Thyrotropin-releasing Hormone Expression Research Articles

Molecular characterization and gene expression of thyrotropin-releasing hormone in Senegalese sole (Solea senegalensis)

We describe the cloning of a cDNA encoding TRH in Solea senegalensis. Phylogenetic analysis clustered it with other fish counterparts. Eight conserved TRH progenitors and highly divergent spacers were identified. Expression profiles of TRH were analyzed in juvenile tissues, during larval development, and in post-larvae grew under four photoperiods (12L:12D, 12D:12L, 24L:0D and 24D:0L) during four weeks using a real-time PCR approach. In juvenile fish, TRH mRNA was expressed ubiquitously although with the highest transcript levels in brain. During larval development, TRH mRNA levels were higher at 2-3days after hatching decreasing progressively until metamorphosis. Hormonal treatments using thiourea and T4 showed no regulation at transcriptional level by thyroid hormones. Moreover, no significant changes in mRNA levels between photoperiods at any time were detected. Overall, these results indicate that TRH do not participate in the regulation of HPT nor adaptation to photoperiod in sole.

Prepro-orexin and feeding-related peptide receptor expression in dehydration-induced anorexia

Food-restricted animals present metabolic adaptations that facilitate food-seeking behavior and decelerate energy utilization by reducing the hypothalamus–pituitary–thyroid (HPT) axis function. Stress by dehydration induces an anorexic behavior in rats, loss of weight and reduced food intake when compared to ad libitum fed animals, however these alterations are accompanied by HPT axis changes such as increased serum thyrotropin levels and enhanced expression of thyrotropin-releasing hormone (TRH) in the paraventricular nucleus of the hypothalamus, which is considered as anorexigenic peptide. In contrast, a pair-fed group conformed by forced-food-restricted animals (FFR) (eating the exact same amount of food as dehydration-induced anorexic rats—DIA rats) present decreased TRH mRNA levels. NPY synthesis in the arcuate nucleus and orexin-expressing neurons from the lateral hypothalamic area (LHA) are activated during food restriction. These brain structures project into PVN, suggesting that NPY and orexins are possible factors involved in TRHergic neuron activation in DIA rats. Leptin signaling is another likely factor to be involved in TRH differential expression. Therefore, to gain more insight into the regulation of the feeding behavior in the experimental models, we analyzed Y1, Y5, Ox1-R and Ob-R b mRNA levels in PVN and prepro-orexin in LHA, since their signaling to the PVN might be altering TRH synthesis and feeding in DIA animals. Prepro-orexinergic cells were activated in FFR animals; Ox1-R and Y1 expression was reduced in FFR vs. controls or DIA group. Compensatory changes in PVN receptor expression of some feeding-related peptides in anorexic rats may alter TRHergic neural response to energy demands.

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 TRα1 and TRβ2 mRNA expression. TRα1 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 TRα1.

Dissociation of Thyrotropin and Leptin Secretion in Acute Surgical Stress in Severely Obese Patients

During illness, thyroid parameters undergo acute changes, which are known as non-thyroidal illness syndrome, the cause of which has not been elucidated. In vitro and in vivo data demonstrate that leptin regulates the expression of thyrotropin-releasing hormone (TRH)-mRNA in the paraventricular nucleus as well as the secretion of thyrotropin (TSH) in response to fasting in humans and animals. Moreover, in healthy adults, TSH and leptin have almost identical circadian rhythms. Our aim was to investigate the secretion of leptin and TSH, and their probable interaction, during the acute stress that is induced by surgery. We studied 18 severely obese but otherwise healthy men. All participants were admitted to the hospital in the morning after an overnight fast. On the following day, 14 of the participants underwent bariatric surgery at 0900. The remaining four participants did not undergo surgery and served as controls. Serum samples to measure the levels of TSH and leptin were collected from all participants, as follows: upon admission to the hospital (baseline values) and on the following day at 0900 and every 10 min, thereafter for 9 h. The serum TSH increased during the first hour after skin incision (si) and then decreased gradually throughout the rest of the observation period. In contrast, during the first hour after si, the leptin levels remained unaltered. The leptin levels then decreased and reached a nadir at 4 h and 10 min post si after which they remained constant for approximately 1 h. Thereafter, while TSH continued to decrease, leptin started to increase and reached baseline values at 9 h post si. In control subjects, the TSH and leptin profiles seemed parallel each other. During acute surgical stress, the secretion of TSH and leptin in severely obese men is asynchronous and causality could not be proven.

