GLP-1 Receptor Signaling Has Different Effects on the Perikarya and Axons of the Hypophysiotropic Thyrotropin-Releasing Hormone Synthesizing Neurons in Male Mice

Background: Glucagon-like peptide 1 (GLP-1) is involved in the regulation of energy and glucose homeostasis. As GLP-1 has similar effects on the energy homeostasis as the hypophysiotropic thyrotropin-releasing hormone (TRH) neurons that regulate the hypothalamic-pituitary-thyroid (HPT) axis, we raised the possibility that the TRH neurons are involved in the mediation of the effects of GLP-1. Therefore, the relationship and interaction of the GLP-1 system and the TRH neurons of the hypothalamic paraventricular nucleus (PVN) were studied. Methods: To examine the anatomical and functional relationship of TRH neurons and the GLP-1 system in the PVN, immunocytochemistry, in situ hybridization, in vitro patch-clamp electrophysiology, metabolic phenotyping, and explant experiments were performed. Results: Our data demonstrate that the TRH neurons of the PVN are innervated by GLP-1 producing neurons and express the GLP-1 receptor (GLP-1R). However, not only do the GLP-1-innervated TRH neurons express GLP-1R but the receptor is also present in the axons of the hypophysiotropic TRH neurons in the blood-brain barrier free median eminence (ME) suggesting that peripherally derived GLP-1 may also influence the TRH neurons. In vitro, GLP-1 increased the firing rate of TRH neurons and depolarized them. In addition, GLP-1 directly stimulated the GABAergic input of a population of TRH neurons. Furthermore, GLP-1 inhibited the release of TRH from the hypophysiotropic axons in the ME. In vivo, peripheral GLP-1R agonist administration markedly inhibited the food intake and the energy expenditure, but had no effect on the TRH expression in the PVN and resulted in lower circulating free T4 levels. Conclusions: Our results indicate that GLP-1R activation has a direct stimulatory effect on TRH neurons in the PVN, but the activation of GLP-1R may also inhibit TRH neurons by facilitating their inhibitory inputs or by inhibiting the axon terminals of these cells in the ME. The innervation of TRH neurons by GLP-1 neurons suggests that TRH neurons might be influenced by both circulating GLP-1 and by GLP-1 neurons of the nucleus tractus solitarii. The lack of GLP-1R agonist-induced regulation of TRH neurons in vivo suggests that the HPT axis does not mediate the GLP-1R agonist-induced weight loss.

Different Hypothalamic Mechanisms Control Decreased Circulating Thyroid Hormone Levels in Infection and Fasting-Induced Non-Thyroidal Illness Syndrome in Male Thyroid Hormone Action Indicator Mice

Background: Non-Thyroidal Illness Syndrome (NTIS) caused by infection or fasting is hallmarked by reduced circulating thyroid hormone (TH) levels. To better understand the role of local TH-action in the development of NTIS, we assessed tissue-specific changes of TH signaling in Thyroid Hormone Action Indicator (THAI) mice. Methods: NTIS was induced in young adult THAI mice by bacterial lipopolysaccharide (LPS)-administration or by 24 or 48 hours' fasting. Tissue-specific TH-action was assessed by the detection of changes of the Luciferase reporter of THAI mice with quantitative polymerase chain reaction along with tissue-specific examination of regulators of TH metabolism and signaling. Age dependence of revealed alterations of hypothalamic TH-action was also studied in 1-year-old male THAI mice. Results: LPS-treatment increased TH-action in the hypothalamic arcuate nucleus-median eminence (ARC-ME) region preceded by an increase of type 2 deiodinase (D2) expression in the same region and followed by the suppression of proTrh expression in the hypothalamic paraventricular nucleus (PVN). In contrast, LPS decreased both TH-action and D2 activity in the pituitary at both ages. Tshβ expression and serum free thyroxine (fT4) and free triiodothyronine (fT3) levels decreased in LPS-treated young adults. Tshβ expression and serum fT4 levels were not significantly affected by LPS treatment in aged animals. In contrast to LPS treatment, TH-action remained unchanged in the ARC-ME of 24 and 48 hours fasted animals accompanied with a modest decrease of proTrh expression in the PVN in the 24-hour group. Tshβ expression and fT3 level were decreased in both fasted groups, but the fT4 decreased only in the 48 hours fasted animals. Conclusions: Although the hypothalamo-pituitary-thyroid (HPT) axis is inhibited both in LPS and fasting-induced NTIS, LPS achieves this by centrally inducing local hyperthyroidism in the ARC-ME region, while fasting acts without affecting hypothalamic TH signaling. Lack of downregulation of Tshβ and fT4 in LPS-treated aged THAI mice suggests age-dependent alterations in the responsiveness of the HPT axis. The LPS-induced tissue-specific hypo-, eu-, and hyperthyroidism in different tissues of the same animal indicate that under certain conditions TH levels alone could be a poor marker of tissue TH signaling. In conclusion, decreased circulating TH levels in these two forms of NTIS are associated with different patterns of hypothalamic TH signaling.

