Typology, early differentiation, and exuberant growth of a set of cortical axons

The functional characterization of callosal connections

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The functional characterization of callosal connections is informed by anatomical data.

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Callosal connections play a conditional driving role depending on the brain state and behavioral demands.

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Callosal connections play a modulatory function, in addition to a driving role.

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The corpus callosum participates in learning and interhemispheric transfer of sensorimotor habits.

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The corpus callosum contributes to language processing and cognitive functions.

The brain operates through the synaptic interaction of distant neurons within flexible, often heterogeneous, distributed systems. Histological studies have detailed the connections between distant neurons, but their functional characterization deserves further exploration. Studies performed on the corpus callosum in animals and humans are unique in that they capitalize on results obtained from several neuroscience disciplines. Such data inspire a new interpretation of the function of callosal connections and delineate a novel road map, thus paving the way toward a general theory of cortico-cortical connectivity. Here we suggest that callosal axons can drive their post-synaptic targets preferentially when coupled to other inputs endowing the cortical network with a high degree of conditionality. This might depend on several factors, such as their pattern of convergence-divergence, the excitatory and inhibitory operation mode, the range of conduction velocities, the variety of homotopic and heterotopic projections and, finally, the state-dependency of their firing. We propose that, in addition to direct stimulation of post-synaptic targets, callosal axons often play a conditional driving or modulatory role, which depends on task contingencies, as documented by several recent studies.

Environmental Factors Shape Sleep EEG Connectivity During Early Adolescence

Quantifying the degree to which genetic and environmental factors shape brain network connectivity is critical to furthering our understanding of the developing human brain. Sleep, a state of sensory disengagement, provides a unique opportunity to study brain network activity noninvasively by means of sleep electroencephalography (EEG) coherence. We conducted a high-density sleep EEG study in monozygotic (MZ; n = 38; mean age = 12.46; 20 females) and dizygotic (DZ; n = 24; mean age = 12.50; 12 females) twins to assess the heritability of sleep EEG coherence in early adolescence—a period of significant brain rewiring. Structural equation modeling was used to estimate three latent factors: genes, environmental factors shared between twins and environmental factors unique to each twin. We found a strong contribution of unique environmental factors (66% of the variance) and moderate genetic influence (19% of the variance) on sleep EEG coherence across frequencies and sleep states. An exception to this was sleep spindle activity, an index of the thalamocortical network, which showed on average a genetic contribution of 48% across connections. Furthermore, we observed high intraindividual stability of coherence across two consecutive nights suggesting that despite only a modest genetic contribution, sleep EEG coherence is like a trait. Our findings in adolescent humans are in line with earlier findings in animals that show the primordial cerebral map and its connections are plastic and it is through interaction with the environment that the pattern of brain network connectivity is shaped. Therefore, even in twins living together, small differences in the environment may cascade into meaningful differences in brain connectivity.

The Target of Exuberant Projections in Development

In addition to neuronal death and elimination of synapses, the production of transient, exuberant axons, and axonal branches is a general phenomenon in development across species and systems. To understand what drives the decision of which axons are maintained and which are eliminated, it is important to monitor the interaction of juvenile axons at their target. As old and more recent work show, unlike what is claimed by Ribeiro Gomez et al. (2019), in the cerebral cortex, both classes of axons branch in the white matter near the target; axons destined to be maintained massively invade the gray matter where they develop terminal arbors and synapses. Axons destined to elimination remain in the white matter although a few transient, exploratory branches can enter the cortex. Axonal behavior and fate seem dictated by positional information probably conveyed by thalamic afferents and activity. Unlike what is suggested by Ribeiro Gomez et al. (2019), axonal selection should not be confused with synaptic reduction, which is a later event with minor or no impact on the topography of the connection.

The anatomy, organisation and development of contralateral callosal projections of the mouse somatosensory cortex

Background:

Alterations in the development of neuronal connectivity can result in dramatic outcomes for brain function. In the cerebral cortex, most sensorimotor and higher-order functions require coordination between precise regions of both hemispheres through the axons that form the corpus callosum. However, little is known about how callosal axons locate and innervate their contralateral targets.

