Cortical local circuit axons do not mature after early deafferentation

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.

Shifts in developmental timing, and not increased levels of experience-dependent neuronal activity, promote barrel expansion in the primary somatosensory cortex of rats enucleated at birth

Birth-enucleated rodents display enlarged representations of whiskers (i.e., barrels of the posteromedial subfield) in the primary somatosensory cortex. Although the historical view maintains that barrel expansion is due to incremental increases in neuronal activity along the trigeminal pathway during postnatal development, recent evidence obtained in experimental models of intramodal plasticity challenges this view. Here, we re-evaluate the role of experience-dependent neuronal activity on barrel expansion in birth-enucleated rats by combining various anatomical methods and sensory deprivation paradigms. We show that barrels in birth-enucleated rats were already enlarged by the end of the first week of life and had levels of metabolic activity comparable to those in control rats at different ages. Dewhiskering after the postnatal period of barrel formation did not prevent barrel expansion in adult, birth-enucleated rats. Further, dark rearing and enucleation after barrel formation did not lead to expanded barrels in adult brains. Because incremental increases of somatosensory experience did not promote barrel expansion in birth-enucleated rats, we explored whether shifts of the developmental timing could better explain barrel expansion during the first week of life. Accordingly, birth-enucleated rats show earlier formation of barrels, accelerated growth of somatosensory thalamocortical afferents, and an earlier H4 deacetylation. Interestingly, when H4 deacetylation was prevented with a histone deacetylases inhibitor (valproic acid), barrel specification timing returned to normal and barrel expansion did not occur. Thus, we provide evidence supporting that shifts in developmental timing modulated through epigenetic mechanisms, and not increased levels of experience dependent neuronal activity, promote barrel expansion in the primary somatosensory cortex of rats enucleated at birth.

Neonatal sensory deprivation and the development of cortical function: unilateral and bilateral sensory deprivation result in different functional outcomes

The normal development of sensory perception in mammals depends on appropriate sensory experience between birth and maturity. Numerous reports have shown that trimming some or all of the large mystacial vibrissa (whiskers) on one side of the face after birth has a detrimental effect on the maturation of cortical function. The objective of the present study was to understand the differences that occur after unilateral whisker trimming compared with those that occur after bilateral deprivation. Physiological deficits produced by bilateral trimming (BD) of all whiskers for 2 mo after birth were compared with the deficits produced by unilateral trimming (UD) for the same period of time using extracellular recording under urethan anesthesia from single cells in rat barrel cortex. Fast spiking (FSUs) and regular spiking (RSUs) units were separated and their properties compared in four subregions identified by histological reconstructions of the electrode penetrations, namely: layer IV barrel and septum, and layers II/III above a barrel and above a septum. UD upregulated responses in layer IV septa and in layers II/III above septa and perturbed the timing of responses to whisker stimuli. After BD, nearly all responses were decreased, and poststimulus latencies were increased. Circuit changes are proposed as an argument for how inputs arising from the spared whiskers project to the undeprived cortex and, via commissural fibers, could upregulate septal responses after UD. Following BD, more global neural deficits create a signature difference in the outcome of UD and BD in rat barrel cortex.

Functional deficit and recovery of developing sensorimotor networks following neonatal hypoxic-ischemic injury in the rat

Neonatal hypoxia-ischemia (HI) is the most important cause of brain injury in the newborn. Here we studied structural alterations and functional perturbations of developing large-scale sensorimotor cortical networks in a rat model of moderate HI at postnatal day 3 (P3). At the morphological level, HI led to a disorganized barrel pattern in the somatosensory cortex without detectable histological changes in the motor cortex. Functional effects were addressed by means of epicranial mapping of somatosensory-evoked potentials (SEPs) during the postischemic recovery period. At P10, SEPs were immature and evoked activity was almost restricted to the somatosensory and motor cortices of the contralateral hemisphere. Peak and topographic analyses of epicranial potentials revealed that responses were profoundly depressed in both sensory and motor areas of HI-lesioned animals. At the end of the postnatal period at P21, responses involved networks in both hemispheres. SEP amplitude was still depressed in the injured sensory region, but it completely recovered in the motor area. These results suggest a process of large-scale network plasticity in sensorimotor circuits after perinatal ischemic injury. The model provides new perspectives for investigating the temporal and spatial characteristics of the recovery process following HI and eventually developing therapeutic interventions.

