Rapid developmental switch in the mechanisms driving early cortical columnar networks

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.

Regulation of AMPA receptor recruitment at developing synapses

Fast synaptic current at most excitatory synapses in the brain is carried by AMPA and NMDA subtypes of ionotropic glutamate receptors (AMPARs and NMDARs). During development there is an increase in the ratio of AMPAR- to NMDAR-mediated current at these synapses. Recent studies indicate that NMDAR signaling early in development negatively regulates AMPAR expression and function at multiple levels, which likely accounts for the small AMPAR current at developing synapses. This contrasts with the positive role of NMDAR signaling in recruiting AMPARs to synapses during long-term potentiation in the adult brain. Thus, NMDARs exert differential effects on the recruitment of AMPA receptors to synapses depending on the developmental state of the neural circuit.

Inferring connection proximity in networks of electrically coupled cells by subthreshold frequency response analysis

Electrical synapses continuously transfer signals bi-directionally from one cell to another, directly or indirectly via intermediate cells. Electrical synapses are common in many brain structures such as the inferior olive, the subcoeruleus nucleus and the neocortex, between neurons and between glial cells. In the cortex, interneurons have been shown to be electrically coupled and proposed to participate in large, continuous cortical syncytia, as opposed to smaller spatial domains of electrically coupled cells. However, to explore the significance of these findings it is imperative to map the electrical synaptic microcircuits, in analogy with in vitro studies on monosynaptic and disynaptic chemical coupling. Since "walking" from cell to cell over large distances with a glass pipette is challenging, microinjection of (fluorescent) dyes diffusing through gap-junctions remains so far the only method available to decipher such microcircuits even though technical limitations exist. Based on circuit theory, we derive analytical descriptions of the AC electrical coupling in networks of isopotential cells. We then suggest an operative electrophysiological protocol to distinguish between direct electrical connections and connections involving one or more intermediate cells. This method allows inferring the number of intermediate cells, generalizing the conventional coupling coefficient, which provides limited information. We validate our method through computer simulations, theoretical and numerical methods and electrophysiological paired recordings.

GABA: a pioneer transmitter that excites immature neurons and generates primitive oscillations

Developing networks follow common rules to shift from silent cells to coactive networks that operate via thousands of synapses. This review deals with some of these rules and in particular those concerning the crucial role of the neurotransmitter gamma-aminobuytric acid (GABA), which operates primarily via chloride-permeable GABA(A) receptor channels. In all developing animal species and brain structures investigated, neurons have a higher intracellular chloride concentration at an early stage leading to an efflux of chloride and excitatory actions of GABA in immature neurons. This triggers sodium spikes, activates voltage-gated calcium channels, and acts in synergy with NMDA channels by removing the voltage-dependent magnesium block. GABA signaling is also established before glutamatergic transmission, suggesting that GABA is the principal excitatory transmitter during early development. In fact, even before synapse formation, GABA signaling can modulate the cell cycle and migration. The consequence of these rules is that developing networks generate primitive patterns of network activity, notably the giant depolarizing potentials (GDPs), largely through the excitatory actions of GABA and its synergistic interactions with glutamate signaling. These early types of network activity are likely required for neurons to fire together and thus to "wire together" so that functional units within cortical networks are formed. In addition, depolarizing GABA has a strong impact on synaptic plasticity and pathological insults, notably seizures of the immature brain. In conclusion, it is suggested that an evolutionary preserved role for excitatory GABA in immature cells provides an important mechanism in the formation of synapses and activity in neuronal networks.

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.

