Selective breeding for variations in patterns of mystacial vibrissae of mice. Bilaterally symmetrical strains derived from ICR stock

How the Barrel Cortex Became a Working Model for Developmental Plasticity: A Historical Perspective

For half a century now, the barrel cortex of common laboratory rodents has been an exceptionally useful model for studying the formation of topographically organized maps, neural patterning, and plasticity, both in development and in maturity. We present a historical perspective on how barrels were discovered, and how thereafter, they became a workhorse for developmental neuroscientists and for studies on brain plasticity and activity-dependent modeling of brain circuits. What is particularly remarkable about this sensory system is a cellular patterning that is induced by signals derived from the sensory receptors surrounding the snout whiskers and transmitted centrally to the brainstem (barrelettes), the thalamus (barreloids), and the neocortex (barrels). Injury to the sensory receptors shortly after birth leads to predictable pattern alterations at all levels of the system. Mouse genetics have increased our understanding of how barrels are constructed and revealed the interplay of the molecular programs that direct axon growth and cell specification, with activity-dependent mechanisms. There is an ever-rising interest in this sensory system as a neurobiological model to study development of somatotopy, patterning, and plasticity at both the morphologic and physiological levels. This article is part of a group of articles commemorating the 50th anniversary of the Society for Neuroscience.

Precision weighting of cortical unsigned prediction error signals benefits learning, is mediated by dopamine, and is impaired in psychosis

Recent theories of cortical function construe the brain as performing hierarchical Bayesian inference. According to these theories, the precision of prediction errors plays a key role in learning and decision-making, is controlled by dopamine and contributes to the pathogenesis of psychosis. To test these hypotheses, we studied learning with variable outcome-precision in healthy individuals after dopaminergic modulation with a placebo, a dopamine receptor agonist bromocriptine or a dopamine receptor antagonist sulpiride (dopamine study n = 59) and in patients with early psychosis (psychosis study n = 74: 20 participants with first-episode psychosis, 30 healthy controls and 24 participants with at-risk mental state attenuated psychotic symptoms). Behavioural computational modelling indicated that precision weighting of prediction errors benefits learning in health and is impaired in psychosis. FMRI revealed coding of unsigned prediction errors, which signal surprise, relative to their precision in superior frontal cortex (replicated across studies, combined n = 133), which was perturbed by dopaminergic modulation, impaired in psychosis and associated with task performance and schizotypy (schizotypy correlation in 86 healthy volunteers). In contrast to our previous work, we did not observe significant precision-weighting of signed prediction errors, which signal valence, in the midbrain and ventral striatum in the healthy controls (or patients) in the psychosis study. We conclude that healthy people, but not patients with first-episode psychosis, take into account the precision of the environment when updating beliefs. Precision weighting of cortical prediction error signals is a key mechanism through which dopamine modulates inference and contributes to the pathogenesis of psychosis.

Development of rat female genital cortex and control of female puberty by sexual touch

Rat somatosensory cortex contains a large sexually monomorphic genital representation. Genital cortex undergoes an unusual 2-fold expansion during puberty. Here, we investigate genital cortex development and female rat sexual maturation. Ovariectomies and estradiol injections suggested sex hormones cause the pubertal genital cortex expansion but not its maintenance at adult size. Genital cortex expanded by thalamic afferents invading surrounding dysgranular cortex. Genital touch was a dominant factor driving female sexual maturation. Raising female rats in contact with adult males promoted genital cortex expansion, whereas contact to adult females or nontactile (audio-visual-olfactory) male cues did not. Genital touch imposed by human experimenters powerfully advanced female genital cortex development and sexual maturation. Long-term blocking of genital cortex by tetrodotoxin in pubescent females housed with males prevented genital cortex expansion and decelerated vaginal opening. Sex hormones, sexual experience, and neural activity shape genital cortex, which contributes to the puberty promoting effects of sexual touch.

We recently identified the somatosensory representation of rat genitals; remarkably, this cortical region—genital cortex—is sexually monomorphic, despite the marked sexual dimorphism of external genitals in rats. Most intriguing was the observation that genital cortex doubles in size during puberty. In order to shed light on this unusual expansion, we studied genital cortex development and sexual maturation in the female rat. We first showed that sex hormones are likely to cause the pubertal expansion of genital cortex. Next, we examined whether sexual experience affects the development of female genital cortex. Raising females together with adult males advanced genital cortex expansion, but cohousing with adult females or exposure to nontactile male cues was not sufficient to drive genital cortex growth. Surprisingly, artificial genital touch led to an early onset of female puberty and growth of genital cortex. In line with this finding, we find that if genital cortex activity is blocked, the advancing effects of adult males on puberty and genital cortex growth are inhibited. Together, our results point to an important role of genital cortex in the puberty-promoting effects of sexual touch.

Ctip1 Controls Acquisition of Sensory Area Identity and Establishment of Sensory Input Fields in the Developing Neocortex

While transcriptional controls over the size and relative position of cortical areas have been identified, less is known about regulators that direct acquisition of area-specific characteristics. Here, we report that the transcription factor Ctip1 functions in primary sensory areas to repress motor and activate sensory programs of gene expression, enabling establishment of sharp molecular boundaries defining functional areas. In Ctip1 mutants, abnormal gene expression leads to aberrantly motorized corticocortical and corticofugal output connectivity. Ctip1 critically regulates differentiation of layer IV neurons, and selective loss of Ctip1 in cortex deprives thalamocortical axons of their receptive "sensory field" in layer IV, which normally provides a tangentially and radially defined compartment of dedicated synaptic territory. Therefore, although thalamocortical axons invade appropriate cortical regions, they are unable to organize into properly configured sensory maps. Together, these data identify Ctip1 as a critical control over sensory area development.