Methods:

Here, we use a combination of in utero electroporation, retrograde tracing, sensory deprivation and high-resolution axonal quantification to investigate the development, organisation and activity dependence of callosal axons arising from the primary somatosensory cortex of mice.

Results:

We show that distinct contralateral projections arise from different neuronal populations and form homotopic and heterotopic circuits. Callosal axons innervate the contralateral hemisphere following a dorsomedial to ventrolateral and region-specific order. Furthermore, we identify two periods of region- and layer-specific developmental exuberance that correspond to initial callosal axon innervation and subsequent arborisation. Early sensory deprivation affects only the latter of these events.

Conclusion:

Taken together, these results reveal the main developmental events of contralateral callosal targeting and may aid future understanding of the formation and pathologies of brain connectivity.

Larger corpus callosum and reduced orbitofrontal cortex homotopic connectivity in codeine cough syrup-dependent male adolescents and young adults

OBJECTIVES

To characterize interhemispheric functional and anatomical connectivity and their relationships with impulsive behaviour in codeine-containing cough syrup (CCS)-dependent male adolescents and young adults.

METHODS

We compared volumes of corpus callosum (CC) and its five subregion and voxel-mirrored homotopic functional connectivity (VMHC) in 33 CCS-dependent male adolescents and young adults and 38 healthy controls, group-matched for age, education and smoking status. Barratt impulsiveness scale (BIS.11) was used to assess participant impulsive behaviour. Abnormal CC subregions and VMHC revealed by group comparison were extracted and correlated with impulsive behaviour and duration of CCS use.

RESULTS

We found selective increased mid-posterior CC volume in CCS-dependent male adolescents and young adults and detected decreased homotopic interhemispheric functional connectivity of medial orbitofrontal cortex (OFC). Moreover, impairment of VMHC was associated with the impulsive behaviour and correlated with the duration of CCS abuse in CCS-dependent male adolescents and young adults.

CONCLUSIONS

These findings reveal CC abnormalities and disruption of interhemispheric homotopic connectivity in CCS-dependent male adolescents and young adults, which provide a novel insight into the impact of interhemispheric disconnectivity on impulsive behaviour in substance addiction pathophysiology.

KEY POINTS

• CCS-dependent individuals (patients) had selective increased volumes of mid-posterior corpus callosum • Patients had attenuated interhemispheric homotopic FC (VMHC) of bilateral orbitofrontal cortex • Impairment of VMHC correlated with impulsive behaviour in patients • Impairment of VMHC correlated with the CCS duration in patients.

Development and evolution: two determinants of cortical connectivity

The production of genotypic and phenotypic diversity (differentiation) is the final outcome of both development and evolution, of nervous systems and of their components. Cortical axons, in particular, differentiate into a variety of phenotypes which are responsible for computational transformations of messages exchanged among neurons. One aspect of this differentiation concerns axon diameters whose diversity in development, but also within and across species, is enhanced by the addition of a relatively small proportion of thicker axons to some axonal pathways. This, combined with differences in the length of pathways and in brain volumes, has rescaled and expanded the temporal range of interneuronal communication. In both development and evolution, this and other aspects of axonal phenotypes, namely the structure and distribution of axonal arbors, differentiate by the combined action of cell intrinsic (genetic) variation, overproduction, and selection by the environment.