Origin of thyrotropin-releasing hormone neurons that innervate the tuberomammillary nuclei

Hypophysiotropic thyrotropin-releasing hormone (TRH) neurons function as metabolic sensors that regulate the thyroid axis and energy homeostasis. Less is known about the role of other hypothalamic TRH neurons. As central administration of TRH decreases food intake and increases histamine in the tuberomammillary nuclei (TMN), and TMN histamine neurons are densely innervated by TRH fibers from an unknown origin, we mapped the location of TRH neurons that project to the TMN. The retrograde tracer, cholera toxin B subunit (CTB), was injected into the TMN E1–E2, E4–E5 subdivisions of adult Sprague–Dawley male rats. TMN projecting neurons were observed in the septum, preoptic area, bed nucleus of the stria terminalis (BNST), perifornical area, anterior paraventricular nucleus, peduncular and tuberal lateral hypothalamus (TuLH), suprachiasmatic nucleus and medial amygdala. However, CTB/pro-TRH178-199 double-labeled cells were only found in the TuLH. The specificity of the retrograde tract-tracing result was confirmed by administering the anterograde tracer, Phaseolus vulgaris leuco-agglutinin (PHAL) into the TuLH. Double-labeled PHAL-pro-TRH boutons were identified in all subdivisions of the TMN. TMN neurons double-labeled for histidine decarboxylase (Hdc)/PHAL, Hdc/Trh receptor (Trhr), and Hdc/Trh. Further confirmation of a TuLH-TRH neuronal projection to the TMN was established in a transgenic mouse that expresses Cre recombinase in TRH-producing cells following microinjection of a Cre recombinase-dependent AAV that expresses mCherry into the TuLH. We conclude that, in rodents, the TRH innervation of TMN originates in part from TRH neurons in the TuLH, and that this TRH population may contribute to regulate energy homeostasis through histamine Trhr-positive neurons of the TMN.

Adult-born proopiomelanocortin neurons derived from Rax-expressing precursors mitigate the metabolic effects of congenital hypothalamic proopiomelanocortin deficiency

OBJECTIVE

Proopiomelanocortin (POMC) neurons of the hypothalamic arcuate nucleus are essential regulators of energy balance. Selective loss of POMC production in these cells results in extreme obesity and metabolic comorbidities. Neurogenesis occurs in the adult hypothalamus, but it remains uncertain whether functional POMC neurons emerge in physiologically significant numbers during adulthood. Here, we tested whether Rax-expressing precursors generate POMC neurons in adult mice and rescue the metabolic phenotype caused by congenital hypothalamic POMC deficiency.

METHODS

Initially, we identified hypothalamic Rax-expressing cell types using wild-type and Rax-CreERT2:Ai34D mice. Then we generated compound Rax-CreERT2:ArcPomcloxTB/loxTB mice in which endogenous hypothalamic Pomc expression is silenced, but can be restored by tamoxifen administration selectively in neurons derived from Rax+ progenitors. The number of POMC neurons generated by Rax+ progenitors in adult mice and their axonal projections was determined. The metabolic effects of these neurons were assessed by measuring food intake, bodyweight, and body composition, along with glucose and insulin levels.

RESULTS

We found that Rax is expressed by tanycytes and a previously unrecognized cell type in the hypothalamic parenchyma of adult mice. Rax+ progenitors generated ~10% of the normal adult hypothalamic POMC neuron population within two weeks of tamoxifen treatment. The same rate and steady state of POMC neurogenesis persisted from young adult to aged mice. These new POMC neurons established terminal projections to brain regions that were involved in energy homeostasis. Mice with Rax+ progenitor-derived POMC neurons had reduced body fat mass, improved glucose tolerance, increased insulin sensitivity, and decreased bodyweight in proportion to the number of new POMC neurons.

CONCLUSIONS

These data demonstrate that Rax+ progenitors generate POMC neurons in sufficient numbers during adulthood to mitigate the metabolic abnormalities of hypothalamic POMC-deficient mice. The findings suggest that adult hypothalamic neurogenesis is a robust phenomenon in mice that can significantly impact energy homeostasis.