Review : Evolution of the Motor System: Why the Elephant's Trunk Works Like a Human's Hand

The nucleus of Darkschewitsch, the nucleus accessorius medialis of Bechterew, and the parvicellular red nucleus in the mammalian mesodiencephalon fuse with each other and thus have borders that are not always distinct. These structures project topographically to the inferior olive and receive inputs from motor cortex, premotor cortex, substantia nigra, and cerebellar nuclei, which suggests that these nuclei play an important role in mammalian motor control. Furthermore, the nuclei show developmental differences that correspond with species-specialized body parts, such as the human's hand, the axial muscular system of the whale, and the elephant's trunk, to name just a few. We focus here on the differences in these meso diencephalo-olivo-cerebellar projections among certain mammals and propose that these brain structures are altered as the animal's gross anatomy alters. We also suggest that well-developed mesodiencephalo olivo-cerebellar projections may be an important factor for the differentiation of the large neocortex of the human, primate, elephant, and whale during evolutionary progress. NEUROSCIENTIST 5:217-226, 1999

Proximity of excitatory synapses and astroglial gap junctions in layer IV of the mouse barrel cortex

Neurons and astrocytes, the two major cell populations in the adult brain, are characterized by their own mode of intercellular communication--the synapses and the gap junctions (GJ), respectively. In addition, there is increasing evidence for dynamic and metabolic neuroglial interactions resulting in the modulation of synaptic transmission at the so-called "tripartite synapse". Based on this, we have investigated at the ultrastructural level how excitatory synapses (ES) and astroglial GJ are spatially distributed in layer IV of the barrel cortex of the adult mouse. We used specific antibodies for connexin (Cx) 30 and 43 to identify astroglial GJ, these two proteins are known to be present in the majority of astroglial GJ in the cerebral cortex. In electron-microscopic images, we measured the distance between two ES, between two GJ and between a GJ and its nearest ES. We found a ratio of two GJ per three ES in the hollow and septal areas. Taking into account the size of an astrocyte domain, the high density of GJ suggests the occurrence of reflexive type, i.e. GJ between processes of the same astrocyte. Interestingly, the distance between an ES and an astroglial GJ was found to be significantly lower than that between either two synapses or between two GJ. These observations indicate that the two modes of cell-to-cell communication are not randomly distributed in layer IV of the barrel cortex. Consequently, this feature may provide the morphological support for the recently reported functional interactions between neuronal circuits and astroglial networks.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Role of adenylate cyclase 1 in retinofugal map development

The development of topographic maps of the sensory periphery is sensitive to the disruption of adenylate cyclase 1 (AC1) signaling. AC1 catalyzes the production of cAMP in a Ca2+/calmodulin-dependent manner, and AC1 mutant mice (AC1−/−) have disordered visual and somatotopic maps. However, the broad expression of AC1 in the brain and the promiscuous nature of cAMP signaling have frustrated attempts to determine the underlying mechanism of AC1-dependent map development. In the mammalian visual system, the initial coarse targeting of retinal ganglion cell (RGC) projections to the superior colliculus (SC) and lateral geniculate nucleus (LGN) is guided by molecular cues, and the subsequent refinement of these crude projections occurs via an activity-dependent process that depends on spontaneous retinal waves. Here, we show that AC1−/− mice have normal retinal waves but disrupted map refinement. We demonstrate that AC1 is required for the emergence of dense and focused termination zones and elimination of inaccurately targeted collaterals at the level of individual retinofugal arbors. Conditional deletion of AC1 in the retina recapitulates map defects, indicating that the locus of map disruptions in the SC and dorsal LGN of AC1−/− mice is presynaptic. Finally, map defects in mice without AC1 and disrupted retinal waves (AC1−/−;β2−/− double KO mice) are no worse than those in mice lacking only β2−/−, but loss of AC1 occludes map recovery in β2−/− mice during the second postnatal week. These results suggest that AC1 in RGC axons mediates the development of retinotopy and eye-specific segregation in the SC and dorsal LGN.

Sostdc1 defines the size and number of skin appendage placodes

Mammary glands and hair follicles develop as ectodermal organs sharing common features during embryonic morphogenesis. The molecular signals controlling the initiation and patterning of skin appendages involve the bone morphogenetic proteins and Wnt family members, which are commonly thought to serve as inhibitory and activating cues, respectively. Here, we have examined the role of the Bmp and Wnt pathway modulator Sostdc1 in mammary gland, and hair and vibrissa follicle development using Sostdc1-null mice. Contrary to previous speculations, loss of Sostdc1 did not affect pelage hair cycling. Instead, we found that Sostdc1 limits the number of developing vibrissae and other muzzle hair follicles, and the size of primary hair placodes. Sostdc1 controls also the size and shape of mammary buds. Furthermore, Sostdc1 is essential for suppression of hair follicle fate in the normally hairless nipple epidermis, but its loss also promotes the appearance of supernumerary nipple-like protrusions. Our data suggest that functions of Sostdc1 can be largely attributed to its ability to attenuate Wnt/β-catenin signaling.

