Synaptic connections between puramidal cells and interneurons in superficial layers of the neocortex
To understand how sensory information is processed in the neocortex it is essential to investigate the neuronal microcircuits and the dynamics of cortical signalling. Layer 1 of the neocortex is a cell-sparse and synaptic dense zone. Layer 1 neurons are exclusively GABAergic. Here, we recorded from layer 1 interneurons in the adult neocortex and we found that layer 1 interneurons displayed highly diverse morphological and electrophysiological properties. Electrophysiologically, L1 interneurons showed accommodating, non-accommodating, accelerating, bursting, irregular and stuttering behaviour upon a sustained current injection. Based on their morphology, various types including neurogliaform cells, basket cells and axo-axonic cells could be identifi ed. Layer 1 interneurons are electrically coupled and provide inhibitory synaptic input to fellow layer 1 interneurons and layer 2/3 pyramidal neurons. However, little is known about the translaminar excitatory innervation of layer 1 interneurons. We performed pairedrecordings from layer 1 and layer 2/3 neurons in adult neocortical slices. The properties of unitary EPSPs were found to be dependent on the identity of the postsynaptic neuron. Our results suggest that L2/3 pyramidal neurons effi ciently recruit layer 1 interneurons providing a substrate for feedback inhibition, which might contribute to the sparse fi ring of L2/3 pyramidal neurons observed in-vivo.