Kinetic properties of Cl uptake mediated by Na+-dependent K+-2Cl cotransport in immature rat neocortical neurons

GABA, the main inhibitory neurotransmitter in the adult nervous system, evokes depolarizing membrane responses in immature neurons, which are crucial for the generation of early network activity. Although it is well accepted that depolarizing GABA actions are caused by an elevated intracellular Cl- concentration ([Cl-]i), the mechanisms of Cl- accumulation in immature neurons are still a matter of debate. Using patch-clamp, microfluorimetric, immunohistochemical, and molecular biological approaches, we studied the mechanism of Cl- uptake in Cajal-Retzius (CR) cells of immature [postnatal day 0 (P0) to P3] rat neocortex. Gramicidin-perforated patch-clamp and 6-methoxy-N-ethylquinolinium-microfluorimetric measurements revealed a steady-state [Cl-]i of approximately 30 mM that was reduced to values close to passive distribution by bumetanide or Na+-free solutions, suggesting a participation of Na+-K+-2Cl- cotransport isoform 1 (NKCC1) in maintaining elevated [Cl-]i. Expression of NKCC1 was found in CR cells on the mRNA and protein levels. To determine the contribution of NKCC1 to [Cl-]i homeostasis in detail, Cl- uptake rates were analyzed after artificial [Cl-]i depletion. Active Cl- uptake was relatively slow (47.2 +/- 5.0 microM/s) and was abolished by bumetanide or Na+-free solution. Accordingly, whole-cell patch-clamp recordings revealed a low Cl- conductance in CR cells. The low capacity of NKCC1-mediated Cl- uptake was sufficient to maintain excitatory GABAergic membrane responses, however, only at low stimulation frequencies. In summary, our results demonstrate that NKCC1 is abundant in CR cells of immature rat neocortex and that the slow Cl- uptake mediated by this transporter is sufficient to maintain high [Cl-]i required to render GABA responses excitatory.

Cholinergic modulation of spindle bursts in the neonatal rat visual cortex in vivo

Acetylcholine (ACh) is known to shape the adult neocortical activity related to behavioral states and processing of sensory information. However, the impact of cholinergic input on the neonatal neuronal activity remains widely unknown. Early during development, the principal activity pattern in the primary visual (V1) cortex is the intermittent self-organized spindle burst oscillation that can be driven by the retinal waves. Here, we assessed the relationship between this early activity pattern and the cholinergic drive by either blocking or augmenting the cholinergic input and investigating the resultant effects on the activity of the rat visual cortex during the first postnatal week in vivo. Blockade of the muscarinic receptors by intracerebroventricular, intracortical, or supracortical atropine application decreased the occurrence of V1 spindle bursts by 50%, both the retina-independent and the optic nerve-mediated spindle bursts being affected. In contrast, blockade of acetylcholine esterase with physostigmine augmented the occurrence, amplitude, and duration of V1 spindle bursts. Whereas direct stimulation of the cholinergic basal forebrain nuclei increased the occurrence probability of V1 spindle bursts, their chronic immunotoxic lesion using 192 IgG-saporin decreased the occurrence of neonatal V1 oscillatory activity by 87%. Thus, the cholinergic input facilitates the neonatal V1 spindle bursts and may prime the developing cortex to operate specifically on relevant early (retinal waves) and later (visual input) stimuli.

Developmental expression of endogenous oscillations and waves in the auditory cortex involves calcium, gap junctions, and GABA

Neuronal oscillations and population waves (OWs) may be important for the maturation of neural circuits in the cortex and other developing areas of the CNS. We examined endogenous network activity by whole-cell and paired extracellular recordings in the thalamorecipient auditory cortex (ACx) in slices of gerbil pups during the first three postnatal weeks. Separately, we examined network ensemble correlates of the OWs using population intracellular free calcium (Ca2+) imaging in slices bulk-loaded with fura-2 AM. In slices devoid of physiological or pharmacological manipulations, spontaneous multi-neuronal bursts recorded extracellularly at the perirhinal cortex precede bursts simultaneously recorded at the ACx, suggesting their caudorostral propagation. OWs waned after postnatal day (P) 7, ceased following hearing onset (P12), and accompanied altered membrane properties. Population imaging from P2-5 slices with fura-2 AM revealed endogenously generated waves that spread from the perirhinal cortex toward the thalamorecipient ACx. Wave incidence varied between 5 waves/min to 0.4 waves/min. OWs were disrupted by treatment of slices with [Ca2+]i chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, the gap junction blocker mefloquine or the GABAA receptor blocker bicuculline. These results suggest that propagating activity involving calcium, gap junctions and GABAergic transmission exists in the gerbil ACx and it correlates with key developmental events in vivo. We speculate such activity may be integral to postnatal maturation of ACx.

