Monocarboxylate transporters contribute to the adaptation of neuronal activity to repeated glucose deprivation in the rat lateral septal nucleus

Induction of Monocarboxylate Transporter 2 Expression and Ketone Transport following Traumatic Brain Injury in Juvenile and Adult Rats

Based on recent work demonstrating age-dependent ketogenic neuroprotection after traumatic brain injury (TBI), it was hypothesized that the neuroprotection among early post-weaned animals was related to induced cerebral transport of ketones after injury. Regional changes in monocarboxylate transporter 2 (MCT2) were acutely examined with immunohistochemistry after sham surgery or controlled cortical impact injury among postnatal day 35 and adult rats. Both ages showed elevated MCT2 expression in the ipsilateral cerebral vasculature after TBI. Using Western blotting, MCT2 expression was 80-88% greater in microvessels isolated from postnatal day 35 rats at all time points relative to adults. The increased MCT2 expression was temporally correlated with an age-related increase in cerebral uptake of ketones, when ketones were made available after injury.

Recurrent hypoglycemia reduces the glucose sensitivity of glucose-inhibited neurons in the ventromedial hypothalamus nucleus

Recurrent hypoglycemia blunts the brain's ability to sense and respond to subsequent hypoglycemic episodes. Glucose-sensing neurons in the ventromedial hypothalamus nucleus (VMN) are well situated to play a role in hypoglycemia detection. VMN glucose-inhibited (GI) neurons, which decrease their firing rate as extracellular glucose increases, are extremely sensitive to decreased extracellular glucose. We hypothesize that recurrent hypoglycemia decreases the glucose sensitivity of VMN GI neurons. To test our hypothesis, 14- to 21-day-old Sprague-Dawley rats were subcutaneously injected with regular human insulin (4 U/kg) or saline (control) for three consecutive days. Blood glucose levels 1 h after insulin injection on day 3 were significantly lower than on day 1, reflecting an impaired ability to counteract hypoglycemia. On day 4, the glucose sensitivity of VMN GI neurons was measured using conventional whole cell current-clamp recording. After recurrent insulin-induced hypoglycemia, VMN GI neurons only responded to a glucose decrease from 2.5 to 0.1, but not 0.5, mM. Additionally, lactate supplementation also decreased glucose sensitivity of VMN GI neurons. Thus our findings suggest that decreases in glucose sensitivity of VMN GI neurons may contribute to the impairments in central glucose-sensing mechanisms after recurrent hypoglycemia.

Dual Effect of Pyruvate in Isolated Nerve Terminals: Generation of Reactive Oxygen Species and Protection of Aconitase

Generation of reactive oxygen species (ROS) in synaptosomes was investigated in the presence of different substrates. When pyruvate was used as a substrate an increased rate of hydrogen peroxide formation was detected by the Amplex Red fluorescent assay, but aconitase, which is known to be a highly sensitive enzyme to ROS was not inhibited. In contrast, pyruvate exerted a partial protection on aconitase against a time-dependent inactivation that occurred when synaptosomes were incubated in the absence of substrates. Disruption of synaptosomal membranes with Triton X-100 prevented the protective effect of pyruvate. It is suggested that citrate and/or isocitrate formed in the metabolism of pyruvate could be responsible for a partial protection of aconitase. Therefore while pyruvate could have a prooxidant effect it could also exert a protective effect on the aconitase.