Induction of somatic sensory inputs to the lateral geniculate nucleus in congenitally blind mice and in phenotypically normal mice

Cross-Modal Plasticity in Brains Deprived of Visual Input Before Vision

Unimodal sensory loss leads to structural and functional changes in both deprived and nondeprived brain circuits. This process is broadly known as cross-modal plasticity. The evidence available indicates that cross-modal changes underlie the enhanced performances of the spared sensory modalities in deprived subjects. Sensory experience is a fundamental driver of cross-modal plasticity, yet there is evidence from early-visually deprived models supporting an additional role for experience-independent factors. These experience-independent factors are expected to act early in development and constrain neuronal plasticity at later stages. Here we review the cross-modal adaptations elicited by congenital or induced visual deprivation prior to vision. In most of these studies, cross-modal adaptations have been addressed at the structural and functional levels. Here, we also appraise recent data regarding behavioral performance in early-visually deprived models. However, further research is needed to explore how circuit reorganization affects their function and what brings about enhanced behavioral performance.

Sensitive timing of undifferentiation in oligodendrocyte progenitor cells and their enhanced maturation in primary visual cortex of binocularly enucleated mice

Sensory experience modulates proliferation, differentiation, and migration of oligodendrocyte progenitor cells (OPCs). In the mouse primary visual cortex (V1), visual deprivation-dependent modulation of OPCs has not been demonstrated. Here, we demonstrate that undifferentiated OPCs developmentally peaked around postnatal day (P) 25, and binocular enucleation (BE) from the time of eye opening (P14-15) elevated symmetrically-divided undifferentiated OPCs in a reversible G0/G1 state even more at the bottom lamina of the cortex by reducing maturing oligodendrocyte (OL) lineage cells. Experiments using the sonic hedgehog (Shh) signaling inhibitor cyclopamine in vivo suggested that Shh signaling pathway was involved in the BE-induced undifferentiation process. The undifferentiated OPCs then differentiated within 5 days, independent of the experience, becoming mostly quiescent cells in control mice, while altering the mode of sister cell symmetry and forming quiescent as well as maturing cells in the enucleated mice. At P50, BE increased mature OLs via symmetric and asymmetric modes of cell segregation, resulting in more populated mature OLs at the bottom layer of the cortex. These data suggest that fourth postnatal week, corresponding to the early critical period of ocular dominance plasticity, is a developmentally sensitive period for OPC state changes. Overall, the visual loss promoted undifferentiation at the early period, but later increased the formation of mature OLs via a change in the mode of cell type symmetry at the bottom layer of mouse V1.

Visual deprivation induces transient upregulation of oligodendrocyte progenitor cells in the subcortical white matter of mouse visual cortex

Sensory experience influences proliferation and differentiation of oligodendrocyte progenitor cells (OPCs). Enhanced sensorimotor experience promoted the lineage progression of OPCs and myelination in the gray matter and white matter (WM) of sensorimotor cortex. In the visual cortex, reduced experience reportedly delayed the maturation of myelination in the gray matter, but whether and how such experience alters the subcortical WM is unclear. Here we investigated if binocular enucleation from the onset of eye opening (i.e., P15) affects the cell state of OPCs in mouse primary visual cortex (V1). Proliferative cells in the WM declined nearly half over 3 days from postnatal day (P) 25. A 3-day BrdU-labeling showed gradual decline in proliferation rates from P19 to P28. Binocular enucleation resulted in an increase in the cycling state of the OPCs that were proliferated from P22 to P25 but not before or after this period. This increase in proliferative OPCs was not associated with lineage progression toward differentiated oligodendrocytes. Proliferative OPCs arose mostly due to symmetric cell division but also asymmetric formation of proliferative and quiescent OPCs. By P30, almost all the proliferated cells exited the cell cycle. Maturing oligodendrocytes among the proliferated cells increased at this age, but most of them disappeared over 25 days. The cell density of the maturing oligodendrocytes was unaffected by binocular enucleation, however. These data suggest that binocular enucleation transiently elevates proliferative OPCs in the subcortical WM of V1 during a specific period of the fourth postnatal week without subsequently affecting the number of maturing oligodendrocytes several days later.

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Binocular enucleation increased proliferative OPCs during P22-25 in the V1 WM.

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Proliferative OPCs decrease in half from P25 over 3 days.

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P22-25 proliferated cells nearly all exited the cell cycle by P30.

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Some P22-25 proliferated OPCs matured over 5 days but disappeared over 25 days.

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Visual loss did not influence oligodendrocyte maturation or its disappearance.

Structural brain plasticity induced by early blindness

It is well established that early blindness results in behavioural adaptations. While the functional effects of visual deprivation have been well researched, anatomical studies are scarce. The aim of this study was to investigate whole brain structural plasticity in a mouse model of congenital blindness. Volumetric analyses were conducted on high-resolution MRI images and histological sections from the same brains. These morphometric measurements were compared between anophthalmic and sighted ZRDBA mice obtained by breeding ZRDCT and DBA mice. Results from MRI analyses using the Multiple Automatically Generated Templates (MAGeT) method showed smaller volume for the primary visual cortex and superior colliculi in anophthalmic compared with sighted mice. Deformation-based morphometry revealed smaller volumes within the dorsal lateral geniculate nuclei and the lateral secondary visual cortex and larger volumes within olfactory areas, piriform cortex, orbital areas and the amygdala, in anophthalmic compared with sighted mice. Histological analyses revealed a larger volume for the amygdala and smaller volume for the superior colliculi, primary visual cortex and medial secondary visual cortex, in anophthalmic compared with sighted mice. The absence of superficial visual layers of the superior colliculus and the thinner cortical layer IV of the primary and secondary visual cortices may explain the smaller volume of these areas, although this was observed in a limited sample. The present study shows large-scale brain plasticity in a mouse model of congenital blindness. In addition, the congruence of MRI and histological findings support the use of MRI to investigate structural brain plasticity in the mouse.

Development of the thalamus: From early patterning to regulation of cortical functions

The thalamus is a brain structure of the vertebrate diencephalon that plays a central role in regulating diverse functions of the cerebral cortex. In traditional view of vertebrate neuroanatomy, the thalamus includes three regions, dorsal thalamus, ventral thalamus, and epithalamus. Recent molecular embryological studies have redefined the thalamus and the associated axial nomenclature of the diencephalon in the context of forebrain patterning. This new view has provided a useful conceptual framework for studies on molecular mechanisms of patterning, neurogenesis and fate specification in the thalamus as well as the guidance mechanisms for thalamocortical axons. Additionally, the availability of genetic tools in mice has led to important findings on how thalamic development is linked to the development of other brain regions, particularly the cerebral cortex. This article will give an overview of the organization of the embryonic thalamus and how progenitor cells in the thalamus generate neurons that are organized into discrete nuclei. I will then discuss how thalamic development is orchestrated with the development of the cerebral cortex and other brain regions. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles.

