Major glutamatergic projection from subplate into visual cortex during development

Axonal connections between S1 barrel, M1, and S2 cortex in the newborn mouse

The development of functionally interconnected networks between primary (S1), secondary somatosensory (S2), and motor (M1) cortical areas requires coherent neuronal activity via corticocortical projections. However, the anatomical substrate of functional connections between S1 and M1 or S2 during early development remains elusive. In the present study, we used ex vivo carbocyanine dye (DiI) tracing in paraformaldehyde-fixed newborn mouse brain to investigate axonal projections of neurons in different layers of S1 barrel field (S1Bf), M1, and S2 toward the subplate (SP), a hub layer for sensory information transfer in the immature cortex. In addition, we performed extracellular recordings in neocortical slices to unravel the functional connectivity between these areas. Our experiments demonstrate that already at P0 neurons from the cortical plate (CP), layer 5/6 (L5/6), and the SP of both M1 and S2 send projections through the SP of S1Bf. Reciprocally, neurons from CP to SP of S1Bf send projections through the SP of M1 and S2. Electrophysiological recordings with multi-electrode arrays in cortical slices revealed weak, but functional synaptic connections between SP and L5/6 within and between S1 and M1. An even lower functional connectivity was observed between S1 and S2. In summary, our findings demonstrate that functional connections between SP and upper cortical layers are not confined to the same cortical area, but corticocortical connection between adjacent cortical areas exist already at the day of birth. Hereby, SP can integrate early cortical activity of M1, S1, and S2 and shape the development of sensorimotor integration at an early stage.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Impaired Hearing and Altered Subplate Circuits During the First and Second Postnatal Weeks of Otoferlin-Deficient Mice

Sensory deprivation from the periphery impacts cortical development. Otoferlin deficiency leads to impaired cochlear synaptic transmission and is associated with progressive hearing loss in adults. However, it remains elusive how sensory deprivation due to otoferlin deficiency impacts the early development of the auditory cortex (ACX) especially before the onset of low threshold hearing. To test that, we performed in vivo imaging of the ACX in awake mice lacking otoferlin (Otof-/-) during the first and second postnatal weeks and found that spontaneous and sound-driven cortical activity were progressively impaired. We then characterized the effects on developing auditory cortical circuits by performing in vitro recordings from subplate neurons (SPN), the first primary targets of thalamocortical inputs. We found that in Otof-/- pups, SPNs received exuberant connections from excitatory and inhibitory neurons. Moreover, as a population, SPNs showed higher similarity with respect to their circuit topology in the absence of otoferlin. Together, our results show that otoferlin deficiency results in impaired hearing and has a powerful influence on cortical connections and spontaneous activity in early development even before complete deafness. Therefore, peripheral activity has the potential to sculpt cortical structures from the earliest ages, even before hearing impairment is diagnosed.

Neonatal Hypoxia-Ischemia Causes Persistent Intracortical Circuit Changes in Layer 4 of Rat Auditory Cortex

The connection between early brain injury and subsequent development of disorders is unknown. Neonatal hypoxia-ischemia (HI) alters circuits associated with subplate neurons (SPNs). SPNs are among the first maturing cortical neurons, project to thalamorecipient layer 4 (L4), and are required for the development of thalamocortical connections. Thus, early HI might influence L4 and such influence might persist. We investigated functional circuits to L4 neurons in neonatal rat HI models of different severities (mild and moderate) shortly after injury and at adolescence. We used laser-scanning photostimulation in slices of auditory cortex during P5-10 and P18-23. Mild injuries did not initially (P6/P7) alter the convergence of excitatory inputs from L2/3, but hyperconnectivity emerged by P8-10. Inputs from L4 showed initial hypoconnectivity which resolved by P8-10. Moderate injuries resulted in initial hypoconnectivity from both layers which resolved by P8-10 and led to persistent strengthening of connections. Inhibitory inputs to L4 cells showed similar changes. Functional changes were mirrored by reduced dendritic complexity. We also observed a persistent increase in similarity of L4 circuits, suggesting that HI interferes with developmental circuit refinement and diversification. Altogether, our results show that neonatal HI injuries lead to persistent changes in intracortical connections.

Transient Subplate Sublayer Forms Unique Corridor for Differential Ingrowth of Associative Pulvinar and Primary Visual Projection in the Prospective Visual Cortical Areas of the Human Fetal Occipital Lobe

Cytoarchitectonical parcellation of the visual cortex into the striate and extrastriate cortex requires complex histogenetic events within a precise spatio-temporal frame to attain the specification of areal domains and associated thalamocortical connections during the fetal brain development. We analyzed a deep subplate cellular monolayer (subplate "corridor" cells) present during a restricted period of 13-15 postconceptional weeks, showing the 3D caudo-ventro-medial position in the human fetal occipital lobe, corresponding to the segregation point of pulvinocortical and geniculocortical fibers at the prospective area 17/18 border. Immunofluorescence stainings revealed subplate "corridor" cells as the specific class of the deepest subplate neurons (NeuN+, Tbr1+, Cplx3+) expressing axon guidance molecules (Sema-3A+, EphA6+), presumably for the attraction of pulvinocortical axons and the repulsion of geniculocortical axons growing at that time (SNAP25+, Syn+, FN+). Furthermore, quantitative analysis of the subplate "corridor" region of interest, considering cell number, immunofluorescence signal intensity per cell and per region, revealed significant differences to other regions across the tangential circumference of the developing cerebral wall. Thus, our study sheds new light on the deepest subplate sublayer, strategically aligned along the growing axon systems in the prospective visual system, suggesting the establishment of the area 17/18 border by differential thalamocortical input during the fetal brain development.

A 'Marginal' tale: the development of the neocortical layer 1

The development of neocortical layer 1 is a very dynamic process and the scene of multiple transient events, with Cajal-Retzius cell death being one of the most characteristic ones. Layer 1 is also the route of migration for a substantial number of GABAergic interneurons during embryogenesis and where some of which will ultimately remain in the adult. The two cell types, together with a diverse set of incoming axons and dendrites, create an early circuit that will dramatically change in structure and function in the adult cortex to give prominence to inhibition. Through the engagement of a diverse set of GABAergic inhibitory cells by bottom-up and top-down inputs, adult layer 1 becomes a powerful computational platform for the neocortex.

