Direct measurement of oxidative metabolism in the living brain by microdialysis: a review

Abstract

This review summarizes microdialysis studies that address the question of which compounds serve as energy sources in the brain. Microdialysis was used to introduce 14C-labeled glucose, lactate, pyruvate, glutamate, glutamine, and acetate into the interstitial fluid of the brain to observe their metabolism to 14CO2. Although glucose uptake from the systemic system supplies the carbon source for these compounds, compounds synthesized from glucose by the brain are subject to recycling including complete metabolism to CO2. Therefore, the brain utilizes multiple compounds in its domain to provide the energy needed to fulfill its function. The physiological conditions controlling metabolism and the contribution of compartmentation into different brain regions, cell types, and subcellular spaces are still unresolved. The aconitase inhibitor fluorocitrate, with a lower inhibition threshold in glial cells, was used to identify the proportion of lactate and glucose that was oxidized in glial cells versus neurons. The fluorocitrate data suggest that glial and neuronal cells are capable of utilizing both lactate and glucose for energy metabolism.

Fig. 1

Effect of acetate and pyruvate on in vivo oxidation. The mean ± stdev.is expressed as percent, normalized to an infusion rate of 1 µCi/hr. The 100% values for each compound, expressed as dpm/1µCi/hr, are: ¹⁴C-U-glucose 391 ± 96, n=5; ¹⁴C-1-pyruvate, 3,349 ± 198, n=5; ¹⁴C-U-lactate, 713 ± 220, n=7; ¹⁴C-U-glutamate, 1,775 ± 438, n=5; and ¹⁴C-U-glutamine, 573 ± 126, n=4 (*P<0.05). From Zielke et al. 2007a with permission.

Fig. 2

Nissl stain of rat brain 24 hrs after perfusion with 100 µM fluorocitrate for 6 hrs. The brains were perfused in situ with 4% paraformaldehyde and processed for Nissl staining (FD Neurotechnologies, Inc, Ellicott City, MD).

Fig. 3

Concentration effect of fluorocitrate on ¹⁴CO₂ recovery following perfusion with [U-¹⁴C]lactate. Mean ± stdev. (0 µmol/L, n= 13; 5 µmol/L, n=4; 20 µmol/L, n=5; 100 µmol/L, n=4; #p<0.001). From Zielke et al. 2007b with permission.

Fig. 4

Effect of fluorocitrate and lactate on recovery of ¹⁴CO₂ following perfusion of [U-¹⁴C]glucose into the interstitial space by microdialysis. Three one hour dialysate fractions were collected as described in Material and Methods to establish the baseline (Control) rate of ¹⁴CO₂ recovery from [U-¹⁴C]glucose. Perfusion was continued with [U-¹⁴C]glucose plus 20 or 100 µmol/L fluorocitrate (+FC). The perfusate was then changed to [U-¹⁴C]glucose plus fluorocitrate plus 50 mmol/L lactate (+FC+Lactate). In studies with 20 µmol/L fluorocitrate each set of samples was significantly different from each other. In studies with 100 µmol/L fluorocitrate, control values were significantly different only from (+FC+Lactate) samples. (Analysis of variance and Tukey’s test; p<0.05; n=4). From Zielke et al. 2007b with permission.