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The Journal of Neuroscience, August 15, 2002, 22(16):6908-6919
Axonal Projection, Input and Output Synapses, and Synaptic
Physiology of Cajal-Retzius Cells in the Developing Rat Neocortex
Gabriele
Radnikow1,
Dirk
Feldmeyer1, and
Joachim
Lübke2
1 Max Planck Institute of Medical Research, Department
for Cell Physiology, D-69120 Heidelberg, Germany, and
2 Department of Anatomy, Albert Ludwigs University
Freiburg, D-79104 Freiburg, Germany
Cajal-Retzius (CR) cells are among the earliest generated neurons
and are thought to play a role in corticogenesis and early neuronal
migration. However, the role of CR cells in an early cortical
microcircuit is still rather unclear. We therefore have investigated the morphology and physiology of CR cells by using whole-cell patch-clamp recordings combined with intracellular biocytin
filling in acute brain slices of postnatal day 5-11 rats. CR cells are
characterized by a long horizontally oriented dendrite; the axonal
collaterals form a dense horizontally oriented plexus in layer 1 and to
a certain extent in layer 2/3, projecting over >2 mm of cortical
surface. The bouton density is relatively high, and synaptic contacts
are established preferentially with dendritic spines or shafts of
excitatory neurons, presumably terminal tuft dendrites of pyramidal
neurons. In turn, CR cells receive dense GABAergic and non-GABAergic
input on somata, dendritic shafts, and spine-like appendages.
Extracellular stimulation in layer 1 could activate both GABAergic and
glutamatergic synaptic inputs. The GABAergic response was blocked by
the GABAA receptor antagonist bicuculline. The glutamatergic response was mediated solely by NMDA receptors and was
highly sensitive to ifenprodil, indicating that it was mediated mainly
via NR1/NR2B subunit-containing receptors. NMDA EPSPs were apparent in
1 mM extracellular Mg2+, suggesting that
this pure NMDA synapse is not silent functionally. Together, the
long-range horizontal projection of the axon, the high density of
synaptic boutons, and the functional synaptic input of CR cells suggest
that they are an integral part of an early cortical network.
Key words:
Cajal-Retzius cells; layer 1; neocortex; input and
output synapses; long-range horizontal axons; synaptic physiology
Copyright © 2002 Society for Neuroscience 0270-6474/02/22166908-12$05.00/0
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