Axonal Projection, Input and Output Synapses, and Synaptic Physiology of Cajal–Retzius Cells in the Developing Rat Neocortex

Abstract

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.

Early Serotonergic Projections to Cajal-Retzius Cells: Relevance for Cortical Development

“…However, it is not known when and how CR cells can detect serotonergic signals and whether CR cells express serotonin receptors. CR cells express an array of receptors for other neurotransmitters, such as glutamate, GABA, glycine, and norepinephrine (Schwartz et al, 1998;Lu et al, 2001;Radnikow et al, 2002, Luhmann et al, 2003Soda et al, 2003). The absence of published reports on serotonin receptors on CR cells appears to be attributable to the availability of adequate serotonin receptor antibodies.…”

“…Because early serotonergic projections target the MZ, it has been suggested that they may play an important role in cortical development (Whitaker-Azmitia et al, 1996;Persico et al, 2003). Specifically, it has been proposed that they may form contacts with Cajal-Retzius (CR) cells (Radnikow et al, 2002), but no actual synaptic contacts have so far been demonstrated. CR cells are located in the MZ and are considered to be necessary for the correct positioning of neurons in cortical layers (Ogawa et al, 1995;Marín-Padilla, 1998;Dulabon et al, 2000;Hack et al, 2002;Ang et al, 2003;Rakic and Zecevic, 2003), as well as in radial columns (Nishikawa et al, 2002;Luhmann et al, 2003).…”

Effect of depolarizing GABAA-mediated membrane responses on excitability of Cajal-Retzius cells in the immature rat neocortex

“…For gramicidin-perforated patchclamp recordings 10 -50 g/ml Gramicidin D (Sigma, St Louis, MO) was added from a stock solution (1-2 mg/ml in DMSO) to a pipette solution containing 100 mM K-gluconate, 50 mM KCl, and 10 mM HEPES (pH adjusted to 7.4 with KOH and osmolarity to 300 mosM with sucrose) directly before the experiments. All potentials were corrected for liquid junction potentials, which were calculated with Clampex 8.1 (Axon Instruments, Burlingame, CA) and amounted to 14.8, 14.4, 14.1, 13.9, 13.4, 13.1, and 10.4 mV for 10, 15, 17.5, 20, 25, 30, and 50 To minimize any interference with spontaneously occurring synaptic inputs, all experiments were performed on CRc, which are known to receive only sparse synaptic input (Kilb and Luhmann 2001;Kirmse and Kirischuk 2006;Radnikow et al 2002;Soda et al 2003 (Mienville 1998;Pozas et al 2008). CRc were easily identified in tangential slices by DIC-videomicroscopy according to their typical morphological properties: an ovoid soma with one thick tapered dendrite and long horizontal axonal projections.…”

Rapid Arrival and Integration of Ascending Sensory Information in Layer 1 Nonpyramidal Neurons and Tuft Dendrites of Layer 5 Pyramidal Neurons of the Neocortex

“…A complex cortical circuitry involved in processing the inputs has now been dissected (for recent reviews, see Ferster and Miller, 2000;Ghazanfar and Nicolelis, 2001;Fox, 2002). In contrast, the layer 1 neuronrelated circuits have only recently become the subjects of investigation in vitro (Christophe et al, 2002;Radnikow et al, 2002;Chu et al, 2003;Soda et al, 2003). In the intact brain, both layer 1 neurons and tuft dendrites of layer 5 pyramidal neurons receive and integrate sensory inputs from multiple whiskers.…”