Photoperiod and acute energy deficits interact on components of the thyroid hormone system in hypothalamic tanycytes of the Siberian hamster

In the Siberian hamster, seasonal weight loss occurs gradually over many weeks during autumn and winter. This is driven by a regulatory mechanism that is able to integrate duration of exposure to short days (SDs) with the size of body energy reserves. After food restriction in SDs, followed by ad libitum refeeding, body weight of the hamster does not return to its former level; rather, it increases to a level defined by the length of time spent in SDs. In this report, we show that components of the thyroid hormone system that are involved in seasonal weight loss change expression in response to 48 h of starvation. Eight weeks in an SD photoperiod induced weight loss in the Siberian hamster. In the hypothalamus of these hamsters, type II deiodinase expression was decreased and type III deiodinase expression was induced, but there was no change in hypothalamic neuropeptide Y or thyrotropin-releasing hormone gene expression. For the first time, we show that the thyroid hormone transporter monocarboxylate transporter 8 is expressed in tanycytes and is increased in response to an SD photoperiod. Food restriction (48 h of starvation) reversed the direction of gene expression change for type II and III deiodinase and monocarboxylate transporter 8 induced by SD photoperiods. Furthermore, fasting increased neuropeptide Y expression and decreased thyrotropin-releasing hormone expression. VGF, a gene upregulated in SDs in the dorsal region of the medial posterior area of the arcuate nucleus, was not changed by starvation. These data point to a mechanism whereby energy deprivation can interact with an SD photoperiod on hypothalamic tanycytes to regulate components of the thyroid hormone system involved in photoperiodic regulation of seasonal physiology.

Effects of BDNF, T3, and corticosterone on expression of the hypothalamic obesity gene network in vivo and in vitro

Hypothalamic neuropeptides, neurotrophins, and systemic hormones modulate food intake and body composition. Although advances toward elucidating these interactions have been made, many aspects of the underlying mechanisms remain vague. Hypothalami from fat and lean chicken lines were assessed for differential expression of anabolic/orexigenic and catabolic/anorexigenic genes. Effects of triiodothyronine (T(3)), corticosterone (Cort), and brain-derived neurotrophic factor (BDNF) on expression of anabolic/orexigenic and catabolic/anorexigenic genes were tested in cultures of hypothalamic neurons. From this, we found that BDNF increased and T(3) decreased gene expression for BDNF, leptin receptor (LEPR), pro-opiomelanocortin (POMC), thyrotropin releasing hormone (TRH), and agouti-related protein (AGRP). Thyroid hormone levels were manipulated during development to show that T(3) inhibited BDNF, TRH, and BDNF receptor gene expression. Delivery of T(3), Cort, T(3) plus Cort, or vehicle in vivo continuously for 72 h indicated that Cort and T(3) have overlapping roles in regulating TRH, LEPR, and POMC gene expression and that Cort and T(3) regulate BDNF, neuropeptide Y, and AGRP in opposite directions. Collectively, these findings suggest that interactions between the neuropeptide BDNF and the hormones T(3) and/or Cort may constitute a homeostatic mechanism that links hypothalamic energy regulation controlling body composition.

Thyrotropin-Releasing Hormone Precursor Gene Knocking Down Impedes Melanocortin-Induced Hypertension in Rats

To the Editor: Recently, da Silva et al1 reported that endogenous melanocortin may cause elevation of arterial blood pressure (ABP) in spontaneously hypertensive rats. We invite authors to consider that the hypothalamic thyrotropin-releasing hormone (TRH) system may be involved, because spontaneously hypertensive rats show an hyperactivity of this system,2 and intracerebroventricular (ICV) injections of a prepro-TRH antisense oligonucleotide (AS) decreases both elevated TRH content and ABP independent of thyroid status.3 Leptin effects include increases in sympathetic activity …