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

Ferret brain possesses young interneuron collections equivalent to human postnatal migratory streams

The human early postnatal brain contains late migratory streams of immature interneurons that are directed to cortex and other focal brain regions. However, such migration is not observed in rodent brain, and whether other small animal models capture this aspect of human brain development is unclear. Here, we investigated whether the gyrencephalic ferret cortex possesses human‐equivalent postnatal streams of doublecortin positive (DCX+) young neurons. We mapped DCX+ cells in the brains of ferrets at P20 (analogous to human term gestation), P40, P65, and P90. In addition to the rostral migratory stream, we identified three populations of young neurons with migratory morphology at P20 oriented toward: (a) prefrontal cortex, (b) dorsal posterior sigmoid gyrus, and (c) occipital lobe. These three neuronal collections were all present at P20 and became extinguished by P90 (equivalent to human postnatal age 2 years). DCX+ cells in such collections all expressed GAD67, identifying them as interneurons, and they variously expressed the subtype markers SP8 and secretagogin (SCGN). SCGN+ interneurons appeared in thick sections to be oriented from white matter toward multiple cortical regions, and persistent SCGN‐expressing cells were observed in cortex. These findings indicate that ferret is a suitable animal model to study the human‐relevant process of late postnatal cortical interneuron integration into multiple regions of cortex.

Kcnab1 Is Expressed in Subplate Neurons With Unilateral Long-Range Inter-Areal Projections

Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel (Kcnab1) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.

The Integrative Function of Silent Synapses on Subplate Neurons in Cortical Development and Dysfunction

The thalamocortical circuit is of central importance in relaying information to the cortex. In development, subplate neurons (SPNs) form an integral part of the thalamocortical pathway. These early born cortical neurons are the first neurons to receive thalamic inputs and excite neurons in the cortical plate. This feed-forward circuit topology of SPNs supports the role of SPNs in shaping the formation and plasticity of thalamocortical connections. Recently it has been shown that SPNs also receive inputs from the developing cortical plate and project to the thalamus. The cortical inputs to SPNs in early ages are mediated by N-methyl-D-aspartate (NMDA)-receptor only containing synapses while at later ages α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptors are present. Thus, SPNs perform a changing integrative function over development. NMDA-receptor only synapses are crucially influenced by the resting potential and thus insults to the developing brain that causes depolarizations, e.g., hypoxia, can influence the integrative function of SPNs. Since such insults in humans cause symptoms of neurodevelopmental disorders, NMDA-receptor only synapses on SPNs might provide a crucial link between early injuries and later circuit dysfunction. We thus here review subplate associated circuits, their changing functions, and discuss possible roles in development and disease.

Neural histology and neurogenesis of the human fetal and infant brain

The human brain develops slowly and over a long period of time which lasts for almost three decades. This enables good spatio-temporal resolution of histogenetic and neurogenetic events as well as an appropriate and clinically relevant timing of these events. In order to successfully apply in vivo neuroimaging data, in analyzing both the normal brain development and the neurodevelopmental origin of major neurological and mental disorders, it is important to correlate these neuroimaging data with the existing data on morphogenetic, histogenetic and neurogenetic events. Furthermore, when performing such correlation, the genetic, genomic, and molecular biology data on phenotypic specification of developing brain regions, areas and neurons should also be included. In this review, we focus on early developmental periods (form 8 postconceptional weeks to the second postnatal year) and describe the microstructural organization and neural circuitry elements of the fetal and early postnatal human cerebrum.

Sublaminar organization of the human subplate: developmental changes in the distribution of neurons, glia, growing axons and extracellular matrix

The objective of this paper was to collect normative data essential for analyzing the subplate (SP) role in pathogenesis of developmental disorders, characterized by abnormal circuitry, such as hypoxic-ischemic lesions, autism and schizophrenia. The main cytological features of the SP, such as low cell density, early differentiation of neurons and glia, plexiform arrangement of axons and dendrites, presence of synapses and a large amount of extracellular matrix (ECM) distinguish this compartment from the cell-dense cortical plate (CP; towards pia) and large fiber bundles of external axonal strata of fetal white matter (towards ventricle). For SP delineation from these adjacent layers based on combined cytological criteria, we analyzed the sublaminar distribution of different microstructural elements and the associated maturational gradients throughout development, using immunocytochemical and histological techniques on postmortem brain material (Zagreb Neuroembryological Collection). The analysis revealed that the SP compartment of the lateral neocortex shows changes in laminar organization throughout fetal development: the monolayer in the early fetal period (presubplate) undergoes dramatic bilaminar transformation between 13 and 15 postconceptional weeks (PCW), followed by subtle sublamination in three 'floors' (deep, intermediate, superficial) of midgestation (15-21 PCW). During the stationary phase (22-28 PCW), SP persists as a trilaminar compartment, gradually losing its sublaminar organization towards the end of gestation and remains as a single layer of SP remnant in the newborn brain. Based on these sublaminar transformations, we have documented developmental changes in the distribution, maturational gradients and expression of molecular markers in SP synapses, transitional forms of astroglia, neurons and ECM, which occur concomitantly with the ingrowth of thalamo-cortical, basal forebrain and cortico-cortical axons in a deep to superficial fashion. The deep SP is the zone of ingrowing axons - 'entrance (ingrowth) zone'. The process of axonal ingrowth begins with thalamo-cortical fibers and basal forebrain afferents, indicating an oblique geometry. During the later fetal period, deep SP receives long cortico-cortical axons exhibiting a tangential geometry. Intermediate SP ('proper') is the navigation and 'nexus' sublamina consisting of a plexiform arrangement of cellular elements providing guidance and substrate for axonal growth, and also containing transient connectivity of dendrites and axons in a tangential plane without radial boundaries immersed in an ECM-rich continuum. Superficial SP is the axonal accumulation ('waiting compartment') and target selection zone, indicating a dense distribution of synaptic markers, accumulation of thalamo-cortical axons (around 20 PCW), overlapping with dendrites from layer VI neurons. In the late preterm brain period, superficial SP contains a chondroitin sulfate non-immunoreactive band. The developmental dynamics for the distribution of neuronal, glial and ECM markers comply with sequential ingrowth of afferents in three levels of SP: ECM and synaptic markers shift from deep to superficial SP, with transient forms of glia following this arrangement, and calretinin neurons are concentrated in the SP during the formation phase. These results indicate developmental and morphogenetic roles in the SP cellular (transient glia, neurons and synapses) and ECM framework, enabling the spatial accommodation, navigation and establishment of numerous connections of cortical pathways in the expanded human brain. The original findings of early developmental dynamics of transitional subtypes of astroglia, calretinin neurons, ECM and synaptic markers presented in the SP are interesting in the light of recent concepts concerning its functional and morphogenetic role and an increasing interest in SP as a prospective substrate of abnormalities in cortical circuitry, leading to a cognitive deficit in different neurodevelopmental disorders.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Early Somatostatin Interneuron Connectivity Mediates the Maturation of Deep Layer Cortical Circuits