What can we get from 'barrels': the rodent barrel cortex as a model for studying the establishment of neural circuits

Sensory inputs triggered by external stimuli are projected into discrete arrays of neuronal modules in the primary sensory cortex. This whisker-to-barrel pathway has gained in popularity as a model system for studying the development of cortical circuits and sensory processing because its clear patterns facilitate the identification of genetically modified mice with whisker map deficits and make possible coordinated in vitro and in vivo electrophysiological studies. Numerous whisker map determinants have been identified in the past two decades. In this review, we summarize what have we learned from the detailed studies conducted in various mutant mice with cortical whisker map deficits. We will specifically focus on the anatomical and functional establishment of the somatosensory thalamocortical circuits.

Proper formation of whisker barrelettes requires periphery-derived Smad4-dependent TGF-beta signaling

Mammalian somatosensory topographic maps contain specialized neuronal structures that precisely recapitulate the spatial pattern of peripheral sensory organs. In the mouse, whiskers are orderly mapped onto several brainstem nuclei as a set of modular structures termed barrelettes. Using a dual-color iontophoretic labeling strategy, we found that the precise topography of barrelettes is not a result of ordered positions of sensory neurons within the ganglion. We next explored another possibility that formation of the whisker map is influenced by periphery-derived mechanisms. During the period of peripheral sensory innervation, several TGF-β ligands are exclusively expressed in whisker follicles in a dynamic spatiotemporal pattern. Disrupting TGF-β signaling, specifically in sensory neurons by conditional deletion of Smad4 at the late embryonic stage, results in the formation of abnormal barrelettes in the principalis and interpolaris brainstem nuclei and a complete absence of barrelettes in the caudalis nucleus. We further show that this phenotype is not derived from defective peripheral innervation or central axon outgrowth but is attributable to the misprojection and deficient segregation of trigeminal axonal collaterals into proper barrelettes. Furthermore, Smad4-deficient neurons develop simpler terminal arbors and form fewer synapses. Together, our findings substantiate the involvement of whisker-derived TGF-β/Smad4 signaling in the formation of the whisker somatotopic maps.

EphA4 is necessary for spatially selective peripheral somatosensory topography

Somatosensation is the primary sensory modality employed by rodents in navigating their environments, and mystacial vibrissae on the snout are the primary conveyors of this information to the murine brain. The layout of vibrissae is spatially stereotyped and topographic connections faithfully maintain this layout throughout the neuraxis. Several factors have been shown to influence general vibrissal innervation by trigeminal neurons. Here, the role of a cell surface receptor, EphA4, in directing position-dependent vibrissal innervation is examined. EphA4 is expressed in the ventral region of the presumptive whisker pad and EphA4(-/-) mice lack the ventroposterior-most vibrissae. Analyses reveal that ventral trigeminal axons are abnormal, failing to innervate emerging vibrissae, and resulting in the absence of a select group of vibrissae in EphA4(-/-) mice. EphA4's selective effect on a subset of whiskers implicates cell-based signaling in the establishment of position-dependent connectivity and topography in the peripheral somatosensory system.

Roles of mGluR5 in synaptic function and plasticity of the mouse thalamocortical pathway

The group I metabotropic glutamate receptor 5 (mGluR5) has been implicated in the development of cortical sensory maps. However, its precise roles in the synaptic function and plasticity of thalamocortical (TC) connections remain unknown. Here we first show that in mGluR5 knockout (KO) mice bred onto a C57BL6 background cytoarchitectonic differentiation into barrels is missing, but the representations for large whiskers are identifiable as clusters of TC afferents. The altered dendritic morphology of cortical layer IV spiny stellate neurons in mGluR5 KO mice implicates a role for mGluR5 in the dendritic morphogenesis of excitatory neurons. Next, in vivo single-unit recordings of whisker-evoked activity in mGluR5 KO adults demonstrated a preserved topographical organization of the whisker representation, but a significantly diminished temporal discrimination of center to surround whiskers in the responses of individual neurons. To evaluate synaptic function at TC synapses in mGluR5 KO mice, whole-cell voltage-clamp recording was conducted in acute TC brain slices prepared from postnatal day 4-11 mice. At mGluR5 KO TC synapses, N-methyl-D-aspartate (NMDA) currents decayed faster and synaptic strength was more easily reduced, but more difficult to strengthen by Hebbian-type pairing protocols, despite a normal developmental increase in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated currents and presynaptic function. We have therefore demonstrated that mGluR5 is required for synaptic function/plasticity at TC synapses as barrels are forming, and we propose that these functional alterations at the TC synapse are the basis of the abnormal anatomical and functional development of the somatosensory cortex in the mGluR5 KO mouse.

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.

