning process in alert behaving animals. Control and recovery rats presented similar learning curves, whereas hypothyroid animals presented lower values. A single pulse presented to the perforant pathway during the conditioned-unconditioned inter-stimulus interval evoked a monosynaptic field excitatory postsynaptic potential in dentate granule cells (whose slope was linearly related to the rate of acquisition in the control group), but not in hypothyroid and recovery animals. Input-output relationships and long-term potentiation evoked by train stimulation of the perforant pathway were significantly depressed in hypothyroid animals. Thyroid hormone treatment failed to normalize these two neurophysiological abnormalities observed in hypothyroid animals. In contrast, paired-pulse facilitation was not affected by thyroidectomy. The results indicate that thyroid hormone treatment after a short period of adult hypothyroidism helps to restore some hippocampally dependent functions, such as classical conditioning, but not other hippocampal properties, such as the synaptic plasticity evoked during associative learning and during experimentally induced long-term potentiation. The present results have important clinical implications for the handling of patients with adult-onset thyroid diseases.

Participation of NMDA-glutamatergic receptors in hippocampal neuronal damage caused by adult-onset hypothyroidism

We analyzed the participation of N-methyl-d-aspartate (NMDA) receptors in the neuronal damage caused by adult-onset hypothyroidism. Wistar rats were randomly assigned into four groups. The euthyroid group received tap water. The hypothyroid group received methimazole (60 mg/kg) in their drinking water to induce hypothyroidism. Two more groups of rats received the antithyroid treatment and were injected daily with the NMDA antagonist ketamine (15 mg/kg, sc) or MK-801 (0.5mg/kg, ip). Treatments were administered during 4 weeks. At the end of the respective treatments rats were deeply anaesthetized and perfused intracardially with 0.9% NaCl followed by 4% paraformaldehyde. The brains were removed from the skull, and coronal brain sections (7microm thick) were obtained. Neurons were counted in the CA1, CA2, CA3, and CA4 hippocampal regions differentiating between normal and atrophic cells by an experimenter blind to the treatment. The percentage of neuronal damage found in the MMI group was significantly greater in the hippocampal regions compared to the euthyroid group. In contrast, both NMDA antagonists were able to prevent the neuronal damage secondary to hypothyroidism in all hippocampal regions. Our results suggest that the neuronal damage caused in the hippocampus of adult-onset hypothyroid rats requires activation of NMDA channels.

Neural response to transcranial magnetic stimulation in adult hypothyroidism and effect of replacement treatment

PURPOSE

Despite clinical evidences that hypothyroidism is often associated with cognitive dysfunction, affective disorders and psychosis, the effects of thyroid hormone deficiency on the adult brain have been largely unexplored. We investigated the hypothesis that hypothyroidism might affect cortical excitability and modulates inhibitory and excitatory cortical circuits by using Transcranial Magnetic Stimulation.

MATERIALS AND METHODS

Cortical excitability was probed in 10 patients with overt hypothyroidism and 10 age-matched healthy controls. We tested motor thresholds and corticospinal excitability, cortical silent period and peripheral silent period, short interval intracortical inhibition, intracortical facilitation. Patients were evaluated at the time of diagnosis, as well as after 3 and 6 months replacement therapy with l-thyroxin.

RESULTS

At baseline, patients showed decreased cortical excitability, with increased resting and active motor threshold and decreased steepness of the motor evoked potential recruitment curves. These changes were paralleled by longer cortical silent period and decreased short interval intracortical inhibition. After 3 months replacement therapy, all the parameters but short interval intracortical inhibition were restored to normal values. Short interval intracortical inhibition returned to normal values only after 6 months of replacement therapy.

CONCLUSIONS

Thyroid hormones are needed to modulate cortical excitability and cortical inhibitory circuits in adults.

Changes in acetylcholinesterase, Na+,K+-ATPase, and Mg2+-ATPase activities in the frontal cortex and the hippocampus of hyper- and hypothyroid adult rats

The thyroid hormones (THs) are crucial determinants of normal development and metabolism, especially in the central nervous system. The metabolic rate is known to increase in hyperthyroidism and decrease in hypothyroidism. The aim of this work was to investigate how changes in metabolism induced by THs could affect the activities of acetylcholinesterase (AChE), (Na+,K+)- and Mg2+-adenosinetriphosphatase (ATPase) in the frontal cortex and the hippocampus of adult rats. Hyperthyroidism was induced by subcutaneous administration of thyroxine (25 microg/100 g body weight) once daily for 14 days, and hypothyroidism was induced by oral administration of propylthiouracil (0.05%) for 21 days. All enzyme activities were evaluated spectrophotometrically in the homogenated brain regions of 10 three-animal pools. A region-specific behavior was observed concerning the examined enzyme activities in hyper- and hypothyroidism. In hyperthyroidism, AChE activity was significantly increased only in the hippocampus (+22%), whereas Na+,K+-ATPase activity was significantly decreased in the hyperthyroid rat hippocampus (-47%) and remained unchanged in the frontal cortex. In hypothyroidism, AChE activity was significantly decreased in the frontal cortex (-23%) and increased in the hippocampus (+21%). Na+,K+-ATPase activity was significantly decreased in both the frontal cortex (-35%) and the hippocampus (-43%) of hypothyroid rats. Mg2+-ATPase remained unchanged in the regions of both hyper- and hypothyroid rat brains. Our data revealed that THs affect the examined adult rat brain parameters in a region- and state-specific way. The TH-reduced Na+,K+-ATPase activity may increase the synaptic acetylcholine release and, thus, modulate AChE activity. Moreover, the above TH-induced changes may affect the monoamine neurotransmitter systems in the examined brain regions.

Adult-onset hypothyroidism facilitates and enhances LTD: Reversal by chronic nicotine treatment

Chronic nicotine treatment reverses hypothyroidism-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). We investigated the effect of hypothyroidism on long-term depression (LTD) and possible protection by nicotine. Following paired pulse stimulation, LTD was expressed in hippocampal area CA1 of anesthetized thyroidectomized, euthyroid (sham control), nicotine-treated and nicotine-treated thyroidectomized (hypothyroid) rats. In hypothyroid rats, a significantly higher LTD magnitude was seen compared with that in control rats. A brief train of stimuli (5 pulses at 100 Hz), which did not affect synaptic transmission in control rats, induced a robust LTD in hypothyroid rats suggesting facilitation of LTD expression in these rats. Chronic nicotine treatment (1 mg/kg, 2x day) of hypothyroid rats reversed hypothyroidism-induced enhancement and facilitation of LTD. Western blot analysis of the NMDA receptor subunits in the membranous fractions of hippocampal area CA1 neurons revealed that hypothyroidism reduced NR1 and increased NR2B without affecting NR2A protein levels. These changes in NMDA receptors in hypothyroid rats were reversed by chronic nicotine treatment. Hypothyroidism did not alter BDNF or nicotinic acetylcholine receptor (nAChR) levels. However, nicotine treatment increased protein levels of these molecules in both euthyroid and hypothyroid rats. Our results suggest that alterations in the levels of NMDA receptor subunits may account for the facilitation and enhancement of LTD in hypothyroidism.

Age-related effects of ethanol consumption on triiodothyronine and retinoic acid nuclear receptors, neurogranin and neuromodulin expression levels in mouse brain

The effects of ethanol consumption and ageing were investigated on the expression levels of retinoic acid (RA) and triiodothyronine (T3) nuclear receptors (RAR, RXR and TR) and of associated target genes involved in synaptic plasticity, neurogranin (RC3) and neuromodulin (GAP-43) in mice brain. For this purpose, C57BL/6 adult and aged mice were subjected to 5-month ethanol consumption and the mRNA expression of RAR, RXR, TR, RC3 and GAP-43 was measured using a real-time RT-PCR method. GAP-43 and RC3 protein levels also were measured by Western blot. Results showed that 12% ethanol consumption in adult mice (11 months) induced an increase in RARbeta, RXRbetagamma and TRalphabeta mRNA level in the brain with only an increase in RC3 expression. The same ethanol consumption in aged mice (22 months) reversed the age-related hypo-expression in brain RARbeta, TRalphabeta and target genes RC3 and GAP-43. Compared with our previous behavioral data showing that ethanol is able to partially suppress a selective age-related cognitive deficit, these results suggest that the ethanol-induced increase in RA and T3 nuclear receptors expression could be one of the mechanisms involved in the normalization of synaptic plasticity-associated gene expression altered in aging brain.

Adult-onset hypothyroidism impairs paired-pulse facilitation and long-term potentiation of the rat dorsal hippocampo-medial prefrontal cortex pathway in vivo

Thyroid hormones are critical for the maturation and function of the central nervous system. Insufficiency of thyroid hormones in the adulthood causes a wide range of cognitive dysfunctions, including deficits in learning and memory. The present study investigated whether adult-onset hypothyroidism would alter synaptic functions in the dorsal hippocampo-medial prefrontal cortex (mPFC) pathway, a neural pathway important for learning and memory. Adult hypothyroidism was induced by oral administration of 1% (g/l) antithyroid acting drug 6-n-propyl-2-thiouracil (PTU) to adult male Sprague-Dawley rats for 4 weeks. Postsynaptic potentials (PSP) were recorded in the mPFC by stimulating the dorsal hippocampal CA1 region in vivo. Basal synaptic transmission was evaluated by comparing input-output relationships. Paired-pulse facilitation and long-term potentiation were recorded to examine short- and long-term synaptic plasticity. Adult-onset hypothyroidism did not change the basal synaptic transmission, but significantly reduced paired-pulse facilitation and long-term potentiation of PSP. These inhibitions can be restored by thyroid hormone replacement. The results suggest that such alterations in synaptic plasticity of the dorsal hippocampo-mPFC pathway might contribute to understanding basic mechanisms underlying learning and memory deficits associated with adult-onset hypothyroidism.

