Mouse somatosensory cortex: alterations in the barrelfield following receptor injury at different early postnatal ages

Type-I-interferon-responsive microglia shape cortical development and behavior

Microglia are brain-resident macrophages that shape neural circuit development and are implicated in neurodevelopmental diseases. Multiple microglial transcriptional states have been defined, but their functional significance is unclear. Here, we identify a type I interferon (IFN-I)-responsive microglial state in the developing somatosensory cortex (postnatal day 5) that is actively engulfing whole neurons. This population expands during cortical remodeling induced by partial whisker deprivation. Global or microglial-specific loss of the IFN-I receptor resulted in microglia with phagolysosomal dysfunction and an accumulation of neurons with nuclear DNA damage. IFN-I gain of function increased neuronal engulfment by microglia in both mouse and zebrafish and restricted the accumulation of DNA-damaged neurons. Finally, IFN-I deficiency resulted in excess cortical excitatory neurons and tactile hypersensitivity. These data define a role for neuron-engulfing microglia during a critical window of brain development and reveal homeostatic functions of a canonical antiviral signaling pathway in the brain.

Olfactory Critical Periods: How Odor Exposure Shapes the Developing Brain in Mice and Flies

Neural networks have an extensive ability to change in response to environmental stimuli. This flexibility peaks during restricted windows of time early in life called critical periods. The ubiquitous occurrence of this form of plasticity across sensory modalities and phyla speaks to the importance of critical periods for proper neural development and function. Extensive investigation into visual critical periods has advanced our knowledge of the molecular events and key processes that underlie the impact of early-life experience on neuronal plasticity. However, despite the importance of olfaction for the overall survival of an organism, the cellular and molecular basis of olfactory critical periods have not garnered extensive study compared to visual critical periods. Recent work providing a comprehensive mapping of the highly organized olfactory neuropil and its development has in turn attracted a growing interest in how these circuits undergo plasticity during critical periods. Here, we perform a comparative review of olfactory critical periods in fruit flies and mice to provide novel insight into the importance of early odor exposure in shaping neural circuits and highlighting mechanisms found across sensory modalities.

Type I interferon responsive microglia shape cortical development and behavior

Microglia are brain resident phagocytes that can engulf synaptic components and extracellular matrix as well as whole neurons. However, whether there are unique molecular mechanisms that regulate these distinct phagocytic states is unknown. Here we define a molecularly distinct microglial subset whose function is to engulf neurons in the developing brain. We transcriptomically identified a cluster of Type I interferon (IFN-I) responsive microglia that expanded 20-fold in the postnatal day 5 somatosensory cortex after partial whisker deprivation, a stressor that accelerates neural circuit remodeling. In situ, IFN-I responsive microglia were highly phagocytic and actively engulfed whole neurons. Conditional deletion of IFN-I signaling (Ifnar1fl/fl) in microglia but not neurons resulted in dysmorphic microglia with stalled phagocytosis and an accumulation of neurons with double strand DNA breaks, a marker of cell stress. Conversely, exogenous IFN-I was sufficient to drive neuronal engulfment by microglia and restrict the accumulation of damaged neurons. IFN-I deficient mice had excess excitatory neurons in the developing somatosensory cortex as well as tactile hypersensitivity to whisker stimulation. These data define a molecular mechanism through which microglia engulf neurons during a critical window of brain development. More broadly, they reveal key homeostatic roles of a canonical antiviral signaling pathway in brain development.

Alcohol and lactation: Developmental deficits in a mouse model

It is well documented that prenatal ethanol exposure via maternal consumption of alcohol during pregnancy alters brain and behavioral development in offspring. Thus, the Centers for Disease Control (CDC) advises against maternal alcohol consumption during pregnancy. However, little emphasis has been placed on educating new parents about alcohol consumption while breastfeeding. This is partly due to a paucity of research on lactational ethanol exposure (LEE) effects in children; although, it has been shown that infants exposed to ethanol via breast milk frequently present with reduced body mass, low verbal IQ scores, and altered sleeping patterns. As approximately 36% of breastfeeding mothers in the US consume alcohol, continued research in this area is critical. Our study employed a novel murine LEE model, where offspring were exposed to ethanol via nursing from postnatal day (P) 6 through P20, a period correlated with infancy in humans. Compared to controls, LEE mice had reduced body weights and neocortical lengths at P20 and P30. Brain weights were also reduced in both ages in males, and at P20 for females, however, female brain weights recovered to control levels by P30. We investigated neocortical features and found that frontal cortex thickness was reduced in LEE males compared to controls. Analyses of dendritic spines in the prelimbic subdivision of medial prefrontal cortex revealed a trend of reduced densities in LEE mice. Results of behavioral tests suggest that LEE mice engage in higher risk-taking behavior, show abnormal stress regulation, and exhibit increased hyperactivity. In summary, our data describe potential adverse brain and behavioral developmental outcomes due to LEE. Thus, women should be advised to refrain from consuming alcohol during breastfeeding until additional research can better guide recommendations of safe maternal practices in early infancy.

Fingolimod increases parvalbumin-positive neurons in adult mice

In recent years, it has been shown that central nervous system agents, such as antidepressants and antiepileptic drugs, reopen a critical period in mature animals. Fingolimod, which is used for the treatment of multiple sclerosis, also restores neuroplasticity. In this study, we investigated the effects of parvalbumin (PV)-positive neurons and perineuronal nets (PNN) on fingolimod administration with respect to neuroplasticity. Fingolimod was chronically administered intraperitoneally to mature mice. PV-positive neurons and PNN in the hippocampus, prefrontal cortex, and somatosensory cortex were analyzed. An increase in PV-positive neurons was observed in the hippocampus, prefrontal cortex, and somatosensory cortex of the fingolimod-treated mice. An increase in Wisteria floribunda agglutinin-positive PNN was confirmed in mice treated with fingolimod in the somatosensory cortex only. Fingolimod increased the density of PV-positive neurons in the brains of mature mice. The results indicate that fingolimod may change the critical period in mature animals.

Spatial organization and transitions of spontaneous neuronal activities in the developing sensory cortex

The sensory cortex underlies our ability to perceive and interact with the external world. Sensory perceptions are controlled by specialized neuronal circuits established through fine-tuning, which relies largely on neuronal activity during the development. Spontaneous neuronal activity is an essential driving force of neuronal circuit refinement. At early developmental stages, sensory cortices display spontaneous activities originating from the periphery and characterized by correlated firing arranged spatially according to the modality. The firing patterns are reorganized over time and become sparse, which is typical for the mature brain. This review focuses mainly on rodent sensory cortices. First, the features of the spontaneous activities during early postnatal stages are described. Then, the developmental changes in the spatial organization of the spontaneous activities and the transition mechanisms involved are discussed. The identification of the principles controlling the spatial organization of spontaneous activities in the developing sensory cortex is essential to understand the self-organization process of neuronal circuits.

