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

Thyrotropin-Releasing Hormone and Food Intake in Mammals: An Update

Thyrotropin-releasing hormone (TRH; pGlu-His-Pro-NH2) is an intercellular signal produced mainly by neurons. Among the multiple pharmacological effects of TRH, that on food intake is not well understood. We review studies demonstrating that peripheral injection of TRH generally produces a transient anorexic effect, discuss the pathways that might initiate this effect, and explain its short half-life. In addition, central administration of TRH can produce anorexic or orexigenic effects, depending on the site of injection, that are likely due to interaction with TRH receptor 1. Anorexic effects are most notable when TRH is injected into the hypothalamus and the nucleus accumbens, while the orexigenic effect has only been detected by injection into the brain stem. Functional evidence points to TRH neurons that are prime candidate vectors for TRH action on food intake. These include the caudal raphe nuclei projecting to the dorsal motor nucleus of the vagus, and possibly TRH neurons from the tuberal lateral hypothalamus projecting to the tuberomammillary nuclei. For other TRH neurons, the anatomical or physiological context and impact of TRH in each synaptic domain are still poorly understood. The manipulation of TRH expression in well-defined neuron types will facilitate the discovery of its role in food intake control in each anatomical scene.

Hypothalamic TRH mediates anorectic effects of serotonin in rats

Among the modulatory functions of thyrotropin-releasing hormone (TRH), an anorectic behavior in rodents is observed when centrally injected. Hypothalamic PVN neurons receive serotonergic inputs from dorsal raphe nucleus and express serotonin (5HT) receptors such as 5HT1A, 5HT2A/2C, 5HT6, which are involved in 5HT-induced feeding regulation. Rats subjected to dehydration-induced anorexia (DIA) model show increased PVN TRH mRNA expression, associated with their decreased food intake. We analyzed whether 5HT input is implicated in the enhanced PVN TRH transcription that anorectic rats exhibit, given that 5HT increases TRH expression and release when studied in vitro By using mHypoA-2/30 hypothalamic cell cultures, we found that 5HT stimulated TRH mRNA, pCREB and pERK1/2 levels. By inhibiting basal PKA or PKC activities or those induced by 5HT, pCREB or pERK1/2 content did not increase suggesting involvement of both kinases in their phosphorylation. 5HT effect on TRH mRNA was not affected by PKA inhibition, but it diminished in the presence of PKCi suggesting involvement of PKC in 5HT-induced TRH increased transcription. This likely involves 5HT2A/2C and the activation of alternative transduction pathways than those studied here. In agreement with the in vitro data, we found that injecting 5HT2A/2C antagonists into the PVN of DIA rats reversed the increased TRH expression of anorectic animals, as well as their decreased food intake; also, the agonist reduced food intake of hungry restricted animals along with elevated PVN TRH mRNA levels. Our results support that the anorectic effects of serotonin are mediated by PVN TRH in this model.Significance statementInteraction between brain peptides and neurotransmitters' pathways regulates feeding behavior, but when altered it could lead to the development of eating disorders, such as anorexia. An abnormal increased TRH expression in hypothalamic PVN results in dehydration-induced anorectic rats, associated to their low food intake. The role of neurotransmitters in that alteration is unknown, and since serotonin inhibits feeding and has receptors in PVN, we analyzed its participation in increasing TRH expression and reducing feeding in anorectic rats. By antagonizing PVN serotonin receptors in anorectic rats, we identify decreased TRH expression and increased feeding, suggesting that the anorectic effects of serotonin are mediated by PVN TRH. Elucidating brain networks involved in feeding regulation would help to design therapies for eating disorders.

The insulin resistance is reversed by exogenous 3,5,3'triiodothyronine in type 2 diabetic Goto-Kakizaki rats by an inflammatory-independent pathway

PURPOSE

Diabetes mellitus (DM) has a multifactorial etiology that imparts a particular challenge to effective pharmacotherapy. Thyroid hormone actions have demonstrated beneficial effects in diabetic as well as obese rats. In both conditions, inflammation status plays a crucial role in the development of insulin resistance. Taking this into consideration, the present study aimed to demonstrate another possible pathway of thyroid hormone action on insulin sensitivity in a spontaneous type 2 diabetic rat model: the Goto-Kakizaki (GK) rats. GK animals present all typical hallmarks of type 2 DM (T2DM), except the usual peripheric inflammatory condition, observed in the other T2DM animal models.

METHODS

GK rats were treated or not with 3,5,3'triiodothyronine (T3). Insulin sensitivity, glucose tolerance, and proteins related to glucose uptake and utilization were evaluated in the skeletal muscle, white adipose tissue, and liver.

