Immunocytochemical delineation of thyroid hormone receptor beta 2-like immunoreactivity in the rat central nervous system

Thyroid Hormones in the Brain and Their Impact in Recovery Mechanisms After Stroke

Thyroid hormones are of fundamental importance for brain development and essential factors to warrant brain functions throughout life. Their actions are mediated by binding to specific intracellular and membranous receptors regulating genomic and non-genomic mechanisms in neurons and populations of glial cells, respectively. Among others, mechanisms include the regulation of neuronal plasticity processes, stimulation of angiogenesis and neurogenesis as well modulating the dynamics of cytoskeletal elements and intracellular transport processes. These mechanisms overlap with those that have been identified to enhance recovery of lost neurological functions during the first weeks and months after ischemic stroke. Stimulation of thyroid hormone signaling in the postischemic brain might be a promising therapeutic strategy to foster endogenous mechanisms of repair. Several studies have pointed to a significant association between thyroid hormones and outcome after stroke. With this review, we will provide an overview on functions of thyroid hormones in the healthy brain and summarize their mechanisms of action in the developing and adult brain. Also, we compile the major thyroid-modulated molecular pathways in the pathophysiology of ischemic stroke that can enhance recovery, highlighting thyroid hormones as a potential target for therapeutic intervention.

Elevated aggressive behavior in male mice with thyroid-specific Prkar1a and global Epac1 gene deletion

Alterations in circulating thyroid hormone concentrations are associated with several psychological and behavioral disorders. In humans, behavioral disorders such as anxiety, depression, and attention-deficit hyperactivity disorder can be associated with thyroid disease. The Tpo-Cre;Prkar1aflox/flox;Epac1-/- (R1A-Epac1KO) mice, originally bred to investigate the role of exchange protein directly activated by cAMP (Epac1) in follicular thyroid cancer, displayed self-mutilating and aggressive behaviors during casual observation. To assess these atypical responses, behavioral testing was conducted with the R1A-Epac1KO mice, as well as their single knockout counterparts, the thyroid-specific Prkar1a-/- and global Epac1-/- mice. Mice of all three genotypes demonstrated increased aggressive behavior against an intruder mouse. In addition, Epac1-/- mice increased response to an auditory stimulus, and the Prkar1a-/- and R1A-Epac1KO mice increased swimming behavior in the Porsolt forced swim test. Both Prkar1a-/- mice and R1A-Epac1KO mice have increased circulating thyroxine and corticosterone concentrations. Although hyperthyroidism has not been previously associated with aggression, increased thyroid hormone signaling might contribute to the increased aggressive response to the intruder mouse, as well as the increased swimming response. Mice with a genetic background of Tpo-Cre;Prkar1aflox/flox;Epac1-/- are aggressive, and both the thyroid-specific knockout of Prkar1a and global knockout of Epac1 likely contribute to this aggressive behavior. This study supports the hypothesis that altered thyroid signaling and aggressive behavior are linked.

Stimulating effect of thyroid hormones in peripheral nerve regeneration: research history and future direction toward clinical therapy

Injury to peripheral nerves is often observed in the clinic and severe injuries may cause loss of motor and sensory functions. Despite extensive investigation, testing various surgical repair techniques and neurotrophic molecules, at present, a satisfactory method to ensuring successful recovery does not exist. For successful molecular therapy in nerve regeneration, it is essential to improve the intrinsic ability of neurons to survive and to increase the speed of axonal outgrowth. Also to induce Schwann cell phenotypical changes to prepare the local environment favorable for axonal regeneration and myelination. Therefore, any molecule that regulates gene expression of both neurons and Schwann cells could play a crucial role in peripheral nerve regeneration. Clinical and experimental studies have reported that thyroid hormones are essential for the normal development and function of the nervous system, so they could be candidates for nervous system regeneration. This review provides an overview of studies devoted to testing the effect of thyroid hormones on peripheral nerve regeneration. Also it emphasizes the importance of combining biodegradable tubes with local administration of triiodothyronine for future clinical therapy of human severe injured nerves. We highlight that the local and single administration of triiodothyronine within biodegradable nerve guide improves significantly the regeneration of severed peripheral nerves, and accelerates functional recovering. This technique provides a serious step towards future clinical application of triiodothyronine in human severe injured nerves. The possible regulatory mechanism by which triiodothyronine stimulates peripheral nerve regeneration is a rapid action on both axotomized neurons and Schwann cells.

Nuclear receptors in neural stem/progenitor cell homeostasis

In the central nervous system, embryonic and adult neural stem/progenitor cells (NSCs) generate the enormous variety and huge numbers of neuronal and glial cells that provide structural and functional support in the brain and spinal cord. Over the last decades, nuclear receptors and their natural ligands have emerged as critical regulators of NSC homeostasis during embryonic development and adult life. Furthermore, substantial progress has been achieved towards elucidating the molecular mechanisms of nuclear receptors action in proliferative and differentiation capacities of NSCs. Aberrant expression or function of nuclear receptors in NSCs also contributes to the pathogenesis of various nervous system diseases. Here, we review recent advances in our understanding of the regulatory roles of steroid, non-steroid, and orphan nuclear receptors in NSC fate decisions. These studies establish nuclear receptors as key therapeutic targets in brain diseases.

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce 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). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Hypothalamic control of seasonal changes in food intake and body weight

Seasonal cycles of fattening and body weight reflecting changes in both food intake and energy expenditure are a core aspect of the biology of mammals that have evolved in temperate and arctic latitudes. Identifying the neuroendocrine mechanisms that underlie these cycles has provided new insights into the hypothalamic control of appetite and fuel oxidation. Surprisingly, seasonal cycles do not result from changes in the leptin-responsive and homeostatic pathways located in the mediobasal and lateral hypothalamus that regulate meal timing and compensatory responses to starvation or caloric restriction. Rather, they result from changes in tanycyte function, which locally regulates transport and metabolism of thyroid hormone and retinoic acid. These signals are crucial for the initial development of the brain, so it is hypothesized that seasonal neuroendocrine cycles reflect developmental mechanisms in the adult hypothalamus, manifest as changes in neurogenesis and plasticity of connections.

Central regulation of hypothalamic-pituitary-thyroid axis under physiological and pathophysiological conditions

TRH is a tripeptide amide that functions as a neurotransmitter but also serves as a neurohormone that has a critical role in the central regulation of the hypothalamic-pituitary-thyroid axis. Hypophysiotropic TRH neurons involved in this neuroendocrine process are located in the hypothalamic paraventricular nucleus and secrete TRH into the pericapillary space of the external zone of the median eminence for conveyance to anterior pituitary thyrotrophs. Under basal conditions, the activity of hypophysiotropic TRH neurons is regulated by the negative feedback effects of thyroid hormone to ensure stable, circulating, thyroid hormone concentrations, a mechanism that involves complex interactions between hypophysiotropic TRH neurons and the vascular system, cerebrospinal fluid, and specialized glial cells called tanycytes. Hypophysiotropic TRH neurons also integrate other humoral and neuronal inputs that can alter the setpoint for negative feedback regulation by thyroid hormone. This mechanism facilitates adaptation of the organism to changing environmental conditions, including the shortage of food and a cold environment. The thyroid axis is also affected by other adverse conditions such as infection, but the central mechanisms mediating suppression of hypophysiotropic TRH may be pathophysiological. In this review, we discuss current knowledge about the mechanisms that contribute to the regulation of hypophysiotropic TRH neurons under physiological and pathophysiological conditions.