Experience-dependent plasticity mechanisms for neural rehabilitation in somatosensory cortex

Functional rehabilitation of the cortex following peripheral or central nervous system damage is likely to be improved by a combination of behavioural training and natural or therapeutically enhanced synaptic plasticity mechanisms. Experience-dependent plasticity studies in the somatosensory cortex have begun to reveal those synaptic plasticity mechanisms that are driven by sensory experience and might therefore be active during behavioural training. In this review the anatomical pathways, synaptic plasticity mechanisms and structural plasticity substrates involved in cortical plasticity are explored, focusing on work in the somatosensory cortex and the barrel cortex in particular.

Neocortical plasticity deficits in fetal alcohol spectrum disorders: lessons from barrel and visual cortex

Fetal Alcohol Spectrum Disorder (FASD) is characterized by a constellation of behavioral and physiological abnormalities, including learning and sensory deficits. There is growing evidence that abnormalities of neuronal plasticity underlie these deficits. However, the cellular and molecular mechanisms by which prenatal alcohol exposure disrupts neuronal plasticity remain elusive. Recently, studies with the barrel and the visual cortex as models to study the effects of early alcohol exposure on neuronal plasticity shed light on this subject. In this Mini-Review, we discuss the effects of ethanol exposure during development on neuronal plasticity and suggest environmental and pharmacological approaches to ameliorate these problems.

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.

Development of layer-specific axonal arborizations in mouse primary somatosensory cortex

In the developing neocortex, pyramidal neurons use molecular cues to form axonal arbors selectively in the correct layers. Despite the utility of mice for molecular and genetic studies, little work has been done on the development of layer-specific axonal arborizations of pyramidal neurons in mice. We intracellularly labeled and reconstructed the axons of layer 2/3 and layer 5 pyramidal neurons in slices of primary somatosensory cortex from C57Bl6 mice on postnatal days 7-21. For all neurons studied, the development of the axonal arborizations in mice follows a pattern similar to that seen in other species; laminar specificity of the earliest axonal branches is similar to that of mature animals. At P7, pyramidal neurons are very simple, having only a main descending axon and few primary branches. Between P7 and P10, there is a large increase in the total number of axonal branches, and axons continue to increase in complexity and total length from P10 to P21. Unlike observations in ferrets, cats, and monkeys, two types of layer 2/3 pyramidal neurons are present in both mature and developing mice; cells in superficial layer 2/3 lack axonal arbors in layer 4, and cells close to the layer 4 border have substantial axonal arbors within layer 4. We also describe axonal and dendritic arborization patterns of three pyramidal cell types in layer 5. The axons of tall-tufted layer 5 pyramidal neurons arborize almost exclusively within deep layers while tall-simple, and short layer 5 pyramidal neurons also project axons to superficial layers.

Use-dependent plasticity in barrel cortex: intrinsic signal imaging reveals functional expansion of spared whisker representation into adjacent deprived columns

We used optical imaging of intrinsic cortical signals, elicited by whisker stimulation, to define areas of activation in primary sensory cortex of normal hamsters and hamsters subjected to neonatal follicle ablation at postnatal day seven (P7). Follicle ablations were unilateral, and spared either C-row whiskers or the second whisker arc. This study was done to determine if the intrinsic cortical connectivity pattern of the barrel cortex, established during the critical period, affects the process of representational plasticity that follows whisker follicle ablation. Additionally, we tested the ability to monitor such changes in individual cortical whisker representations using intrinsic signal imaging. Stimulation of a single whisker yielded peak activation of a barrel-sized patch in the somatotopically appropriate location in normal cortex. In both row and arc-spared animals, functional representations corresponding to spared follicles were significantly stronger and more oblong than normal. The pattern of activation differed in the row-sparing and arc-sparing groups, in that the expansion was preferentially into deprived, not spared areas. Single whisker stimulation in row-spared cases preferentially activated the corresponding barrel arc, while stimulation of one whisker in arc-spared cases produced elongated activation down the barrel row. Since whisker deflection normally has a net inhibitory effect on neighboring barrels, our data suggest that intracortical inhibition fails to develop normally in deprived cortical columns. Because thalamocortical projections are not affected by follicle ablation after P7, we suggest that the effects we observed are largely cortical, not thalamocortical.