Genetic and activity dependent-mechanisms wiring the cortex: Two sides of the same coin

The cerebral cortex is responsible for the higher-order functions of the brain such as planning, cognition, or social behaviour. It provides us with the capacity to interact with and transform our world. The substrates of cortical functions are complex neural circuits that arise during development from the dynamic remodelling and progressive specialization of immature undefined networks. Here, we review the genetic and activity-dependent mechanisms of cortical wiring focussing on the importance of their interaction. Cortical circuits emerge from an initial set of neuronal types that engage in sequential forms of embryonic and postnatal activity. Such activities further complement the cells' genetic programs, increasing neuronal diversity and modifying the electrical properties while promoting selective connectivity. After a temporal window of enhanced plasticity, the main features of mature circuits are established. Failures in these processes can lead to neurodevelopmental disorders whose treatment remains elusive. However, a deeper dissection of cortical wiring will pave the way for innovative therapies.

The follicle-sinus complex of the bottlenose dolphin (Tursiops truncatus). Functional anatomy and possible evolutional significance of its somato-sensory innervation

Vibrissae are tactile hairs found mainly on the rostrum of most mammals. The follicle, which is surrounded by a large venous sinus, is called "follicle-sinus complex" (FSC). This complex is highly innervated by somatosensitive fibers and reached by visceromotor fibers that innervate the surrounding vessels. The surrounding striated muscles receive somatomotor fibers from the facial nerve. The bottlenose dolphin (Tursiops truncatus), a frequently described member of the delphinid family, possesses this organ only in the postnatal period. However, information on the function of the vibrissal complex in this latter species is scarce. Recently, psychophysical experiments on the river-living Guiana dolphin (Sotalia guianensis) revealed that the FSC could work as an electroreceptor in murky waters. In the present study, we analyzed the morphology and innervation of the FSC of newborn (n = 8) and adult (n = 3) bottlenose dolphins. We used Masson's trichrome stain and antibodies against neurofilament 200 kDa (NF 200), protein gene product (PGP 9.5), substance P (SP), calcitonin gene-related peptide, and tyrosine hydroxylase (TH) to characterize the FSC of the two age classes. Masson's trichrome staining revealed a structure almost identical to that of terrestrial mammals except for the fact that the FSC was occupied only by a venous sinus and that the vibrissal shaft lied within the follicle. Immunostaining for PGP 9.5 and NF 200 showed somatosensory fibers finishing high along the follicle with Merkel nerve endings and free nerve endings. We also found SP-positive fibers mostly in the surrounding blood vessels and TH both in the vessels and in the mesenchymal sheath. The FSC of the bottlenose dolphin, therefore, possesses a rich somatomotor innervation and a set of peptidergic visceromotor fibers. This anatomical disposition suggests a mechanoreceptor function in the newborns, possibly finalized to search for the opening of the mother's nipples. In the adult, however, this structure could change into a proprioceptive function in which the vibrissal shaft could provide information on the degree of rotation of the head. In the absence of psychophysical experiments in this species, the hypothesis of electroreception cannot be rejected.

Modelling the emergence of whisker barrels

Brain development relies on an interplay between genetic specification and self-organization. Striking examples of this relationship can be found in the somatosensory brainstem, thalamus, and cortex of rats and mice, where the arrangement of the facial whiskers is preserved in the arrangement of cell aggregates to form precise somatotopic maps. We show in simulation how realistic whisker maps can self-organize, by assuming that information is exchanged between adjacent cells only, under the guidance of gene expression gradients. The resulting model provides a simple account of how patterns of gene expression can constrain spontaneous pattern formation to faithfully reproduce functional maps in subsequent brain structures.

Defining a critical period for inhibitory circuits within the somatosensory cortex

Although experience-dependent changes in brain inhibitory circuits are thought to play a key role during the “critical period” of brain development, the nature and timing of these changes are poorly understood. We examined the role of sensory experience in sculpting an inhibitory circuit in the primary somatosensory cortex (S1) of mice by using optogenetics to map the connections between parvalbumin (PV) expressing interneurons and layer 2/3 pyramidal cells. Unilateral whisker deprivation decreased the strength and spatial range of inhibitory input provided to pyramidal neurons by PV interneurons in layers 2/3, 4 and 5. By varying the time when sensory input was removed, we determined that the critical period closes around postnatal day 14. This yields the first precise time course of critical period plasticity for an inhibitory circuit.

Large Scale Cortical Functional Networks Associated with Slow-Wave and Spindle-Burst-Related Spontaneous Activity

Cortical sensory systems are active with rich patterns of activity during sleep and under light anesthesia. Remarkably, this activity shares many characteristics with those present when the awake brain responds to sensory stimuli. We review two specific forms of such activity: slow-wave activity (SWA) in the adult brain and spindle bursts in developing brain. SWA is composed of 0.5-4 Hz resting potential fluctuations. Although these fluctuations synchronize wide regions of cortex, recent large-scale imaging has shown spatial details of their distribution that reflect underlying cortical structural projections and networks. These networks are regulated, as prior awake experiences alter both the spatial and temporal features of SWA in subsequent sleep. Activity patterns of the immature brain, however, are very different from those of the adult. SWA is absent, and the dominant pattern is spindle bursts, intermittent high frequency oscillations superimposed on slower depolarizations within sensory cortices. These bursts are driven by intrinsic brain activity, which act to generate peripheral inputs, for example via limb twitches. They are present within developing sensory cortex before they are mature enough to exhibit directed movements and respond to external stimuli. Like in the adult, these patterns resemble those evoked by sensory stimulation when awake. It is suggested that spindle-burst activity is generated purposefully by the developing nervous system as a proxy for true external stimuli. While the sleep-related functions of both slow-wave and spindle-burst activity may not be entirely clear, they reflect robust regulated phenomena which can engage select wide-spread cortical circuits. These circuits are similar to those activated during sensory processing and volitional events. We highlight these two patterns of brain activity because both are prominent and well-studied forms of spontaneous activity that will yield valuable insights into brain function in the coming years.

Reduced Cortical Activity Impairs Development and Plasticity after Neonatal Hypoxia Ischemia

Survivors of preterm birth are at high risk of pervasive cognitive and learning impairments, suggesting disrupted early brain development. The limits of viability for preterm birth encompass the third trimester of pregnancy, a "precritical period" of activity-dependent development characterized by the onset of spontaneous and evoked patterned electrical activity that drives neuronal maturation and formation of cortical circuits. Reduced background activity on electroencephalogram (EEG) is a sensitive marker of brain injury in human preterm infants that predicts poor neurodevelopmental outcome. We studied a rodent model of very early hypoxic-ischemic brain injury to investigate effects of injury on both general background and specific patterns of cortical activity measured with EEG. EEG background activity is depressed transiently after moderate hypoxia-ischemia with associated loss of spindle bursts. Depressed activity, in turn, is associated with delayed expression of glutamate receptor subunits and transporters. Cortical pyramidal neurons show reduced dendrite development and spine formation. Complementing previous observations in this model of impaired visual cortical plasticity, we find reduced somatosensory whisker barrel plasticity. Finally, EEG recordings from human premature newborns with brain injury demonstrate similar depressed background activity and loss of bursts in the spindle frequency band. Together, these findings suggest that abnormal development after early brain injury may result in part from disruption of specific forms of brain activity necessary for activity-dependent circuit development.

