Euthyroid Rats Research Articles

Hyperthyroidism-Induced Upregulation of Neurodegeneration-Related Gene Expression in Metaplasticity-Induced Hippocampus

Introduction: Thyroid hormones, which produce critical changes in our bodies even when their physiological levels alter slightly, are crucial hormones that influence gene transcription. Neuronal plasticity, on the other hand, requires both the activation of local proteins as well as protein translation and transcription in response to external signals. So far, no study has examined metaplastic long-term potentiation (LTP) and related gene expression levels in a hyperthyroid experimental model. Methods: The Wistar male rats were administered 0.2 mg/kg/day of <sc>l</sc>-thyroxine for 21 days to induce hyperthyroidism. Perforant path was primed with 1-Hz low-frequency stimuli (LFS) for 900 s to investigate metaplasticity responses. The LFS was followed by high-frequency stimuli (HFS, 100 Hz) after 5 min. Excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude were recorded from the granule cell layer of the dentate gyrus. The mRNA levels of genes related to neurodegeneration (Gsk-3β, Cdk5, Akt1, Mapt, p35, Capn1, Bace1, and Psen2) were measured using the RT-PCR method for the stimulated hippocampus. Results: Similar to euthyroid rats, hyperthyroid animals had a lower EPSP slope and PS after LFS. Depression of EPSP prevented subsequently induced EPSP-LTP, although HFS was able to elicit PS-LTP despite depression of PS amplitude in both groups. Despite similarities in metaplastic LTP responses, these electrophysiological findings were accompanied by increased Akt, Bace1, Cdk5, and p35-mRNA expressions and decreased Gsk-3β mRNA expression in hyperthyroid rats’ hippocampus. Conclusion: These data support the view that in thyroid hormone excess, the mechanism that keeps synaptic efficacy within a dynamic range occurs concurrently with increased mRNA expression of neurodegeneration-related genes. Our study encourages further examination of the increased risk of neurodegenerative disease in hyperthyroidism.

Mast Cell Population and Histamine Content in Hypothyroid Rat Tissues.

Simple SummaryIn this study, we investigated the putative regulatory role of the pituitary–thyroid axis on mast cells (MCs). We found that hypothyroidism resulted in a significant increase in number of MCs and the histamine content in the skin and exorbital lacrimal gland of Wistar rats, Rattus norvegicus. Furthermore, an increase in the percentage of degranulating MCs suggests that the thyroid status also influences the activation state of these cells.The morphological features and relative number of mast cells (MCs) were studied in the skin and exorbital lacrimal glands of hypothyroid Wistar rats, Rattus norvegicus. Hypothyroidism significantly increased the number of MCs (up to 4.5-fold) and histamine content (up to 50%) in the examined tissues. The magnitude of the increase in the number of MCs was greater in the cheek skin and exorbital lacrimal glands than in the back skin. In the skin, the MCs were mainly located within the hypodermis and closely associated with the blood vessels, nerve fascicles, and adipocytes. In the exorbital lacrimal gland, which is a seromucous gland located lateral to the cheek below the ear, the MCs were distributed in the connective tissue surrounding the acini. The secretory granules of MCs showed histochemical characteristics of connective tissue MCs. They were metachromatic with Toluidine blue and safranin positive with the Alcian blue/safranin reactions. Finally, a significant increase in degranulating MCs was observed in hypothyroid tissues, relative to euthyroid tissues. At the ultrastructural level, the MCs of euthyroid rats were predominantly non-degranulating (Stage I). In hypothyroid animals, numerous MCs showed partial degranulation (Stage II–III) or were in a stage of complete degranulation. Our results concerning the skin and exorbital lacrimal gland suggested that the thyroid status might be involved in regulating the frequency and activation state of MCs.