RESULTS

GK rats T3-treated presented enhanced insulin sensitivity, increased GLUT-4 content in the white adipose tissue and skeletal muscle, and increased hexokinase and citrate synthase content in skeletal muscle. Both non-treated and T3-treated GK rats did not present alterations in cytokine content in white adipose tissue, skeletal muscle, liver, and serum.

CONCLUSIONS

These results indicate that T3 improves insulin sensitivity in diabetic rats by a novel inflammatory-independent mechanism.

Tanycytes and the Control of Thyrotropin-Releasing Hormone Flux Into Portal Capillaries

Central and peripheral mechanisms that modulate energy intake, partition and expenditure determine energy homeostasis. Thyroid hormones (TH) regulate energy expenditure through the control of basal metabolic rate and thermogenesis; they also modulate food intake. TH concentrations are regulated by the hypothalamus-pituitary-thyroid (HPT) axis, and by transport and metabolism in blood and target tissues. In mammals, hypophysiotropic thyrotropin-releasing hormone (TRH) neurons of the paraventricular nucleus of the hypothalamus integrate energy-related information. They project to the external zone of the median eminence (ME), a brain circumventricular organ rich in neuron terminal varicosities and buttons, tanycytes, other glial cells and capillaries. These capillary vessels form a portal system that links the base of the hypothalamus with the anterior pituitary. Tanycytes of the medio-basal hypothalamus express a repertoire of proteins involved in transport, sensing, and metabolism of TH; among them is type 2 deiodinase, a source of 3,3',5-triiodo-L-thyronine necessary for negative feedback on TRH neurons. Tanycytes subtypes are distinguished by position and phenotype. The end-feet of β2-tanycytes intermingle with TRH varicosities and terminals in the external layer of the ME and terminate close to the ME capillaries. Besides type 2 deiodinase, β2-tanycytes express the TRH-degrading ectoenzyme (TRH-DE); this enzyme likely controls the amount of TRH entering portal vessels. TRH-DE is rapidly upregulated by TH, contributing to TH negative feedback on HPT axis. Alterations in energy balance also regulate the expression and activity of TRH-DE in the ME, making β2-tanycytes a hub for energy-related regulation of HPT axis activity. β2-tanycytes also express TRH-R1, which mediates positive effects of TRH on TRH-DE activity and the size of β2-tanycyte end-feet contacts with the basal lamina adjacent to ME capillaries. These end-feet associations with ME capillaries, and TRH-DE activity, appear to coordinately control HPT axis activity. Thus, down-stream of neuronal control of TRH release by action potentials arrival in the external layer of the median eminence, imbricated intercellular processes may coordinate the flux of TRH into the portal capillaries. In conclusion, β2-tanycytes appear as a critical cellular element for the somatic and post-secretory control of TRH flux into portal vessels, and HPT axis regulation in mammals.

Impaired hypothalamic cocaine- and amphetamine-regulated transcript expression in lateral hypothalamic area and paraventricular nuclei of dehydration-induced anorexic rats

Negative energy balance promotes physiological adaptations that ensure the survival of animals. The hypothalamic-pituitary-thyroid axis regulates basal energy expenditure and its down-regulating adaptation to negative energy balance is well described: in fasting, the serum content of thyrotrophin (TSH) and thyroid hormones (TH) decreases, enhancing the survival odds of individuals. By contrast, dehydration-induced anorexic (DIA) rats present an impaired hypothalamic-pituitary-thyroid (HPT) axis adaptation despite their negative energy balance: increased circulating TSH levels. The implication of cocaine- and amphetamine-regulated transcript (CART), an anorexic peptide, in HPT axis function impairment and food-avoidance behaviour displayed by DIA animals is unknown. Because CART is co-expressed with the peptide that regulates the HPT axis in hypophysiotrophic paraventricular nucleus (PVN) neurones (TSH-releasing hormone), we analysed CART expression and possible implications with respect to high TSH levels of DIA animals. We examined whether changes in CART expression from the lateral hypothalamic area (LHA) and arcuate nucleus (ARC) could participate in food-avoidance of DIA rats. DIA and forced-food restricted (FFR) animals reduced their body weight and food intake. FFR rats had a down-regulation of their HPT axis (reduced serum TH and TSH content), whereas DIA animals had reduced TH but increased TSH levels. CART mRNA expression in the ARC decreased similarly between experimental groups and diminished in anterior, medial PVN and in LHA of FFR animals, whereas DIA animals showed unchanged levels. This impaired CART mRNA expression in the anterior PVN and LHA could be related to the aberrant feeding behaviour of DIA rats but not to their deregulated HPT axis function.