SIGNIFICANCE STATEMENT

Preterm birth and term birth asphyxia result in brain injury from inadequate oxygen delivery and constitute a major and growing worldwide health problem. Poor outcomes are noted in a majority of very premature (<25 weeks gestation) newborns, resulting in death or life-long morbidity with motor, sensory, learning, behavioral, and language disabilities that limit academic achievement and well-being. Limited progress has been made to develop therapies that improve neurologic outcomes. The overall objective of this study is to understand the effect of early brain injury on activity-dependent brain development and cortical plasticity to develop new treatments that will optimize repair and recovery after brain injury.

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.

Fibroblast growth factor 8 organizes the neocortical area map and regulates sensory map topography

The concept of an "organizer" is basic to embryology. An organizer is a portion of the embryo producing signals that lead to the creation of a patterned mature structure from an embryonic primordium. Fibroblast growth factor 8 (FGF8) is a morphogen that disperses from a rostromedial source in the neocortical primordium (NP), forms a rostral-to-caudal (R/C) gradient, and regulates embryonic and neonatal R/C patterns of gene expression in neocortex. Whether FGF8 also has organizer activity that generates the postnatal neocortical area map is uncertain. To test this possibility, new sources of FGF8 were introduced into the mouse NP with in utero microelectroporation at embryonic day 10.5, close to the estimated peak of area patterning. Results differed depending on the position of ectopic FGF8. Ectopic FGF8 in the caudalmost NP could duplicate somatosensory cortex (S1) and primary visual cortex (V1). FGF8 delivered to the midlateral NP generated a sulcus separating rostral and caudal portions of the NP, in effect creating duplicate NPs. In the caudal NP, ectopic FGF8 induced a second, inclusive area map, containing frontal cortex, S1, V1, and primary auditory areas. Moreover, duplicate S1 showed plasticity to sensory deprivation, and duplicate V1 responded to visual stimuli. Our findings implicate FGF8 as an organizer signal, and its source in the rostromedial telencephalon as an organizer of the neocortical area map.

Effects of brief stress exposure during early postnatal development in Balb/CByJ mice: II. Altered cortical morphology

Early life experience can significantly determine later mental health status and cognitive function. Neonatal stress, in particular, has been linked to the etiology of mental health disorders as divergent as mood disorder, schizophrenia, and autism. Our study uses a Balb/CByJ mouse model to test the hypothesis, that neonatal stress will alter development and subsequent environmental modulation of neocortex. Using a split litter design, we generated stressed mice (STR) and within litter controls (LMC) along with age-matched, untreated animals (AMC), to serve as across litter controls. Short, daily exposure to a psychosocial/physical stressor, during the first week of life, resulted by adulthood in significant changes in neocortical thickness and architecture, which were further modulated by exposure to behavioral testing. Surprisingly, cortical size in LMC mice was also affected. These observations were compared to the effects of environmental enrichment in the same mouse strain. Our data indicate that LMC and STR males share with environmentally enriched males, an increase in thickness in infra-granular cortical layers, while STR also display a stress selective decrease in supragranular layers, in response to behavioral training as adults.

The emergence of somatotopic maps of the body in S1 in rats: the correspondence between functional and anatomical organization

Most of what we know about cortical map development and plasticity comes from studies in mice and rats, and for the somatosensory cortex, almost exclusively from the whisker-dominated posteromedial barrel fields. Whiskers are the main effector organs of mice and rats, and their representation in cortex and subcortical pathways is a highly derived feature of murine rodents. This specialized anatomical organization may therefore not be representative of somatosensory cortex in general, especially for species that utilize other body parts as their main effector organs, like the hands of primates. For these reasons, we examined the emergence of whole body maps in developing rats using electrophysiological recording techniques. In P5, P10, P15, P20 and adult rats, multiple recordings were made in the medial portion of S1 in each animal. Subsequently, these functional maps were related to anatomical parcellations of S1 based on a variety of histological stains. We found that at early postnatal ages (P5) medial S1 was composed almost exclusively of the representation of the vibrissae. At P10, other body part representations including the hindlimb and forelimb were present, although these were not topographically organized. By P15, a clear topographic organization began to emerge coincident with a reduction in receptive field size. By P20, body maps were adult-like. This study is the first to describe how topography of the body develops in S1 in any mammal. It indicates that anatomical parcellations and functional maps are initially incongruent but become tightly coupled by P15. Finally, because anatomical and functional specificity of developing barrel cortex appears much earlier in postnatal life than the rest of the body, the entire primary somatosensory cortex should be considered when studying general topographic map formation in development.

Lesion-induced synaptic plasticity in the somatosensory cortex of prenatally stressed rats

Prenatal stress exposure causes long-lasting impairments of the behavioral and neuroendocrine responses to later stressors of the offspring. Although mechanisms underlying these effects remain largely unknown, abnormalities in the neuronal plasticity might be responsible for neurobiological alterations. This study used the whisker-to-barrel pathway as a model system to investigate the effects of prenatal stress on lesion-induced plasticity of neurons. Pregnant rats were subjected to immobilization stress during the trigeminal neurogenesis period, corresponding to gestational days 12 to 17, for three hours a day. After birth, the middle row (C) whisker follicles of pups from the control and stressed groups were electrocauterized. Ten days later, tangentially sectioned cortical hemispheres were stained with cytochrome oxidase histochemistry to calculate the volumes of each barrel row (A-E) in both lesioned and intact sides of the cortex, using stereological methods. The adrenal to body weight ratios were significantly increased in stressed animals, when compared to the controls. The pattern and total volume of the barrel subfield remained unaltered, but the lesion-induced map plasticity index, calculated as the D/C ratio, decreased in stressed animals. In addition, the BDNF (Brain Derived Neurotrophic Factor), NT-3 (neurotrophin-3) and the cyclic AMP response element binding protein (CREB) phosphorylation levels in tissue homogenates of the barrel cortices were measured using the ELISA method. In prenatally stressed animals, the BDNF and NT-3 levels were reduced on the lesioned side, but significant CREB activation was observed on the intact side of the barrel cortex. Taken together, the results show that prenatal stress exposure negatively affects critical period plasticity by reducing the expansion of active barrels following peripheral whisker lesion. These changes arise independent of CREB phosphorylation and appear to be mediated by reduced levels of neurotrophins.

