In and out from the cortex: development of major forebrain connections.

In this review we discuss recent advances in the understanding of the development of forebrain projections attending to their origin, fate determination, and axon guidance. Major forebrain connections include callosal, corticospinal, corticothalamic and thalamocortical projections. Although distinct transcriptional programs specify these subpopulations of projecting neurons, the mechanisms involved in their axonal development are similar. Guidance by short- and long-range molecular cues, interaction with intermediate target populations and activity-dependent mechanisms contribute to their development. Moreover, some of these connections interact with each other showing that the development of these axonal tracts is a well-orchestrated event. Finally, we will recapitulate recent discoveries that challenge the field of neural wiring that show that these forebrain connections can be changed once formed. The field of reprogramming has arrived to postmitotic cortical neurons and has showed us that forebrain connectivity is not immutable and might be changed by manipulations in the transcriptional program of matured cells.

Glutamate and GABA receptor signalling in the developing brain.

Our understanding of the role played by neurotransmitter receptors in the developing brain has advanced in recent years. The major excitatory and inhibitory neurotransmitters in the brain, glutamate and GABA, activate both ionotropic (ligand-gated ion channels) and metabotropic (G protein-coupled) receptors, and are generally associated with neuronal communication in the mature brain. However, before the emergence of their role in neurotransmission in adulthood, they also act to influence earlier developmental events, some of which occur prior to synapse formation: such as proliferation, migration, differentiation or survival processes during neural development. To fulfill these actions in the constructing of the nervous system, different types of glutamate and GABA receptors need to be expressed both at the right time and at the right place. The identification by molecular cloning of 16 ionotropic glutamate receptor subunits, eight metabotropic glutamate receptor subtypes, 21 ionotropic and two metabotropic GABA receptor subunits, some of which exist in alternatively splice variants, has enriched our appreciation of how molecular diversity leads to functional diversity in the brain. It now appears that many different types of glutamate and GABA receptor subunits have prominent expression in the embryonic and/or postnatal brain, whereas others are mainly present in the adult brain. Although the significance of this differential expression of subunits is not fully understood, it appears that the change in subunit composition is essential for normal development in particular brain regions. This review focuses on emerging information relating to the expression and role of glutamatergic and GABAergic neurotransmitter receptors during prenatal and postnatal development.

Expression and distribution of metabotropic GABA receptor subtypes GABABR1 and GABABR2 during rat neocortical development.

To understand the possible contribution of metabotropic gamma-aminobutyric acid receptors (GABABR) in cortical development, we investigated the expression pattern and the cellular and subcellular localization of the GABABR1 and GABABR2 subtypes in the rat neocortex from embryonic day 14 (E14) to adulthood. At the light microscopic level, both GABABR1 and GABABR2 were detected as early as E14. During prenatal development, both subtypes were expressed highly in the cortical plate. Using double immunofluorescence, GABABR1 colocalized with GABABR2 in neurons of the marginal zone and subplate, indicating that these proteins are coexpressed and could be forming functional GABABRs during prenatal development in vivo. In contrast, only GABABR1 but not GABABR2 was detected in the tangentially migratory cells in the lower intermediate zone. During postnatal development, immunoreactivity for GABABR1 and GABABR2 was distributed mainly in pyramidal cells. Discrete GABABR1-immunopositive cell bodies of interneurons were present throughout the neocortex. In addition, GABABR1 but not GABABR2 was found in identified Cajal-Retzius cells in layer I. At the electron microscopic level, immunoreactivity for GABABR1 and GABABR2 was found in dendritic spines and dendritic shafts at extrasynaptic and perisynaptic sites throughout postnatal development. We further demonstrated the presynaptic localization of GABABR1 and GABABR2, as well as the association of the receptors with asymmetrical synaptic junctions. These results indicate potentially important roles for the GABABRs in the regulation of migratory processes during corticogenesis and in the modulation of synaptic transmission during early development of cortical circuitry.

Differential distribution of group I metabotropic glutamate receptors during rat cortical development.

Neurons in the rat cerebral cortex are enriched in group I metabotropic glutamate receptor (mGluR) subtypes and respond to their activation during development. To understand better the mechanisms by which mGluR1 and mGluR5 mediate these effects, the goal of this study was to elucidate the expression pattern and to determine the cellular and the precise subcellular localization of these two receptor subtypes in the rat neocortex and hippocampus during late prenatal and postnatal development. At the light microscopic level, mGluR1alpha and mGluR5 were first detected in the cerebral cortex with different expression levels at embryonic day E18. Thus, mGluR5 had a moderate expression, whereas mGluR1alpha was detected as a diffuse and weak labeling. mGluR5 was localized in some Cajal- Retzius cells as well as in other cell types, such as pioneer neurons of the marginal zone. During postnatal development, the distribution of the receptors dramatically changed. From P0 to around P10, mGluR1alpha was localized in identified, transient Cajal-Retzius cells of neocortex and hippocampus, until these cells disappear. In addition, a population of interneurons localized the receptor from the second/third postnatal week. In contrast, mGluR5 was localized mainly in pyramidal cells and in some interneurons, with a neuropilar staining throughout the cerebral cortex. At the electron microscopic level, the immunoreactivity for both group I mGluR subtypes was expressed postsynaptically. Using immunogold methods, mGluR1alpha and mGluR5 immunoreactivities were found throughout postnatal development at the edge of postsynaptic specialization of asymmetrical synapses. These results show that the two group I mGluRs have a differential expression pattern in neocortex and hippocampus that may suggest roles for the receptors in the early processing of cortical information and in the control of cortical developmental events.

Developmental changes in the localisation of the mGluR1alpha subtype of metabotropic glutamate receptors in Purkinje cells.

The regulation of neurotransmitter receptors during synapse formation has been studied extensively at the neuromuscular junction, but little is known about the development of excitatory neurotransmitter receptors during synaptogenesis in central synapses. In this study we show qualitatively and quantitatively that a receptor undergoes changes in localisation on the surface of rat Purkinje cells during development in association with its excitatory synapses. The presence of mGluR1alpha at parallel and climbing fibre synapses on developing Purkinje cells was studied using high-resolution immunoelectron microscopy. Immunoreactivity for mGluR1alpha was detected from embryonic day 18 in Purkinje cells, and showed dramatic changes in its localisation with age. At early postnatal ages (P0 and P3), mGluR1alpha was found both in somata and stem dendrites but was not usually associated with synaptic contacts. At P7, mGluR1alpha became concentrated in somatic spines associated with climbing fibres and in the growing dendritic arborisation even before innervation by parallel fibres. During the second and third postnatal week, when spines and parallel fibre synapses were generated, mGluR1alpha became progressively concentrated in the molecular layer, particularly in the synaptic specialisations. As a result, during the fourth postnatal week, the pattern and level of mGluR1alpha expression became similar to the adult and mGluR1alpha appeared in high density in perisynaptic sites. Our results indicate that mGluR1alpha is present in the developing Purkinje cells prior to their innervation by climbing and parallel fibres and demonstrate that this receptor undergoes a dynamic and specific regulation during postnatal development in association with the establishment of synaptic inputs to Purkinje cell.

Cajal-Retzius cells in early postnatal mouse cortex selectively express functional metabotropic glutamate receptors.

Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca(2+)](i)) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca(2+)](i) increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca(2+), repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca(2+)](i) responses that were restored by refilling a thapsigargin-sensitive Ca(2+) store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1 alpha, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons.