Autapses enhance bursting and coincidence detection in neocortical pyramidal cells

Autapses are synaptic contacts of a neuron's axon onto its own dendrite and soma. In the neocortex self-inhibiting autapses in GABAergic interneurons are abundant in number and play critical roles in regulating spike precision and network activity. Here we examine whether the principal glutamatergic pyramidal cells (PCs) also form functional autapses. In patch-clamp recording from both rodent and human PCs we isolated autaptic responses and found that these occur predominantly in layer-5 PCs projecting to subcortical regions with very few in those projecting to contralateral prefrontal cortex and layer 2/3 PCs. Moreover PC autapses persist during development into adulthood. Surprisingly they produce giant post -synaptic responses (similar to 5 fold greater than recurrent PC-PC synapses) that are exclusively mediated by AMPA receptors. Upon activation autapses enhance burst firing neuronal responsiveness and coincidence detection of synaptic inputs. These findings indicate that PC autapses are functional and represent an important circuit element in the neocortex.