title: Efficacy and connectivity of intracolumnar pairs of layer 2/3 pyramidal cells in the barrel cortex of juvenile rats.
abstract: Synaptically coupled layer 2/3 (L2/3) pyramidal neurones located above the same layer 4 barrel ('barrel-related') were investigated using dual whole-cell voltage recordings in acute slices of rat somatosensory cortex. Recordings were followed by reconstructions of biocytin-filled neurones. The onset latency of unitary EPSPs was 1.1 +/- 0.4 ms, the 20-80% rise time was 0.7 +/- 0.2 ms, the average amplitude was 1.0 +/- 0.7 mV and the decay time constant was 15.7 +/- 4.5 ms. The coefficient of variation (c.v.) of unitary EPSP amplitudes decreased with increasing EPSP peak and was 0.33 +/- 0.18. Bursts of APs in the presynaptic pyramidal cell resulted in EPSPs that, over a wide range of frequencies (5-100 Hz), displayed amplitude depression. Anatomically the barrel-related pyramidal cells in the lower half of layer 2/3 have a long apical dendrite with a small terminal tuft, while pyramidal cells in the upper half of layer 2/3 have shorter and often more 'irregularly' shaped apical dendrites that branch profusely in layer 1. The number of putative excitatory synaptic contacts established by the axonal collaterals of a L2/3 pyramidal cell with a postsynaptic pyramidal cell in the same column varied between 2 and 4, with an average of 2.8 +/- 0.7 (n = 8 pairs). Synaptic contacts were established predominantly on the basal dendrites at a mean geometric distance of 91 +/- 47 mum from the pyramidal cell soma. L2/3-to-L2/3 connections formed a blob-like innervation domain containing 2.8 mm of the presynaptic axon collaterals with a bouton density of 0.3 boutons per mum axon. Within the supragranular layers of its home column a single L2/3 pyramidal cell established about 900 boutons suggesting that 270 pyramidal cells in layer 2/3 are innervated by an individual pyramidal cell. In turn, a single pyramidal cell received synaptic inputs from 270 other L2/3 pyramidal cells. The innervation domain of L2/3-to-L2/3 connections superimposes almost exactly with that of L4-to-L2/3 connections. This suggests that synchronous feed-forward excitation of L2/3 pyramidal cells arriving from layer 4 could be potentially amplified in layer 2/3 by feedback excitation within a column and then relayed to the neighbouring columns.
authors.affiliation: Institut für Neurowissenschaften und Biophysik, AG Zelluläre Neurobiologie-Medizin, Forschungszentrum Jülich GmbH, Leo-Brandt-Strasse, D-52425 Jülich, Germany. d.feldmeyer@fz-juelich.de
authors.forename: Dirk
authors.initials: D
authors.lastname: Feldmeyer
authors.forename: Joachim
authors.initials: J
authors.lastname: Lübke
authors.forename: Bert
authors.initials: B
authors.lastname: Sakmann
mesh_terms: D000200:Action Potentials / Q000502:physiology; D000818:Animals; D000831:Animals, Newborn / Q000033:anatomy & histology / Q000502:physiology; D001369:Axons / Q000502:physiology; D002450:Cell Communication / Q000502:physiology; D003712:Dendrites / Q000502:physiology; D019706:Excitatory Postsynaptic Potentials / Q000502:physiology; D009474:Neurons / Q000166:cytology / Q000502:physiology; D017966:Pyramidal Cells / Q000166:cytology* / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D013003:Somatosensory Cortex / Q000166:cytology* / Q000502:physiology*; D013569:Synapses / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology*; D013997:Time Factors