Mice with Deletion of Neuromedin B Receptor Exhibit Decreased Oral Glucose-Stimulated Insulin Release

Neuromedin B (NB) and gastrin-releasing peptide (GRP) are bombesin-like peptides, found in the gastrointestinal tube and pancreas, among other tissues. Consistent data proposed that GRP stimulates insulin secretion, acting directly in pancreatic cells or in the release of gastrointestinal hormones that are incretins. However, the role of NB remains unclear. We examined the glucose homeostasis in mice with deletion of NB receptor (NBR-KO). Female NBR-KO exhibited similar fasting basal glucose with lower insulinemia (48.4%) and lower homeostasis model assessment of insulin resistance index (50.5%) than wild type (WT). Additionally, they were more tolerant to oral glucose, demonstrated by a decrease in the area under the glucose curve (18%). In addition, 15 min after an oral glucose load, female and male NBR-KO showed lower insulin serum levels (45.6 and 26.8%, respectively) than WT, even though blood glucose rose to similar levels in both groups. Single injection of NB, one hour before the oral glucose administration, tended to induce higher serum insulin in WT (28.9%, p=0.3), however the same did not occur in NBR-KO. They showed no changes in fasting insulin content in pancreatic islets by immunohistochemistry, however, the fasting serum levels of glucagon-like peptide, a potent incretin, exhibited a strong trend to reduction (40%, p=0.07). Collectively, mice with deletion of NB receptor have lower insulinemia, especially in response to oral glucose, and females also exhibited a better glucose tolerance, suggesting the involvement of NB and its receptor in regulation of insulin secretion induced by incretins, and also, in insulin sensitivity.

Blocking leptin action one week after weaning reverts most of the programming caused by neonatal hyperleptinemia in the adult rat

Hyperleptinemia during lactation programs for higher serum leptin in 30-day-old and adult rats, associated with metabolic changes. Here we evaluated the inhibition of serum leptin at 29 and 30 days on the metabolic phenotype of rats programmed with leptin during lactation. Pups from Wistar rats were saline-injected or leptin-injected from postnatal day 1 to day 10. At 29 and 30 days old, animals were injected with anti-leptin antibody (LA and CA) or saline (LS and CS). In adult animals, higher visceral (+53%) and total fat mass (+33%), hyperleptinemia (+67%), hypertriglyceridemia (+47%), and hypoadiponectinemia (-44%) observed in LS group compared to CS were prevented by immunoneutralization of leptin, since LA group had those parameters values similar to CS group. However, immunoblockade of leptin in normal animals led to the same metabolic changes seen in leptin-treated animals, in addition to lower serum adiponectin (-77% vs. CS) and higher insulin resistance index (+37%). Liver sirtuin1 (SIRT1) was higher (+41%) only in LA group, suggesting a role for SIRT1 in the prevention of leptin programming. Hypothalamic OBR was lower and SOCS3 higher in LS group and these changes were normalized in LA group. In conclusion, blocking leptin action one week after weaning seems to revert most of the alterations observed in rats programmed by neonatal hyperleptinemia. Higher liver SIRT1 expression may be one of the mechanisms involved, leading to a better glucose and lipid metabolism. Our data suggest that the lack or the excess of leptin programs an adverse metabolic phenotype in adulthood.

Peptide YY (PYY)3-36 modulates thyrotropin secretion in rats

Peptide YY (PYY)3-36 is a gut-derived hormone, with a proposed role in central mediation of postprandial satiety signals, as well as in long-term energy balance. In addition, recently, the ability of the hormone to regulate gonadotropin secretion, acting at pituitary and at hypothalamus has been reported. Here, we examined PYY3-36 effects on thyrotropin (TSH) secretion, both in vitro and in vivo. PYY3-36-incubated rat pituitary glands showed a dose-dependent decrease in TSH release, with 44 and 62% reduction at 10(-8) and 10(-6) M (P < 0.05 and P < 0.001 respectively), and no alteration in TSH response to thyrotropin-releasing hormone. In vivo, PYY3-36 i.p. single injection in the doses of 3 or 30 cg/kg body weight, administered to rats fed ad libitum, was not able to change serum TSH after 15 or 30 min. However, in fasted rats, PYY3-36 at both doses elicited a significant rise (approximately twofold increase, P < 0.05) in serum TSH observed 15 min after the hormone injection. PYY3-36 treatment did not modify significantly serum T4, T3, or leptin. Therefore, in the present paper, we have demonstrated that the gut hormone PYY3-36 acts directly on the pituitary gland to inhibit TSH release, and in the fasting situation, in vivo, when serum PYY3-36 is reduced, the activity of thyroid axis is reduced as well. In such a situation, systemically injected PYY3-36 was able to acutely activate the thyrotrope axis, suggesting a new role for PYY3-36 as a regulator of the hypothalamic-pituitary-thyroid axis.