Early development of the thalamic inhibitory feedback loop in the primary somatosensory system of the newborn mice

Spontaneous neuronal activity plays an important role during the final development of the brain circuits and the formation of the primary sensory maps. In young rats, spindle bursts have been recorded in the primary somatosensory cortex. They are correlated with spontaneous muscle twitches and occur before active whisking. They bear similarities with the spindles recorded in adult brain that occur during early stages of sleep and rely on a thalamic feedback loop between the glutamatergic nucleus ventroposterior medialis (nVPM) and the GABAergic nucleus reticularis thalami (nRT). However, whether a functional nVPM-nRT loop exists in newborn rodents is unknown. We studied the reciprocal synaptic connections between nVPM and nRT in thalamic acute slices from mice from birth [postnatal day 0 (P0)] until P9. We first demonstrated that nVPM-to-nRT EPSCs could be distinguished from corticothalamic EPSCs by their inhibition by 5-HT attributable to the transient expression of functional presynaptic serotonin 1B receptors. The nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs were both detected the first day after birth; their amplitude near 2 nS was relatively stable until P5. At P6-P7, there was a rapid and simultaneous increase of both nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs that reached 8 and 9 nS, respectively. Our results show that the thalamic synapses implicated in spindle activity are functional shortly after birth, suggesting that they could already generate spindles during the first postnatal week. Our results also suggest an inhibitory action of 5-HT on the spindle bursts of the newborn mice.

Glutamate transporters regulate lesion-induced plasticity in the developing somatosensory cortex

Glutamate transporters are involved in neural differentiation, neuronal survival, and synaptic transmission. In the present study, we examined glutamate transporter 1 (GLT1) expression in the neonatal somatosensory cortex of C57BL/6 mice, and pursued its role in somatosensory development by comparing barrel development between GLT1 knock-out and control mice. During the first few neonatal days, a critical period for barrels, GLT1 expression is strikingly upregulated in cortical astrocytes, whereas it was downregulated in neuronal elements to below the detection threshold. GLT1 knock-out neonates developed normally in terms of body growth, cortical histoarchitecture, barrel formation, and critical period termination. However, when row C whiskers were lesioned during the critical period, reduction of lesioned row C barrels and reciprocal expansion of intact row B/D barrels were both milder in GLT1 knock-out mice than in control littermates. Accordingly, the map plasticity index, calculated as (B + D)/2C, was significantly lowered in GLT1 knock-out mice. We also found that extracellular glutamate levels in the neonatal somatosensory cortex were significantly elevated in GLT1 knock-out mice. Diminished lesion-induced plasticity was further found in mutant mice lacking glutamate-aspartate transporter (GLAST), an astrocyte-specific glutamate transporter throughout development. Therefore, glutamate transporters regulate critical period plasticity by enhancing expansion of active barrels and shrinkage of inactive barrels. Because cortical contents of glutamate receptors and GLAST were unaltered in GLT1 knock-out mice, this action appears to be mediated, at least partly, by keeping the ambient glutamate level low. Considering an essential role of glutamate receptors in the formation of whisker-related thalamocortical synapse patterning, glutamate transporters thus facilitate their activity-dependent remodeling.

Mapping the matrix: the ways of neocortex

While we know that the neocortex occupies 85% of our brains and that its circuits allow an enormous flexibility and repertoire of behavior (not to mention unexplained phenomena like consciousness), a century after Cajal we have very little knowledge of the details of the cortical circuits or their mode of function. One simplifying hypothesis that has existed since Cajal is that the neocortex consists of repeated copies of the same fundamental circuit. However, finding that fundamental circuit has proved elusive, although partial drafts of a "canonical circuit" appear in many different guises of structure and function. Here, we review some critical stages in the history of this quest. In doing so, we consider the style of cortical computation in relation to the neuronal machinery that supports it. We conclude that the structure and function of cortex honors two major computational principles: "just-enough" and "just-in-time."

Defects in embryonic neurogenesis and initial synapse formation in the forebrain of the Ts65Dn mouse model of Down syndrome

Trisomy 21, one of the most prevalent congenital birth defects, results in a constellation of phenotypes collectively termed Down syndrome (DS). Mental retardation and motor and sensory deficits are among the many debilitating symptoms of DS. Alterations in brain growth and synaptic development are thought to underlie the cognitive impairments in DS, but the role of early brain development has not been studied because of the lack of embryonic human tissue and because of breeding difficulties in mouse models of DS. We generated a breeding colony of the Ts65Dn mouse model of DS to test the hypothesis that early defects in embryonic brain development are a component of brain dysfunction in DS. We found substantial delays in prenatal growth of the Ts65Dn cerebral cortex and hippocampus because of longer cell cycle duration and reduced neurogenesis from the ventricular zone neural precursor population. In addition, the Ts65Dn neocortex remains hypocellular after birth and there is a lasting decrease in synaptic development beginning in the first postnatal week. These results demonstrate that specific abnormalities in embryonic forebrain precursor cells precede early deficits in synaptogenesis and may underlie the postnatal disabilities in Ts65Dn and DS. The early prenatal period is therefore an important new window for possible therapeutic amelioration of the cognitive symptoms in DS.

Functional barrel cortex in 'hairless', nude mice

Although nude mice are not truly hairless, they demonstrate abnormal hair structure and growth patterns, which are related to their genetic state. Whereas wild-type mice are born with visible vibrissae, nude mice are distinguishable at birth by the lack of visible vibrissae, which do not appear until approximately postnatal day 6. Additionally, adult nude mice have abnormal whisker cycling patterns in which structurally normal whisker follicles produce fragile whiskers which break or fallout leaving follicles whiskerless for several days before a fine replacement whisker appears and develops. The current study shows that despite these abnormal periods of whisker deprivation, the barrel cortex of nude mice develops a normal structural appearance viewed with cytochrome oxidase staining. Additionally, intrinsic optical imaging studies of barrel cortex responses to single whisker stimulation do not appear altered from normal despite periodic loss of adjacent whiskers.

Development of structural and functional connectivity in the thalamocortical somatosensory pathway in the wallaby

Neuronal activity is implicated as a driving force in the development of sensory systems. In order for it to play a developmental role, however, the pathways involved must be capable of transmitting this activity. The relationship between afferent arrival, synapse formation and the onset of chemical neurotransmission has been examined using the advantageous model of a marsupial mammal, the wallaby (Macropus eugenii), to determine at what stage activity has the capacity to influence cortical development. It is known that thalamocortical afferents arrive in the somatosensory cortex on postnatal day (P)15 and that their growth cones reach to the base of the compact cell zone of the cortical plate. However, electronmicroscopy showed that thalamocortical synapses were absent at this stage. Glutamatergic responses were recorded in the cortex following stimulation of the thalamus in slices at this time but only in magnesium-free conditions. The responses were mediated entirely by N-methyl-d-aspartate (NMDA) receptors. From P28, responses could be recorded in normal magnesium and comprised a dominant NMDA-mediated component and a non-NMDA mediated component. At this time thalamocortical synapses were first identified and they were in the cortical plate. By P63 the non-NMDA-mediated component had increased relative to the NMDA-mediated component, and by P70 layer IV began to emerge and contained thalamocortical synapses. By P76 a fast non-NMDA-mediated peak dominated the response. This coincides with the appearance of cortical whisker-related patches and the onset in vivo of responses to peripheral stimulation of the whiskers.

