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Volume 16, Number 10,
Issue of May 15, 1996
pp. 3209-3218
Copyright ©1996 Society for Neuroscience
Frequency and Dendritic Distribution of Autapses Established by
Layer 5 Pyramidal Neurons in the Developing Rat Neocortex: Comparison
with Synaptic Innervation of Adjacent Neurons of the Same Class
Received Dec. 22, 1995; revised Feb. 16, 1996; accepted Feb. 23, 1996.
Joachim Lübke1,
Henry Markram2,
Michael Frotscher1, and
Bert Sakmann2
1 Anatomisches Institut der Albert-Ludwigs
Universität Freiburg, D-79104 Freiburg, Germany, and
2 Max-Planck Institut für Medizinische Forschung,
Abteilung Zellphysiologie, D-69120 Heidelberg, Germany
Synaptic contacts formed by the axon of a neuron on its own
dendrites are known as autapses. Autaptic contacts occur frequently in
cultured neurons and have been considered to be aberrant structures. We
examined the regular occurrence, dendritic distribution, and fine
structure of autapses established on layer 5 pyramidal neurons in the
developing rat neocortex. Whole-cell recordings were made from single
neurons and synaptically coupled pairs of pyramidal cells, which were
filled with biocytin, morphologically reconstructed, and quantitatively
analyzed. Autapses were found in most neurons (in 80% of all cells
analyzed; n = 41). On average, 2.3 ± 0.9 autapses per
neuron were found, located primarily on basal dendrites (64%; 50-70
µm from the soma), to a lesser extent on apical oblique dendrites
(31%; 130-200 µm from the soma), and rarely on the main apical
dendrite (5%; 480-540 µm from the soma). About three times more
synaptic than autaptic contacts (ratio 2.4:1) were formed by a single
adjacent synaptically coupled neuron of the same type. The dendritic
locations of these synapses were remarkably similar to those of
autapses. Electron microscopic examination of serial ultrathin sections
confirmed the formation of autapses and synapses, respectively, and
showed that both types of contacts were located either on dendritic
spines or shafts. The similarities between autapses and synapses
suggest that autaptic and synaptic circuits are governed by some common
principles of synapse formation.
Key words:
autapses;
somatosensory cortex;
layer 5 pyramidal
neurons;
whole-cell patch clamp;
biocytin;
camera lucida
reconstruction
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