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.
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