Hippocampus Of Hypothyroid Rats Research Articles

The Expression of Neurodegeneration-Related Genes in the Hippocampus of Hypothyroid Rats Following Long-Term Potentiation

The Expression of Neurodegeneration-Related Genes in the Hippocampus of Hypothyroid Rats Following Long-Term Potentiation

The Expression of Neurodegeneration-Related Genes in the Hippocampus of Hypothyroid Rats Following Long-Term Potentiation

The Expression of Neurodegeneration-Related Genes in the Hippocampus of Hypothyroid Rats Following Long-Term Potentiation

Thyroid hormone-dependent development of the GABAergic pre- and post-synaptic components in the rat hippocampus

Thyroid hormone (TH) plays essential roles in normal brain development mainly by regulating gene expression through binding to specific nuclear receptors which serve as transcription factors. Previous studies showed that perinatal deficiency of TH or impairment of its signaling severely affect brain development, especially the development of the γ-aminobutyric acid (GABA) system, but cellular and molecular targets of the hormone are only partly uncovered. In the present study, we focused on the developing rat hippocampus which was confirmed to be one of the regions highly sensitive to TH status, and found two new targets of the hormone among the pre- and post-synaptic components of the GABAergic system. One was glutamic acid decarboxylase 65 (GAD65), the protein level of which was reduced to less than 50% of control in the hippocampus of hypothyroid rats (obtained by administering 0.025% methimazole in drinking water to pregnant dams from gestational day 15 until 4 weeks postpartum) and recovered to control levels by daily thyroxine-replacement after birth. Reduction in GAD65 protein was correlated immunohistochemically with a 37% reduction in the number of GAD65-positive cells as well as a reduction in GAD65-positive processes. In contrast, the other GAD isotype, GAD67, was not affected by TH status. A subpopulation of GABAergic neurons containing parvalbumin was also confirmed to be highly dependent on TH status. The second target of thyroid hormone was neuron-specific K+/Cl− co-transporter, KCC2, which is responsible for switching of GABA action from excitatory to inhibitory. In the euthyroid hippocampus, a sharp rise of kcc2 expression was observed at postnatal day (PND)10 which was followed by a large increase in KCC2 protein at PND15. This transient rise in kcc2 expression was completely suppressed by hypothyroidism, resulting in nearly 80% reduction in KCC2 protein at PND15. These results indicate that the development of GABAergic terminals and the excitatory to inhibitory maturation of GABA signaling are strongly dependent on TH.

Hypothyroidism in the Adult Rat Causes Incremental Changes in Brain-Derived Neurotrophic Factor, Neuronal and Astrocyte Apoptosis, Gliosis, and Deterioration of Postsynaptic Density

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. 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. 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. 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.

HYPOTHYROIDISM IN THE ADULT RAT CAUSES AN INCREMENT OF BDNF IN THE BRAIN, NEURONAL AND ASTROCYTE APOPTOSIS, GLIOSIS AND DETERIORATION OF THE POSTSYNAPTIC DENSITY.

Background: Adult hypothyroidism is a highly prevalent condition that impairs processes, such as learning and memory. Even though T4 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 6Propyl-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 ELISA and in situ hybridization. The glutamatergic synapse and the postsynaptic density (PSD) were analyzed by electron microscopy. The content of postsynaptic density 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, as well as reactive gliosis as compared to 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 as compared 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. Page 3 of 36

Monoamine concentrations changes in the PTU-induced hypothyroid rat brain and the ameliorating role of folic acid

Thyroid hormones are recognized as the key metabolic hormones that play a critical role in the development of central nervous system (CNS) throughout life. The present study was designed to determine the changes in brain monoamine concentrations in 6-n-propyl thiouracil (PTU)-induced hypothyroid rats, in addition to the ameliorating role of folic acid treatment. Fifty male albino rats were equally divided into five groups; first and second groups were the control and folic acid groups, respectively, while the third group was the hypothyroid group in which the rats received PTU in drinking water for 6 weeks. The fourth and fifth groups were co- and post-treated folic acid groups with hypothyroid rats, respectively. Our results revealed that serotonin and norepinephrine concentrations were significantly decreased in the hypothalamus and cortex, while it significantly increased in the hippocampus of hypothyroid rats when compared with control group. Serotonin and norepinephrine concentrations were decreased in hypothalamus and cortex in co- and post-treated folic acid groups with hypothyroid rats, while the concentration of dopamine were significantly increased in the hypothalamus and hippocampus of the hypothyroid rats and co-treated folic acid group with hypothyroid rats. In cortex, the dopamine concentration was significantly increased in hypothyroid rats and post-treated folic acid group with hypothyroid rats, while it significantly decreased in co-treated folic acid group with hypothyroid rats when compared with the control group. Also, our results revealed that, folic acid treatment was better if it is administered as an adjuvant after returning to the euthyroid state by withdrawing PTU from the drinking water.