Modified Sensory Processing in the Barrel Cortex of the Adult Mouse After Chronic Whisker Stimulation

Chronic stimulation of a mystacial whisker follicle for 24 h induces structural and functional changes in layer IV of the corresponding barrel, with an insertion of new inhibitory synapses on spines and a depression of neuronal responses to the stimulated whisker. Under urethane anesthesia, we analyzed how sensory responses of single units are affected in layer IV and layers II & III of the stimulated barrel column as well as in adjacent columns. In the stimulated column, spatiotemporal characteristics of the activation evoked by the stimulated whisker are not altered, although spontaneous activity and response magnitude to the stimulated whisker are decreased. The sensitivity of neurons for the deflection of this whisker is not altered but the dynamic range of the response is reduced as tested by varying the amplitude and repetition rate of the deflection. Responses to deflection of nonstimulated whiskers remain unaltered with the exception of in-row whisker responses that are depressed in the column corresponding to the stimulated whisker. In adjacent nonstimulated columns, neuronal activity remains unaltered except for a diminished response of units in layer II/III to deflection of the stimulated whisker. From these results we propose that an increased inhibition within the stimulated barrel reduced the magnitude of its excitatory output and accordingly the flow of excitation toward layers II & III and the subsequent spread into adjacent columns. In addition, the period of uncorrelated activity between pathways from the stimulated and nonstimulated whiskers weakens synaptic inputs from in-row whiskers in the stimulated barrel column.

Plasticity of astrocytic coverage and glutamate transporter expression in adult mouse cortex

Astrocytes play a major role in the removal of glutamate from the extracellular compartment. This clearance limits the glutamate receptor activation and affects the synaptic response. This function of the astrocyte is dependent on its positioning around the synapse, as well as on the level of expression of its high-affinity glutamate transporters, GLT1 and GLAST. Using Western blot analysis and serial section electron microscopy, we studied how a change in sensory activity affected these parameters in the adult cortex. Using mice, we found that 24 h of whisker stimulation elicited a 2-fold increase in the expression of GLT1 and GLAST in the corresponding cortical column of the barrel cortex. This returns to basal levels 4 d after the stimulation was stopped, whereas the expression of the neuronal glutamate transporter EAAC1 remained unaltered throughout. Ultrastructural analysis from the same region showed that sensory stimulation also causes a significant increase in the astrocytic envelopment of excitatory synapses on dendritic spines. We conclude that a period of modified neuronal activity and synaptic release of glutamate leads to an increased astrocytic coverage of the bouton-spine interface and an increase in glutamate transporter expression in astrocytic processes.

Instructive role of a peripheral pattern for the central patterning of the trigeminal projection at the brainstem and thalamus revealed by an artificially altered whisker pattern

The central patterning mechanism of neuronal circuits is an important issue in developmental neuroscience. We report here the role of a peripheral whisker pattern for the patterning of the trigeminal projection at the brainstem and thalamus in the mouse somatosensory system. The whisker pattern was manipulated by infecting the embryonic epidermis with adenovirus harboring Shh. The ectopic expression of Shh led to the induction of extra whiskers and displacement of whiskers, where these whiskers were histologically normal. The altered whisker pattern was isomorphically represented in the brainstem (barrelette: subnuclei principalis and subnuclei interpolaris), thalamus (barreloid) and cortex (barrel) as revealed by cytochrome oxidase staining. The barrelette-like pattern of the parvalbumin became discernible by immunostaining at P7 in subnuclei principalis and at P4 in subnuclei interpolaris in normal mice. These are the barrelette neurons projecting to the thalamus and the local circuit within the barrelette. The barrelette-like parvalbumin pattern also exhibits the altered whisker pattern induced by the adenovirus harboring Shh. These results highlight the role the peripheral whisker pattern for the central patterning of the brainstem, thalamus, and cortex in the mouse somatosensory system.

Role of efficient neurotransmitter release in barrel map development

Cortical maps are remarkably precise, with organized arrays of thalamocortical afferents (TCAs) that project into distinct neuronal modules. Here, we present evidence for the involvement of efficient neurotransmitter release in mouse cortical barrel map development using barrelless mice, a loss-of-function mutant of calcium/calmodulin-activated adenylyl cyclase I (AC1), and mice with a mutation in Rab3-interacting molecule 1alpha (RIM1alpha), an active zone protein that regulates neurotransmitter release. We demonstrate that release efficacy is substantially decreased in barrelless TCAs. We identify RIMs as important phosphorylation targets for AC1 in the presynaptic terminal. We further show that RIM1alpha mutant mice have reduced TCA neurotransmitter release efficacy and barrel map deficits, although not as severe as those found in barrelless mice. This supports the role of RIM proteins in mediating, in part, AC1 signaling in barrel map development. Finally, we present a model to show how inadequacies in presynaptic function can interfere with activity-dependent processes in neuronal circuit formation. These results demonstrate how efficient synaptic transmission mediated by AC1 function contributes to the development of cortical barrel maps.

Molecular determinants of the face map development in the trigeminal brainstem

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

A digital atlas to characterize the mouse brain transcriptome

Massive amounts of data are being generated in an effort to represent for the brain the expression of all genes at cellular resolution. Critical to exploiting this effort is the ability to place these data into a common frame of reference. Here we have developed a computational method for annotating gene expression patterns in the context of a digital atlas to facilitate custom user queries and comparisons of this type of data. This procedure has been applied to 200 genes in the postnatal mouse brain. As an illustration of utility, we identify candidate genes that may be related to Parkinson disease by using the expression of a dopamine transporter in the substantia nigra as a search query pattern. In addition, we discover that transcription factor Rorb is down-regulated in the barrelless mutant relative to control mice by quantitative comparison of expression patterns in layer IV somatosensory cortex. The semi-automated annotation method developed here is applicable to a broad spectrum of complex tissues and data modalities.