The precise connectivity of somatostatin and parvalbumin cortical interneurons is generated during development. An understanding of how these interneuron classes incorporate into cortical circuitry is incomplete but essential to elucidate the roles they play during maturation. Here, we report that somatostatin interneurons in infragranular layers receive dense but transient innervation from thalamocortical afferents during the first postnatal week. During this period, parvalbumin interneurons and pyramidal neurons within the same layers receive weaker thalamocortical inputs, yet are strongly innervated by somatostatin interneurons. Further, upon disruption of the early (but not late) somatostatin interneuron network, the synaptic maturation of thalamocortical inputs onto parvalbumin interneurons is perturbed. These results suggest that infragranular somatostatin interneurons exhibit a transient early synaptic connectivity that is essential for the establishment of thalamic feedforward inhibition mediated by parvalbumin interneurons.

Cortical Gray-White Matter Blurring and Cognitive Morbidity in Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is a malformation of cortical development that is associated with high rates of cognitive morbidity. However, the degree to which specific irregularities of dysplastic tissue directly impact cognition remains unknown. This study investigates the relationship between blurring of the cortical gray and white matter boundary on magnetic resonance imaging (MRI) and global cognitive abilities in FCD. Gray-white blurring (GWB) is quantified by sampling the non-normalized T1 image intensity contrast above and below the gray and white matter interface along the cortical mantle. Spherical averaging is used to compare resulting GWB for patients with histopathologically verified FCD with matched controls. Whole-brain correlational analyses are used to investigate the relationship between blurring and general cognitive abilities, controlling for epilepsy duration. Results show that cognitive performance is reduced in patients with FCD relative to controls. Patients show increased GWB in bilateral temporal, parietal, and frontal regions. Furthermore, increased GWB in these regions is linearly related to decreased cognition and mediates group differences in cognitive performance. These findings demonstrate that GWB is a marker of reduced cognitive efficiency in FCD that can potentially be used to probe general and domain-specific cognitive functions in other neurological disorders.

Expression of EAAT2 in neurons and protoplasmic astrocytes during human cortical development

The major regulators of synaptic glutamate in the cerebral cortex are the excitatory amino acid transporters 1-3 (EAAT1-3). In this study, we determined the cellular and temporal expression of EAAT1-3 in the developing human cerebral cortex. We applied single- and double-label immunocytochemistry to normative frontal or parietal (associative) cortex samples from 14 cases ranging in age from 23 gestational weeks to 2.5 postnatal years. The most striking finding was the transient expression of EAAT2 in layer V pyramidal neuronal cell bodies up until 8 postnatal months prior to its expression in protoplasmic astrocytes at 41 postconceptional weeks onward. EAAT2 was also expressed in neurons in layer I (presumed Cajal-Retzius cells), and white matter (interstitial) neurons. This expression in neurons in the developing human cortex contrasts with findings by others of transient expression exclusively in axon tracts in the developing sheep and rodent brain. With western blotting, we found that EAAT2 was expressed as a single band until 2 postnatal months, after which it was expressed as two bands. The expression of EAAT2 in pyramidal neurons during human brain development may contribute to cortical vulnerability to excitotoxicity during the critical period for perinatal hypoxic-ischemic encephalopathy. In addition, by studying the expression of EAAT1 and EAAT2 glutamate transporters, it was possible to document the development of protoplasmic astrocytes.

Zinc histochemistry reveals circuit refinement and distinguishes visual areas in the developing ferret cerebral cortex

A critical question in brain development is whether different brain circuits mature concurrently or with different timescales. To characterize the anatomical and functional development of different visual cortical areas, one must be able to distinguish these areas. Here, we show that zinc histochemistry, which reveals a subset of glutamatergic processes, can be used to reliably distinguish visual areas in juvenile and adult ferret cerebral cortex, and that the postnatal decline in levels of synaptic zinc follows a broadly similar developmental trajectory in multiple areas of ferret visual cortex. Zinc staining in all areas examined (17, 18, 19, 21, and Suprasylvian) is greater in the 5-week-old than in the adult. Furthermore, there is less laminar variation in zinc staining in the 5-week-old visual cortex than in the adult. Despite differences in staining intensity, areal boundaries can be discerned in the juvenile as in the adult. By 6 weeks of age, we observe a significant decline in visual cortical synaptic zinc; this decline was most pronounced in layer IV of areas 17 and 18, with much less change in higher-order extrastriate areas during the important period in visual cortical development following eye opening. By 10 weeks of age, the laminar pattern of zinc staining in all visual areas is essentially adultlike. The decline in synaptic zinc in the supra- and infragranular layers in all areas proceeds at the same rate, though the decline in layer IV does not. These results suggest that the timecourse of synaptic zinc decline is lamina specific, and further confirm and extend the notion that at least some aspects of cortical maturation follow a similar developmental timecourse in multiple areas. The postnatal decline in synaptic zinc we observe during the second postnatal month begins after eye opening, consistent with evidence that synaptic zinc is regulated by sensory experience.

