Thyrotropin-releasing hormone precursor: characterization in rat brain

Pleiotropic Nanostructures Built from l-Histidine Show Biologically Relevant Multicatalytic Activities

The essential amino acid histidine plays a central role in the manifestation of several metabolic processes, including protein synthesis, enzyme-catalysis, and key biomolecular interactions. However, excess accumulation of histidine causes histidinemia, which shows brain-related medical complications, and the molecular mechanism of such histidine-linked complications is largely unknown. Here, we show that histidine undergoes a self-assembly process, leading to the formation of amyloid-like cytotoxic and catalytically active nanofibers. The kinetics of histidine self-assembly was favored in the presence of Mg(II) and Co(II) ions. Molecular dynamics data showed that preferential noncovalent interactions dominated by H-bonds between histidine molecules facilitate the formation of histidine nanofibers. The histidine nanofibers induced amyloid cross-seeding reactions in several proteins and peptides including pathogenic Aβ1-42 and brain extract components. Further, the histidine nanofibers exhibited oxidase activity and enhanced the oxidation of neurotransmitters. Cell-based studies confirmed the cellular internalization of histidine nanofibers in SH-SY5Y cells and subsequent cytotoxic effects through necrosis and apoptosis-mediated cell death. Since several complications including behavioral abnormality, developmental delay, and neurological disabilities are directly linked to abnormal accumulation of histidine, our findings provide a foundational understanding of the mechanism of histidine-related complications. Further, the ability of histidine nanofibers to catalyze amyloid seeding and oxidation reactions is equally important for both biological and materials science research.

TRH and NPY Interact to Regulate Dynamic Changes in Energy Balance in the Male Zebra Finch

In contrast to mammals, birds have a higher basal metabolic rate and undertake wide range of energy-demanding activities. As a consequence, food deprivation for birds, even for a short period, poses major energy challenge. The energy-regulating hypothalamic homeostatic mechanisms, although extensively studied in mammals, are far from clear in the case of birds. We focus on the interplay between neuropeptide Y (NPY) and thyrotropin-releasing hormone (TRH), 2 of the most important hypothalamic signaling agents, in modulating the energy balance in a bird model, the zebra finch, Taeniopygia guttata. TRH neurons were confined to a few nuclei in the preoptic area and hypothalamus, and fibers widely distributed. The majority of TRH neurons in the hypothalamic paraventricular nucleus (PVN) whose axons terminate in median eminence were contacted by NPY-containing axons. Compared to fed animals, fasting significantly reduced body weight, PVN pro-TRH messenger RNA (mRNA) and TRH immunoreactivity, but increased NPY mRNA and NPY immunoreactivity in the infundibular nucleus (IN, avian homologue of mammalian arcuate nucleus) and PVN. Refeeding for a short duration restored PVN pro-TRH and IN NPY mRNA, and PVN NPY innervation to fed levels. Compared to control tissues, treatment of the hypothalamic superfused slices with NPY or an NPY-Y1 receptor agonist significantly reduced TRH immunoreactivity, a response blocked by treatment with a Y1-receptor antagonist. We describe a detailed neuroanatomical map of TRH-equipped elements, identify new TRH-producing neuronal groups in the avian brain, and demonstrate rapid restoration of the fasting-induced suppression of PVN TRH following refeeding. We further show that NPY via Y1 receptors may regulate PVN TRH neurons to control energy balance in T. guttata.

Sex-dependent and -independent regulation of thyrotropin-releasing hormone expression in the hypothalamic dorsomedial nucleus by negative energy balance, exercise, and chronic stress

The dorsomedial nucleus of the hypothalamus (DMH) is part of the brain circuits that modulate organism responses to the circadian cycle, energy balance, and psychological stress. A large group of thyrotropin-releasing hormone (Trh) neurons is localized in the DMH; they comprise about one third of the DMH neurons that project to the lateral hypothalamus area (LH). We tested their response to various paradigms. In male Wistar rats, food restriction during adulthood, or chronic variable stress (CVS) during adolescence down-regulated adult DMH Trh mRNA levels compared to those in sedentary animals fed ad libitum; two weeks of voluntary wheel running during adulthood enhanced DMH Trh mRNA levels compared to pair-fed rats. Except for their magnitude, female responses to exercise were like those in male rats; in contrast, in female rats CVS did not change DMH Trh mRNA levels. A very strong negative correlation between DMH Trh mRNA levels and serum corticosterone concentration in rats of either sex was lost in CVS rats. CVS canceled the response to food restriction, but not that to exercise in either sex. TRH receptor 1 (Trhr) cells were numerous along the rostro-caudal extent of the medial LH. In either sex, fasting during adulthood reduced DMH Trh mRNA levels, and increased LH Trhr mRNA levels, suggesting fasting may inhibit the activity of TRH^DMH->LH neurons. Thus, in Wistar rats DMH Trh mRNA levels are regulated by negative energy balance, exercise and chronic variable stress through sex-dependent and -independent pathways.

Molecular Cloning and Expression Analysis of Thyrotropin-Releasing Hormone, and Its Possible Role in Gonadal Differentiation in Rice Field eel Monopterus albus

Simple Summary

Thyrotropin-releasing hormone (TRH) is an important upstream regulator in the hypothalamus-pituitary-thyroid (HPT) axis in mammals. In this study, we isolated and characterized trh gene from a protogynous hermaphrodite fish rice field eel Monopterus albus. TRH had no significant effect on serum thyroid hormone levels in rice field eel. However, we found that TRH was involved in the regulation gonadal differentiation-related gene expression and serum sex steroid hormone secretion. Our results indicated that TRH may play a novel role in gonadal differentiation in rice field eel.

Abstract

Rice field eel (Monopterus albus), a protogynous hermaphrodite fish, is a good model for the research of sex determination and gonadal differentiation in teleosts. In this study, we cloned the full-length cDNA sequence of trh, which encoded a predicted protein with 270 amino acids. Trh mainly expressed in the brain, followed by the ovary, testis, muscle and pituitary, and had low levels in other peripheral tissues. During natural sex reversal, trh mRNA expression levels exhibited a significant increase at the late intersexual stage in the hypothalamus. In the gonad, trh mRNA expression levels showed a trend of increase followed by decrease, and only increased significantly at the middle intersexual stage. No matter static incubation or intraperitoneal (IP) injection, TRH had no significant effect on trh and thyroid-stimulating hormone βsubunit (tshβ) mRNA expression levels, and serum T3, T4 and TRH release. After static incubation of ovarian fragments by TRH, the expression of gonadal soma derived factor (gsdf) was up-regulated significantly at both the doses of 10 and 100 nM. IP injection of TRH stimulated the expression of gsdf, and inhibited the expression of ovarian aromatase gene (cyp19a1a), accompanied by the increase of serum 11-KT levels. The results indicated that TRH may play a novel role in gonadal differentiation by the regulation of gonadal differentiation-related gene expression and sex steroid hormone secretion in rice field eel.

