Neuronal activity during development: permissive or instructive?

N-methyl-d-aspartate receptor activation exerts a dual control on postnatal development of nucleus tractus solitarii neurons in vivo

We have used a morphological approach to evaluate the role of NMDA receptors (NMDAR) in postnatal development of brainstem neurons in awake rats. Chronic NMDAR blockade was performed by placing drug-impregnated Elvax implants over the brainstem at the fifth postnatal day (P5). Compared with control, NMDAR blockade led to a transient increase in dendritic arbor area and filopodium density until P12 followed by a rapid decline in both parameters. Electron microscopy observations showed that these changes correlated with an increase in synapse density at P14 followed by a decrease in synapse density at P28 if chronic NMDAR blockade was maintained until P21. These results support the hypothesis that synapse formation does not require NMDAR activation. In addition, our data suggest a dual role for NMDAR in controlling the synapse number. Early in development NMDARs may be involved in controlling the rate of synapse elimination. Later on, they may subserve synapse stabilization. The physiological significance of these results is discussed.

Development of columnar topography in the excitatory layer 4 to layer 2/3 projection in rat barrel cortex

The excitatory feedforward projection from layer (L) 4 to L2/3 in rat primary somatosensory (S1) cortex exhibits precise, columnar topography that is critical for columnar processing of whisker inputs. Here, we characterize the development of axonal topography in this projection using single-cell reconstructions in S1 slices. In the mature projection [postnatal day (P) 14-26], axons of L4 cells extending into L2/3 were confined almost entirely to the home barrel column, consistent with previous results. At younger ages (P8-11), however, axonal topography was significantly less columnar, with a large proportion of branches innervating neighboring barrel columns representing adjacent whisker rows. Mature topography developed from this initial state by targeted axonal growth within the home column and by growth of barrel columns themselves. Raising rats with all or a subset of whiskers plucked from P8-9, manipulations that induce reorganization of functional whisker maps and synaptic depression at L4 to L2/3 synapses, did not alter normal anatomical development of L4 to L2/3 axons. Thus, development of this projection does not require normal sensory experience after P8, and deprivation-induced reorganization of whisker maps at this age is unlikely to involve physical remodeling of L4 to L2/3 axons.

Developmental modulation of retinal wave dynamics: shedding light on the GABA saga

Embryonic spontaneous activity, in the form of propagating waves, is crucial for refining visual connections. To study what aspects of this correlated activity are instructive, we must first understand how their dynamics change with development and what factors trigger their disappearance after birth. Here we report that in the turtle retina, GABA, rather than glutamate and acetylcholine, influences developmental changes in wave dynamics. Using calcium imaging of the ganglion cell layer, we report how waves switch from fast and broad, when they emerge, to slow and narrow a few days before hatching, coinciding with the emergence of excitatory GABA(A) receptor-mediated activity. Around hatching, waves gradually become stationary patches, whereas GABA(A) shifts from excitatory to inhibitory, coinciding with the upregulation of the cotransporter KCC2, suggesting that changes in intracellular chloride underlie the shift. Dark-rearing from hatching causes correlated spontaneous activity to persist, whereas GABA(A) responses remain excitatory, and KCC2 expression is weaker. We conclude that GABA plays an important regulatory role during the maturation of retinal neural activity. Using a simple and elegant mechanism, namely the switch from excitatory to inhibitory, GABA(A) receptor-mediated activity is necessary and sufficient to cause retinal waves to stop propagating, ultimately leading to the disappearance of correlated spontaneous activity. Moreover, our results suggest that visual experience modulates the GABAergic switch.

Adenylyl cyclase I regulates AMPA receptor trafficking during mouse cortical 'barrel' map development

Cortical map formation requires the accurate targeting, synaptogenesis, elaboration and refinement of thalamocortical afferents. Here we demonstrate the role of Ca2+/calmodulin-activated type-I adenylyl cyclase (AC1) in regulating the strength of thalamocortical synapses through modulation of AMPA receptor (AMPAR) trafficking using barrelless mice, a mutant without AC1 activity or cortical 'barrel' maps. Barrelless synapses are stuck in an immature state that contains few functional AMPARs that are rarely silent (NMDAR-only). Long-term potentiation (LTP) and long-term depression (LTD) at thalamocortical synapses require postsynaptic protein kinase A (PKA) activity and are difficult to induce in barrelless mice, probably due to an inability to properly regulate synaptic AMPAR trafficking. Consistent with this, both the extent of PKA phosphorylation on AMPAR subunit GluR1 and the expression of surface GluR1 are reduced in barrelless neurons. These results suggest that activity-dependent mechanisms operate through an AC1/PKA signaling pathway to target some synapses for consolidation and others for elimination during barrel map formation.