Development of Cholinergic Modulation and Graded Persistent Activity in Layer V of Medial Entorhinal Cortex

During muscarinic modulation, principal neurons from layer V of rat medial entorhinal cortex (mEC) respond to repeated applications of a brief stimulus with a graded change in persistent firing frequency. This pattern of discharge has been proposed to represent an intrinsic mechanism for short-term memory operations. To investigate the implementation of persistent activity in mEC during development, we characterized the electrophysiological properties of layer V principal neurons in the mEC over a range of postnatal stages. We observed significant differences in both passive (resistance, time constant, and resting membrane potential) and active properties (threshold, action potential, and adaptation) of principal neurons from rats aged 5–7, 10–13, 16–19, and 21–23 days. We also examined the properties of muscarinic-dependent persistent activity in EC slices from different age groups. Recordings were conducted using the perforated-patch whole cell technique because persistent activity runs down in the ruptured-patch configuration. Although no neuron in the youngest group exhibited graded persistent activity in response to muscarinic receptor activation, this activity was recorded in the 10- to 13-day-old group and its occurrence increased from 69% in the 16- to 19-day-old group to 76% in the 21- to 23-day-old group. This postnatal increase in neurons endowed with persistent firing properties in mEC was found to parallel the increase in density of ChAT-positive immunostaining of fibers and the developmental changes in M1 muscarinic receptor mRNA levels. All these data suggest that the implementation of mnemonic properties in mEC principal neurons matches the ontogenic development of afferent cholinergic circuits and their signaling components.

Removal of GABA within adult modulatory systems alters electrical coupling and allows expression of an embryonic-like network

The maturation and operation of neural networks are known to depend on modulatory neurons. However, whether similar mechanisms may control both adult and developmental plasticity remains poorly investigated. To examine this issue, we have used the lobster stomatogastric nervous system (STNS) to investigate the ontogeny and role of GABAergic modulatory neurons projecting to small pattern generating networks. Using immunocytochemistry, we found that modulatory input neurons to the stomatogastric ganglion (STG) express GABA only after metamorphosis, a time that coincides with the developmental switch from a single to multiple pattern generating networks within the STNS. We demonstrate that blocking GABA synthesis with 3-mercapto-propionic acid within the adult modulatory neurons results in the reconfiguration of the distinct STG networks into a single network that generates a unified embryonic-like motor pattern. Using dye-coupling experiments, we also found that gap-junctional coupling is greater in embryos and GABA-deprived adults exhibiting the unified motor pattern compared with control adults. Furthermore, GABA was found to diminish directly the extent and strength of electrical coupling within adult STG networks. Together, these observations suggest the acquisition of a GABAergic phenotype by modulatory neurons after metamorphosis may induce the reconfiguration of the single embryonic network into multiple adult networks by directly decreasing electrical coupling. The findings also suggest that adult neural networks retain the ability to express typical embryonic characteristics, indicating that network ontogeny can be reversed and that changes in electrical coupling during development may allow the segregation of multiple distinct functional networks from a single large embryonic network.

Genetic interplay between the transcription factors Sp8 and Emx2 in the patterning of the forebrain

BACKGROUND

The forebrain consists of multiple structures necessary to achieve elaborate functions. Proper patterning is, therefore, a prerequisite for the generation of optimal functional areas. Only a few factors have been shown to control the genetic networks that establish early forebrain patterning.

