Functional synaptic projections onto subplate neurons in neonatal rat somatosensory cortex

Thalamic network oscillations synchronize ontogenetic columns in the newborn rat barrel cortex

Neocortical areas are organized in columns, which form the basic structural and functional modules of intracortical information processing. Using voltage-sensitive dye imaging and simultaneous multi-channel extracellular recordings in the barrel cortex of newborn rats in vivo, we found that spontaneously occurring and whisker stimulation-induced gamma bursts followed by longer lasting spindle bursts were topographically organized in functional cortical columns already at the day of birth. Gamma bursts synchronized a cortical network of 300-400 µm in diameter and were coherent with gamma activity recorded simultaneously in the thalamic ventral posterior medial (VPM) nucleus. Cortical gamma bursts could be elicited by focal electrical stimulation of the VPM. Whisker stimulation-induced spindle and gamma bursts and the majority of spontaneously occurring events were profoundly reduced by the local inactivation of the VPM, indicating that the thalamus is important to generate these activity patterns. Furthermore, inactivation of the barrel cortex with lidocaine reduced the gamma activity in the thalamus, suggesting that a cortico-thalamic feedback loop modulates this early thalamic network activity.

Termination and initial branch formation of SNAP-25-deficient thalamocortical fibres in heterochronic organotypic co-cultures

We are interested in the role of neural activity mediated through regulated vesicular release in the stopping and early branching of the thalamic projections in the cortex. Axon outgrowth, arrival at the cortical subplate, side-branch formation during the waiting period and cortical plate innervation of embryonic thalamocortical projections occurs without major abnormalities in the absence of regulated release in Snap25 (-/-) null mutant mice [Washbourne et al. (2002) Nat. Neurosci. 5:19-26; Molnár et al. (2002) J. Neurosci. 22:10313-10323]. The fact that Snap25 (-/-) null mutant mice die at birth limited our previous experiments to the prenatal period. We therefore investigated the behaviour of thalamic projections in co-culture paradigms by using heterochronic thalamic [embryonic day (E)16-E18] and cortical [postnatal day (P)0-P3] explants, in which the stopping and branching behaviour has been previously documented. Our current co-culture experiments established that thalamic projections from E16-E18 Snap25(+/+) or Snap25 (-/-) explants behaved in an identical fashion in P0-P3 Snap25 (+/+) cortical explants after 7 days in vitro. Thalamic projections from Snap25 (-/-) explants developed similar patterns of fibre ingrowth to the cortex, and stopped and formed branches at a similar depth in the Snap25(+/+) cortical slice as in control cultures. These results imply that thalamic projections can reach their ultimate target cells in layer 4, stop, and start to develop branches in the absence of regulated vesicular transmitter release from their own terminals.

Is fetal pain a real evidence?

Due to the progress in fetal surgery, it is important to acquire data about fetal pain.

MATERIAL AND METHODS

We performed a Medline research from 1995, matching the following key words: "pain" and "fetus", with the following: "subplate", "thalamocortical", "myelination", "analgesia", "anesthesia", "brain", "behavioral states", "substance p". We focused on: (a) fetal development of nociceptive pathways; (b) fetal electrophysiological, endocrinological and behavioral reactions to stimuli and pain.

RESULTS

We retrieved 217 papers of which 157 were highly informative; some reported similar data or were only case-reports, and were not quoted. Most endocrinological, behavioral and electrophysiological studies of fetal pain are performed in the third trimester, and they seem to agree that the fetus in the 3rd trimester can experience pain. But the presence of fetal pain in the 2nd trimester is less evident. In favor of a 2nd trimester perception of pain is the early development of spino-thalamic pathways (approximately from the 20th week), and the connections of the thalamus with the subplate (approximately from the 23rd week). Against this possibility, some authors report the immaturity of the cortex with the consequent lack of awareness, and the almost continuous state of sleep of the fetus.

CONCLUSIONS

Most studies disclose the possibility of fetal pain in the third trimester of gestation. This evidence becomes weaker before this date, though we cannot exclude its increasing presence since the beginning of the second half of the gestation.

Evidence for structural and functional changes of subplate neurons in developing rat barrel cortex

In the developing sensory cortex, the subplate could serve as a transient relay station between the thalamus and cortical plate and assists the formation of thalamocortical projection. While the thalamus-layer IV connection is formed, the thalamic activation of subplate is diminished. In the present study, we aimed to explore the mechanism which may attribute to the decline of subplate activity. To resolve this issue, the developmental changes of subplate neurons (SPns) in rat somatosensory cortex were examined during the first two postnatal weeks which covers the stages prior and subsequent to the establishment of thalamocortical connection. During development, more SPns exhibited regular-spiking firing pattern and the membrane properties of SPns displayed a continual trend of maturation. In the meantime, the excitability of SPns decreased as revealed by increasing rheobase and rightwardly shifted frequency-current curves. On the other hand, increasing paired-pulse ratio and slowing MK-801 blocking rate were noted during development, implying the reduction of presynaptic transmitter release. Morphologically, the size of SPn soma increased with age while the shape became flat. The total length, branching nodes and segments of dendrites increased significantly during the first week. However, after peaking around day 10, these values decreased, implying a pruning process. Our findings here propose that the reduction of neuronal excitability, synaptic transmission and dendritic complexity may attribute to the decline of functional connectivity between thalamus and subplate and reduction of subplate activity while the thalamocortical pathway is established.

