Insulin activates microglia and increases COX-2/IL-1ß expression in young but not in aged hippocampus.

Brain insulin resistance and neuroinflammation are known to increase with age. Insulin exerts metabolic roles on neurons and astrocytes, but its effects on microglia is unclear. In this study we investigated whether insulin affected microglia in the hippocampus of young and aged rats. We injected intracerebroventricular (i.c.v.) insulin (20 mU) or vehicle for five days and evaluated microglial inflammatory markers in the hippocampus of young (3 months) Wistar rats. Increased microglial activation (Iba-1+CD68+cells) and COX-2/IL-1ß levels in the hippocampus were found. Since the aged brain is an experimental model for brain insulin resistance and chronic neuroinflammation we submitted aged rats (22 months) to i.c.v. insulin/vehicle administration and found no significant increase in Iba-1+CD68+ microglia or COX-2/IL-1ß levels. To further investigate whether insulin triggered transient or persistent proinflammatory responses, young rats were evaluated eight-days after the last insulin injection. Microglia were persistently activated, and COX-2 levels remained elevated in the hippocampus, which paralleled increased spatial memory performance in the Morris Water Maze behavioral task. To determine if microglia were directly responsive to insulin, primary microglia were challenged with insulin and increased Akt Ser473 phosphorylation, a protein activated by the insulin receptor, was detected. These data suggest that microglia in the hippocampus integrate insulin signaling and neuroinflammatory responses and that this signal is disrupted during chronic inflammation. In our concept, the disruption between microglia activation by insulin signaling is a new pathological mechanism behind insulin resistance in the aging brain.

Hyperpalatable Diet and Physical Exercise Modulate the Expression of the Glial Monocarboxylate Transporters MCT1 and 4.

Hyperpalatable diets (HP) impair brain metabolism, and regular physical exercise has an apparent opposite effect. Here, we combined a prior long-term exposure to HP diet followed by physical exercise and evaluated the impact on some neuroenergetic components and on cognitive performance. We assessed the extracellular lactate concentration, expression of monocarboxylate transporters (MCTs), pyruvate dehydrogenase (PDH), and mitochondrial function in the hippocampus. Male C57BL/6J mice were fed 4 months with HP or a control diet. Subsequently, they were divided in the following groups: control diet sedentary (CDS), control diet exercise (CDE), HP diet sedentary (HPS), and HP diet exercise (HPE) (n = 15 per group) and were engaged for an additional 30-day period of voluntary exercise and HP diet. Relative to the control situation, exercise increased MCT1, MCT4, and PDH protein levels, while the HP diet increased MCT1 and MCT4 protein levels. The production of hydrogen peroxide (H2O2) and the mitochondrial membrane potential (∆Ñ°m) stimulated by succinate in hippocampal homogenates were not significantly different between groups. ADP phosphorylation and the maximal respiratory rate induced by FCCP showed similar responses between groups, implying a normal mitochondrial function. Also, extracellular brain lactate levels were increased in the HPE group compared to other groups soon after performing the Y-maze task. However, such enhanced lactate levels were not associated with improved memory performance. In summary, hippocampal protein expression levels of MCT1 and 4 were increased by physical exercise and HP diet, whereas PDH was only increased by exercise. These observations indicate that a hippocampal metabolic reprogramming takes place in response to these environmental factors.

Prior Exercise Training Prevent Hyperglycemia in STZ Mice by Increasing Hepatic Glycogen and Mitochondrial Function on Skeletal Muscle.

Diabetes mellitus is a metabolic disorder characterized by hyperglycemia. We investigated the effect of a prior 30 days voluntary exercise protocol on STZ-diabetic CF1 mice. Glycemia, and the liver and skeletal muscle glycogen, mitochondrial function, and redox status were analyzed up to 5 days after STZ injection. Animals were engaged in the following groups: Sedentary vehicle (Sed Veh), Sedentary STZ (Sed STZ), Exercise Vehicle (Ex Veh), and Exercise STZ (Ex STZ). Exercise prevented fasting hyperglycemia in the Ex STZ group. In the liver, there was decreased on glycogen level in Sed STZ group but not in EX STZ group. STZ groups showed decreased mitochondrial oxygen consumption compared to vehicle groups, whereas mitochondrial H2 O2 production was not different between groups. Addition of ADP to the medium did not decrease H2 O2 production in Sed STZ mice. Exercise increased GSH level. Sed STZ group increased nitrite levels compared to other groups. In quadriceps muscle, glycogen level was similar between groups. The Sed STZ group displayed decreased O2 consumption, and exercise prevented this reduction. The H2 O2 production was higher in Ex STZ when compared to other groups. Also, GSH level decreased whereas nitrite levels increased in the Sed STZ compared to other groups. The PGC1 α levels increased in Sed STZ, Ex Veh, and Ex STZ groups. In summary, prior exercise training prevents hyperglycemia in STZ-mice diabetic associated with increased liver glycogen storage, and oxygen consumption by the mitochondria of skeletal muscle implying in increased oxidative/biogenesis capacity, and improved redox status of both tissues. J. Cell. Biochem. 118: 678-685, 2017. © 2016 Wiley Periodicals, Inc.

Intracerebroventricular metformin decreases body weight but has pro-oxidant effects and decreases survival.

Metformin (Met), which is an insulin-sensitizer, decreases insulin resistance and fasting insulin levels. The precise molecular target of Met is unknown; however, several reports have shown an inhibitory effect on mitochondrial complex I of the electron transport chain (ETC), which is a related site for reactive oxygen species production. In addition to peripheral effects, Met is capable of crossing the blood-brain barrier, thus regulating the central mechanism involved in appetite control. The present study explores the effects of intracerebroventricular (i.c.v.) infusion of Met on ROS production on brain, insulin sensitivity and metabolic and oxidative stress outcomes in CF1 mice. Metformin (Met 50 and 100 µg) was injected i.c.v. in mice daily for 7 days; the brain mitochondrial H2O2 production, food intake, body weight and fat pads were evaluated. The basal production of H2O2 of isolated mitochondria from the hippocampus and hypothalamus was significantly increased by Met (100 µg). There was increased peripheral sensitivity to insulin (Met 100 µg) and glucose tolerance tests (Met 50 and 100 µg). Moreover, Met decreased food intake, body weight, body temperature, fat pads and survival rates. Additionally, Met (1, 4 or 10 mM) decreased mitochondrial viability and increased the production of H2O2 in neuronal cell cultures. In summary, our data indicate that a high dose of Met injected directly into the brain has remarkable neurotoxic effects, as evidenced by hypothermia, hypoglycemia, disrupted mitochondrial ETC flux and decreased survival rate.