Thyroid hormone's role in regulating brain glucose metabolism and potentially modulating hippocampal cognitive processes

Cognitive performance is dependent on adequate glucose supply to the brain. Insulin, which regulates systemic glucose metabolism, has been recently shown both to regulate hippocampal metabolism and to be a mandatory component of hippocampally-mediated cognitive performance. Thyroid hormones (TH) regulate systemic glucose metabolism and may also be involved in regulation of brain glucose metabolism. Here we review potential mechanisms for such regulation. Importantly, TH imbalance is often encountered in combination with metabolic disorders such as diabetes, and may cause additional metabolic dysregulation and hence worsening of disease states. TH's potential as a regulator of brain glucose metabolism is heightened by interactions with insulin signaling, but there have been relatively few studies on this topic or on the actions of TH in a mature brain. This review discusses evidence for mechanistic links between TH, insulin, cognitive function, and brain glucose metabolism, and reaches the conclusion that TH may modulate memory processes, likely at least in part by modulation of central insulin signaling and glucose metabolism.

Contribution of TNF-alpha and nuclear factor-kappaB signaling to type 2 iodothyronine deiodinase activation in the mediobasal hypothalamus after lipopolysaccharide administration

To determine whether signaling through TNF and/or nuclear factor-kappaB contributes to bacterial lipopolysaccharide (LPS)-induced activation of type 2 iodothyronine deiodinase (D2) in tanycytes lining the floor and infralateral walls of the third ventricle, the effect of a TNF antagonist on D2 gene expression and LPS-induced Ikappa-Balpha expression in tanycytes were studied. Animals treated with soluble, rat, polyethylene glycol-conjugated TNF receptor type 1 (4 mg/kg body weight) before a single ip injection of LPS showed a significant reduction in circulating IL-6 levels but no effect on LPS-induced D2 mRNA in the majority of tanycytes with the exception of a subpopulation of alpha tanycytes in the wall of the third ventricle. LPS induced a rapid increase in Ikappa-Balpha mRNA in the pars tuberalis and a delayed response in alpha tanycytes but absent in all other tanycyte subsets. The LPS-induced increase in Ikappa-Balpha in the pars tuberalis was associated with increased TSHbeta gene expression in this tissue, but cAMP response element-binding protein (CREB) phosphorylation was observed only in a subset of alpha tanycytes. These data suggest that TNF and nuclear factor-kappaB signaling are not the primary, initiating mechanisms mediating the LPS-induced D2 response in tanycytes, but may contribute in part to sustaining the LPS-induced D2 response in a subset of alpha tanycytes. We hypothesize that in addition to TSH, other factors derived from the pars tuberalis may contribute to LPS-induced D2 activation in tanycytes.

Tanycyte pyroglutamyl peptidase II contributes to regulation of the hypothalamic-pituitary-thyroid axis through glial-axonal associations in the median eminence

Pyroglutamyl peptidase II (PPII), a highly specific membrane-bound metallopeptidase that inactivates TRH in the extracellular space, is tightly regulated by thyroid hormone in cells of the anterior pituitary. Whether PPII has any role in the region where axons containing hypophysiotropic TRH terminate, the median eminence, is unknown. For this purpose, we analyzed the cellular localization and regulation of PPII mRNA in the mediobasal hypothalamus in adult, male rats. PPII mRNA was localized in cells lining the floor and infralateral walls of the third ventricle and coexpressed with vimentin, establishing these cells as tanycytes. PPII mRNA extended in a linear fashion from the tanycyte cell bodies in the base of the third ventricle to its cytoplasmic and end-feet processes in the external zone of the median eminence in close apposition to pro-TRH-containing axon terminals. Compared with vehicle-treated, euthyroid controls, animals made thyrotoxic by the i.p. administration of 10 microg L-T(4) daily for 1-3 d, showed dramatically increased accumulation of silver grains in the mediobasal hypothalamus and an approximately 80% increase in enzymatic activity. PPII inhibition in mediobasal hypothalamic explants increased TRH secretion, whereas i.p. injection of a specific PPII inhibitor increased cold stress- and TRH-induced TSH levels in plasma. We propose that an increase in circulating thyroid hormone up-regulates PPII activity in tanycytes and enhances degradation of extracellular TRH in the median eminence through glial-axonal associations, contributing to the feedback regulation of thyroid hormone on anterior pituitary TSH secretion.

Hypothalamic thyroid hormone in energy balance regulation

Thyroid hormone has been known for decades as a hormone with profound effects on energy expenditure and ability to control weight. The regulation of energy expenditure by thyroid hormone primarily occurs via regulation of the activity, or expression, of uncoupling proteins in peripheral tissues. However, mechanistically this requires a signal from the brain to change circulating levels of thyroxine and thyroid hormone or increased sympathetic drive to peripheral tissues to alter local thyroid hormone levels via increased expression of type 2 deiodinase. However, little consideration has been given to the potential role and involvement of thyroid hormones action in the brain in the regulation of energy balance. Recent evidence implicates thyroid hormone as a shortterm signal of energy deficit imposed by starvation. Furthermore, thyroid hormone action within the hypothalamus is involved in adjusting long-term energy expenditure in seasonal animals which endure food shortages in winter. Evidence from several studies suggests that regulation of type 2 and type 3 deiodinase enzymes in tanycytes of the third ventricle are gatekeepers of thyroid hormone levels in the hypothalamus. This paper reviews some of the evidence for the role of deiodinase enzymes and the actions of thyroid hormone in the hypothalamus in the regulation of energy balance.

A central thermogenic-like mechanism in feeding regulation: an interplay between arcuate nucleus T3 and UCP2

The active thyroid hormone, triiodothyronine (T3), regulates mitochondrial uncoupling protein activity and related thermogenesis in peripheral tissues. Type 2 deiodinase (DII), an enzyme that catalyzes active thyroid hormone production, and mitochondrial uncoupling protein 2 (UCP2) are also present in the hypothalamic arcuate nucleus, where their interaction and physiological significance have not been explored. Here, we report that DII-producing glial cells are in direct apposition to neurons coexpressing neuropeptide Y (NPY), agouti-related protein (AgRP), and UCP2. Fasting increased DII activity and local thyroid hormone production in the arcuate nucleus in parallel with increased GDP-regulated UCP2-dependent mitochondrial uncoupling. Fasting-induced T3-mediated UCP2 activation resulted in mitochondrial proliferation in NPY/AgRP neurons, an event that was critical for increased excitability of these orexigenic neurons and consequent rebound feeding following food deprivation. These results reveal a physiological role for a thyroid-hormone-regulated mitochondrial uncoupling in hypothalamic neuronal networks.

