It is well known that amino acids, one of the three major classes of nutrients, are involved in the metabolism of carbohydrates.1,2) Among various amino acids, L-leucine (leucine) and L-isoleucine (isoleucine) show potent activity for controlling blood glucose. In normal or cirrhotic rats, acute administration of leucine or isoleucine improves the glucose tolerance curve after an oral glucose challenge, 3,4) while chronic leucine supplementation reduces obesity and improves glucose metabolism in mice on a high-fat diet (HFD).5) The glucose-lowering activity of isoleucine beyond normoglycemia seems to be relatively mild and becomes saturated as its blood level increases.6) If the glucose-lowering activity of isoleucine in diabetic animals was as strong as in normal animals, this amino acid or another agent with the same target molecule could be a promising candidate as an anti-diabetic agent. Accordingly, we determined the acute effects of isoleucine in glucose-intolerant and diabetic mice. We also investigated the chronic effect of isoleucine supplementation on glucose metabolism in mice fed a highfat/high-sucrose diet. MATERIALS AND METHODS AnimalsThe study protocol was designed to comply with the relevant institutional guidelines and was approved by the Animal Care Committee of Ajinomoto Co., Inc. Male or female C57BL/6J mice (10-16 weeks) and male BKS.Cg-mϩ/ϩLepr db /J(ϩLeprdb/ϩLeprdb) mice (db/db mice, 7-8 weeks) were purchased from Charles River Japan (Yokohama, Japan). The animals were maintained in an airconditioned room (24Ϯ1°C) with a 12 : 12-h light-dark cycle and were given free access to regular chow (CRF-1; Oriental Yeast Co., Tokyo, Japan). In the HFD study, female C57BL/6J mice were fed control chow (D12450B; 4% w/w fat, Research Diet, New Brunswick, NJ, U.S.A.) or an HFD (D12492; 35% w/w fat, Research Diet) for 8 weeks before undergoing a glucose tolerance test. In the high-fat/high-sucrose (HFHS) diet study, female C57BL/6J mice also had free access to drinking water containing 20% sucrose without or with 1 or 2% of isoleucine for 6 weeks. Mice were deprived of food and had access to isoleucine-and sucrose-free water 18 h before undergoing a glucose tolerance test. Since it was reported that female C57BL/6J mice on a HFD develop impaired glucose tolerance after a relatively short period, 7) we used female mice for our glucose-intolerance model.Oral Glucose Tolerance Test Mice fasted for 18 h were given isoleucine orally, which was immediately followed by bolus administration of glucose (1 or 2 g/kg as indicated). Distilled water or 0.5% methylcellulose was given to mice in the control group. To determine the blood glucose and plasma insulin levels, approximately 25 ml of blood was taken from the tail vein at each time indicated. Blood glucose levels were measured by the glucose oxidase method using a Fuji Dri-Chem 5500 autoanalyzer (Fuji Medical Systems, Tokyo, Japan). Plasma was separated by centrifugation and the plasma insulin level was measured by an ELISA kit (Morinaga, Tokyo, Japan). The p...

Nutritional epidemiology shows that insufficient protein intake is related to senile dementia. The levels of protein intake in aged people are positively associated with memory function, and elderly people with high protein intake have a low risk of mild cognitive impairment. Although the beneficial roles of protein nutrition in maintaining brain function in aged people are well demonstrated, little is known about the mechanism by which dietary intake of protein affects memory and brain conditions. We fed aged mice a low protein diet (LPD) for 2 months, which caused behavioral abnormalities, and examined the nutritional effect of essential amino acid administration under LPD conditions. The passive avoidance test revealed that LPD mice demonstrated learning and memory impairment. Similarly, the LPD mice showed agitation and hyperactive behavior in the elevated plus maze test. Moreover, LPD mice exhibited decreased concentrations of gamma-aminobutyric acid (GABA), glutamate, glycine, dopamine, norepinephrine, serotonin and aspartate in the brain. Interestingly, oral administration of seven essential amino acids (EAAs; valine, leucine, isoleucine, lysine, phenylalanine, histidine, and tryptophan) to LPD mice, which can be a source of neurotransmitters, reversed those behavioral changes. The oral administration of EAAs restored the brain concentration of glutamate, which is involved in learning and memory ability and may be associated with the observed behavioral changes. Although the details of the link between decreased amino acid and neurotransmitter concentrations and behavioral abnormalities must be examined in future studies, these findings suggest the importance of dietary protein and essential amino acids for maintaining brain function.

The use of glycine as a therapeutic option for improving sleep quality is a novel and safe approach. However, despite clinical evidence of its efficacy, the details of its mechanism remain poorly understood. In this study, we investigated the site of action and sleep-promoting mechanisms of glycine in rats. In acute sleep disturbance, oral administration of glycine-induced non-rapid eye movement (REM) sleep and shortened NREM sleep latency with a simultaneous decrease in core temperature. Oral and intracerebroventricular injection of glycine elevated cutaneous blood flow (CBF) at the plantar surface in a dose-dependent manner, resulting in heat loss. Pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists AP5 and CGP78608 but not the glycine receptor antagonist strychnine inhibited the CBF increase caused by glycine injection into the brain. Induction of c-Fos expression was observed in the hypothalamic nuclei, including the medial preoptic area (MPO) and the suprachiasmatic nucleus (SCN) shell after glycine administration. Bilateral microinjection of glycine into the SCN elevated CBF in a dose-dependent manner, whereas no effect was observed when glycine was injected into the MPO and dorsal subparaventricular zone. In addition, microinjection of D-serine into the SCN also increased CBF, whereas these effects were blocked in the presence of L-701324. SCN ablation completely abolished the sleep-promoting and hypothermic effects of glycine. These data suggest that exogenous glycine promotes sleep via peripheral vasodilatation through the activation of NMDA receptors in the SCN shell.

Intake of specific seven essential amino acids affects the pathology of the brain.

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