Optic nerve transection affects development and use-dependent plasticity in neocortex of the rat: Quantitative acetylcholinesterase imaging

We investigated the effects of neonatal optic nerve transection on cortical acetylcholinesterase (AChE) activity in hooded rats during postnatal development and following behavioral manipulation after weaning. AChE reaction product was quantified on digitized images of histochemically stained sections in layer IV of primary somatic sensory, primary visual and visual association cortex. Rats with optic nerve transection were compared to sham-operated littermates. In all cortical regions of both types of animal, AChE reaction product was increased to peak 2 weeks after birth and decreased thereafter, reaching adult levels at the end of the third postnatal week. During postnatal development, reaction product in primary visual cortex was lower in rats deprived of retinal input than in sham-operated littermates and the area delineated by reaction product was smaller. However, optic nerve transection did not modify the time course of postnatal development or statistically significantly diminish adult levels of AChE activity. Behavioral manipulations after weaning statistically significantly increased enzyme activity in sham-operated rats in all cortical areas examined. Compared with cage rearing, training in a discrimination task with food reward had a greater impact than environmental enrichment. By contrast, in the rats with optic nerve transection enrichment and training resulted in statistically significantly increased AChE activity only in lateral visual association cortex. Our findings provide evidence for intra- and supramodal influences of the neonatal removal of retinal input on neural activity- and use-dependent modifications of cortical AChE activity. The laminar distribution of the AChE reaction product suggests that the observed changes in AChE activity were mainly related to cholinergic basal forebrain afferents. These afferents may facilitate the stabilization of transient connections between the somatic sensory and the visual pathway.

Fibroblast growth factor 8 regulates neocortical guidance of area-specific thalamic innervation

Thalamic innervation of each neocortical area is vital to cortical function, but the developmental strategies that guide axons to specific areas remain unclear. We took a new approach to determine the contribution of intracortical cues. The cortical patterning molecule fibroblast growth factor 8 (FGF8) was misexpressed in the cortical primordium to rearrange the area map. Thalamic axons faithfully tracked changes in area position and innervated duplicated somatosensory barrel fields induced by an ectopic source of FGF8, indicating that thalamic axons indeed use intracortical positional information. Because cortical layers are generated in temporal order, FGF8 misexpression at different ages could be used to shift regional identity in the subplate and cortical plate either in or out of register. Thalamic axons showed strikingly different responses in the two different conditions, disclosing sources of positional guidance in both subplate and cortical plate. Unexpectedly, axon trajectories indicated that an individual neocortical layer could provide not only laminar but also area-specific guidance. Our findings demonstrate that thalamocortical axons are directed by sequential, positional cues within the cortex and implicate FGF8 as an indirect regulator of thalamocortical innervation.

Dissociating barrel development and lesion-induced plasticity in the mouse somatosensory cortex

In the mouse somatosensory cortex, thalamocortical axons (TCAs) corresponding to individual whiskers cluster into restricted barrel domains during the first days of life. If whiskers are lesioned before that time, the cortical space devoted to the afferents from the damaged whisker shrinks and becomes occupied by thalamocortical afferents from neighboring unlesioned whiskers. This plasticity ends by postnatal day 3 (P3) to P4 when barrels emerge. To test whether TCA development and lesion-induced plasticity are linked, we used monoamine oxidase A knock-out (MAOA-KO) mice in which normal TCA development is halted by an excess of serotonin. Normal TCA development can be restored when serotonin levels are lowered by parachlorophenylalanine (PCPA). By varying the time of PCPA administration, we found that barrel development can be reinitiated until P11, although the emergence of TCA clusters becomes gradually slower and less complete. In mice in which barrels emerge 3 d later than the normal schedule, at P6 instead of P3, we examined lesion-induced plasticity. We find a progressive decline of the lesion-induced plasticity and a closure at P3, similar to normal mice, showing that this plasticity is not influenced by an excess of serotonin levels. Thus, in MAOA-KO mice, the emergence of barrel patterning can be delayed without a concomitant delay in lesion-induced plasticity, and the cortical space devoted to one whisker representation cannot be modified by the periphery once patterning is imprinted in the subcortical relays. We conclude that the closure of the lesion-induced plasticity period in the barrelfield is probably not determined at the cortical level.

Prenatal alcohol exposure alters the size, but not the pattern, of the whisker representation in neonatal rat barrel cortex

Maternal alcohol exposure results in a variety of neurodevelopmental abnormalities that include cognitive and sensorimotor dysfunctions that often persist into adulthood. Many reports of central nervous system disturbances associated within a clinical diagnosis of fetal alcohol syndrome point toward disturbances in central information processing. In this study, we used the rat barrel field cortex as a model system to examine the effects of prenatal alcohol exposure (PAE) on the organization and size of the large whisker representation in layer IV of the posteromedial barrel subfield (PMBSF) in somatosensory cortex. Pregnant rats (Sprague-Dawley) were intragastrically gavaged daily with alcohol doses (6 gm/kg body weight) from gestational day 1 to day 20 in a chronic binge pattern which produced blood alcohol levels ranging between 260 mg/dl and 324 mg/dl. Chow-fed (CF), pair-fed (PF), and cross-foster (XF) groups served as normal, nutritionally matched, and maternal controls, respectively, for the ethanol-exposed (EtOH) treatment group. All pups were examined on gestational day 32 corresponding approximately to postnatal day 9. EtOH and control group pups were weighed, anesthetized, and perfused. Brains were removed and weighed, with and without cerebellum and olfactory bulbs, and the neocortex was removed and weighed. Cortices were then flattened, sectioned tangentially, and stained with a metabolic marker-cytochrome oxidase-to reveal the barrel field. A subset of 27 cortical barrels, associated with the representation of the large whisker pad, was selected to examine in detail. The major results were: (i) the total barrel field area comprising the PMBSF was significantly reduced in EtOH (by 17%) and XF (by 16%) pups compared with CF pups, (ii) the sizes of individual barrels within the PMBSF were also significantly reduced in EtOH (16%) and XF (18%) pups, (iii) the septal region between barrels was also significantly reduced in EtOH (18%) and XF (12%) pups, (iv) anteriorly located barrels underwent greater reduction in size relative to the posteriorly located barrels, (v) body weights were also significantly reduced in EtOH (21%) and XF (27%) pups, (vi) total brain weight [with and without (forebrain) cerebellum/olfactory bulbs] and cortical weights were also significantly reduced in EtOH (total brain weight 15%, forebrain weight 16%, cortical weight 15%) and XF (18%, 19%, 20%) pups, and in contrast (vi) neither the overall barrel field pattern nor the pattern of individual barrels in the PMBSF was altered. These findings suggest that PAE reduces body and brain weight as well as the central cortical representation of the whisker pad, while leaving the overall barrel field pattern unperturbed. While these results might appear to support a miniaturization hypothesis (smaller PMBSF, smaller brain, smaller body weight), PAE also shows regional vulnerability within the PMBSF whereby anteriorly located barrels are most affected.