Fasting-Induced Changes in the Hypothalamus–Pituitary–Thyroid Axis

Fasting induces profound changes in the hypothalamus-pituitary-thyroid (HPT) axis. The alterations observed in humans and rodents are similar in many ways, although they may be more pronounced and more acute in rodents. The molecular mechanisms underlying the resetting of HPT axis regulation in the framework of caloric deprivation are still incompletely understood. Fascinating studies in rats and mice have shown a dramatic downregulation of thyrotropin-releasing hormone (TRH) gene expression in hypophysiotropic paraventricular nucleus (PVN) neurons during fasting. Direct and indirect effects of decreased serum leptin, as well as effects of increased local triiodothyronine (T3) concentrations, in the hypothalamus during food deprivation contribute to the decreased activity of TRH neurons in the PVN. However, the relative contributions of these complex determinants remain to be defined in more detail. Pituitary thyroid-stimulating hormone (TSH) beta mRNA expression decreases during fasting, and this may be relatively independent of leptin and/or TRH, since leptin administration in this setting does not fully restore pituitary TSH expression, while it does restore TRH expression in the PVN. There may be a role for pituitary peptides, such as neuromedin B, in altered TSH gene expression during fasting. The observed decrease in serum thyroid hormone concentrations results to some extent from diminished thyroidal secretion of thyroid hormones, especially in rodents. Decreased thyroxine (T4) and T3 contribute to the downregulation of T3-responsive genes such as liver D1. The overall result of these complex HPT axis changes in various tissues during fasting is downregulation of the HPT axis, which is assumed to represent an energy-saving mechanism, instrumental in times of food shortage.

Analysis of the anxiolytic-like effect of TRH and the response of amygdalar TRHergic neurons in anxiety

Thyrotropin-releasing hormone (TRH) was first described for its neuroendocrine role in controlling the hypothalamus-pituitary-thyroid axis (HPT). Anatomical and pharmacological data evidence its participation as a neuromodulator in the central nervous system. Administration of TRH induces various behavioural effects including arousal, locomotion, analepsy, and in certain paradigms, it reduces fear behaviours. In this work we studied the possible involvement of TRHergic neurons in anxiety tests. We first tested whether an ICV injection of TRH had behavioural effects on anxiety in the defensive burying test (DBT). Corticosterone serum levels were quantified to evaluate the stress response and, the activity of the HPT axis to distinguish the endocrine response of TRH injection. Compared to a saline injection, TRH reduced cumulative burying, and decreased serum corticosterone levels, supporting anxiolytic-like effects of TRH administration. The response of TRH neurons was evaluated in brain regions involved in the stress circuitry of animals submitted to the DBT and to the elevated plus maze (EPM), tests that allow to correlate biochemical parameters with anxiety-like behaviour. In the DBT, the response of Wistar rats was compared with that of the stress-hypersensitive Wistar Kyoto (WKY) strain. Behavioural parameters were analysed in recorded videos. Animals were sacrificed 30 or 60min after test completion. In various limbic areas, the relative mRNA levels of TRH, its receptors TRH-R1 and -R2, and its inactivating ectoenzyme pyroglutamyl peptidase II (PPII), were determined by RT-PCR, TRH tissue content by radioimmunoassay (RIA). The extent of the stress response was evaluated by measuring the expression profile of CRH, CRH-R1 and GR mRNA in the paraventricular nucleus (PVN) of the hypothalamus and in amygdala, corticosterone levels in serum. As these tests demand increased physical activity, the response of the HPT axis was also evaluated. Both tasks increased the levels of serum corticosterone. WKY rats showed higher anxiety-like behaviour in the DBT than Wistar, as well as increased PVN mRNA levels of CRH and GR. TRH mRNA levels increased in the PVN and TSH values remained unchanged in both strains although TRH content decreased in the medial basal hypothalamus of Wistar rats only. TRH content was measured in several limbic regions but only amygdala showed specific task-related changes after DBT exposure of both strains: increased TRH content. Expression of TRH mRNA decreased in the amygdala of Wistar, suggesting inhibition of TRHergic neuronal activity in this region. The participation of amygdalar TRH neurons in anxiety was confirmed in the EPM where TRH expression and release correlated with the number of entries, and the % of time spent in open arms, supporting an anxiolytic role of these TRH-neurons. These results contribute to the understanding of the involvement of TRH during emotionally charged situations and shed light on the participation of particular circuits in related behaviours.