Selective Restoration of Pomc Expression in Glutamatergic POMC Neurons: Evidence for a Dynamic Hypothalamic Neurotransmitter Network

Hypothalamic POMC deficiency leads to obesity and metabolic deficiencies, largely due to the loss of melanocortin peptides. However, POMC neurons in the arcuate nucleus (ARC) are comprised of glutamatergic and GABAergic subpopulations. The developmental program, relative proportion and function of these two subpopulations are unresolved. To test whether glutamatergic POMC neurons serve a distinct role in maintaining energy homeostasis, we activated Pomc expression Cre- dependently in Vglut2-expressing neurons of mice with conditionally silenced Pomc alleles. The Vglut2-Pomc restored mice had normal ARC Pomc mRNA levels, POMC immunoreactivity, as well as body weight and body composition at age 12 weeks. Unexpectedly, the cumulative total of Vglut2+ glutamatergic- and Gad67+ GABAergic-Pomc neurons detected by in situ hybridization (ISH) exceeded 100% in both Vglut2- Pomc restored and control mice, indicating that a subpopulation of Pomc neurons must express both neuronal markers. Consistent with this hypothesis, triple ISH of C57BL/6J hypothalami revealed that 35% of ARC Pomc neurons were selectively Gad67+, 21% were selectively Vglut2+, and 38% expressed both Gad67 and Vglut2. The single Gad67+ and Vglut2+Pomc neurons were most prevalent in the rostral ARC, while the Vglut2/Gad67+ dual-phenotype cells predominated in the caudal ARC. A lineage trace using Ai9-tdTomato reporter mice to label fluorescently all Vglut2-expressing neurons showed equal numbers of tdTomato+ and tdTomato- POMC immunoreactive neurons. Together, these data suggest that POMC neurons exhibit developmental plasticity in their expression of glutamatergic and GABAergic markers, enabling re-establishment of normal energy homeostasis in the Vglut2-Pomc restored mice.

Prss56 expression in the rodent hypothalamus: Inverse correlation with pro-opiomelanocortin suggests oscillatory gene expression in adult rat tanycytes

We recently reported that the number of hypothalamic tanycytes expressing pro-opiomelanocortin (Pomc) is highly variable among brains of adult rats. While its cause and significance remain unknown, identifying other variably expressed genes in tanycytes may help understand this curious phenomenon. In this in situ hybridization study, we report that the Prss56 gene, which encodes a trypsin-like serine protease and is expressed in neural stem/progenitor cells, shows a similarly variable mRNA expression in tanycytes of adult rats and correlates inversely with tanycyte Pomc mRNA. Prss56 was expressed in α1, β1, subsets of α2, and some median eminence γ tanycytes, but virtually absent from β2 tanycytes. Prss56 was also expressed in vimentin positive tanycyte-like cells in the parenchyma of the ventromedial and arcuate nuclei, and in thyrotropin beta subunit-expressing cells of the pars tuberalis of the pituitary. In contrast to adults, Prss56 expression was uniformly high in tanycytes in adolescent rats. In mice, Prss56-expressing tanycytes and parenchymal cells were also observed but fewer in number and without significant variations. The results identify Prss56 as a second gene that is expressed variably in tanycytes of adult rats. We propose that the variable, inversely correlating expression of Prss56 and Pomc reflect periodically oscillating gene expression in tanycytes rather than stable expression levels that vary between individual rats. A possible functional link between Prss56 and POMC, and Prss56 as a potential marker for migrating tanycytes are discussed.

[When A is not A anymore: problems and pitfalls with blood group typing]

Correct blood group typing is a prerequisite for transfusion. In most cases blood group determination is without problems; however, in individual cases various factors can complicate blood group determination and sometimes lead to confusing findings. For a better understanding the clinician should have basic knowledge of blood typing. Blood group determination usually covers the AB0 blood groups, Rhesus and Kell systems; in addition, a direct Coombs test and an antibody screening test for the detection of irregular antibodies in the recipient are performed. Confusion of patients, blood samples, results or preparations can lead to severe consequences due to incompatible transfusion and must be prevented. In this context, bedside blood type testing before transfusion is of utmost importance. Problems in laboratory analysis as well as patient-related factors, such as the existence of irregular antibodies against red blood cells can complicate the immunohematology diagnostics. Certain medications, such as daratumumab, lead to a significantly increased complexity in laboratory analyses. Massive transfusions can lead to chimerism with more than one population of circulating red blood cells. Hematopoetic stem cell transplantation can also lead to a change in blood groups as well as chimerism. In addition, there are various other rare causes that can result in difficulties in blood group determination, such as rare blood groups or rare disease-associated phenomena. In the case of problems in blood group determination, early and close cooperation with transfusion medicine is essential for the clinician.

