Novel two-dimensional morphometric maps and quantitative analysis reveal marked growth and structural recovery of the rat hippocampal regions from early hypothyroid retardation

Thyroid Disorders and Development of Cognitive Impairment: A Review Study

Dementia is a neurological disorder that is spreading with increasing human lifespan. In this neurological disorder, memory and cognition are declined and eventually impaired. Various factors can be considered as the background of this disorder, one of which is endocrine disorders. Thyroid hormones are involved in various physiological processes in the body; one of the most important of them is neuromodulation. Thyroid disorders, including hyperthyroidism or hypothyroidism, can affect the nervous system and play a role in the development of dementia. Despite decades of investigation, the nature of the association between thyroid disorders and cognition remains a mystery. Given the enhancing global burden of dementia, the principal purpose of this study was to elucidate the association between thyroid disturbances as a potentially modifiable risk factor of cognitive dysfunction. In this review study, we have tried to collect almost all of the reported mechanisms demonstrating the role of hypothyroidism and hyperthyroidism in the pathogenesis of dementia.

Hypothyroidism and Cognitive Disorders during Development and Adulthood: Implications in the Central Nervous System

Thyroid hormones (THs) play a critical function in fundamental signaling of the body regulating process such as metabolism of glucose and lipids, cell maturation and proliferation, and neurogenesis, to name just a few. THs trigger biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). For years, a close relationship between the THs hormones and the central nervous system (CNS) has been described, not only for neuronal cells but also for glial development and differentiation. A deficit in thyroid hormones triiodothyronine (T₃) and thyroxine (T₄) is observed in the hypothyroid condition, generated by a iodine deficiency or an autoimmune response of the body. In the hypothyroid condition, several cellular deregulation and alterations have been described in dendrite spine morphology, cell migration and proliferation, and impaired synaptic transmission in the hippocampus, among others. The aim of this review is to describe the role of the thyroid hormones with focus in brain function and neurodegenerative disorders.

Postnatal growth hormone deficiency in growing rats causes marked decline in the activity of spinal cord acetylcholinesterase but not butyrylcholinesterase

The effects of growth hormone (GH) deficiency on the developmental changes in the abundance and activity of cholinesterase enzymes were studied in the developing spinal cord (SC) of postnatal rats by measuring the specific activity of acetylcholinesterase (AChE), a marker for cholinergic neurons and their synaptic compartments, and butyrylcholinesterase (BuChE), a marker for glial cells and neurovascular cells. Specific activities of these two enzymes were measured in SC tissue of 21- and 90 day-old (P21, weaning age; P90, young adulthood) GH deficient spontaneous dwarf (SpDwf) mutant rats which lack anterior pituitary and circulating plasma GH, and were compared with SC tissue of normal age-matched control animals. Assays were carried out for AChE and BuChE activity in the presence of their specific chemical inhibitors, BW284C51 and iso-OMPA, respectively. Results revealed that mean AChE activity was markedly and significantly reduced [28% at P21, 49% at P90, (p<0.01)] in the SC of GH deficient rats compared to age-matched controls. GH deficiency had a higher and more significant effect on AChE activity of the older (P90) rats than the younger ones (P21) ones. In contrast, BuChE activity in SC showed no significant changes in GH deficient rats at either of the two ages studied. Results imply that, in the absence of pituitary GH, the postnatal proliferation of cholinergic synapses in the rat SC, a CNS structure, where AChE activity is abundant, is markedly reduced during both the pre- and postweaning periods; more so in the postweaning than preweaning ages. In contrast, the absence of any effects on BuChE activity implies that GH does not affect the development of non-neuronal elements, e.g., glia, as much as the neuronal and synaptic compartments of the developing rat SC.

Effects of increasing durations of immobilization stress on plasma corticosterone level, learning and memory and hippocampal BDNF gene expression in rats