Effects of thyroxine on apoptosis and proliferation of mammary tumors

Hypothyroidism is a protective factor against breast cancer but long-term exposure or overdoses of thyroid replacement therapy with thyroxine (T4) may increase breast cancer risk. Objective: to study, in vivo and in vitro, the effects of T4 on the proliferation and apoptosis of mammary tumors of hypo- and euthyroid rats, and the possible mechanisms involved in these effects. Material and Methods: Female Sprague-Dawley rats were treated with a single dose of dimethylbenzathracene (15 mg/rat) at 55 days of age and were divided into three groups: hypothyroidism (HypoT; 0.01% 6-N-propyl-2-thiouracil -PTU- in drinking water, n = 20), hypothyroidism treated with T4 (HypoT + T4; 0.01% PTU in drinking water and 0.25 mg/kg/day T4 via sc; n = 20) and EUT (untreated control, n = 20). At sacrifice, tumor explants from HypoT and EUT rats were obtained and treated either with 10−10 M T4 in DMEM/F12 without phenol red with 1% Charcoalized Fetal Bovine Serum or DMEM/F12 only for 15 min to evaluate intracellular signaling pathways associated with T4, and 24 h to evaluate changes in the expression of hormone receptors and proteins related to apoptosis and proliferation by immunohistochemistry and Western Blot. Results: In vivo, hypothyroidism retards mammary carcinogenesis but its treatment with T4 reverted the protective effects. In vitro, the proliferative and anti-apoptosis mechanisms of T4 were different regarding the thyroid status. In EUT tumors, the main signaling pathway involved was the cross-talk with other receptors, such as ERα, PgR, and HER2. In HypoT tumors, the non-genomic signaling pathway of T4 was the chief mechanism involved since αvβ3 integrin, HER2, β-catenin and, downstream, PI3K/AKT and ERK signaling pathways were activated. Conclusion: T4 can regulate mammary carcinogenesis by mainly activating its non-genomic signaling pathway and by interacting with other hormone or growth factor pathways endorsing that overdoses of thyroid replacement therapy with T4 can increase the risk of breast cancer.

Alterations in atrial ion channels and tissue structure promote atrial fibrillation in hypothyroid rats.

It is well known that hyperthyroidism is associated with atrial fibrillation (AF); however, the relationship between hypothyroidism and AF remains controversial. Hypothyroidism was established in rats by two methods: methimazole-induced (MMI) and thyroidectomy (TX). MMI model includes control (n = 10), MMI (n = 10), and MMI + L-thyroxine (T4, n = 10). Methimazole was given intragastrically in MMI and MMI + T4 for 12 weeks, and T4 was added intragastrically in MMI + T4 at week 5. TX model includes sham (n = 10), TX (n = 10), and TX + T4 (n = 10). Four weeks after surgery, rats in TX + T4 received T4 for 8 weeks. Triiodothyronine (T3), T4, and thyroid-stimulating hormone (TSH) were measured. Electrophysiology, tissue structure and function, and protein levels of potassium and L-type calcium channels were assessed in the atria. Severe changes in the atrial structure of hypothyroid rats were observed. Compared with euthyroid rats, atrial effective refractory period (AERP) in hypothyroid rats was significantly shortened; accordingly, inducibility and duration of AF were considerably increased. Protein levels of minK, Kv1.5, Kv4.2, Kv4.3, Kv7.1, and Cav1.2 were upregulated in hypothyroid rats, whereas there was only a tendency toward increased Kir2.1. Kv11.1 was statistically upregulated in the MMI model and had an increasing tendency in the TX model. Conversely, Kir3.1 and Kir3.4 were downregulated in hypothyroid rats. The above changes could be partially inhibited by T4 treatment. AERP shortening due to altered protein levels of ion channels and atrial structural changes increased the susceptibility to AF in hypothyroidism. Thyroid replacement therapy could prevent electrical and structural remodeling under hypothyroid condition.

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 95min after priming onset. We found that priming stimulation at 5Hz for 3s 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.

Effects of hypothyroidism on the mesenteric and omental adipose tissue in rats

To characterize the influence of hypothyroidism on the endocrine activity of mesenteric and omental adipose tissue (MOAT) and the peripheral regulation of energy balance (EB) in rats, we analyzed food intake (FI); basal metabolic rate (BMR); locomotor activity; body weight (BW); serum hormone concentrations and the expression of their receptors in MOAT. We evaluated the morphology and differentiation of adipocytes. Hypothyroidism decreased FI, BMR and BW. The percentage of visceral white adipose tissue (WAT) depots and the morphology of adipocytes were similar to euthyroid rats. Serum leptin and adiponectin expression in MOAT were altered by hypothyroidism. The expression of Perilipin 1, HSL, UCP1 and PRDM16 was significantly lower in MOAT of hypothyroid animals. Hypothyroidism in rats leads to a compensated EB by inducing a white adipocyte dysfunction and a decrease in BW, BMR, FI and adipokine secretions without changing the percentage of WAT depots and the morphology of the MOAT.