Expression of thyroid hormone receptor isoforms down-regulated by thyroid hormone in human medulloblastoma cells

The role of thyroid hormone (T3) in the regulation of growth and development of the central nervous system including the cerebellum has been well established. However, the effects of thyroid hormone on malignant tumors derived from the cerebellum remain poorly understood. Our analysis mainly focused on expression levels of TR isoforms and the effects of thyroid hormone in human medulloblastoma HTB-185 cells. Northern blot analysis revealed TRalpha2 mRNA but not TRalpha1, beta1 or beta2 mRNA in the cell. The TRalpha1 and TRbeta1 mRNAs were detected only by RT-PCR method and TRbeta2 was not expressed. Incubation of T3 for 24 h decreased TRalpha1, TRalpha2 and TRbeta1 mRNA. Addition of actinomycin D caused an acute increase in the basal TR mRNA levels and the rate of decrease of all kinds of TR isoform mRNA was accelerated in the T3-treated groups compared to controls, indicating that the stability of TR mRNA was affected by T3. Incubation with cycloheximide also blocked a decrease in TR mRNA levels in the T3-treated HTB-185 cells suggesting that down-regulation of TR mRNA required the synthesis of new protein. Our data provide novel evidence for the expression of TRs down-regulated by T3 in HTB-185 cells, suggesting that TR expression is post-transcriptionally regulated by T3 at the level of RNA stability.

Hypothyroidism increases Fos immunoreactivity in cholinergic neurons of brain medullary dorsal vagal complex in rats

Hypo- or hyperthyroidism is associated with autonomic disorders. We studied Fos expression in the medullary dorsal motor nucleus of the vagus (DMV), nucleus tractus solitarii (NTS), and area postrema (AP) in four groups of rats with different thyroid states induced by a combination of drinking water and daily intraperitoneal injection for 1-4 wk: 1) tap water and vehicle; 2) 0.1% propylthiouracil (PTU) and vehicle; 3) PTU and thyroxine (T4; 2 microg/100 g); and 4) tap water and T4 (10 microg/100 g). The numbers of Fos immunoreactive (IR) positive neurons in the DMV, NTS, and AP were low in euthyroid rats but significantly higher in the 4-wk duration in hypothyroid rats, which were prevented by simultaneous T4 replacement. Hyperthyroidism had no effect on Fos expression in these areas. There were significant negative correlations between T4 levels and the numbers of Fos-IR-positive neurons in the DMV (r = -0.6388, P < 0.008), NTS (r = -0.6741, P < 0.003), and AP (r = -0.5622, P < 0.004). Double staining showed that Fos immunoreactivity in the DMV of hypothyroid rats was mostly localized in choline acetyltransferase-containing neurons. Thyroid hormone receptors alpha1 and beta2 were localized in the observed nuclei. These results indicate that thyroid hormone influences the DMV/NTS/AP neuronal activity, which may contribute to the vagal-related visceral disorders observed in hypothyroidism.

Suppression of hypothalamic deiodinase type II activity blunts TRH mRNA decline during fasting

Fasting is characterized by disrupted thyroid feedback, with suppressed levels of thyroid hormones and paraventricular thyrotropin releasing hormone (TRH). We found that third ventricle administration of the deiodinase inhibitor, iopanoic acid, dose-dependently reduced deiodinase type II (DII) activity selectively in the hypothalamus. This suppression of DII by iopanoic acid during fasting prevented elevated DII activity and blunted the decline in hypothalamic TRH mRNA levels. Because fasting-induced elevation in hypothalamic DII activity is paralleled by increased hypothalamic T3 concentration, our study suggests that T3 formation by DII in the hypothalamus is the cause of disrupted thyroid feedback during fasting.

Effects of hyper- and hypothyroidism on thyroid hormone concentrations in regions of the rat brain

The purpose of this study was to investigate the effects of hyper- and hypothyroidism on thyroid hormone concentrations and deiodinase activities in nine regions of the rat brain. Four weeks of treatment with 75 microg thyroxine (T4)/kg body wt induced a two- to threefold increase in T4 levels in all of these brain regions, whereas the 3,5,3'-triiodothyronine (T3) concentrations were reduced in five brain regions and remained unchanged in four. Even after 8 wk treatment with 300 microg T4/kg, the T3 concentrations remained normal in cortical areas, the hippocampus and amygdala, and were elevated only in areas in which inner-ring deiodinase activity was low or absent, and in the hypothalamus. At the subcellular level, nuclear concentrations of T3 were diminished in hypothyroidism but remained unaltered in hyperthyroidism in all areas except the hypothalamus, where they were enhanced. Cortical mitochondrial succinate dehydrogenase activity was reduced in both hypo- and hyperthyroidism in spite of normal T3 concentrations in hyperthyroid animals. The results show that nuclear T3 concentrations fall in hypothyroidism but do not change during severe hyperthyroidism in any brain region except the hypothalamus. Further research is thus needed to clarify the mechanisms mediating the numerous biochemical and psychological effects of hyperthyroidism.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

Effects of excess thyroid hormone on cell death, cell proliferation, and new neuron incorporation in the adult zebra finch telencephalon

Widespread telencephalic neuronal replacement occurs throughout life in birds. We explored the potential relationship between thyroxine (T4) and cell turnover in the adult male zebra finch. We found that many cells in the zebra finch brain, including long-projection neurons in the high vocal center (HVC), stained positively with an antibody to thyroid hormone receptors (TR). Labeling was generally weak in the ventricular zone (VZ) that gives rise to new neurons but some proliferative VZ cells and/or their progeny, identified by [3H]-thymidine labeling, co-labeled with anti-TR antibody. Acute T4 treatment dramatically increased the number of pyknotic and TUNEL-positive cells in HVC and other telencephalic regions. In contrast, degenerating cells were never observed in the archistriatum or sub-telencephalic regions, suggesting that excess T4 augments cell death selectively in regions that show naturally occurring neuronal turnover. VZ mitotic activity was not altered shortly after acute T4 treatment at a dosage that stimulated cell death, although [3H]-labeling intensity per cell was slightly reduced. Moreover, the incorporation rates for neurons formed shortly before or after acute hormone treatment were no different from control values. Chronic T4 treatment resulted in a reduction in the total number of HVC neurons. Thus, hyperthyroidism augmented neuronal death, which was not compensated for by neuronal replacement. Collectively, these results indicate that excess T4 affects adult neuronal turnover in birds, and raises the possibility that thyroxine plays an important role in the postnatal development of the avian brain and vocal behavior.

Action of thyroid hormone in brain

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

Regulation of microglial development: a novel role for thyroid hormone

The postnatal development of rat microglia is marked by an important increase in the number of microglial cells and the growth of their ramified processes. We studied the role of thyroid hormone in microglial development. The distribution and morphology of microglial cells stained with isolectin B4 or monoclonal antibody ED1 were analyzed in cortical and subcortical forebrain regions of developing rats rendered hypothyroid by prenatal and postnatal treatment with methyl-thiouracil. Microglial processes were markedly less abundant in hypothyroid pups than in age-matched normal animals, from postnatal day 4 up to the end of the third postnatal week of life. A delay in process extension and a decrease in the density of microglial cell bodies, as shown by cell counts in the developing cingulate cortex of normal and hypothyroid animals, were responsible for these differences. Conversely, neonatal rat hyperthyroidism, induced by daily injections of 3,5,3'-triiodothyronine (T3), accelerated the extension of microglial processes and increased the density of cortical microglial cell bodies above physiological levels during the first postnatal week of life. Reverse transcription-PCR and immunological analyses indicated that cultured cortical ameboid microglial cells expressed the alpha1 and beta1 isoforms of nuclear thyroid hormone receptors. Consistent with the trophic and morphogenetic effects of thyroid hormone observed in situ, T3 favored the survival of cultured purified microglial cells and the growth of their processes. These results demonstrate that thyroid hormone promotes the growth and morphological differentiation of microglia during development.

