Layer 6 Cortico-thalamic Pyramidal Cells Preferentially Innervate Interneurons and Generate Facilitating EPSPs

The properties of the connections made by the axons of pyramidal cells with cortico-thalamic (CT)-like morphology with a range of postsynaptic layer 6 targets were studied with dual intracellular recordings in slices of adult rat and cat neocortex. The cells were filled with biocytin and identified morphologically and, where appropriate, immunofluorescently. CT-like pyramids contacted interneurons with a very high probability (up to 1:2) but contacted other layer 6 pyramidal cells only rarely (∼1:80). The excitatory postsynaptic potentials (EPSPs) that they elicited both in pyramidal cells and in a variety of types of interneurons (including those immunopositive for parvalbumin and for somatostatin) facilitated, the second EPSP being larger than the first over a range of interspike intervals. Facilitation was not, however, maximal at the shortest intervals; in fact, depression was apparent at some connections at short interspike intervals. Facilitation in the majority of connections peaked at intervals of 25–35 ms and then declined slowly. Nor did these connections display the augmentation typical of many other strongly facilitating connections. Third EPSPs were smaller on average than second EPSPs, and fourth and subsequent EPSPs could be depressed (relative to first EPSPs). The properties of the outputs of these CT-like pyramidal cells are therefore quite distinct from those of other pyramidal cells, both within layer 6 and in other layers, possibly reflecting their unique role as both first order thalamo-cortical recipient and cortico-thalamic output neurons.