Area specificity and topography of thalamocortical projections are controlled by ephrin/Eph genes

The mechanisms generating precise connections between specific thalamic nuclei and cortical areas remain poorly understood. Using axon tracing analysis of ephrin/Eph mutant mice, we provide in vivo evidence that Eph receptors in the thalamus and ephrins in the cortex control intra-areal topographic mapping of thalamocortical (TC) axons. In addition, we show that the same ephrin/Eph genes unexpectedly control the inter-areal specificity of TC projections through the early topographic sorting of TC axons in an intermediate target, the ventral telencephalon. Our results constitute the first identification of guidance cues involved in inter-areal specificity of TC projections and demonstrate that the same set of mapping labels is used differentially for the generation of topographic specificity of TC projections between and within individual cortical areas.

Absence of Whisker-related pattern formation in mice with NMDA receptors lacking coincidence detection properties and calcium signaling

Precise refinement of synaptic connectivity is the result of activity-dependent mechanisms in which coincidence-dependent calcium signaling by NMDA receptors (NMDARs) under control of the voltage-dependent Mg2+ block might play a special role. In the developing rodent trigeminal system, the pattern of synaptic connections between whisker-specific inputs and their target cells in the brainstem is refined to form functionally and morphologically distinct units (barrelettes). To test the role of NMDA receptor signaling in this process, we introduced the N598R mutation into the native NR1 gene. This leads to the expression of functional NMDARs that are Mg2+ insensitive and Ca2+ impermeable. Newborn mice expressing exclusively NR1 N598R-containing NMDARs do not show any whisker-related patterning in the brainstem, whereas the topographic projection of trigeminal afferents and gross brain morphology appear normal. Furthermore, the NR1 N598R mutation does not affect expression levels of NMDAR subunits and other important neurotransmitter receptors. Our results show that coincidence detection by, and/or Ca2+ permeability of, NMDARs is necessary for the development of somatotopic maps in the brainstem and suggest that highly specific signaling underlies synaptic refinement.

Activity blockade increases the number of functional synapses in the hippocampus of newborn rats

During development neuronal circuitries are refined by activity. Here we studied the role of spontaneous electrical activity in the regulation of synapse formation in the intact newborn (Postnatal Day 3; P3) rat hippocampus in vitro. The blockade of the spontaneous network activity with TTX led to an increase in the number of functional excitatory synapses in the CA3 area of the developing hippocampus. In parallel, there was a substantial increase in the expression levels of the presynaptic markers synaptophysin, synaptotagmin, and synapsin I and of GluR1 AMPA receptor subunits. These changes were associated with an increase in the frequency and amplitude of AMPA receptor-mediated miniature excitatory postsynaptic currents (mEPSCs). Our correlated immunocytochemical, electronmicroscopical, and electrophysiological experiments indicate that in the developing hippocampus spontaneous network activity controls the number of functional synapses.

Visual experience before eye-opening and the development of the retinogeniculate pathway

Visual experience before eye-opening is not usually thought to have any developmental significance. Here we show that naturalistic visual stimuli presented through unopened eyelids robustly activate neurons in the ferret dorsal lateral geniculate nucleus. Further, dark-rearing prior to natural eye-opening has striking effects upon geniculate physiology. Receptive field maps after dark-rearing show increased convergence of On- and Off-center responses, and neurons frequently respond to both bright and dark phases of drifting gratings. There is also increased selectivity for the orientation of the gratings. These abnormalities of On-Off segregation can be explained by the finding that the responses of immature On and Off cells to naturalistic stimuli are strongly anticorrelated.

The role of nAChR-mediated spontaneous retinal activity in visual system development

In the developing vertebrate retina, nAChR synapses are among the first to appear. This early cholinergic circuitry plays a key role in generating "retinal waves," spontaneously generated waves of action potentials that sweep across the ganglion cell layer. These retinal waves exist for a short period of time during development when several circuits within the visual system are being established. Here I review the cholinergic circuitry of the developing retina and the role these early circuits play in the development of the retina itself and of retinal projections to the lateral geniculate nucleus of the thalamus.

