Compartmental investigation of brain energy metabolism using radiotracer kinetics

Abstract

The measurement of metabolism-related processes in vivo is a critical prerequisite for a better understanding of brain metabolism and its relationship to the brain at work. For a long time it was supposed that blood-borne glucose was the sole energy substrate of brain cells and that it was metabolized by the neurons and the astrocytes more or less independently. Only during the last few decades did one begin to appreciate the compartmentalization of energy metabolism between neurons and astrocytes. The finding of an activity-dependent glutamate mediated activation of the astrocytic glycolysis to generate and release lactate has led to the idea of an astrocyte neuron lactate shuttle (ANLS). The ANLS hypothesis claims that neurons take up the astrocytic lactate and use it for oxidation. This hypothesis has provoked a vivid debate about lactate use of neurons as an energy substrate. This project aimed at measuring the kinetics of positron emitter labeled tracers for the investigation of specific metabolic compartments of the neuron-astrocyte unit. In vivo radiotracer studies are useful tools to estimate the exchange of specific substances between different compartments. They can be performed both in animals and humans. The project was divided into three main parts: First, astrocytic oxidative metabolism was investigated using 1-11C-acetate. It turned out that this is a promising tracer to investigate astrocytic oxidative metabolism in rats and humans. An increased radioactivity washout was found during brain activation pointing to an increase in astrocytic oxidative metabolism during increased synaptic activity. Different pharmacological interventions supported the hypothesis that the measured acetate turnover is indeed related to oxidative metabolism. In a second part, we focused on neuronal oxidative metabolism. Inspired by previous reports demonstrating preferential lactate uptake by neurons, we evaluated the newly synthesized radiotracer 1-11C-L-lactate with respect to its kinetic properties in the rodent brain and were able to Summary 2 demonstrate its feasibility to quantify cerebral lactate oxidation in vivo. We could show increased cerebral - most probably neuronal - lactate oxidation during increased brain activity. We further hypothesize, that the kinetics of 1-11C-L-lactate is not only measuring lactate metabolism, but is in addition an indicator of total neuronal oxidative metabolism. In a final method-oriented part of the project, we introduced a novel high-sensitivity surface probe for the measurement of radiotracer concentration through the intact although thinned skull. We demonstrated its ability to measure glucose utilization and blood flow in the rat cerebral cortex. This new development potentially broadens the field of applications for beta probes compared to conventional intracortical beta scintillators. Due to its decreased invasiveness it was successfully applied in the conscious animal. In summary, we demonstrated the usefulness of radiotracer methods for the investigation of specific aspects of cerebral energy metabolism and improved the methodology of beta probes.