In vivo evidence for lactate as a neuronal energy source

Abstract

Cerebral energy metabolism is a highly compartmentalized and complex process in which transcellular trafficking of metabolites plays a pivotal role. Over the past decade, a role for lactate in fueling the energetic requirements of neurons has emerged. Furthermore, a neuroprotective effect of lactate during hypoglycemia or cerebral ischemia has been reported. The majority of the current evidence concerning lactate metabolism at the cellular level is based on in vitro data; only a few recent in vivo results have demonstrated that the brain preferentially utilizes lactate over glucose. Using voltage-sensitive dye (VSD) imaging, beta-probe measurements of radiotracer kinetics, and brain activation by sensory stimulation in the anesthetized rat, we investigated several aspects of cerebral lactate metabolism. The present study is the first in vivo demonstration of the maintenance of neuronal activity in the presence of lactate as the primary energy source. The loss of the voltage-sensitive dye signal found during severe insulin-induced hypoglycemia is completely prevented by lactate infusion. Thus, lactate has a direct neuroprotective effect. Furthermore, we demonstrate that the brain readily oxidizes lactate in an activity-dependent manner. The washout of 1-[(11)C]L-lactate, reflecting cerebral lactate oxidation, was observed to increase during brain activation from 0.077 ± 0.009 to 0.105 ± 0.007 min(-1). Finally, our data confirm that the brain prefers lactate over glucose as an energy substrate when both substrates are available. Using [(18)F]fluorodeoxyglucose (FDG) to measure the local cerebral metabolic rate of glucose, we demonstrated a lactate concentration-dependent reduction of cerebral glucose utilization during experimentally increased plasma lactate levels.

Figure 1.

Lactate is able to sustain neuronal activity in the absence of glucose. A, A single electrical pulse applied to the rat's hindpaw evoked a transient increase in VSD fluorescence in the primary somatosensory cortex that was abolished after only 150 min in animals receiving saline after the insulin injection (top row) but remained stable in animals infused with glucose (middle row) or lactate (bottom row) after 240 min. B–D, The time courses of the VSD signal displayed a similar result in animals supplied with glucose (C; gluc) or lactate (D; lac) with sustained amplitude 240 min after insulin injection. In contrast, removal of the signal was observed in animals receiving saline infusion only (B; sal). In the group supplied with lactate, the signal onset was delayed (example shown in D). The vertical black line represents the time point of hindpaw stimulation. E, Comparison of amplitudes in all three groups injected with insulin at 0 min (mean ± SE; n = 5). F–H, Mean plasma levels of glucose and lactate plotted for saline (F), glucose (G), and lactate (H) animals.

Figure 2.

1-[¹¹C]l-lactate is a suitable tracer to study cerebral lactate oxidation. A, Measured radioactivity concentration in the brain (open circles), model fit (black line), and arterial input curve (gray line). The inset displays the residuals of the fitting to a one-tissue compartment model. The absence of any distribution bias supports the adequacy of the applied one-tissue compartment model. B, Fraction of native radiolabeled lactate over 40 min after intravenous injection of 1-[¹¹C]l-lactate in blood. The filled circles represent data points from baseline experiments (n = 7) performed to characterize the radiotracer, and the solid line is the corresponding fit of a biexponential curve.

Figure 3.

The brain oxidizes lactate in an activity-dependent manner. A, Schematic of the biochemical pathways involved in the degradation of 1-[¹¹C]l-lactate (¹¹C-Lac) and the proposed interpretation of the rate constants K₁ and k₂ describing the kinetics of the radiotracer (for details, see Results). Briefly, K₁ represents delivery of 1-[¹¹C]l-lactate (orange arrow), and k₂ reflects kinetically the loss of radiolabel after conversion of lactate to pyruvate and of pyruvate to acetyl-coenzyme A (purple arrows). During activation (B; S; n = 5), delivery (K₁) and washout (k₂) increased, whereas during MCT blockade (C; CIN; n = 5), the transfer of 1-[¹¹C]l-lactate slowed down in both directions (Pyr, pyruvate; Ac CoA, acetyl coenzyme A; TCA cycle, tricarboxylic acid cycle). *p < 0.05, n.s., not significant.

Figure 4.

Lactate is preferred over glucose by the brain. A and D detail the protocol during baseline (A) and stimulation (D) conditions. Overall, hyperlactemia reduced LCMR_glu by on average 38% in a dose-dependent manner (B, C) during baseline conditions. In activated cortex (S1_contra), cerebral glucose utilization was further decreased. The effectiveness of stimulation was controlled autoradiographically (D). The ratio S1_contra/S1_ipsi decreased from 1.65 ± 0.47 to 1.26 ± 0.37 (E). 1, Acquisition 1; 2, acquisition 2; LAC, hyperlactemia; SAL, saline. *p < 0.05, n.s., not significant. n = 13 for LAC group, n = 4 for SAL group at baseline conditions, n = 5 for LAC group, and n = 2 for SAL at stimulation conditions.

Figure 5.

Cerebral integrity is not disrupted during hyperlactemia. A, B, A single example demonstrating the spread (A) and the amplitude over time (B) during saline (SAL) and lactate (LAC) infusion is shown. C–E, Mean results of changes in amplitude (C), spread (D), and time-to-peak (E) (n = 4; n.s., not significant compared with baseline).