Development of ocularity domains and growth behaviour of axon terminals

Ontogenetic development of ocularity domains — stripes, patches and layers in cortex, colliculus superior and lateral geniculate nucleus — is the result of organization that may either be intrinsic to the postsynaptic structure or induced to it by the afferents. A specific type of axonal growth behaviour that was recently proposed as a basis for ontogenetic development of retinotopy is sufficient to account also for ocularity domains. No intrinsic organization in the postsynaptic structure is required. The latter merely serves as a propagating medium for markers carried by the presynaptic terminals. Computer simulations demonstrate the mechanism to be complete and consistent.     

Effects of the thalamus on the development of cerebral cortical efferents in vitro

The cerebral cortex is a multilayered tissue, with each layer differing in its cellular composition and connections. Axons from deep layer neurons project subcortically, many to the thalamus, whereas superficial layer neurons target other cortical areas. The mechanisms that regulate the development of this pattern of connections are not fully understood. Our experiments examined the potential of the thalamus to attract and/or select neurites from appropriate cortical layers. First, we cocultured murine cortical slices in close proximity to thalamic explants in collagen gels. The amount of neurite outgrowth from deep layer cells was enhanced by, but not attracted to, the thalamic explants. Second, we cocultured cortical slices in contact with thalamic or cortical explants to test for laminar specificity of connections. Specificity was apparent after culture for about a week, in that deep cortical layers contained the highest proportions of corticothalamic cells and superficial cortical layers contained the highest proportions of corticocortical cells. After shorter culture of only a few days, however, specificity was not apparent and there were larger numbers of corticothalamic projections from the superficial layers than after a week. To study how the early nonspecific pattern of corticothalamic connections was transformed into the more specific pattern, we labeled corticothalamic cells early, after 2 days, but let the cultures survive for 8 days. On day 8, the nonspecific pattern of early-labeled cells was still seen. We conclude that although the thalamus does not block the initial entry of inappropriate axons from the superficial layers, many of these axons are subsequently lost. This suggests that contact-mediated interactions between cortical axons and the thalamus allow cortical efferents from appropriate layers to be distinguished from those arising in inappropriate layers. This may contribute to the development of layer-specific cortical connections in vivo.     

Long-range neuronal circuits underlying the interaction between sensory and motor cortex

In the rodent vibrissal system, active sensation and sensorimotor integration are mediated in part by connections between barrel cortex and vibrissal motor cortex. Little is known about how these structures interact at the level of neurons. We used Channelrhodopsin-2 (ChR2) expression, combined with anterograde and retrograde labeling, to map connections between barrel cortex and pyramidal neurons in mouse motor cortex. Barrel cortex axons preferentially targeted upper layer (L2/3, L5A) neurons in motor cortex; input to neurons projecting back to barrel cortex was particularly strong. Barrel cortex input to deeper layers (L5B, L6) of motor cortex, including neurons projecting to the brainstem, was weak, despite pronounced geometric overlap of dendrites with axons from barrel cortex. Neurons in different layers received barrel cortex input within stereotyped dendritic domains. The cortico-cortical neurons in superficial layers of motor cortex thus couple motor and sensory signals and might mediate sensorimotor integration and motor learning.     

Spiking neural network model for memorizing sequences with forward and backward recall

We present an oscillatory network of conductance based spiking neurons of Hodgkin–Huxley type as a model of memory storage and retrieval of sequences of events (or objects). The model is inspired by psychological and neurobiological evidence on sequential memories. The building block of the model is an oscillatory module which contains excitatory and inhibitory neurons with all-to-all connections. The connection architecture comprises two layers. A lower layer represents consecutive events during their storage and recall. This layer is composed of oscillatory modules. Plastic excitatory connections between the modules are implemented using an STDP type learning rule for sequential storage. Excitatory neurons in the upper layer project star-like modifiable connections toward the excitatory lower layer neurons. These neurons in the upper layer are used to tag sequences of events represented in the lower layer. Computer simulations demonstrate good performance of the model including difficult cases when different sequences contain overlapping events. We show that the model with STDP type or anti-STDP type learning rules can be applied for the simulation of forward and backward replay of neural spikes respectively.     