Stress effects on learning and memory are widely recognized, but less agreement exists on whether they are positive or negative as well as on their neuronal and neuromolecular correlates. Stress involves expression of certain genes such as neurotrophin BDNF (brain derived neurotrophic factor), which is also involved in learning, but results are not consistent. Here effects of stress on memory and BDNF expression were studied using on adult male rats exposed to "immobilization stress" for various "short" durations, i.e., 1-h, 3-h, 5-h and "long-term" ones (2-h/day for 1 week). Learning and memory was measured using passive avoidance testing (STL=step-through-latency scores) as well as plasma corticosterone (CSt) levels and hippocampal BDNF gene expression. CSt increased in the 3-h and longer stressed groups but differences were significant in the 5-h and 1-week stressed subgroups. Three and 5-h of stress markedly and significantly (60-69%, p<0.01) decreased memory retention in the stressed animals, while 1-h of stress had no effect; prolonged stress (2-h daily for 1-week) increased memory significantly (33%, p<0.05). Hippocampal BDNF gene expression increased in the 1-h and 3-h stressed groups (44%, p<0.05 and 71%, p<0.01); but this parameter steadily declined in the 5-h stressed group (26%, p<0.05) and weeklong stressed group (6%, not significant). Statistical analysis revealed an apparent but significant negative correlation between changes in memory and those of BDNF gene expression, indicating that BDNF may possibly play a compensatory role, reversing deleterious effects of stress on hippocampal memory functions.

Stimulatory effects of thyroid hormone on brain angiogenesis in vivo and in vitro

Thyroid hormone is critical for the proper development of the central nervous system. However, the specific role of thyroid hormone on brain angiogenesis remains poorly understood. Treatment of rats from birth to postnatal day 21 (P21) with propylthiouracil (PTU), a reversible blocker of triiodothyronine (T3) synthesis, resulted in decreased brain angiogenesis, as indicated by reduced complexity and density of microvessels. However, when PTU was withdrawn at P22, these parameters were fully recovered by P90. These changes were paralleled by an altered expression of vascular endothelial growth factor A (Vegfa) and basic fibroblast growth factor (Fgf2). Physiologic concentrations of T3 and thyroxine (T4) stimulated proliferation and tubulogenesis of rat brain-derived endothelial (RBE4) cells in vitro. Protein and mRNA levels of VEGF-A and FGF-2 increased after T3 stimulation of RBE4 cells. The thyroid hormone receptor blocker NH-3 abolished T3-induced Fgf2 and Vegfa upregulation, indicating a receptor-mediated effect. Thyroid hormone inhibited the apoptosis in RBE4 cells and altered mRNA levels of apoptosis-related genes, namely Bcl2 and Bad. The present results show that thyroid hormone has a substantial impact on vasculature development in the brain. Pathologically altered vascularization could, therefore, be a contributing factor to the neurologic deficits induced by thyroid hormone deficiency.

Marked recovery of functional metabolic activity and laminar volumes in the rat hippocampus and dentate gyrus following postnatal hypothyroid growth retardation: A quantitative cytochrome oxidase study

Similar to cretinism in human children, absence or deficiency of thyroid hormones in rats and mice during early postnatal development results in marked retardation of brain development along with behavioral and cognitive deficits. Less is known about brain recovery from postnatal hypothyroidism. [Farahvar, A., Meisami, E., 2007. Novel two-dimensional morphometric maps and quantitative analysis reveal marked growth and structural recovery of the rat hippocampal regions from early hypothyroid retardation. Experimental Neurology.] found, by means of morphometric maps, that surface areas of hippocampal cortex and its CA1-CA4 regions which were significantly reduced in developing hypothyroid rats, show nearly complete growth recovery upon restoration of thyroid function. Here we explore the ability of hippocampal synapse-rich neuronal fiber layers to show recovery from early hypothyroid growth retardation. Rat pups were made hypothyroid from birth to day 25 (weaning) or up to young adulthood (day 90) by a treatment with the reversible goitrogen, PTU (n-propylthiouracil), in the drinking water. Recovery was induced by withdrawal of PTU at weaning and analysis of cytochrome oxidase (CytOx)-stained serial sections of the hippocampus and dentate gyrus at the ages of 25 and 90 days. CytOx stains the synapse-rich fiber layers of the hippocampal formation (HCF). Volumetric growth of molecular layer, stratum oriens and radiatum and dentate hilar region showed complete or nearly complete recovery from marked and significant growth retardation induced by early postnatal hypothyroidism. Also the reduced CytOx staining intensity in the hypothyroid rat HCF layers showed marked recovery following hormonal restoration. Results indicate remarkable growth plasticity of the HCF and ability of the synapse-rich fiber layers to show complete recovery of metabolic and functional neural activity from deleterious effects of early hypothyroidism. Mitochondrial CytOx is highly localized to the synapse-rich fiber layers of the HCF and its activity and histochemical staining intensity correlates positively with functional metabolic activity of neural tissue. Thus hippocampus and dentate gyrus neuronal fiber layers and their oxidative activity show remarkable ability to recover from the postnatal hypothyroid growth retardation. The results indicate that some brain regions are less vulnerable to early developmental insults and can recover.