Thyroid Hormone Deficiency Adversely Affects Cardiomyocyte T‐tubule Structure and Function

Thyroid hormone deficiency causes adverse changes in cardiomyocyte shape leading to increased wall stress, chamber dilatation and in the long‐term can result in heart failure. We hypothesized that thyroid hormones regulate cardiac contractile function in part by maintaining T‐tubule (TT) structures and efficient excitation‐contraction (EC)‐coupling.MethodsFemale Sprague‐Dawley rats (aged 12 wks) were surgically thyroidectomized (Tx) or were maintained as euthyroid (Eu) controls (6 rats/group). The study was terminated after 12 mo. at which time echocardiographic and hemodynamic measurements were recorded. Ventricular myocytes were isolated by collagenase digestion and plated onto laminin‐coated glass coverslips for TT analysis or were analyzed immediately for contractile and calcium dynamic using videobased edge detection of sarcomere length changes coupled to a fluorescence imaging system (IonOptix). Live cells were labeled with wheat germ agglutinin‐coupled to Alexa‐Fluor (WGA‐488) to image the sarcolemma and TT by confocal microscopy. Multiple z‐stack images were captured per cell and TT periodicity was analyzed using an algorithm to calculate periodicity based on the peak amplitude in the Fourier spectrum of the image at the TT frequency, or TT power. This software program is available as TTorg plugin on ImageJ.ResultsEchocardiography and in vivo hemodynamic measurements showed significantly reduced systolic and diastolic function in Tx vs euthyroid rats. In response to 1 Hz field stimulation at 4V, isolated cardiomyocyte contractile cycles showed significant dysfunction in Tx vs Eu cardiomyocytes: slower max contraction velocity (+Vmax), −2.047±0.30 vs −3.503±0.50 um/sec, Tx vs Eu, respectively, p&lt;0.001; slower max relaxation velocity (‐Vmax), 1.369±0.27 vs 2.44±0.73 um/sec, p&lt;0.01; longer time to peak contraction, 0.279±0.03 vs 0.182±0.03 sec, p&lt;0.001. Myocytes were loaded with Fura2/AM (1uM) and intracellular calcium measured by fluorescence intensity ratio at 340/380 nm. Ca2+ transients showed slower kinetics in Tx vs Eu cardiomyocytes: time to Ca2+ peak, 0.076±0.01 vs 0.0516±0.003 sec, Tx vs Eu, respectively, p&lt;0.0001; time to Ca2+ max return velocity, 0.261 0.03 vs 0.184 0.005, Tx vs Eu, respectively, p&lt;0.0001. TTorg analysis indicated significant disorganization of T‐tubules in Tx cardiomyocytes compared to Eu (TT power = 61±2 vs 80±2, Tx vs Eu, respectively, p&lt;0.00001; arbitrary units with higher values indicate increased periodicity). In conclusion, TH‐deficiency disrupts T‐tubule structures causing cardiomyocyte contractile dysfunction and slower calcium kinetics indicating altered EC‐coupling. Therefore maintaining normal thyroid function is necessary for optimal heart health.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Thyroid Disorders Change the Pattern of Response of Angiotensinase Activities in the Hypothalamus-Pituitary-Adrenal Axis of Male Rats.