Postnatal development of rat hippocampal gamma rhythm in vivo

Network oscillations in the gamma-frequency band (20-100 Hz) may have a central role in the timing and coordination of neural activity in the adult brain, yet their appearance in the course of development has remained unexplored. Moreover, electroencephalogram (EEG)-based classification of the vigilance states [active sleep (AS), quiet sleep (QS), or awake (W)] has been thought to be possible only after the second postnatal week. We now report the presence of spontaneous hippocampal gamma oscillations in the area CA3 of freely moving rats at postnatal days (P) 5-10. Initially, at P5, the gamma oscillations were seen in time-frequency analyses of intrahippocampal EEG recordings as brief (<500 ms) bursts at 20-30 Hz. The early gamma rhythmicity was most pronounced during periods of AS but was occasionally detected also during QS. Toward P10, the gamma oscillations gained amplitude and extended also to higher (<or=60 Hz) frequencies. In parallel, the gamma oscillations were progressively more and more confined to AS. To further consolidate these findings, we compared amplitude spectra averaged within the behavioral categories. AS was characterized by the appearances of gamma (20-30 Hz) and theta (3-5 Hz) peaks at P6 and at P8, respectively. QS, on the other hand, had considerably smoother amplitude distributions between 1 and 100 Hz for P5-P10, with no peaks in gamma or theta bands. Hippocampal gamma rhythm thus seems to hallmark early AS. Our data provide the first in vivo evidence for both the presence and the behavioral correlate of spontaneous gamma oscillations in the newborn rat.

Retinal waves: implications for synaptic learning rules during development

Neural activity is often required for the final stages of synaptic refinement during brain development. It is thought that learning rules acting at the individual synapse level, which specify how pre- and postsynaptic activity lead to changes in synaptic efficacy, underlie such activity-dependent development. How such rules might function in vivo can be addressed in the retinogeniculate system because the input activity from the retina and its importance in development are both known. In fact, detailed studies of retinal waves have revealed their complex spatiotemporal properties, providing insights into the mechanisms that use such activity to guide development. First of all, the information useful for development is contained in the retinal waves and can be quantified, placing constraints on synaptic learning rules that use this information. Furthermore, knowing the distribution of activity over the entire set of inputs makes it possible to address a necessary component of developmental refinement: rules governing competition between synaptic inputs. In this way, the detailed knowledge of retinal input and lateral geniculate nucleus development provides a unique opportunity to relate the rules of synaptic plasticity directly to their role in development.

Visually driven modulation of glutamatergic synaptic transmission is mediated by the regulation of intracellular polyamines

Ca2+-permeable AMPARs are inwardly rectifying due to block by intracellular polyamines. Neuronal activity regulates polyamine synthesis, yet whether this affects Ca2+-AMPAR-mediated synaptic transmission is unknown. We test whether 4 hr of increased visual stimulation regulates glutamatergic retino-tectal synapses in Xenopus tadpoles. Tectal neurons containing Ca2+-AMPARs form a gradient along the rostro-caudal developmental axis. These neurons had inwardly rectifying AMPAR-mediated EPSCs. Four hours of visual stimulation or addition of intracellular spermine increased rectification in immature neurons. Polyamine synthesis inhibitors blocked the effect of visual stimulation, suggesting that visual activity regulates AMPARs via the polyamine synthesis pathway. This modulation resulted in changes in the integrative properties of tectal neurons. Regulation of polyamine synthesis by physiological stimuli is a novel form of modulation of synaptic transmission important for understanding the short-term effects of enhanced sensory experience during development.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Ocular dominance development revisited

New approaches to the study of ocular dominance development, a model system for the development of neural architecture, indicate that eye-specific columns in primary visual cortex emerge substantially before the onset of the critical period, during which neural connections can be altered by visual experience. The timing, speed and specificity of column emergence implicate molecular patterning mechanisms, along with patterns of neural activity, in the generation of this columnar architecture.

An instructive role for retinal waves in the development of retinogeniculate connectivity

A central hypothesis of neural development is that patterned activity drives the refinement of initially imprecise connections. We have examined this hypothesis directly by altering the frequency of spontaneous waves of activity that sweep across the mammalian retina prior to vision. Activity levels were increased in vivo using agents that elevate cAMP. When one eye is made more active, its layer within the LGN is larger despite the other eye having normal levels of activity. Remarkably, when the frequency of retinal waves is increased in both eyes, normally sized layers form. Because relative, rather than absolute, levels of activity between the eyes regulate the amount of LGN territory devoted to each eye, we conclude that activity acts instructively to guide binocular segregation during development.