Post translational modifications of Trifolitoxin: a blue fluorescent peptide antibiotic

Trifolitoxin (TFX, C₄₁H₆₃N₁₅O₁₅S) is a selective, ribosomally-synthesized, post-translationally modified, peptide antibiotic, produced by Rhizobium leguminosarum bv. trifolii T24. TFX specifically inhibits α-proteobacteria, including the plant symbiont Rhizobium spp., the plant pathogen Agrobacterium spp. and the animal pathogen Brucella abortus. TFX-producing strains prevent legume root nodulation by TFX-sensitive rhizobia. TFX has been isolated as a pair of geometric isomers, TFX1 and TFX2, which are derived from the biologically inactive primary amino acid sequence: Asp-Ile-Gly-Gly-Ser-Arg-Gln-Gly-Cys-Val-Ala. Gly-Cys is present as a thiazoline ring and the Arg-Gln-Gly sequence is extensively modified to a UV absorbing, blue fluorescent chromophore. The chromophore consists of a conjugated, 5-membered heterocyclic ring and side chain of modified glutamine.

Synthesis and Evaluation of in Vivo Anti-hypothermic Effect of the N- and C-Terminus Modified Thyrotropin-Releasing Hormone Mimetic: [(4S,5S)-(5-Methyl-2-oxooxazolidine-4-yl)carbonyl]-[3-(thiazol-4-yl)-L-alanyl]-L-prolinamide

We explored orally effective thyrotropin-releasing hormone (TRH) mimetics, which show high central nervous system effects in structure-activity relationship studies based on in vivo antagonistic activity on reserpine-induced hypothermia (anti-hypothermic effect) in mice starting from TRH. This led us to the TRH mimetic: [(4S,5S)-(5-methyl-2-oxooxazolidine-4-yl)carbonyl]-[3-(thiazol-4-yl)-L-alanyl]-L-prolinamide 1, which shows a higher anti-hypothermic effect compared with that of TRH after oral administration. We next attempted further chemical modification of the N- and C-terminus of 1 to find more orally effective TRH mimetics. As a result, we obtained several N- and C-terminus modified TRH mimetics which showed high anti-hypothermic effects.

Characterization of a novel thyrotropin-releasing hormone receptor, TRHR3, in chickens

The physiological roles of thyrotropin-releasing hormone (TRH) are proposed to be mediated by TRH receptors (TRHR), which have been divided into 3 subtypes, namely, TRHR1, TRHR2, and TRHR3, in vertebrates. Although 2 TRH receptors (TRHR1 and TRHR3) have been predicted to exist in birds, it remains unclear whether TRHR3 is a functional TRH receptor similar to TRHR1. Here, we reported the functionality and tissue expression of TRHR3 in chickens. The cloned chicken TRHR3 (cTRHR3) encodes a receptor of 387 amino acids, which shares high-amino-acid identities (63–80%) to TRHR3 of parrots, lizards, Xenopus tropicalis, and tilapia and comparatively lower sequence identities to chicken TRHR1 or mouse TRHR2. Using cell-based luciferase reporter assays and Western blot, we demonstrated that similar to chicken TRHR1 (cTRHR1), cTRHR3 expressed in HEK 293 cells can be potently activated by TRH and that its activation stimulates multiple signaling pathways, indicating both TRH receptors are functional. Quantitative real-time PCR revealed that cTRHR1 and cTRHR3 are widely, but differentially, expressed in chicken tissues, and their expression is likely controlled by promoters located upstream of exon 1, which display strong promoter activities in cultured DF-1 cells. cTRHR1 is highly expressed in the anterior pituitary and testes, while cTRHR3 is highly expressed in the muscle, testes, fat, pituitary, spinal cord, and many brain regions (including hypothalamus). These findings indicate that TRH actions are likely mediated by 2 TRH receptors in chickens. In conclusion, our data provide the first piece of evidence that both cTRHR3 and cTRHR1 are functional TRH receptors, which helps to elucidate the physiological roles of TRH in birds.

Of Molecules and Mechanisms

Without question, molecular biology drives modern neuroscience. The past 50 years has been nothing short of revolutionary as key findings have moved the field from correlation toward causation. Most obvious are the discoveries and strategies that have been used to build tools for visualizing circuits, measuring activity, and regulating behavior. Less flashy, but arguably as important are the myriad investigations uncovering the actions of single molecules, macromolecular structures, and integrated machines that serve as the basis for constructing cellular and signaling pathways identified in wide-scale gene or RNA studies and for feeding data into informational networks used in systems biology. This review follows the pathways that were opened in neuroscience by major discoveries and set the stage for the next 50 years.

CLARITY-BPA: Bisphenol A or Propylthiouracil on Thyroid Function and Effects in the Developing Male and Female Rat Brain

The CLARITY-BPA experiment, a large collaboration between the National Institute of Environmental Health Sciences, the National Toxicology Program, and the US Food and Drug Administration, is designed to test the effects of bisphenol A (BPA) on a variety of endocrine systems and end points. The specific aim of this subproject was to test the effect of BPA exposure on thyroid functions and thyroid hormone action in the developing brain. Timed-pregnant National Center for Toxicological Research Sprague-Dawley rats (strain code 23) were dosed by gavage with vehicle control (0.3% carboxymethylcellulose) or one of five doses of BPA [2.5, 25, 250, 2500, or 25,000 µg/kg body weight (bw) per day] or ethinyl estradiol (EE) at 0.05 or 0.50 µg/kg bw/d (n = 8 for each group) beginning on gestational day 6. Beginning on postnatal day (PND) 1 (day of birth is PND 0), the pups were directly gavaged with the same dose of vehicle, BPA, or EE. We also obtained a group of animals treated with 3 ppm propylthiouracil in the drinking water and an equal number of concordant controls. Neither BPA nor EE affected serum thyroid hormones or thyroid hormone‒sensitive end points in the developing brain at PND 15. In contrast, propylthiouracil (PTU) reduced serum T4 to the expected degree (80% reduction) and elevated serum TSH. Few effects of PTU were observed in the male brain and none in the female brain. As a result, it is difficult to interpret the negative effects of BPA on the thyroid in this rat strain because the thyroid system appears to respond differently from that of other rat strains.

Thyrotropin-releasing hormone (TRH) in the brain and pituitary of the teleost, Clarias batrachus and its role in regulation of hypophysiotropic dopamine neurons

Thyrotropin-releasing hormone (TRH) regulates the hypothalamic-pituitary-thyroid axis in mammals and also regulates prolactin secretion, directly or indirectly via tuberoinfundibular dopamine neurons. Although TRH is abundantly expressed in teleost brain and believed to mediate neuronal communication, empirical evidence is lacking. We analyzed pro-TRH-mRNA expression, mapped TRH-immunoreactive elements in the brain and pituitary, and explored its role in regulation of hypophysiotropic dopamine (DA) neurons in the catfish, Clarias batrachus. Partial pro-TRH transcript from C. batrachus transcriptome showed six TRH progenitors repeats. Quantitative real-time polymerase chain reaction (qRT-PCR) identified pro-TRH transcript in a number of different brain regions and immunofluorescence showed TRH-immunoreactive cells/fibers in the olfactory bulb, telencephalon, preoptic area (POA), hypothalamus, midbrain, hindbrain, and spinal cord. In the pituitary, TRH-immunoreactive fibers were seen in the neurohypophysis, proximal pars distalis, and pars intermedia but not rostral pars distalis. In POA, distinct TRH-immunoreactive cells/fibers were seen in nucleus preopticus periventricularis anterior (NPPa) that demonstrated a significant increase in TRH-immunoreactivity when collected during preparatory and prespawning phases, reaching a peak in the spawning phase. Although tyrosine hydroxylase (TH)-immunoreactive neurons in NPPa are hypophysiotropic, none of the TRH-immunoreactive neurons in NPPa accumulated neuronal tracer DiI following implants into the pituitary. However, 87 ± 1.6% NPPa TH-immunoreactive neurons were surrounded by TRH-immunoreactive axons that were seen in close proximity to the somata. Superfused POA slices treated with TRH (0.5-2 μM) significantly reduced TH concentration in tissue homogenates and the percent TH-immunoreactive area in the NPPa. We suggest that TRH in the brain of C. batrachus regulates a range of physiological functions but in particular, serves as a potential regulator of hypophysiotropic DA neurons and reproduction.