PYY transgenic mice are protected against diet-induced and genetic obesity

The gut-derived hormone, peptide YY (PYY) reduces food intake and enhances satiety in both humans and animals. Obese individuals also have a deficiency in circulating peptide YY, although whether this is a cause or a consequence of obesity is unclear. Our aims were to determine whether peptide YY (PYY) over-expression may have therapeutic effects for the treatment of obesity by altering energy balance and glucose homeostasis. We generated PYY transgenic mice and measured body weight, food intake, temperature, adiposity, glucose tolerance, circulating hormone and lipid concentrations and hypothalamic neuropeptide levels (neuropeptide Y; proopiomelanocortin, and thyrotropin-releasing hormone) under chow and high-fat feeding and after crossing these mice onto the genetically obese leptin-deficient ob/ob mouse background. PYY transgenic mice were protected against diet-induced obesity in association with increased body temperature (indicative of increased thermogenesis) and sustained expression of thyrotropin-releasing hormone in the paraventricular nucleus of the hypothalamus. Moreover, PYY transgenic mice crossed onto the genetically obese ob/ob background had significantly decreased weight gain and adiposity, reduced serum triglyceride levels and improved glucose tolerance compared to ob/ob controls. There was no effect of PYY transgenic over expression on basal or fasting-induced food intake measured at 11-12 weeks of age. Together, these findings suggest that long-term administration of PYY, PYY-like compounds or agents that stimulate PYY synthesis in vivo can reduce excess adiposity and improve glucose tolerance, possibly via effects on the hypothalamo-pituitary-thyroid axis and thermogenesis.

Amygdala kindling differentially regulates the expression of the elements involved in TRH transmission

Subthreshold electrical stimulation of the amygdala (kindling) activates neuronal pathways increasing the expression of several neuropeptides including thyrotropin releasing-hormone (TRH). Partial kindling enhances TRH expression and the activity or its inactivating ectoenzyme; once kindling is established (stage V), TRH and its mRNA levels are further increased but TRH-binding and pyroglutamyl aminopeptidase II (PPII) activity decreased in epileptogenic areas. To determine whether variations in TRH receptor binding or PPII activity are due to regulation of their synthesis, mRNA levels of TRH receptors (R1, R2) and PPII were semi-quantified by RT-PCR in amygdala, frontal cortex and hippocampus of kindled rats sacrificed at stage II or V. Increased mRNA levels of PPII were found at stage II in amygdala and frontal cortex, and of pro-TRH and TRH-R2, in amygdala and hippocampus. At stage V, pro-TRH mRNA levels increased and those of PPII, decreased in the three regions; TRH-R2 mRNA levels diminished in amygdala and frontal cortex and of TRH-R1 only in amygdala. In situ hybridization analyses revealed, at stage II, enhanced TRH-R1 mRNA levels in dentate gyrus and amygdala while decreased in piriform cortex; those of TRH-R2 increased in amygdala, CA2, dentate gyrus, piriform cortex, thalamus and subiculum and of PPII, in CAs and piriform cortex. In contrast, at stage V decreased expression of TRH-R1 occurred in amygdala, CA2/3, dentate gyrus and piriform cortex; of TRH-R2 in CA2, thalamus and piriform cortex, and of PPII in CA2, and amygdala. The magnitude of changes differed between ipsi and contralateral side. These results support a trans-synaptic modulation of all elements involved in TRH transmission in conditions that stimulate the activity of TRHergic neurons. They show that reported changes in PPII activity or TRH-binding caused by kindling relate to regulation of the expression of TRH receptors and degrading enzyme.

Chronic ethanol or glucose consumption alter TRH content and pyroglutamyl aminopeptidase II activity in rat limbic regions

Thyrotropin-releasing hormone (TRH), its receptors and inactivating enzyme (PPII) are present in limbic regions. Nutritional changes or acute ethanol administration in male rats differentially modulate TRH or PPII expression. Chronic ethanol effect was studied in male (3, 6 and 8 weeks) and female rats (6 weeks) including naïve and pair-fed (glucose) groups. Daily solid food and liquid intake, serum TSH and corticosterone, TRH content and PPII activity in limbic regions, were quantified. Gender differences were found in ethanol and total caloric intake and body weight gain, TSH and corticosterone levels. Ethanol consumption decreased TRH content and PPII activity in frontal cortex of male rats after 3–6 weeks. In contrast, glucose ingestion altered, by the third week, TRH content in amygdala, hippocampus, hypothalamus and nucleus accumbens, PPII activity in hippocampus and frontal cortex; by the sixth week, TRH content in amygdala and n. accumbens of male and females. Withdrawal at 24 h after 3-week ethanol ingestion decreased TRH content in amygdala and PPII activity in n. accumbens, while withdrawal from glucose reverted some of the effects produced by chronic glucose ingestion. Variations in TRH content or PPII activity support a region specific involvement of TRH neurons that depend on the treatment.

