Synaptic Plasticity at Hippocampal Synapses: Experimental Background

Abstract

Glutamatergic synapses in the hippocampus undergo activity-dependent bidirectional persistent changes in synaptic strength known as long-term potentiation (LTP) and long-term depression (LTD). This bidirectionality is important for the maintenance of equilibrium within a neuronal network, and distinct activity patterns need to be sensed by the synapse to initiate either LTP or LTD. Donald Hebb originally proposed that coincident firing of inputs onto a neuron or coincident firing of the presynaptic and postsynaptic neurons would strengthen synaptic connections. This theory is broadly correct for associative or Hebbian LTP and has been modified to include a description of LTD induction by uncorrelated firing patterns. However, it does not apply to non-associative or non-Hebbian synaptic plasticity which requires activity in only one neuron. In addition, these theories do not incorporate the role of homeostatic or heterosynaptic plasticity. Glutamatergic synapses in the hippocampus also undergo transient changes in synaptic strength known as short-term potentiation (STP) and short-term depression (STD), which operate on timescales of generally less than a second. Short-term changes in synaptic strength are important for the processing of information in the hippocampus, although their role in learning and memory may be primarily through their impact on long-term forms of synaptic plasticity.