Critical period for activity-dependent elimination of corticospinal synapses in vitro

There is no in vitro model of the critical periods for developmental plasticity, the time windows of plastic changes during development, which may hinder in-depth mechanistic analysis. We have shown previously that the corticospinal tract with synaptic connections can be reconstructed in in vitro co-cultures using slices of the sensorimotor cortex and spinal cord of the rat. In our in vitro system, corticospinal synapses form widely over spinal gray matter during early development, after which those on the ventral side are eliminated in an activity and N-methyl-D-aspartate (NMDA)-dependent manner. A detailed quantitative analysis of the time course of sensitivity to an NMDA blocker was made with this system. Synapse distribution was evaluated by recording field excitatory post-synaptic potentials evoked by deep cortical layer stimulation. Corticospinal axon terminal distribution was examined by anterograde labeling with biocytin. We showed that the D-2-amino-5-phosphonovaleric acid (APV) effect is irreversible for at least the length of culture. When APV was removed from the medium before 6 days in vitro(DIV) or after 11 DIV, elimination of ventral synapses was not blocked. APV sensitivity showed a clearly defined time window. A 6-11 DIV application was necessary and sufficient for the full, irreversible block of synapse elimination. From 6-11 DIV, APV sensitivity seems to decrease gradually but not linearly. This system provides an in vitro model of critical periods for developmental plasticity of central synapses which up to now has not been available.

The size of the whisker barrel field in adult rats: minimal nondirectional asymmetry and limited modifiability by chronic changes of the sensory input

We have evaluated quantitatively the whisker barrel field (posteromedial barrel subfield, PMBSF) size in rats raised in standard cages and in rats chronically exposed to an enriched sensory environment. Some animals were subjected to either chronic trimming of the right whiskers, or permanent transection of the right infraorbital nerve. Coronal brain sections were Nissl-stained or reacted for cytochrome oxidase. All, except the IoN-transected rats, showed +/-5% variation in mean PMBSF thickness, with no consistent side bias. In the transected animals, however, the left PMBSF was a significant 3.1% shallower than the right. This denervation-dependent radial shrinkage was consistent with an 11% volume shrinkage of the deafferented PMBSF. The mean volume of the PMBSF ranged between 8.7 and 9.5 mm(3), with moderate interindividual variability (3.5% to 11%). No significant differences in PMBSF volume were found between groups in the right hemisphere, nor in the right vs. left ratios. However, the PMBSF volume was a significant 6.6% larger in the enriched animals without whisker trimming. The PMBSF volume correlated positively with neocortical volume, and with PMBSF cortex thickness, in rats exposed to enriched environment. These data show that: (1) there is a moderate interanimal and lateral variability in the PMBSF volume, with no side preference; (2) exposing young adult rats to an enriched environment induces a discrete but significant enlargement of the PMBSF; (3) the effects of whisker trimming on the contralateral PMBSF, if any, are lost in the interanimal and lateral variability; and (4) such changes reach significance, however, when studied in combination with exposure to an enriched environment.

Naturalistic experience transforms sensory maps in the adult cortex of caged animals

Much of what is known about the functional organization and plasticity of adult sensory cortex is derived from animals housed in standard laboratory cages. Here we report that the transfer of adult rats reared in standard laboratory cages to a naturalistic habitat modifies the functional and morphological organization of the facial whisker representation in the somatosensory 'barrel' cortex. Cortical whisker representations, visualized with repeated intrinsic signal optical imaging in the same animals, contracted by 46% after four to six weeks of exposure to the naturalistic habitat. Acute, multi-site extracellular recordings demonstrated suppressed evoked neuronal responses and smaller, sharper constituent receptive fields in the upper cortical layers (II/III), but not in the thalamic recipient layer (IV), of rats with naturalistic experience. Morphological plasticity of the layer IV barrel field was observed, but on a substantially smaller scale than the functional plasticity. Thus, transferring animals to an environment that promotes the expression of natural, innate behaviours induces a large-scale functional refinement of cortical sensory maps.

Members of theWnt,Fz, andFrp gene families expressed in postnatal mouse cerebral cortex

The functions of Wingless-Int (Wnt) signaling, studied intensely in embryonic brain development, have been comparatively little investigated in the postnatal brain. We report remarkably patterned gene expression of Wnt signaling components in postnatal mouse cerebral cortex, lasting into young adulthood. Wnt genes are expressed in gene-specific regional and lamina patterns in each of the major subdivisions of the cerebral cortex: the olfactory bulb (OB), the hippocampal formation, and the neocortex. Genes encoding Frizzled (Fz) Wnt receptors, or secreted Frizzled-related proteins (sFrps), are also expressed in regional and lamina patterns. These findings suggest that Wnt signaling is active and regulated in the postnatal cortex and that different cortical cell populations have varying requirements for a Wnt signal. The OB, in particular, shows gene expression of a large variety of Wnt signaling components, making it a prime target for future functional studies. The penultimate components of the canonical Wnt pathway are the Tcf/Lef1 transcription factors, which regulate transcription of Wnt signaling target genes. Surprisingly, we found little Tcf/Lef1 expression in the postnatal neocortex. These observations suggest that noncanonical Wnt pathways predominate, which will require functional testing. However, Lef1 is widely expressed in the dorsal thalamus, and Wnt ligands and receptors are expressed, respectively, in cortical areas and thalamic nuclei that are interconnected. Thus, canonical Wnt signaling could be utilized in a major cortical input by Fz- and Lef1-expressing thalamic cells that innervate the Wnt-expressing cortex.

Generating the cerebral cortical area map

The view that the cortical primordium is initially patterned in similar ways to the rest of the embryo has been a conceptual breakthrough. We now have a new starting point for understanding how the cortical area map is established and how maps may change and evolve. Here we review findings that signaling molecules secreted from distinct cortical signaling centers establish positional information in the cortical primordium and regulate regional growth. In other embryonic systems, positional signals would regulate the patterned expression of transcription factors, leading, in a gene regulatory cascade, to the patterned differentiation of the tissue. We discuss candidate transcription factors with respect to such a model of cortical patterning. Finally, embryonic structures interact to pattern one another. We review data suggesting that the thalamus and cortex are patterned independently then interact to generate the final cortical area map.

A morphogenetic role for acetylcholine in mouse cerebral neocortex

Recently, cholinergic afferents to cerebral cortex have met renewed attention regarding the regulation of plasticity as well as cognitive processing. My laboratory has developed a mouse neonatal basal forebrain lesion paradigm that has contributed considerably to the understanding of cholinergic mechanisms in cortical development. We have shown that transient cholinergic deafferentation, beginning at birth, precipitates alterations in neuronal differentiation and synaptic connectivity that persist into maturity, and contribute to altered cognitive behavior. These data are in general agreement with studies in rats in which the cholinergic basal forebrain is lesioned very early in development but contrast with effects of later developmental lesions. Moreover, in mouse, both morphological and behavioral consequences of the lesion are sex dependent. Studies of receptors and secondary messengers that are instrumental in morphogenesis and plasticity suggest that sex dependent molecular alterations occur within days if not hours following cortical cholinergic deafferentation.