Expression of thyrotropin-releasing hormone by human melanoma and nevi.

Thyrotropin-releasing hormone (TRH) is a tripeptide hormone produced by the hypothalamus in response to hypothyroidism. RNA transcripts for the TRH prohormone have recently been described in melanoma cell lines. To expand these findings, we have examined cultured melanoma cells and melanocytes, human melanoma tumors, and nevi for the expression of TRH. Five melanoma cell lines were analyzed by reverse transcription-PCR/Southern blotting for preproTRH message. The same melanoma lines and two melanocyte lines were examined by immunocytochemistry for TRH protein expression and for growth response to exogenous TRH. Immunohistochemistry was used to test for TRH protein in sections of 19 melanomas, 33 dysplastic nevi, and 27 benign nevi. TRH message and protein were detected in all melanoma cell lines examined. Melanocytes were also found to express TRH protein. Four of the five melanoma cell lines but neither melanocyte line responded with a increase in proliferation to low concentrations of exogenous TRH. TRH immunoreactivity was observed in 12 of 19 melanomas (63%), 23 of 33 (69.7%) dysplastic nevi, and 14 of 27 (51.9%) benign nevi. Expression in dysplastic nevi was significantly greater than in benign nevi. Upon separate analysis of nevi from melanoma patients, the difference between dysplastic and benign nevi was even more significant. However, in healthy individuals, no difference between dysplastic and benign nevi was observed. Furthermore, dysplastic nevi from melanoma patients had a significantly higher percentage of TRH-positive cells when compared with healthy individuals. TRH is commonly expressed by melanomas and dysplastic nevi and may function as a melanoma autocrine growth factor. The presence of TRH in dysplastic nevi may be predictive for the development of melanoma. Our findings have significant clinical and biological implications for future research into the early stages of melanoma initiation and progression.

Central administration of cocaine- and amphetamine-regulated transcript increases phosphorylation of cAMP response element binding protein in corticotropin-releasing hormone-producing neurons but not in prothyrotropin-releasing hormone-producing neurons in the hypothalamic paraventricular nucleus

Cocaine- and amphetamine-regulated transcript (CART) has an important action on hypophysiotropic thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH) neurons to regulate the hypothalamic–pituitary–thyroid and adrenal axis, respectively. To elucidate the mechanisms by which CART mediates its effect on TRH and CRH neurons, we determined whether the exogenous administration of CART into the cerebrospinal fluid (CSF) phosphorylates the transcription factor, cyclic adenosine 5′-monophosphate response element binding protein (CREB), in the nucleus of TRH and CRH neurons. CART dramatically increased the percentage of phosphoCREB (PCREB) immunolabeled cell nuclei in the hypothalamic paraventricular nucleus (PVN) in fasted as well as fed rats at 10-min postinjection, particularly in the medial parvocellular subdivision of the PVN. Double immunolabelling with CRH antiserum revealed that CART increased the number of CRH neurons containing PCREB from 10.5±1.2 % to 87±1.2% ( P<0.001) in fasting animals and from 3.7±0.8% to 74±5.3% ( P<0.001) in fed animals. In contrast, no significant change was observed in the percentage of proTRH neurons colocalizing with PCREB either in the fasted (11.7±1.85%) or fed animals (4.2±2.2%) as compared to their respective vehicle controls (2.5±1.4% and 4.6±1%). Ultrastructural analysis revealed that CART establishes axosomatic and axodendritic contacts with CRH neurons in the PVN. These data demonstrate a selective effect of CART to phosphorylate CREB in CRH, but not TRH neurons in the PVN. Since CART is capable of increasing the gene expression of both CRH and TRH in hypophysiotropic neurons, and CART-containing axon terminals establish synaptic relationships with hypophysiotropic CRH and TRH neurons, we propose that CART may signal to the nucleus by more than one pathway.