Neonatal fluoxetine exposure affects the action potential properties and dendritic development in cortical subplate neurons of rats

Selective serotonin reuptake inhibitor (SSRI)-type antidepressants might be given to depressive pregnant women and the developing fetuses are thus exposed to these drugs. Since serotonin plays important roles in the maturation of the nervous system, early SSRI exposure might influence the fetal brain development. To test this hypothesis, we treated the neonatal rat pups with fluoxetine (Flx) from the day of birth to postnatal day (P) 4, comparable to the third trimester of human gestation, and observed the physiological and morphological features of subplate neurons (SPns), a group of cells important for early cortical development and vulnerable to neonatal neural insults. Using whole-cell patch-clamp recording technique, we examined the passive membrane properties and characteristics of action potential (AP). In SPns of Flx-treated rats, the rheobase for generating an AP was increased and the width of APs was reduced, especially in the falling phase. In the morphological aspect, the dendritic remodeling of SPns including dendritic branching, elongation and pruning were affected by early Flx treatment. Together, our results demonstrate that the teratogenic effect of early SSRI exposure on the structure and function of developing SPns and these changes may lead to undesired brain activity and distorted behaviors later in life.

Gene expression analysis of the embryonic subplate

The subplate layer of the cerebral cortex is comprised of a heterogeneous population of cells and contains some of the earliest-generated neurons. In the embryonic brain, subplate cells contribute to the guidance and areal targeting of thalamocortical axons. At later developmental stages, they are predominantly involved in the maturation and plasticity of the cortical circuitry and the establishment of functional modules. We aimed to further characterize the embryonic murine subplate population by establishing a gene expression profile at embryonic day (E) 15.5 using laser capture microdissection and microarrays. The microarray identified over 300 transcripts with higher expression in the subplate compared with the cortical plate at this stage. Using quantitative reverse transcription-polymerase chain reaction, in situ hybridization (ISH), and immunohistochemistry (IHC), we have confirmed specific expression in the E15.5 subplate for 13 selected genes, which have not been previously associated with this compartment (Abca8a, Cdh10, Cdh18, Csmd3, Gabra5, Kcnt2, Ogfrl1, Pls3, Rcan2, Sv2b, Slc8a2, Unc5c, and Zdhhc2). In the reeler mutant, the expression of the majority of these genes (9 of 13) was shifted in accordance with the altered position of subplate. These genes belong to several functional groups and likely contribute to synapse formation and axonal growth and guidance in subplate cells.

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.

Developmental history of the subplate zone, subplate neurons and interstitial white matter neurons: relevance for schizophrenia

The subplate zone is a transient cytoarchitectonic compartment of the fetal telencephalic wall and contains a population of subplate neurons which are the main neurons of the fetal neocortex and play a key role in normal development of cerebral cortical structure and connectivity. While the subplate zone disappears during the perinatal and early postnatal period, numerous subplate neurons survive and remain embedded in the superficial (gyral) white matter of adolescent and adult brain as so-called interstitial neurons. In both fetal and adult brain, subplate/interstitial neurons belong to two major classes of cortical cells: (a) projection (glutamatergic) neurons and (b) local circuit (GABAergic) interneurons. As interstitial neurons remain strategically positioned at the cortical/white matter interface through which various cortical afferent systems enter the deep cortical layers, they probably serve as auxiliary interneurons involved in differential "gating" of cortical input systems. It is widely accepted that prenatal lesions which alter the number of surviving subplate neurons (i.e., the number of interstitial neurons) and/or the nature of their involvement in cortical circuitry represent an important causal factor in pathogenesis of at least some types of schizophrenia--e.g., in the subgroup of patients with cognitive impairment and deficits of frontal lobe functions. The abnormal functioning of cortical circuitry in schizophrenia becomes manifest during the adolescence, when there is an increased demand for proper functioning of the prefrontal cortex. In this review, we describe developmental history of subplate zone, subplate neurons and surviving interstitial neurons, as well as presumed consequences of the increased number of GABAergic interstitial neurons in the prefrontal cortex. We propose that the increased number of GABAergic interstitial neurons leads to the increased inhibition of prefrontal cortical neurons. This inhibitory action of GABAergic interstitial neurons is facilitated by their strategic position at the cortical/white matter interface where limbic and modulatory afferent pathways enter the prefrontal cortex. Thus, enlarged population of inhibitory interstitial neurons (even if they represent a minor fraction of total neuron number, as in the cerebral cortex itself) may alter the differential "gating" of limbic and modulatory inputs (as well as other cortical and subcortical inputs) and cause a functional disconnectivity between the prefrontal and limbic cortex in the adolescent brain. In conclusion, fetal subplate neurons and surviving postnatal interstitial neurons are important modulators of cortical functions in both normal and schizophrenic cerebral cortex.

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.