Receptive field properties and sensitivity to edges defined by motion in the postero-lateral lateral suprasylvian (PLLS) area of the cat

The present study investigated the spatial properties of cells in the postero-lateral lateral suprasylvian (PLLS) area of the cat and assessed their sensitivity to edges defined by motion. A total of one hundred and seventeen (117) single units were isolated. First, drifting sinusoidal gratings were used to assess the spatial properties of the cells' receptive fields and to determine their spatial frequency tuning functions. Second, random-dot kinematograms were used to create illusory edges by drifting textured stimuli (i.e. a horizontal bar) against a similarly textured but static background. Almost all the cells recorded in PLLS (96.0%) were binocular, and a substantial majority of receptive fields (79.2%) were end-stopped. Most units (81.0%) had band-pass spatial frequency tuning functions and responded optimally to low spatial frequencies (mean spatial frequency: 0.08 c./degree). The remaining units (19.0%) were low-pass. All the recorded cells responded vigorously to edges defined by motion. The vast majority (96.0%) of cells responded optimally to large texture elements; approximately half the cells (57.3%) also responded to finer texture elements. Moreover, 38.5% of the cells were selective to the width of the bar (i.e., the distance between the leading and the trailing edges). Finally, some (9.0%) cells responded in a transient fashion to leading and to trailing edges. In conclusion, cells in the PLLS area are low spatial frequency analyzers that are sensitive to texture and to the distance between edges defined by motion.

Exuberance in the development of cortical networks

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

Somatodendritic minicolumns of output neurons in the rat visual cortex

The apical dendrites of the pyramidal neurons of the cerebral cortex form radial bundles in all species and areas. Using microtubule-associated protein (MAP)2 immunostaining and Voronoi tessellation analysis in the rat visual cortex, we obtained objective criteria to define dendritic bundles in tangential sections: in supragranular layers of the rat visual cortex we found bundles of 6-6.4 dendrites, at a density of 1929 bundles/mm(2) and a centre-to-centre distance of 27 micro m. Using lipophilic tracers to label different pyramidal cell populations, based on the same criteria as in MAP2-immunostained material, we found that in the rat visual cortex the bundles consist of neurons with specific targets. Neurons projecting to the ipsi- or contralateral cortex form bundles together and with neurons projecting to the striatum, but not with those projecting to the superior colliculus, dorsal division of the lateral geniculate nucleus or through the cerebral peduncle. The latter neurons form bundles with neurons projecting to the striatum. Thus, the cerebral cortex is organized in minicolumns of output neurons visible at the earliest ages studied (P3), which might have a higher probability of being interconnected than those outside.

Immature cortex lesions alter retinotopic maps and interhemispheric connections

Unilateral lesions of the occipital visual areas performed on postnatal day 5 (P5) in the ferret are not compensated by the appearance, in the lesioned hemisphere, of visual responses at ectopic locations. Instead, when parts of the visual areas are spared, they show abnormal retinotopic organizations; furthermore, callosal connections are abnormally distributed in relation to the retinotopic maps. Lesions that completely eliminate the visual areas including the posterior parietal cortex cause the appearance of abnormal callosal connections from the primary somatosensory cortex on the lesion side to the contralateral, intact, posterior parietal cortex. The occipital visual areas (17, 18, 19, and 21) of the intact hemisphere show a normal retinotopy but lose callosal connections in territories homotopic to the lesions. These findings clarify the nature and limits of structural developmental plasticity in the visual cortex. Early in life, certain regions of cortex have been irreversibly allocated to the visual areas, but two properties defining the areas, that is, retinotopy and connections, remain modifiable. The findings might be relevant for understanding the consequences of early-onset visual cortical lesions in humans.

Schizophrenia, neurodevelopment and corpus callosum

The Zeitgeist favors an interpretation of schizophrenia as a condition of abnormal connectivity of cortical neurons, particularly in the prefrontal and temporal cortex. The available evidence points to reduced connectivity, a possible consequence of excessive synaptic pruning in development. A decreased thalamic input to the cerebral cortex appears likely, and developmental studies predict that this decrease should entail a secondary loss of both long- and short-range cortico-cortical connections, including connections between the hemispheres. Indeed, morphological, electrophysiological and neuropsychological studies over the last two decades suggest that the callosal connections are altered in schizophrenics. However, the alterations are subtle and sometimes inconsistent across studies, and need to be investigated further with new methodologies.