Expression of neurotransmitter genes in motor regions of the dyskinetic rat after iminodipropionitrile

Rats treated with iminodipropionitrile develop a neurobehaviour syndrome with dyskinesia. Searching for the molecular correlates, we have examined the expression of selected genes involved in neurotransmission in motor regions using hybridization histochemistry. Frontal cortical and thalamic vasoactive intestinal peptide (VIP) expression, and striatal dynorphin, enkephalin (ENK) and substance P expression were increased. No change in cortical cholecystokinin (CCK), ENK, glutamic acid decarboxylase (GAD) and somatostatin (SRIF) expression, in striatal GAD, SRIF, nitric oxide synthase (NOS) and guanylate cyclase expression, and in thalamic CCK, GAD and thyrotropin-releasing hormone expression was found. NOS expression in the subthalamic nucleus as well as tyrosine hydroxylase, GAD and CCK expression in the substantia nigra were unchanged. These results confirm the involvement of striatal projection neurons in dyskinesia and suggest a novel role for VIP.

An improved method for the expression of TRH in serum-supplemented primary cultures of fetal hypothalamic cells

Primary cultures of dispersed cells from fetal nervous tissue are extensively used for studying multiple neuronal properties. Analyses of the developmental expression of thyrotropin releasing hormone (TRH; pglu-his-pro-NH 2) biosynthesis in primary cultures of fetal dissociated hypothalamic cells have shown that cellular TRH levels per dish increase with time in culture, after a lag period of a few days, but do not attain the values observed in vivo, hampering its use as a model system for the study of peptide biosynthesis and release. We have demonstrated that homologous conditioned medium (CM) enhances TRH expression in dissociated cell cultures from fetal mice hypothalamus, maintained in presence of serum. We report here experimental conditions that allow the expression, during the second or third week in vitro, of higher cellular TRH levels than previously described in primary cultures of dissociated hypothalamic cells from 17-day rat fetuses. The medium used was Dulbecco’s Modified Eagle Medium (DMEM) supplemented with fetal bovine serum (10%), vitamins, glucose, glutamine and insulin (DMEM-S). Cellular levels of TRH/mg protein increased with cell density between 1 and 2.7×10 6 cells per 35-mm dish. Addition of 10 −5 M cytosine arabinoside (CAr) at the 4th day in vitro (DIV) improved TRH cell content per dish compared to addition at 5 DIV; 2.5–5×10 −5 M bromodeoxyuridine added at seeding reduced cell survival and did not enhance TRH levels, in comparison to CAr-treated cultures. Addition of ascorbic acid (0.5–1×10 −4 M) increased TRH levels per dish. Substitution of DMEM by DMEM-F12 (1:1) did not improve TRH levels. Cellular levels of TRH, in Neurobasal plus B27 (a serum-free medium), were similar to levels in serum-supplemented media. In the optimized conditions, a small number of pro-TRH mRNA expressing cells (2% of total cells) was detected by in situ hybridization; 40% coexpressed the pro-protein convertase PC1 mRNA. Conditioning the medium, controlling glial proliferation, and adding ascorbic acid improved the expression of TRH in primary culture of hypothalamic cells in DMEM-S.

The hypothalamic-pituitary-thyroid axis in critical illness.

In severe illness, profound changes occur in the hypothalamic-pituitary-thyroid axis. The observed decrease in serum concentration of both thyroid hormones and thyrotropin (TSH) are not compatible with a negative feedback loop and suggest a major change in setpoint regulation of the hypothalamic-pituitary-thyroid axis. This is supported by post mortem studies showing a decreased expression of thyrotropin-releasing hormone in the hypothalamic paraventricular nucleus of patients with a decreased serum T3 level. In critical illness, serum T3 may even become undetectable without giving rise to an elevated concentration of serum TSH. It is currently not clearly established whether this reflects an adaptation of the organism to illness or instead a potentially harmful condition leading to hypothyroidism at tissue level. There is thus a need for randomized clinical trials in critically ill patients to investigate whether they may benefit from a normalization of thyroid hormone concentration. Recent clinical studies in these patients involving the administration of hypothalamic peptides open up new ways of achieving this.

