Sweet Taste Receptor Serves to Activate Glucose- and Leptin-Responsive Neurons in the Hypothalamic Arcuate Nucleus and Participates in Glucose Responsiveness

The hypothalamic feeding center plays an important role in energy homeostasis. In the feeding center, whole-body energy signals including hormones and nutrients are sensed, processed, and integrated. As a result, food intake and energy expenditure are regulated. Two types of glucose-sensing neurons exist in the hypothalamic arcuate nucleus (ARC): glucose-excited neurons and glucose-inhibited neurons. While some molecules are known to be related to glucose sensing in the hypothalamus, the mechanism underlying glucose sensing in the hypothalamus are not fully understood. The sweet taste receptor is a heterodimer of taste type 1 receptor 2 (T1R2) and taste type 1 receptor 3 (T1R3) and senses sweet tastes. T1R2 and T1R3 receptors are distributed in multiple organs including the tongue, pancreas, adipose tissue, and hypothalamus. However, the role of sweet taste receptors in the ARC remains to be clarified. To examine the role of sweet taste receptors in the ARC, cytosolic Ca2+ concentration ([Ca2+]i) in isolated single ARC neurons were measured using Fura-2 fluorescent imaging. An artificial sweetener, sucralose at 10-5 M-10-2 M dose dependently increased [Ca2+]i in 12-16% of ARC neurons. The sucralose-induced [Ca2+]i increase was suppressed by a sweet taste receptor inhibitor, gurmarin. The sucralose-induced [Ca2+]i increase was inhibited under an extracellular Ca2+-free condition and in the presence of an L-type Ca2+ channel blocker, nitrendipine. Sucralose-responding neurons were activated by high-concentration of glucose. This response to glucose was markedly suppressed by gurmarin. More than half of sucralose-responding neurons were activated by leptin but not ghrelin. Percentage of proopiomelanocortin (POMC) neurons among sucralose-responding neurons and sweet taste receptor expressing neurons were low, suggesting that majority of sucralose-responding neurons are non-POMC neurons. These data suggest that sweet taste receptor-mediated cellular activation mainly occurs on non-POMC leptin-responding neurons and contributes to glucose responding. Endogenous sweet molecules including glucose may regulate energy homeostasis through sweet taste receptors on glucose-and leptin-responsive neurons in the ARC

Exogenous and endogenous ghrelin counteracts GLP-1 action to stimulate cAMP signaling and insulin secretion in islet β-cells

AbstractWe studied interactive effects of insulinotropic GLP-1 and insulinostatic ghrelin on rat pancreatic islets. GLP-1 potentiated glucose-induced insulin release and cAMP production in isolated islets and [Ca2+]i increases in single β-cells, and these potentiations were attenuated by ghrelin. Ghrelin suppressed [Ca2+]i responses to an adenylate cyclase activator forskolin. Moreover, GLP-1-induced insulin release and cAMP production were markedly enhanced by [d-lys3]-GHRP-6, a ghrelin receptor antagonist, in isolated islets. These results indicate that both exogenous and endogenous islet-derived ghrelin counteracts glucose-dependent GLP-1 action to increase cAMP production, [Ca2+]i and insulin release in islet β-cells, positioning ghrelin as a modulator of insulinotropic GLP-1

Nesfatin-1 evokes Ca2+ signaling in isolated vagal afferent neurons via Ca2+ influx through N-type channels

金沢大学医薬保健研究域医学系Nesfatin-1, processed from nucleobindin 2, is an anorexigenic peptide expressed in the brain and several peripheral tissues including the stomach and pancreas. Peripheral, as well as intracerebroventricular, administration of nesfatin-1 suppresses feeding behavior, though underlying mechanisms are unknown. In this study, we examined effects of nesfatin-1 on cytosolic Ca2+ concentration ([Ca2+]i) in the neurons isolated from the vagal afferent nodose ganglion of mice. Nesfatin-1 at 10-10-10-8 M increased [Ca2+]i in the isolated neurons in a concentration-dependent manner, and at 10-8 M it increased [Ca2+]i in 33 out of 263 (12.5%) neurons. These responses were inhibited under Ca2+-free conditions and by N-type Ca2+ channel blocker, ω-conotoxin GVIA. All the nesfatin-1-responsive neurons also exhibited [Ca2+]i responses to capsaicin and cholecystokinin-8. These results provide direct evidence that nesfatin-1 activates vagal afferent neurons by stimulating Ca2+ influx through N-type channels, demonstrating the machinery through which peripheral nesfatin-1 can convey signals to the brain. © 2009 Elsevier Inc. All rights reserved

Central Glucagon-like Peptide-1 Receptor Signaling via Brainstem Catecholamine Neurons Counteracts Hypertension in Spontaneously Hypertensive Rats