The role of neuronal death during the development of topographically ordered projections: a computational approach

At the time of synaptogenesis typically 50% of the neurons die. The biological role of this is still unclear, but there is evidence in the visual system that many neurons projecting to topographically inappropriate parts of their target are eliminated to improve the accuracy of the mapping. The signaling that determines neuronal survival involves electrical activity and trophic factors. Based on these observations, we have elaborated a computational model for the self-organization of a two-layered neural network. We observe changes in the topographical organization between the two layers. In layer 1, a traveling wave of electrical activity is used as input. Activity transmission to layer 2 can generate, according to a Hebbian rule, a retrograde death signal that is compensated by a trophic survival signal generated by the target cells. Approximately 50% of the neurons die, and we observe refinement in the topography between the two layers. In alternative versions of the model, we show that an equivalent reorganization can occur through Hebbian synaptic modification alone, but with less precision and efficiency. When the two mechanisms are combined, synaptic modification provides no further improvement over that produced by neuronal death alone. This computational study supports the hypothesis that neuronal death during development can play a role in the refinement of topographical projections in the nervous system. Received: 9 November 1998 / Accepted in revised form: 14 April 1999     

Intracortical connections between neuron groups in the somatosensory cortex studied by the retrograde horseradish peroxidase transport method in rats

Short corticocortical connections between specialized groups of neurons (so-called barrels) were studied in the somatosensory cortex. After microinjections of horseradish peroxidase into a definite "barrel" labeled neurons were found in nearby groups within a radius of up to 400 µ. Labeled neurons were located chiefly in cortical layers V and III; 90% of them were pyramidal cells. Intracortical connection of labeled neurons were 1.6 times more numerous than thalamocortical connections. It is postulated that connections between neighboring cortical neuron groups are effected through their output cells, i.e., through pyramidal neurons of layers V and III.     

Intrinsic versus extrinsic influences in the development of neuronal maps

Accumulating evidence suggests that the plasticity of extrinsic thalamocortical inputs in cortical layer IV may be guided or instructed by earlier plasticity events in the intrinsic, horizontal connections within the extragranular cortical layers. We analyse a rate-based model of the plasticity of a set of extrinsic afferents in the presence of a pre-existing (and fixed) plexus of intrinsic, overall excitatory horizontal connections between a set of target neurons. We determine conditions under which afferent synaptic pattern formation respects this pre-existing lateral structure. We find three broad regimes under which extrinsic afferent plasticity may violate this structure: the initial pattern of extrinsic afferent innervation of the target cells is far from balanced; the gain of the extrinsic afferents greatly exceeds the overall scale of the strength of lateral excitation; the target cell horizontal coupling matrix is sparse. If none of these conditions is satisfied, then extrinsic afferent plasticity respects the pre-existing lateral connectivity, so that afferent synaptic pattern formation conforms to the pattern of lateral excitation.     

Target and temporal pattern selection at neocortical synapses

We attempt to summarize the properties of cortical synaptic connections and the precision with which they select their targets in the context of information processing in cortical circuits. High-frequency presynaptic bursts result in rapidly depressing responses at most inputs onto spiny cells and onto some interneurons. These 'phasic' connections detect novelty and changes in the firing rate, but report frequency of maintained activity poorly. By contrast, facilitating inputs to interneurons that target dendrites produce little or no response at low frequencies, but a facilitating-augmenting response to maintained firing. The neurons activated, the cells they in turn target and the properties of those synapses determine which parts of the circuit are recruited and in what temporal pattern. Inhibitory interneurons provide both temporal and spatial tuning. The 'forward' flow from layer-4 excitatory neurons to layer 3 and from 3 to 5 activates predominantly pyramids. 'Back' projections, from 3 to 4 and 5 to 3, do not activate excitatory cells, but target interneurons. Despite, therefore, an increasing complexity in the information integrated as it is processed through these layers, there is little 'contamination' by 'back' projections. That layer 6 acts both as a primary input layer feeding excitation 'forward' to excitatory cells in other layers and as a higher-order layer with more integrated response properties feeding inhibition to layer 4 is discussed.     