Hypothyroidism changes adenine nucleotide hydrolysis in synaptosomes from hippocampus and cerebral cortex of rats in different phases of development

The influence of the thyroid hormones on the normal function of the mammalian central nervous system depends on the brain region and on the developmental stage. Adenine nucleotides and their products also affect the brain function; ATP is an excitatory neurotransmitter, and adenosine has inhibitory effects on neurotransmission. Thus, this study aimed to evaluate the effects of hypothyroidism on the hydrolysis of ATP to adenosine in hippocampal and cortical synaptosomes and blood serum of rats during different phases of development. Rats aged 60 and 420 days old were divided into three groups: control, sham-operated and hypothyroid. Hypothyroidism was induced in these rats by thyroidectomy and methimazole (0.05%) added to their drinking water for 14 days. Neonatal hypothyroidism was induced by adding 0.02% methimazole in the drinking water from day 9 of gestation, and continually until 14 days old. Hypothyroidism increased the AMP hydrolysis in both hippocampus and cerebral cortex synaptosomes of rats in all aged tested. In blood serum, thyroid hormones deficiency increased the AMP hydrolysis in 14-day-old rats and the hydrolysis of ATP, ADP and AMP in 60-day-old rats; however, no alteration was observed in 420-day-old rats. Thus, our results suggest the involvement of the 5'-nucleotidase in synaptic function control in hypothyroidism throughout brain development.

Regional distribution of SK3 mRNA-containing neurons in the adult and adolescent rat ventral midbrain and their relationship to dopamine-containing cells

SK3 small conductance, calcium-activated potassium channels play an important role in regulating the activity of mesencephalic dopamine (DA) neurons. In the present series of experiments, in situ hybridization techniques were used to compare SK3 and tyrosine hydroxylase (TH) mRNA expression throughout the rostrocaudal extent of the ventral midbrain in juvenile and adult rats. SK3 mRNA was found exclusively in areas that also contained large numbers of DA neurons including the substantia nigra (SN), the ventral tegmental area, and related cell groups (VTA-A10). An anteroposterior and mediolateral gradient in SK3 mRNA hybridization was apparent in the VTA-A10 but not in the SN. Younger rats appeared to possess higher levels and less regional variation in TH and SK3 transcripts. These results are consistent with previous studies reporting differential expression of SK3 protein within the midbrain and suggest that variations in SK3 channel distribution could contribute to differences in dopamine-related functions in the rat.

Corticosterone alters N-methyl-D-aspartate receptor subunit mRNA expression before puberty

Stress and stress hormones alter the expression of mRNA for the NR1, NR2A and NR2B subunits of the N-methyl-D-aspartate (NMDA) receptor in brain regions associated with the stress response. Early life stress contributes to the risk and pathophysiology of mental illness. Examining how stress hormones modulate NMDA receptor subunit gene expression before and after pubertal onset will further contribute to the understanding of how stress during adolescence relates to adult mental illness. Using in situ hybridization histochemistry, we measured NR1, NR2A and NR2B mRNA expression in the hippocampus and in the hypothalamic paraventricular nucleus (PVN) of rats that had undergone adrenalectomy (ADX) or sham surgery before or after puberty. Some ADX rats received corticosterone pellets that released either normal or stress levels of corticosterone for 14 days prior to sacrifice. There was a significant increase in NR1 subunit mRNA expression throughout the subfields of the hippocampus and in the PVN of ADX prepubertal rats. However, similar changes in hippocampal NR1 expression were not observed in postpubertal ADX rats. Pre- and postpubertal ADX rats implanted with a high-dose corticosterone pellet had decreased expression of PVN NR1 mRNA. Only prepubertal rats had an increase in dentate gyrus NR2A mRNA and CA3 region NR2B mRNA following high-dose replacement. These results provide evidence that glucocorticoids have differential effects on the regional expression of mRNA NMDA receptor subunits and elucidate a window during adolescence in which the NR1, NR2A and NR2B genes are responsive to glucocorticoids.