Mechanisms underlying developmental changes in the firing patterns of ON and OFF retinal ganglion cells during refinement of their central projections

Patterned neuronal activity is implicated in the refinement of connectivity during development. Calcium-imaging studies of the immature ferret visual system demonstrated previously that functionally separate ON and OFF retinal ganglion cells (RGCs) develop distinct temporal patterns of spontaneous activity as their axonal projections undergo refinement. OFF RGCs become spontaneously more active compared with ON cells, resulting in a decrease in synchronous activity between these cell types. This change in ON and OFF activity patterns is suitable for driving the activity-dependent refinement of their axonal projections. Here, we used whole-cell and perforated-patch recording techniques to elucidate the mechanisms that underlie the developmental alteration in the ON and OFF RGC activity patterns. First, we show that before the refinement period, ON and OFF RGCs have similar spike patterns; however, during the period of segregation, OFF RGCs demonstrate significantly higher spike rates relative to ON cells. With increasing age, OFF cells require less depolarization to reach their action potential threshold and fire more spikes in response to current injection compared with ON cells. In addition, spontaneous postsynaptic currents and potentials are greater in magnitude in OFF cells than ON cells. In contrast, before axonal refinement, there are no differences in the intrinsic excitability or synaptic drive onto ON and OFF cells. Together, our results show that developmental changes in ON and OFF RGC excitability and in the strength of their synaptic drives act together to reshape the spike patterns of these cells in a manner appropriate for the refinement of their connectivity.

Barrel cortex critical period plasticity is independent of changes in NMDA receptor subunit composition

The regulation of NMDA receptor (NMDAR) subunit composition and expression during development is thought to control the process of thalamocortical afferent innervation, segregation, and plasticity. Thalamocortical synaptic plasticity in the mouse is dependent on NMDARs containing the NR2B subunit, which are the dominant form during the "critical period" window for plasticity. Near the end of the critical period there is a gradual increase in the contribution of NR2A subunits that happens in parallel to changes in NMDAR-mediated current kinetics. However, no extension of the critical period occurs in NR2A knockout mice, despite the fact that NMDA subunit composition and current kinetics remain immature past the end of the critical period. These data suggest that regulation of NMDAR subunit composition is not essential for closing the critical period plasticity window in mouse somatosensory barrel cortex.

UNC-119 suppresses axon branching inC. elegans

The architecture of the differentiated nervous system is stable but the molecular mechanisms that are required for stabilization are unknown. We characterized the gene unc-119 in the nematode Caenorhabditis elegans and demonstrate that it is required to stabilize the differentiated structure of the nervous system. In unc-119 mutants, motor neuron commissures are excessively branched in adults. However, live imaging demonstrated that growth cone behavior during extension was fairly normal with the exception that the overall rate of migration was reduced. Later, after development was complete, secondary growth cones sprouted from existing motor neuron axons and cell bodies. These new growth cones extended supernumerary branches to the dorsal nerve cord at the same time the previously formed axons retracted. These defects could be suppressed by expressing the UNC-119 protein after embryonic development; thus demonstrating that UNC-119 is required for the maintenance of the nervous system architecture. Finally, UNC-119 is located in neuron cell bodies and axons and acts cell-autonomously to inhibit axon branching.