Variable proopiomelanocortin expression in tanycytes of the adult rat hypothalamus and pituitary stalk

It is generally believed that proopiomelanocortin (POMC) is expressed exclusively by neurons in the adult rodent brain. Unbeknownst to most researchers, however, Pomc in situ hybridization studies in the rat show specific labeling in the ventral wall of the hypothalamic third ventricle, which is formed by specialized ependymal cells, called tanycytes. Here we characterized this non-neuronal POMC expression in detail using in situ hybridization and immunohistochemical techniques, and report two unique characteristics. First, POMC mRNA and precursor protein expression in non-neuronal cells varies to a great degree as to the extent and abundance of expression. In brains with low-level expression, POMC mRNA and protein was largely confined to a population of tanycytes within the infundibular stalk/caudal median eminence, termed here γ tanycytes, and a subset of closely located β and α2 tanycytes. In brains with high-level expression, POMC mRNA and protein was observed in the vast majority of α2, β, and γ tanycytes. This variability was observed in both adult males and females; of 41 rats between 8 and 15 weeks of age, 17 had low-, 9 intermediate-, and 15 high-level POMC expression in tanycytes. Second, unlike other known POMC-expressing cells, tanycytes rarely contained detectable levels of adrenocorticotropin or α-melanocyte-stimulating hormone. The results indicate either a dynamic spatiotemporal pattern whereby low and high POMC syntheses in tanycytes occur periodically in each brain, or marked interindividual differences that may persist throughout adulthood. Future studies are required to examine these possibilities and elucidate the physiologic importance of POMC in tanycytes. J. Comp. Neurol. 525:411-441, 2017. © 2016 Wiley Periodicals, Inc.

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice

Background

We recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.

Methods

LPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.

Results

In both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.

Conclusions

The results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Parallel regulation of thyroid hormone transporters OATP1c1 and MCT8 during and after endotoxemia at the blood-brain barrier of male rodents

There is increasing evidence that local thyroid hormone (TH) availability changes profoundly in inflammatory conditions due to altered expression of deiodinases that metabolize TH. It is largely unknown, however, how inflammation affects TH availability via the expression of TH transporters. In this study we examined the effect of bacterial lipopolysaccharide (LPS) administration on two TH transporters that are critically important for brain TH homeostasis, organic anion-transporting polypeptide 1c1 (OATP1c1), and monocarboxylate transporter 8 (MCT8). MRNA levels were studied by in situ hybridization and qPCR as well as protein levels by immunofluorescence in both the rat and mouse forebrain. The mRNA of both transporters decreased robustly in the first 9 hours after LPS injection, specifically in brain blood vessels; OATP1c1 mRNA in astrocytes and MCT8 mRNA in neurons remained unchanged. At 24 and/or 48 hours after LPS administration, OATP1c1 and MCT8 mRNAs increased markedly above control levels in brain vessels. OATP1c1 protein decreased markedly in vessels by 24 hours whereas MCT8 protein levels did not decrease significantly. These changes were highly similar in mice and rats. The data demonstrate that OATP1c1 and MCT8 expression are regulated in a parallel manner during inflammation at the blood-brain barrier of rodents. Given the indispensable role of both transporters in allowing TH access to the brain, the results suggest reduced brain TH uptake during systemic inflammation.

Inflammation-inducible type 2 deiodinase expression in the leptomeninges, choroid plexus, and at brain blood vessels in male rodents

Thyroid hormone regulates immune functions and has antiinflammatory effects. In promoter assays, the thyroid hormone-activating enzyme, type 2 deiodinase (D2), is highly inducible by the inflammatory transcription factor nuclear factor-κ B (NF-κB), but it is unknown whether D2 is induced in a similar fashion in vivo during inflammation. We first reexamined the effect of bacterial lipopolysaccharide (LPS) on D2 expression and NF-κB activation in the rat and mouse brain using in situ hybridization. In rats, LPS induced very robust D2 expression in normally non-D2-expressing cells in the leptomeninges, adjacent brain blood vessels, and the choroid plexus. These cells were vimentin-positive fibroblasts and expressed the NF-κB activation marker, inhibitor κ B-α mRNA, at 2 hours after injection, before the increase in D2 mRNA. In mice, LPS induced intense D2 expression in the choroid plexus but not in leptomeninges, with an early expression peak at 2 hours. Moderate D2 expression along numerous brain blood vessels appeared later. D2 and NF-κB activation was induced in tanycytes in both species but with a different time course. Enzymatic assays from leptomeningeal and choroid plexus samples revealed exceptionally high D2 activity in LPS-treated rats and Syrian hamsters and moderate but significant increases in mice. These data demonstrate the cell type-specific, highly inducible nature of D2 expression by inflammation, and NF-κB as a possible initiating factor, but also warrant attention for species differences. The results suggest that D2-mediated T₃ production by fibroblasts regulate local inflammatory actions in the leptomeninges, choroid plexus and brain blood vessels, and perhaps also in other organs.