Gene expression of thyrotropin- and corticotrophin-releasing hormones is regulated by environmental salinity in the euryhaline teleost Sparus aurata

In euryhaline teleosts, the hypothalamus-pituitary-thyroid and hypothalamus-pituitary-interrenal axes (HPT and HPI, respectively) are regulated in response to environmental stimuli such as salinity changes. However, the molecular players participating in this physiological process in the gilthead seabream (Sparus aurata), a species of high value for aquaculture, are still not identified and/or fully characterized in terms of gene expression regulation. In this sense, this study identifies and isolates the thyrotropin-releasing hormone (trh) mRNA sequence from S. aurata, encoding prepro-Trh, the putative factor initiating the HPT cascade. In addition, the regulation of trh expression and of key brain genes in the HPI axis, i.e., corticotrophin-releasing hormone (crh) and corticotrophin-releasing hormone-binding protein (crhbp), was studied when the osmoregulatory status of S. aurata was challenged by exposure to different salinities. The deduced amino acid structure of trh showed 65-81% identity with its teleostean orthologs. Analysis of the tissue distribution of gene expression showed that trh mRNA is, though ubiquitously expressed, mainly found in brain. Subsequently, regulation of gene expression of trh, crh, and crhbp was characterized in fish acclimated to 5-, 15-, 40-, and 55-ppt salinities. In this regard, the brain gene expression pattern of trh mRNA was similar to that found for the crh gene, showing an upregulation of gene expression in seabream acclimated to the highest salinity tested. Conversely, crhbp did not change in any of the groups tested. Our results suggest that Trh and Crh play an important role in the acclimation of S. aurata to hypersaline environments.

Transcriptomic identification of starfish neuropeptide precursors yields new insights into neuropeptide evolution

Neuropeptides are evolutionarily ancient mediators of neuronal signalling in nervous systems. With recent advances in genomics/transcriptomics, an increasingly wide range of species has become accessible for molecular analysis. The deuterostomian invertebrates are of particular interest in this regard because they occupy an ‘intermediate' position in animal phylogeny, bridging the gap between the well-studied model protostomian invertebrates (e.g. Drosophila melanogaster, Caenorhabditis elegans) and the vertebrates. Here we have identified 40 neuropeptide precursors in the starfish Asterias rubens, a deuterostomian invertebrate from the phylum Echinodermata. Importantly, these include kisspeptin-type and melanin-concentrating hormone-type precursors, which are the first to be discovered in a non-chordate species. Starfish tachykinin-type, somatostatin-type, pigment-dispersing factor-type and corticotropin-releasing hormone-type precursors are the first to be discovered in the echinoderm/ambulacrarian clade of the animal kingdom. Other precursors identified include vasopressin/oxytocin-type, gonadotropin-releasing hormone-type, thyrotropin-releasing hormone-type, calcitonin-type, cholecystokinin/gastrin-type, orexin-type, luqin-type, pedal peptide/orcokinin-type, glycoprotein hormone-type, bursicon-type, relaxin-type and insulin-like growth factor-type precursors. This is the most comprehensive identification of neuropeptide precursor proteins in an echinoderm to date, yielding new insights into the evolution of neuropeptide signalling systems. Furthermore, these data provide a basis for experimental analysis of neuropeptide function in the unique context of the decentralized, pentaradial echinoderm bauplan.

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.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary-thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms which control hypothalamus-pituitary-thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4 and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques facilitated the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary-thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3 concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels. Trh expression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.

Regulation of TRH neurons and energy homeostasis-related signals under stress

Energy homeostasis relies on a concerted response of the nervous and endocrine systems to signals evoked by intake, storage, and expenditure of fuels. Glucocorticoids (GCs) and thyroid hormones are involved in meeting immediate energy demands, thus placing the hypothalamo-pituitary-thyroid (HPT) and hypothalamo-pituitary-adrenal axes at a central interface. This review describes the mode of regulation of hypophysiotropic TRHergic neurons and the evidence supporting the concept that they act as metabolic integrators. Emphasis has been be placed on i) the effects of GCs on the modulation of transcription of Trh in vivo and in vitro, ii) the physiological and molecular mechanisms by which acute or chronic situations of stress and energy demands affect the activity of TRHergic neurons and the HPT axis, and iii) the less explored role of non-hypophysiotropic hypothalamic TRH neurons. The partial evidence gathered so far is indicative of a contrasting involvement of distinct TRH cell types, manifested through variability in cellular phenotype and physiology, including rapid responses to energy demands for thermogenesis or physical activity and nutritional status that may be modified according to stress history.

Lateral hypothalamic thyrotropin-releasing hormone neurons: distribution and relationship to histochemically defined cell populations in the rat

The lateral hypothalamic area (LHA) constitutes a large component of the hypothalamus, and has been implicated in several aspects of motivated behavior. The LHA is of particular relevance to behavioral state control and the maintenance of arousal. Due to the cellular heterogeneity of this region, however, only some subpopulations of LHA cells have been properly anatomically characterized. Here, we have focused on cells expressing thyrotropin-releasing hormone (TRH), a peptide found in the LHA that has been implicated as a promoter of arousal. Immunofluorescence and in situ hybridization were used to map the LHA TRH population in the rat, and cells were observed to form a large ventral cluster that extended throughout almost the entire rostro-caudal axis of the hypothalamus. Almost no examples of coexistence were seen when sections were double-stained for TRH and markers of other LHA populations, including the peptides hypocretin/orexin, melanin-concentrating hormone and neurotensin. In the juxtaparaventricular area, however, a discrete group of TRH-immunoreactive cells were also stained with antisera against enkephalin and urocortin 3. Innervation from the metabolically sensitive hypothalamic arcuate nucleus was investigated by double-staining for peptide markers of the two centrally projecting groups of arcuate neurons, agouti gene-related peptide and α-melanocyte-stimulating hormone, respectively; both populations of terminals were observed forming close appositions on TRH cells in the LHA. The present study indicates that TRH-expressing cells form a unique population in the LHA that may serve as a link between metabolic signals and the generation of arousal.

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.

American Thyroid Association Guide to investigating thyroid hormone economy and action in rodent and cell models

BACKGROUND

An in-depth understanding of the fundamental principles that regulate thyroid hormone homeostasis is critical for the development of new diagnostic and treatment approaches for patients with thyroid disease.

SUMMARY

Important clinical practices in use today for the treatment of patients with hypothyroidism, hyperthyroidism, or thyroid cancer are the result of laboratory discoveries made by scientists investigating the most basic aspects of thyroid structure and molecular biology. In this document, a panel of experts commissioned by the American Thyroid Association makes a series of recommendations related to the study of thyroid hormone economy and action. These recommendations are intended to promote standardization of study design, which should in turn increase the comparability and reproducibility of experimental findings.