Differential effect of thyroid hormone deficiency on the growth of calretinin-expressing neurons in rat spinal cord and dorsal root ganglia

The development of spinal cord or dorsal root ganglia neurons expressing calretinin (CR) was studied in thyroid hormone-deficient rats. Immunocytochemical and morphometric analyses showed that the hypothyroidism induced a significant decrease in the number and size of immunoreactive neurons in the spinal cord, as well as stunted growth and arborization of the axons and dendrites. These alterations were observed at different embryonic ages and persisted during the whole postnatal life. In adult hypothyroid rats, the mean number of CR-positive neurons per spinal cord section (31.2 +/- 2.3 in laminae I and II and 30.5 +/- 5.5 in laminae III-X) was significantly decreased (P < 0.001 and P = 0.024, respectively) compared with adult normal rats (68.7 +/- 8.9 and 50.0 +/- 11.0, respectively). In the peripheral nervous system, hypothyroidism altered the growth of sensory neurons expressing CR protein mainly during embryonic life. In comparison with normal rats, hypothyroid embryonic animals showed not only reduced cell size but also a significantly decreased percentage of CR-positive neurons (6.6 +/- 0. 9% in normal, 2.1 +/- 0.3% in hypothyroid rats, P < 0.001). In contrast, although the size of neurons was reduced in hypothyroid young and adult rats, there was no reduction in the percentage of CR-positive neurons. These results showed that thyroid hormone deficiency altered differentially the development of neurons expressing CR protein in the central and peripheral nervous systems. This suggests that central and peripheral neurons are heterogeneous in their sensitivity to thyroid hormone.

Localization of thyroid hormone receptor beta2 in the ventral medullary neurons that synthesize thyrotropin-releasing hormone

Altered thyroid statuses are associated with autonomic disorders. Thyrotropin-releasing hormone (TRH) synthesized in medullary raphe pallidus (Rpa), raphe obscurus (Rob) and the parapyramidal regions (PPR) regulates vagal and sympathetic preganglionic motoneurons. Hypothyroidism increased TRH gene expression and c-Fos immunoreactivity (IR) in these nuclei. Whether these increases represent a direct action of thyroid hormone was studied by detecting the presence of thyroid hormone receptor beta2 (TRbeta2) in pro-TRH-synthesizing neurons in the Rpa, Rob and the PPR using immunohistochemistry with specific TRbeta2 antiserum and in situ hybridization with digoxigenin-labeled pro-TRH cRNA probe. TRbeta2 IR was widely distributed throughout the medulla and primarily localized within the cell nuclei. Particularly intense immunostaining was presented in the Rpa, Rob and the PPR neurons. The combination of immunohistochemistry with in situ hybridization revealed that all pro-TRH mRNA-positive neurons in these ventral medullary nuclei were also TRbeta2 IR positive. The numbers of TRbeta2 IR-positive neurons in each nucleus were identical in both euthyroid rats and hypothyroid rats induced by 6-n-propyl-2-thiouracil in drinking water for 4 weeks. The finding that TRbeta2 localized in pro-TRH-synthesizing neurons in the ventral medullary nuclei provides an anatomical substrate for a direct thyroid hormone action on these neurons in the regulation of TRH gene expression, which may contribute to the altered autonomic activity in different thyroid statuses.

Transcriptional repression of TRH promoter function by T3: analysis by in vivo gene transfer

We consider how an integrated in vivo model can be used to study the specific transcriptional effects of specific receptors in neuroendocrine systems. Our example is the role of thyroid receptor (TR) isoforms in mediating negative feedback effects of T3 on TRH (thyrotropin releasing hormone) expression. The in vivo transfection method employed polyethylenimine (PEI) to introduce genes directly into specifc regions of the brains of mice, rats, and Xenopus tadpoles. In the mouse model, the technique has served to examine TR effects on TRH transcription and on the pituitary-thyroid axis end point: thyroid hormone secretion. When a TRH-luciferase construct is introduced into the hypothalami of newborn mice TRH-luciferase transcription is regulated physiologically, being significantly increased in hypothyroidism and decreased in T3-treated animals. When various T3-binding forms of TRβ or TRα are expressed in the hypothalamus, all TRβ isoforms give T3-dependent regulation of TRH transcription, whereas TRα isoforms block T3-dependent transcription. Moreover, TR transcriptional effects are correlated with physiological consequences on circulating T4. Thus, somatic gene transfer shows TR subtypes to have distinct, physiologically relevant effects on TRH transcription. The approach is an appealing alternative to germinal transgenesis for studying specific neuroendocrine regulations at defined developmental stages in different species.Key words: thyroid hormone, TRH, mouse central nervous system, non viral gene transfer, polyethylenimine.

Transcriptional anti-repression. Thyroid hormone receptor beta-2 recruits SMRT corepressor but interferes with subsequent assembly of a functional corepressor complex

Thyroid hormone receptors (T3Rs) are hormone-regulated transcription factors. Different T3R isoforms are expressed in a tissue-specific and developmentally regulated manner. The T3Ralpha-1, beta-0, and beta-1 isoforms typically repress target gene expression in the absence of hormone and activate transcription in the presence of hormone. Intriguingly, however, the T3Rbeta-2 isoform fails to repress, and instead is able to activate transcription in both the absence and presence of hormone. We investigated the molecular mechanism behind this absence of repression by T3Rbeta-2. Repression by T3Ralpha-1, beta-0, and beta-1 is mediated by the ability of these isoforms to physically recruit a SMRT/N-CoR corepressor complex. We determined that the unliganded T3Rbeta-2 also recruits the SMRT corepressor; in contrast to the alpha-1, beta-0, and beta-1 isoforms, however, the T3Rbeta-2 protein interacts not only with the C-terminal "receptor-interaction domain" of SMRT, but also makes additional contacts with the N-terminal "silencing domain" of the SMRT corepressor. These additional, T3Rbeta-2-specific contacts interfere with the subsequent association of SMRT with mSin3, a crucial second subunit of the corepressor holo-complex. Our results suggest that T3Rbeta-2 regulates transcription through a novel anti-repression mechanism, recruiting SMRT, but preventing the subsequent formation of a functional corepressor complex.