Developmental Hypothyroidism Increases the Expression of Kainate Receptors in the Hippocampus and the Sensitivity to Kainic Acid-Induced Seizures in the Rat

Thyroid hormones are essential for normal brain development, and multiple alterations at behavioral, cognitive, cellular, and molecular levels have been described in animals made hypothyroid during development. Here we analyzed the effect of developmental hypothyroidism in the rat on the sensitivity to kainic acid-induced limbic seizures and the expression of kainate receptors in the hippocampus. Our results show that hypothyroid rats are extremely sensitive to the proconvulsant and neurotoxic effects of kainic acid (KA). Hypothyroid rats entered in status epilepticus at a dose of KA three times lower than that required to reach status epilepticus in control animals. In accordance with this, high levels of glial activation and neuronal loss after low KA dose injections were observed only in the hippocampus of hypothyroid rats. These effects correlated with an increased expression of kainate receptor subunits, excluding GluR5, in the hippocampus of hypothyroid animals. The concentrations of GluR6, GluR7, KAR1, and KAR2 (ionotropic glutamate receptor subunits of the kainic acid subtype) mRNAs were increased between 50 and 250% in hypothyroid animals relative to the values in controls. In agreement with these results, Western blot and immunohistochemical analysis showed a clear increase in the hippocampal content of GluR6/7 proteins in hypothyroid animals.

Hypothyroidism induces selective oxidative stress in amygdala and hippocampus of rat

The effects of hypothyroidism on lipid peroxidation (LP), reactive oxygen species (ROS), and nitric oxide synthase (NOS), levels and expression, in rat brain were examined. Hypothyroidism was induced by administering methimazole in drinking water (60 mg/kg/day). In striatum, motor cortex and cerebellum of hypothyroid rats LP was not modified, whereas LP and ROS increased in amygdala and hippocampus of hypothyroid rats at the third week of treatment with methimazole as compared to euthyroid group values. Regarding NOS participation, only hippocampal constitutive-NOS activity was increased, accompanied by an augmentation in nNOS expression. Results show that hypothyroidism induces selective oxidative stress in both the hippocampus and amygdala, where the nitrergic system is involved.

Neonatal Hypothyroidism Affects the Adenine Nucleotides Metabolism in Astrocyte Cultures from Rat Brain

Neonatal hypothyroidism is associated with multiple and severe brain alterations. We recently demonstrated a significant increase in hydrolysis of AMP to adenosine in brain of hypothyroid rats at different ages. However, the origin of this effect was unclear. Considering the effects of adenine nucleotides to brain functions and the harmful effects of neonatal hypothyroidism to normal development of the central nervous system, in this study we investigated the metabolism of adenine nucleotides in hippocampal, cortical and cerebellar astrocyte cultures from rats submitted to neonatal hypothyroidism. ATP and AMP hydrolysis were enhanced by 52 and 210%, respectively, in cerebellar astrocytes from hypothyroid rats. In hippocampus of hypothyroid rats, the 47% increase in AMP hydrolysis was significantly reverted when the astrocytes were treated with T3. Therefore, the imbalance in the ATP and adenosine levels in astrocytes, during brain development, may contribute to some of the effects described in neonatal hypothyroidism.

Temporal, regional and cellular selectivity of neonatal alteration of the thyroid state on neurochemical maturation in the rat

The effects of alteration of thyroid state on neurochemical maturation have been studied in rats made hypothyroid by daily injections of methimazole or hyperthyroid by daily supplementation with thyroid hormone (T3) from postnatal days 1 to 27. Biochemical assays on seven brain regions plus the spinal cord were carried out on 14 and 28 day-old rats as well as in adult rats after at least 40 days of recovery. 2',3'cyclic nucleotide phosphohydrolase (CNPase), a specific marker for oligodendrocytes and myelination was significantly decreased in all regions except the spinal cord of hypothyroid rats. The astrocytic marker glutamine synthetase (GS) was slightly increased in the hippocampus of hypothyroid rats. Choline acetyltransferase (ChAT), a specific marker for cholinergic neurons, was decreased in the prefrontal and visual cortices, the striatum and the superior colliculus and increased in the cerebellum of hypothyroid rats; in addition, the enzyme activity was increased in the prefrontal cortex and striatum and decreased in the cerebellum of hyperthyroid rats. Acetylcholinesterase (AChE) activity was decreased in the prefrontal cortex and in the striatum of hypothyroid rats while 3H-quinuclidinyl benzilate (QNB) muscarinic binding was decreased in all cortical areas and in the hippocampus of hypothyroid rats. Glutamate decarboxylase (GAD), a specific marker for GABAergic neurons, was decreased in the cortical areas of hypothyroid rats. Aromatic amino acid decarboxylase (AAD), a general marker for monoaminergic neurons, was unaffected. Alteration of neurochemical parameters was never observed in the spinal cord. Under our experimental conditions, the effects of alteration of thyroid state appeared graded and selective with respect to temporal, regional and cellular parameters.