RESULTS AND CONCLUSION

Using conditional inactivation, we show that the transcription factor Sp8 has an essential role in the molecular and functional patterning of the developing telencephalon along the anteroposterior axis by modulating the expression gradients of Emx2 and Pax6. Moreover, Sp8 is essential for the maintenance of ventral cell identity in the septum and medial ganglionic eminence (MGE). This is probably mediated through a positive regulatory interaction with Fgf8 in the medial wall, and Nkx2.1 in the rostral MGE anlage, and independent of SHH and WNT signaling. Furthermore, Sp8 is required during corticogenesis to sustain a normal progenitor pool, and to control preplate splitting, as well as the specification of cellular diversity within distinct cortical layers.

A Parturition-Associated Nonsynaptic Coherent Activity Pattern in the Developing Hippocampus

Correlated neuronal activity is instrumental in the formation of networks, but its emergence during maturation is poorly understood. We have used multibeam two-photon calcium microscopy combined with targeted electrophysiological recordings in order to determine the development of population coherence from embryonic to postnatal stages in the hippocampus. At embryonic stages (E16-E19), synchronized activity is absent, and neurons are intrinsically active and generate L-type channel-mediated calcium spikes. At birth, small cell assemblies coupled by gap junctions spontaneously generate synchronous nonsynaptic calcium plateaus associated to recurrent burst discharges. The emergence of coherent calcium plateaus at birth is controlled by oxytocin, a maternal hormone initiating labour, and progressively shut down a few days later by the synapse-driven giant depolarizing potentials (GDPs) that synchronize the entire network. Therefore, in the developing hippocampus, delivery is an important signal that triggers the first coherent activity pattern, which is silenced by the emergence of synaptic transmission.

Development of the spontaneous activity transients and ongoing cortical activity in human preterm babies

Recent experimental studies have shown that developing cortex in several animals species, including humans, exhibits spontaneous intermittent activity that is believed to be crucial for the proper wiring of early brain networks. The present study examined the developmental changes in these spontaneous activity transients (SAT) and in other ongoing cortical activities in human preterm babies. Full-band electroencephalography (FbEEG) recordings were obtained from 16 babies at conceptional ages between 32.8 and 40 wk. We examined the SATs and the intervening ongoing cortical activities (inter-SAT; iSAT) with average waveforms, their variance and power, as well as with wavelet-based time-frequency analyses. Our results show, that the low frequency power and the variance of the average waveform of SAT decrease during development. There was a simultaneous increase in the activity at higher frequencies, with most pronounced increase at theta-alpha range (4-9 Hz). In addition to the overall increase, the activity at higher frequencies showed an increased grouping into bursts that are nested in the low frequency (0.5-1 Hz) waves. Analysis of the iSAT epochs showed a developmental increase in power at lower frequencies in quiet sleep. There was an increase in a wide range of higher frequencies (4-16 Hz), whereas the ratio of beta (16-30 Hz) and theta-alpha (4-9 Hz) range activity declined, indicating a preferential increase at theta-alpha range activity. Notably, SAT and iSAT activities remained distinct throughout the development in all measures used in our study. The present results are consistent with the idea that SAT and the other ongoing cortical activities are distinct functional entities. Recognition of these two basic mechanisms in the cortical activity in preterm human babies opens new rational approaches for an evaluation and monitoring of early human brain function.

Transient electrical coupling regulates formation of neuronal networks

Electrical synapses are abundant before and during developmental windows of intense chemical synapse formation, and might therefore contribute to the establishment of neuronal networks. Transient electrical coupling develops and is then eliminated between regenerating Helisoma motoneurons 110 and 19 during a period of 48-72 h in vivo and in vitro following nerve injury. An inverse relationship exists between electrical coupling and chemical synaptic transmission at these synapses, such that the decline in electrical coupling is coincident with the emergence of cholinergic synaptic transmission. In this study, we have generated two- and three-cell neuronal networks to test whether predicted synaptogenic capabilities were affected by previous synaptic interactions. Electrophysiological analyses demonstrated that synapses formed in three-cell neuronal networks were not those predicted based on synaptogenic outcomes in two-cell networks. Thus, new electrical and chemical synapse formation within a neuronal network is dependent on existing connectivity of that network. In addition, new contacts formed with established networks have little impact on these existing connections. These results suggest that network-dependent mechanisms, particularly those mediated by gap junctional coupling, regulate synapse formation within simple neural networks.