Role of thyroid hormones and their receptors in peripheral nerve regeneration

After peripheral nerve injury in adult mammals, reestablishment of functional connections depends on several parameters including neurotrophic factors, the extracellular matrix, and hormones. However, little is known about the contribution of hormones to peripheral nerve regeneration. Thyroid hormones, which are required for the development and maturation of the central nervous system, are also important for the development of peripheral nerves. The action of triiodothyronine (T3) on responsive cells is mediated through nuclear thyroid hormone receptors (TRs) which modulate the expression of specific genes in target cells. Thus, to study the effect of T3, it is first necessary to know whether the target tissues possess TRs. The fact that sciatic nerve cells possess functional TRs suggests that these cells can respond to T3 and, as a consequence, that thyroid hormone may be involved in peripheral nerve regeneration. The silicone nerve guide model provides an excellent system to study the action of local administration of T3. Evidence from such studies demonstrate that animals treated locally with T3 at the level of transection have more complete regeneration of sciatic nerve and better functional recovery. Among the possible regulatory mechanisms by which T3 enhances peripheral nerve regeneration is rapid action on both axotomized neurons and Schwann cells which, in turn, produce a lasting and stimulatory effect on peripheral nerve regeneration. It is probable that T3 up- or down-regulates gene expression of one or more growth factors, extracellular matrix, or cell adhesion molecules, all of which stimulate peripheral nerve regeneration. This could explain the greater effect of T3 on nerve regeneration compared with the effect of any one growth factor or adhesion molecule.

Physiological regulation of hypothalamic TRH transcription in vivo is T3 receptor isoform specific

Thyroid hormone (tri-iodo-thyronine, T3) exerts transcriptional effects on target genes in responsive cells. These effects are determined by DNA/protein interactions governed by the type of T3 receptors (TRs) in the cell. As TRs show tissue and developmental variations, regulation is best addressed in an integrated in vivo model. We examined TR subtype effects on thyrotropin-releasing hormone (TRH) transcription and on the pituitary/thyroid axis end point: thyroid hormone secretion. Polyethylenimine served to transfect a TRH-luciferase construct containing 554 bp of the rat TRH promoter into the hypothalami of newborn mice. Transcription from the TRH promoter was regulated in a physiologically faithful manner, being significantly increased in hypothyroidism and decreased in T3-treated animals. Moreover, when various ligand binding forms of mouse or chicken TRbeta and TRalpha were expressed with TRH-luciferase, all forms of TRbeta gave T3-dependent regulation of TRH transcription, whereas transcription was T3 insensitive with each TRalpha tested. Moreover, chicken TRalpha increased TRH transcription sixfold, whereas mouse TRalpha decreased transcription. These transcriptional effects had correlated physiological consequences: expression of the chicken TRalpha in the hypothalamus of newborn mice raised circulating T4 levels by fourfold, whereas mouse TRalpha had opposite effects. Thus, TR subtypes have distinct, physiologically relevant effects on TRH transcription.

Monosynaptic pathway between the arcuate nucleus expressing glial type II iodothyronine 5'-deiodinase mRNA and the median eminence-projective TRH cells of the rat paraventricular nucleus

Recent evidence suggests that the thyroid regulation of thyrotropin-releasing hormone (TRH)-containing neurons in the paraventricular nucleus of the hypothalamus involves the activation of other hypothalamic neural circuits. For example, the arcuate nucleus and not the paraventricular nucleus contains the highest enzyme activity of 5'-deiodinase type II, an enzyme that is pivotal for the local synthesis of T3. This experiment was undertaken to demonstrate whether a monosynaptic pathway exists between the arcuate nucleus and those TRH cells of the paraventricular nucleus that are neuroendocrine, i.e. project to the external layer of the median eminence. A specific cRNA probe derived from the coding region of deiodinase type II was used for the in situ hybridization histochemistry which was combined with immunocytochemistry for a specific marker of glial cells, glial fibrillary acidic protein (GFAP). The hybridization signals were present within the hypothalamus in the arcuate nucleus-median eminence region and in the periventricular area. The periventricular labeling was localized to the ependymal layer of the third ventricle and no hybridization product was detected in the paraventricular nucleus and other hypothalamic nuclei adjacent to the third ventricle. Within the median eminence, numerous cells containing the hybridization product were located in the internal layer adjacent to the floor of the third ventricle and in the external layer adjacent to the surface of the brain. In the dorso- and ventromedial regions of the arcuate nucleus, deiodinase type II mRNA-containing cells were also detected. Numerous type II deiodinase mRNA-containing cells in the median eminence and arcuate nucleus were also found to be immunopositive for GFAP. The abundance of arcuate cells expressing the hybridization product was lower than those in the periventricular region or in the median eminence. The anterograde tracer, Phaseolus vulgaris leucoagglutinin, was injected into the medial parts of the arcuate nucleus where the in situ hybridization experiment detected deiodinase type II mRNA. Simultaneously with the anterograde tracing, the retrograde tracer, Fluoro-Gold, was injected into either the median eminence or the general circulation. Light and electron microscopic double and triple immunolabeling experiments on vibratome sections of colchicine-pretreated animals revealed that arcuate fibers innervate TRH cells within the parvicellular region of the paraventricular nucleus. Populations of these TRH cells receiving afferents from the arcuate nucleus were also retrogradely labelled from either the median eminence or the general circulation indicating their direct role in the regulation of thyrotropin secretion from the anterior pituitary. The majority of arcuate nucleus efferents on TRH cells were found to establish symmetrical synaptic connections. The present results provided direct evidence of a monosynaptic pathway between the hypothalamic site of local thyroid hormone production, the arcuate nucleus, and neuroendocrine TRH cells in the paraventricular nucleus. This signalling modality may play an important role in thyroid feedback on TRH cells. Since the arcuate nucleus is involved in the regulation of central mechanisms controlling diverse homeostatic functions, including reproduction and feeding, the pathway described in this study may also carry integrated signals related to reproduction and ingestion to TRH-producing cells.

Physiological and pathophysiological aspects of thyrotropin-releasing hormone gene expression in the human hypothalamus

Although the tripeptide thyrotropin-releasing hormone (TRH) was the first hypothalamic hormone to be isolated and characterized, only very few data were available on the central component of the hypothalamus-pituitary-thyroid (HPT) axis in the human brain until recently. We used immunocytochemistry to describe, for the first time, the distribution of TRH-containing cells and fibers in the human hypothalamus. Brain material was obtained with a short postmortem delay followed by fixation in paraformaldehyde, glutaraldehyde, and picric acid. Many TRH-containing cells were present in the paraventricular nucleus (PVN), especially in its dorsocaudal part. Some TRH cells were found in the suprachiasmatic nucleus (SCN), which is the circadian clock of the brain, and in the sexually dimorphic nucleus (SDN), which is in agreement with earlier observations in the rat hypothalamus. Dense TRH-containing fiber networks were present not only in the median eminence but also in a number of other hypothalamic areas, suggesting a physiological function of TRH as a neuromodulator or neurotransmitter in the human brain, in addition to its neuroendocrine role in pituitary secretion of thyroid-stimulating hormone (TSH). As a next step, we developed a technique for TRH mRNA in situ hybridization using a [35S] CTP-labeled TRH cRNA antisense probe in formalin-fixed paraffin-embedded sections. Numerous heavily labeled TRH mRNA-containing neurons were detected in the caudal part of the PVN, while some cells were present in the SCN and in the perifornical area. These results demonstrated the value of in situ hybridization for elucidating the chemoarchitecture of the human hypothalamus in routinely fixed autopsy tissue and enabled us to perform quantitative studies. As part of the neuroendocrine response to disease, serum concentrations of thyroid hormone decrease without giving rise to elevated concentrations of TSH, suggesting altered feedback control at the level of the hypothalamus and/or pituitary. In order to establish whether decreased activity of TRH cells in the PVN contributes to the persistence of low TSH levels in nonthyroidal illness (NTI), hypothalamic TRH gene expression was investigated in patients whose plasma concentrations of thyroid hormones had been measured just before death. Quantitative in situ hybridization showed a positive correlation of total TRH mRNA in the PVN and serum concentrations of TSH and triiodothyronine (T3) less than 24 hours before death, supporting our hypothesis. Current experiments aim at elucidating the mechanism by which hypothalamic thyroid hormone feedback control in TRH cells of patients with NTI is changed.