Between molecules and experience: role of early patterns of coordinated activity for the development of cortical maps and sensory abilities

Sensory systems processing information from the environment rely on precisely formed and refined neuronal networks that build maps of sensory receptor epithelia at different subcortical and cortical levels. These sensory maps share similar principles of function and emerge according to developmental processes common in visual, somatosensory and auditory systems. Whereas molecular cues set the coarse organization of cortico-subcortical topography, its refinement is known to succeed under the influence of experience-dependent electrical activity during critical periods. However, coordinated patterns of activity synchronize the cortico-subcortical networks long before the meaningful impact of environmental inputs on sensory maps. Recent studies elucidated the cellular and network mechanisms underlying the generation of these early patterns of activity and highlighted their similarities across species. Moreover, the experience-independent activity appears to act as a functional template for the maturation of sensory networks and cortico-subcortical maps. A major goal for future research will be to analyze how this early activity interacts with the molecular cues and to determine whether it is permissive or rather supporting for the establishment of sensory topography.

Fetal pain and intrauterine analgesia/anesthesia: neuroanatomy, ontogenesis and physiology of pain perception

A magzat intrauterin élete során és perinatalisan számos, potenciálisan fájdalmas beavatkozáson eshet át. Az elmúlt években a magzati fájdalom létével és csillapításának szükségességével foglalkozó tudományos állásfoglalások kereteit túllépve széles körű társadalmi, politikai, vallási, erkölcsi, filozófiai vita bontakozott ki – leginkább a művi abortusz során a magzat által megélt szenvedést helyezve a középpontba. Munkánkban összefoglaljuk a magzati fájdalommal kapcsolatos orvosi szakirodalomban fellelhető ismereteket. Definiáljuk a fájdalmat, majd a szubjektív fájdalomérzet kialakulásában szerepet játszó neuroanatómiai struktúrák rövid ismertetése után utóbbiak ontogenezis során bekövetkező fejlődését ismertetjük. Ezt követően a noxa kiváltotta stresszválasz objektív (neuroendokrin-vegetatív, cardiovascularis, reflexes és viselkedési) komponensének intrauterin megfigyelhető bizonyítékait vesszük sorra.

AMPA and metabotropic excitoxicity explain subplate neuron vulnerability

Cerebral hypoxia-ischemia results in unique patterns of injury during development owing to selective vulnerability of specific cell populations including subplate neurons. To evaluate the contribution of glutamate excitotoxicity, we studied enriched cultures of subplate neurons in comparison with cortical neurons, deriving expression profiles for glutamate receptor subunits by microarray and immunoblot. The excitotoxic potency of specific glutamate receptors was tested with selective agonists and antagonists. After 1 week in culture, subplate neurons are more sensitive to oxygen-glucose deprivation than cortical neurons, confirming in vivo observations. Subplate and cortical neurons are equally sensitive to glutamate and insensitive to NMDA. Subplate neurons are more sensitive than cortical neurons to AMPA and express twofold less GluR2. Subplate neurons express significantly more mGluR3, a receptor proposed to be protective. Despite this increased expression, group II mGluR agonists increase subplate neuron death and antagonists lessen glutamate excitotoxicity, suggesting a novel mechanism for subplate vulnerability.

Electrical excitability of early neurons in the human cerebral cortex during the second trimester of gestation

Information about development of the human cerebral cortex (proliferation, migration, and differentiation of neurons) is largely based on postmortem histology. Physiological properties of developing human cortical neurons are difficult to access experimentally and therefore remain largely unexplored. Animal studies have shown that information about the arousal of electrical activity in individual cells within fundamental cortical zones (subventricular zone [SVZ], intermediate zone, subplate [SP], and cortical plate [CP]) is necessary for understanding normal brain development. Here we ask where, in what cortical zone, and when, in what gestational week (gw), human neurons acquire the ability to generate nerve impulses (action potentials [APs]). We performed electrical recordings from individual cells in acute brain slices harvested postmortem from the human fetal cerebral cortex (16-22 gw). Tetrodotoxin-sensitive Na(+) current occurs more frequently among CP cells and with significantly greater peak amplitudes than in SVZ. As early as 16 gw, a relatively small population of CP neurons (27%) was able to generate sodium APs upon direct current injection. Neurons located in the SP exhibited the highest level of cellular differentiation, as judged by their ability to fire repetitive APs. At 19 gw, a fraction of human CP and SP neurons possess beta IV spectrin-positive axon initial segments populated with voltage-gated sodium channels (PanNav). These results yield the first physiological characterization of developing human fetal cortical neurons with preserved morphologies in intact surrounding brain tissue.

Synaptogenesis in purified cortical subplate neurons

An ideal preparation for investigating events during synaptogenesis would be one in which synapses are sparse, but can be induced at will using a rapid, exogenous trigger. We describe a culture system of immunopurified subplate neurons in which synaptogenesis can be triggered, providing the first homogeneous culture of neocortical neurons for the investigation of synapse development. Synapses in immunopurified rat subplate neurons are sparse, and can be induced by a 48-h exposure to feeder layers of neurons and glia, an induction more rapid than any previously reported. Induced synapses are electrophysiologically functional and ultrastructurally normal. Microarray and real-time PCR experiments reveal a new program of gene expression accompanying synaptogenesis. Surprisingly few known synaptic genes are upregulated during the first 24 h of synaptogenesis; Gene Ontology annotation reveals a preferential upregulation of synaptic genes only at a later time. In situ hybridization confirms that some of the genes regulated in cultures are also expressed in the developing cortex. This culture system provides both a means of studying synapse formation in a homogeneous population of cortical neurons, and better synchronization of synaptogenesis, permitting the investigation of neuron-wide events following the triggering of synapse formation.

