Cerebral Cortex Advance Access originally published online on January 12, 2005
Cerebral Cortex 2005 15(10):1485-1496; doi:10.1093/cercor/bhi027
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© Oxford University Press 2005; all rights reserved
Excitatory Connections Made by Presynaptic Cortico-Cortical Pyramidal Cells in Layer 6 of the Neocortex
Department of Pharmacology, The School of Pharmacy, University of London, 2939 Brunswick Square, London WC1N 1AX, UK
Address correspondence to A.M. Thomson, Department of Pharmacology, The School of Pharmacy, 2939 Brunswick Square, London WC1N 1AX, UK. Email: alex.thomson{at}ulsop.ac.uk.
Paired intracellular recordings with biocytin labelling were made in slices of adult rat somatosensory and visual cortex and in cat visual cortex to examine the properties of synaptic connections made by layer 6 pyramidal cells, to determine whether cortico-cortical pyramids more commonly provide input to other layer 6 pyramids than cortico-thalamic cells, and whether these connections exhibit paired pulse and brief train depression. Pyramidal cells with cortico-cortical like morphology were 24 times more likely to innervate other pyramidal cells than were cortico-thalamic like cells, but less likely to innervate inhibitory interneurons. The excitatory postsynaptic potentials elicited by presynaptic, phasically firing cortico-cortical pyramids in all classes of postsynaptic infragranular layer pyramidal cells exhibited strong, presynaptically mediated paired pulse and brief train depression. Those with larger paired pulse ratios also exhibited post-tetanic potentiation, but this was accompanied by stronger paired pulse and brief train depression. Both the firing characteristics and the outputs of cortico-cortical pyramidal cells display pronounced phasic characteristics, indicating that these cells respond most effectively to and preferentially pass on information related to novelty.
Key Words: cortico-cortical EPSP (excitatory postsynaptic potential) layer 6 pyramidal cell microcircuitry paired pulse depression synapse
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