Alterations in cortical and thalamic connections of somatosensory cortex following early loss of vision

Early loss of vision produces dramatic changes in the functional organization and connectivity of the neocortex in cortical areas that normally process visual inputs, such as the primary and second visual area. This loss also results in alterations in the size, functional organization, and neural response properties of the primary somatosensory area, S1. However, the anatomical substrate for these functional changes in S1 has never been described. In the present investigation, we quantified the cortical and subcortical connections of S1 in animals that were bilaterally enucleated very early in development, prior to the formation of retino-geniculate and thalamocortical pathways. We found that S1 receives dense inputs from novel cortical fields, and that the density of existing cortical and thalamocortical connections was altered. Our results demonstrate that sensory systems develop in tandem and that alterations in sensory input in one system can affect the connections and organization of other sensory systems. Thus, therapeutic intervention following early loss of vision should focus not only on restoring vision, but also on augmenting the natural plasticity of the spared systems.

Sensory experience-dependent formation of perineuronal nets and expression of Cat-315 immunoreactive components in the mouse somatosensory cortex

Perineuronal nets (PNNs) are structures of extracellular matrix molecules surrounding the cell bodies and proximal dendrites of certain neurons. While PNNs are present throughout the mouse cerebral cortex, recent studies have shown that the components differ among cortical sub-regions and layers, suggesting region-specific functions. Parvalbumin-expressing interneurons (PV neurons) may be important regulators of cortical plasticity during the early "critical period" that is sensitive to sensory input. Here we examined the distribution and developmental functions of PNN components associated with PV neurons in the somatosensory cortex during the critical period. Aggrecan, brevican, neurocan, phosphacan, and tenascin-R were identified as PNN components in the mouse somatosensory cortex. High-magnification analysis revealed that some lectin Wisteria floribunda agglutinin (WFA)-reactive molecules did not co-localize with monoclonal antibody Cat-315 recognition molecules around the cell body. During postnatal development, Cat-315-positive (Cat-315⁺) PNNs appeared later than PNNs binding to the lectin WFA (WFA⁺ PNNs). These WFA⁺ PNNs changed from granular-like to reticular-like structures during normal cortical development, while this transition was delayed by sensory deprivation. This study indicates that the formation of reticular-like WFA⁺ PNNs is dependent on sensory experience in the mouse somatosensory cortex. We suggest that Cat-315⁺ molecules and WFA expression in PNNs are involved in the early critical period of input-dependent cortical plasticity.

The Current Status of Somatostatin-Interneurons in Inhibitory Control of Brain Function and Plasticity

The mammalian neocortex contains many distinct inhibitory neuronal populations to balance excitatory neurotransmission. A correct excitation/inhibition equilibrium is crucial for normal brain development, functioning, and controlling lifelong cortical plasticity. Knowledge about how the inhibitory network contributes to brain plasticity however remains incomplete. Somatostatin- (SST-) interneurons constitute a large neocortical subpopulation of interneurons, next to parvalbumin- (PV-) and vasoactive intestinal peptide- (VIP-) interneurons. Unlike the extensively studied PV-interneurons, acknowledged as key components in guiding ocular dominance plasticity, the contribution of SST-interneurons is less understood. Nevertheless, SST-interneurons are ideally situated within cortical networks to integrate unimodal or cross-modal sensory information processing and therefore likely to be important mediators of experience-dependent plasticity. The lack of knowledge on SST-interneurons partially relates to the wide variety of distinct subpopulations present in the sensory neocortex. This review informs on those SST-subpopulations hitherto described based on anatomical, molecular, or electrophysiological characteristics and whose functional roles can be attributed based on specific cortical wiring patterns. A possible role for these subpopulations in experience-dependent plasticity will be discussed, emphasizing on learning-induced plasticity and on unimodal and cross-modal plasticity upon sensory loss. This knowledge will ultimately contribute to guide brain plasticity into well-defined directions to restore sensory function and promote lifelong learning.

Cross-modal plasticity in sensory deprived animal models: From the thalamocortical development point of view

Over recent decades, our understanding of the plasticity of the central nervous system has expanded enormously. Accordingly, it is now widely accepted that the brain can adapt to changes by reorganizing its circuitry, both in response to external stimuli and experience, as well as through intrinsic mechanisms. A clear example of this is the activation of a deprived sensory area and the expansion of spared sensory cortical regions in individuals who suffered peripheral sensory loss. Despite the efforts to understand these neuroplastic changes, the mechanisms underlying such adaptive remodeling remains poorly understood. Progress in understanding these events may be hindered by the highly varied data obtained from the distinct experimental paradigms analyzed, which include different animal models and neuronal systems, as well as studies into the onset of sensory loss. Here, we will establish the current state-of-the-art describing the principal observations made according to the time of sensory deprivation with respect to the development of the thalamocortical connectivity. We will review the experimental data obtained from animal models where sensory deprivation has been induced either before or after thalamocortical axons reach and invade their target cortical areas. The anatomical and functional effects of sensory loss on the primary sensory areas of the cortex will be presented. Indeed, we consider that the comparative approach of this review is a necessary step in order to help deciphering the processes that underlie sensory neuroplasticity, for which studies in animal models have been indispensable. Understanding these mechanisms will then help to develop restorative strategies and prostheses that will overcome the functional loss.

Rapid Changes in Cortical and Subcortical Brain Regions after Early Bilateral Enucleation in the Mouse

Functional sensory and motor areas in the developing mammalian neocortex are formed through a complex interaction of cortically intrinsic mechanisms, such as gene expression, and cortically extrinsic mechanisms such as those mediated by thalamic input from the senses. Both intrinsic and extrinsic mechanisms are believed to be involved in cortical patterning and the establishment of areal boundaries in early development; however, the nature of the interaction between intrinsic and extrinsic processes is not well understood. In a previous study, we used a perinatal bilateral enucleation mouse model to test some aspects of this interaction by reweighting sensory input to the developing cortex. Visual deprivation at birth resulted in a shift of intraneocortical connections (INCs) that aligned with ectopic ephrin A5 expression in the same location ten days later at postnatal day (P) 10. A prevailing question remained: Does visual deprivation first induce a change in gene expression, followed by a shift in INCs, or vice versa? In the present study, we address this question by investigating the neuroanatomy and patterns of gene expression in post-natal day (P) 1 and 4 mice following bilateral enucleation at birth. Our results demonstrate a rapid reduction in dorsal lateral geniculate nucleus (dLGN) size and ephrin A5 gene expression 24-hours post-enucleation, with more profound effects apparent at P4. The reduced nuclear size and diminished gene expression mirrors subtle changes in ephrin A5 expression evident in P1 and P4 enucleated neocortex, 11 and 8 days prior to natural eye opening, respectively. Somatosensory and visual INCs were indistinguishable between P1 and P4 mice bilaterally enucleated at birth, indicating that perinatal bilateral enucleation initiates a rapid change in gene expression (within one day) followed by an alteration of sensory INCs later on (second postnatal week). With these results, we gain a deeper understanding of how gene expression and sensory input together regulate cortical arealization and plasticity during early development.