Dynamic integration of subplate neurons into the cortical barrel field circuitry during postnatal development in the Golli-tau-eGFP (GTE) mouse

In the Golli-tau-eGFP (GTE) transgenic mouse the reporter gene expression is largely confined to the layer of subplate neurons (SPn), providing an opportunity to study their intracortical and extracortical projections. In this study, we examined the thalamic afferents and layer IV neuron patterning in relation to the SPn neurites in the developing barrel cortex in GTE mouse at ages embryonic day 17 (E17) to postnatal day 14 (P14). Serotonin transporter immunohistochemistry or cytochrome oxydase histochemistry was used to reveal thalamic afferent patterning. Bizbenzimide staining identified the developing cytoarchitecture in coronal and tangential sections of GTE brains. Enhanced green fluorescent protein (GFP)-labelled neurites and thalamic afferents were both initially diffusely present in layer IV but by P4-P6 both assumed the characteristic periphery-related pattern and became restricted to the barrel hollows. This pattern gradually changed and by P10 the GFP-labelled neurites largely accumulated at the layer IV-V boundary within the barrel septa whereas thalamic afferents remained in the hollows. To investigate whether this reorganisation is dependent on sensory input, the whiskers of row 'a' or 'c' were removed at P0 or P5 and the organisation of GFP-labelled neurites in the barrel cortex was examined at P6 or P10. In the contralateral region corresponding to row 'a' or 'c' the lack of hollow to septa rearrangement of the GFP-labelled neurites was observed after P0 row removal at P10 but not at P6. Our findings suggest a dynamic, sensory periphery-dependent integration of SPn neurites into the primary somatosensory cortex during the period of barrel formation.

Developmental alterations in the functional properties of excitatory neocortical synapses

In the neocortex, most excitatory, glutamatergic synapses are established during the first 4-5 weeks after birth. During this period profound changes in the properties of synaptic transmission occur. Excitatory postsynaptic potentials (EPSPs) at immature synaptic connections are profoundly and progressively reduced in response to moderate to high frequency (5-100 Hz) stimulation. With maturation, this frequency-dependent depression becomes progressively weaker and may eventually transform into a weak to moderate EPSP facilitation. In parallel to changes in the short-term plasticity, a reduction in the synaptic reliability occurs at most glutamatergic neocortical synapses: immature synapses show a high probability of neurotransmitter release as indicated by their low failure rate and small EPSP amplitude variation. This high reliability is reduced in mature synapses, which show considerably higher failure rates and more variable EPSP amplitudes. During early neocortical development synaptic vesicle pools are not yet fully differentiated and their replenishment may be slow, thus resulting in EPSP amplitude depression. The decrease in the probability of neurotransmitter release may be the result of an altered Ca(2+) control in the presynaptic terminal with a reduced Ca(2+) influx and/or a higher Ca(2+) buffering capacity. This may lead to a lower synaptic reliability and a weaker short-term synaptic depression with maturation.

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.

Oxidative injury in the cerebral cortex and subplate neurons in periventricular leukomalacia

We previously identified immunocytochemical evidence of nitrative and oxidative injury in premyelinating oligodendrocytes in periventricular leukomalacia (PVL). Here, we tested the hypothesis that free radical injury occurs in the overlying cerebral cortex and subplate neurons in PVL. We immunostained for nitrotyrosine, malondialdehyde, and hydroxynonenal adducts and scored neuron staining density in PVL (n = 11) and non-PVL (n = 15) cases (postconceptional ages from 34 to 109 weeks). Analysis of covariance controlled for age. Mean malondialdehyde scores in PVL cases were increased over controls (p = 0.005). Hydroxynonenal scores increased with age only in PVL cases (diagnosis vs age interaction; p = 0.024). Nitrotyrosine scores were not significantly increased. In 11 PVL and 23 control cases between 20 and 183 postconceptional weeks, cells morphologically consistent with subplate and Cajal-Retzius neurons showed qualitatively increased free radical modification in PVL over control cases with statistically significant odds ratios for hydroxynonenal and nitrotyrosine in both subplate neurons and Cajal-Retzius cells. Glial fibrillary acidic protein and CD68 scores for reactive astrocytes and microglia, respectively, were not significantly increased, suggesting a minimal inflammatory response. Thus, oxidative/nitrative damage to cortical and "pioneer" neurons, although mild overall, may contribute to cortical volume loss and cognitive/behavioral impairment in survivors of prematurity.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.