Thyroid disorders affect the hypothalamic-pituitary-adrenal axis with important consequences on the cardiovascular function in which the renin-angiotensin system plays a major role. Hypo and hyperthyroidism influence the classic main components of the renin-angiotensin system. However, the behavior of other elements of the renin-angiotensin system such as Ang III, Ang 2-10, Ang IV, or AT4, regulated by angiotensinase enzymes such as alanyl- (AlaAP), cystinyl- (CysAP), glutamyl- (GluAP), or aspartyl-aminopeptidase (AspAP), has not yet been described. In order to obtain a comprehensive view on the response of the renin-angiotensin system in the hypothalamic-pituitary-adrenal axis of animals with thyroid disorders, these enzyme activities were simultaneously analyzed fluorometrically, using arylamide derivatives as substrates in hypothalamus, anterior and posterior pituitary, adrenals and plasma of euthyroid, hypothyroid, and hyperthyroid rats, and their intra- and inter-tissue correlations were evaluated. The response is depending on the type of enzyme studied, its location and the thyroid status. Anterior pituitary, adrenals and plasma were mainly affected by the thyroid disorders. In the anterior pituitary, GluAP and AspAP increased in hypothyroid rats. In adrenals, AlaAP and CysAP decreased in hypothyroid whereas GluAP and AspAP decreased in hyperthyroid rats. In plasma, while AlaAP increased in hypo- and hyperthyroid rats, CysAP and GluAP decreased only in hyperthyroid. In comparison with euthyroid, intra-tissue correlations decreased in hypothyroid but inter-tissue correlations decreased mainly in hyperthyroid rats. Thyroid disorders also produced a disruption in the pattern of inter-tissue correlations observed in euthyroid. These results suggest that thyroid hormone levels hit components of the renin-angiotensin system and may influence the paracrine and endocrine cross talk between cells.

Beneficial effects of Cinnamon on hepatic lipid metabolism are impaired in hypothyroid rats

Hypolipidemic effects of cinnamon (Cinnamomum zeylanicum) have been described in humans and in experimental animals. Hypothyroidism is frequently associated with hyperlipidemia. Here we investigated the effects of chronic ingestion of cinnamon on lipid metabolism of hypothyroid male Wistar rats. Rats received methimazole for 7 weeks, and treated either with cinnamon as powder added to the chow or aqueous extract of cinnamon, during the last 4 weeks of protocol, and compared to untreated hypothyroid and euthyroid rats. Cinnamon intake reduced body mass increasing fat mass and reducing body protein content of hypothyroid rats. Cinnamon ingestion did not revert the hypercholesterolemia of hypothyroid rats and promoted a further reduction in serum T3, suggesting a worsening of the hypothyroid condition. Our results indicate that the beneficial effects of cinnamon on lipid metabolism are impaired in hypothyroidism.

Does hypothyroidism augment sun-induced skin damage?

ABSTRACTObjectives: We investigated the mutual effects of overt hypothyroidism and prolonged sunlight exposure on free radical accumulation and oxidative skin damage.Methods: Free radical accumulation was evaluated by monitoring the transformation of 3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide (MTT) into MTT-formazan. The pro-oxidant enzymes xanthine oxidase (XO) and NADPH-diaphorase were measured in the skin. XO activity was estimated based on the yield of uric acid, while NADPH-diaphorase reactivity was monitored histochemically as an indirect marker of nitric oxide synthase and nitric oxide activity. Cellular damage was determined by malondialdehyde formation, a marker for lipid peroxidation.Results: In the skin of both euthyroid and hypothyroid animals, solar simulated ultraviolet irradiance increased the activity of XO and the NADPHdiaphorase reactivity as a protective response to formation of free radicals, such as reactive oxygen or nitrogen species. These pro-oxidant enzymes diminished in hypothyroid rats. Accumulation of the same amount of free radicals led to similar peroxidation in both hypothyroid and irradiated euthyroid rats. Hypothyroid skin after UV-exposure showed even greater lipid peroxidation.Discussion: The hypothyroid state could be a risk factor for enhanced oxidative skin damage in chronic photo-exposed skin due to oxidative stress. The lipid peroxidation is one of the major pathways by which photo-oxidative stress promotes photocarcinogenesis and photo-aging.

