Introduction
Exogenous acetate is preferentially taken in astrocytes by monocarboxylate transporter-1 (MCT1) mediated process. Recently, we found that 14C-acetate uptake in the rat brain appears to occur in parallel with glial energy metabolism and reflects glial conditions. In this study, we examined in vivo 14C-acetate uptake after ischemia/reperfusion of middle cerebral artery (MCA). Effects of microinjection of excitotoxic amino acid into rat striatum were also studied.
Methods
Male Wistar rats (8–9 weeks old) were used, and middle cerebral artery occlusion (MCAO) was made under halothane anesthesia. After 3, 10, 30 and 90 minutes of MCAO, the intraluminal suture was removed for reperfusion. Immediately after the reperfusion, rats were given an intravenous bolus injection of 14C-acetate, 14C-IMP, or 14C-deoxyglucose (DG) and decapitated 5 minutes, 0.5 minute, or 45 minutes after the tracer injection. Coronal sections (20 μm) were prepared and autoradiograms were obtained. The radioactivity concentrations in regions of interest (ROIs) were determined as the photo-stimulated luminescence (PSL) values and expressed as the proportion of the value on the ischemic side to that on the contralateral side. For excitotoxic amino acid experiments, rats were microinjected with ibotenic acid (16 μg/2 ìl) into the left striatum. At 3, 24 hours and 2 weeks after the microinjection, autoradiograms for 14C-acetate and 14C-DG were prepared by the same method. The sections were stained with cresyl violet for estimating cell injury.
Results and Discussion
The brain uptake of 14C-acetate was very sensitive to brain ischemia. Immediately after the 3 minutes-MCAO and reperfusion, 14C-acetate uptake showed a significant reduction (50%) in the rat striatum (ischemic core). In contrast, 14C-DG uptake was almost unaltered by 3 minutes-MCAO and reperfusion. By 90 minutes occlusion, 14C-acetate uptake in the striatum was reduced to 30%, whereas 14C-DG uptake was decreased to 70%. The reduction areas were expanded to cerebral cortex (penumbra) with increase in occlusion period. A significant increase in regional blood flow was seen in the striatum and cerebral cortex in rat brain occluded for 10 and 30 minutes, which indicated 14C-acetate uptake was independent on changes in blood flow. Transient down regulation of MCT-1 function or glial metabolism is the mechanism for sensitive reduction in 14C-acetate uptake in ischemic rat brain. Ibotenic acid caused significant reduction in 14C-acetate uptake at 3, 24 hours and 2 weeks after the microinjection. An increase in 14C-DG uptake was seen at 3 hours after the ibotenic acid injection, however decreases in glucose metabolism were observed at 24 hours and 2 weeks after the microinjection. These results indicated that excitotoxic amino acid might have important roles on regulation of 14C-acetate uptake in intact rat brain. In conclusion, 14C-acetate is a very sensitive probe to detect glial dysfunction in ischemic rat brain.