Long-distance corticocortical GABAergic neurons in the adult monkey white and gray matter

A subgroup of GABAergic neurons has been reported to project over long distances in several species. Here we demonstrate that long-distance cortically projecting nonpyramidal neurons occur in monkeys in both white and gray matter. Nonpyramidal neurons were first identified morphologically. Visualization of Golgi-like details was achieved by retrograde infection from injections of an adenovirus vector, producing enhanced green fluorescent protein (EGFP) under control of a neuron-specific promoter. Injections in areas V1, V4, TEO, and posterior TE resulted in EGFP-expressing nonpyramidal neurons up to 1.5 cm distant from the injections, mainly in the white matter. Some neurons occurred in the gray matter, mainly in layer 3, but also in layers 5 and 6, and, very occasionally, layer 1. As control, we injected cholera toxin subunit B, a standard retrograde tracer, in V4, and observed a similarly wide distribution of neurons in the white matter. Second, the GABAergic identity of EGFP-expressing nonpyramidal neurons was established by colabeling for EGFP and GAD67 in selected tissue sections. Most neurons positive for EGFP and GAD67 were positive for somatostatin (SS; 90%). Of those neurons positive for EGFP and SS, almost all were also positive for neuronal nitric oxide synthase or m2 muscarinic receptor, but only 23% were also positive for calretinin. None were positive for parvalbumin. We conclude that long-distance projecting GABAergic neurons 1) are phylogenetically conserved, although in monkeys most gray matter neurons are in the upper layers, and 2) are heterogeneous in terms of their neurochemistry, location, and potentially function.

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.

Neurodevelopmental changes of fetal pain

Pain in the developing fetus is controversial because of the difficulty in measuring and interpreting pain during gestation. It has received increased attention lately because of recently introduced legislation that would require consideration of fetal pain during intentional termination of pregnancy. During development, sensory fibers are abundant by 20 weeks; a functional spinal reflex is present by 19 weeks; connections to the thalamus are present by 20 weeks; and connections to subplate neurons are present by 17 weeks with intensive differentiation by 25 weeks. These cells are important developmentally, but decline as a result of natural apoptosis. Mature thalamocortical projections are not present until 29 to 30 weeks, which has led many to believe the fetus does not experience emotional "pain" until then. Pain requires both nociception and emotional reaction or interpretation. Nociception causes physiologic stress, which in turn causes increases in catecholamines, cortisol, and other stress hormones. Physiological stress is different from the emotional pain felt by the more mature fetus or infant, and this stress is mitigated by pain medication such as opiates. The plasticity of the developing brain makes it vulnerable to the stressors that cause long-term developmental changes, ultimately leading to adverse neurological outcomes. Whereas evidence for conscious pain perception is indirect, evidence for the subconscious incorporation of pain into neurological development and plasticity is incontrovertible. Scientific data, not religious or political conviction, should guide the desperately needed research in this field. In the meantime, it seems prudent to avoid pain during gestation.

Spatio-temporal dynamics of oscillatory network activity in the neonatal mouse cerebral cortex

We used a 60-channel microelectrode array to study in thick (600-1000 microm) somatosensory cortical slices from postnatal day (P)0-P3 mice the spatio-temporal properties of early network oscillations. We recorded local non-propagating as well as large-scale propagating spontaneous oscillatory activity. Both types of activity patterns could never be observed in neocortical slices of conventional thickness (400 microm). Local non-propagating spontaneous oscillations with an average peak frequency of 15.6 Hz, duration of 1.7 s and maximal amplitude of 66.8 microV were highly synchronized in a network of approximately 200 microm in diameter. Spontaneous oscillations of lower frequency (10.4 Hz), longer duration (23.8 s) and larger amplitude (142.9 microV) propagated with 0.11 mm/s in the horizontal direction over at least 1 mm. These propagating oscillations were also synchronized in a columnar manner, but these waves synchronized the activity in a larger neuronal network of 300-400 microm in diameter. Both types of spontaneous network activity could be blocked by the gap junction antagonist carbenoxolone. Electrical stimulation of the subplate (SP) or bath application of the cholinergic agonist carbachol also elicited propagating network oscillations, emphasizing the role of the SP and the cholinergic system in the generation of early cortical network oscillations. Our data demonstrate that a sufficiently large network in thick neocortical slice preparations is capable of generating spontaneous and evoked network oscillations, which are highly synchronized via gap junctions in 200-400-microm-wide columns. These via synchronized oscillations coupled networks may represent a self-organized functional template for the activity-dependent formation of neocortical modules during the earliest stages of development.

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.

Embryonic and Postnatal Development of the Layer I–Directed (“Matrix”) Thalamocortical System in the Rat

Inputs to the layer I apical dendritic tufts of pyramidal cells are crucial in "top-down" interactions in the cerebral cortex. A large population of thalamocortical cells, the "matrix" (M-type) cells, provides a direct robust input to layer I that is anatomically and functionally different from the thalamocortical input to layer VI. The developmental timecourse of M-type axons is examined here in rats aged E (embryonic day) 16 to P (postnatal day) 30. Anterograde techniques were used to label axons arising from 2 thalamic nuclei mainly made up of M-type cells, the Posterior and the Ventromedial. The primary growth cones of M-type axons rapidly reached the subplate of dorsally situated cortical areas. After this, interstitial branches would sprout from these axons under more lateral cortical regions to invade the overlying cortical plate forming secondary arbors. Moreover, retrograde labeling of M-type cell somata in the thalamus after tracer deposits confined to layer I revealed that large numbers of axons from multiple thalamic nuclei had already converged in a given spot of layer I by P3. Because of early ingrowth in such large numbers, interactions of M-type axons may significantly influence the early development of cortical circuits.