The effects of lifelong blindness on murine neuroanatomy and gene expression

Mammalian neocortical development is regulated by neural patterning mechanisms, with distinct sensory and motor areas arising through the process of arealization. This development occurs alongside developing central or peripheral sensory systems. Specifically, the parcellation of neocortex into specific areas of distinct cytoarchitecture, connectivity and function during development is reliant upon both cortically intrinsic mechanisms, such as gene expression, and extrinsic processes, such as input from the sensory receptors. This developmental program shifts from patterning to maintenance as the animal ages and is believed to be active throughout life, where the brain’s organization is stable yet plastic. In this study, we characterize the long-term effects of early removal of visual input via bilateral enucleation at birth. To understand the long-term effects of early blindness we conducted anatomical and molecular assays 18 months after enucleation, near the end of lifespan in the mouse. Bilateral enucleation early in life leads to long-term, stable size reductions of the thalamic lateral geniculate nucleus (LGN) and the primary visual cortex (V1) alongside a increase in individual whisker barrel size. Neocortical gene expression in the aging brain has not been previously identified; we document cortical expression of multiple regionalization genes. Expression patterns of Ephrin A5, COUP-TFI, and RZRβ and patterns of intraneocortical connectivity (INC) are altered in the neocortices of aging blind mice. Sensory inputs from different modalities during development likely play a major role in the development of cortical areal and thalamic nuclear boundaries. We suggest that early patterning by prenatal retinal activity combined with persistent gene expression within the thalamus and cortex is sufficient to establish and preserve a small but present LGN and V1 into late adulthood.

Sensory Deprivation during Early Postnatal Period Alters the Density of Interneurons in the Mouse Prefrontal Cortex

Early loss of one sensory system can cause improved function of other sensory systems. However, both the time course and neuronal mechanism of cross-modal plasticity remain elusive. Recent study using functional MRI in humans suggests a role of the prefrontal cortex (PFC) in cross-modal plasticity. Since this phenomenon is assumed to be associated with altered GABAergic inhibition in the PFC, we have tested the hypothesis that early postnatal sensory deprivation causes the changes of inhibitory neuronal circuit in different regions of the PFC of the mice. We determined the effects of sensory deprivation from birth to postnatal day 28 (P28) or P58 on the density of parvalbumin (PV), calbindin (CB), and calretinin (CR) neurons in the prelimbic, infralimbic, and dorsal anterior cingulate cortices. The density of PV and CB neurons was significantly increased in layer 5/6 (L5/6). Moreover, the density of CR neurons was higher in L2/3 in sensory deprived mice compared to intact mice. These changes were more prominent at P56 than at P28. These results suggest that long-term sensory deprivation causes the changes of intracortical inhibitory networks in the PFC and the changes of inhibitory networks in the PFC may contribute to cross-modal plasticity.

Visual system plasticity in mammals: the story of monocular enucleation-induced vision loss

The groundbreaking work of Hubel and Wiesel in the 1960’s on ocular dominance plasticity instigated many studies of the visual system of mammals, enriching our understanding of how the development of its structure and function depends on high quality visual input through both eyes. These studies have mainly employed lid suturing, dark rearing and eye patching applied to different species to reduce or impair visual input, and have created extensive knowledge on binocular vision. However, not all aspects and types of plasticity in the visual cortex have been covered in full detail. In that regard, a more drastic deprivation method like enucleation, leading to complete vision loss appears useful as it has more widespread effects on the afferent visual pathway and even on non-visual brain regions. One-eyed vision due to monocular enucleation (ME) profoundly affects the contralateral retinorecipient subcortical and cortical structures thereby creating a powerful means to investigate cortical plasticity phenomena in which binocular competition has no vote.In this review, we will present current knowledge about the specific application of ME as an experimental tool to study visual and cross-modal brain plasticity and compare early postnatal stages up into adulthood. The structural and physiological consequences of this type of extensive sensory loss as documented and studied in several animal species and human patients will be discussed. We will summarize how ME studies have been instrumental to our current understanding of the differentiation of sensory systems and how the structure and function of cortical circuits in mammals are shaped in response to such an extensive alteration in experience. In conclusion, we will highlight future perspectives and the clinical relevance of adding ME to the list of more longstanding deprivation models in visual system research.

Strain differences of the effect of enucleation and anophthalmia on the size and growth of sensory cortices in mice

Anophthalmia is a condition in which the eye does not develop from the early embryonic period. Early blindness induces cross-modal plastic modifications in the brain such as auditory and haptic activations of the visual cortex and also leads to a greater solicitation of the somatosensory and auditory cortices. The visual cortex is activated by auditory stimuli in anophthalmic mice and activity is known to alter the growth pattern of the cerebral cortex. The size of the primary visual, auditory and somatosensory cortices and of the corresponding specific sensory thalamic nuclei were measured in intact and enucleated C57Bl/6J mice and in ZRDCT anophthalmic mice (ZRDCT/An) to evaluate the contribution of cross-modal activity on the growth of the cerebral cortex. In addition, the size of these structures were compared in intact, enucleated and anophthalmic fourth generation backcrossed hybrid C57Bl/6J×ZRDCT/An mice to parse out the effects of mouse strains and of the different visual deprivations. The visual cortex was smaller in the anophthalmic ZRDCT/An than in the intact and enucleated C57Bl/6J mice. Also the auditory cortex was larger and the somatosensory cortex smaller in the ZRDCT/An than in the intact and enucleated C57Bl/6J mice. The size differences of sensory cortices between the enucleated and anophthalmic mice were no longer present in the hybrid mice, showing specific genetic differences between C57Bl/6J and ZRDCT mice. The post natal size increase of the visual cortex was less in the enucleated than in the anophthalmic and intact hybrid mice. This suggests differences in the activity of the visual cortex between enucleated and anophthalmic mice and that early in-utero spontaneous neural activity in the visual system contributes to the shaping of functional properties of cortical networks.

Reorganization of the connectivity of cortical field DZ in congenitally deaf cat

Psychophysics and brain imaging studies in deaf patients have revealed a functional crossmodal reorganization that affects the remaining sensory modalities. Similarly, the congenital deaf cat (CDC) shows supra-normal visual skills that are supported by specific auditory fields (DZ-dorsal zone and P-posterior auditory cortex) but not the primary auditory cortex (A1). To assess the functional reorganization observed in deafness we analyzed the connectivity pattern of the auditory cortex by means of injections of anatomical tracers in DZ and A1 in both congenital deaf and normally hearing cats. A quantitative analysis of the distribution of the projecting neurons revealed the presence of non-auditory inputs to both A1 and DZ of the CDC which were not observed in the hearing cats. Firstly, some visual (areas 19/20) and somatosensory (SIV) areas were projecting toward DZ of the CDC but not in the control. Secondly, A1 of the deaf cat received a weak projection from the visual lateral posterior nuclei (LP). Most of these abnormal projections to A1 and DZ represent only a small fraction of the normal inputs to these areas. In addition, most of the afferents to DZ and A1 appeared normal in terms of areal specificity and strength of projection, with preserved but smeared nucleotopic gradient of A1 in CDCs. In conclusion, while the abnormal projections revealed in the CDC can participate in the crossmodal compensatory mechanisms, the observation of a limited reorganization of the connectivity pattern of the CDC implies that functional reorganization in congenital deafness is further supported also by normal cortico-cortical connectivity.