Effects of Selenium and Thyroid Hormone Deficiency on Peritoneal Macrophages Adhesion and Occurrence of Natural IGM Antibodies in Juvenile Rats

Abstract Both selenium, as an effector and regulator of antioxidative enzymes activity, and thyroid hormones are potent immunomodulators. Besides, selenium incorporated into iodothyronine deiodinases is involved in the thyroid function and thus indirectly regulates the immune response. Studies of the mutual influence of selenium and thyroid hormones on the immune response are scarce, hence we analyzed the effects of an iodothyronine deiodinases blocker, propylthiouracil (PTU), and selenium deficiency on the function of peritoneal macrophages, and titer of naturally occurring anti-sheep red blood cells (SRBC) IgM antibodies in juvenile rats. The experiment was carried out on 64 Wistar male rats allotted to 4 groups: controlselenium adequate PTU- group; selenium adequate, PTU+ group; selenium defi cient, PTU- group; and selenium defi cient, PTU+. The selenium adequate and selenium defi cient groups were fed a diet containing 0.334 and 0.031 mg Se/kg, respectively. PTU+ groups received PTU (150 mg/L) in drinking water. After 3 weeks, thyroxine (T4), triiodothyronine (T3), and thyroid stimulating hormone (TSH) were determined. The animals having “intermediate” concentrations of T3 (1.56-1.69 nmol/L) and T4 (41-50 nmol/L) were excluded from further analysis. Thus, PTU+ groups included hypothyroid animals (T3≤1.55 nmol/L; T4≤40 nmol/L), while PTU- groups included euthyroid rats (T3≥1.70 nmol/L; T4≥50 nmol/L). Both groups of selenium deficient rats had, when compared to the control group, a significantly lower activity of glutathione peroxidase GPx1 and GPx3. Neither selenium deficiency nor PTU influenced the adherence of peritoneal macrophages. Selenium deficiency significantly decreased the peroxide synthesis in macrophages and significantly increased the titer of anti-SRBC IgM. Hypotyroidism alone or in combination with selenium deficiency had no influence on these parameters.

Синдром ендогенної інтоксикації в щурів із гострим поширеним перитонітом на тлі мерказоліл-індукованого гіпотиреозу

Introduction. The growth of endogenous toxicosis occurs when the admission intensity of toxic metabolic products increased sharply, or when the activity of antitoxic systems or elimination of toxic metabolites inhibits. The syndrome of endogenous intoxication develops at various pathological conditions, when the number of natural waste products of the body, which appeared in large quantities in biological environments, as well as aggressive components exceeds the capacity of biotransformation. The aim of the study – to learn the effect of reduced production of thyroid hormones on the indices of endogenous intoxication of rats with acute widespread peritonitis.Research Methods. The study used white Wistar rats. Hypothyroidism was simulated by the administration of Mercazolil at a dose of 25 mg / kg for 21 days. Acute widespread peritonitis was simulated by injecting 0.5 ml of 10 % filtered fecal suspension into the abdominal cavity of the studied animals. The determination of molecules of average weight, erythrocyte and leukocyte indices of intoxication, level of circulating immune complexes and catepsin D was carried out.Results and Discussion. It was established that the presence of hypothyroidism in experimental animals to which was simulated acute widespread faecal peritonitis was accompanied by, more expressed than in euthyroid rats, increase in endotoxicosis – medium molecular weight peptides, erythrocyte and leukocyte indices of intoxication, circulating immune complexes and worse dynamics of their normalization. Phase changes in cathepsin D activity were observed.Conclusions. Deficiency of iodine-containing thyroid hormones is accompanied by a reliable increase in the indices characterizing the syndrome of endogenous intoxication in comparison with the euthyroid animals. This may be one of the reasons for the development of multiple organ failure in these conditions.

Особливості кисень-залежних процесів у щурів з гострим поширеним перитонітом на тлі мерказоліл-індукованого гіпотиреозу

Мета роботи: дослідження впливу зниженої продукції тиреоїдних гормонів на стан кисень-залежних процесів.Матеріали і методи. У роботі використовували білих щурів лінії Вістар. Гіпотиреоз моделювали уведенням мерказолілу в дозі 25 мг/кг протягом 21-ї доби. Гострий поширений перитоніт моделювали уведенням 0,5 мл 10 % профільтрованої калової суспензії в черевну порожнину досліджуваних тварин. Визначали співвідношення фагоцитуючих клітин у місці ураження, продукування активних форм кисню системою фагоцитуючих мононуклеарів, спонтанний та індукований тест із нітросинім тетразолієм (НСТ-тест).Результати досліджень та їх обговорення. Проведеними дослідженнями встановлено, що наявність гіпотиреозу в експериментальних тварин, яким моделювали гострий поширений каловий перитоніт, супроводжувалась порушенням співвідношення кількості фагоцитуючих клітин у місці ушкодження, менш вираженим, ніж в еутиреоїдних щурів, зростанням активних форм кисню, а також показників спонтанного та індукованого НСТ-тесту і, як наслідок, зменшенням інтенсивності продукування ними активних форм кисню. За умов гіпотиреозу знижується кисень-залежна біоцидність нейтрофілів та зменшуються функціональні резерви фагоцитуючих клітин.