The Laminar Cortex Model: A New Continuum Cortex Model Incorporating Laminar Architecture

Local field potentials (LFPs) are widely used to study the function of local networks in the brain. They are also closely correlated with the blood-oxygen-level-dependent signal, the predominant contrast mechanism in functional magnetic resonance imaging. We developed a new laminar cortex model (LCM) to simulate the amplitude and frequency of LFPs. Our model combines the laminar architecture of the cerebral cortex and multiple continuum models to simulate the collective activity of cortical neurons. The five cortical layers (layer I, II/III, IV, V, and VI) are simulated as separate continuum models between which there are synaptic connections. The LCM was used to simulate the dynamics of the visual cortex under different conditions of visual stimulation. LFPs are reported for two kinds of visual stimulation: general visual stimulation and intermittent light stimulation. The power spectra of LFPs were calculated and compared with existing empirical data. The LCM was able to produce spontaneous LFPs exhibiting frequency-inverse (1/ƒ) power spectrum behaviour. Laminar profiles of current source density showed similarities to experimental data. General stimulation enhanced the oscillation of LFPs corresponding to gamma frequencies. During simulated intermittent light stimulation, the LCM captured the fundamental as well as high order harmonics as previously reported. The power spectrum expected with a reduction in layer IV neurons, often observed with focal cortical dysplasias associated with epilepsy was also simulated.     

Synaptic organization of the indoleamine-accumulating neurons in the cyprinid retina

The distribution and synaptic connections of the indoleamine-accumulating neurons in the retinae of the goldfish and carp were studied by means of fluorescence and electron microscopy. The indoleamine-accumulating neurons were visualized after intravitreal injection and uptake of the indoleamine 5,6-dihydroxytryptamine. This labeling procedure produced a characteristic yellow fluorescence of the indoleamine-accumulating neurons and also characteristic ultrastructural changes in these cells. To avoid interference from the dopaminergic neurons of the retina, their processes were either removed by prior treatment with 5-hydroxydopamine or prevented from taking up 5,6-dihydroxytryptamine by the simultaneous injection of the catecholamine alpha-methyl-noradrenaline. Fluorescence-microscopic studies confirmed earlier reports that the indoleamine-accumulating perikarya and processes are distributed similar to those of amacrine cells. The indoleamine-accumulating processes ramify in three bands in the inner plexiform layer, the outermost one being the densest. Electron-microscopic investigations showed the indoleamine-accumulating neurons to have synapses of the conventional type, similar to amacrine cells. Their main synaptic contacts are with other amacrine cells, but synapses with bipolar cell terminals are also present. Both the distribution of the indoleamine-accumulating processes and their synaptic arrangement in the cyprinid retina differ from those found in mammalian retinae investigated previously.     

Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections

The development of connections between thalamic afferents and their cortical target cells occurs in a highly precise manner. Thalamic axons enter the cortex through deep cortical layers, then stop their growth in layer 4 and elaborate terminal arbors specifically within this layer. The mechanisms that underlie target layer recognition for thalamocortical projections are not known. We compared the growth pattern of thalamic explants cultured on membrane substrates purified from cortical layer 4, the main recipient layer for thalamic axons, and cortical layer 5, a non-target layer. Thalamic axons exhibited a reduced growth rate and an increased branching density on their appropriate target membranes compared with non-target substrate. When confronted with alternating stripes of both membrane substrates, thalamic axons grew preferentially on their target membrane stripes. Enzymatic treatment of cortical membranes revealed that growth, branching and guidance of thalamic axons are independently regulated by attractive and repulsive cues differentially expressed in distinct cortical layers. These results indicate that multiple membrane-associated molecules collectively contribute to the laminar targeting of thalamic afferents. Furthermore, we found that interfering with the function of Eph tyrosine kinase receptors and their ligands, ephrins, abolished the preferential branching of thalamic axons on their target membranes, and that recombinant ephrin-A5 ligand elicited a branch-promoting activity on thalamic axons. We conclude that interactions between Eph receptors and ephrins mediate branch formation of thalamic axons and thereby may play a role in the establishment of layer-specific thalamocortical connections.     