CONCLUSIONS

It is expected that adherence to these recommendations by investigators in the field will facilitate progress towards a better understanding of the thyroid gland and thyroid hormone dependent processes.

Food-restricted and dehydrated-induced anorexic rats present differential TRH expression in anterior and caudal PVN. Role of type 2 deiodinase and pyroglutamyl aminopeptidase II

TRH synthesized in hypothalamic paraventricular nucleus (PVN) regulates thyroid axis function and is also implicated in anorexigenic effects. Under energy deficit, animals present decreased PVN TRH expression and release, low TSH levels, and increased appetite. Dehydration-induced anorexia (DIA) model allows insight into underlying mechanisms of feeding regulation. Animals drinking a 2.5% NaCl solution for 7 d present body weight reduction; despite their negative energy balance, they avoid food and have increased PVN TRH expression and TSH serum levels. These findings support an inhibiting role of PVN TRH in feeding control. We compared TRH expression by in situ hybridization in PVN subdivisions of 7-d dehydrated male rats to those of a pair-fed group (forced food-restricted) with similar metabolic changes than DIA, but motivated to eat, and to controls. We measured peripheral deiodinase activities, and expression and activity of medial basal hypothalamic type 2 deiodinase and pyroglutamyl-aminopeptidase II, to understand their regulating role in PVN TRH changes between food restriction and anorexia. TRH mRNA levels increased in anterior (aPVN) and medial-caudal subdivisions in DIA rats, whereas it decreased in medial PVN in both experimental groups. We confirmed the nonhypophysiotropic nature of aPVN TRHergic cells by injecting ip fluorogold tracer. Findings support a subspecialization of TRHergic hypophysiotrophic cells that responded differently between anorexic and food-restricted animals; also, that aPVN TRH participates in food intake regulation. Increased type 2 deiodinase activity seemed responsible for low medial PVN TRH synthesis, whereas increased medial basal hypothalamic pyroglutamyl-aminopeptidase II activity in DIA rats might counteract their high TRH release.

Biosynthesis of proTRH-derived peptides in prohormone convertase 1 and 2 knockout mice

Prohormone convertases (PCs) 1 and 2 are the primary endoproteases involved in the post-translational processing of proThyrotropin Releasing Hormone (proTRH) to give rise to TRH and other proposed biologically active non-TRH peptides. Previous evidence suggests that PC1 is responsible for most proTRH cleavage events. Here, we used the PC1 and PC2 knockout (KO) mouse models to examine the effects of PC1 or PC2 loss on proTRH processing. The PC1KO mouse presented a decrease in five proTRH-derived peptides, whereas the PC2KO mouse showed only lesser reduction in three TRH (Gln-His-Pro), TRH-Gly (Gln-His-Pro-Gly), and the short forms preproTRH(178-184) (pFQ(7)) and preproTRH(186-199) (pSE(14)) of pFE(22) (preproTRH(178-199)). Also, PC1KO and not PC2KO showed a decrease in pEH(24) indicating that PC1 is more important in generating this peptide in the mouse, which differs from previous studies using rat proTRH. Furthermore, downstream effects on thyroid hormone levels were evident in PC1KO mice, but not PC2KO mice suggesting that PC1 plays the more critical role in producing bioactive hypophysiotropic TRH. Yet loss of PC1 did not abolish TRH entirely indicating a complementary action for both enzymes in the normal processing of proTRH. We also show that PC2 alone is responsible for catalyzing the conversion of pFE(22) to pFQ(7) and pSE(14), all peptides implicated in regulation of suckling-induced prolactin release. Collectively, results characterize the specific roles of PC1 and PC2 in proTRH processing in vivo.

The acute response of the amygdalar TRH system to psychogenic stressors varies dependent on the paradigm and circadian condition

Central administration of thyrotropin releasing hormone (TRH) reduces anxiety; amygdalar TRH expression is inversely proportional to the anxious behavior displayed in the elevated plus maze performed during the dark phase (EPM-D). To better understand the role of TRH in amygdala function, we evaluated the expression of TRH and the elements involved in its transmission in various stressful paradigms and how they associated with behavior. Wistar male rats were exposed to restraint (RES), EPM, or the open field test (OFT) and sacrificed 0-60 min afterwards; OFT, RES and EPM were performed during the light (L), and OFT during the dark phase. Restraint increased amygdalar levels of proCRH mRNA, without change in proTRH. All paradigms augmented corticosterone release, highest after OFT-L that also enhanced proCRH mRNA levels and decreased those of proTRH. OFT-D activated the TRH system. Levels of anxiety or locomotion were similar in animals tested in light or dark phases but their association with biochemical parameters differed. ProTRH expression and TRH release correlated positively with decreased anxiety in EPM-L and in OFT-D. No association with anxiety was detected in OFT-L where proCRH and proTRH expression correlated with locomotion supporting their involvement in arousal. The responses of TRH amygdalar systems appeared modulated by the extent of the stress response and by the circadian conditions. Increased proTRH expression of animals exposed to OFT-D was specifically observed in the cortical nucleus of the amygdala, area involved in processing fear stimuli; these TRH neurons may thus be part of a circuit with anxiolytic properties.

Levodopa-induced dyskinesia is associated with increased thyrotropin releasing hormone in the dorsal striatum of hemi-parkinsonian rats

Background

Dyskinesias associated with involuntary movements and painful muscle contractions are a common and severe complication of standard levodopa (L-DOPA, L-3,4-dihydroxyphenylalanine) therapy for Parkinson's disease. Pathologic neuroplasticity leading to hyper-responsive dopamine receptor signaling in the sensorimotor striatum is thought to underlie this currently untreatable condition.

Methodology/Principal Findings

Quantitative real-time polymerase chain reaction (PCR) was employed to evaluate the molecular changes associated with L-DOPA-induced dyskinesias in Parkinson's disease. With this technique, we determined that thyrotropin releasing hormone (TRH) was greatly increased in the dopamine-depleted striatum of hemi-parkinsonian rats that developed abnormal movements in response to L-DOPA therapy, relative to the levels measured in the contralateral non-dopamine-depleted striatum, and in the striatum of non-dyskinetic control rats. ProTRH immunostaining suggested that TRH peptide levels were almost absent in the dopamine-depleted striatum of control rats that did not develop dyskinesias, but in the dyskinetic rats, proTRH immunostaining was dramatically up-regulated in the striatum, particularly in the sensorimotor striatum. This up-regulation of TRH peptide affected striatal medium spiny neurons of both the direct and indirect pathways, as well as neurons in striosomes.

Conclusions/Significance

TRH is not known to be a key striatal neuromodulator, but intrastriatal injection of TRH in experimental animals can induce abnormal movements, apparently through increasing dopamine release. Our finding of a dramatic and selective up-regulation of TRH expression in the sensorimotor striatum of dyskinetic rat models suggests a TRH-mediated regulatory mechanism that may underlie the pathologic neuroplasticity driving dopamine hyper-responsivity in Parkinson's disease.