Influence of discontinuance of methimazole and propylthiouracil on 24 h radioactive iodine uptake in normal rats

Objective To investigate the influence of discontinuance of methimazole (MMI) and propylthiouracil (PTU) on 24 h radioactive iodine uptake( RAIU) in normal rats. Methods A total of 66 female SD rats were randomly divided into 3 groups by simple random sampling method. MMI group (30 rats) received MMI with a dose of 0.6 mg·ml-1·d-1 orally for 2 weeks, PTU group (30 rats) received PTU with a dose of 6 mg·ml-1·d-1 orally for 2 weeks. At 1, 3, 5, 7 and 9 d after MMI or PTU was discontinued, 6 rats were sacrificed to measure 24 h RAIU respectively. The control group (6 rats) received normal saline with a dose of 1 ml/d orally for 2 weeks, and then they were sacrificed to measure 24 h RAIU too. Data were analyzed with two-sample t test or t′ test. Results Thyroid 24 h RAIU in normal rats was (14.6±4.3)%. The 24 h RAIU on 1, 3, 5, 7 and 9 d was (4.6±0.7)%, (24.2±7.2)%, (13.1±2.9)%, (14.1±2.6)%, (16.1±3.7)% respectively after MMI withdrawal. The 24 h RAIU was significantly lower than that in control group at day 1 (t′=5.55, P 0.05). The 24 h RAIU on 1, 3, 5, 7 and 9 d was (1.9±0.8)%, (3.3±1.2)%, (7.7±2.4)%, (29.0±4.0)%, (17.1±4.8)% respectively after PTU withdrawal. It was lower than that in control group (t=7.01, 6.10, 3.40, all P 0.05). Conclusions MMI and PTU can inhibit the iodine uptake of thyroid, the 24 h RAIU returns to normal level at day 5 after withdrawal of MMI and at day 9 after withdrawal of PTU in euthyroid rats. Key words: Thyroid gland; Iodine uptake; Iodine radioisotopes; Methimazole; Propylthiouracil; Rats

The state of free-radical oxidation and antioxidant system at the experimental acute peritonitis associated with hypothyroidism

To study the influence of hypothyroidism on the course of free-radical processes and state of antioxidant system in rats with acute extensive peritonitis, the production of active forms of oxygen by mononuclear leukocytes, concentration of lipid hydroperoxides, TBA-active products, Schiff bases, SOD and catalase activity were determined. Simulation of extensive peritonitis with associated hypothyroidism led to the less obvious, than in euthyroid rats, increase of active forms of oxygen and lipid peroxidation products. Although the reduction of activity of enzymes of the first line of antioxidant defense (superoxide dismutase and catalase) was more evident.

Hypothyroidism reduces mammary tumor progression via Β-catenin-activated intrinsic apoptotic pathway in rats.

