Axonal projection, input and output synapses, and synaptic physiology of Cajal-Retzius cells in the developing rat neocortex.

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 GABA(A) 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.