Cortical GABAergic interneurons in cross-modal plasticity following early blindness

Early loss of a given sensory input in mammals causes anatomical and functional modifications in the brain via a process called cross-modal plasticity. In the past four decades, several animal models have illuminated our understanding of the biological substrates involved in cross-modal plasticity. Progressively, studies are now starting to emphasise on cell-specific mechanisms that may be responsible for this intermodal sensory plasticity. Inhibitory interneurons expressing γ-aminobutyric acid (GABA) play an important role in maintaining the appropriate dynamic range of cortical excitation, in critical periods of developmental plasticity, in receptive field refinement, and in treatment of sensory information reaching the cerebral cortex. The diverse interneuron population is very sensitive to sensory experience during development. GABAergic neurons are therefore well suited to act as a gate for mediating cross-modal plasticity. This paper attempts to highlight the links between early sensory deprivation, cortical GABAergic interneuron alterations, and cross-modal plasticity, discuss its implications, and further provide insights for future research in the field.

Bilateral enucleation alters gene expression and intraneocortical connections in the mouse

Background

Anatomically and functionally distinct sensory and motor neocortical areas form during mammalian development through a process called arealization. This process is believed to be reliant on both activity-dependent and activity-independent mechanisms. Although both mechanisms are thought to function concurrently during arealization, the nature of their interaction is not understood. To examine the potential interplay of extrinsic activity-dependent mechanisms, such as sensory input, and intrinsic activity-independent mechanisms, including gene expression in mouse neocortical development, we performed bilateral enucleations in newborn mice and conducted anatomical and molecular analyses 10 days later. In this study, by surgically removing the eyes of the newborn mouse, we examined whether early enucleation would impact normal gene expression and the development of basic anatomical features such as intraneocortical connections and cortical area boundaries in the first 10 days of life, before natural eye opening. We examined the acute effects of bilateral enucleation on the lateral geniculate nucleus of the thalamus and the neocortical somatosensory-visual area boundary through detailed analyses of intraneocortical connections and gene expression of six developmentally regulated genes at postnatal day 10.

Results

Our results demonstrate short-term plasticity on postnatal day 10 resulting from the removal of the eyes at birth, with changes in nuclear size and gene expression within the lateral geniculate nucleus as well as a shift in intraneocortical connections and ephrin A5 expression at the somatosensory-visual boundary. In this report, we highlight the correlation between positional shifts in ephrin A5 expression and improper refinement of intraneocortical connections observed at the somatosensory-visual boundary in enucleates on postnatal day 10.

Conclusions

Bilateral enucleation induces a positional shift of both ephrin A5 expression and intraneocortical projections at the somatosensory-visual border in only 10 days. These changes occur prior to natural eye opening, suggesting a possible role of spontaneous retinal activity in area border formation within the neocortex. Through these analyses, we gain a deeper understanding of how extrinsic activity-dependent mechanisms, particularly input from sensory organs, are integrated with intrinsic activity-independent mechanisms to regulate neocortical arealization and plasticity.

Evidence for cross-modal plasticity in adult mouse visual cortex following monocular enucleation

The goal of this study was to assess cortical reorganization in the visual system of adult mice in detail. A combination of deprivation of one eye and stimulation of the remaining eye previously led to the identification of input-specific subdivisions in mouse visual cortex. Using this information as a reference map, we established to what extent each of these functional subdivisions take part in cortical reactivation and reorganization upon unilateral enucleation. A recovery experiment revealed a differential laminar and temporal reactivation profile. Initiation of infragranular recovery of molecular activity near the border with nonvisual cortex and simultaneous hyperactivation of this adjacent cortex implied a partial nonvisual contribution to this plasticity. The strong effect of somatosensory deprivation as well as stimulation on infragranular visual cortex activation in long-term enucleated animals support this view. Furthermore, targeted tracer injections in visual cortex of control and enucleated animals revealed preexisting connections between the visual and somatosensory cortices of adult mice as possible mediators. In conclusion, this study supports an important cross-modal component in reorganization of adult mouse visual cortex upon monocular enucleation.

Altered expression of parvalbumin and calbindin in interneurons within the primary visual cortex of neonatal enucleated hamsters

In the present study, we tested the hypothesis that the expression of calcium binding proteins (CaBPs), parvalbumin (PV), calretinin (CR) and calbindin (CB), is dependent upon sensory experience as emphasized in visual deprivation and deafferentation studies. The expression of CaBPs was studied in interneurons within the primary and extrastriate visual cortices (V1, V2M, V2L) and auditory cortex (AC) of adult hamsters enucleated at birth. The effects of enucleation were mainly confined to area V1 where there was a significant volume reduction (26%) and changes in the laminar distribution of PV and CB immunoreactive (IR) cells. The density of PV-IR cell bodies was significantly increased in layer IV and reduced in layer V. Moreover, the density of CB-IR neurons was inferior in layer V of V1 in enucleated hamsters (EH) compared to controls. These results suggest that some features of the laminar distribution of specific CaBPs, in primary sensory cortices, are dependent upon or modulated by sensory input.

What are the Effects of Severe Visual Impairment on the Cortical Organization and Connectivity of Primary Visual Cortex?

The organization and connections of the primary visual area (V1) were examined in mice that lacked functional rods (Gnat−/−), but had normal cone function. Because mice are nocturnal and rely almost exclusively on rod vision for normal behaviors, the Gnat−/− mice used in the present study are considered functionally blind. Our goal was to determine if visual cortex is reorganized in these mice, and to examine the neuroanatomical connections that may subserve reorganization. We found that most neurons in V1 responded to auditory, or some combination of auditory, somatosensory, and/or visual stimulation. We also determined that cortical connections of V1 in Gnat−/− mice were similar to those in normal animals, but even in normal animals, there is sparse input from auditory cortex (AC) to V1. An important observation was that most of the subcortical inputs to V1 were from thalamic nuclei that normally project to V1 such as the lateral geniculate (LG), lateral posterior (LP), and lateral dorsal (LD) nuclei. However, V1 also received some abnormal subcortical inputs from the anterior thalamic nuclei, the ventral posterior, the ventral lateral and the posterior nuclei. While the vision generated from the small number of cones appears to be sufficient to maintain most of the patterns of normal connectivity, the sparse abnormal thalamic inputs to VI, existing inputs from AC, and possibly abnormal inputs to LG and LP may be responsible for generating the alterations in the functional organization of V1.