Differential distribution of neurons in the gyral white matter of the human cerebral cortex

The neurons in the cortical white matter (WM neurons) originate from the first set of postmitotic neurons that migrates from the ventricular zone. In particular, they arise in the subplate that contains the earliest cells generated in the telencephalon, prior to the appearance of neurons in gray matter cortical layers. These cortical WM neurons are very numerous during development, when they are thought to participate in transient synaptic networks, although many of these cells later die, and relatively few cells survive as WM neurons in the adult. We used light and electron microscopy to analyze the distribution and density of WM neurons in various areas of the adult human cerebral cortex. Furthermore, we examined the perisomatic innervation of these neurons and estimated the density of synapses in the white matter. Finally, we examined the distribution and neurochemical nature of interneurons that putatively innervate the somata of WM neurons. From the data obtained, we can draw three main conclusions: first, the density of WM neurons varies depending on the cortical areas; second, calretinin-immunoreactive neurons represent the major subpopulation of GABAergic WM neurons; and, third, the somata of WM neurons are surrounded by both glutamatergic and GABAergic axon terminals, although only symmetric axosomatic synapses were found. By contrast, both symmetric and asymmetric axodendritic synapses were observed in the neuropil. We discuss the possible functional implications of these findings in terms of cortical circuits.

The subplate and early cortical circuits

The developing mammalian cerebral cortex contains a distinct class of cells, subplate neurons (SPns), that play an important role during early development. SPns are the first neurons to be generated in the cerebral cortex, they reside in the cortical white matter, and they are the first to mature physiologically. SPns receive thalamic and neuromodulatory inputs and project into the developing cortical plate, mostly to layer 4. Thus SPns form one of the first functional cortical circuits and are required to relay early oscillatory activity into the developing cortical plate. Pathophysiological impairment or removal of SPns profoundly affects functional cortical development. SPn removal in visual cortex prevents the maturation of thalamocortical synapses, the maturation of inhibition in layer 4, the development of orientation selective responses and the formation of ocular dominance columns. SPn removal also alters ocular dominance plasticity during the critical period. Therefore, SPns are a key regulator of cortical development and plasticity. SPns are vulnerable to injury during prenatal stages and might provide a crucial link between brain injury in development and later cognitive malfunction.

The development of the subplate and thalamocortical connections in the human foetal brain

The aim of this review is to present clinically relevant data on prenatal development of thalamocortical connections in the human brain. The analysis is based on extensive Zagreb Neuroembryological Collection, including more than 500 prenatal human brains stained with various classical neurohistological, as well as modern histochemical and immunohistochemical methods. The connection of thalamocortical axons during the 'waiting' period with transient cortical subplate zone and subsequent synaptic engagement in the cortical plate is the main connectivity event in the late foetus and preterm infant. This connectivity is the structural substrate for the endogeneous subplate and sensory-driven circuitry generating transient electrical phenomena and may represent a transient network in the developmental history of consciousness.

CONCLUSION

Findings presented in this review should be considered in the management of pain in preterm infants, in searching for the vulnerability of the subplate zone in diagnostic procedures using the in vivo MRI and in revealing the developmental origin of cognitive and mental disorders.

Progestin receptor is transiently expressed perinatally in neurons of the rat isocortex

Steroid hormones influence the development of numerous brain regions, including some that are not classically considered steroid-sensitive. For example, nuclear receptors for both androgen and estrogen have been detected in neonatal cortical cells. High levels of progestin binding and progestin receptor (PR) mRNA have also been reported in early perinatal isocortex. PR expression coincides with high levels of de novo progesterone produced within the cortex, suggesting that PR and its ligand influence the important developmental cortical processes occurring shortly after birth. In order to better understand the role PR plays in cortical development, we used the cellular-level resolution of immunohistochemistry and in situ hybridization (ISH) to characterize changes in perinatal PR expression within specific cortical lamina. PR immunoreactivity (PR-ir) was examined at embryonic days (E) 18, 20, 21, 22, and postnatal days (P) 1, 3, 6, 9, 13, and 27. We find that PR-ir is transiently expressed in specific lamina of frontal, parietal, temporal, and occipital cortex. PR-ir was observed in subplate cells on E18, in increasingly superficial lamina (primarily lamina V, then II/III) during early postnatal development, and was absent by P27. Double-labeling immunohistochemistry indicated that PR-ir colocalizes with the neuronal marker, microtubule associated protein-2, but not with the glial marker, nestin, nor with gamma-aminobutyric acid. These results suggest that specific subpopulations of cortical neurons may be transiently sensitive to progesterone, and that progesterone and its receptor may play a critical role in the fundamental mechanisms underlying normal cortical development.

Lifespan longitudinal multitasking by cortical neurons

The large number of neurons (1011) and synapses (1014) in the mammalian brain provides a rich anatomical substrate for information processing. Many neurons perform very specialized functions, such as detecting or processing sensory stimuli, relaying or amplifying attributes of an afferent input to another brain area or making decisions to convert inputs into action. Some cell types, including the early-generated subplate cells of the developing cerebral cortex, play a special role during a restricted period of early brain development, acting transiently as scaffolds for the formation of thalamocortical and corticothalamic connections. However, many of these neurons (10–20%) survive elimination and become functionally integrated into the mature cortical circuitry. Thus, a single neuron type can perform different functions in the brain at different periods of life, potentially increasing the combinatorial capacity of the functional cellular architecture of the brain over the lifespan.

Cajal-Retzius cells and subplate neurons differentially express vesicular glutamate transporters 1 and 2 during development of mouse cortex

In the light of the various neurobiological effects of glutamate in brain development, although some embryonic cells are a probable source of glutamate involved in the development of precursor cells and/or immature neurons, little is known about when and where glutamate plays its crucial roles during corticogenesis. To investigate these roles, we focused on the developmental expression of vesicular glutamate transporter (VGLUT)1 and VGLUT2, which are regarded as the best markers for verifying glutamatergic neuron identity, especially the spatiotemporal distributions of their transcripts and proteins in the developing mouse cortex and hippocampus. In situ hybridization studies revealed that VGLUT1 mRNA is expressed in preplate and marginal zone cells at embryonic day (E)10 and in subplate cells by E13, whereas VGLUT2 mRNA is expressed in preplate and marginal zone cells at E10 and in cells of the subventricular zone by E13. Reverse transcriptase-polymerase chain reaction analysis detected full-length VGLUT1 and VGLUT2 gene transcripts in the embryonic brain. By dual labeling combined with immunostaining for microtubule-associated protein 2 (MAP2) or reelin, we showed that MAP2-positive preplate and marginal zone neurons and subplate neurons express VGLUT1, while reelin-positive preplate and marginal zone cells and MAP2-negative subventricular zone cells express VGLUT2. The present study is the first to provide morphologically reliable evidence showing that Cajal-Retzius cells and subplate neurons are glutamatergic, and that the two cells differentially express VGLUT1 and VGLUT2, respectively, as the specific transport system of glutamate in some events orchestrated by these cells during the cortical development of mice.

