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

Reversal of Age-Related Changes in Cortical Sound-Azimuth Selectivity with Training

The compromised abilities to understand speech and localize sounds are two hallmark deficits in aged individuals. Earlier studies have shown that age-related deficits in cortical neural timing, which is clearly associated with speech perception, can be partially reversed with auditory training. However, whether training can reverse aged-related cortical changes in the domain of spatial processing has never been studied. In this study, we examined cortical spatial processing in ~21-month-old rats that were trained on a sound-azimuth discrimination task. We found that animals that experienced 1 month of training displayed sharper cortical sound-azimuth tuning when compared to the age-matched untrained controls. This training-induced remodeling in spatial tuning was paralleled by increases of cortical parvalbumin-labeled inhibitory interneurons. However, no measurable changes in cortical spatial processing were recorded in age-matched animals that were passively exposed to training sounds with no task demands. These results that demonstrate the effects of training on cortical spatial domain processing in the rodent model further support the notion that age-related changes in central neural process are, due to their plastic nature, reversible. Moreover, the results offer the encouraging possibility that behavioral training might be used to attenuate declines in auditory perception, which are commonly observed in older individuals.

Cross-Talk of Low-Level Sensory and High-Level Cognitive Processing: Development, Mechanisms, and Relevance for Cross-Modal Abilities of the Brain

The emergence of cross-modal learning capabilities requires the interaction of neural areas accounting for sensory and cognitive processing. Convergence of multiple sensory inputs is observed in low-level sensory cortices including primary somatosensory (S1), visual (V1), and auditory cortex (A1), as well as in high-level areas such as prefrontal cortex (PFC). Evidence shows that local neural activity and functional connectivity between sensory cortices participate in cross-modal processing. However, little is known about the functional interplay between neural areas underlying sensory and cognitive processing required for cross-modal learning capabilities across life. Here we review our current knowledge on the interdependence of low- and high-level cortices for the emergence of cross-modal processing in rodents. First, we summarize the mechanisms underlying the integration of multiple senses and how cross-modal processing in primary sensory cortices might be modified by top-down modulation of the PFC. Second, we examine the critical factors and developmental mechanisms that account for the interaction between neuronal networks involved in sensory and cognitive processing. Finally, we discuss the applicability and relevance of cross-modal processing for brain-inspired intelligent robotics. An in-depth understanding of the factors and mechanisms controlling cross-modal processing might inspire the refinement of robotic systems by better mimicking neural computations.

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.

Regional Specificity of GABAergic Regulation of Cross-Modal Plasticity in Mouse Visual Cortex after Unilateral Enucleation

In adult mice, monocular enucleation (ME) results in an immediate deactivation of the contralateral medial monocular visual cortex. An early restricted reactivation by open eye potentiation is followed by a late overt cross-modal reactivation by whiskers (Van Brussel et al., 2011). In adolescence (P45), extensive recovery of cortical activity after ME fails as a result of suppression or functional immaturity of the cross-modal mechanisms (Nys et al., 2014). Here, we show that dark exposure before ME in adulthood also prevents the late cross-modal reactivation component, thereby converting the outcome of long-term ME into a more P45-like response. Because dark exposure affects GABAergic synaptic transmission in binocular V1 and the plastic immunity observed at P45 is reminiscent of the refractory period for inhibitory plasticity reported by Huang et al. (2010), we molecularly examined whether GABAergic inhibition also regulates ME-induced cross-modal plasticity. Comparison of the adaptation of the medial monocular and binocular cortices to long-term ME or dark exposure or a combinatorial deprivation revealed striking differences. In the medial monocular cortex, cortical inhibition via the GABAA receptor α1 subunit restricts cross-modal plasticity in P45 mice but is relaxed in adults to allow the whisker-mediated reactivation. In line, in vivo pharmacological activation of α1 subunit-containing GABAA receptors in adult ME mice specifically reduces the cross-modal aspect of reactivation. Together with region-specific changes in glutamate acid decarboxylase (GAD) and vesicular GABA transporter expression, these findings put intracortical inhibition forward as an important regulator of the age-, experience-, and cortical region-dependent cross-modal response to unilateral visual deprivation.

SIGNIFICANCE STATEMENT

In adult mice, vision loss through one eye instantly reduces neuronal activity in the visual cortex. Strengthening of remaining eye inputs in the binocular cortex is followed by cross-modal adaptations in the monocular cortex, in which whiskers become a dominant nonvisual input source to attain extensive cortical reactivation. We show that the cross-modal component does not occur in adolescence because of increased intracortical inhibition, a phenotype that was mimicked in adult enucleated mice when treated with indiplon, a GABAA receptor α1 agonist. The cross-modal versus unimodal responses of the adult monocular and binocular cortices also mirror regional specificity in inhibitory alterations after visual deprivation. Understanding cross-modal plasticity in response to sensory loss is essential to maximize patient susceptibility to sensory prosthetics.

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.

Perceptual Training Restores Impaired Cortical Temporal Processing Due to Lead Exposure

Low-level lead exposure is a risk factor for cognitive and learning disabilities in children and has been specifically associated with deficits in auditory temporal processing that impair aural language and reading abilities. Here, we show that rats exposed to low levels of lead in early life display a significant behavioral impairment in an auditory temporal rate discrimination task. Lead exposure also results in a degradation of the neuronal repetition-rate following capacity and response synchronization in primary auditory cortex. A modified go/no-go repetition-rate discrimination task applied in adult animals for ∼50 days nearly restores to normal these lead-induced deficits in cortical temporal fidelity. Cortical expressions of parvalbumin, brain-derived neurotrophic factor, and NMDA receptor subunits NR2a and NR2b, which are down-regulated in lead-exposed animals, are also partially reversed with training. These studies in an animal model identify the primary auditory cortex as a novel target for low-level lead exposure and demonstrate that perceptual training can ameliorate lead-induced deficits in cortical discrimination between sound sequences.

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.

The nature of consciousness in the visually deprived brain

Vision plays a central role in how we represent and interact with the world around us. The primacy of vision is structurally imbedded in cortical organization as about one-third of the cortical surface in primates is involved in visual processes. Consequently, the loss of vision, either at birth or later in life, affects brain organization and the way the world is perceived and acted upon. In this paper, we address a number of issues on the nature of consciousness in people deprived of vision. Do brains from sighted and blind individuals differ, and how? How does the brain of someone who has never had any visual perception form an image of the external world? What is the subjective correlate of activity in the visual cortex of a subject who has never seen in life? More in general, what can we learn about the functional development of the human brain in physiological conditions by studying blindness? We discuss findings from animal research as well from recent psychophysical and functional brain imaging studies in sighted and blind individuals that shed some new light on the answers to these questions.

Insights from darkness: what the study of blindness has taught us about brain structure and function

Vision plays a central role in how we represent and interact with the world around us. Roughly, one-third of the cortical surface in primates is involved in visual processes. The loss of vision, either at birth or later in life, must therefore have profound consequences on brain organization and on the way the world is perceived and acted upon. In this chapter, we formulate a number of critical questions. Do blind individuals indeed develop supra-normal capacities for the remaining senses in order to compensate for their loss of vision? Do brains from sighted and blind individuals differ, and how? How does the brain of someone who has never had any visual perception form an image of the external world? We discuss findings from animal research as well from recent psychophysical and functional brain imaging studies in sighted and blind individuals that shed some new light on the answers to these questions.