Molecular evolution of the thyrotrophin-releasing hormone precursor in vertebrates: insights from comparative genomics

Human preprothyrotrophin-releasing hormone (ppTRH) includes six copies of the TRH sequence, the rat and mouse precursors have five, and those of non-mammalian vertebrates have up to eight. In the present study, the evolutionary basis of this variation was investigated using ppTRH gene sequences extracted from available vertebrate genomic databases. A structure based on eight TRH repeats appears to be the norm for non-mammalian vertebrates, but in all mammals except monotremes this number is reduced to a maximum of six. In some species, one (or more) of the TRH repeats has been mutated, probably rendering it functionless and, in a few species, one or two copies of the TRH sequence have been deleted completely. Sequences of regions between the TRH sequences are poorly conserved, despite reports that several active peptides are produced from these regions. The 5' untranslated region of ppTRH is also very variable but, in eutherians, the promoter region immediately upstream of the gene is quite strongly conserved. In particular, those sequences identified as being involved in transcriptional regulation are well conserved in most eutherians, although they are largely absent from other vertebrates. In most species, gene order around the ppTRH locus is conserved, although exceptions include man and chimpanzee, as well as rat and mouse. The comparative genomics approach thus provides a wider view than previously available of the range of ppTRH genes in vertebrates, and of the species specificity displayed by this molecule.

Regulation of the hypothalamic thyrotropin releasing hormone (TRH) neuron by neuronal and peripheral inputs

The hypothalamic-pituitary-thyroid (HPT) axis plays a critical role in mediating changes in metabolism and thermogenesis. Thus, the central regulation of the thyroid axis by Thyrotropin Releasing Hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) is of key importance for the normal function of the axis under different physiological conditions including cold stress and changes in nutritional status. Before the TRH peptide becomes biologically active, a series of tightly regulated processes occur including the proper folding of the prohormone for targeting to the secretory pathway, its post-translational processing, and targeting of the processed peptides to the secretory granules near the plasma membrane of the cell ready for secretion. Multiple inputs coming from the periphery or from neurons present in different areas of the brain including the hypothalamus are responsible for the activation or inhibition of the TRH neuron and in turn affect the output of TRH and the set point of the axis.

The thyrotropin-releasing hormone gene is regulated by thyroid hormone at the level of transcription in vivo

The expression of the TRH gene in the paraventricular nucleus (PVH) of the hypothalamus is required for the normal production of thyroid hormone (TH) in rodents and humans. In addition, the regulation of TRH mRNA expression by TH, specifically in the PVH, ensures tight control of the set point of the hypothalamic-pituitary-thyroid axis. Although many studies have assumed that the regulation of TRH expression by TH is at the level of transcription, there is little data available to demonstrate this. We used two in vivo model systems to show this. In the first model system, we developed an in situ hybridization (ISH) assay directed against TRH heteronuclear RNA to measure TRH transcription directly in vivo. We show that in the euthyroid state, TRH transcription is present both in the PVH and anterior/lateral hypothalamus. In the hypothyroid state, transcription is activated in the PVH only and can be shut off within 5 h by TH. In the second model system, we employed transgenic mice that express the Cre recombinase under the control of the genomic region containing the TRH gene. Remarkably, TH regulates Cre expression in these mice in the PVH only. Taken together, these data affirm that TH regulates TRH at the level of transcription in the PVH only and that genomic elements surrounding the TRH gene mediate its regulation by T(3). Thus, it should be possible to identify the elements within the TRH locus that mediate its regulation by T(3) using in vivo approaches.

Distribution and axonal projections of neurons coexpressing thyrotropin-releasing hormone and urocortin 3 in the rat brain

Thyrotropin-releasing hormone (TRH) decreases food intake when administered intracerebroventricularly or into the ventromedial hypothalamus. However, it is unknown which population of TRH neurons exerts this anorexigenic function. In the rostral perifornical area, the pattern of TRH-expressing neurons is reminiscent of the distribution of neurons expressing urocortin3 (Ucn3) that also inhibits feeding when injected into the hypothalamic ventromedial nucleus (VMN). Since colocalization of TRH and Ucn3 may help to identify feeding-related TRH neurons, the putative coexpression of the two peptides was examined using fluorescent in situ hybridization combined with immunofluorescence. Almost all (95.5 +/- 0.2%) Ucn3-immunoreactive neurons in the perifornical area expressed pro-TRH mRNA, while 50.2 +/- 1.6% Ucn3 neurons were double-labeled in the bed nucleus of the stria terminalis (BNST). Only a few Ucn3/pro-TRH neurons were found outside these two areas. The distribution of axons containing both Ucn3 and TRH was examined by dual immunofluorescence. Ucn3/TRH fibers heavily innervated the VMN. In addition, high densities of double-labeled axons were observed in the lateral septal nucleus, posterior division of the BNST, medial amygdaloid nucleus, amygdalohippocampal area, and ventral hippocampus, forebrain areas associated with psychological stress and anxiety. We conclude that Ucn3 and TRH are coexpressed in a discrete, continuous population of neurons in the perifornical area and BNST, making Ucn3 a neurochemical marker to define a distinct subset of TRH neurons. The distribution of their axons suggests that Ucn3/TRH neurons may coordinate feeding and behavioral responses to stressful stimuli.

TRH acts as a multifunctional hypophysiotropic factor in vertebrates

Thyrotropin-releasing hormone (TRH) is the first hypothalamic hypophysiotropic neuropeptide whose sequence has been chemically characterized. The primary structure of TRH (pGlu-His-Pro-NH(2)) has been fully conserved across the vertebrate phylum. TRH is generated from a large precursor protein that contains multiple repeats of the TRH progenitor tetrapeptide Gln-His-Pro-Gly. In all tetrapods, TRH-expressing neurons located in the hypothalamus project towards the external zone of the median eminence while in teleosts they directly innervate the pars distalis of the pituitary. In addition, in frogs and teleosts, a bundle of TRH-containing fibers terminate in the neurointermediate lobe of the pituitary gland. Although TRH was originally named for its ability to trigger the release of thyroid-stimulating hormone (TSH) in mammals, it later became apparent that it exerts multiple, species-dependent hypophysiotropic activities. Thus, in fish TRH stimulates growth hormone (GH) and prolactin (PRL) release but does not affect TSH secretion. In amphibians, TRH is a marginal stimulator of TSH release in adult frogs, not in tadpoles, and a major releasing factor for GH and PRL. In birds, TRH triggers TSH and GH secretion. In mammals, TRH stimulates TSH, GH and PRL release. In fish and amphibians, TRH is also a very potent stimulator of alpha-melanocyte-stimulating hormone release. Because the intermediate lobe of the pituitary of amphibians is composed by a single type of hormone-producing cells, the melanotrope cells, it is a suitable model in which to investigate the mechanism of action of TRH at the cellular and molecular level. The occurrence of large amounts of TRH in the frog skin and high concentrations of TRH in frog plasma suggests that, in amphibians, skin-derived TRH may exert hypophysiotropic functions.

T3 differentially regulates TRH expression in developing hypothalamic neurons in vitro

Triiodothyronine (T3) plays an important role during development of the central nervous system. T3 effects on gene expression are determined in part by the type of thyroid hormone receptors (TRs) expressed in a given cell type. Previous studies have demonstrated that thyrotropin releasing hormone (TRH) transcription in the adult hypothalamus is subjected to negative regulation by thyroid hormones. However, the role of T3 on the development of TRH expression is unknown. In this study we used primary cultures derived from 17-day-old fetal rat hypothalamus to analyze the effects of T3 on TRH gene expression during development. T3 increased TRH mRNA expression in immature cultures, but decreased it in mature cultures. In addition, T3 up-regulated TRalpha1 and TRbeta2 mRNA expression. TRalpha1 expression coincided chronologically with that of TRH in the rat hypothalamus in vivo. Maturation of TRH expression in the hypothalamus may involve T3 acting through TRalpha1.