The role of early neural activity in the maturation of turtle retinal function

In the developing vertebrate retina, ganglion cells fire spontaneous bursts of action potentials long before the eye becomes exposed to sensory experience at birth. These early bursts are synchronised between neighbouring retinal ganglion cells (RGCs), yielding unique spatiotemporal patterns: 'waves' of activity sweep across large retinal areas every few minutes. Both at retinal and extraretinal levels, these embryonic retinal waves are believed to guide the wiring of the visual system using hebbian mechanisms of synaptic strengthening. In the first part of this review, we recapitulate the evidence for a role of these embryonic spontaneous bursts of activity in shaping developing complex receptive field properties of RGCs in the turtle embryonic retina. We also discuss the role of visual experience in establishing RGC visual functions, and how spontaneous activity and visual experience interact to bring developing receptive fields to maturation. We have hypothesised that the physiological changes associated with development reflect modifications in the dendritic arbours of RGCs, the anatomical substrate of their receptive fields. We demonstrate that there is a temporal correlation between the period of receptive field expansion and that of dendritic growth. Moreover, the immature spontaneous activity contributes to dendritic growth in developing RGCs. Intracellular staining of RGCs reveals, however, that immature receptive fields only rarely show direct correlation with the layout of the corresponding dendritic tree. To investigate the possibility that not only the presence of the spontaneous activity, but even the precise spatiotemporal patterns encoded in retinal waves might contribute to the refinement of retinal neural circuitry, first we must clarify the mechanisms mediating the generation and propagation of these waves across development. In the second part of this review, we present evidence that turtle retinal waves, visualised using calcium imaging, exhibit profound changes in their spatiotemporal patterns during development. From fast waves sweeping across large retinal areas and recruiting many cells on their trajectory at early stages, waves become slower and eventually stop propagating towards hatching, when they become stationary patches of neighbouring coactive RGCs. A developmental switch from excitatory to inhibitory GABAA responses appears to mediate the modification in spontaneous activity patterns while the retina develops. Future chronic studies using specific spatiotemporal alterations of the waves will shed a new light on how the wave dynamics help in sculpting retinal receptive fields.

The information content of spontaneous retinal waves

Spontaneous neural activity that is present in the mammalian retina before the onset of vision is required for the refinement of retinotopy in the lateral geniculate nucleus and superior colliculus. This paper explores the information content of this retinal activity, with the goal of determining constraints on the nature of the developmental mechanisms that use it. Through information-theoretic analysis of multielectrode and calcium-imaging experiments, we show that the spontaneous retinal activity present early in development provides information about the relative positions of retinal ganglion cells and can, in principle, be used at retinogeniculate and retinocollicular synapses to refine retinotopy. Remarkably, we find that most retinotopic information provided by retinal waves exists on relatively coarse time scales, suggesting that developmental mechanisms must be sensitive to timing differences from 100 msec up to 2 sec to make optimal use of it. In fact, a simple Hebbian-type learning rule with a correlation window on the order of seconds is able to extract the bulk of the available information. These findings are consistent with bursts of action potentials (rather than single spikes) being the unit of information used during development and suggest new experimental approaches for studying developmental plasticity of the retinogeniculate and retinocollicular synapses. More generally, these results demonstrate how the properties of neuronal systems can be inferred from the statistics of their input.

Development of retinal ganglion cell structure and function

In this review, we summarize the main stages of structural and functional development of retinal ganglion cells (RGCs). We first consider the various mechanisms that are involved in restructuring of dendritic trees. To date, many mechanisms have been implicated including target-dependent factors, interactions from neighboring RGCs, and afferent signaling. We also review recent evidence showing how rapidly such dendritic remodeling might occur, along with the intracellular signaling pathways underlying these rearrangements. Concurrent with such structural changes, the functional responses of RGCs also alter during maturation, from sub-threshold firing to reliable spiking patterns. Here we consider the development of intrinsic membrane properties and how they might contribute to the spontaneous firing patterns observed before the onset of vision. We then review the mechanisms by which this spontaneous activity becomes correlated across neighboring RGCs to form waves of activity. Finally, the relative importance of spontaneous versus light-evoked activity is discussed in relation to the emergence of mature receptive field properties.

Functional requirement for class I MHC in CNS development and plasticity

Class I major histocompatibility complex (class I MHC) molecules, known to be important for immune responses to antigen, are expressed also by neurons that undergo activity-dependent, long-term structural and synaptic modifications. Here, we show that in mice genetically deficient for cell surface class I MHC or for a class I MHC receptor component, CD3zeta, refinement of connections between retina and central targets during development is incomplete. In the hippocampus of adult mutants, N-methyl-D-aspartate receptor-dependent long-term potentiation (LTP) is enhanced, and long-term depression (LTD) is absent. Specific class I MHC messenger RNAs are expressed by distinct mosaics of neurons, reflecting a potential for diverse neuronal functions. These results demonstrate an important role for these molecules in the activity-dependent remodeling and plasticity of connections in the developing and mature mammalian central nervous system (CNS).