Thyroid hormone affects Schwann cell and oligodendrocyte gene expression at the glial transition zone of the VIIIth nerve prior to cochlea function

All cranial nerves, as well as the VIIIth nerve which invades the cochlea, have a proximal end in which myelin is formed by Schwann cells and a distal end which is surrounded by oligodendrocytes. The question which arises in this context is whether peripheral and central parts of these nerves myelinate simultaneously or subsequently and whether the myelination of either of the parts occurs simultaneously at the onset of the cochlea function and under the control of neuronal activity. In the present paper, we examined the relative time course of the myelinogenesis of the distal part of the VIIIth nerve by analyzing the expression of peripheral protein P0, proteolipid protein and myelin basic protein. To our surprise, we observed that the expression of myelin markers in the peripheral and central part of the intradural part of the VIIIth nerve started simultaneously, from postnatal day 2 onwards, long before the onset of cochlea function. The expression rapidly achieved saturation levels on the approach to postnatal day 12, the day on which the cochlea function commenced. Because of its importance for the neuronal and morphological maturation of the cochlea during this time, an additional role of thyroid hormone in cochlear myelinogenesis was considered. Indeed, it transpires that this hormone ensures the rapid accomplishment of glial gene expression, not only in the central but also in the peripheral part of the cochlea. Furthermore, an analysis of the thyroid hormone receptors, TRaplha and TRbeta, indicates that TRbeta is necessary for myelinogenesis of the VIIIth nerve. Rapid thyroid hormone-dependent saturation of myelin marker gene expression in Schwann cells and oligodendrocytes of the VIIIth nerve may guarantee nerve conduction and synchronized impulse transmission at the onset of hearing. The thyroid hormone-dependent commencement of nerve conduction is discussed in connection with the patterning refinement of central auditory pathways and the acquisition of deafness.

Thyroid hormone receptor (alpha) distribution in hamster and sheep brain: colocalization in gonadotropin-releasing hormone and other identified neurons

Thyroid hormones appear to play an important role in the seasonal reproductive transitions of a number of mammalian and avian species. These seasonal transitions as well as the effects of thyroid hormones on the reproductive neuroendocrine axis are mediated by the GnRH system. How thyroid hormones affect the GnRH system is unclear. Double label immunocytochemistry was used to examine GnRH- and other neurotransmitter/neuropeptide-containing neurons for thyroid hormone receptor (alphaTHR) colocalization in two seasonal breeders, the golden hamster and the sheep. AlphaTHR was identified in hamster and sheep brain by Western blot analysis. Furthermore, alphaTHR immunoreactivity was widely distributed in brain and was colocalized in identified populations: GnRH neurons (hamster, 28%; sheep, 46%); dopaminergic neurons of the A14 (hypothalamic) and A16 (olfactory bulb) cell groups, but not in the hypothalamic A13 cell group; and neurophysin-immunoreactive neurons of the supraoptic and paraventricular nuclei. The finding of alphaTHR in GnRH and A14 dopamine neurons provides an anatomical substrate for direct thyroid hormone action on the reproductive neuroendocrine system of these two seasonally breeding species. It remains to be determined whether the GnRH gene itself or the gene of another constituent within the same GnRH neuron is responsive to thyroid hormones.

A unique role of the beta-2 thyroid hormone receptor isoform in negative regulation by thyroid hormone. Mapping of a novel amino-terminal domain important for ligand-independent activation

Negative regulation by thyroid hormone is mediated by nuclear thyroid hormone receptors (TRs) acting on thyroid hormone response elements (TREs). We examine here the role of human TR-beta2, a TR isoform with central nervous system-restricted expression, in the regulation of target genes whose expression are decreased by triiodothyronine (T3). Using transient transfection studies, we found that TR-beta2 achieved significantly greater ligand-independent activation on the thyrotropin-releasing hormone (TRH) and common glycoprotein alpha-subunit genes than either TR-beta1 or TR-alpha1. A chimeric TR-beta isoform containing the TR-beta2 amino terminus linked to the TR-alpha1 DNA- and ligand-binding domains functioned like the TR-beta2 isoform on these promoters, confirming that the amino terminus of TR-beta2 was both necessary and sufficient to mediate this effect. By constructing deletion mutants of the TR-beta2 amino terminus, we demonstrate that amino acids 89-116 mediate this function. This domain, important in ligand-independent activation on negative TREs, is discrete from a previously described activation domain in the amino-terminal portion of TR-beta2. We conclude that the central nervous system-restricted TR-beta2 isoform has a unique effect on negative regulation by T3 that can be mapped to amino acids 89-116 of the amino terminus of the human TR-beta2.

Interactions of estrogen- and thyroid hormone receptors on a progesterone receptor estrogen response element (ERE) sequence: a comparison with the vitellogenin A2 consensus ERE

The identification of hormone response elements in the promoter regions of hormonally regulated genes has revealed a striking similarity between the half-site of the estrogen-response element (ERE) and a consensus sequence constituting the thyroid hormone-response element. Because of the potential for thyroid hormone (T3) to affect estrogen (E)- and progesterone-dependent female reproductive behavior via EREs, we have begun to investigate the activity of an ERE identified in the progesterone receptor (PR) proximal promoter and its interactions with the estrogen receptor (ER) and thyroid hormone receptors (TR). In addition, we have compared ER and TR interactions on the PR ERE construct with that of the vitellogenin A2 (vit A2) consensus ERE. Electrophoretic mobility shift assays demonstrated that TR binds to the PR ERE as well as to the consensus ERE sequence in vitro. Further, these two EREs were differentially regulated by T3 in the presence of TR. T3 in the presence of TR alpha increased transcription from a PR ERE construct approximately 5-fold and had no inhibitory effect on E induction. Similarly, T3 also activated a beta-galactosidase reporter construct containing PR promoter sequences spanning -1400 to +700. In addition, the TR isoforms beta1 and beta2 also stimulated transcription from the PR ERE construct by 5- to 6-fold. A TR alpha mutant lacking the ability to bind AGGTCA sequences in vitro failed to activate transcription from the PR ERE construct, demonstrating dependence on DNA binding. In contrast to its actions on the PR ERE construct, TR alpha did not activate transcription from the vit A2 consensus ERE but rather attenuated E-mediated transcriptional activation. Attenuation from the vit A2 consensus ERE is not necessarily dependent on DNA binding as the TR alpha DNA binding mutant was still able to inhibit E-dependent transactivation. In contrast to TR alpha, the isoforms TRbeta1 and TRbeta2 failed to inhibit E-induced activation from the vit A2 consensus ERE. These results demonstrate that the PR ERE construct differs from the vit A2 consensus ERE in its ability to respond to TRs and that divergent pathways exist for activation and inhibition by TR. Since ERs, PRs, and TRs are all present in hypothalamic neurons, these findings may be significant for endocrine integration, which is important for reproductive behavior.