Ligand trapping in epithelial layers and cell cultures

We analyze a stochastic model that describes receptor-mediated ligand trapping in epithelial layers and cell culture assays. In both cases, the problem is reduced to diffusion of a Brownian particle between the partially absorbing and reflective surfaces. We derive an analytical expression for the spatial distribution of the trapping points and identify the domains of applicability of the two limiting regimes. We conclude that a thin layer approximation is applicable for ligand trapping in epithelial layers while a typical cell culture experiment is appropriately described within an infinite layer approximation.     

Distinct origin of GABA-ergic neurons in forebrain of man, nonhuman primates and lower mammals

In this mini-review we present recent data about origin of GABA-ergic (gama-aminobutyric acid) neurons in the mammalian forebrain, including the diencephalon and telencephalon. The interest in GABA-ergic neurons, which in cerebral cortex mostly correspond to local circuit neurons (interneurons), has increased in the past decade. Many studies have shown that in lower mammals all hippocampal and almost all neo-cortical GABA-ergic neurons are born in the specific region named ganglionic eminence, and not locally in proliferative layers all around telencephalic vesicle. The ganglionic eminence, that represents a region with thick proliferative-subventricular layer in the ventral (basal) part of telencephalon, was classically thought to give neurons to basal ganglia and septal nuclei, whereas proliferative layers of dorsal telencephalon give neurons to cerebral cortex including hippocampus. It was thought that neurons migrate from proliferative layer to their target region following a radial orientation. However, data in lower mammals showed that this is the case only for glutamatergic principal cells, i.e. projection neurons. GABA-ergic neurons use long distance tangentional migration, parallel to pial surface to reach, from ganglionic eminence, their targeting layer in the cerebral cortex. Especially intriguing, but frequently neglecting, several studies suggest that mammalian evolution might use different developmental rules to provide GABA-ergic neurons to an expending brain. In this review we focus on specific events underlying GABA-ergic neuron development in human and non-human primates. Disturbances of the GABAergic network are found in many neurological and psychiatric disorders, some of them might result from altered production or migration of these neurons during development. Therefore, it is crucial to understand human-specific mechanisms that regulate the development of GABA-ergic neurons.     

Differentiated horizontal interneurons clonally expand to form metastatic retinoblastoma in mice

During neurogenesis, the progression from a progenitor cell to a differentiated neuron is believed to be unidirectional and irreversible. The Rb family of proteins (Rb, p107, and p130) regulates cell-cycle exit and differentiation during retinogenesis. Rb and p130 are redundantly expressed in the neurons of the inner nuclear layer (INL) of the retina. We have found that in the adult Rb;p130-deficient retinae p107 compensation prevents ectopic proliferation of INL neurons. However, p107 is haploinsufficient in this process. Differentiated Rb(-/-);p107(+/-);p130(-/-) horizontal interneurons re-entered the cell cycle, clonally expanded, and formed metastatic retinoblastoma. Horizontal cells were not affected in Rb(+/-);p107(-/-);p130(-/-) or Rb(-/-);p107(-/-);p130(+/-), retinae suggesting that one copy of Rb or p130 was sufficient to prevent horizontal proliferation. We hereby report that differentiated neurons can proliferate and form cancer while maintaining their differentiated state including neurites and synaptic connections.     