Differential expression of the PSD-95 gene family in electrosensory neurons

The PSD-95 family of membrane-associated guanylate kinase (MAGUK) proteins are involved in the assembly and organization of neurotransmitter receptors at excitatory synapses in the vertebrate nervous system. We have isolated partial cDNAs for five PSD-95 family members from Apteronotus leptorhynchus brain RNA using a degenerate PCR method. The amino acid sequences deduced indicate that A. leptorhynchus neurons express homologues of the mammalian PSD-93, SAP-97, and SAP-102 MAGUKs and two homologues of mammalian PSD-95. In situ hybridization experiments have been carried out to localize the cellular expression of all five MAGUK mRNAs in the central nervous system of A. leptorhynchus. In the cerebellum the expression patterns are highly similar to patterns reported for mammalian cerebellum, suggesting an evolutionary conservation of the functional roles in this gene family. Cellular levels of expression of the PSD-95 MAGUK mRNAs and the NMDAR-1 mRNA were highly correlated in neurons of the dorsal forebrain but were not correlated in neurons of the electrosensory lateral line lobe (ELL) or the cerebellum. These results suggest that the expression of PSD-95 MAGUK genes in forebrain neurons may provide mechanisms for synaptic organization that are not shared by neurons in the ELL and cerebellum.

Development. The decade of the developing brain

Our understanding of neural development has advanced dramatically over the past decade. Significant insights have now been obtained into seven fundamental developmental processes: first, induction of the neural plate; second, regionalization of the neural tube along the dorsoventral and anteroposterior axes; third, generation of neurons and glia from multipotential precursors; fourth, apoptotic cell death; fifth, migration of neurons; sixth, guidance of axons to their targets; and seventh, formation of synapses.

Altered development of visual subcortical projections following neonatal thalamic ablation in the hamster

Previous research has demonstrated that precise patterns of axonal connectivity often develop during a series of stages characterized by pathfinding, target recognition, and address selection. This last stage involves the focusing of projections to a precisely defined region within the target. Because thalamic projections begin to innervate cortex before the latter stages are reached, these projections may be important in the establishment of adult-like patterns of cortical connectivity. To address this issue, we examined the mature corticopontine and corticospinal projections of visual cortex deprived of early thalamic input by visual thalamic ablation. Although ablations on the day of birth in hamsters did not disrupt the targeting of appropriate subcortical structures by visual cortical axons, they did alter the organization of projections within the basilar pons and spinal cord. The density and spread of visual corticopontine connections in lesioned animals was greatly increased relative to unlesioned animals, suggesting that thalamic afferents are required during address selection, when the topographic specificity of projections is established. To determine whether early visual thalamic ablation increases connectivity by stabilizing an exuberant developmental projection, we examined the normal development of visual corticopontine connections in hamsters ages postnatal days 1-17 (P1-P17). From the earliest ages, visual cortical axons innervate the pontine nucleus in regions specific to their adult projection zones and show progressive growth within these zones. At no time during development do projections exist that are equivalent to the projections found after thalamic ablation, suggesting that removal of thalamic input does not simply stabilize a developmental projection.

Malformation of the functional organization of somatosensory cortex in adult ephrin-A5 knock-out mice revealed by in vivo functional imaging

The molecular mechanisms that coordinate the functional organization of the mammalian neocortex are largely unknown. We tested the involvement of a putative guidance label, ephrin-A5, in the functional organization of the somatosensory cortex by quantifying the functional representations of individual whiskers in vivo in adult ephrin-A5 knock-out mice, using intrinsic signal optical imaging. In wild-type mice ephrin-A5 is expressed in a gradient in the somatosensory cortex during development. In adult ephrin-A5 knock-out mice, we found a spatial gradient of change in the amount of cortical territory shared by individual whisker functional representations across the somatosensory cortex, as well as a gradient of change in the distance between the functional representations. Both gradients of change were in correspondence with the developmental expression gradient of ephrin-A5 in wild-type mice. These changes involved malformations of the cortical spacing of the thalamocortical components, without concurrent malformations of the intracortical components of individual whisker functional representations. Overall, these results suggest that a developmental guidance label, such as ephrin-A5, is involved in establishing certain spatial relationships of the functional organization of the adult neocortex, and they underscore the advantage of investigating gene manipulation using in vivo functional imaging.