In search of a unifying theory of complex brain evolution

The neocortex is the part of the brain that is involved in perception, cognition, and volitional motor control. In mammals it is a highly dynamic structure that has been dramatically altered in different lineages, and these alterations account for the remarkable variations in behavior that species exhibit. When we consider how this structure changes and becomes more complex in some mammals such as humans, we must also consider how the alterations that occur at macro levels of organization, such as the level of the individual and social system, as well as micro levels of organization, such as the level of neurons, synapses and molecules, impact the neocortex. It is also important to consider the constraints imposed on the evolution of the neocortex. Observations of highly conserved features of cortical organization that all mammals share, as well as the convergent evolution of similar features of organization, indicate that the constraints imposed on the neocortex are pervasive and restrict the avenues along which evolution can proceed. Although both genes and the laws of physics place formidable constraints on the evolution of all animals, humans have evolved a number of mechanisms that allow them to loosen these constraints and often alter the course of their own evolution. While this cortical plasticity is a defining feature of mammalian neocortex, it appears to be exaggerated in humans and could be considered a unique derivation of our species.

Morphological alterations in the congenital blind based on the analysis of cortical thickness and surface area

To explore the morphological aspects of the functional reorganization of the blind's visual cortex, we analyzed the regional cortical thickness and cortical surface area in the congenitally blind subjects (CB) compared to the late-onset blind (LB) and sighted controls (SC). Cortical thickness was calculated from high-resolution T1-weighted magnetic resonance images of 21 young CB (blind from birth, mean age=27.1 yr), 12 LB, and 35 young SC. Analysis of covariance of cortical layer thickness with global thickness, age, and gender as covariates was done node-by-node on the entire cortical surface. Further analysis of mean thickness and surface area was performed for 33 automatically parceled cortical regions. Voxel-based morphometry was also conducted to compare results with cortical thickness and surface area. We found increased cortical thickness in the regions involved in vision and eye movement, such as the pericalcarine sulcus, cingulate cortex, and right frontal eye field, but cortical thinning in the left somatosensory cortex and right auditory cortex of CB compared to SC. CB had significantly reduced surface extent in the primary and associated visual areas, which explains volumetric atrophies in the visual cortex of CB despite increased cortical thickness. Conversely, LB tended to have cortical thinning in the primary visual cortex with a slight or no significant reduction in the surface extent. These morphological alterations in CB may indicate cortical reorganization at the visual cortex in connection with other sensory cortices.

Effects of bilateral enucleation on the size of visual and nonvisual areas of the brain

Alterations in the activity of one sensory system can affect the development of cortical and subcortical structures in all sensory systems. In this study, we characterize the changes that occur in visual and nonvisual areas of the brain following bilateral enucleation in short-tailed opossums. We demonstrate that bilateral enucleation early in development can significantly decrease brain size. This change is driven primarily by a decrease in the size of the thalamus, midbrain, and hindbrain, rather than a decrease in the size of the cortical hemispheres. We also found a significant decrease in the size of the lateral geniculate nucleus in bilaterally enucleated animals. Although the overall size of the neocortex was the same, the percentage of neocortex devoted to visual areas V1 (primary visual area) and caudotemporal area were significantly smaller in bilaterally enucleated opossums and the percentage of neocortex devoted to the primary somatosensory area (S1) was significantly larger, although S1 did not change in size to the same extent as V1. Our data suggest that during development the relative activity patterns between sensory systems, which are driven by activity from unique sets of sensory receptor arrays, play a major role in determining the relative size and organization of cortical and subcortical areas.

Genetic and epigenetic contributions to the cortical phenotype in mammals

One aspect of cortical organization, cortical field size, is variable both within and across species. The observed variability arises from a variety of sources, including genes intrinsic to the neocortex and a number of extrinsic and epigenetic factors. Genes intrinsic to the cortex are directly involved in the development and specification of cortical fields and are regulated from both signaling centers located outside of the neocortex, which secrete diffusible molecules, and the expression of transcription factors within the neocortex. In addition, extrinsic factors such as the type, location and density of sensory receptor arrays and how these receptor arrays are utilized, are also strongly related to cortical field size. Epigenetic factors including the relative activity patterns generated by the different types of physical stimuli in a given environment also contribute to differences in cortical organization, including cortical field size. Since both genetic and epigenetic factors contribute to cortical organization, some aspects of the cortical phenotype evolve, while other aspects of the cortical phenotype persist only if the environment in which an individual develops is relatively stable.

Visual activity and cortical rewiring: activity-dependent plasticity of cortical networks

The mammalian cortex is organized anatomically into discrete areas, which receive, process, and transmit neural signals along functional pathways. These pathways form a system of complex networks that wire up through development and refine their connections into adulthood. Understanding the processes of cortical-pathway formation, maintenance, and experience-dependent plasticity has been among the major goals of contemporary neurobiology. In this chapter, we will discuss an experimental model used to investigate the role of activity in the patterning of cortical networks during development. This model involves the "rewiring" of visual inputs into the auditory thalamus and subsequent remodeling of the auditory cortex to process visual information. We review the molecular, cellular, and physiological mechanisms of visual "rewiring" and activity-dependent shaping of cortical networks.

Early blindness results in abnormal corticocortical and thalamocortical connections

Studies in congenitally blind and bilaterally enucleated individuals show that an early loss of sensory driven activity can lead to massive functional reorganization. However, the anatomical substrate for this functional reorganization is unknown. In the present study, we examined patterns of corticocortical and thalamocortical connections in adult opossums that had been bilaterally enucleated neonatally, prior to the formation of retinogeniculate and geniculocortical connections. We show that in addition to normal thalamocortical projection patterns from visual nuclei, enucleated animals also receive input from nuclei associated with the somatosensory (ventral posterior nucleus, VP), auditory (medial geniculate nucleus, MGN), motor (ventrolateral nucleus, VL), and limbic/hippocampal systems (anterior dorsal nucleus, AD; and anterior ventral nucleus, AV). Likewise, in addition to normal corticocortical projections to area 17, bilaterally enucleated opossums also receive input from auditory, somatosensory, and multimodal cortex. These aberrant patterns of thalamocortical and corticocortical connections can account for alterations in functional organization observed in the visual cortex of bilateral enucleated animals, and indicate that factors extrinsic to the cortex play a large role in cortical field development and evolution. On the other hand, the maintenance of normal patterns of connections in the absence of visual input suggests that there are formidable constraints imposed on the developing cortex that highly restrict the types of evolutionary change possible.

Aberrant retinal projections in congenitally deaf mice: how are phenotypic characteristics specified in development and evolution?