Subplate neurons regulate maturation of cortical inhibition and outcome of ocular dominance plasticity

Synaptic plasticity during critical periods of development requires intact inhibitory circuitry. We report that subplate neurons are needed both for maturation of inhibition and for the proper sign of ocular dominance (OD) plasticity. Removal of subplate neurons prevents the developmental upregulation of genes involved in mature, fast GABAergic transmission in cortical layer 4, including GABA receptor subunits and KCC2, and thus prevents the switch to a hyperpolarizing effect of GABA. To understand the implications of these changes, a realistic circuit model was formulated. Simulations predicted that without subplate neurons, monocular deprivation (MD) paradoxically favors LGN axons representing the deprived (less active) eye, exactly what was then observed experimentally. Simulations also account for published results showing that OD plasticity requires mature inhibition. Thus, subplate neurons regulate molecular machinery required to establish an adult balance of excitation and inhibition in layer 4, and thereby influence the outcome of OD plasticity.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

Ancestry of the mammalian preplate and its derivatives: evolutionary relicts or embryonic adaptations?

Mammalian cortical development is preceded by the elaboration of a transient preplate, which is split into a superficial marginal zone and a deep subplate after the arrival of the cortical plate. There has been some controversy in the evolutionary interpretation of this transient structure, as some propose it to represent the ancestral cortex or pallium of non-mammals, while others consider it to be a phylogenetic novelty. The preplate and its derivatives contain components derived by both tangential and radial migration. Tangentially migrating elements include pioneer neurons and interneurons, both of subpallial origin, and Cajal-Retzius cells, mostly of pallial origin. Pioneer neurons were probably present in the ancestors of mammals, but may have changed their original superficial position to one below the developing cortex, thus attracting thalamic afferents in the subcortical white matter, and making them penetrate the cortex radially. In mammals, Cajal-Retzius cells appear to have increased both in number and on their level of reelin expression, perhaps in the context of controlling the final stages of migration in a radially expanding neoocortex. Radial-migrating cells are partly represented by the pyramidal-like cells of the subplate. These neurons resemble the excitatory elements of the adult reptilian cortex, but is not clear whether they are their true homologues. One possibility is that these cells appeared by virtue of a heterochronic process in which the earliest radial elements of the cortical plate began to be produced at progressively earlier developmental stages. Thus, we conclude that the mammalian preplate and its derivatives contain both ancestral and derived elements, all of which have been modified in the course of mammalian evolution to support an increasingly complex cortical plate development.

Systemic prenatal insults disrupt telencephalon development: implications for potential interventions

Infants born prematurely are prone to chronic neurologic deficits including cerebral palsy, epilepsy, cognitive delay, behavioral problems, and neurosensory impairments. In affected children, imaging and neuropathological findings demonstrate significant damage to white matter. The extent of cortical damage has been less obvious. Advances in the understanding of telencephalon development provide insights into how systemic intrauterine insults affect the developing white matter, subplate, and cortex, and lead to multiple neurologic impairments. In addition to white matter oligodendrocytes and axons, other elements at risk for perinatal brain injury include subplate neurons, GABAergic neurons migrating through white matter and subplate, and afferents of maturing neurotransmitter systems. Common insults including hypoxia-ischemia and infection often affect the developing brain differently than the mature brain, and insults precipitate a cascade of damage to multiple neural lineages. Insights from development can identify potential targets for therapies to repair the damaged neonatal brain before it has matured.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

Demonstration of long-range GABAergic connections distributed throughout the mouse neocortex

Gamma-aminobutyric acid (GABA)ergic neurons in the neocortex have been mainly regarded as interneurons and thought to provide local interactions. Recently, however, glutamate decarboxylase (GAD) immunocytochemistry combined with retrograde labeling experiments revealed the existence of GABAergic projection neurons in the neocortex. We further studied the network of GABAergic projection neurons in the neocortex by using GAD67-green fluorescent protein (GFP) knock-in mice for retrograde labeling and a novel neocortical GABAergic neuron labeling method for axon tracing. Many GFP-positive neurons were retrogradely labeled after Fast Blue injection into the primary somatosensory, motor and visual cortices. These neurons were labeled not only around the injection site, but also at a long distance from the injection site. Of the retrogradely labeled GABAergic neurons remote from the injection sites, the vast majority (91%) exhibited somatostatin immunoreactivity, and were preferentially distributed in layer II, layer VI and in the white matter. In addition, most of GABAergic projection neurons were positive for neuropeptide Y (82%) and neuronal nitric oxide synthase (71%). We confirmed the long-range projections by tracing GFP-labeled GABAergic neurons with axon branches traveled rostro-caudally and medio-laterally. Axon branches could be traced up to 2 mm. Some (n = 2 of 4) were shown to cross the areal boundaries. The GABAergic projection neurons preferentially received neocortical inputs. From these results, we conclude that GABAergic projection neurons are distributed throughout the neocortex and are part of a corticocortical network.

Transient microcircuits formed by subplate neurons and their role in functional development of thalamocortical connections

Subplate neurons are a transient population of neurons in the brain forming one of the first functional cortical circuits. Past experiments have demonstrated their importance in growth of thalamocortical afferents into the cortical plate and later segregation of thalamocortical afferents. Recently, subplate neurons have been shown to be required for the functional maturation of both thalamocortical connections and mature visual responses in visual cortex. These findings suggest that thalamocortical afferents might not segregate properly in the absence of subplate neurons because the thalamocortical synapse does not mature. Subplate neurons are unique in that they form a circuit that appears to promote synaptic scaling and maturation. Although the precise contribution of subplate neurons within the context of cortical development is unknown, they might play an early role in providing thalamic input to cortex that then interacts with learning rules governing synaptic strengthening at the thalamocortical synapse. Because they appear to play multiple key roles at different stages of development, subplate neurons might also play a role in the pathology of developmental disorders, such as epilepsy and schizophrenia.