Disruption of neuromedin B receptor gene results in dysregulation of the pituitary-thyroid axis

The level of thyrotropin (TSH) secretion is determined by the balance of TSH-releasing hormone (TRH) and thyroid hormones. However, neuromedin B (NB), a bombesin-like peptide, highly concentrated in the pituitary, has been postulated to be a tonic inhibitor of TSH secretion. We studied the pituitary-thyroid axis in adult male mice lacking NB receptor (NBR-KO) and their wild-type (WT) littermates. At basal state, NBR-KO mice presented serum TSH slightly higher than WT (18%, P< 0.05), normal intra-pituitary TSH content, and no significant changes in alpha and beta TSH mRNA levels. Serum thyroxine was normal but serum triiodothyronine (T3) was reduced by 24% (P< 0.01) in NBR-KO mice. Pituitaries of NBR-KO mice exhibited no alteration in prolactin mRNA expression but type I and II deiodinase mRNA levels were reduced by 53 and 42% respectively (P< 0.05), while TRH receptor mRNA levels were importantly increased (78%, P< 0.05). The TSH-releasing effect of TRH was significantly higher in NBR-KO than in WT mice (7.1-and 4.0-fold respectively), but, while WT mice presented a 27% increase in serum T3 (P< 0.05) after TRH, NBR-KO mice showed no change in serum T3 after TRH. NBR-KO mice did not respond to exogenous NB, while WT showed a 30% reduction in serum TSH. No compensatory changes in mRNA expression of NB or other bombesin-related peptides and receptors (gastrin-releasing peptide (GRP), GRP-receptor and bombesin receptor subtype-3) were found in the pituitary of NBR-KO mice. Therefore, the data suggest that NB receptor pathways are importantly involved in thyrotroph gene regulation and function, leading to a state where TSH release is facilitated especially in response to TRH, but probably with a less-bioactive TSH. Therefore, the study highlights the important role of NB as a physiological regulator of pituitary-thyroid axis function and gene expression.

Inhibition of thyroid type 1 deiodinase activity by flavonoids

Some dietary flavonoids inhibit thyroperoxidase and hepatic deiodinase activity, indicating that these compounds could be classified as anti-thyroid agents. In this study, we evaluated the in vitro effect of various flavonoids on thyroid type 1 iodothyronine deiodinase activity (D1). D1 activity was measured in murine thyroid microsome fractions by the release of 125I from 125I-reverse T3. D1 activity was significantly inhibited by all the flavonoids tested; however, the inhibitory potencies on thyroid D1 activity differed greatly among them. A 50% inhibition of D1 activity (IC(50)) was obtained at 11 microM baicalein, 13 microM quercetin, 17 microM catechin, 55 microM morin, 68 microM rutin, 70 microM fisetin, 72 microM kaempferol and 77 microM biochanin A. Our data reinforce the concept that dietary flavonoids might behave as antithyroid agents, and possibly their chronic consumption could alter thyroid function.

The role of leptin in the regulation of TSH secretion in the fed state: in vivo and in vitro studies

Leptin has been shown to stimulate the hypothalamus-pituitary-thyroid axis in fasting rodents; however, its role in thyroid axis regulation under physiological conditions is still under investigation. Here it was investigated in freely fed rats whether leptin modulates thyrotroph function in vivo and whether leptin has direct pituitary effects on TSH release. Since leptin is produced in the pituitary, the possibility was also investigated that leptin may be a local regulator of TSH release. TSH was measured by specific RIA. Freely fed adult rats 2 h after being injected with a single s.c. injection of 8 microg leptin/100 g body weight showed a 2-fold increase in serum TSH (P<0.05). Hemi-pituitary explants incubated with 10(-9) and 10(-7) M leptin for 2 h showed a reduced TSH release of 40 and 50% respectively (P<0.05). Conversely, incubation of hemi-pituitary explants with antiserum against leptin, aiming to block the action of locally produced leptin, resulted in higher TSH release (45%, P<0.05). In conclusion, also in the fed state, leptin has an acute stimulatory effect on TSH release in vivo, acting probably at the hypothalamus. However, the direct pituitary effect of leptin is inhibitory and data also provide evidence that in the rat pituitary leptin may act as an autocrine/paracrine inhibitor of TSH release.

Sex steroids modulate rat anterior pituitary and liver iodothyronine deiodinase activities

In this study, we investigated the sex hormone regulation of 5'-iodothyronine deiodinase activity, which is responsible for enzymatic conversion of thyroxine into the bioactive form, triiodothyronine. Pituitary homogenates and liver microsomes from: 1) ovariectomized rats injected with 17-beta-estradiol benzoate and/or progesterone (0.7 and 250 microg/100 g body weight, respectively, subcutaneously, over 10 days); 2) male castrated rats treated or not with 0.4 mg/100 g body weight testosterone propionate, intramuscular, over 7 days, were assayed for type 1 and type 2 deiodinase activity in the pituitary. Enzyme activities were measured by release of (125)I from deiodination of (125)I reverse triiodothyronine under varying assay conditions. Estrogen stimulated anterior pituitary and liver type 1 deiodinase activity in ovariectomized rats (45 and 30 %, p < 0.05). Progesterone inhibited the liver enzyme (40 %, p < 0.05), and had no effect on the pituitary, but in both tissues, blocked estrogen stimulatory effect on type 1 deiodinase. In males, testosterone normalized the reduced liver type 1 deiodinase of castrated rats. However, in the pituitary, castration increased (50 %) type 1 deiodinase independent of testosterone treatment, suggesting the existence of a inhibitory testicular regulator of pituitary type 1 enzyme. Treatments did not alter pituitary type 2 deiodinase activity. In conclusion, gonads and sex steroids differentially modulate type 1 deiodinase activity in rat pituitary and liver.