Burst firing induces a rebound of synaptic strength at unitary neocorticalsynapses

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