Glucagon-like peptide-1 receptor (GLP-1R) agonists, widely used to treat type 2 diabetes, reduce blood pressure (BP) in hypertensive patients. Whether this action involves central mechanisms is unknown. We here report that repeated lateral ventricular (LV) injection of GLP-1R agonist, liraglutide, once daily for 15 days counteracted the development of hypertension in spontaneously hypertensive rats (SHR). In parallel, it suppressed urinary norepinephrine excretion, and induced c-Fos expressions in the area postrema (AP) and nucleus tractus solitarius (NTS) of brainstem including the NTS neurons immunoreactive to dopamine beta-hydroxylase (DBH). Acute administration of liraglutide into fourth ventricle, the area with easy access to the AP and NTS, transiently decreased BP in SHR and this effect was attenuated after lesion of NTS DBH neurons with anti-DBH conjugated to saporin (anti-DBH-SAP). In anti-DBH-SAP injected SHR, the antihypertensive effect of repeated LV injection of liraglutide for 14 days was also attenuated. These findings demonstrate that the central GLP-1R signaling via NTS DBH neurons counteracts the development of hypertension in SHR, accompanied by attenuated sympathetic nerve activity

Reconstruction-Dependent Recovery from Anorexia and Time-Related Recovery of Regulatory Ghrelin System in Gastrectomized Rats

Gastrectomy reduces food intake and body weight (BW) hampering recovery of physical conditions. It also reduces plasma levels of stomach-derived orexigenic ghrelin. This study explored changes in orexigenic ghrelin system in rats receiving total gastrectomy with Billroth II (B-II) or Roux-en-Y (R-Y) method. Feeding and BW were reduced by gastrectomy and subsequently recovered to a greater extent with R-Y than B-II while plasma ghrelin decreased similarly. At postoperative 12th week, ghrelin contents increased in the duodenum and pancreas, plasma ghrelin levels increased upon fasting, and ghrelin injection promoted feeding but not in earlier periods. In summary, gastrectomized rats partially recover feeding and BW, in a reconstruction-dependent manner. At 12th week, ghrelin is upregulated in extra-stomach tissues, plasma ghrelin levels are physiologically regulated, and orexigenic effect of exogenous ghrelin is restored. This time-related recovery of ghrelin system may provide a strategy for promoting feeding, BW, and thereby physical conditions in gastrectomized patients

Activation of AMPK-Regulated CRH Neurons in the PVH is Sufficient and Necessary to Induce Dietary Preference for Carbohydrate over Fat

Food selection is essential for metabolic homeostasis and is influenced by nutritional state, food palatability, and social factors such as stress. However, the mechanism responsible for selection between a high-carbohydrate diet (HCD) and a high-fat diet (HFD) remains unknown. Here, we show that activation of a subset of corticotropin-releasing hormone (CRH)-positive neurons in the rostral region of the paraventricular hypothalamus (PVH) induces selection of an HCD over an HFD in mice during refeeding after fasting, resulting in a rapid recovery from the change in ketone metabolism. These neurons manifest activation of AMP-activated protein kinase (AMPK) during food deprivation, and this activation is necessary and sufficient for selection of an HCD over an HFD. Furthermore, this effect is mediated by carnitine palmitoyltransferase 1c (CPT1c). Thus, our results identify the specific neurons and intracellular signaling pathway responsible for regulation of the complex behavior of selection between an HCD and an HFD

NEUROTOXIC EFFECTS OF α-FLUORO-β-ALANINE (FBAL) AND FLUOROACETIC ACID (FA) ON DOGS

In order to clarify the mechanism of the neurotoxicity of 5-FU and/or its masked compounds, we studied the effects of α-fluoro-β-alanine (FBAL) and fluoroacetic acid (FA) on the formation of vacuolar changes in the dog cerebrum, using the dosage of 3.0 mg/kg/day of FBAL・HCl (FBAL・HCl) and 0.03 mg/kg/day of FA・Na (FA・Na), respectively. These 2 compounds were selected because they are the metabolites of 5-FU claimed to be responsible for the neurotoxic effects of 5-FU and/or its masked compounds, and we wanted to confirm their effects. Tegafur-uracil mixture (UFT) was used as a positive control drug for the formation of vacuolar changes in the dog cerebrum. All compounds were orally administered daily for 3 months to beagle dogs. Each study group consisted of 3 males. Neurotoxic signs such as hyperesthesia and/or excitement, as well as convulsions, were observed in both FBAL・HCl and FA・Na groups ; these toxic signs were also found in the UFT group. Slight loss of body weight gain and of food consumption was observed in the FBAL・HCl and UFT groups. Neuropathologically, vacuolar changes were detected in several areas of the dog cerebrum following administration of FBAL・HCl, FA・Na or UFT. In terms of morphology, the neuropathological effects of these 2 drugs were very similar to those induced by UFT. In conclusion, we clearly showed that FBAL is one of the main substances that cause neurotoxic signs and neuropathological changes in dogs intoxicated by 5-FU or its masked compounds. Moreover, FA might be considered to be a causative factor in addition to FBAL