Experimental hypothyroidism retards mammary carcinogenesis promoting apoptosis of tumor cells. β-catenin plays a critical role in cell adhesion and intracellular signaling pathways conditioning the prognosis of breast cancer. However, the mechanistic connections associated with the expression of β-catenin in thyroid status and breast cancer are not known. Therefore, we studied the relationship between the expression and localization of β-catenin and apoptosis in mammary tumors induced by 7,12-dimethylbenz(a)anthracene (DMBA) in hypothyroid (Hypot) and euthyroid (EUT) rats. Female Sprague Dawley rats were treated with a dose of DMBA (15mg/rat) at 55 days of age and were then divided into two groups: HypoT (0.01% 6-N-propyl-2-thiouracil in drinking water, n = 54) and EUT (untreated control, n = 43). Latency, incidence and progression of tumors were determined. At sacrifice, tumors were obtained for immunohistological studies and Western Blot. The latency was longer (p < 0.05), the incidence was lower (p < 0.0001) and tumor growth was slower (p < 0.01) in HypoT rats compared to EUT. The expression of Bax, cleaved caspase-9 and caspase-3 was significantly higher in tumors of HypoT than in EUT (p < 0.05) indicating the activation of the intrinsic pathway. In this group, β-catenin was expressed in the plasma membrane and with less intensity, while its expression was nuclear and with greater intensity in the EUT (p < 0.05). Moreover, the expression of survivin was reduced in tumors of HypoT rats (p < 0.05). In conclusion, decreased expression of β-catenin and its normal location in membrane of mammary tumors are associated with augmented apoptosis via activation of the intrinsic pathway in HypoT rats.

Low-frequency stimulation induces a durable long-term depression in young adult hyperthyroid rats: the role of p38 mitogen-activated protein kinase and protein phosphatase 1.

Long-term potentiation and long-term depression (LTD) are cellular mechanisms of learning and memory in the mammalian brain. We have previously shown that adult hyperthyroid rats showed a delay in the acquisition of a place learning task and attenuated long-term potentiation. However, changes in LTD in hyperthyroidism remain unclear. Rats were administered 0.2 mg/kg/day of L-thyroxine for 21 days starting at postnatal day 40 to induce hyperthyroidism. LTD was induced in the dentate gyrus using low-frequency stimulation (LFS) of the perforant pathway. The mRNA expressions of p38 mitogen-activated protein kinase (p38-MAPK) and protein phosphatase 1 (PP1) were evaluated using a quantitative reverse transcriptase PCR. In control rats, a standard LFS protocol induced a slight depression of the population spike (PS) amplitude during the induction phase of LTD (76±13% vs. baseline), but a small potentiation of the PS amplitude was observed in the early (107±18%) and late (111±20%) phases of LTD. Interestingly, in the hyperthyroid rats, the same LFS protocol induced a reliable LTD in the dentate gyrus of the hippocampus as evidenced by a marked depression in the PS amplitude during the induction (54±6% vs. baseline) and the early phases (56±8%) of LTD. Elevated mRNA levels of p38-MAPK and PP1 were observed in the hippocampus of the LFS-treated hyperthyroid rats compared with the hippocampus of the vehicle-treated hyperthyroid rats. No significant change in p38-MAPK or PP1 mRNA expression was observed in the euthyroid rats. The present study shows that a standard LFS protocol can induce a durable depression of synaptic strength and an upregulation of PP1 and p38-MAPK mRNA in hyperthyroid rats. We conclude that hyperthyroidism can induce molecular changes associated with degeneration of the hippocampus. The relationship between the levels of thyroid hormone and dementia requires further investigation.

Effects of selenium treatment on 6-n-propyl-2-thiouracil-induced impairment of long-term potentiation

The goal of this study was to evaluate whether sodium selenite could afford protection against the effects of hypothyroidism on long-term potentiation (LTP), which is thought to be the cellular basis for learning and memory. Hypothyroidism was induced in young-adult rats by the administration of 6-n-propyl-2-thiouracil (PTU) in tap water for 21 days. Half of these hypothyroid and euthroid rats were given 10ppM selenium with their drinking water. Field potentials were recorded from the dentate gyrus in response to stimulation of the medial perforant pathway in vivo. PTU treatment resulted in a significant reduction in both free T3 and free T4 levels, whereas selenium administration to PTU-treated rats restored only the levels of free T3 to their control values. Thyroid hormone levels were not affected by selenium in euthyroid rats. PTU-treated rats exhibited an attenuation of population spike (PS) – LTP, but a comparable potentiation of excitatory postsynaptic potential (EPSP) was found among these rats. The administration of selenium to PTU-treated rats was partially able to attenuate impairment of LTP, but not of potentiation during the LTP induction protocol in hypothyroid rats. Interestingly, the hypothyroid rats that were supplemented with selenium had a lower EPSP potentiation during induction protocol than the control rats. The present study suggests a possible importance of T3 in Se-induced rescue of impaired PS-LTP in hypothyroidism.