The mammalian brain is a complex organ with hundreds of functional parts. Describing when and where genes are expressed in the brain is thus a potentially powerful method for understanding the function of gene products. In recent years, several mammalian genomes including those of human and mouse have been characterized. There are now efforts around the world that aim to determine the expression patterns for all genes in the mouse brain. To search these expression data readily, they must be placed into an atlas. The authors propose a new method for bringing such genetic data into a common spatial framework so that one can perform spatial searches and comparisons of gene expression patterns. To create this atlas, the authors developed a series of maps of the brain using a graphical modeling method called subdivision. These maps were deformed to match the shape of tissue sections, and genetic activity information was associated with the appropriate coordinates on the map. After placing 200 genes into the context of this atlas, the authors illustrate its application in discovering genes potentially involved in diseases and brain development.

The evolution of the neocortex in mammals: how is phenotypic diversity generated?

Evolution of the mammalian neocortex is difficult to examine directly. For this reason, comparative studies and developmental studies are the best way of gaining insight into the evolutionary process. Comparative studies indicate that neocortical evolution is constrained, and that the types of systems-level modifications made to the neocortex are limited. Developmental studies of gene expression suggest that genetic contingencies set up aspects of cortical organization and connectivity, and that the complex spatial and temporal interactions of genes constrain development and evolution. Although genes obviously contribute to phenotypic variability, variability can also be achieved through alterations in the sensory receptor arrays, or changes in sensory driven activity. The intracellular mechanisms that enable phenotypic variability might evolve, but often the phenotypic characteristic in question is context-dependent.

Cortical origin of functional recovery in the somatosensory cortex of the adult mouse after thalamic lesion

To study the degree and time course of the functional recovery in the somatosensory cortex (SI) after an excitotoxic lesion in the adult mouse thalamus, metabolic activity was determined in SI at various times points post-lesion. Immediately after the lesion, metabolic activity in the thalamically deafferented part of SI was at its lowest value but increased progressively at subsequent time points. This was seen in all cortical layers; however, layers I and Vb recovered more rapidly than layers II, III, IV, Va and VI. Removal of the mystacial whiskers corresponding to the deafferented area, 5 weeks after cortical recovery, produced a subsequent 32% drop in metabolic activity, demonstrating peripheral sensory activation of this part of the cortex. Tracing experiments revealed that the deafferented cortex did not receive a novel thalamic input but that cortico-cortical and contralateral barrel cortex projections to this area were reinforced. We conclude that the cortical functional recovery after a thalamic lesion is, at least partially, due to modified cortico-cortical and callosal projections to the deafferented cortical area.

Adenylate Cyclase 1 dependent refinement of retinotopic maps in the mouse

Development of the retino-collicular pathway has served as an important model system for examining the cellular mechanisms responsible for the establishment of neuronal maps of the sensory periphery. A consensus has emerged that molecular or chemical cues are responsible for the initial establishment of gross topography in this map, and that activity dependent factors sharpen this initial rough topography into precision. However, there is little evidence available concerning the biochemical signaling mechanisms that are responsible for topographic map refinement in the retino-collicular system. Using a combination of anatomical and biochemical techniques in normal and mutant mice, we provide evidence that Ca2+/Calmodulin regulated Adenylate Cyclase 1 (AC1), which is strongly expressed in the superficial layers of the colliculus, is an important downstream signaling agent for activity dependent map refinement in the superior colliculus.

Differential distribution of MAP1A isoforms in the adult mouse barrel cortex and comparison with the serotonin 5-HT2A receptor

Microtubule-associated protein 1A (MAP1A) is essential during the late differentiation phase of neuronal development. Here, we demonstrated the presence of two MAP1A isoforms with a differential spatial distribution in the adult mouse barrel cortex. Antibody A stained MAP1A in pyramidal and stellate cells, including dendrites that crossed layer IV in the septa between barrels. The other antibody, BW6 recognized a MAP1A isoform that was mainly confined to the barrel hollow and identified smaller caliber dendrites. Previously, an interaction of MAP1A and the serotonin 5-hydroxytryptamine 2A (5-HT(2A)) receptor was shown in the rat cortex. Here, we identified, by double-immunofluorescent labeling, MAP1A isoform and serotonin 5-HT(2A) receptor distribution. MAP1A co-localized mainly with 5-HT(2A) receptor in larger apical dendrites situated in septa. This differential staining of MAP1A and a serotonin receptor in defined barrel compartments may be due to changes in the expression or processing of MAP1A during dendritic transport as a consequence of functional differences in processing of whisker-related sensory input.