Circuit analysis of experience-dependent plasticity in the developing rat barrel cortex

Sensory deprivation during a critical period reduces spine motility and disrupts receptive field structure of layer 2/3 neurons in rat barrel cortex. To determine the locus of plasticity, we used laser scanning photostimulation, allowing us to rapidly map intracortical synaptic connectivity in brain slices. Layer 2/3 neurons differed in their spatial distributions of presynaptic partners: neurons directly above barrels received, on average, significantly more layer 4 input than those above the septa separating barrels. Complementary connectivity was found in deprived cortex: neurons above septa were now strongly coupled to septal regions, while connectivity between barrel regions and layer 2/3 was reduced. These results reveal competitive interactions between barrel and septal circuits in the establishment of precise intracortical circuits.

Somatosensory cortical barrel dendritic abnormalities in a mouse model of the fragile X mental retardation syndrome

The Fragile X mental retardation syndrome is the largest source of inherited mental retardation. The syndrome usually results from the transcriptional silencing of the fragile X mental retardation gene (FMR1). To date the most prominent reported neuronal abnormalities for the fragile X mental retardation syndrome include a higher density of long thin spines similar to those found in sensory deprived and developing tissue, suggesting a possible deficit in pruning of immature spines. Dendrites on spiny stellate cells in the inner 1/3 of the barrel wall in layer IV of the rodent somatosensory cortex have been shown to exhibit developmental pruning similar to that affecting spines. To determine if FMRP plays a role in dendritic development, these neurons were examined in two strains of adult FMRP knockout (FraX) mice. FraX mice in both strains exhibited a greater amount of septa-oriented dendritic material, a morphology consistent with pre-pruning status early in development. This observation suggests that FMRP could be necessary for normal developmentally regulated dendritic pruning.

Altered zincergic innervation of the developing primary somatosensory cortex in monoamine oxidase-A knockout mice

Genetic inactivation of monoamine oxidase-A (MAO-A) significantly elevates levels of serotonin (5-HT) during early development and causes a disruption in the compartmented organization of thalamocortical axon terminals in layer 4 of the somatosensory cortex. In order to determine whether corticocortical innervation of the primary somatosensory cortex is also affected by this mutation, we examined the distribution of zinc-containing axon terminals (terminals known to originate from within the cortex) in the developing somatosensory cortex of MAO-A knockout mice, at postnatal days (PD) 3, 5, 6, 8, 10, 12, 15, 28, and 60. In layer 4 of wild-type mice, histochemical staining for zinc respected barrel-specific compartments at all ages beyond PD 5. By contrast, zinc staining in MAO-A knockout mice did not exhibit signs of barrel compartmentation at any age. Across cortical layers, substantial developmental changes in the distribution of zinc-containing terminals were observed in wild-type mice up until PD 12, at which time the mature lamina-specific pattern of zinc staining was achieved. Similar changes were observed in the somatosensory cortex of MAO-A knockout mice, except that its developmental time course was significantly compressed, with zincergic innervation achieving a mature appearance by PD 8. These results provide evidence that an excess of monoamines, most likely 5-HT, dramatically perturbs the columnar organization of intracortical zincergic afferents in layer 4 and significantly accelerates the appearance of a mature laminar pattern of zinc-containing corticocortical terminals.

Neonatal electrolytic lesions of the basal forebrain stunt plasticity in mouse barrel field cortex

Previous studies have shown that neonatal electrolytic lesions of basal forebrain cholinergic projections in mice lead to a transient cholinergic depletion of neocortex and to permanent alterations in cortical cytoarchitecture and in cognitive performance. The present study examines whether neonatal electrolytic lesions of the basal forebrain modify neocortical plasticity. Using cytochrome oxidase histochemistry, we compared cross-sectional areas of individual barrels in the barrel field of four groups of postnatal day 8 (P8) old mice that on P1 received either (1) right electrolytic lesions of the basal forebrain, (2) left C row 1-4 whisker follicle ablations, (3) combined lesion treatments or (4) ice anesthesia only. The size of barrels in basal forebrain lesioned animals was not significantly different from controls. However, the plastic response to whisker removal was compromised in basal forebrain lesioned animals. An index of plasticity, the ratio of row D/row C areas, was reduced significantly in the combined nBM lesioned/follicle ablation group. Compared to whisker-lesioned mice, the expansion in rows B and D and the shrinkage in the lesioned row C area were diminished in the combined treatment group. The present findings correspond to those from a study of rats injected with a cholinergic immunotoxin [Cereb. Cortex 8 (1998) 63]. These results suggest that cholinergic inputs play a role in regulating plasticity as well as in the morphogenesis of mouse sensory-motor cortex.

Testing the role of the cell-surface molecule Thy-1 in regeneration and plasticity of connectivity in the CNS

Thy-1 is a cell-surface signaling molecule of the Ig superfamily implicated in the regulation of neurite outgrowth, synaptic function and plasticity. There is, however, no consensus as to its precise function in the nervous system, and it remains unclear or untested as to what its role is in the development, maintenance and plasticity of neuronal connectivity in the intact brain and whether it is essential for any of the purported functions which have been attributed to it based largely on in vitro bioassays. Here, we have engineered transgenic mice with a targeted deletion of the Thy-1 gene and, after characterizing the development of their corticospinal and thalamocortical pathways, subjected them at adulthood to paradigms of axonal regeneration and plasticity which can be readily induced during development. Quantitative analyses of the brains and spinal cords of adult null mutants showed normal cellular organization, normal anatomical features of the corticospinal and thalamocortical pathways, and basic neurophysiological properties of thalamocortical synaptic transmission which were quantitatively indistinguishable from wild-type mice. Despite the absence of Thy-1, corticospinal axons in adult mutants failed to exhibit overt regeneration following spinal cord lesion; likewise, the terminal arbors of ventrobasal thalamocortical axons also failed to reorganize in adult barrel cortex in response to whisker cautery, although they did so during a developmental critical period identical to that displayed by wild-type mice.Taken together, these results suggest that Thy-1 is not essential for the normal development and maintenance of major axon pathways and functional synaptic connections, nor would it appear to be critically important for inhibiting or promoting axonal growth, regeneration and plasticity in the developing and mature CNS.

Role of thyroid hormones in the maturation and organisation of rat barrel cortex

The influence of thyroid hormones on cortical development was analysed in rat somatosensory cortex. Maternal and foetal hypothyroidism was induced and maintained by methimazole treatment from embryonic day 13 onwards. 5-Bromo-2'-deoxyuridine (BrdU) labelling in hypothyroid rats showed that cell positioning during corticogenesis followed an inside-out pattern. The radial neurogenetic gradients were more diffuse at all ages with respect to normal rats due to the inappropriate location of many cells, including those of the subcortical white matter. Most (62%) of the cells in the subcortical white matter of hypothyroid rats were labelled at embryonic day 15. Nissl staining of the primary somatosensory cortex showed blurred cortical layer boundaries and an abnormal barrel cytoarchitecture. Cytochrome oxidase and peanut agglutinin staining showed that the tangential organisation of the posteromedial barrel subfield and its layer IV specificity was not lost in hypothyroid rats. However the temporal pattern of peanut agglutinin labelling was delayed 3-4 days with respect to normal rats. In hypothyroid rats, the total barrelfield tangential area was reduced by 27% with respect to normal. The total tangential barrel area, corresponding to peanut agglutinin-negative labelling, occupied 77% of the barrelfield area and only 66% in hypothyroid rats. This reduction was larger with cytochrome oxidase staining where the total barrel area occupied 69% of the barrelfield area in normal and 46% in hypothyroid rats. Our data stress the importance of maternal and foetal thyroid hormones during development, and demonstrate the irreversible effects that maternal and foetal hypothyroidism may have on the intrinsic organisation and maturation of the neocortex.