The contribution of sensory input to the formation of sensory system-specific (sensoritopic) connections of the thalamus and midbrain was investigated using mice lacking the Na+-K+-2Cl- cotransporter (NKCC1) or the plasma membrane Ca2+-ATPase isoform2 (PMCA2). Because these mice are congenitally deaf, the developing nervous system has no exposure to sensory-driven neural activity from the auditory system. Here we compared the retinofugal pathway in normal and congenitally deaf mice using intraocular injections of neuroanatomical tracers into each eye, and relating tracer patterns to identified thalamic nuclei and superior colliculus layers. We demonstrate that loss of such activity results in aberrant projections of the retina into nonvisual auditory structures such as the medial geniculate nucleus and the intermediate layers of the superior colliculus. These findings indicate that activity from peripheral sensory receptor arrays is necessary not only for the refinement of developing connections within a unimodal structure, but for the establishment of sensoritopic or sensory-specific connections of unimodal and multimodal structures. We hypothesize that specification of such connections may occur through the modulation of spatial expression patterns of molecules known to be involved in the development of topography of connections between brain structures, such as the ephrins, via activity-dependent, CRE-mediated gene expression.

Environmental enrichment enhances auditory takeover of the occipital cortex in anophthalmic mice

Enrichment of the environment is an effective means of enhancing neuronal development and plasticity but its effect on the cross-modal compensation resulting from sensory deprivation has never been investigated. The present study used c-Fos immunohistochemistry and dextran-biotin neuronal tracing to examine the reorganization of sensory modalities in the brain of anophthalmic mutant mice (ZRDCT/An) raised in either enriched or standard environments. Auditory stimulation was found to elicit strong neuronal activation in thalamic and cortical structures that are normally visual. An important finding was that the latter auditory-evoked cortical activity was considerably enhanced in blind mice raised in the enriched environment. The axonal tracing study demonstrated auditory inputs from the inferior colliculus to the visual thalamus. This animal model will be useful for understanding neuronal mechanisms underlying some cross-modal sensory phenomena observed in blind or deaf humans.

Nature versus nurture revisited: an old idea with a new twist

The nature versus nurture debate has recently resurfaced with the emergence of the field of developmental molecular neurobiology. The questions associated with "nature" have crystallized into testable hypotheses regarding patterns of gene expression during development, and those associated with "nurture" have given over to activity-dependent cellular mechanisms that give rise to variable phenotypes in developing nervous systems. This review focuses on some of the features associated with complex brains and discusses the evolutionary and activity-dependent mechanisms that generate these features. These include increases in the size of the cortical sheet, changes in cortical domain and cortical field specification, and the activity-dependent intracellular mechanisms that regulate the structure and function of neurons during development. We discuss which features are likely to be genetically mediated, which features are likely to be regulated by activity, and how these two mechanisms act in concert to produce the wide variety of phenotypes observed for the mammalian neocortex. For example, the size of the cortical sheet is likely to be under genetic control, and regulation of cell-cycle kinetics through upregulation of genes such as beta-catenin can account for increases in the size of the cortical sheet. Similarly, intrinsic signaling genes or gene products such as Wnt, Shh, Fgf2, Fgf8 and BMP may set up a combinatorial coordinate system that guides thalamic afferents. Changes in peripheral morphology that regulate patterned activity are also likely to be under genetic control. Finally, the intracellular machinery that allows for activity-dependent plasticity in the developing CNS may be genetically regulated, although the specific phenotype they generate are not. On the other hand, aspects of neocortical organization such as sensory domain assignment, the size and shape of cortical fields, some aspects of connectivity, and details of functional organization are likely to be activity-dependent. Furthermore, the role of genes versus activity, and their interactions, may be different for primary fields versus non-primary fields.

Expanding the biological basis of tinnitus: crossmodal origins and the role of neuroplasticity

Tinnitus is most often initiated by modality specific otopathologic disturbances affecting peripheral and central auditory pathways. However, there is growing evidence indicating that the anatomical location generating tinnitus occurs at sites different from the initial pathology. Support for this notion is found in individuals where tinnitus can be triggered or modulated by inputs from other sensory modalities or sensorimotor systems (somatosensory, somatomotor, visual-motor). The use of functional imaging methods combined with psychophysics, detailed physical examinations and questionnaire-based assessments has reinforced and validated these observations. Available data suggest that tinnitus-related crossmodal interactions are more common than previously anticipated. This communication reviews these advancements and suggests that a relatively broad multimodal network of neurons is involved in generating and sustaining the tinnitus perception in some forms of the disorder. Also implicated as part of the tinnitus experience are interactions within large-scale neural networks subserving attention, cognition, and emotion. Incorporating this knowledge into contemporary psychophysiological models will help facilitate the conceptualization of this phantom perception in a more comprehensive manner.

Massive cross-modal cortical plasticity and the emergence of a new cortical area in developmentally blind mammals

In the current investigation, the neurophysiological organization of the neocortex was examined in adult animals that were bilaterally enucleated very early in life, before the retino-geniculo-cortical pathway was established. Our results indicate that some aspects of development of cortical fields are not mediated by specific sensory inputs. However, the current study also demonstrates that peripheral innervation plays a large role in the organization of the neocortex, as cortical territories normally involved in visual processing are completely captured by the auditory and somatosensory system. Thus, a large degree of phenotypic variability in cortical organization can be accomplished solely by removing or modifying sensory inputs.

Auditory activation of "visual" cortical areas in the blind mole rat (Spalax ehrenbergi)

The mole rat (Spalax ehrenbergi) is a subterranean rodent whose adaptations to its fossorial life include an extremely reduced peripheral visual system and an auditory system suited for the perception of vibratory stimuli. We have previously shown that in this blind rodent the dorsal lateral geniculate nucleus, the primary visual thalamic nucleus of sighted mammals, is activated by auditory stimuli. In this report we focus on the manifestation of this cross-modal compensation at the cortical level. Cyto- and myeloarchitectural analyses of the occipital area showed that despite the almost total blindness of the mole rat this area has retained the organization of a typical mammalian primary visual cortex. Application of the metabolic marker 2-deoxyglucose and electrophysiological recording of evoked field potentials and single-unit activity disclosed that a considerable part of this area is activated by auditory stimuli. Previous neuronal tracing studies had revealed the origin of the bulk of this auditory input to be the dorsal lateral geniculate nucleus which itself receives auditory input from the inferior colliculus.

Cross-modal neuroplasticity in neonatally enucleated hamsters: structure, electrophysiology and behaviour

Potential auditory compensation in neonatally bilaterally enucleated Syrian hamsters was explored anatomically, electrophysiologically and behaviourally. Gross morphology of the visual cortex appeared normal and no obvious cytoarchitectural malformation was discerned. However, enucleation induced a significant increase in the spontaneous firing rate of visual cortex cells. Further, auditory stimuli elicited field potentials and single unit responses in the visual cortex of enucleated, but not normal, animals. About 63% of the cells isolated in the visual cortex of 16 enucleated hamsters responded to at least one type of auditory stimulus. Most of the responses were less vigorous and less time-locked than those of auditory cortex cells, and thresholds were typically higher. Projection tracing with WGA-HRP disclosed reciprocal connections between the visual cortex and the dorsal lateral geniculate nucleus in both intact and enucleated animals. However, in the enucleated animals retrogradely labelled cells were also found in the inferior colliculus, the major midbrain auditory nucleus. Behaviourally determined auditory sensitivity across the hearing range did not differ between enucleated and intact hamsters. Minimum audible angle, as determined by a conditioned suppression task, ranged from around 17 to 22 degrees, with no significant difference between normal and enucleated animals. The two groups also did not differ with regard to the direction of their unconditioned head orientating response to intermittent noise. However, the enucleated animals showed a more vigorous response and were slower to habituate to the noise. These results show that bilateral enucleation of newborn hamsters results in auditory activation of visual targets, in addition to the typical activation of the intact auditory pathway. Behaviourally it appears that enucleated hamsters, compared with their normal littermates, are slower to habituate in their response to an unexpected source of sound.