Exuberance in the development of cortical networks

The cerebral cortex is the largest and most intricately connected part of the mammalian brain. Its size and complexity has increased during the course of evolution, allowing improvements in old functions and causing the emergence of new ones, such as language. This has expanded the behavioural and cognitive repertoire of different species and has determined their competitive success. To allow the relatively rapid emergence of large evolutionary changes in a structure of such importance and complexity, the mechanisms by which cortical circuitry develops must be flexible and yet robust against changes that could disrupt the normal functions of the networks.

Developmental regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit expression in forebrain and relationship to regional susceptibility to hypoxic/ischemic injury. I. Rodent cerebral white matter and cortex

This is the first part of a two-part study to investigate the cellular distribution and temporal regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunits in the developing white matter and cortex in rat (part I) and human (part II). Western blot and immunocytochemistry were used to evaluate the differential expression of AMPAR subunits on glial and neuronal subtypes during the first 3 postnatal weeks in the Long Evans and Sprague Dawley rat strains. In Long Evans rats during the first postnatal week, GluR2-lacking AMPARs were expressed predominantly on white matter cells, including radial glia, premyelinating oligodendrocytes, and subplate neurons, whereas, during the second postnatal week, these AMPARs were highly expressed on cortical neurons, coincident with decreased expression on white matter cells. Immunocytochemical analysis revealed that cell-specific developmental changes in AMPAR expression occurred 2-3 days earlier by chronological age in Sprague Dawley rats compared with Long Evans rats, despite overall similar temporal sequencing. In both white and gray matter, the periods of high GluR2 deficiency correspond to those of regional susceptibility to hypoxic/ischemic injury in each of the two rat strains, supporting prior studies suggesting a critical role for Ca2+-permeable AMPARs in excitotoxic cellular injury and epileptogenesis. The developmental regulation of these receptor subunits strongly suggests that Ca2+ influx through GluR2-lacking AMPARs may play an important role in neuronal and glial development and injury in the immature brain. Moreover, as demonstrated in part II, there are striking similarities between rat and human in the regional and temporal maturational regulation of neuronal and glial AMPAR expression.

The development of cortical connections

The cortex receives its major sensory input from the thalamus via thalamocortical axons, and cortical neurons are interconnected in complex networks by corticocortical and callosal axons. Our understanding of the mechanisms generating the circuitry that confers functional properties on cortical neurons and networks, although poor, has been advanced significantly by recent research on the molecular mechanisms of thalamocortical axonal guidance and ordering. Here we review recent advances in knowledge of how thalamocortical axons are guided and how they maintain order during that process. Several studies have shown the importance in this process of guidance molecules including Eph receptors and ephrins, members of the Wnt signalling pathway and members of a novel planar cell polarity pathway. Signalling molecules and transcription factors expressed with graded concentrations across the cortex are important in establishing cortical maps of the topography of sensory surfaces. Neural activity, both spontaneous and evoked, plays a role in refining thalamocortical connections but recent work has indicated that neural activity is less important than was previously thought for the development of some early maps. A strategy used widely in the development of corticocortical and callosal connections is the early overproduction of projections followed by selection after contact with the target structure. Here we discuss recent work in primates indicating that elimination of juvenile projections is not a major mechanism in the development of pathways feeding information forward to higher levels of cortical processing, although its use is common to developing feedback pathways.

Prolonged coexistence of transient and permanent circuitry elements in the developing cerebral cortex of fetuses and preterm infants

The aim of this paper is to evaluate correlative magnetic resonance imaging (MRI) and histological parameters of development of cortical afferents during pathfinding and target selection in transient fetal cerebral laminas in human fetuses and preterm infants. The transient fetal subplate zone, situated between the fetal white matter (i.e. intermediate zone) and the cortical plate, is the crucial laminar compartment for development of thalamocortical and corticocortical afferents. The prolonged coexistence of transient (endogenously active) and permanent (sensory-driven) circuitry within the transient fetal zones is a salient feature of the fetal and preterm cortex; this transient circuitry is the substrate of cerebral functions in preterm infants. Another transient aspect of organization of developing fibre pathways is the abundance of extracellular matrix and guidance molecules in periventricular crossroads of projection and corticocortical pathways. Both the subplate zone and periventricular crossroads are visible on MRI in vivo and in vitro. Hypoxic-ischaemic lesions of periventricular crossroads are the substrate for motor, sensory, and cognitive deficits after focal periventricular leukomalacia (PVL). Lesions of distal portions of the white matter and the subplate zone are the substrate for diffuse PVL. The neuronal elements in transient fetal zones form a developmental potential for plasticity after perinatal cerebral lesions.

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.

New data about embryonic and fetal neurodevelopment and behavior obtained by 3D and 4D sonography

The development of the human central nervous system (CNS) begins in the early embryonic period and proceeds through a sequence of very complicated processes long after delivery. Although the neurodevelopmental processes are genetically determined, their complexity and intensity implicates the vulnerability of the CNS to number of environmental factors. There is substantial evidence to show that many neurological problems, from minor cerebral dysfunction to the cerebral palsy, originate form the prenatal, rather than perinatal or postnatal periods of life. A variety of neurological and neuropsychiatric diseases is nowadays considered to originate, at least partly, from the prenatal incidents. In most of these conditions, there is no reliable parameter for detection or prediction of cerebral lesions and there is an urgent need to develop strategies that would enable the early detection of cerebral lesions or indications that such lesions might occur. The new, advanced, imaging techniques such as 3-dimensional and 4-dimensional sonography, opened a new perspective for the investigation of structural and functional development of fetal CNS. The application of these techniques might improve our understanding of the prenatal neurodevelopmental events and possibly facilitate the development of diagnostic strategies for early detection or prevention of brain dysfunctions and damage.