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

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

Adenylate cyclase 1 as a key actor in the refinement of retinal projection maps

cAMP occupies a strategic position to control neuronal responses to a large variety of developmental cues. We have analyzed the role of calcium-stimulated adenylate cyclase 1 (AC1) in the development of retinal topographic maps. AC1 is expressed in retinal ganglion cells (RGCs) from embryonic day 15 to adulthood with a peak during the first postnatal week. At that time, the other calcium-stimulated AC, AC8, is expressed in the superior colliculus (SC) but not in the RGCs. In mice of the barrelless strain, which carry an inactivating mutation of the AC1 gene, calcium-stimulated AC activity is reduced by 40-60% in the SC and retina. RGC projection maps were analyzed with a variety of anterograde and retrograde tracers. After an initially normal development until postnatal day 3, retinal fibers from the ipsilateral and contralateral eye fail to segregate into eye-specific domains in the lateral geniculate nucleus and the SC. Topographic defects in the fine tuning of the retinotectal and retinogeniculate maps are also observed with abnormalities in the confinement of the retinal axon arbors in the anteroposterior and mediolateral dimensions. This is attributable to the lack of elimination of misplaced axon collaterals and to the maintenance of a transient ipsilateral projection. These results establish an essential role of AC1 in the fine patterning of the retinal map. Calcium-modulated cAMP production in the RGCs could constitute an important link between activity-dependent changes and the anatomical restructuring of the retinal terminal arbors within central targets.

Formation of dendritic spines with GABAergic synapses induced by whisker stimulation in adult mice

During development, alterations in sensory experience modify the structure of cortical neurons, particularly at the level of the dendritic spine. Are similar adaptations involved in plasticity of the adult cortex? Here we show that a 24 hr period of single whisker stimulation in freely moving adult mice increases, by 36%, the total synaptic density in the corresponding cortical barrel. This is due to an increase in both excitatory and inhibitory synapses found on spines. Four days after stimulation, the inhibitory inputs to the spines remain despite total synaptic density returning to pre-stimulation levels. Functional analysis of layer IV cells demonstrated altered response properties, immediately after stimulation, as well as four days later. These results indicate activity-dependent alterations in synaptic circuitry in adulthood, modifying the flow of sensory information into the cerebral cortex.

The development of inhibitory synaptic specializations in the mouse deep cerebellar nuclei

Using confocal laser scanning microscopy and immunohistochemistry, this study shows the complete morphological development of GABAergic synaptic contacts between Purkinje cells and neurons of the deep cerebellar nuclei of the mouse. Firstly, presynaptic varicosities visualized with antibodies against synaptophysin, synapsin or glutamic acid decarboxylase, were detected when the postsynaptic GABA(A) receptors were not yet aggregated in the membrane but had a diffuse cytoplasmic distribution, which indicated a lead in maturation of presynaptic terminals over target cells. Secondly, receptor aggregates developed suddenly after an initial week of diffuse expression and these clusters matured into more numerous and larger synaptic aggregates. During this postsynaptic maturation, the presynaptic varicosities develop into numerous and well-defined spots. As soon as both pre- and postsynaptic clusters were detectable, these sites are always colocalized. We therefore consider the aggregation of postsynaptic receptor during development as a landmark of synapse formation. Our observations are consistent with a developmental model in which the presynaptic neuron differentiates its axon before the target neuron expresses the mature form of its receptors on the membrane. The presynaptic neuron can therefore instruct the target neuron about the distribution and aggregation of the postsynaptic receptors at the synapse.

Partial denervation of the whiskerpad in adult mice: pattern and origin of reinnervation

We studied sensory organ reinnervation after nerve transection in the mouse whisker-to-barrel pathway. In one set of adult mice, we determined at light microscopy level the number of fibres reaching the caudal whisker follicles 5, 15, 20, 60, 100 days and 1 year after transection of the sensory nerve of row C. Regenerated fibres were first detected 15 days post lesionem (p.l.) and myelin first observed at 20 days. Between 60 and 100 days, the number of fibres stayed at approximately 80% of the values obtained in control animals. At that time, myelinated fibres reached only 58% of their number in controls. At the electron microscopy level, these fibres differ from control ones by a smaller fibre diameter. The innervation of follicles of adjacent rows was not modified, indicating that follicular reinnervation is row specific. We checked this feature by injecting in another set of mice the denervated follicles and the adjacent ones with distinct retrograde tracers 45 days and 1 year after nerve transection. The percentage of double-labelled neurons in the Gasserian ganglion did not increase in experimental animals. This confirms the absence of colonization of intact follicles by regenerating fibres and indicates that reinnervation of the whisker follicles takes place by regeneration of the degenerated axons without collateral reinnervation. The companion paper describes the pattern of activation of the barrel cortex relative to the present findings.

Upregulation of BDNF mRNA expression in the barrel cortex of adult mice after sensory stimulation

Upregulation of brain-derived neurotrophic factor (BDNF) mRNA expression by neuronal activity has been reported in cultured hippocampal cells and in different in vivo excitotoxic paradigms. The aim of the present study was to determine whether sensory stimulation of the whisker-to-barrel pathway alters BDNF mRNA expression in the cortex and, if so, to evaluate the specificity of this effect. To this end, a set of mystacial whiskers was unilaterally stimulated by mechanical deflection, and the expression of BDNF mRNA was analyzed in the barrel cortex by in situ hybridization (ISH) using a 35S-labeled antisense BDNF riboprobe and emulsion autoradiography. A clear-cut and specific upregulation of the BDNF mRNA expression was found at the level of the somatosensory cortex after the increased peripheral stimulation. In the barrel cortex of control mice, BDNF mRNA was present in a few cells in layers II/III and VI, whereas it was almost undetectable in layer IV. After 6 hr of whisker stimulation, increased levels of BDNF mRNA were found in layers II to VI of the contralateral barrel cortex. In layer IV, BDNF upregulation was confined to the barrels corresponding to the stimulated follicles. ISH combined with immunocytochemistry against the three calcium-binding proteins parvalbumin, calretinin, and calbindin-D28K revealed that BDNF mRNA-expressing cells do not belong to the GABAergic cell population of the barrel cortex. The present results support a role for BDNF in activity-dependent modifications of the adult cerebral cortex.

