Preserved cerebral oxygen metabolism against astrocytic dysfunction: a combination study of O-15-gas PET with C-14-acetate autoradiography

270 Background: Fluorocitrate (FC) is a metabolic inhibitor of tricarbonic acid (TCA) cycle specifically in astrocyte. In previous studies, intrastriatal injection of FC induced significant reduction in 14C-acetate uptake. Therefore, acetate uptake is a marker of astrocytic TCA cycle activity in brain. Oxygen-15 (15O) PET is the reference standard for quantitative assessment of cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2). The primary purpose of this study was to evaluate whether the inhibition of astrocytic TCA cycle metabolism by FC will affect the oxygen metabolism in the rat brain. METHODS: A total of 6 male Wistar rats (BW: 198 ± 20g) under anesthesia were used. All rats were injected with FC solution intrastriatally [first 3 rats with a dose of 0.33 nmol/ul (low dose) in the right striatum and the other 3 rats with a dose of 1.0 nmol/ul (high) in the left striatum]. Saline solution with the same dose was also infused into the contralateral side. After 4 hours of intrastriatal FC infusion, the rats were investigated by 15O labeled gas PET. CBF, CMRO2, and cerebral blood volume (CBV) were measured with gases 15O-CO2, 15O-O2, and 15O-CO, respectively. Arterial blood sampling was performed during each study to measure the whole blood and plasma counts. After the PET scans, CT imaging was performed. After 15O-gas PET (six hours later from the time of FC injection), the rats were given 14C-acetate intravenously. Five minutes later, the rats were sacrificed by euthanasia and the brains were removed and frozen. Coronal sections were prepared in a cryostat and placed in contact with an imaging plate for autoradiography. Analyses of low dose and high dose of the parameters (CBF, CMRO2, Oxygen extraction fraction (OEF), and CBV) were calculated by OPM and PMOD 3.6 software using rat brain template. Differences were evaluated using paired t test. Autoradiography images were analyzed using Image J software for control-lesion ratio. RESULTS: There were no significant differences between the ipsilateral and contralateral striatum in each parameter using the 15O-gas PET. The following are the results of the high dose group: [Ipsilateral and contralateral striatum: CBF (54.4 ± 8.8 and 55.3 ± 11.6 ml/100ml/min), CMRO2 (7.0 ± 0.9 and 7.1 ± 1.2 ml/100ml/min), OEF (72.0 ± 8.9 and 70.8 ± 8.2 %), and CBV (4.1 ± 0.8 and 4.2 ± 0.9 ml/100ml), respectively]. In 14C-acetate autoradiography results, there is significant inhibition in the astrocyte metabolism after FC injection in the ipsilateral striatum. The percentage reduction rate in low and high doses of FC was 17.5 ± 2.0 % and 38.6 ± 7.2 %, respectively. Conclusions: Regional cerebral oxygen consumption as well as hemodynamic parameters was maintained against the inhibition of astrocytic TCA cycle metabolism in the rat brain.