Development and plasticity of cortical areas and networks

The development of cortical layers, areas and networks is mediated by a combination of factors that are present in the cortex and are influenced by thalamic input. Electrical activity of thalamocortical afferents has a progressive role in shaping cortex. For early thalamic innervation and patterning, the presence of activity might be sufficient; for features that develop later, such as intracortical networks that mediate emergent responses of cortex, the spatiotemporal pattern of activity often has an instructive role. Experiments that route projections from the retina to the auditory pathway alter the pattern of activity in auditory thalamocortical afferents at a very early stage and reveal the progressive influence of activity on cortical development. Thus, cortical features such as layers and thalamocortical innervation are unaffected, whereas features that develop later, such as intracortical connections, are affected significantly. Surprisingly, the behavioural role of 'rewired' cortex is also influenced profoundly, indicating the importance of patterned activity for this key aspect of cortical function.

Auditory activation of cortical visual areas in cats after early visual deprivation

Auditory activation of the primary visual cortex (area 17) and two extrastriate visual cortical areas - the anterolateral lateral suprasylvian area (ALLS) and anteromedial lateral suprasylvian area (AMLS), was investigated in visually impaired cats. Impairment was accomplished shortly after birth by bilateral eyelid suturing (binocularly deprived cats, BD) or bilateral enucleation (binocularly enucleated cats, BE). In BE cats, the optic nerve and chiasm were entirely degenerated. No cortical atrophy or cytoarchitectural malformation was noticed in either BD or BE cats. In both normal and impaired cats we found auditory-responsive cells in the ALLS and AMLS, areas that are considered strictly visual. The most remarkable finding was an increase in the relative number of these auditory cells in the BD and BE cats, which was more prominent in the latter. Some auditory-responsive cells were also found in area 17 of BE cats. On the basis of formal calculation, it is tempting to suggest that the increase in relative number of auditory cells in these areas reflects the transformation of all the visual cells in the ALLS of BD and BE cats into auditory cells. In BE cats, all bimodal cells and an appreciable percentage of non-responsive cells also had transformed to auditory cells. In the AMLS of BD cats, it is primarily the bimodal cells that become auditory cells, whereas in BE cats all the visual and bimodal cells as well as non-responsive cells undergo this transformation. This assumption, however, is one possible interpretation of our results but not the only one. Other modes of neuronal plasticity that might yield similar results in the visually deprived cats can not be ruled out.

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

Following specific neonatal brain lesions in rodents and ferrets, retinal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retinal targets along with deafferentation of the MGN are two concurrent factors required for the induction of novel retino-MGN projections. We have examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of neonatal lesions to deafferent this nucleus, by injecting retrograde tracers into the MGN of normal and neonatally operated adult ferrets, respectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN afferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterograde tracers. We found that the extent of retino-MGN projections correlates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at least some normal retinal targets seems necessary for the formation of retino-MGN connections. The diameters of retino-MGN axons suggest that more than one type of retinal ganglion cells innervate the MGN under a lesion paradigm that spares the visual cortex and lateral geniculate nucleus. We also found that, after extensive deafferentation of MGN, other axonal systems in addition to retinal axons project ectopically to the MGN. These data are consistent with the idea that ectopic retino-MGN projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons compete with other sets of axons for terminal space in the MGN.

Effects of increased neural activity on brain growth

We have measured the effects of regionally increased metabolic activity--and by inference electrical activity--on cortical growth in the developing rat brain. Cortical growth is significantly and specifically greater in regions of chronically increased activity. This effect of activity on cortical growth may help explain the permanent storage of early experience in the developing nervous system.

Use, disuse, and growth of the brain

It is well known that across species, the relative size of the cortical area representing a particular sensory surface is proportional to how important that sense is for the animal. Furthermore, we are commonly aware of the observation that the loss of one sense, such as sight, appears to lead to an increase in sensitivity of the remaining senses, although the physiological basis for this is not entirely clear. Now, several studies, including that of Zheng and Purves (11), have suggested that the cortical area devoted to a particular sensory system can be modulated by neuronal activity during development. The fact that use, or disuse, of a sensory organ can lead to significant changes in its area of representation in the developing cortex is intriguing and calls for further investigations aimed at understanding the functional significance and the mechanisms underlying these changes. What remains to be determined is whether enhanced "growth" also means enhanced performance by that sensory system and, if so, whether this is the result of selective changes in neuronal connectivity and/or synaptic efficacy. It is too early to tell, but, whatever the outcome, it is refreshing to consider neuronal growth in the light of enhanced neural activity, in parallel to the results of activity deprivation, to which we are more accustomed.

Neonatal monocular enucleation-induced cross-modal effects observed in the cortex of adult rat

The cortices of neonatally enucleated rats were explored for somatosensory responses with special reference to an extension into the occipital cortex. Monocular enucleation was performed on rats at birth. The animals were raised and from the age of three months the activity evoked by either electric stimulation of the vibrissa pad or bending of the vibrissae was tested in the contralateral cortex by electric recording and autoradiography. It was found that early enucleation caused an expansion of the somatosensory responses, among others into the visual area. Neurons responsive to visual and somatosensory stimuli were demonstrated in the anterior part of the primary and secondary visual areas, contralateral to the enucleation. Electrophysiological and autoradiographic studies unambiguously proved that early enucleation exerted a significant cross-modal effect on the somatosensory responsive area.

Neonatal enucleation induces cross-modal changes in the barrel cortex of rat. A behavioural and electrophysiological study

The present study was undertaken to determine whether neonatal enucleation leads to functional changes in the somato-sensory system and whether it has any behavioural effects. Binocular enucleation was performed on newborn rats. The effects of enucleation were tested versus controls in a rectangular maze on 10 successive days starting on postnatal day 80. Immediately after the 10-day behavioural study session, electro-physiological experiments were performed on 5 enucleated and 5 control rats. All the whiskers of the remaining animals were clipped off on both sides, and these animals continued the maze running for four additional sessions. The behavioural study demonstrated that the maze performance achieved by the neonatally enucleated animals was better than that of the controls. This suggested that cross-modal compensatory changes took place in other sensory systems, presumably somatosensory too. This was supported by the result of the experiment preceded by bilateral vibrissa clipping. The electrophysiological experiments clearly revealed that functional changes took place in the somatosensory system of enucleated rats. In these animals, the cells in some barrels (C1 and E3) displayed enlarged receptive fields, while in an other barrel (A3) an increased angular sensitivity for deflection of its related whisker was observed. This combined study clearly demonstrates that neonatal enucleation is able to induce cross-modal compensatory changes in the somatosensory system of the rat.