Altered sensory processing in the somatosensory cortex of the mouse mutant barrelless

Mice homozygous for the barrelless (brl) mutation, mapped here to chromosome 11, lack barrel-shaped arrays of cell clusters termed "barrels" in the primary somatosensory cortex. Deoxyglucose uptake demonstrated that the topology of the cortical whisker representation is nevertheless preserved. Anterograde tracers revealed a lack of spatial segregation of thalamic afferents into individual barrel territories, and single-cell recordings demonstrated a lack of temporal discrimination of center from surround information. Thus, structural segregation of thalamic inputs is not essential to generate topological order in the somatosensory cortex, but it is required for discrete spatiotemporal relay of sensory information to the cortex.

Remodelling in the array of cell aggregates in somatotopic representation of the facial vibrissae through the trigeminal sensory system of the mouse

The disposition of the facial vibrissae of the mouse is represented as a matrix-like array of cell aggregates in rows and columns at every station of the whisker-to-barrel pathway. In order to evaluate the role of each station in this pathway, lesions were made in the facial vibrissae of the mystacial group on P0-P3, and the animals were sacrificed on P8. The effects of the lesions on the cell aggregates in the array were analyzed by using cytochrome oxidase and gallocyanin cell-staining methods. Division of cell aggregates in the array was controlled by row basis interactions through the pathway up to the cerebral cortex. In this organization, affected cell aggregates which corresponded to the damaged vibrissae were eliminated and/or fused together in the array of the thalamic relay nucleus. On the basis of thalamic modification, the final array of cell aggregates was remodelled in the cerebral cortex. In contrast, affected cell aggregates remained degenerative spaces at the original sites in the array in relation to the damaged vibrissae in the brain stem trigeminal nuclear complex. These results indicate that a protoframework with row basis orientation for the division of cell aggregates is prepared in every station of the pathway at the time of lesioning, and adjustment of subcortical alterations in the thalamic relay nucleus is a decisive process to let the cerebral cortex remodel the topographic array of cell aggregates.

Vision influences paw-preference in mice

We studied the influence of vision on the expression of handedness in mice. In one experiment we submitted adult mice that had an opaque scleral contact lens fitted to one eye, to a paw-preference testing procedure. When the eye was occluded before training, the animals showed a clear preference for the paw ipsilateral to the open eye; however, we could not induce a shift in a previously determined, natural, paw-preference when the lens was placed over the eye ipsilateral to the spontaneously preferred paw; these results indicate that vision plays a role in the animal's choice of a paw during the learning phase of the paw-preference test. In a second experiment adult mice that had been subjected to unilateral eye removal at birth, underwent the same test. The enucleation did not appear to influence handedness with respect to both direction and strength. The latter result--we propose--reflects a reorganization of the visual system induced by neonatal enucleation.

Genetic analysis of cerebellar folial pattern in crosses of C57BL/6J and DBA/2J inbred mice

Variation in the cerebellar folial pattern of mice is influenced by genetic elements [Inouye, M. and Oda, S., J. Comp. Neurol., 190 (1980) 357-362]. In crosses of C57BL/6J and DBA/2J inbred mice, the presence or absence of a specific fissure, the intraculminate fissure, is largely determined by a single genetic locus (Cfp-1), which is located on distal Chromosome 4 [Neumann et al., Brain Res., 524 (1990) 85-89]. In the present study, the mid-sagittal cerebellar folial pattern has been examined in crosses of C57BL/6J and DBA/2J mice and in BXD recombinant inbred strains. At least three loci, including Cfp-1, are involved in variation in vermian pattern formation. Genetic variation in thyroid hormone function may be involved in the inheritance of folial pattern. A locus (Cfp-2) that appears to be partially responsible for this negative genetic correlation in mice may be linked to Afp on Chromosome 5. This hypothesis was suggested by the negative correlation between neonatal serum T4 level and the number of folia in rats given neonatal injections of thyroxine or propylthiouracil [Lauder, J.M. et al., Brain Res., 76 (1974) 33-40].

Direction of handedness linked to hereditary asymmetry of a sensory system

Studies on the role of heredity in the transmission of handedness in nonhuman mammals have, so far, led to the isolation of mouse strains that differed in the lateralized versus ambidextrous use of the forepaw in a food-retrieval task (strength of paw preference). Here we report that left versus right use of the forepaw (direction of paw preference) is associated with a genetically expressed structural asymmetry of a sensory system, the whisker-to-barrel pathway. Mice that express whisker pad asymmetry of a direction that corresponds with the asymmetry for which they were bred demonstrate an opposite shift in the distribution of handedness: a right or left dominance of the whisker pad predicts a high proportion of left-handers or right-handers, respectively. Is an altered brain circuit--that is, a consequence of the asymmetry of the whisker pad--associated with a change in the circuitry that governs handedness? Or, alternatively, are there two gene sets responsible for the phenomena that we report--one that causes "whiskeredness" and another that causes handedness?