Regional Dendritic Variation in Neonatal Human Cortex: A Quantitative Golgi Study

The present study quantitatively compared the basilar dendritic/spine systems of lamina V pyramidal neurons across four hierarchically arranged regions of neonatal human neocortex. Tissue blocks were removed from four Brodmann's areas (BAs) in the left hemisphere of four neurologically normal neonates (mean age=41+/- 40 days): primary (BA4 and BA3-1-2), unimodal (BA18), and supramodal cortices (BA10). Tissue was stained with a modified rapid Golgi technique. Ten cells per region (N=160) were quantified. Despite the small sample size, significant differences in dendritic/spine extent obtained across cortical regions. Most apparent were substantial differences between BA4 and BA10: total dendritic length was 52% greater in BA4 than BA10, and dendritic spine number was 67% greater in BA4 than BA10. Neonatal patterns were compared to adult patterns, revealing that the relative regional pattern of dendritic complexity in the neonate was roughly the inverse of that established in the adult, with BA10 rather than BA4 being the most complex area in the adult. Overall, regional dendritic patterns suggest that the developmental time course of basilar dendritic systems is heterochronous and is more protracted for supramodal BA10 than for primary or unimodal regions (BA4, BA3-1-2, BA18).

Organization and development of corticocortical associative neurons expressing the orphan nuclear receptor Nurr1

The developmental mechanism that contributes to the highly organized axonal connections within the cerebral cortex is not well understood. This is partly due to the lack of molecular markers specifically expressed in corticocortical associative neurons during the period of circuit formation. We have shown previously that latexin, a carboxypeptidase A inhibitor, is expressed in intrahemispheric corticocortical neurons from the second postnatal week in the rat (Arimatsu et al. [1999] Cereb. Cortex 9:569-576). In the present study, we first demonstrate in the adult rat that the orphan nuclear receptor Nurr1 is coexpressed in latexin-expressing neurons located in layer V, sublayer VIa, and the white matter of the lateral sector of the neocortex, and also in latexin-negative early born neurons in sublayer VIb of the entire neocortex. Virtually all Nurr1-expressing neurons exhibit immunoreactivity for phosphate-activated glutaminase but not for gamma-aminobutyric acid, suggesting that they are glutamatergic-excitatory neurons. By combining Nurr1 immunohistochemistry and 5-bromo-2'-deoxyuridine-birthdating, we then show that Nurr1 is expressed in (early born) subplate neurons and (later born) presumptive latexin-expressing neurons from embryonic day 18 onward. Finally, by combination of Nurr1 immunohistochemistry and retrograde tracing, we show that Nurr1-expressing neurons, including those in sublayer VIb, contribute predominantly to long-range intrahemispheric corticocortical projections. These results raise the possibility that Nurr1 plays a role in the establishment and maintenance of normal corticocortical circuitry and function.

Role of subplate neurons in functional maturation of visual cortical columns

The subplate forms a transient circuit required for development of connections between the thalamus and the cerebral cortex. When subplate neurons are ablated, ocular dominance columns do not form in the visual cortex despite the robust presence of thalamic axons in layer 4. We show that subplate ablation also prevents formation of orientation columns. Visual responses are weak and poorly tuned to orientation. Furthermore, thalamocortical synaptic transmission fails to strengthen, whereas intracortical synapses are unaffected. Thus, subplate circuits are essential not only for the anatomical segregation of thalamic inputs but also for key steps in synaptic remodeling and maturation needed to establish the functional architecture of visual cortex.

Cellular physiology of the neonatal rat cerebral cortex

The early development of the cerebral cortex is characterized by neurogenesis, neuronal migration, cellular differentiation and programmed cell death. Cajal-Retzius cells, developing cortical plate neurons and subplate cells form a transient synaptic circuit which may serve as a template for the formation of cortical layers and columns. These three neuronal cell types show distinct electrophysiological properties and synaptic inputs. Endogenous or exogenous harmful disturbances during this developmental period may lead to the preservation of early cortical circuits, which may act as trigger zones for the initiation of pathophysiological activity.

Neurochemical phenotypes of MRF neurons influencing diaphragm and rectus abdominis activity

In prior studies that used transneuronal transport of isogenic recombinants of pseudorabies virus, we established that medial medullary reticular formation (MRF) neurons sent collateralized projections to both diaphragm and abdominal muscle motoneurons. Furthermore, inactivation of MRF neurons in cats and ferrets increased the excitability of diaphragm and abdominal motoneurons, suggesting that MRF neurons controlling respiratory activity are inhibitory. To test this hypothesis, the present study determined the neurochemical phenotypes of MRF premotor respiratory neurons in the ferret by using immunohistochemical procedures. Dual-labeling immunohistochemistry combining pseudorabies virus injections into respiratory muscles with the detection of glutamic acid decarboxylase-like immunoreactive and glutamate-like immunoreactive cells showed that both GABAergic and glutamatergic MRF neurons project to respiratory motoneurons, although the latter are more common. These data suggest that the role of the MRF in respiratory regulation is multifaceted, as this region provides both inhibitory and excitatory influences on motoneuron activity.

Development of laminar organization in the mushroom bodies of the cockroach: Kenyon cell proliferation, outgrowth, and maturation

The mushroom bodies of the insect brain are lobed integration centers made up of tens of thousands of parallel-projecting axons of intrinsic (Kenyon) cells. Most of the axons in the medial and vertical lobes of adult cockroach mushroom bodies derive from class I Kenyon cells and are organized into regular, alternating pairs (doublets) of pale and dark laminae. Organization of Kenyon cell axons into the adult pattern of laminae occurs gradually over the course of nymphal development. Newly hatched nymphs possess tiny mushroom bodies with lobes containing a posterior lamina of ingrowing axons, followed by a single doublet, which is flanked anteriorly by a gamma layer composed of class II Kenyon cells. Golgi impregnations show that throughout nymphal development, regardless of the number of doublets present, the most posterior lamina serves as the "ingrowth lamina" for axons of newborn Kenyon cells. Axons of the ingrowth lamina are taurine- and synaptotagmin-immunonegative. They produce fine growth cone tipped filaments and long perpendicularly oriented collaterals along their length. The maturation of these Kenyon cells and the formation of a new lamina are marked by the loss of filaments and collaterals, as well as the onset of taurine and synaptotagmin expression. Class I Kenyon cells thus show plasticity in both morphology and transmitter expression during development. In a hemimetabolous insect such as the cockroach, juvenile stages are morphologically and behaviorally similar to the adult. The mushroom bodies of these insects must be functional from hatching onward, while thousands of new neurons are added to the existing structure. The observed developmental plasticity may serve as a mechanism by which extensive postembryonic development of the mushroom bodies can occur without disrupting function. This contrasts with the more evolutionarily derived holometabolous insects, such as the honey bee and the fruit fly, in which nervous system development is accomplished in a behaviorally simple larval stage and a quiescent pupal stage.