Distinct glutamatergic and GABAergic subsets of hypothalamic pro-opiomelanocortin neurons revealed by in situ hybridization in male rats and mice

Pro-opiomelanocortin (POMC) and agouti-related protein (AGRP) neurons in the hypothalamus regulate various aspects of energy homeostasis and metabolism. POMC and AGRP neurons, respectively, agonize and antagonize melanocortin receptors on their common downstream neurons. However, it is unknown whether they also reciprocally stimulate and inhibit the same neurons by amino acid transmitters. Whereas AGRP neurons are mostly GABAergic, surprisingly, only a small population of POMC neurons has been found to be glutamatergic, and a significantly larger subpopulation to be GABAergic. To further examine amino acid phenotypes of POMC neurons, we studied mRNA expression for the glutamatergic marker, type 2 vesicular glutamate transporter (VGLUT2), and the GABA synthetic enzyme, glutamic acid decarboxylase 67 (GAD67), in POMC neurons of both rats and mice by using in situ hybridization techniques. In rats, approximately 58% of POMC neurons were labeled for VGLUT2 and 37% for GAD67 mRNA. In mice, approximately 43% of POMC neurons contained VGLUT2, and 54% contained GAD67 mRNA. In both species, a prominent mediolateral distribution pattern was observed at rostral and mid levels of the POMC cell group with VGLUT2-POMC neurons dominating in lateral portions and GAD67-POMC neurons in medial portions. These data demonstrate that both glutamatergic and GABAergic cells are present in comparably significant numbers among POMC neurons. Their glutamatergic or GABAergic phenotype may represent a major functional division within the POMC cell group.

Activation of anorexigenic pro-opiomelanocortin neurones during refeeding is independent of vagal and brainstem inputs

After fasting, satiety is observed within 2 h after reintroducing food, accompanied by activation of anorexigenic, pro-opiomelanocortin (POMC)-synthesising neurones in the arcuate nucleus (ARC), indicative of the critical role that α-melanocyte-stimulating hormone has in the regulation of meal size during refeeding. To determine whether refeeding-induced activation of POMC neurones in the arcuate is dependent upon the vagus nerve and/or ascending brainstem pathways, bilateral subdiaphragmatic vagotomy or transection of the afferent brainstem input to one side of the ARC was performed. One day after vagotomy or 2 weeks after brain surgery, animals were fasted and then refed for 2 h. Sections containing the ARC from vagotomised animals or animals with effective transection were immunostained for c-Fos and POMC to detect refeeding-induced activation of POMC neurones. Quantitative analyses of double-labelled preparations demonstrated that sham-operated and vagotomised animals markedly increased the number of c-Fos-immunoreactive (-IR) POMC neurones with refeeding. Furthermore, transection of the ascending brainstem pathway had no effect on diminishing c-Fos-immunoreactivity in POMC neurones on either side of the ARC, although it did diminish activation in a separate, subpopulation of neurones in the dorsomedial posterior ARC (dmpARC) on the transected side. We conclude that inputs mediated via the vagus nerve and/or arising from the brainstem do not have a primary role in refeeding-induced activation of POMC neurones in the ARC, and propose that these neurones may be activated solely by direct effects of circulating hormones/metabolites during refeeding. Activation of the dmpARC by refeeding indicates a previously unrecognised role for these neurones in appetite regulation in the rat.

Distribution of type 1 cannabinoid receptor-expressing neurons in the septal-hypothalamic region of the mouse: colocalization with GABAergic and glutamatergic markers

Type 1 cannabinoid receptor (CB1) is the principal mediator of retrograde endocannabinoid signaling in the brain. In this study, we addressed the topographic distribution and amino acid neurotransmitter phenotype of endocannabinoid-sensitive hypothalamic neurons in mice. The in situ hybridization detection of CB1 mRNA revealed high levels of expression in the medial septum (MS) and the diagonal band of Broca (DBB), moderate levels in the preoptic area and the hypothalamic lateroanterior (LA), paraventricular (Pa), ventromedial (VMH), lateral mammillary (LM), and ventral premammillary (PMV) nuclei, and low levels in many other hypothalamic regions including the suprachiasmatic (SCh) and arcuate (Arc) nuclei. This regional distribution pattern was compared with location of γ-aminobutyric acid (GABA)ergic and glutamatergic cell groups, as identified by the expression of glutamic acid decarboxylase 65 (GAD65) and type 2 vesicular glutamate transporter (VGLUT2) mRNAs, respectively. The MS, DBB, and preoptic area showed overlaps between GABAergic and CB1-expressing neurons, whereas hypothalamic sites with moderate CB1 signals, including the LA, Pa, VMH, LM, and PMV, were dominated by glutamatergic neurons. Low CB1 mRNA levels were also present in other glutamatergic and GABAergic regions. Dual-label in situ hybridization experiments confirmed the cellular co-expression of CB1 with both glutamatergic and GABAergic markers. In this report we provide a detailed anatomical map of hypothalamic glutamatergic and GABAergic systems whose neurotransmitter release is controlled by retrograde endocannabinoid signaling from hypothalamic and extrahypothalamic target neurons. This neuroanatomical information contributes to an understanding of the role that the endocannabinoid system plays in the regulation of endocrine and metabolic functions.

Refeeding-activated glutamatergic neurons in the hypothalamic paraventricular nucleus (PVN) mediate effects of melanocortin signaling in the nucleus tractus solitarius (NTS)

We previously demonstrated that refeeding after a prolonged fast activates a subset of neurons in the ventral parvocellular subdivision of the paraventricular nucleus (PVNv) as a result of increased melanocortin signaling. To determine whether these neurons contribute to satiety by projecting to the nucleus tractus solitarius (NTS), the retrogradely transported marker substance, cholera toxin-β (CTB), was injected into the dorsal vagal complex of rats that were subsequently fasted and refed for 2 h. By double-labeling immunohistochemistry, CTB accumulation was found in the cytoplasm of the majority of refeeding-activated c-Fos neurons in the ventral parvocellular subdivision of the hypothalamic paraventricular nucleus (PVNv). In addition, a large number of refeeding-activated c-Fos-expressing neurons were observed in the lateral parvocellular subdivision (PVNl) that also contained CTB and were innervated by axon terminals of proopiomelanocortin neurons. To visualize the location of neuronal activation within the NTS by melanocortin-activated PVN neurons, α-MSH was focally injected into the PVN, resulting in an increased number of c-Fos-containing neurons in the PVN and in the NTS, primarily in the medial and commissural parts. All refeeding-activated neurons in the PVNv and PVNl expressed the mRNA of the glutamatergic marker, type 2 vesicular glutamate transporter (VGLUT2), indicating their glutamatergic phenotype, but only rare neurons contained oxytocin. These data suggest that melanocortin-activated neurons in the PVNv and PVNl may contribute to refeeding-induced satiety through effects on the NTS and may alter the sensitivity of NTS neurons to vagal satiety inputs via glutamate excitation.

Distribution of hypophysiotropic thyrotropin-releasing hormone (TRH)-synthesizing neurons in the hypothalamic paraventricular nucleus of the mouse

Hypophysiotropic thyrotropin-releasing hormone (TRH) neurons, the central regulators of the hypothalamic-pituitary-thyroid axis, are located in the hypothalamic paraventricular nucleus (PVN) in a partly overlapping distribution with non-hypophysiotropic TRH neurons. The distribution of hypophysiotropic TRH neurons in the rat PVN is well understood, but the localization of these neurons is unknown in mice. To determine the distribution and phenotype of hypophysiotropic TRH neurons in mice, double- and triple-labeling experiments were performed on sections of intact mice, and mice treated intravenously and intraperitoneally with the retrograde tracer Fluoro-Gold. TRH neurons were located in all parts of the PVN except the periventricular zone. Hypophysiotropic TRH neurons were observed only at the mid-level of the PVN, primarily in the compact part. In this part of the PVN, TRH neurons were intermingled with oxytocin and vasopressin neurons, but based on their size, the TRH neurons were parvocellular and did not contain magnocellular neuropeptides. Co-localization of TRH and cocaine- and amphetamine-regulated transcript (CART) were observed only in areas where hypophysiotropic TRH neurons were located. In accordance with the morphological observations, hypothyroidism increased TRH mRNA content of neurons only at the mid-level of the PVN. These data demonstrate that the distribution of hypophysiotropic TRH neurons in mice is vastly different from the pattern in rats, with a dominant occurrence of these neurosecretory cells in the compact part and adjacent regions at the mid-level of the PVN. Furthermore, our data demonstrate that the organization of the PVN is markedly different in mice and rats.

Inhibition of the type 2 iodothyronine deiodinase underlies the elevated plasma TSH associated with amiodarone treatment

The widely prescribed cardiac antiarrhythmic drug amiodarone (AMIO) and its main metabolite, desethylamiodarone (DEA), have multiple side effects on thyroid economy, including an elevation in serum TSH levels. To study the AMIO effect on TSH, mice with targeted disruption of the type 2 deiodinase gene (D2KO) were treated with 80 mg/kg AMIO for 4 wk. Only wild-type (WT) mice controls developed the expected approximate twofold rise in plasma TSH, illustrating a critical role for D2 in this mechanism. A disruption in the D2 pathway caused by AMIO could interfere with the transduction of the T4 signal, generating less T3 and softening the TSH feedback mechanism. When added directly to sonicates of HEK-293 cells transiently expressing D2, both AMIO and DEA behaved as noncompetitive inhibitors of D2 [IC(50) of >100 μm and ∼5 μm, respectively]. Accordingly, D2 activity was significantly decreased in the median eminence and anterior pituitary sonicates of AMIO-treated mice. However, the underlying effect on TSH is likely to be at the pituitary gland given that in AMIO-treated mice the paraventricular TRH mRNA levels (which are negatively regulated by D2-generated T3) were decreased. In contrast, AMIO and DEA both exhibited dose-dependent inhibition of D2 activity and elevation of TSH secretion in intact TαT1 cells, a pituitary thyrotroph cell line used to model the TSH feedback mechanism. In conclusion, AMIO and DEA are noncompetitive inhibitors of D2, with DEA being much more potent, and this inhibition at the level of the pituitary gland contributes to the rise in TSH seen in patients taking AMIO.