Phosphodiesterase-7 inhibition affects accumbal and hypothalamic thyrotropin-releasing hormone expression, feeding and anxiety behavior of rats

Thyrotropin-releasing hormone (TRH) has anorexigenic and anxiolytic functions when injected intraventricularly. Nucleus accumbens (NAcc) is a possible brain region involved, since it expresses proTRH. TRH from hypothalamic paraventricular nucleus (PVN) has a food intake-regulating role. TRHergic pathways of NAcc and PVN are implicated in anxiety and feeding. Both behaviors depend on cAMP and phosphorylated-cAMP response element binding protein (pCREB) intracellular levels. Intracellular levels of cAMP are controlled by the degrading activity of phosphodiesterases (PDEs). Since TRH transcription is activated by pCREB, a specific inhibitor of PDE7B may regulate TRH-induced effects on anxiety and feeding. We evaluated the effectiveness of an intra-accumbal and intraperitoneal (i.p.) administration of a PDE7 inhibitor (BRL-50481) on rats' anxiety-like behavior and food intake; also on TRH mRNA and protein expression in NAcc and PVN to define its mediating role on the PDE7 inhibitor-induced behavioral changes. Accumbal injection of 4μg/0.3μL of PDE7 inhibitor decreased rats' anxiety. The i.p. injection of 0.2mg/kg of the inhibitor was able to increase the PVN TRH mRNA expression and to decrease feeding but did not change animals' anxiety levels; in contrast, 2mg/kg b.w inhibitor enhanced accumbal TRH mRNA, induced anxiolysis with no change in food intake. PDE7 inhibitor induced anxiolytic and anorexigenic like behavior depending on the dose used. Results supported hypothalamic TRH mediated feeding-reduction effects, and accumbal TRH mediation of inhibitor-induced anxiolysis. Thus, an i.p dose of this inhibitor might be reducing anxiety with no change in feeding, which could be useful for obese patients.

Prepuberal light phase feeding induces neuroendocrine alterations in adult rats

Feeding patterns are important factors in obesity evolvement. Time-restricted feeding schedules (tRF) during resting phase change energy homeostasis regulation, disrupting the circadian release of metabolism-regulating hormones, such as leptin, insulin and corticosterone and promoting body weight gain. Thyroid (HPT) and adrenal (HPA) axes exhibit a circadian regulation and are involved in energy expenditure, thus studying their parameters in tRF paradigms will elucidate their role in energy homeostasis impairments under such conditions. As tRF in young animals is poorly studied, we subjected prepuberal rats to a tRF either in light (LPF) or in darkness phase (DPF) and analyzed HPT and HPA response when they reach adulthood, as well as their arcuate (ARC) and paraventricular (PVN) hypothalamic nuclei neurons' sensitivity to leptin in subsets of 10-week-old animals after fasting and with i.p. leptin treatment. LPF group showed high body weight and food intake, along with increased visceral fat pads, corticosterone, leptin and insulin serum levels, whereas circulating T₄ decreased. HPA axis hyperactivity was demonstrated by their high PVN Crf mRNA expression; the blunted activity of HPT axis, by the decreased hypophysiotropic PVN Trh mRNA expression. Trh impaired expression to the positive energy balance in LPF, accounted for their ARC leptin resistance, evinced by an increased Npy and Socs3 mRNA expression. We concluded that the hyperphagia of prepuberal LPF animals could account for the HPA axis hyperactivity and for the HPT blocked function due to the altered ARC leptin signaling and impaired NPY regulation on PVN TRH neurons.

Mct8 and trh co-expression throughout the hypothalamic paraventricular nucleus is modified by dehydration-induced anorexia in rats

Thyrotropin-releasing hormone (TRH) is a neuropeptide with endocrine and neuromodulatory effects. TRH from the paraventricular hypothalamic nucleus (PVN) participates in the control of energy homeostasis; as a neuromodulator TRH has anorexigenic effects. Negative energy balance decreases PVN TRH expression and TSH concentration; in contrast, a particular model of anorexia (dehydration) induces in rats a paradoxical increase in TRH expression in hypophysiotropic cells from caudal PVN and high TSH serum levels, despite their apparent hypothalamic hyperthyroidism and low body weight. We compared here the mRNA co-expression pattern of one of the brain thyroid hormones' transporters, the monocarboxylate transporter-8 (MCT8) with that of TRH in PVN subdivisions of dehydration-induced anorexic (DIA) and control rats. Our aim was to identify whether a low MCT8 expression in anorexic rats could contribute to their high TRH mRNA content.We registered daily food intake and body weight of 7-day DIA and control rats and analyzed TRH and MCT8 mRNA co-expression throughout the PVN by double in situ hybridization assays. We found that DIA rats showed increased number of TRHergic cells in caudal PVN, as well as a decreased percentage of TRH-expressing neurons that co-expressed MCT8 mRNA signal. Results suggest that the reduced proportion of double TRH/MCT8 expressing cells may be limiting the entry of hypothalamic triiodothyronine to the greater number of TRH-expressing neurons from caudal PVN and be in part responsible for the high TRH expression in anorexia rats and for the lack of adaptation of their hypothalamic-pituitary-thyroid axis to their low food intake.