Assembly of lamina-specific neuronal connections by slit bound to type IV collagen

The mechanisms that generate specific neuronal connections in the brain are under intense investigation. In zebrafish, retinal ganglion cells project their axons into at least six layers within the neuropil of the midbrain tectum. Each axon elaborates a single, planar arbor in one of the target layers and forms synapses onto the dendrites of tectal neurons. We show that the laminar specificity of retinotectal connections does not depend on self-sorting interactions among RGC axons. Rather, tectum-derived Slit1, signaling through axonal Robo2, guides neurites to their target layer. Genetic and biochemical studies indicate that Slit binds to Dragnet (Col4a5), a type IV Collagen, which forms the basement membrane on the surface of the tectum. We further show that radial glial endfeet are required for the basement-membrane anchoring of Slit. We propose that Slit1 signaling, perhaps in the form of a superficial-to-deep gradient, presents laminar positional cues to ingrowing retinal axons.     

Opposing roles for neurotrophin-3 in targeting and collateral formation of distinct sets of developing cortical neurons.

Neurotrophin-3 and its receptor TrkC are expressed during the development of the mammalian cerebral cortex. To examine whether neurotrophin-3 might play a role in the elaboration of layer-specific cortical circuits, slices of layer 6 and layers 2/3 neurons were cultured in the presence of exogenously applied neurotrophin-3. Results indicate that neurotrophin-3 promotes axonal branching of layer 6 axons, which target neurotrophin-3-expressing layers in vivo, and that it inhibits branching of layers 2/3 axons, which avoid neurotrophin-3-expressing layers. Such opposing effects of neurotrophin-3 on axonal branching were also observed with embryonic cortical neurons, indicating that the response to neurotrophin-3 is specified at early developmental stages, prior to cell migration. In addition to its effects on fiber branching, axonal guidance assays also indicate that neurotrophin-3 is an attractive signal for layer 6 axons and a repellent guidance cue for layers 2/3 axons. Experiments with specific antibodies to neutralize neurotrophin-3 in cortical membranes revealed that endogenous levels of neurotrophin-3 are sufficient to regulate branching and targeting of cortical axons. These opposing effects of neurotrophin-3 on specific populations of axons demonstrate that it could serve as one of the signals for the elaboration of local cortical circuits.     

A recurrent neural network for partitioning of hand drawn characters into strokes of different orientations.

A neural network is described which is intended to extract orientation features that should be used for recognition of hand drawn characters. The network partitions the input hand drawn characters into separate line segments (strokes) according to their orientations. The network consists of several neural layers; each layer serves for extracting strokes of a certain orientation. Every neural layer has one-to-one correspondence with an input screen. The network uses an iterative update procedure which includes interactions of neurons inside each layer through oriented excitatory connections and inhibitory interrelations between the corresponding neurons of different layers. Computer simulation of the network was performed. Experiments showed that the network efficiently classifies all pixels of any hand drawn characters according to the orientations of the strokes constituting these characters and performs, as a result of that, a reasonable segmentation of characters.     

Neuronal composition and interneuronal connection of area 5 in the cat parietal association cortex

Area 5 of the cat cortex was studied by Nissl's method and by Golgi's chromate-silver impregnation method. Its typical six-layered structure with well-developed layers of pyramidal cells was revealed. The characteristic features of area 5 are: predominance of pyramidal cells in layers II–III and the presence of large forms (40×26 µ) among them (in layer III); giant pyramidal neurons (70×23 µ) arranged singly or nidally in layer V; large (diameter 25–30 µ) and giant (diameter 40–45 µ) stellate cells with radial dendrites, arranged singly or in groups in layers V–VI; infrequent efferent fusiform neurons (40×20 µ) in layers V–VI. Stellate cells connecting pyramidal neurons in the same or in different layers were found in layers II–VI. Some stellate cells in layers II–III form long horizontal connections within area 5. Interneuronal connections are effected by axosomatic and axodendritic terminals, the latter being more numerous; Dendrodendritic and axoaxonal synapses are less common.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 1, pp. 35–42, January–February, 1979.     

Calcium helps neurons identify synaptic targets during development

Correct wiring of brain circuitry during development involves the selective formation and retention of synaptic connections between neurons. In this issue of Neuron, Lohmann and Bonhoeffer show that dendritic filopodia can distinguish among prospective presynaptic axonal targets during development. Contact with the appropriate target triggers local calcium signals that stabilize the filopodia and tell them to form mature synapses.