Early postnatal development of functional ocular dominance columns in cat primary visual cortex

During postnatal development of the visual cortex the thalamocortical afferents serving the two eyes segregate into alternating patches called ocular dominance (OD) columns. Interested in the dynamics of this segregation process we studied the appearance of functional OD columns in the primary visual cortex of normally raised and strabismic kittens aged 2-6 weeks using 2-deoxyglucose labelling in awake animals. In both experimental groups, OD columns covering the entire area 17 and spanning all cortical laminae are first visible at 3 weeks and appear already adult-like at 4 weeks, much earlier than thought on the basis of previous anatomical studies. We hypothesize that a small and anatomically undetectable imbalance between the afferents from the two eyes is amplified by intracortical interactions so that their activity patterns become different and may guide the segregation process of the afferents in cortical layer IV.

Dynamic regulation of BDNF and NT-3 expression during visual system development

Recent studies have proposed roles for neurotrophins in the formation and plasticity of ocular dominance columns as well as in the regulation of dendritic arborization in visual cortex of higher mammals. To assess potential roles for neurotrophins in these processes, we have examined the developmental expression of BDNF and NT-3 mRNA in the cat's visual system using in situ hybridization. BDNF and NT-3 mRNAs are dynamically regulated in many CNS structures during embryonic and postnatal development, and both mRNAs undergo striking developmental changes in laminar specificity and levels of expression within primary visual cortex during the critical period for ocular dominance column formation. Within visual cortex, BDNF mRNA is found in neurons in deep cortical layers (5 and 6) prior to eye opening, and in both deep and superficial layers (2 and 3) shortly afterwards. Within layer 4, the target of thalamocortical axons, BDNF mRNA is low initially and rises to high levels by the end of the critical period for ocular dominance column formation. NT-3 mRNA is first detectable in small stellate neurons at the base of layer 4 (4c) after eye opening, and levels decrease near the end of the critical period. BDNF and NT-3 mRNAs can be detected in the lateral geniculate nucleus at birth, and levels peak during the critical period. In both structures, BDNF mRNA expression is maintained into adulthood, while NT-3 is undetectable in the adult. The presence and dynamic regulation of these neurotrophins in visual structures is consistent with suggested roles for both of these neurotrophins in axonal and dendritic remodeling known to accompany the formation of ocular dominance columns.

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

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

A mapping label required for normal scale of body representation in the cortex

The neocortical primary somatosensory area (S1) consists of a map of the body surface. The cortical area devoted to different regions, such as parts of the face or hands, reflects their functional importance. Here we investigated the role of genetically determined positional labels in neocortical mapping. Ephrin-A5 was expressed in a medial > lateral gradient across S1, whereas its receptor EphA4 was in a matching gradient across the thalamic ventrobasal (VB) complex, which provides S1 input. Ephrin-A5 had topographically specific effects on VB axon guidance in vitro. Ephrin-A5 gene disruption caused graded, topographically specific distortion in the S1 body map, with medial regions contracted and lateral regions expanded, changing relative areas up to 50% in developing and adult mice. These results provide evidence for within-area thalamocortical mapping labels and show that a genetic difference can cause a lasting change in relative scale of different regions within a topographic map.

Opposite changes in synaptic activity of organotypic rat spinal cord cultures after chronic block of AMPA/kainate or glycine and GABAA receptors

1. The well-developed cytoarchitecture of rat organotypic spinal cord culture makes it a suitable model to explore how persistent suppression of certain synaptic inputs might be compensated by increased synaptic efficacy (homeostatic plasticity). 2. Spontaneous or electrically evoked synaptic transmission of patch-clamped ventral horn interneurons was studied in control solution after blocking, for the second week in culture, AMPA/kainate receptors with CNQX or glycine and GABAA receptors with strychnine and bicuculline, or indiscriminately removing inputs with tetrodotoxin (TTX). 3. In untreated cells, spontaneous postsynaptic currents (PSCs) had fast (tau < 5 ms) or slow (tau > 10 ms) decay. A similar separation was observed when recording miniature currents (mPSCs). Slow decay PSCs were suppressed by strychnine plus bicuculline while fast decay events were eliminated by CNQX. 4. After chronic CNQX treatment the frequency of spontaneous, fast PSCs (of larger amplitude) or mPSCs was almost doubled with respect to control. These events were blocked by acutely applied CNQX, which unmasked slow PSCs. 5. After chronic TTX treatment neither the frequency nor the amplitude of spontaneous events was changed. 6. After chronic strychnine and bicuculline treatment the frequency and amplitude of all PSCs was decreased in most cells. mPSCs were also decreased in frequency. Spontaneous or electrically evoked currents acquired a larger component mediated by NMDA receptor activity. 7. The developing spinal network thus operated distinct homeostatic processes which led to strong enhancement in glutamatergic transmission after CNQX block or to broad downregulation of synaptic activity following chronic exposure to strychnine and bicuculline.

