Lorraine Jaimes-Hoy

Differential response of TRHergic neurons of the hypothalamic paraventricular nucleus (PVN) in female animals submitted to food-restriction or dehydration-induced anorexia and cold exposure

TRH neurons of the hypothalamic paraventricular nucleus (PVN), regulate pituitary-thyroid axis (HPT). Fasting activates expression of orexigenic peptides from the arcuate nucleus, increases corticosterone while reduces leptin, and pro-TRH mRNA levels despite low serum thyroid hormone concentration (tertiary hypothyroidism). TRH synthesis is positively regulated by anorexigenic peptides whose expression is reduced in fasting. The model of dehydration-induced anorexia (DIA) leads to decreased voluntary food intake but peptide expression in the arcuate is similar to forced-food restriction (FFR), where animals remain hungered. We compared the response of HPT axis of female Wistar rats submitted to DIA (2.5% saline solution, food ad libitum, 7 days) with FFR (provided with the amount of food ingested by DIA) and naïve (N) group fed ad libitum, as well as their response to acute cold exposure. Pro-TRH and pro-CRH mRNA levels in the PVN were measured by RT-PCR, TRH content, serum concentration of TSH and thyroid hormones by radioimmunoassay. DIA rats reduced 80% their food consumption compared to N, decreased PVN pro-CRH expression, serum estradiol and leptin levels, increased corticosterone similar to FFR. HPT axis of DIA animals failed to adapt: FFR presented tertiary hypothyroidism and DIA, primary. Response to cold stimulation leading to increased pro-TRH mRNA levels and TRH release was preserved under reduced energy availability in FFR rats but not in DIA, although the dynamics of hormonal release differed: TSH release augmented only in naïve; thyroxine in all but highest in DIA, and triiodothyronine in FFR and DIA suggesting a differential regulation of deiodinases.

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.

TRH, first hypophysiotropic releasing hormone isolated: its control of 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.

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.

Involvement of CRH-R2 receptor in eating behavior and in the response of the HPT axis in rats subjected to dehydration-induced anorexia

Wistar rats subjected to dehydration-induced anorexia (DIA), with 2.5% NaCl solution as drinking water for 7 days, decrease by 80% their food intake and present some changes common to pair-fed food restricted rats (FFR) such as: weight loss, decreased serum leptin and expression of orexigenic arcuate peptides, increasing the anorexigenic ones and serum corticosterone levels. In contrast, the response of the HPT axis differs: DIA animals have increased TRH expression in PVN and present primary as opposed to the tertiary hypothyroidism of the FFR. Exclusive to DIA is the activation of CRHergic neurons in the lateral hypothalamus (LH) that project to PVN. Since TRH neurons of the PVN contain CRH receptors, we hypothesized that the differences in the response of the HPT axis to DIA could be due to CRH regulating TRHergic neurons. CRH effect was first evaluated on TRH expression of cultured hypothalamic cells where TRH mRNA levels increased after 1h with 0.1nM of CRH. We then measured the mRNA levels of CRH receptors in the PVN of male and female rats subjected to DIA; only those of CRH-R2 were modulated (down-regulated). The CRH-R2 antagonist antisauvagine-30 was therefore injected into the PVN of male rats, during the 7 days of DIA. Antisauvagine-30 induced a higher food intake than controls, and impeded the changes produced by DIA on the HPT axis: PVN TRH mRNA, and serum TH and TSH levels were decreased to similar values of FFR animals. Results corroborate the anorexigenic effect of CRH and show its role, acting through CRH-R2 receptors, in the activation of TRHergic PVN neurons caused by DIA. These new data further supports clinical trials with CRH-R2 antagonists in anorexia nervosa patients.