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

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

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.

The E3 ubiquitin ligase TEB4 mediates degradation of type 2 iodothyronine deiodinase

The endoplasmic reticulum resident thyroid hormone-activating type 2 deiodinase (D2) is inactivated by ubiquitination via the hedgehog-inducible WSB-1. Ubiquitinated D2 can then be subsequently taken up by the proteasomal system or be reactivated by USP-33/20-mediated deubiquitination. Given that heterologously expressed D2 accumulates in Saccharomyces cerevisiae lacking the E3 ligase Doa10, we tested whether the human Doa10 ortholog, TEB4, plays a role in D2 ubiquitination and degradation. In a setting of transient coexpression in HEK-293 cells, TEB4 and D2 could be coimmunoprecipitated, and additional TEB4 expression decreased D2 activity by approximately 50% (P < 0.05). A highly efficient TEB4 knockdown (>90% reduction in mRNA and protein levels) decreased D2 ubiquitination and increased D2 activity and protein levels by about fourfold. The other activating deiodinase, D1, or a truncated D2 molecule (Delta18-D2) that lacks a critical instability domain was not affected by TEB4 knockdown. Furthermore, TEB4 knockdown prolonged D2 activity half-life at least fourfold, even under conditions known to promote D2 ubiquitination. Neither exposure to 1 microM of the proteasomal inhibitor MG132 for 24 h nor RNA interference WSB-1 knockdown resulted in additive effects on D2 expression when combined with TEB4 knockdown. Similar results were obtained with MSTO-211 cells, which endogenously express D2, after TEB4 knockdown using a lentivirus-based transduction strategy. While TEB4 expression predominates in the hematopoietic lineage, both WSB-1 and TEB4 are coexpressed with D2 in a number of tissues and cell types, except the thyroid and brown adipose tissue, where TEB4 expression is minimal. We conclude that TEB4 interacts with and mediates loss of D2 activity, indicating that D2 ubiquitination and degradation can be tissue specific, depending on WSB-1 and TEB4 expression levels.

Improved method for combination of immunocytochemistry and Nissl staining

Nissl staining is a widely used method to study morphology and pathology of neural tissue. After standard immunocytochemistry, the Nissl staining labels only the nucleus of neurons and the characteristic staining of the neuronal perikarya is absent or very weak. We hypothesized that the RNA degradation during the immunocytochemical treatment results in the loss of cytoplasmic staining with Nissl-dyes. To test this hypothesis, we used RNAse-free conditions for all steps of immunostaining. To further prevent the RNA-degradation by RNAse contaminations, the RNAse inhibitor heparin was added to all antibody-containing solutions. The efficiency of Nissl staining after standard and RNAse-free double-labeling immunocytochemistry was compared using antibodies against c-Fos and neuropeptide Y (NPY) on tissues of rats refed after 3 days of fasting. After standard immunocytochemistry, the Nissl-staining labeled the nuclei of neurons and only very faintly the cytoplasm of these cells. The RNAse-free treatment did not alter the distribution of immunoreaction signal, but preserved the staining of neuronal perikarya by the Nissl-dyes. In conclusion, the RNAse-free conditions during immunocytochemistry allow the labeling of neuronal perikarya by Nissl-dyes. The described method facilitates the mapping of immunocytochemical signals and makes possible the light microscopic examination of the innervation of neurons identified by their nuclear protein content.

Efferent projections of thyrotropin-releasing hormone-synthesizing neurons residing in the anterior parvocellular subdivision of the hypothalamic paraventricular nucleus

The anterior parvocellular subdivision of the PVN (aPVN) contains nonhypophysiotropic thyrotropin-releasing hormone (TRH) neurons that are densely innervated by feeding-related neuronal groups of the hypothalamic arcuate nucleus. To determine how these TRH neurons are integrated within the brain, the major projection fields of this cell group were studied by anterograde and retrograde tract-tracing methods. Projection sites were identified by injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHAL) into the aPVN, and subsequent double immunofluorescent staining was used to visualize axons containing both PHAL and pro-TRH. To distinguish between the projection sites of TRH neurons residing in the aPVN and the closely situated perifornical area, the retrograde tracer cholera toxin B subunit (CTB) was injected into regions where PHAL/pro-TRH-containing axons were densely accumulated. TRH neurons in the aPVN were found to project to the hypothalamic arcuate, dorsomedial and ventral premammillary nuclei, medial preoptic region, tuber cinereum area, paraventricular thalamic nucleus, bed nucleus of the stria terminalis, lateral septal nucleus, and central amygdaloid nucleus. Projection fields of perifornical TRH neurons were in partial overlap with those of the aPVN TRH cells. In addition, these neurons also innervated the hypothalamic ventromedial nucleus, the medial amygdaloid nucleus, and the amygdalohippocampal area. The data suggest that, through its efferent connections, aPVN TRH neurons may be involved in the regulation of energy homeostasis coordinately with effects on behavior, locomotor activity, and thermogenesis. In addition, the major differences in the projection fields of aPVN and perifornical TRH neurons suggest that these two TRH-synthesizing neuronal groups are functionally different.