Critical periods during sensory development

Recent studies have made progress in characterizing the determinants of critical periods for experience-dependent plasticity. They highlight the role of neurotrophins, NMDA receptors and GABAergic inhibition. In particular, genetic manipulation of a single molecule, brain-derived neurotrophic factor (BDNF), has been shown to alter the timing of the critical period of plasticity in mouse visual cortex, establishing a causal relation between neurotrophin action, the development of visual function, and the duration of the critical period.

BDNF modulates, but does not mediate, activity-dependent branching and remodeling of optic axon arbors in vivo

The proper development of axon terminal arbors and their recognition of target neurons depend, in part, on neuronal activity. Neurotrophins are attractive candidate signals to participate in activity-dependent development and refinement of neuronal connectivity. In the visual system, brain-derived neurotrophic factor (BDNF) has been shown to modulate the elaboration and refinement of axonal arbors and to participate in the establishment of topographically ordered visual maps. By examining in vivo with time-lapse microscopy the effects of activity blockade and BDNF on optic axon arborization, I show that the dynamic mechanisms by which neurotrophins and neuronal activity regulate axon arborization differ. Acute retinal activity blockade by intraocular injection of tetrodotoxin (TTX) rapidly and significantly increased branch addition and elimination, thus interfering with axon branch stabilization. The effects of activity blockade on branch dynamics resulted in increased arbor complexity in the long term and were prevented by altering endogenous BDNF levels at the target. BDNF promoted axon arborization by increasing branch addition and lengthening, without affecting branch elimination. Activity blockade, however, did not prevent the growth-promoting effects of BDNF, indicating that BDNF can affect axon arborization even in the absence of activity. Together this evidence indicates that BDNF acts as a modulator, but not as a direct mediator, of activity during the morphological development of neurons. Consequently, neuronal activity and BDNF use distinct but interactive mechanisms to control the development of neuronal connectivity; BDNF modulates axon arborization by promoting growth, neuronal activity participates in axon branch stabilization, and together these two signals converge to shape axon form.

Dynamics of retinal waves are controlled by cyclic AMP

Waves of spontaneous activity sweep across the developing mammalian retina and influence the pattern of central connections made by ganglion cell axons. These waves are driven by synaptic input from amacrine cells. We show that cholinergic synaptic transmission during waves is not blocked by TTX, indicating that release from starburst amacrine cells is independent of sodium action potentials. The spatiotemporal properties of the waves are regulated by endogenous release of adenosine, which sets intracellular cAMP levels through activation of A2 receptors present on developing amacrine and ganglion cells. Increasing cAMP levels increase the size, speed, and frequency of the waves. Conversely, inhibiting adenylate cyclase or PKA prevents wave activity. Together, these results imply a novel mechanism in which levels of cAMP within an immature retinal circuit regulate the precise spatial and temporal patterns of spontaneous neural activity.

Cross-modal reorganization of horizontal connectivity in auditory cortex without altering thalamocortical projections

The development of the different, highly specialized regions of the mammalian cerebral cortex depends in part on neural activity, either intrinsic spontaneous activity or externally driven sensory activity. To determine whether patterned sensory activity instructs the development of intrinsic cortical circuitry, we have experimentally altered the modality of sensory inputs to cerebral cortex. Neonatal diversion of retinal axons to the auditory thalamus (cross-modal rewiring) results in a primary auditory cortex (AI) that resembles visual cortex in its response properties and topography (Roe et al., 1990, 1992). To test the hypothesis that the visual response properties are created by a visually driven reorganization of auditory cortical circuitry, we investigated the effect of early visual experience on the development of intrinsic, horizontal connections within AI. Horizontal connections are likely to play an important role in the construction of visual response properties in AI as they do in visual cortex. Here we show that early visual inputs to auditory thalamus can reorganize horizontal connections in AI, causing both an increase in their extent and a change in pattern, so that projections are not restricted to the isofrequency axis, but extend in a more isotropic pattern around the injection site. Thus, changing afferent modality, without altering the source of the thalamocortical axons, can profoundly alter cortical circuitry. Similar changes may underlie cortical compensatory processes in deaf or blind humans and may also have played a role in the parcellation of neocortex during mammalian evolution.