Voluntary Exercise Adapts the Hypothalamus-Pituitary-Thyroid Axis in Male Rats

The hypothalamic-pituitary thyroid (HPT) axis modulates energy homeostasis. Its activity decreases in conditions of negative energy balance but the effects of chronic exercise on the axis are controversial and unknown at hypothalamic level. Wistar male rats were exposed for up to 14 days to voluntary wheel running (WR), or pair-feeding (PF; 18% food restriction), or to repeated restraint (RR), a mild stressor. WR and RR diminished food intake; body weight gain decreased in the 3 experimental groups, but WAT mass and serum leptin more intensely in the WR group. WR, but not RR, produced a delayed inhibition of central markers of HPT axis activity. At day 14, in WR rats paraventricular nucleus-pro-TRH mRNA and serum TSH levels decreased, anterior pituitary TRH-receptor 1 mRNA levels increased, but serum thyroid hormone levels were unaltered, which is consistent with decreased secretion of TRH and clearance of thyroid hormones. A similar pattern was observed if WR animals were euthanized during their activity phase. In contrast, in PF animals the profound drop of HPT axis activity included decreased serum T3 levels and hepatic deiodinase 1 activity; these changes were correlated with an intense increase in serum corticosterone levels. WR effects on HPT axis were not associated with changes in the activity of the hypothalamic-pituitary adrenal axis, but correlated positively with serum leptin levels. These data demonstrate that voluntary WR adapts the status of the HPT axis, through pathways that are distinct from those observed during food restriction or repeated stress.

An Acute Injection of Corticosterone Increases Thyrotrophin-Releasing Hormone Expression in the Paraventricular Nucleus of the Hypothalamus but Interferes with the Rapid Hypothalamus Pituitary Thyroid Axis Response to Cold in Male Rats

The activity of the hypothalamic‐pituitary‐thyroid (HPT) axis is rapidly adjusted by energy balance alterations. Glucocorticoids can interfere with this activity, although the timing of this interaction is unknown. In vitro studies indicate that, albeit incubation with either glucocorticoid receptor (GR) agonists or protein kinase A (PKA) activators enhances pro‐thyrotrophin‐releasing hormone (pro‐TRH) transcription, co‐incubation with both stimuli reduces this enhancement. In the present study, we used primary cultures of hypothalamic cells to test whether the order of these stimuli alters the cross‐talk. We observed that a simultaneous or 1‐h prior (but not later) activation of GR is necessary to inhibit the stimulatory effect of PKA activation on pro‐TRH expression. We tested these in vitro results in the context of a physiological stimulus on the HPT axis in adult male rats. Cold exposure for 1 h enhanced pro‐TRH mRNA expression in neurones of the hypophysiotrophic and rostral subdivisions of the paraventricular nucleus (PVN) of the hypothalamus, thyrotrophin (TSH) serum levels and deiodinase 2 (D2) activity in brown adipose tissue (BAT). An i.p. injection of corticosterone stimulated pro‐TRH expression in the PVN of rats kept at ambient temperature, more pronouncedly in hypophysiotrophic neurones that no longer responded to cold exposure. In corticosterone‐pretreated rats, the cold‐induced increase in pro‐TRH expression was detected only in the rostral PVN. Corticosterone blunted the increase in serum TSH levels and D2 activity in BAT produced by cold in vehicle‐injected animals. Thus, increased serum corticosterone levels rapidly restrain cold stress‐induced activation of TRH hypophysiotrophic neurones, which may contribute to changing energy expenditure. Interestingly, TRH neurones of the rostral PVN responded to both corticosterone and cold exposure with an amplified expression of pro‐TRH mRNA, suggesting that these neurones integrate stress and temperature distinctly from the hypophysiotrophic neurones.