Neonatal SEP - back to bedside with basic science

Scalp-recorded somatosensory evoked potentials (SEPs) have been successfully used in neonatal assessment for several decades. The current routine SEP paradigm is markedly predictive for future cerebral palsy (CP) or other neurocognitive sequelae in brain-injured babies. Recent advances in basic science have dramatically increased our knowledge about structural-functional development of SEP-related brain mechanisms. It has thereby become apparent that preterm SEP differs from that in more mature counterparts in that it also comprises responses from transient brain structures, and hence being unique to the preterm period. It is now obvious also that several aspects in the current SEP paradigm, ranging from the type of stimulation to the methods of recording and analysis, are suboptimal for preterm babies. Recent progress in recording and analysis techniques have made it possible to combine SEP studies with EEG recordings, as well as to implement advanced analyses (e.g. time-frequency analysis) into routine practice. This review summarizes literature from relevant areas in basic science, and proposes a novel, integrated approach in neonatal SEP studies in order to significantly increase the fidelity of testing somatosensory system.

Development of neonatal EEG activity: from phenomenology to physiology

After having been in routine use for about half a century, neonatal EEG is currently facing unprecedented challenges in assessing and monitoring brain function during intensive care of preterm babies. It has therefore become increasingly important to understand the neurophysiological processes underlying EEG activity, as well as to identify those features of brain activity that are essential for brain development. By integrating the existing literature from basic neuroscience to neonatal EEG, the present review proposes a simple, neurophysiologically and neuroanatomically based framework for neonatal EEG interpretation. This is composed of two developmental trajectories: one related to discrete spontaneous activity transients (SAT) and the other to the ongoing, apparently oscillatory EEG activity. This framework can readily be applied to clinical use. It may open novel avenues to automated analysis in EEG monitoring and, moreover, it may facilitate genuine translational research.

The development of cerebral connections during the first 20-45 weeks' gestation

We have correlated data on neuroanatomical organization and magnetic resonance imaging of transient fetal zones shown to contain connectivity elements (growing axons, synapses, dendrites). In the fetal phase, afferent fibres 'wait' within the subplate zone which is the most prominent lamina on histological and magnetic resonance images and is a substrate of endogenous neuronal activity. In early preterm the thalamocortical afferents accumulate within the superficial subplate and grow into cortical plate developing synapses. In late preterm, the resolution of the subplate and growth of cortico-cortical fibres into the cortical plate occur simultaneously with gyration. Both preterm phases characterize the coexistence of endogenous and sensory-driven circuitries and occurrence of the transient electrical phenomena. In neonates, the long cortico-cortical pathways stop growth, and the main histogenetic events are an elaboration of intracortical circuitry and synaptogenesis. In conclusion, the growth of the axonal pathways preterm explains their vulnerability and plasticity. In neonates the vulnerability is related to the intracortical circuitry.

Network mechanisms of spindle-burst oscillations in the neonatal rat barrel cortex in vivo

Early in development, cortical networks generate particular patterns of activity that participate in cortical development. The dominant pattern of electrical activity in the neonatal rat neocortex in vivo is a spatially confined spindle-burst. Here, we studied network mechanisms of generation of spindle-bursts in the barrel cortex of neonatal rats using a superfused cortex preparation in vivo. Both spontaneous and sensory-evoked spindle-bursts were present in the superfused barrel cortex. Pharmacological analysis revealed that spindle-bursts are driven by glutamatergic synapses with a major contribution of AMPA/kainate receptors, but slight participation of NMDA receptors and gap junctions. Although GABAergic synapses contributed minimally to the pacing the rhythm of spindle-burst oscillations, surround GABAergic inhibition appeared to be crucial for their compartmentalization. We propose that local spindle-burst oscillations, driven by glutamatergic synapses and spatially confined by GABAergic synapses, contribute to the development of barrel cortex during the critical period of developmental plasticity.