Thyroid hormone and estrogen interact to regulate behavior

Environmental perturbations that increase plasma thyroid hormone (T3) concentrations also profoundly affect female reproductive behavior and physiology. We explored whether these effects were mediated by interactions between T3 receptor (TR) and estrogen receptor (ER). This hypothesis was of interest because the half-site of a consensus T3 response element DNA sequence is identical to an ER response element (ERE), and TRs bind to a consensus ERE. Molecular data presented in the accompanying paper [Zhu, Y.-S., Yen, P.M., Chin, W.W.& Pfaff, D.W. (1996) Proc. Natl. Acad. Sci. USA 93, 12587-12592] demonstrate that TRs and ERs are both present in rat hypothalamic nuclear extracts and that both can bind to the promoter the hypothalamic gene preproenkephalin and that interations between liganded TRs and ERs affect preproenkephalin transcription. In this paper, we show that molecular interactions between TRs and ERs are sufficient to mediate environmental effects on estrogen-controlled reproductive behavior. Ovariectomized (OVX) rats treated with high doses of T3 showed significantly lower levels of lordosis behavior in response to estradiol benzoate (EB) compared with OVX females treated with EB alone. Conversely, thyroidectomized/OVX females treated with EB showed significantly greater levels of lordosis behavior compared with OVX females treated with EB, showing the effect of endogenous T3. Thyroid hormone interference with EB-induced behavior could not be explained by a reduction in plasma E2 concentrations or by a general reduction in responsiveness of EB-sensitive tissues. Moreover, numbers of hypothalamic ER-immunoreactive cells increased dramatically following T3 treatment. These data suggest that T3 may reduce EB-dependent sexual behavior through interactions between TR and ER in the nuclei of behaviorally relevant hypothalamic neurons, envisioning for the first time a functional consequence of interactions between two nuclear hormone receptors in brain. These results also open up the possibility of molecular interactions on DNA encoding environmental signals, a new field for the study of neuronal integration.

Estrogen and thyroid hormone interaction on regulation of gene expression

Estrogen receptor (ER) and thyroid hormone receptors (TRs) are ligand-dependent nuclear transcription factors that can bind to an identical half-site, AGGTCA, of their cognate hormone response elements. By in vitro transfection analysis in CV-1 cells, we show that estrogen induction of chloramphenicol acetyltransferase (CAT) activity in a construct containing a CAT reporter gene under the control of a minimal thymidine kinase (tk) promoter and a copy of the consensus ER response element was attenuated by cotransfection of TR alpha 1 plus triiodothyronine treatment. This inhibitory effect of TR was ligand-dependent and isoform-specific. Neither TR beta 1 nor TR beta 2 cotransfection inhibited estrogen-induced CAT activity, although both TR alpha and TR beta can bind to a consensus ER response element. Furthermore, cotransfection of a mutated TR alpha 1 that lacks binding to the AGGTCA sequence also inhibited the estrogen effect. Thus, the repression of estrogen action by liganded TR alpha 1 may involve protein-protein interactions although competition of ER and TR at the DNA level cannot be excluded. A similar inhibitory effect of liganded TR alpha 1 on estrogen induction of CAT activity was observed in a construct containing the preproenkephalin (PPE) promoter. A study in hypophysectomized female rats demonstrated that the estrogen-induced increase in PPE mRNA levels in the ventromedial hypothalamus was diminished by coadministration of triiodothyronine. These results suggest that ER and TR may interact to modulate estrogen-sensitive gene expression, such as for PPE, in the hypothalamus.

Immunohistochemical mapping of brain triiodothyronine reveals prominent localization in central noradrenergic systems

Many lines of evidence support a close association between thyroid hormones and noradrenergic systems in peripheral tissues. However, there is little certainty regarding interactions of the two systems in brain. We now report that triiodothyronine is concentrated in both nuclei and projection sites of central noradrenergic systems. Immunohistochemical mapping of the hormone revealed the following: (1) Locus coeruleus and all other noradrenergic cell groups identified were the most prominently labeled neural centers in the brain. (2) The hormone was also concentrated in the widely dispersed targets of noradrenergic projections. (3) Triiodothyronine labeling in noradrenergic target cells was most prominent over the cell nuclei, indicating that the hormone was bound to its receptors. Therefore, targets of noradrenergic innervation should be responsive to triiodothyronine. (4) Unlike that in noradrenergic target cells, triiodothyronine staining was decidedly perikaryal in locus coeruleus (A-6) and the other A-1 to A-7 cell groups; the staining pattern in locus coeruleus cytosol and processes was heavy, clumped and similar to that seen in contiguous sections immunostained for tyrosine hydroxylase. Results of radio-immunoassay, immunoabsorption and pharmacological tests demonstrated the specificity of the antibody for triiodothyronine and ruled against cross-reactivity with norepinephrine or its metabolites as the basis for the staining reactions. Although other possibilities consistent with these new observations are given consideration, it appears that the structure and activity of central noradrenergic systems may be major determinants of triiodothyronine distribution patterns and actions in brain. If the noradrenergic system processes both triiodothyronine and norepinephrine and conducts them both to nerve cell groups receiving its terminal arborizations, specific postsynaptic receptors would be available for transduction of both sets of messages. The evidence provides a morphological basis for earlier proposals that triiodothyronine may play a neuromodulatory or neurotransmitter role in the adrenergic nervous system.

Ligand-dependent and -independent transactivation by thyroid hormone receptor beta 2 is determined by the structure of the hormone response element

Chicken thyroid hormone receptor beta 2 (cTR beta 2) is likely to serve specific functions in gene regulation since it possesses a unique N-terminal domain and is expressed in very few tissues. We demonstrate here that TR beta 2 exhibits distinct transactivation properties which are dependent on the availability of ligand and on the structure of the hormone response element. First, a strong ligand-independent transactivation was observed with hormone response elements composed of direct repeats and everted repeats. Second, TR beta 2 was induced by triiodothyronine to transactivate more efficiently than TR beta 0 on palindromic and everted-repeat types of hormone response elements. However, coexpression of the retinoid X receptor reduced the strong transactivation by TR beta 2 but not by TR beta 0 via palindromic response elements, suggesting that TR beta 2 can transactivate as a homodimer. Finally, the N terminus of TR beta 2 contains two distinct transactivation regions rich in tyrosines, which are essential for transactivation. Our results thus show that the activity of the novel transactivating region of TR beta 2 is dependent on the organization of the half-sites in the response element.