Critical role for thyroid hormone receptor beta2 in the regulation of paraventricular thyrotropin-releasing hormone neurons.

Thyroid hormone thyroxine (T(4)) and tri-iodothyronine (T(3)) production is regulated by feedback inhibition of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) synthesis in the pituitary and hypothalamus when T(3) binds to thyroid hormone receptors (TRs) interacting with the promoters of the genes for the TSH subunit and TRH. All of the TR isoforms likely participate in the negative regulation of TSH production in vivo, but the identity of the specific TR isoforms that negatively regulate TRH production are less clear. To clarify the role of the TR-beta2 isoform in the regulation of TRH gene expression in the hypothalamic paraventricular nucleus, we examined preprothyrotropin-releasing hormone (prepro-TRH) expression in mice lacking the TR-beta2 isoform under basal conditions, after the induction of hypothyroidism with propylthiouracil, and in response to T(3) administration. Prepro-TRH expression was increased in hypothyroid wild-type mice and markedly suppressed after T(3) administration. In contrast, basal TRH expression was increased in TR-beta2-null mice to levels seen in hypothyroid wild-type mice and did not change significantly in response to induction of hypothyroidism or T(3) treatment. However, the suppression of TRH mRNA expression in response to leptin reduction during fasting was preserved in TR-beta2-null mice. Thus TR-beta2 is the key TR isoform responsible for T(3)-mediated negative-feedback regulation by hypophysiotropic TRH neurons.

Decreased neuropeptide Y (NPY) expression in the infundibular nucleus of patients with nonthyroidal illness

In patients with a variety of illnesses, serum concentrations of T3 decrease without giving rise to elevated serum levels of TSH, a phenomenon known as the sick euthyroid syndrome or nonthyroidal illness (NTI). Our previous studies in postmortem brain material showed decreased thyrotropin-releasing hormone (TRH) messenger RNA (mRNA) in the paraventricular nucleus (PVN) of patients with NTI, suggesting a role for TRH cells in the persistence of low TSH levels in NTI. In the present study, we hypothesized that changes in neuropeptide Y (NPY) input from the infundibular nucleus (IFN) to TRH cells in the PVN might be a determinant of decreased TRH expression in NTI. We investigated the hypothalamus of nine patients whose endocrine status had been assessed in a serum sample taken less than 24h before death and we examined NPY expression in the IFN by means of immunocytochemistry and mRNA in situ hybridization using an image analysis system. There was a negative correlation ( r = −0.88; p = 0.01) between serum leptin concentrations and total NPY mRNA in the IFN. The total amount of NPY immunoreactivity in the IFN correlated with total NPY mRNA ( r = 0.69; p = 0.04). In contrast to the situation in food-deprived rodents, total NPY immunoreactivity in the IFN showed a positive correlation with total TRH mRNA in the PVN ( r = 0.77; p = 0.02). The results suggest a role for decreased NPY input from the IFN in the resetting of thyroid hormone feedback on hypothalamic TRH cells in NTI.

Transcriptional regulation of the thyrotropin-releasing hormone gene by leptin and melanocortin signaling.

Starvation causes a rapid reduction in thyroid hormone levels in rodents. This adaptive response is caused by a reduction in thyrotropin-releasing hormone (TRH) expression that can be reversed by the administration of leptin. Here we examined hypothalamic signaling pathways engaged by leptin to upregulate TRH gene expression. As assessed by leptin-induced expression of suppressor of cytokine signaling-3 (SOCS-3) in fasted rats, TRH neurons in the paraventricular nucleus are activated directly by leptin. To a greater degree, they also contain melanocortin-4 receptors (MC4Rs), implying that leptin can act directly or indirectly by increasing the production of the MC4R ligand, alpha-melanocyte stimulating hormone (alpha-MSH), to regulate TRH expression. We further demonstrate that both pathways converge on the TRH promoter. The melanocortin system activates the TRH promoter through the phosphorylation and DNA binding of the cAMP response element binding protein (CREB), and leptin signaling directly regulates the TRH promoter through the phosphorylation of signal transducer and activator of transcription 3 (Stat3). Indeed, a novel Stat-response element in the TRH promoter is necessary for leptin's effect. Thus, the TRH promoter is an ideal target for further characterizing the integration of transcriptional pathways through which leptin acts.