Burst firing induces a rebound of synaptic strength at unitary neocortical synapses

High-frequency activity produces transient depression at many synapses but also, as recently demonstrated, may accelerate the recovery from use-dependent depression. We have examined the possible consequences of this synaptic mechanism in neocortical excitatory synapses by recording simultaneously from presynaptic pyramidal neurons and their postsynaptic targets. Brief bursts of high-frequency spikes produced a strong depression of the amplitude of unitary excitatory postsynaptic currents (uEPSCs). However, when burst firing was combined with low-frequency ongoing activity, we found that the strong synaptic depression was followed by a transient rebound of synaptic strength. This rebound overshot the low-frequency baseline values and lasted 1-2 s. These results suggest that in the presence of ongoing activity, neocortical synapses may functionally facilitate following burst firing.