Oxygen and glucose deprivation induces major dysfunction in the somatosensory cortex of the newborn rat

The mechanisms and functional consequences of ischemia-induced injury during perinatal development are poorly understood. Subplate neurons (SPn) play a central role in early cortical development and a pathophysiological impairment of these neurons may have long-term detrimental effects on cortical function. The acute and long-term consequences of combined oxygen and glucose deprivation (OGD) were investigated in SPn and compared with OGD-induced dysfunction of immature layer V pyramidal cortical neurons (PCn) in somatosensory cortical slices from postnatal day (P)0-4 rats. OGD for 50 min followed by a 10-24-h period of normal oxygenation and glucose supply in vitro or in culture led to pronounced caspase-3-dependent apoptotic cell death in all cortical layers. Whole-cell patch-clamp recordings revealed that the majority of SPn and PCn responded to OGD with an initial long-lasting ischemic hyperpolarization accompanied by a decrease in input resistance (R(in)), followed by an ischemic depolarization (ID). Upon reoxygenation and glucose supply, the recovery of the membrane potential and R(in) was followed by a Na+/K+-ATPase-dependent postischemic hyperpolarization, and in almost half of the investigated SPn and PCn by a postischemic depolarization. Whereas neither a moderate (2.5 mm) nor a high (4.8 mm) increase in extracellular magnesium concentration protected the SPn from OGD-induced dysfunction, blockade of NMDA receptors with MK-801 led to a significant delay and decrease of the ID. Our data demonstrate that OGD induces apoptosis and a profound dysfunction in SPn and PCn, and underline the critical role of NMDA receptors in early ischemia-induced neuronal damage.

Rapid developmental switch in the mechanisms driving early cortical columnar networks

The immature cerebral cortex self-organizes into local neuronal clusters long before it is activated by patterned sensory inputs. In the cortical anlage of newborn mammals, neurons coassemble through electrical or chemical synapses either spontaneously or by activation of transmitter-gated receptors. The neuronal network and the cellular mechanisms underlying this cortical self-organization process during early development are not completely understood. Here we show in an intact in vitro preparation of the immature mouse cerebral cortex that neurons are functionally coupled in local clusters by means of propagating network oscillations in the beta frequency range. In the newborn mouse, this activity requires an intact subplate and is strongly synchronized within a cortical column by gap junctions. With the developmental disappearance of the subplate at the end of the first postnatal week, activation of NMDA (N-methyl-D-aspartate) receptors in the immature cortical network is essential to generate this columnar activity pattern. Our findings show that during a brief developmental period the cortical network switches from a subplate-driven, gap-junction-coupled syncytium to a synaptic network acting through NMDA receptors to generate synchronized oscillatory activity, which may function as an early functional template for the development of the cortical columnar architecture.

Role of glutamate receptors in periventricular leukomalacia

Periventricular leukomalacia is a form of white-matter injury that occurs in the setting of either primary or secondary hypoxia-ischemia in the premature infant. Hypoxia-ischemia induces increases in cerebral extracellular glutamate levels, thereby activating glutamate receptors on a variety of cell types within the white matter. This review examines the evidence of a role for glutamate receptors in white-matter injury and periventricular leukomalacia. Multiple glutamate receptor subtypes exist, and these appear to play differential roles depending on cell type and time after injury. Glutamate receptors are developmentally regulated on neurons and glia, and certain subtypes are transiently overexpressed in developing rodent brain and are expressed on immature oligodendrocytes in human white matter in the premature period. Pharmacologic agents acting on glutamate receptors might represent age-specific therapeutic strategies for the treatment of periventricular leukomalacia.

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.

Involvement of gap junctions in the development of the neocortex

Gap junctions play an important role during the development of the mammalian brain. In the neocortex, gap junctions are already expressed at very early stages of development and they seem to be involved in many processes like neurogenesis, migration and synapse formation. Gap junctions are found in all cell types including progenitor cells, glial cells and neurons. These direct cell-to-cell connections form clusters consisting of a distinct number of cells of a certain type. These clusters can be considered as communication compartments in which the information transfer is mediated electrically by ionic currents and/or chemically by, e.g., small second messenger molecules. Within the neocortex, four such communication compartments can be identified: (1) gap junction-coupled neuroblasts of the ventricular zone and gap junctions in migrating cells and radial glia, (2) gap junction-coupled glial cells (astrocytes and oligodendrocytes), (3) gap junction-coupled pyramidal cells (only during the first two postnatal weeks) and (4) gap junction-coupled inhibitory interneurons. These compartments can consist of sub-compartments and they may overlap to some degree. The compartments 1 and 3 disappear with ongoing develop, whereas compartments 2 and 4 persist in the mature neocortex. Gap junction-mediated coupling of glial cells seems to be important for stabilization of the extracellular ion homeostasis, uptake of neurotransmitters, migration of neurons and myelination of axons. Electrical synapses between inhibitory interneurons facilitate the synchronization of pyramidal cells. In this way, they contribute to the generation of oscillatory network activity correlated with higher cortical functions. The role of gap junctions present in neuroblasts of the ventricular zone as well as the role of gap junctions found in pyramidal cells during the early postnatal stages is less clear. It is assumed that they might help to form precursors of the functional columns observed in the mature neocortex. Although recent developments of new techniques led to the solution of many problems concerning gap junction-coupling between neurons and glial cells in the neocortex, there are many open questions which need to be answered before we can achieve a comprehensive understanding of the role of gap junctions in the development of the neocortex.