Activity-dependent expression of Egr1 mRNA in somatosensory cortex of developing rats

The rat barrel field in somatosensory cortex is a well-characterized model of neocortical development, with activity-dependent and activity-independent components. Egr1 encodes an inducible transcription factor that is required for certain forms activity-dependent plasticity. This study examines Egr1 mRNA expression in the developing barrel field under basal conditions and after short-term deprivation or stimulation of whiskers. Egr1 mRNA was measured with in situ hybridization at postnatal Day (P) 6, P9, P12, P15, and P21. For short-term deprivation, whiskers were trimmed close to the skin and Egr1 mRNA was examined 3 hr later. For controlled stimulation of a single whisker, surrounding whiskers were trimmed, a wire was glued to the designated whisker, and animals were placed in an AC magnetic field pulsed at 2 Hz, 10 mT rms for 15 min. Egr1 mRNA was examined 30 min later. At P6, basal Egr1 mRNA in the barrel field was very low and was increased only slightly by stimulation (P < 0.05). At each of the later ages, there was a large increase in Egr1 mRNA in stimulated versus deprived barrels (P < 0.001). Egr1 mRNA expression after whisker stimulation increased exponentially with age through P15 (P < 0.001) and then declined between P15 and P21. The onset of Egr1 responses to whisker stimulation at P9 and the striking increase in activity-dependent Egr1 mRNA expression in the second postnatal week suggest that this transcription factor may play a role in activity-dependent processes that occur in this developmental period, such as maturation of barrel cortex circuitry.

Large-scale plasticity in barrel cortex following repeated whisker trimming in young adult hamsters

Using the 2DG/immunostaining method [McCasland, J.S., Graczyk, G.M., 2000. Metabolic mapping-Unit 1.6. In: Gerfen, C.R. (Ed.), Current Protocols in Neuroscience. Wiley, New York, pp 1.6.1-1.6.15], we have previously demonstrated large-scale plasticity in whisker/barrel fields of young adult hamsters subject to follicle ablation on postnatal day 7 (P7) [Somatosens. Motor Res. 13 (1996) 245]. This plasticity occurs after the barrel field has formed, but before neuronal differentiation and synaptogenesis are complete. The present study tested for similar large-scale plasticity following whisker deprivation in young adult hamsters, when neuronal and synaptic development are more mature. Beginning around P40, animals had all whiskers except row C trimmed on alternating days for periods ranging from 1 h to 2 weeks, after which they were administered (3)H 2DG (i.p.) and allowed to explore a fresh empty cage. Autoradiograms from these animals showed a clear expansion in the zone of heavy 2DG labeling with continued whisker trimming. Hamsters with row C spared overnight showed markedly higher labeling in the row C barrels, as expected. After 2 weeks of repeated trimming, the pattern of 2DG labeling in the barrel field ranged from complete activation of all large-whisker columns, as in a previous study of P7 follicle ablation, down to a more localized activation of rows B, C, and D. Intermediate periods of trimming produced more localized label in the region of row C. There was a clear trend toward larger areas of activation with longer periods of trimming. Because inhibitory neurons are strongly activated in all cases, this large-scale neuronal plasticity must take place in the presence of strong inhibition. The data show that simple trimming of all but a few whiskers in normally reared adults leads to abnormally widespread metabolic labeling encompassing virtually the entire barrel field. Taken together, our findings suggest that a large-scale synaptic reorganization occurs in barrel fields deprived of normal sensory input in the adult as well as during postnatal development.

Role of development in reorganization of the SI forelimb-stump representation in fetally, neonatally, and adult amputated rats

Rats that sustain forelimb removal on postnatal day (P) 0 exhibit numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to hindlimb stimulation when cortical GABAA+B receptors are blocked. Most of these hindlimb inputs originate in the medial SI hindlimb representation. Although many forelimb-stump sites in these animals respond to hindlimb stimulation, very few respond to stimulation of the face (vibrissae or lower jaw), which is represented in SI just lateral to the forelimb. The lateral to medial development of SI may influence the capacity of hindlimb (but not face) inputs to "invade" the forelimb-stump region in neonatal amputees. The SI forelimb-stump was mapped in adult (>60 days) rats that had sustained amputation on embryonic day (E) 16, on P0, or during adulthood. GABA receptors were blocked and subsequent mapping revealed increases in nonstump inputs in E16 and P0 amputees: fetal amputees exhibited forelimb-stump sites responsive to face (34%), hindlimb (10%), and both (22%); neonatal amputees exhibited 10% face, 39% hindlimb, and 5% both; adult amputees exhibited 10% face, 5% hindlimb, and 0% both, with approximately 80% stump-only sites. These results indicate age-dependent differences in receptive-field reorganization of the forelimb-stump representation, which may reflect the spatiotemporal development of SI. Results from cobalt chloride inactivation of the SI vibrissae region and electrolesioning of the dysgranular cortex suggest that normally suppressed vibrissae inputs to the SI forelimb-stump area originate in the SI vibrissae region and synapse in the dysgranular cortex.

