Extracellular levels of glucose in the hippocampus and striatum during maze training for food or water reward in male rats

Abstract

AbstractPeripheral and central administration of glucose potently enhance cognitive functions. The present experiments examined changes in brain extracellular glucose levels while rats were trained to solve hippocampus-sensitive place or striatum-sensitive response learning tasks for food or water reward. During the first minutes of either place or response training, extracellular glucose levels declined in both the hippocampus and striatum, an effect not seen in untrained, rewarded rats. Subsequently, glucose increased in both brain areas under all training conditions, approaching asymptotic levels ∼15-25 min into training. Compared to untrained-food controls, training with food reward resulted in significant glucose increases in the hippocampus but not striatum; striatal glucose levels exhibited large increases to food intake in both trained and untrained groups. In rats trained to find water, glucose levels increased significantly above the values seen in untrained rats in both hippocampus and striatum. In contrast to results seen with lactate measurements, the magnitude of training-related increases in hippocampus and striatum glucose levels did not differ by task under either reward condition. The decreases in glucose early in training might reflect an increase in brain glucose consumption, perhaps triggering increased brain uptake of glucose from blood, as evident in the increases in glucose later in training. Together with past findings measuring lactate levels under the same conditions, the initial decreases in glucose may also stimulate increased production of lactate from astrocytes to support neural metabolism directly and/or to act as a signal to increase blood flow and glucose uptake into the brain.HighlightsGlucose levels in hippocampus and striatum decrease at the start of training.Glucose levels increase in both brain areas later in training.Glucose changes in both brain areas were similar for place and response tasks.Glucose levels responded similarly to training for either food and water rewards.Early decreases in glucose may trigger increased production astrocytic lactate.