Processing of peptide and hormone precursors at the dibasic cleavage sites

Many functionally important cellular peptides and proteins, including hormones, neuropeptides, and growth factors, are synthesized as inactive precursor polypeptides, which require post-translational proteolytic processing to become biologically active polypeptides. This is achieved by the action of a relatively small number of proteases that belong to a family of seven subtilisin-like proprotein convertases (PCs) including furin. In view of this, this review focuses on the importance of privileged secondary structures and of given amino acid residues around basic cleavage sites in substrate recognition by these endoproteases. In addition to their participation in normal cell functions, PCs are crucial for the initiation and progress of many important diseases. Hence, these proteases constitute potential drug targets in medicine. Accordingly, this review also discusses the approaches used to shed light on the cleavage preference and the substrate specificity of the PCs, a prerequisite to select which PCs are promising drug targets in each disease.

Thyrotropin-releasing hormone (TRH) in the cerebellum

Thyrotropin-releasing hormone (TRH) was originally isolated from the hypothalamus. Besides controlling the secretion of TSH from the anterior pituitary, this tripeptide is widely distributed in the central nervous system and regarded as a neurotransmitter or modulator of neuronal activities in extrahypothalamic regions, including the cerebellum. TRH has an important role in the regulation of energy homeostasis, feeding behavior, thermogenesis, and autonomic regulation. TRH controls energy homeostasis mainly through its hypophysiotropic actions to regulate circulating thyroid hormone levels. Recent investigations have revealed that TRH production is regulated directly at the transcriptional level by leptin, one of the adipocytokines that plays a critical role in feeding and energy expenditure. The improvement of ataxic gait is one of the important pharmacological properties of TRH. In the cerebellum, cyclic GMP has been shown to be involved in the effects of TRH. TRH knockout mice show characteristic phenotypes of tertiary hypothyroidism, but no morphological changes in their cerebellum. Further analysis of TRH-deficient mice revealed that the expression of PFTAIRE protein kinase1 (PFTK1), a cdc2-related kinase, in the cerebellum was induced by TRH through the NO-cGMP pathway. The antiataxic effect of TRH and TRH analogs has been investigated in rolling mouse Nagoya (RMN) or 3-acetylpyridine treated rats, which are regarded as a model of human cerebellar degenerative disease. TRH and TRH analogs are promising clinical therapeutic agents for inducing arousal effects, amelioration of mental depression, and improvement of cerebellar ataxia.

17β-Oestradiol indirectly inhibits thyrotrophin-releasing hormone expression in the hypothalamic paraventricular nucleus of female rats and blunts thyroid axis response to cold exposure

Energy expenditure and thermogenesis are regultated by thyroid and sex hormones. Several parameters of hypothalamic-pituitary-thyroid (HPT) axis function are modulated by 17β-oestradiol (E(2)) but its effects on thyrotrophin-releasing hormone (TRH) mRNA levels remain unknown. We evaluated, by in situ hybridisation and Northern bloting, TRH expression in the paraventricular nucleus of the hypothalamus (PVN) of cycling rats, 2 weeks-ovariectomised (OVX) and OVX animals injected s.c. during 1-4 days with E(2) (5, 50, 100 or 200 μg ⁄ kg) (OVX-E). Serum levels of E(2), thyroid-stimulating hormone (TSH), prolactin, corticosterone and triiodothyronine (T(3)) were quantified by radioimmunoassay. Increased serum E(2) levels were observed after 4 days injection of 50 μg ⁄ kg E(2) (to 68.5 ± 4.8 pg ⁄ ml) in OVX rats. PVN-TRH mRNA levels were slightly higher in OVX than in virgin females at dioestrous 1 or pro-oestrous, decreasing proportionally to increased serum E(2) levels. E(2) injections augmented serum T(3), prolactin, and corticosterone levels. Serum TSH levels augmented with 4 days 50 μg ⁄ kg E(2), but not with the higher doses that enhanced serum T(3) levels. Exposure to cold for 1 h resulted in marked HPT axis activation in OVX rats, increasing the levels of TRH mRNA along the rostro-caudal PVN areas, as well as serum TSH, T(3), corticosterone and prolactin levels. By contrast, no significant changes in any of these parameters were observed in cold-exposed OVX-E (50 μg ⁄ kg E(2)) rats. Very few PVN-TRHergic neurones expressed the oestrogen receptor type-α, suggesting that the effects of E(2) on PVN-TRH expression are indirect, most probably as a result of its multiple modulatory effects on circulating hormones and their receptor sensitivity. The blunted response of OVX-E rats to cold coincides with the effects of E(2) on the autonomic nervous system and increased cold tolerance.

Minireview: Thyrotropin-releasing hormone and the thyroid hormone feedback mechanism

Thyroid hormone (TH) plays a critical role in development, growth, and cellular metabolism. TH production is controlled by a complex mechanism of positive and negative regulation. Hypothalamic TSH-releasing hormone (TRH) stimulates TSH secretion from the anterior pituitary. TSH then initiates TH synthesis and release from the thyroid gland. The synthesis of TRH and TSH subunit genes is inhibited at the transcriptional level by TH, which also inhibits posttranslational modification and release of TSH. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback at the pituitary was thought to be the primary regulator of serum TSH levels. However, study of transgenic animals showed an unexpected, dominant role for TRH in regulating the hypothalamic-pituitary-thyroid axis and an unanticipated involvement of the thyroid hormone receptor ligand-dependent activation function (AF-2) domain in TH negative regulation. These results are summarized in the review.

Characterization of the post-translational modification of recombinant human BMP-15 mature protein

Bone morphogenetic protein-15 (BMP-15) is an oocyte-secreted factor critical for the regulation of ovarian physiology. When recombinant human BMP-15 (rhBMP-15) produced in human embryonic kidney 293 cells was subjected to SDS-PAGE analysis, two mature protein forms corresponding to 16 kDa (P16) and 17 kDa (P17) were observed. Despite the physiological relevance and critical function of BMP-15 in female reproduction, little is known about the structure of rhBMP-15. Here, we have analyzed the structure of the rhBMP-15 mature proteins (P16 and P17) using state-of-the-art proteomics technology. Our findings are as follows: (1) the N-terminal amino acid of P16 and P17 is pyroglutamic acid; (2) the Ser residue at the sixth position of P16 is phosphorylated; (3) P17 is O-glycosylated at Thr10; and (4) the C-terminal amino acid of P16 and P17 is truncated. These findings are the first knowledge of the structure of rhBMP-15 mature protein toward understanding the molecular basis of BMP-15 function and could provide an important contribution to the rapidly progressing research area involving oocyte-specific growth factors in modulation of female fertility.

The role of the thyrotropin-releasing hormone (TRH) neuron as a metabolic sensor

Thyroid hormone (TH) plays a critical role in mediating changes in development and metabolism in humans. Thus, circulating TH levels are regulated by a number of distinct mechanisms to allow them to remain at physiologic levels. The central regulation of the thyroid axis by thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVH) is absolutely required for normal function of the axis. Remarkably, the TRH neurons in the PVH are regulated by multiple pathways that allow for the set point of TRH production to be determined. The following review will focus on how the TRH neuron is regulated by TH as well as key pathways that regulate energy expenditure. By integrating these inputs, the TRH neuron is able to set the thyroid axis at the appropriate level given the physiologic demands present.