A computer-controlled Y-maze for the analysis of vibrissotactile discrimination learning in mice

An automatized, computer-controlled Y-maze is described in which mice are trained to discriminate between two vibrissotactile, and/or visual stimuli, other modalities or combinations of modalities being testable as well. Movements of the mouse are recorded by photocells and monitored on a computer screen. Forward passage of the mouse is ensured by movable gates, and, if necessary, by brief air blows. Wrong choices are punished by air blows as well. The controller is a Hewlett-Packard Series 80 microcomputer with a 16 channel parallel input/output interface; programs are in BASIC. The program analyzes start latency, decision and homing time, side preference, inspections and ultimate choice, as well as choice strategies based on discrimination, left/right habits and short-term memory. Thus we can determine the nature of discrimination errors, and establish individual behavioral profiles of the animals. Results are both printed alphanumerically and plotted. The apparatus may be used for studying sensory physiology as well as cerebral lateralization, and drug or gene effects on memory and learning.

Vibrissa-related behavior in mice: transient effect of ablation of the barrel cortex

Knowing that the mystacial vibrissae are an important part of the tactile sensory apparatus of rodents, we investigated the role of the barrel cortex - the endstation of the pathway between whiskerpad and cerebral cortex - in mouse behavior. We tested 15 female adult mice 2 and 10 weeks after both unilateral ablation of the barrel cortex and removal of the vibrissae on the same side in order to assess acute as well as transient effects of the cortical lesion. Two kinds of behavioral tests were performed on animals permanently provided with opaque lenses: one involved a passive stimulation of the vibrissae; the other was the 'gap-crossing' test which required the animal's active use of the vibrissae. Lesioned subjects did not show a deficit during passive stimulation of the vibrissae. On the contrary, there was a deficit during the gap-crossing test 2 weeks after the ablation of the barrel cortex. The deficit partly disappeared when the subjects were tested 10 weeks later. The results show that in mice, the barrel cortex is involved in the performance of complex behavioral tasks. The recovery of function could be due to changes in strategies to solve the gap-crossing test and/or to physical changes in neuronal circuitry. In either case, the results are relevant for the interpretation of cortical transplantation models using the whisker-to-barrel pathway.

Organization of feedback and feedforward projections of the barrel cortex: a PHA-L study in the mouse

In order to analyze the organization of the efferent projections of single barrel columns (BC, i.e. a barrel in layer IV of parietal cortex plus the cortical tissue above and below it), we made small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin in the barrel cortex of 20 adult mice. On the basis of reconstructions of the sites of terminal labelling, the brain regions receiving projections from the barrel cortex could be identified and classified in five groups. Each group is characterized by the topography of the distribution of efferents arising from a single BC. The projections to the trigeminal sensory complex are point to point: i.e. one BC projects only to the site of termination of the primary sensory neurons innervating the corresponding whisker follicle. In the ventrobasal thalamic nucleus BC projections are not restricted to the corresponding barreloid; instead they contract parts of barreloids belonging to one arc. In the reticular and posterior thalamic nuclei the projections from a row of BC's converge to a collective termination site, whereas in the superior colliculus the projections from an arc of BC's converge to a common termination site. There is a complete overlap of BC projections in restricted zones within SII, motor cortex, perirhinal cortex, contralateral barrelfield, caudoputamen and pons. The organization of the efferents from the barrel cortex demonstrates a contrast between feedback and feedforward projections from this important area of neocortex.

Organization of the projections from barrel cortex to thalamus in mice studied with Phaseolus vulgaris-leucoagglutinin and HRP

In order to elucidate the geometric organization of projections from the barrel cortex to the thalamus, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were made. The injections were confined to one barrel column (i.e. barrel in layer IV + cortical tissue above and below it). Axonal terminations could be demonstrated in three thalamic nuclei: reticularis (RT), ventrobasalis (VB) and posterior (PO). Anterograde terminal labelling was obtained in RT + VB; in PO only; or in RT + VB + PO. The terminals labelled in PO were much larger than those in RT and VB. The termination areas in RT, VB and PO were shaped like rods which have a rostro-caudal orientation. These cortico-thalamic projections are discretely and topographically organized. The clearest such arrangement was found in VB. Here, projections from the A row of barrels in BF terminate dorsally, whereas those from the C row end ventrally. Barrel A1 projects to the lateral part of VB, whereas A4, to more medial parts; other rows are arranged similarly. These results were compared with the distribution of thalamo-cortical projection neurons that were labelled after iontophoretic HRP injections in individual barrels. We concluded that the corticothalamic projections originating from one barrel column contact an are of barreloids in VB.

Is areal extent in sensory cerebral cortex determined by peripheral innervation density?

The whisker-to-barrel pathway of mice (an important component of the animal's somatosensory system) was studied in two experiments. In one, the cortical representation of a row of whiskers was caused to be larger by lesioning a neighbouring row of follicles, while the innervation density remained unchanged. In the second experiment mice, selectively bred for particular whisker and barrel patterns, showed for their supernumerary vibrissal follicles a relatively large cortical representation. On the basis of the second experiment we formulate a possible role of the sensory periphery in brain evolution.