Anorexia in human and experimental animal models: physiological aspects related to neuropeptides

Anorexia, a loss of appetite for food, can be caused by various physiological and pathophysiological conditions. In this review, firstly, clinical aspects of anorexia nervosa are summarized in brief. Secondly, hypothalamic neuropeptides responsible for feeding regulation in each hypothalamic nucleus are discussed. Finally, three different types of anorexigenic animal models; dehydration-induced anorexia, cisplatin-induced anorexia and cancer anorexia-cachexia, are introduced. In conclusion, hypothalamic neuropeptides may give us novel insight to understand and find effective therapeutics strategy essential for various kinds of anorexia.

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.

Daily regulation of body temperature rhythm in the camel (Camelus dromedarius) exposed to experimental desert conditions

In the present work, we have studied daily rhythmicity of body temperature (Tb) in Arabian camels challenged with daily heat, combined or not with dehydration. We confirm that Arabian camels use heterothermy to reduce heat gain coupled with evaporative heat loss during the day. Here, we also demonstrate that this mechanism is more complex than previously reported, because it is characterized by a daily alternation (probably of circadian origin) of two periods of poikilothermy and homeothermy. We also show that dehydration induced a decrease in food intake plays a role in this process. Together, these findings highlight that adaptive heterothermy in the Arabian camel varies across the diurnal light–dark cycle and is modulated by timing of daily heat and degrees of water restriction and associated reduction of food intake. The changed phase relationship between the light–dark cycle and the Tb rhythm observed during the dehydration process points to a possible mechanism of internal desynchronization during the process of adaptation to desert environment. During these experimental conditions mimicking the desert environment, it will be possible in the future to determine if induced high‐amplitude ambient temperature (Ta) rhythms are able to compete with the zeitgeber effect of the light–dark cycle.

e12151

In the Arabian camel, the “adaptative heterothermy” is characterized by a daily alteration (probably of circadian origin) of two periods. The findings highlight that adaptative heterothermy varies across the diurnal light–dark cycle and is modulated by timing of daily heat and degrees of water restriction and associated reduction of food intake.

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.

Pro-TRH and pro-CRF expression in paraventricular nucleus of small litter-reared fasted adult rats

Neuroendocrine axes adapt to nutrient availability. During fasting, the function of the hypothalamus-pituitary-thyroid axis (HPT) is reduced, whereas that of the hypothalamus-pituitary-adrenal axis (HPA) is increased. Overfeeding-induced hyperleptinemia during lactation may alter the regulatory set point of neuroendocrine axes and their adaptability to fasting in adulthood. Hyperleptinemia is developed in rodents by litter size reduction during lactation; adult rats from small litters become overweight, but their paraventricular nucleus (PVN) TRH synthesis is unchanged. It is unclear whether peptide expression still responds to nutrient availability. PVN corticotropin-releasing factor (CRF) expression has not been evaluated in this model. We analyzed adaptability of HPT and HPA axes to fasting-induced low leptin levels of reduced-litter adult rats. Offspring litters were reduced to 2-3/dam (early-overfed) or maintained at 8/dam (controls, C). At 10 weeks old, a subset of animals from each group was fasted for 48 h and leptin, corticosterone, and thyroid hormones serum levels were analyzed. In brain, expressions of leptin receptor, NPY and SOCS3, were evaluated in arcuate nucleus, and those of proTRH and proCRF in PVN by real-time PCR. ProTRH expression in anterior and medial PVN subcompartments was assayed by in situ hybridization. Early-overfed adults developed hyperphagia and excessive weight, together with decreased proTRH expression in anterior PVN, supporting the anorexigenic effects of TRH. Early-overfed rats presented low PVN proTRH synthesis, whereas fasting did not induce a further reduction. Fasting-induced stress was unable to increase corticosterone levels, contributing to reduced body weight loss in early-overfed rats. We concluded that early overfeeding impaired the adaptability of HPT and HPA axes to excess weight and fasting in adults.