Visual system of a naturally microphthalmic mammal: the blind mole rat, Spalax ehrenbergi

Retinal projections and visual thalamo-cortical connections were studied in the subterranean mole rat, belonging to the superspecies Spalax ehrenbergi, by anterograde and retrograde tracing techniques. Quantitative image analysis was used to estimate the relative density and distribution of retinal input to different primary visual nuclei. The visual system of Spalax presents a mosaic of both regressive and progressive morphological features. Following intraocular injections of horseradish peroxidase conjugates, the retina was found to project bilaterally to all visual structures described as receiving retinal afferents in non-fossorial rodents. Structures involved in form analysis and visually guided behaviors are reduced in size by more than 90%, receive a sparse retinal innervation, and are cytoarchitecturally poorly differentiated. The dorsal lateral geniculate nucleus, as defined by cyto- and myelo-architecture, cytochrome oxidase, and acetylcholinesterase distribution as well as by afferent and efferent connections, consists of a narrow sheet 3-5 neurons thick, in the dorsal thalamus. Connections with visual cortex are topographically organized but multiple cortical injections result in widespread and overlapping distributions of geniculate neurons, thus indicating that the cortical map of visual space is imprecise. The superficial layers of the superior colliculus are collapsed to a single layer, and the diffuse ipsilateral distribution of retinal afferents also suggests a lack of precise retinotopic relations. In the pretectum, both the olivary pretectal nucleus and the nucleus of the optic tract could be identified as receiving ipsilateral and contralateral retinal projections. The ventral lateral geniculate nucleus is also bilaterally innervated, but distinct subdivisions of this nucleus or the intergeniculate leaflet could not be distinguished. The retina sends a sparse projection to the dorsal and lateral terminal nuclei of the accessory optic system. The medial terminal nucleus is not present. In contrast to the above, structures of the "non-image forming" visual pathway involved in photoperiodic perception are well developed in Spalax. The suprachiasmatic nucleus receives a bilateral projection from the retina and the absolute size, cytoarchitecture, density, and distribution of retinal afferents in Spalax are comparable with those of other rodents. A relatively hypertrophied retinal projection is observed in the bed nucleus of the stria terminalis. Other regions which receive sparse visual input include the lateral and anterior hypothalamic areas, the retrochiasmatic region, the sub-paraventricular zone, the paraventricular hypothalamic nucleus, the anteroventral and anterodorsal nuclei, the lateral habenula, the mediodorsal nucleus, and the basal telencephalon.(ABSTRACT TRUNCATED AT 400 WORDS)

Retinal transplants in congenitally blind mice: patterns of projection and synaptic connectivity

Embryonic retinae were transplanted onto the midbrain of neonatal congenitally anophthalmic mice and neonatal mice from which both eyes had been removed. When donor mice of the AKR strain were used, the detailed patterns of the transplant projections to the host brain were demonstrated with an antibody to Thy-1.1, which specifically stains neural tissue derived from AKR donors. Many of the subcortical visual centers were innervated, and only small differences were encountered between anophthalmic and eye-enucleated mice. The terminal arbors of transplant-derived axons could not be classified as in normal animals, although several distinct arbor types were seen. In the superior colliculus, the laminar arrangements that characterize normal retinal arbors were disrupted. Despite this, the synaptic patterns formed by transplant-derived axons in the superior colliculus of anophthalmic mice compared very closely with those of retinal axons in normal, sighted animals. These observations indicate that the ability of a retinal transplant to innervate the host brain and to form the synaptic arrays characteristic of optic terminals are not dependent on prior innervation, nor do they appear to be influenced by the events that follow eye removal.

Axon substitution in the reorganization of developing neural connections

Insights into the mechanisms of normal and pathological neural development may be gained by studying the reorganization of developing neural connections, caused experimentally or by disease. Many reorganized connections are assumed to arise by the anomalous stabilization of transient connections that occur during normal development. We report that, although the retina projects transiently to the somatosensory system in normal developing hamsters, the permanent retinal projections to the somatosensory system that arise as a consequence of early brain lesions are not formed by the stabilization of the normally transient projection. Instead, the transient retinal axons are replaced by retinal axons that do not normally project to the somatosensory system. The distinction between anomalous stabilization and substitution is significant for determining the cellular mechanisms underlying the development of neural connectivity.

Crossmodal changes in the somatosensory vibrissa/barrel system of visually deprived animals

Cats deprived of vision from birth adapt remarkably well to their situation and show little behavioral impairment. They seem to compensate for their lack of vision by relying more on their auditory and tactile senses. We report here that the facial vibrissae, which are most important for tactile orientation in many animals, show supernormal growth in both cats and mice that have been deprived of vision from birth. Furthermore, the whisker representation in the somatosensory cortical barrel field shows a concomitant enlargement in binocularly enucleated mice: individual barrels are expanded in size by up to one-third. The increased use of the vibrissae in visually deprived animals may stimulate both their own growth and, via activation of the respective neural pathways, the expansion of their central representation.

Neonatal enucleation induces correlated modification in sensory responsive areas and pial angioarchitecture of the parietal and occipital cortex of albino rats

This study was carried out to investigate whether correlations existing in normal adult rats (Ambach et al., '86) between functional characteristics of neocortical areas and their pial angioarchitecture can be correspondingly modified under pathological conditions. The right eyes of albino rats were enucleated on the 1st, 8th, 15th and 30th day after birth, respectively. At the age of 3 to 4 months, the responsiveness of the parieto-occipital cortex to sensory stimuli was studied in enucleated animals and age matched controls. After the mapping of visually and somatosensorily evoked potentials, the vascular system was filled with dye. Monocular enucleation at birth induced bilateral modifications in sensory responsiveness and corresponding changes in pial angioarchitecture, especially in the venous drainage fields. In comparison with the controls, a considerable expansion was observed in the overlapping zone between visually and somatosensorily responsive areas. In contrast, borders of the visual cortex toward the auditory and retrosplenial areas were essentially stable. Corresponding changes were found in the pial distribution patterns of cerebral veins but not of arteries. The major effect of neonatal enucleation on angioarchitecture was a change in the subdivision of the parieto-occipital veins drainage fields. This was due to a significant enlargement of the anterior accessory occipital (O3) vein, which compressed the drainage fields of the parietal and occipital veins and completely separated them from one another. The results suggest that during ontogenesis: (1) alterations in the formation of sensory input may interfere with neocortical angiogenesis, especially the structuring of veins, (2) after monocular enucleation this influence is prominent in parietal and occipital cerebral veins, and (3) these angiogenetic processes are vulnerable only during the perinatal and early postnatal period.