Hypothyroidism Induces Hypophagia Associated with Alterations in Protein Expression of Neuropeptide Y and Proopiomelanocortin in the Arcuate Nucleus, Independently of Hypothalamic Nuclei-Specific Changes in Leptin Signaling.

Thyroid hormone and leptin are essential regulators of energy homeostasis. Both hormones stimulate energy expenditure but have opposite effects on appetite. The mechanisms behind food intake regulation in thyroid dysfunctions are poorly understood. It has been shown that hypothyroid rats exhibited impaired leptin anorexigenic effect and signaling in total hypothalamus, even though they were hypophagic. It was hypothesized that hypothyroidism modulates the expression of neuropeptides: orexigenic neuropeptide Y (NPY) and anorexigenic proopiomelanocortin (POMC), independently of inducing nuclei-specific changes in hypothalamic leptin signaling. Adult male rats were rendered hypothyroid by administration of 0.03% methimazole in the drinking water for 21 days. Protein content of NPY, POMC, and leptin signaling (the signal transducer and activator of transcription 3 [STAT3] pathway) were evaluated by Western blot, and mRNA levels by real time reverse transcription polymerase chain reaction in arcuate (ARC), ventromedial (VMN), and paraventricular (PVN) hypothalamic nuclei isolated from euthyroid (eu) and hypothyroid (hypo) rats. Leptin anorexigenic effect was tested by recording food intake for two hours after intracerebroventricular (i.c.v.) administration of leptin. Statistical differences were considered significant at p ≤ 0.05. Hypothyroidism was confirmed by decreased serum triiodothyronine, thyroxine, and increased thyrotropin, in addition to increased levels of pro-TRH mRNA in PVN and Dio2 mRNA in the ARC of hypo rats. Hypothyroidism decreased body weight and food intake associated with decreased protein content of NPY and increased content of POMC in the ARC. Conversely, hypothyroidism induced central resistance to the acute anorexigenic effect of leptin, since while euthyroid rats displayed reduced food intake after leptin i.c.v. injection, hypothyroid rats showed no response. Hypothyroid rats exhibited decreased leptin receptor (ObRb) protein content in ARC and VMN but not in PVN nucleus. ObRb protein changes were concomitant with decreased phosphorylated STAT3 in the ARC, and decreased total STAT3 in VMN and PVN. However, hypothyroidism did not affect mRNA levels of Lepr or Stat3 in the hypothalamic nuclei. Experimental hypothyroidism induced a negative energy balance accompanied by decreased NPY and increased POMC protein content in the ARC, resulting in predominance of anorexigenic pathways, despite central leptin resistance and impairment of the leptin signaling cascade in a nuclei-specific manner.

Effect of triiodothyronine (T3) excess on fatty acid metabolism in the soleus muscle from endurance-trained rats.

We studied whether short-term administration of triiodothyronine (T3) for the last 3 days of endurance training would influence the rate of uptake of palmitic acid (PA) as well as metabolism in rat soleus muscle, in vitro. Training per se did not affect the rate of PA uptake by the soleus; however, an excess of T3 increased the rate of this process at 1.5 mmol/L PA, as well as the rate that at which PA was incorporated into intramuscular triacylglycerols (TG). The rate of TG synthesis in trained euthyroid rats was reduced after exercise (1.5 mmol/L PA). The rate of PA oxidation in all of the trained rats immediately after exercise was enhanced by comparison with the sedentary values. Hyperthyroidism additionally increased the rate of this process at 1.5 mmol/L PA. After a recovery period, the rate of PA oxidation returned to the control values in both the euthyroid and the hyperthyroid groups. Examination of the high-energy phosphate levels indicated that elevated PA oxidation after exercise-training in euthyroid rats was associated with stable ATP levels and increased ADP and AMP levels, thus reducing energy cellular potential (ECP). In the hyperthyroid rats, levels of ADP and AMP were increased in the sedentary as well as the exercise-trained rats. ECP levels were high as a result of high levels of ATP and decreased levels of ADP and AMP in hyperthyroid rats after the recovery period. In conclusion, short-term hyperthyroidism accelerates PA utilization in well-trained soleus muscle.