Reinforcement, glucose metabolism and memory formation: A possible role for astrocytes

Abstract

In the CNS, information processing, including memory formation, has been rightly viewed as primarily a neuronal function. In recent years, however, considerable evidence has been accumulated suggesting that astrocytes, a subclass of glia cells, may play a critical role in memory processing through their interactions with neurones in the regulation of the cellular microenvironment, in neurotransmitter and neuromodulator synthesis and release, and in energy metabolism. In day-old chicks trained on a single-trial discriminated passive avoidance task, astrocyte-neurone interactions in the regulation of extracellular potassium and calcium levels, in replenishment of neurotransmitter glutamate for glutamatergic and GABAergic neurones, and in glucose metabolism may determine whether a learning experience is consolidated into permanent memory. In particular, while the energy requirements for short-term memory stages are adequately met by glucose metabolism in neuronal and astrocytic citric acid cycles, this energy source appears to be insufficient for consolidation of permanent memory. Energy for the cellular processes underlying permanent memory appears to depend on the exclusively astrocytic metabolism of glycogen, stored in astrocytes but not in neurones. Furthermore, glycogenosis appears to be activated by the adequate release of noradrenaline, contingent on an appropriate level of reinforcement for the learning task.