Distinct mechanisms of spike timing-dependent LTD at vertical and horizontal inputs onto L2/3 pyramidal neurons in mouse barrel cortex

Spike timing-dependent plasticity (STDP) is an attractive candidate to mediate the synaptic changes that support circuit plasticity in sensory cortices during development. STDP is prevalent at excitatory synapses, but it is not known whether the underlying mechanisms are universal, or whether distinct mechanisms underpin STDP at different synapses. Here, we set out to compare and contrast STDP at vertical layer 4 and horizontal layer 2/3 inputs onto postsynaptic layer 2/3 neurons in the mouse barrel cortex. We find that both vertical and horizontal inputs show STDP, but that they display different time windows for induction of timing-dependent long-term depression (t-LTD). Moreover, whereas t-LTD at vertical inputs requires presynaptic NMDA receptors and is expressed presynaptically, using paired recordings we find that t-LTD at horizontal inputs requires postsynaptic NMDA receptors and is expressed postsynaptically. These results demonstrate that similar forms of plasticity on the same postsynaptic neuron can be mediated by distinct mechanisms, and suggest that these forms of plasticity may enable these two types of cortical synapses to support different functions.