BDNF up-regulates pre-pro-TRH mRNA expression in the fetal/neonatal paraventricular nucleus of the hypothalamus. Properties of the transduction pathway

Brain derived neurotrophic factor (BDNF) increases the levels of pre-pro-thyrotropin releasing hormone (TRH) mRNA in fetal rodent hypothalamic neurons that express TrkB receptors. The present studies aimed at better understanding the role of BDNF in establishing and maintaining the TRH phenotype in hypothalamic neurons during early development. To determine where BDNF regulates the expression of pre-pro-TRH mRNA in vivo, we compared the hypothalamic distribution of pre-pro-TRH mRNA to that of TrkB mRNA. Full-length TrkB (FL-TrkB) mRNA was detected earlier in development than pre-pro-TRH mRNA in the region that gives rise to the paraventricular nucleus of the hypothalamus (PVN). We also evaluated the effects of BDNF on the expression of pre-pro-TRH mRNA in vitro. BDNF up-regulated the levels of pre-pro-TRH mRNA in primary cell cultures obtained from the hypothalamus or the PVN of 17 days old fetuses or newborn rats. This effect was abolished by PD98059, an inhibitor of the mitogen-activated protein kinase kinase (MEK) 1/2 or 5. The effect of BDNF on pre-pro-TRH mRNA levels was reversible. The continuous application of BDNF led to a desensitization of the response at day 10 in vitro, an effect that correlated with a drop in the levels of FL-TrkB protein. In conclusion, BDNF enhances the expression of pre-pro-TRH mRNA in PVN neurons. This effect is reversible, decreases with time, and requires an active MEK. BDNF may contribute to the enhancement of pre-pro-TRH mRNA expression in the hypothalamic PVN during development.

Feedback regulation of thyrotropin-releasing hormone gene expression by thyroid hormone in the hypothalamic paraventricular nucleus

Hypothyroidism caused by chemical or surgical thyroidectomy or hypophysectomy causes a substantial increase in the content of thyrotropin-releasing hormone (TRH) mRNA and proTRH exclusively in cells of the medial and periventricular paravocellular subdivisions of the hypothalamic paraventricular nucleus (PVN). This response may be important to raise the anterior pituitary thyrostat to promote increased secretion of thyroid-stimulating hormone (TSH) and to induce the secretion of a more biologically active TSH. The increase in TRH mRNA can be obliterated by stereotaxic implants of hormonally active L-triiodothyronine (T3) placed into the anterior hypothalamus but not by implants of the hormonally inactive 3,5'-diiodo-L-thyronine (T2); we therefore suggested that T3 has a direct action on TRH-containing cells of the PVN. Ablation of brainstem catecholaminergic projection fields to the PVN (known to stimulate TRH secretion) has no effect on TRH mRNA expression; beta 1 thyroid hormone receptor mRNA is present in extracts of the PVN. Euthyroid levels of serum T3 in hypothyroid animals achieved via intraperitoneally implanted osmotic minipumps are not associated with a return of PVN levels of TRH mRNA to normal unless circulating T3 levels are raised into the hyperthyroid range (1.7 times normal). This requirement is similar to that needed to normalize nuclear thyroid hormone receptor levels in the anterior pituitary of hypothyroid animals, suggesting that in addition to circulating T3 monodeiodination of T4 to T3 within the brain must also contribute to feedback inhibition of TRH mRNA. As Type II deiodinase activity is absent or very low in the PVN and does not rise with hypothyroidism, we propose that an alternative source for T4 monodeiodination exists within the central nervous system.

Regulation of prohormone convertases in hypothalamic neurons: implications for prothyrotropin-releasing hormone and proopiomelanocortin

Recent evidence demonstrated that posttranslational processing of neuropeptides is critical in the pathogenesis of obesity. Leptin or other physiological changes affects the biosynthesis and processing of many peptides hormones as well as the regulation of the family of prohormone convertases responsible for the maturation of these hormones. Regulation of energy balance by leptin involves regulation of several proneuropeptides such as proTRH and proopiomelanocortin. These proneuropeptide precursors require for their maturation proteolytic cleavage by the prohormone convertases 1 and 2 (PC1/3 and PC2). Because biosynthesis of mature peptides in response to leptin requires prohormone processing, it is hypothesized that leptin might regulate hypothalamic PC1/3 and PC2 expression, ultimately leading to coordinated processing of prohormones into mature peptides. Leptin has been shown to increase PC1/3 and PC2 promoter activities, and starvation of rats, leading to low serum leptin levels, resulted in a decrease in PC1/3 and PC2 gene and protein expression in the paraventricular and arcuate nucleus of the hypothalamus. Changes in nutritional status also changes proopiomelanocortin processing in the nucleus of the solitary tract, but this is not reversed by leptin. The PCs are also physiologically regulated by states of hyperthyroidism, hyperglycemia, inflammation, and suckling, and a recently discovered nescient helix-loop-helix-2 transcription factor is the first one to show an ability to regulate the transcription of PC1/3 and PC2. Therefore, the coupled regulation of proneuropeptide/processing enzymes may be a common process, by which cells generate more effective processing of prohormones into mature peptides.

Molecular Evolution of Prepro-Thyrotropin-Releasing Hormone in the Chicken (Gallus gallus) and Its Expression in the Brain

A cDNA encoding prepro-thyrotropin-relaesing hormone (ppTRH) in chicken (Gallus gallus) was isolated and the sites of expression in the brain were determined. The chicken ppTRH cDNA encodes 260 amino acids, including four TRH progenitor sequences (-Lys/Arg-Arg-Gln-His-Pro-Gly-Lys/Arg-Arg-). It is interesting to note that chicken ppTRH harbors four TRH progenitor-like sequences. According to the hydropathy profile of chicken ppTRH, not only the TRH progenitor sequences but also the TRH progenitor-like sequences are localized in hydrophilic regions. The TRH progenitor-like sequences might be related to structural conservation in the evolution of ppTRH, although they cannot be processed into TRH due to the mutation of several amino acids. According to the alignment of the deduced amino-acid sequences of known vertebrate ppTRHs and the molecular phylogenetic tree we constructed, we speculate on the molecular evolution of ppTRH in vertebrates. In situ hybridization demonstrated experession of the ppTRH gene in the nucleus preopticus periventricularis, nucleus preopticus medialis, regio lateralis hypothalami, paraventricular nucleus, nucleus periventricularis hypothalami, and nucleus ventromedialis hypothalami in the chicken brain.

Cocaine- and amphetamine-regulated transcript (CART) expression is differentially regulated in the hypothalamic paraventricular nucleus of lactating rats exposed to suckling or cold stimulation

Neural stimuli, such as suckling or cold exposure, increase TRH mRNA in the paraventricular nucleus (PVN) of the rat hypothalamus, yet only suckling induces prolactin secretion. As TRH co-localizes with cocaine- and amphetamine-regulated transcript (CART) in hypophysiotropic neurons of the PVN, and CART inhibits TRH-induced prolactin release but not TRH-induced TSH release in adenohypophyseal cell cultures, we raised the possibility that differential regulation of CART gene expression in the PVN may explain the differences in prolactin secretion following each of the two stimuli. Primiparous female rats were mated and handled daily during the pre- and postpartum periods. After delivery, the litter was adjusted to 8 pups and at mid-lactation, dams were separated from their pups for 8 h and exposed to either 1 h of cold or 30 min of suckling. Long-term effects of suckling were studied by separating pups from their mothers for 24 h, followed by a 12 h period of continuous suckling. Serum TSH levels increased in response to cold exposure, while prolactin levels were increased by suckling and diminished by cold exposure. CART mRNA levels increased in rostral and mid parts of the medial parvocellular PVN following cold exposure but not after suckling stimulation. These data demonstrate a differential regulation of CART gene expression in hypophysiotropic neurons in response to stimuli that increase TRH mRNA levels, and suggest that CART activation in the PVN may contribute to the decrease in PRL release when the thyroid axis is activated by cold exposure.