Structure and projections of white matter neurons in the postnatal rat visual cortex

Transient contributions of subplate neurons to the initial development of the cortex are well-characterized, yet little data are available on a subpopulation of subplate neurons that persist in the white matter (WM) of the cerebral cortex across development. To characterize the WM neurons, differential interference contrast and Nomarski optics were used to visualize individual cells in the WM in slices of rat visual cortex at postnatal ages 9-23. Soma-dendritic morphology and local axonal projection patterns, including probable synaptic innervation sites of their axons, were identified by intracellular filling with biocytin during electrophysiologic recordings. Dendritic branches of all WM neurons, tripartitioned here into upper, middle, and deep divisions, extend throughout the WM and frequently into the overlying cortex. Axonal arborizations from most WM neurons, including apparent boutons, project into adjacent WM with many also innervating overlying cortical layers, whereas some project into the stratum oriens/alveus of the hippocampal formation. Processes of a subset of WM neurons appear to be confined to the WM itself. By using antimicrotubule associated protein (MAP2) immunostaining to quantify the density of WM neurons in rat visual cortex, we find that their overall numbers decrease to approximately 30% of initial levels during postnatal development. During this same developmental period, an increasing percentage of WM neurons contain the synthetic enzyme for nitric oxide, nitric oxide synthase (NOS), as evaluated by immunostaining. Thus, WM neurons that survive the initial perinatal period of cell death are positioned under the laminae of the maturing cortex to potentially modulate the integration of visual signals through either conventional synaptic or nonconventional (diffusible NO signaling) mechanisms.

Conservation of K1 immunoreactivity against early cortical neurones in the vertebrate telencephalon

A monoclonal antibody, K1, immunostains neurones generated earliest in the rat neocortex. The K1 immunoreactivity was found in both mouse and human embryos. In the human marginal zone, the subpial granular layer and the inner sublayer were stained at the 19th and 20th week of gestation, respectively. Western blot analysis revealed that the K1 immunoreactivity was conserved in a variety of vertebrates. While a protein of low molecular weight (200 kDa) reacted dominantly in an amphibian (Xenopus laevis) and a reptile (Agkistrodon blomhoffii), proteins of higher molecular weights (280 and 290 kDa) reacted dominantly in mammals (mouse, rat and macaque). In the brain of the reptile (Lacerta triliniata) embryo, K1 stained a marginal part of the superficial molecular layer in the dorsal cortex that is probably homologous to the mammalian marginal zone in the neocortex. In the chick embryo at the 8th day of incubation, the immunoreactivity was observed on neurones generated earliest in the dorsal cortex but not in the superficial molecular layer. The dorsal ventricular ridge and pallial thickening in either the reptile or chick were not stained. The K1 antigen could be a good marker for evolutional study of the mammalian neocortex.

Auditory thalamocortical projections in the cat: laminar and areal patterns of input

Thalamocortical projections were studied in adult cats using biotinylated dextran amines, wheat germ agglutinin conjugated to horseradish peroxidase, and autoradiography with tritiated leucine and/or proline. The input from 7 architectonically defined nuclei to 14 auditory cortical fields was characterized qualitatively and quantitatively. The principal results were that 1) every thalamic nucleus projected to more than 1 field (range, 4-14 fields; mean, 7 fields); 2) only the projection from the ventral division to some primary fields (primary auditory cortex and posterior auditory cortex) had a periodic, clustered distribution, whereas the input from other divisions to nonprimary areas was continuous; 3) layers III-V received >85% of the total axonal profiles; 4) in most experiments, five or more layers were labeled; 5) the projections to nonprimary auditory areas had many laterally oriented axons; 6) the heaviest input to layer I in all experiments was usually in its upper half, suggesting a sublaminar arrangement; 7) the largest axonal trunks (up to 6 microm in diameter) arose from the medial division and ended in layer Ia, where they ran laterally for long distances; 8) there were three projection patterns: type 1 had its peak in layers III-IV with little input to layer I, and it arose from the ventral division and the dorsal superficial, dorsal, and suprageniculate nuclei of the dorsal division; type 2 had heavy labeling in layer I and less in layers III-IV, arising from the dorsal division nuclei primarily, especially the caudal dorsal and deep dorsal nuclei; and type 3 was a trimodal concentration in layers I, III-IV, and VI that originated chiefly in the medial division and had the lowest density of labeling; and 9) the quantitative profiles with the three methods were very similar. The results suggest that the subdivisions of the auditory thalamus have consistent patterns of laminar distribution to different cortical areas, that an average of five or more layers receive significant input in a specific area, that a given thalamic nucleus can influence areas as far as 20 mm apart, that the first information to arrive at the cortex may reach layer I by virtue of the giant axons, and that several laminar patterns of auditory thalamocortical projection exist. The view that the auditory thalamus (and perhaps other thalamic nuclei) serves mainly a relay function underestimates its many modes for influencing the cortex on a laminar basis.

Nitric oxide, microglial activities and neuronal cell death in the lateral geniculate nucleus of glaucomatous rats

The present study was initiated to investigate neuronal degeneration, microglial reactivity and possible roles of NO in the lateral geniculate nucleus (LGN) of glaucomatous rats. An experimental one-eye glaucoma model was created by cauterization of the limbal-derived veins. Neuronal cell viability was studied by immunostaining with antibody against neuronal nuclei. Changes of expressions of nitric oxide synthase I (NOS I), NOS II, ED 1, OX6 and OX42 in the LGN were studied by immunohistochemistry. NADPH-d histochemistry was also employed. In the experimental glaucomatous rats, the number of NeuN labelled neurons was significantly decreased in both the ipsi- and contra-lateral sides of the ventral LGN (vLGN) but not the dorsal LGN (dLGN) at 1 month post-operation and beyond. Expressions of NOS I and NADPH-d were notably increased from 1 week post-operation in the ipsilateral vLGN. In the contralateral side of the vLGN, however, this change was only observed from 1 month post-operation. No NOS II immunoreaction was observed in LGN of both the normal control and glaucomatous rats. Increased microglial reactivity as indicated by OX-42 immunoreactivity was first observed in both sides of the LGN at 1 week post-operation, and this was most significant especially at 1 and 2 months post-operation. The present results suggest that NO and microglial cells may play some important roles in the pathologic processes of neuronal degeneration in the LGN of glaucomatous rats.