Expression of enkephalin and dynorphin precursor mRNAs in brain areas of hypo-and hyperthyroid rat: effect of kainic acid injection

An abnormal thyroid status induces morphological and neurochemical modifications in the adult brain. In this study we have analyzed the expression of enkephalin (ENK) and dynorphin (DYN) precursor mRNAs by means of in situ hybridization in the brain of hypothyroid and hyperthyroid rats. The influence of thyroid hormones on kainic acid (KA)-induced expression of ENK and DYN mRNAs in the granule cells of the dentate gyrus was also studied. Our results can be summarized as follows: (1) hypothyroidism induces an up-regulation of ENK mRNA in the granule cells of the dentate gyrus and layers V/VI of the cingulate cortex and of DYN mRNA in the granule cells of the dentate gyrus; (2) the up-regulation of ENK mRNA expression in the granule cells induced by KA is not modified by altered thyroid status; (3) in contrast, the KA injection fails to up-regulate DYN precursor mRNA expression in the granule cells of the rostral dentate gyrus of the hypothyroid rats; (4) injection of KA in hyperthyroid rats increases the expression of DYN mRNA in the granule dentate gyrus more than in euthyroid rats. The present results suggest that thyroid hormones exert an inhibitory control of expression of ENK and DYN mRNAs in selected brain areas. This effect could be directly mediated though the thyroid hormone nuclear receptor or could be secondary to changes in glutamatergic transmission in the dentate gyrus, as suggested by the profound alteration of the KA-induced expression of DYN mRNA in the dentate gyrus of hypo-and hyperthyroid rats.

Effect of hypothyroidism on vasoactive intestinal polypeptide-immunoreactive neurons in forebrain-neurohypophysial nuclei of the rat brain

We have recently reported that hypothyroidism increases immunoreactive (IR)-vasoactive intestinal polypeptide (VIP) and VIP mRNA content in both parvocellular and magnocellular neurons of the rat, hypothalamic paraventricular nucleus (PVN). As VIP can stimulate vasopressin (AVP) secretion, we conducted an anatomical investigation to determine whether VIP-containing neurons in other regions of the brain that are involved with homeostatic mechanisms of water and salt conservation are also affected by hypothyroidism. The distribution and intensity of VIP immunostaining in neurons and fibers of the magnocellular-neurohypophysial system, including the hypothalamic PVN, supraoptic nucleus (SON) and accessory magnocellular cell groups, circumventricular subfornical organ (SFO), preoptic and anterior hypothalamus, midline thalamus, subthalamic zona incerta and posterior septal nuclei were studied using a highly sensitive immunocytochemical technique and unbiased neuronal counting methods, based on the optical dissector principle. Hypothyroidism increased the intensity of VIP immunostaining and/or the number/section, percentage and numerical density of IR-VIP neurons in the PVN, SON, nucleus circularis, periventricular preoptic nucleus of the hypothalamus and SFO. In addition, IR-VIP perikarya and/or fibers in the hypothalamic medial preoptic area and anterior periventricular nucleus, nucleus reuniens of the thalamus and dorsal fornix-triangular septal nucleus complex were also apparent in the hypothyroid animals while no immunostaining was seen in these areas in control animals. No quantitative and/or qualitative modifications in IR-VIP neurons and fibers were noted in the anterior hypothalamic area, suprachiasmatic nucleus, thalamic paraventricular nucles an subthalamic zona incerta between hypothyroid and control animals. These findings suggest an inverse relationship between thyroid hormone and VIP content and/or distribution of IR-VIP neurons in specific forebrain regions involved in the control of AVP release, extracellular fluid volume, thirst, blood pressure and anterior pituitary secretion. This raises the possibility that changes in fluid homeostasis and cardiovascular function occurring in hypothyroidism may be mediated, at least in part, by VIP-producing neurons in diverse regions of the brain.

Rapid stimulation of type I 5'-deiodinase in rat pituitaries by 3,3',5-triiodo-L-thyronine

The negative feedback control of thyrotropin production by the anterior pituitary involves local 5'-deiodination of L-thyroxine (L-T4) to the active thyroid hormone 3,3',5-triiodo-L-thyronine (T3) by two 5'-deiodinase isozymes which are distinctly regulated by thyroid hormone. T3 rapidly increased steady-state mRNA levels and activity of type I iodothyronine-5'-deiodinase (5'DI) in rat anterior pituitary and in reaggregate cultures of anterior pituitaries. Type II 5'-deiodinase activity determined in parallel with 3,3',5-triiodo-L-thyronine (rT3) as substrate was markedly lower than that of 5'DI. 5'DI mRNA levels and activity were higher in anterior pituitaries of female compared to male rats. Neither gender differences nor T3 stimulation of 5'DI activity were found in the posterior part. These data demonstrate a T3 dependent expression of 5'DI in euthyroid anterior pituitary and suggest that this isozyme serves a major function within the complex network of thyroid hormone homeostasis.

3,5,3'-L-triiodothyronine promotes survival and axon elongation of embryonic rat septal neurons

The effect of 3,5,3'-L-triiodothyronine (T3) on survival and morphology of primary cultured neurons of the fetal rat brain was studied. In defined conditions of serum-free culture media we found the death preventing effect of T3 in all tested neuronal populations cultivated at high initial densities of plating (10(5) cells/cm2). While the survival of cerebrocortical neurons was improved very slightly, the number of surviving hippocampal and septal neurons reached 127.2 +/- 2.0% or 134.8 +/- 12.3% of their respective controls. The septal neurons responded at normal physiological concentration of T3 (1 nM) in high density as well as in low density cultures (5 x 10(3) cells/cm2). Moreover the treatment with 10 nM of T3 caused significant extension of the axon elongation of septal neurons (194.5 +/- 15.7%). These findings suggest the direct positive effect of T3 on pure cell population of septal neurons derived from embryonic rat brain and support the evidence for the role of this peripheral hormone during neuritogenesis.

A novel role for thyroid hormone, glucocorticoids and retinoic acid in timing oligodendrocyte development

The timing of oligodendrocyte differentiation is thought to depend on an intrinsic clock in oligodendrocyte precursor cells that counts time or cell divisions and limits precursor cell proliferation. We show here that this clock mechanism can be separated into a counting component and an effector component that stops cell proliferation: whereas the counting mechanism is driven by mitogens that activate cell-surface receptors, the effector mechanism depends on hydrophobic signals that activate intracellular receptors, such as thyroid hormones, glucocorticoids and retinoic acid. When purified oligodendrocyte precursor cells are cultured at clonal density in serum-free medium in the presence of mitogens but in the absence of these hydrophobic signals, the cells divide indefinitely and do not differentiate into postmitotic oligodendrocytes. In the absence of mitogens, the precursor cells stop dividing and differentiate prematurely into oligodendrocytes even in the absence of these hydrophobic signals, indicating that these signals are not required for differentiation. The levels of these signals in vivo may normally regulate the timing of oligodendrocyte differentiation, as the maximum number of precursor cell divisions in culture depends on the concentration of such signals and injections of thyroid hormone into newborn rats accelerates oligodendrocyte development. As thyroid hormone, glucocorticoids and retinoic acid have been shown to promote the differentiation of many types of vertebrate cells, it is possible that they help coordinate the timing of differentiation by signalling clocks in precursor cells throughout a developing animal.