Scientists from the Medical Research Council's Anatomical Neuropharmacology Unit (ANU) at the University of Oxford have discovered evidence that sleep is important for the stabilisation of memory.

In a study published in Nature Neuroscience, Dr Jozsef Csicsvari and colleagues describe how memory traces of the latest waking experiences could be reactivated during sleep, helping the formation of long-lasting memories.

The team has discovered that, during sleep, a recent conscious experience can control how neuronal (nerve cell) activity patterns are reactivated in the hippocampus, the brain region responsible for memory.

The research was carried out by observing rats as they explored an environment and then as they slept. The activity of neurons in the animals' hippocampus was recorded to establish which waking neuronal patterns were stored for reactivation during sleep and the mechanism for such storage.

The scientists studied pairs of neurons and looked for brief intervals in which both neurons fired messages to each other. They found that the more frequently these cells fired together during exploration, the stronger their correlated firing became during the sleep period afterwards, when compared to sleep prior to this exploring behaviour.

Dr Csicsvari said: "Neuronal patterns during exploration can lead to changes in neuronal connections that temporarily store reactivated memory traces. The recurrence of these events was governed by where the animal explored and how long it spent there: factors which are required for autobiographical memory."

The ANU team also discovered that reactivation of memories of waking experience were more pronounced after the animal had explored a new environment: suggesting that sleep is vital to the storage of new information and the processing of memory.

Original research paper: Reactivation of experience-dependent cell assembly patterns in the hippocampus is published in Nature Neuroscience, 2008: aop.

The mission of the MRC Anatomical Neuropharmacology Unit is to define the molecular, spatial and temporal organisational principles of networks in the brain at the synaptic and cellular level by analysing a variety of brain regions affected in disease. The ability to develop an understanding of the cellular mechanisms underlying activity in a range of neuronal centres is essential, since insight from one area provides hypotheses and facilitates progress in another. This strategy also aids the discovery of general principles of operation and the definition of local adaptations. Brain areas studied at present include the cerebellum, the basal ganglia, the cerebral cortex and, in particular, the hippocampal formation.

Medical Research Council

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