Molecular cloning of prepro-thyrotropin-releasing hormone cDNA from medaka (Oryzias latipes)

The cDNA encoding prepro-thyrotropin-releasing hormone (ppTRH) in a teleost, medaka (Oryzias latipes) was isolated and characterized. The medaka ppTRH cDNA codes for 270 amino acid residues including eight TRH progenitor sequences (-Lys/Arg-Arg-Gln-His-Pro-Gly-Lys/Arg-Arg-). In silico analyses of the medaka genome database predicted that the structure of the medaka ppTRH gene is similar to the ppTRH genes of the other vertebrate species studied to date; consisting of three exons and two introns. Identity of the medaka ppTRH with the other vertebrates is rather low except the sockeye salmon. A molecular phylogenic tree showed that the ppTRH sequences reflected the predicted pattern of species classification. RT-PCR analysis demonstrated ppTRH gene expression in the brain and retina. These results gave some insight into the molecular evolution of ppTRH and physiological functions of TRH in vertebrates.

Cellular colocalization and coregulation between hypothalamic pro-TRH and prohormone convertases in hypothyroidism

The prohormone convertases (PCs), PC1/3 and PC2, are involved in the tissue-specific endoproteolytic posttranslational processing of many hormonal precursors within the secretory pathway. One important prohormone, pro-thyrotropin-releasing hormone (TRH), is expressed in both hypophysiotropic (where it regulates the secretion of thyroid-stimulating hormone) and nonhypophysiotropic regions of the brain. Pro-TRH is processed at specific sites in the secretory pathway, primarily by PC1/3 followed by PC2. We hypothesized that thyroid hormone status in specific nuclei of the brain would alter pro-TRH processing by inducing changes in PC1/3 and PC2 expression. Therefore, we examined pro-TRH, PC1/3, and PC2 coexpression and coregulation in the paraventricular nucleus (PVN), lateral hypothalamus (LH), and ventromedial nucleus (VMN) of hypothyroid and euthyroid rats. Our results show that 6-n-propyl-2-thiouracil (PTU) treatment producing hypothyroidism induced a significant increase in the expression of PC1/3, PC2, and pro-TRH in the PVN and LH, but not VMN. When confocal studies were performed, an increase in colocalization of PC1/3 or PC2 in pro-TRH was observed only in PVN, a response that was especially prominent in the ventral and medial areas of the PVN. PTU did not regulate colocalization in the VMH or LH. Regulation of colocalization of processing enzyme and prohormone expression is a novel mechanism to alter hormonal biosynthesis.

In situ hybridization localization of TRH precursor and TRH receptor mRNAs in the brain and pituitary of Xenopus laevis

We examined the distribution of the mRNAs encoding proTRH and the three TRH receptor subtypes (xTRHR1, xTRHR2, and xTRHR3) in the Xenopus laevis CNS and pituitary. A positive correlation was generally observed between the expression patterns of proTRH and xTRHR mRNAs. xTRHRs were widely expressed in the telencephalon and diencephalon, where two or even three xTRHR mRNAs were often simultaneously observed within the same brain structures. In the pituitary, xTRHR2 was selectively expressed in the distal lobe, and xTRHR3 was found exclusively in the intermediate lobe of white background-adapted animals, indicating that, in amphibians, the effect of TRH on alpha-melanotropin (alpha-MSH) secretion from melanotrope cells is mediated through the novel receptor subtype xTRHR3.

Cold stress induces metabolic activation of thyrotrophin-releasing hormone-synthesising neurones in the magnocellular division of the hypothalamic paraventricular nucleus and concomitantly changes ovarian sympathetic activity parameters

Recent studies suggest thyrotrophin-releasing hormone (TRH) serves as a neurotransmitter and thereby provides a functional vegetative connection between the brain and the ovary. In the present study, magnocellular neurones of the paraventricular nucleus (PVN) in animals subjected to cold exposure were studied to determine the hypothalamic origin of the TRH involved in this pathway. In situ hybridisation analysis of hypothalamic tissue showed that cold exposure causes a two-fold increase in the total number of neurones expressing TRH mRNA in the PVN. Immunohistochemical studies showed that TRH peptide is localised to the magnocellular PVN and that the number of TRH immunoreactive cells increases two-fold following 64 h of cold exposure. Double-immunostaining for MAP-2 and TRH revealed that TRH peptide is localised in the perikarya of the magnocellular neurones. TRH release was measured in vivo from the magnocellular portion of the PVN using push-pull perfusion. Although controls exhibited a very low level of TRH release, animals subjected to cold showed a pulsatile-like TRH release profile with two different patterns of release: (i) low basal level with small bursts of TRH release and (ii) a profile with an up to seven-fold increase in TRH release compared to controls. The colocalisation of TRH with the specific somato-dendritic marker MAP-2 in processes of the magnocellular neurones suggested a local release of TRH. Additional studies demonstrated a reduction in ovarian noradrenaline content after 48 h of cold exposure, a feature indicative of nerve activation at the terminal organ. After 64 h of cold exposure, the ovarian noradrenaline returned to control values but the noradrenaline content of the coeliac ganglia was increased, suggesting a compensatory effect originating in the cell bodies of the sympathetic neurones that innervate the ovary. The correlation between the local release of TRH from dendrites within the magnocellular PVN in conditions of cold and the activation of the sympathetic nerves supplying the ovary raises the possibility that TRH contributes to the processing regulating sympathetic outflow and may thereby impact on the functional activity of the ovary.

A comprehensive description of the transcriptome of the hypothalamoneurohypophyseal system in euhydrated and dehydrated rats

The hypothalamoneurohypophyseal system (HNS) consists of the large peptidergic magnocellular neurons of the supraoptic hypo thalamic nucleus (SON) and the paraventricular hypothalamic nucleus (PVN), the axons of which course through the internal zone of the median eminence and terminate at blood capillaries of the posterior lobe of the pituitary gland. The HNS is a specialized brain neurosecretory apparatus responsible for the production of the antidiuretic peptide hormone vasopressin (VP). VP maintains water balance by promoting water conservation at the level of the kidney. Dehydration evokes a massive increase in the regulated release of VP from magnocellular neuron axon terminals in the posterior pituitary, which is accompanied by a plethora of changes in the morphology, electrophysiological properties, and biosynthetic and secretory activity of the HNS. We wish to understand this functional plasticity in terms of the differential expression of genes. We have therefore used microarrays to comprehensively catalog the genes expressed in the PVN, the SON and the neurointermediate lobe of the pituitary gland of control and dehydrated rats. Comparison of these gene lists has enabled us to identify transcripts that are regulated as a consequence of dehydration as well as RNAs that are enriched in the PVN or the SON. We suggest that these differentially expressed genes represent candidate regulators and effectors of HNS activity and remodeling.