Fasting Enhances Pyroglutamyl Peptidase II Activity in Tanycytes of the Mediobasal Hypothalamus of Male Adult Rats

Fasting down-regulates the hypothalamus-pituitary-thyroid (HPT) axis activity through a reduction of TRH synthesis in neurons of the parvocellular paraventricular nucleus of the hypothalamus (PVN). These TRH neurons project to the median eminence (ME), where TRH terminals are close to the cytoplasmic extensions of β2 tanycytes. Tanycytes express pyroglutamyl peptidase II (PPII), the TRH-degrading ectoenzyme that controls the amount of TRH that reaches the anterior pituitary. We tested the hypothesis that regulation of ME PPII activity is another mechanism by which fasting affects the activity of the HPT axis. Semiquantitative in situ hybridization histochemistry data indicated that PPII and deiodinase 2 mRNA levels increased in tanycytes after 48 hours of fasting. This increase was transitory, followed by an increase of PPII activity in the ME, and a partial reversion of the reduction in PVN pro-TRH mRNA levels and the number of TRH neurons detected by immunohistochemistry. In fed animals, adrenalectomy and corticosterone treatment did not change ME PPII activity 72 hours later. Methimazole-induced hypothyroidism produced a profound drop in tanycytes PPII mRNA levels, which was reverted by 3 days of treatment with T4. The activity of thyroliberinase, the serum isoform of PPII, was increased at most fasting time points studied. We conclude that delayed increases in both the ME PPII as well as the thyroliberinase activities in fasted male rats may facilitate the maintenance of the deep down-regulation of the HPT axis function, despite a partial reactivation of TRH expression in the PVN.

Advances in TRH signaling

The activity of the hypothalamus-pituitary-thyroid axis (HPT) is coordinated by hypophysiotropic thyrotropin releasing hormone (TRH) neurons present in the paraventricular nucleus of the hypothalamus. Hypophysiotropic TRH neurons act as energy sensors. TRH controls the synthesis and release of thyrotropin, which activates the synthesis and secretion of thyroid hormones; in target tissues, transporters and deiodinases control their local availability. Thyroid hormones regulate many functions, including energy homeostasis. This review discusses recent evidence that covers several aspects of TRH role in HPT axis regulation. Knowledge about the mechanisms of TRH signaling has steadily increased. New transcription factors engaged in TRH gene expression have been identified, and advances made on how they interact with signaling pathways and define the dynamics of TRH neurons response to acute and/or long-term influences. Albeit yet incomplete, the relationship of TRH neurons activity with positive energy balance has emerged. The importance of tanycytes as a central relay for the feedback control of the axis, as well as for HPT responses to alterations in energy balance, and other stimuli has been reinforced. Finally, some studies have started to shed light on the interference of prenatal and postnatal stress and nutrition on HPT axis programing, which have confirmed the axis susceptibility to early insults.

Neonatal Maternal Separation Alters, in a Sex-Specific Manner, the Expression of TRH, of TRH-Degrading Ectoenzyme in the Rat Hypothalamus, and the Response of the Thyroid Axis to Starvation

Hypothalamic-pituitary-thyroid (HPT) axis activity is important for energy homeostasis, and is modified by stress. Maternal separation (MS) alters the stress response and predisposes to metabolic disturbances in the adult. We therefore studied the effect of MS on adult HPT axis activity. Wistar male and female pups were separated from their mothers 3 h/d during postnatal day (PND)2-PND21 (MS), or left nonhandled (NH). Open field and elevated plus maze tests revealed increased locomotion in MS males and anxiety-like behavior in MS females. At PND90, MS females had increased body weight gain, Trh expression in the hypothalamic paraventricular nucleus, and white adipose tissue mass. MS males had increased expression of TRH-degrading enzyme in tanycytes, reduced TSH and T3, and enhanced corticosterone serum concentrations. MS stimulated brown adipose tissue deiodinase 2 activity in either sex. Forty-eight hours of fasting (PND60) augmented serum corticosterone levels similarly in MS or NH females but more in MS than in NH male rats. MS reduced the fasting-induced drop in hypothalamic paraventricular nucleus-Trh expression of males but not of females and abolished the fasting-induced increase in Trh expression in both sexes. Fasting reduced serum concentrations of TSH, T4, and T3, less in MS than in NH males, whereas in females, TSH decreased in MS but not in NH rats, but T4 and T3 decreased similarly in NH and MS rats. In conclusion, MS produced long-term changes in the activity of the HPT axis that were sex specific; response to fasting was partially blunted in males, which could affect their adaptive response to negative energy balance.