A beta2-frequency (20-30 Hz) oscillation in nonsynaptic networks of somatosensory cortex

Beta2 frequency (20-30 Hz) oscillations appear over somatosensory and motor cortices in vivo during motor preparation and can be coherent with muscle electrical activity. We describe a beta2 frequency oscillation occurring in vitro in networks of layer V pyramidal cells, the cells of origin of the corticospinal tract. This beta2 oscillation depends on gap junctional coupling, but it survives a cut through layer 4 and, hence, does not depend on apical dendritic electrogenesis. It also survives a blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors or a blockade of GABA(A) receptors that is sufficient to suppress gamma (30-70 Hz) oscillations in superficial cortical layers. The oscillation period is determined by the M type of K+ current.

Rapid Cortical Oscillations and Early Motor Activity in Premature Human Neonate

Delta-brush is the dominant pattern of rapid oscillatory activity (8-25 Hz) in the human cortex during the third trimester of gestation. Here, we studied the relationship between delta-brushes in the somatosensory cortex and spontaneous movements of premature human neonates of 29-31 weeks postconceptional age using a combination of scalp electroencephalography and monitoring of motor activity. We found that sporadic hand and foot movements heralded the appearance of delta-brushes in the corresponding areas of the cortex (lateral and medial regions of the contralateral central cortex, respectively). Direct hand and foot stimulation also reliably evoked delta-brushes in the same areas. These results suggest that sensory feedback from spontaneous fetal movements triggers delta-brush oscillations in the central cortex in a somatotopic manner. We propose that in the human fetus in utero, before the brain starts to receive elaborated sensory input from the external world, spontaneous fetal movements provide sensory stimulation and drive delta-brush oscillations in the developing somatosensory cortex contributing to the formation of cortical body maps.

Retinal waves trigger spindle bursts in the neonatal rat visual cortex

During visual system development, the light-insensitive retina spontaneously generates waves of activity, which are transmitted to the lateral geniculate nucleus. The crucial question is whether retinal waves are further transmitted to the cortex and influence the early cortical patterns of activity. Using simultaneous recordings from the rat retina and visual cortex during the first postnatal week in vivo, we found that spontaneous retinal bursts are correlated with spindle bursts (intermittent network bursts associated with spindle-shape field oscillations) in the contralateral visual cortex (V1). V1 spindle bursts could be evoked by electrical stimulation of the optic nerve. Intraocular injection of forskolin, which augments retinal waves, increased the occurrence of V1 spindle bursts. Blocking propagation of retinal activity, or removal of the retina reduced the frequency, but did not completely eliminate the cortical spindle bursts. These results indicate that spontaneous retinal waves are transmitted to the visual cortex and trigger endogenous spindle bursts. We propose that the interaction between retinal waves and spindle bursts contributes to the development of visual pathways to the cortex.

Early patterns of electrical activity in the developing cerebral cortex of humans and rodents

During prenatal and early postnatal development, the cerebral cortex exhibits synchronized oscillatory network activity that is believed to be essential for the generation of neuronal cortical circuits. The nature and functional role of these early activity patterns are of central interest in neuroscience. Much of the research is performed in rodents and in vitro, but how closely do these model systems relate to the human fetal brain? In this review, we compare observations in humans with in vivo and in vitro rodent data, focusing on particular oscillatory activity patterns that share many common features: delta brushes, spindle bursts and spindle-like oscillations. There is considerable evidence that the basic functional properties of immature cortical networks are conserved through mammalian evolution, making the neonatal rodent an excellent model for studying early cortical activity and associated plasticity during the developmental period corresponding to the human fetal stage. This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).