Recent techniques for tracing pathways in the central nervous system of developing and adult mammals

Over the last 20 years, the choice of neural tracers has increased manyfold, and includes newly introduced anterograde tracers that allow quantitation of single-axon morphologies, and retrograde tracers that can be combined with intracellular fills for the study of dendritic arbors of neurons which have a specific projection pattern. The combination of several different tracers now permits the comparison of multiple connections in the same animal, both quantitatively and qualitatively. Moreover, the finding of new virus strains, which infect neural cells without killing them, provides a tool for studying multisynaptic connections that participate in a circuit. In this paper, the labeling characteristics, mechanism of transport and advantages/disadvantages of use are discussed for the following recently introduced neural tracers: carbocyanine dyes, fluorescent latex microspheres, fluorescent dextrans, biocytin, dextran amines, Phaseolus vulgaris leucoagglutinin, cholera toxin and viruses. We also suggest the choice of specific tracers, depending on the experimental animal, age and type of connection to be studied, and discuss quantitative methodologies.

Visual stimulus-dependent changes in interhemispheric EEG coherence in ferrets

In recent years, the analysis of the coherence between signals recorded from the scalp [electroencephalographic (EEG) coherence] has been used to assess the functional properties of cortico-cortical connections, both in animal models and in humans. However, the experimental validation of this technique is still scarce. Therefore we applied it to the study of the callosal connections between the visual areas of the two hemispheres, because this particular set of cortico-cortical connections can be activated in a selective way by visual stimuli. Indeed, in primary and in low-order secondary visual areas, callosal axons interconnect selectively regions, which represent a narrow portion of the visual field straddling the vertical meridian and, within these regions, neurons that prefer the same stimulus orientation. Thus only isooriented stimuli located near the vertical meridian are expected to change interhemispheric coherence by activating callosal connections. Finally, if such changes are found and are indeed mediated by callosal connections, they should disappear after transection of the corpus callosum. We perfomed experiments on seven paralyzed and anesthetized ferrets, recording their cortical activity with epidural electrodes on areas 17/18, 19, and lateral suprasylvian, during different forms of visual stimulation. As expected, we found that bilateral iso-oriented stimuli near the vertical meridian, or extending across it, caused a significant increase in interhemispheric coherence in the EEG beta-gamma band. Stimuli with different orientations, stimuli located far from the vertical meridian, as well as unilateral stimuli failed to affect interhemispheric EEG coherence. The stimulus-induced increase in coherence disappeared after surgical transection of the corpus callosum. The results suggest that the activation of cortico-cortical connections can indeed be revealed as a change in EEG coherence. The latter can therefore be validly used to investigate the functionality of cortico-cortical connections.

The role of pattern vision in the development of cortico-cortical connections

The development of cortico-cortical connections was studied in kittens deprived of vision by binocular eyelid suture during the formation of axonal arbors and synaptogenesis, i.e. between the second postnatal week and the end of the third postnatal month. Axons originating in area 17 and terminating either in ipsilateral or contralateral visual areas were visualized with biocytin. In ipsilateral areas 17 and 18, distinct clusters of branches begin to form, distally from the injection, during the second half of the first postnatal month, independently of pattern vision. More proximal clusters differentiate during the second postnatal month, and this seems to involve elimination of exuberant axonal branches. In kittens deprived of vision for 3 or more months, beginning before natural eye opening, the distal clusters regress and the proximal ones fail to differentiate. In extrastriate areas, distinct clusters of branches have segregated by the end of the second postnatal month, independently of visual experience; however, in kittens deprived of vision for 2 or more months, one of the clusters was selectively eliminated. In contralateral areas 17 and 18, we found stunted terminal arbors in kittens continuously deprived of vision. This was already noticeable at the end of the first postnatal month. Apparently, in the absence of pattern vision, most axons undergo only limited growth and do not form their characteristic terminal columns. Many of these axons are subsequently eliminated. In contrast, 8 days of vision beginning at natural eye opening and followed by visual deprivation caused a nearly normal development of intrahemispheric and interhemispheric connections. In conclusion, pattern vision appears to validate connections at early stages of their development; this validation is necessary for their further growth and differentiation that can then continue autonomously.