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.

Behavioral pattern continuity from prenatal to postnatal life--a study by four-dimensional (4D) ultrasonography

AIM

To investigate whether the same behavioral patterns were present pre- and postnatally, and whether there were any differences in the frequency of movements observed in fetal and in early neonatal life.

SUBJECTS AND METHODS

Ten out of 37 pregnant women in the third trimester of pregnancy (median gestational age 34 weeks, range 33 to 35 weeks) in the two-month period (from November 1st to December 31st, 2003) were enrolled in the investigation. Ten term, appropriate for gestational age newborns (seven born vaginally, three by elective SC, six girls, six first-born) and were enrolled in the study. All 4D examinations were performed on Voluson 730 (Kretztechnik, Zipt, Austria) and Acuvix (Medison, Korea) with transabdominal 5 MHz transducer. After standard assessment in 2D B-mode ultrasound, a 4D mode was switched on and live 3D image was reconstructed by selecting the ideal representative 2D image placed in the region of interest (ROI). The recordings of neonatal behavior were made on the Sony P-612 OHMPL videotape by video camera (Sony Camcoder CC DTRV 318 Hv8) and reviewed on the videocassette recorder (Sony VHS SLV-N 900). The median of newborns' age at the moment of recording was 49 hours (range 4 to 112). During the examination, newborns were lying in the bed, separated from other infants in the nursery, dressed, and lying on their backs in a supine position with unrestrained hands. The temperature in the room was 22 to 24 degrees C. The video recording was performed mainly while the children were actively awake or during alert inactivity.

RESULTS

There were no movements observed in fetal life that were not present in neonatal life, while the Moro reflex was present only in neonates. The most frequent fetal and neonatal movements were scowling, eye and mouth opening, and hand to face, hand to eye and hand to head movements. Isolated blinking, mouth to eyelid movement, yawning, tongue expulsion and scowling were more frequent in neonates than in fetuses, although the difference was not statistically significant. Hand to mouth movements were more frequent in neonatal than in fetal life while all other hand movements were less frequent in neonates than in fetuses, although the differences did not reach statistical significance. Spearman rank order correlation reached statistical significance in smiling (R=0.71; t=2.91; P=0.02) and in hand to ear movement (R = 0.80; t= 3.86; P = 0.005), and was almost statistically significant in isolated eye blinking (R=0.61; t=2.17; P =0.06), while the correlations between the rest of the movements were not statistically significant.

CONCLUSIONS

4D ultrasonography is a powerful tool in the assessment of fetal behavior, and our study showed that there is a continuity from fetal to neonatal behavior, especially in terms of isolated eye blinking movements, mouth and eyelid opening, yawning, tongue expulsion, smiling, scowling and hand movements directed to other parts of the face.

Functional nicotinic acetylcholine receptors on subplate neurons in neonatal rat somatosensory cortex

The establishment of cortical synaptic circuits during early development requires the presence of subplate neurons (SPn's), a heterogeneous population of neurons capable to integrate and process synaptic information from the thalamus, cortical plate, and neighboring SPn's. An accumulation of cholinergic afferents and nicotinic acetylcholine receptors (nAChRs) has been documentated in the subplate around birth. To assess the developmental role of the cholinergic innervation onto SPn's, we used whole cell patch-clamp recordings of visually identified and biocytin-labeled SPn's in neonatal rat somatosensory cortex. Functional nAChRs were present in 92% of the investigated SPn's. Activation of postsynaptic nAChRs by local application of agonists elicited a brief membrane depolarization associated with a barrage of action potentials and large inward currents reversing around 0 mV. According to our pharmacological data, excitation of SPn's is mediated by alpha4beta2 receptors. In contrast, functional alpha7 nAChRs could not be identified on SPn's. Activation of nAChRs affected neither the spontaneous synaptic activity of SPn's nor the synaptic connections between thalamus and SPn's and within subplate. Nicotine, at concentrations reaching the developing brain by maternal smoking, induced a severe desensitization of nAChRs and an increase in the baseline noise. These results indicate that nAChR-mediated excitation of SPn's may stabilize the developing synaptic circuits and suggest the involvement of nAChRs located on SPn's in the fetal tobacco syndrome.

Subplate neurons: bridging the gap to function in the cortex

The importance of subplate neurons in controlling multiple steps of cortex development was recognized more than a decade ago. However, knowledge of the precise function of subplate neurons in the establishment of cortical circuitry has been missing. A recent study has now started to dissect the functional and molecular mechanisms underlying the generation of transient neuronal circuitry in the visual cortex.