Neither peripheral nerve input nor cortical NMDA receptor activity are necessary for recovery of a disrupted barrel pattern in rat somatosensory cortex

Elevating cortical serotonin (5-HT) in rats from postnatal day (P-) 0 to P-6 by administering the monoamine oxidase (MAO(A)) inhibitor, clorgyline, produces a dose-dependent spectrum of effects on rat somatosensory organization, ranging from enlarged with indistinct septa to a complete lack of vibrissae-related patterns. However, if clorgyline treatment is stopped on P-6, a qualitatively and quantitatively normal vibrissae-related pattern of thalamocortical afferents appears in somatosensory cortex (S-I) on P-10. We employed high performance liquid chromatography (HPLC), infraorbital nerve (ION) transection, N-methyl-D-aspartate (NMDA) receptor blockade, 1,1'-dioctadecyl-3,3,3"3'-tetramethylindocarbocyanine perchlorate (DiI) labeling of thalamic afferents, and CO histochemistry to determine whether peripheral nerve input and/or cortical NMDA receptor activity were required for the recovery of vibrissae-related patterns in clorgyline-treated animals. Clorgyline administration from P-0 to P-6 produced a 1589.4+/-53.3% increase in cortical 5-HT over control animals on P-6 and a 268.8+/-6.3% elevation over controls at P-10. Postnatal day 6 pups had significantly altered vibrissae-related patterns in S-I following 6 days of clorgyline treatment but by P-10, the characteristic vibrissae-related patterns were restored. Neither transection of the ION nor application of the NMDA antagonist, DL-2-amino-5-phosphonovaleric acid (APV), to the cortices of P-6 pups that were treated with clorgyline from birth had any significant effect on the recovery of the vibrissae-related patterns by P-10. These results indicate that neither peripheral nerve input nor cortical NMDA receptor activity are necessary for the restoration of cortical vibrissae-related patterns in rats that have sustained transient elevations of 5-HT.

Microglia, astrocytes, and macrophages react differentially to central and peripheral lesions in the developing and mature rat whisker-to-barrel pathway: a study using immunohistochemistry for lipocortin1, phosphotyrosine, s100 beta, and mannose receptors

Adult and neonatal rats were subjected to transection of the left infraorbital nerve or ablation of the left parietal cortex. The ensuing glial reaction in the whisker-to-barrel pathway was studied with immunohistochemistry for Lipocortin1- (LC1+), phosphotyrosine- (PY+), S100 beta- (S100 beta+), and mannose receptor- (MR+) immunoreactive microglia, astrocytes, and macrophages. Four days after infraorbital nerve transection in adult rats, LC1+ and PY+ microglia were prominently increased in the trigeminal sensory brain-stem nuclei on the deafferented side compared with the intact side. Changes were negligible at the second synapse of the pathway, i.e., the thalamic ventroposterior medial nucleus. Cortical ablation in adults led to an increase in microglia in the ipsilateral ventroposterior medial nucleus that reciprocally connects with the ablated cortex. Moreover, microglial reactions occurred in the contralateral trigeminal sensory brain-stem nuclei in which corticofugal projections from the ablated cortex terminate. S100 beta+ astrocytes, in contrast, appeared unaltered after both types of lesion in adults. In neonates, LC1+, PY+, and S100 beta+ cells did not have the adult morphology of microglia or astrocytes. Four days after nerve transection, LC1+ and PY+ cells were sparse and remained unchanged. In contrast, S100 beta+ cells substantially increased in the deafferented trigeminal brain-stem nuclei. Four days after cortical ablation in neonates, LC1+, PY+, and S100 beta+ cells had accumulated in the deprived thalamus. In contrast to adults, many of these cells were MR+ macrophages. In the deprived brain-stem, only S100 beta+ cells increased and none were macrophages. Therefore, macrophages do not appear to stem from microglia, and neonatal LC1+, PY+, and S100 beta+ cells may possess functions different from those in adults.

Neonatal lead exposure impairs development of rodent barrel field cortex

Childhood exposure to low-level lead can permanently reduce intelligence, but the neurobiologic mechanism for this effect is unknown. We examined the impact of lead exposure on the development of cortical columns, using the rodent barrel field as a model. In all areas of mammalian neocortex, cortical columns constitute a fundamental structural unit subserving information processing. Barrel field cortex contains columnar processing units with distinct clusters of layer IV neurons that receive sensory input from individual whiskers. In this study, rat pups were exposed to 0, 0.2, 1, 1.5, or 2 g/liter lead acetate in their dam's drinking water from birth through postnatal day 10. This treatment, which coincides with the development of segregated columns in the barrel field, produced blood lead concentrations from 1 to 31 microg/dl. On postnatal day 10, the area of the barrel field and of individual barrels was measured. A dose-related reduction in barrel field area was observed (Pearson correlation = -0.740; P < 0.001); mean barrel field area in the highest exposure group was decreased 12% versus controls. Individual barrels in the physiologically more active caudoventral group were affected preferentially. Total cortical area measured in the same sections was not altered significantly by lead exposure. These data support the hypothesis that lead exposure may impair the development of columnar processing units in immature neocortex. We demonstrate that low levels of blood lead, in the range seen in many impoverished inner-city children, cause structural alterations in a neocortical somatosensory map.

Partial denervation of the whiskerpad in adult mice: altered patterns of metabolic activity in barrel cortex

One hundred days after unilateral C-row nerve transection in the adult mouse whiskerpad, the caudal follicles of row C are reinnervated with approximately 80 % of the original number of axons [Corthésy, M.-E., Bronchti, G. & Welker, E. (1999) Eur. J. Neurosci. , 11, 2835-2846]. To what extent is this reinnervation functional, and how does it interact with the enlargement of the functional representation of neighbouring rows subsequent to the denervation? Using the autoradiographic deoxyglucose method, we studied the whisker representation at the level of the barrel cortex 100 days post lesionem. We stimulated whiskers belonging to the denervated row C, the neighbouring rows B and D, or to all five rows A-E. The deoxyglucose uptake was measured in tangential sections through layer IV. The results indicate that, 100 days post lesionem, whiskers of row C reactivate their cortical barrels. However, (i) the magnitude of this cortical response was reduced; (ii) row C barrels were equivalently activated by the stimulation of the neighbouring rows; and (iii) when all whiskers were stimulated, we observed a significantly reduced deoxyglucose uptake over the representation of nonlesioned whiskers of rows D and E. Therefore, 100 days after the peripheral nerve lesion the reinnervation of the whiskerpad had not restored a normal pattern of activation at the level of the barrel cortex. We propose that this is due to a modified interaction between the representations of the various rows of follicles at the cortical level that does not return to normal.