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Metaplasticity: new insights through electrophysiological investigations.

The term synaptic plasticity describes the ability of excitatory synapses to undergo activity-driven long-lasting changes in the efficacy of basal synaptic transmission. This change may be expressed as a long-term potentiation (LTP) or as a long-term depression (LTD). Metaplasticity is a higher-order form of synaptic plasticity that regulates the expression of both LTP and LTD through processes that are initiated by cellular activity that precedes a later bout of plasticity-inducing synaptic activity. Activation by prior synaptic activity and later expression as a facilitation or inhibition of activity-dependent synaptic plasticity are fundamental properties of metaplasticity. The intracellular mechanisms which support metaplasticity appear to be closely linked to those of synaptic plasticity, hence there are significant technical challenges to overcome in order to elucidate those mechanisms specific to metaplasticity. This review will examine the progress in the characterization of metaplasticity over the last decade or so with a focus on findings gained using electrophysiological techniques. It will look at the techniques applied, the brain regions investigated and the knowledge gained from the application of a wide range of protocols designed to examine the influence of varied forms of prior synaptic activity on later, activity-induced, synaptic plasticity.