Roles of NMDA receptor activity and nitric oxide production in brain development

The concept that neural activity is important for brain maturation has focused much research interest on the developmental role of the NMDA receptor, a key mediator of experience-dependent synaptic plasticity. However, a mechanism able to link spatial and temporal parameters of synaptic activity during development emerged as a necessary condition to explain how axons segregate into a common brain region and make specific synapses on neuronal sub-populations. To comply with this developmental constraint, it was proposed that nitric oxide (NO), or other substances having similar chemical and biological characteristics, could act as short-lived, activity-dependent spatial signals, able to stabilize active synapses by diffusing through a local volume of tissue. The present article addresses this issue, by reviewing the experimental evidence for a correlated role of the activity of the NMDA receptor and the production of NO in key steps of neural development. Evidence for such a functional coupling emerges not only concerning synaptogenesis and formation of neural maps, for which it was originally proposed, but also for some earlier phases of neurogenesis, such as neural cell proliferation and migration. Regarding synaptogenesis and neural map formation in some cases, there is so far no conclusive experimental evidence for a coupled functional role of NMDA receptor activation and NO production. Some technical problems related to the use of inhibitors of NO formation and of gene knockout animals are discussed. It is also suggested that other substances, known to act as spatial signals in adult synaptic plasticity, could have a role in developmental plasticity. Concerning the crucial developmental phase of neuronal survival or elimination through programmed cell death, the well-documented survival role related to NMDA receptor activation also starts to find evidence for a concomitant requirement of downstream NO production. On the basis of the reviewed literature, some of the major controversial issues are addressed and, in some cases, suggestions for possible future experiments are proposed.

Subplate neuron ablation alters neurotrophin expression and ocular dominance column formation

Ocular dominance column formation in visual cortex depends on both the presence of subplate neurons and the endogenous expression of neurotrophins. Here we show that deletion of subplate neurons, which supply glutamatergic inputs to visual cortex, leads to a paradoxical increase in brain-derived neurotrophic factor mRNA in the same region of visual cortex in which ocular dominance columns are absent. Subplate neuron ablation also increases glutamic acid decarboxylase-67 levels, indicating an alteration in cortical inhibition. These observations imply a role for this special class of neurons in modulating activity-dependent competition by regulating levels of neurotrophins and excitability within a developing cortical circuit.

Connections between cells of the internal capsule, thalamus, and cerebral cortex in embryonic rat

The aim of our study is to understand the development of the earliest connections in the mammalian pallium by documenting the distribution of cells and fibres labelled from the dorsal and ventral thalamus, internal capsule, perirhinal, and dorsal cortex during the period between embryonic day (E) 14 and 17 by using carbocyanine dye tracing in fixed embryonic rat brains. Dye placed in the thalamus of E14 brains backlabels cells in the thalamic reticular nucleus and within the primitive internal capsule. Both anterograde and retrograde tracing confirmed that the first corticofugal projections reach the internal capsule by E14. At E15-E16, after the first cortical plate cells have migrated into the lateral cortex, some cells of the cortical plate and subplate and marginal zone, are backlabelled from the internal capsule, but still not from the dorsal thalamus, even with very long incubation periods. Crystal placement into the perirhinal cortex at E14-E15 labels numerous cells within the internal capsule, whereas no such cells are revealed from dorsal cerebral cortex until E17, suggesting that internal capsule cells establish early connections with the perirhinal and ventral but not dorsal cortex. We propose that the growth of axons from cortex to dorsal thalamus is delayed in two regions: first from E14-E15 at the lateral entrance of the internal capsule and then, from E16, closer to the thalamus, probably within the thalamic reticular nucleus. Subplate projections reach the proximity of the diencephalon at an early stage, but they might never enter the dorsal thalamus.

Changing properties of GABA(A) receptor-mediated signaling during early neocortical development

Evidence from several brain regions suggests gamma-aminobutyric acid (GABA) can exert a trophic influence during development, expanding the role of this amino acid beyond its function as an inhibitory neurotransmitter. Proliferating precursor cells in the neocortical ventricular zone (VZ) express functional GABA(A) receptors as do immature postmigratory neurons in the developing cortical plate (CP); however, GABA(A) receptor properties in these distinct cell populations have not been compared. Using electrophysiological techniques in embryonic and early postnatal neocortex, we find that GABA(A) receptors expressed by VZ cells have a higher apparent affinity for GABA and are relatively insensitive to receptor desensitization compared with neurons in the CP. GABA-induced current magnitude increases with maturation with the smallest responses found in recordings from precursor cells in the VZ. No evidence was found that GABA(A) receptors on VZ cells are activated synaptically, consistent with previous data suggesting that these receptors are activated in a paracrine fashion by nonsynaptically released ligand. After neurons are born and migrate to the CP, they begin to demonstrate spontaneous synaptic activity, the majority of which is GABA(A) mediated. These spontaneous GABA(A) postsynaptic currents (sPSCs) first were detected at embryonic day 18 (E18). At birth, approximately 50% of recordings from cortical neurons demonstrated GABA(A)-mediated sPSCs, and this value increased with age. GABA(A)-mediated sPSCs were action potential dependent and arose from local GABAergic interneurons. GABA application could evoke action potential-dependent PSCs in neonatal cortical neurons, suggesting that during the first few postnatal days, GABA can act as an excitatory neurotransmitter. Finally, N-methyl-D-aspartate (NMDA)- but not non-NMDA-mediated sPSCs were also present in early postnatal neurons. These events were not observed in cells voltage clamped at negative holding potentials (-60 to -70 mV) but were evident when the holding potential was set at positive values (+30 to +60 mV). Together these results provide evidence for the early maturation of GABAergic communication in the neocortex and a functional change in GABA(A)-receptor properties between precursor cells and early postmitotic neurons. The change in GABA(A)-receptor properties may reflect the shift from paracrine to synaptic receptor activation.