Type 1 cannabinoid receptor-containing axons innervate hypophysiotropic thyrotropin-releasing hormone-synthesizing neurons

Hypophysiotropic TRH-synthesizing neurons of the hypothalamic paraventricular nucleus (PVN) have a critical role in the regulation of the energy homeostasis through control of the hypothalamic-pituitary-thyroid axis. Recently, endocannabinoids have been shown to exert inhibitory effects on TRH neurons via the type 1 cannabinoid receptor (CB1). To understand the anatomical basis for this regulatory mechanism, we determined whether CB1 is contained in axons innervating hypophysiotropic TRH neurons using a recently developed antiserum against the C-terminal portion of mouse CB1. CB1-immunoreactive axons densely innervated the parvicellular subdivisions of the PVN where the hypophysiotropic TRH neurons are located. By double-labeling immunocytochemistry, CB1-immunoreactive varicosities were observed in juxtaposition to the vast majority of TRH neurons in the PVN. At the ultrastructural level, CB1-immunoreactivity was observed in the preterminal portion of axons establishing both symmetric and asymmetric synaptic specializations with the perikarya and dendrites of TRH neurons in the PVN. These data demonstrate that CB1 is abundantly present in axons that are in synaptic association with hypophysiotropic TRH neurons, indicating an important role for endocannabinoids in the regulation of the hypothalamic-pituitary-thyroid axis. The presence of both symmetric and asymmetric type CB1 synapses on TRH neurons in the PVN suggests that endocannabinoids may influence both excitatory and inhibitory inputs of these neurons.

Regulation of cocaine- and amphetamine-regulated transcript-synthesising neurons of the hypothalamic paraventricular nucleus by endotoxin; implications for lipopolysaccharide-induced regulation of energy homeostasis

Infectious diseases and the administration of bacterial lipopolysaccharide (LPS) result in decreased food intake and increased energy expenditure. Because the hypothalamic paraventricular nucleus (PVN) has pivotal roles in the regulation of energy homeostasis and expresses an anorexic peptide, cocaine- and amphetamine-regulated transcript (CART), we hypothesised that increased CART synthesis in this nucleus may contribute to LPS-induced changes in energy homeostasis. Therefore, we studied the effects of intraperitoneal administration of LPS on CART gene expression in the PVN by semiquantitative in situ hybridisation. LPS caused a rapid increase in CART mRNA levels in the PVN. One hour after treatment, the density of silver grains was increased by three-fold in the PVN, and remained elevated 3 h after treatment. Because the dorsal vagal complex, an important vegetative centre in the brainstem, is heavily innervated by CART-containing axons, we determined whether the retrograde tracer, cholera toxin B subunit (CTB), accumulates in CART neurons in the PVN following stereotaxic injection of the tracer into the dorsal vagal complex. One week after injection, CTB accumulated in CART neurons in the ventral, medial, and lateral parvocellular subdivisions of the PVN. In addition, LPS administration induced c-fos expression in a population of CART neurons in the PVN that project to the dorsal vagal complex. These data indicate that increased CART gene expression in neurons of PVN may contribute to LPS-induced anorexia, and suggest that this action may be mediated, at least in part, through a PVN-dorsal vagal complex pathway.

Regulation of hypophysiotrophic corticotrophin-releasing hormone- and thyrotrophin-releasing hormone-synthesising neurones by brainstem catecholaminergic neurones

Hypophysiotrophic corticotrophin-releasing hormone (CRH)- and thyrotrophin-releasing hormone (TRH)-synthesising neurones are the principal hypothalamic regulators of glucocorticoid and thyroid hormone secretion, respectively. These two neuroendocrine cell populations are closely situated in the hypothalamic paraventricular nucleus and are targets of neuronal afferent pathways that convey important signals for adapting the neurosecretory activity of CRH and TRH neurones to actual demands. The catecholaminergic afferents of CRH and TRH neurones originate from both noradrenaline- and adrenaline-synthesising cell groups located in the brainstem, and collectively represent one of the most well studied neural inputs of these neurones. The present review summarises the data obtained in recent years concerning the functional significance of the catecholaminergic innervation of hypophysiotrophic CRH and TRH neurones in rats.