Developmental regulation of plasticity in the forepaw representation of ferret somatosensory cortex

This study characterized the spatiotemporal responses in ferret somatosensory cortex after sensory deprivation at different phases of cortical development. We hypothesized that cortical responses to stimulation of intact superficial radial nerve in adults will vary systematically according to maturation of thalamocortical relationships at the time of an ulnar nerve transection. Depending on the age of the animal at the time of the lesion, we found differential effects on the spatial distribution of the short- and long-latency components of the cortical response. In animals lesioned at postnatal days 5-7, when thalamic projections are not yet stabilized and layer 4 is not yet formed, we found that initial (short-latency) cortical responses are widespread and fragmented. Ulnar nerve transections performed at postnatal day 20 or 21, when thalamocortical afferents are more stabilized and layer 4 is clearly identifiable, yield moderate expansions in the distribution of short- and long-latency components of the cortical response. Nerve lesions in adults lead to a wider distribution of long-latency cortical activity. Neonatal lesions broaden the spatial distribution and increase the latency of the initial cortical response; interruption of nerve input in older juveniles alters both the early and later components; and nerve lesions in adult animals expand the distribution of later cortical activity only. These findings demonstrate correlation between developmental phase at the time sensory input is interrupted and the latency of affected components of the cortical response. This supports the hypothesis that differential response changes are regulated by functional reorganization of thalamocortical connections after neonatal lesions and alteration of corticocortical dynamics after adult lesions.

Schizophrenia, neurodevelopment and corpus callosum

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

Dopamine D(3) receptor is selectively and transiently expressed in the developing whisker barrel cortex of the rat

The rodent primary somatosensory cortex (SI) contains a map of the body surface, the most conspicuous part of which are "barrels," neuronal aggregates in layer IV that receive somatotopic projections from whiskers on the rodent's snout. We report that the D(3) dopamine receptor (D(3)R) is selectively and transiently expressed in SI during the first 2 weeks of postnatal development. D(3)R binding sites and mRNA overlap completely and are limited to layer IV of SI. D(3)R/mRNA are organized in a pattern corresponding to somatotopic representations of the body (e.g., whiskers, jaws, paws, etc.) with the highest expression in the barrel field. D(3) mRNA is first detected at postnatal day (P)4, increases rapidly until P7-10, and sharply decreases after P14. D(3)R binding sites are detectable at P6, peak at P14, and decline afterwards. D(1), D(2), D(4), or D(5) mRNAs display dissimilar expression pattern. D(1) mRNA is mostly confined to infragranular layers throughout the cortex. D(4) mRNA expression in layer IV rises by 4 weeks postnatal, when D(3)R expression is virtually undetectable. Quantitative analysis of D(3) mRNA expression demonstrates that the proportion of D(3) mRNA-positive cells decreases between P7 and P14, whereas mRNA concentration per cell remains stable. Moreover, D(3)R number continues to rise, whereas mRNA levels begin to decline. Thus, a process limiting D(3)R expression to fewer cells may occur that also induces changes in post-transcriptional regulation of D(3)R expression in remaining cells. These findings indicate that dopamine acting via D(3)R may play an important role in the development or function of the SI.

A mapping label required for normal scale of body representation in the cortex

The neocortical primary somatosensory area (S1) consists of a map of the body surface. The cortical area devoted to different regions, such as parts of the face or hands, reflects their functional importance. Here we investigated the role of genetically determined positional labels in neocortical mapping. Ephrin-A5 was expressed in a medial > lateral gradient across S1, whereas its receptor EphA4 was in a matching gradient across the thalamic ventrobasal (VB) complex, which provides S1 input. Ephrin-A5 had topographically specific effects on VB axon guidance in vitro. Ephrin-A5 gene disruption caused graded, topographically specific distortion in the S1 body map, with medial regions contracted and lateral regions expanded, changing relative areas up to 50% in developing and adult mice. These results provide evidence for within-area thalamocortical mapping labels and show that a genetic difference can cause a lasting change in relative scale of different regions within a topographic map.

Neonatal whisker clipping alters intracortical, but not thalamocortical projections, in rat barrel cortex

Retrograde axonal transport of cholera toxin B subunit (CTB) was used to compare the development of intracortical and thalamocortical connections in normal rats with those in rats in which all of the whiskers were trimmed continuously from birth. In normal animals, injections of CTB into a single barrel column resulted in an asymmetrical labeling of cells that were distributed preferentially within columns related to the same row in which the injection was placed. This anisotropy in the patterns of intracortical connections was not observed in whisker-clipped animals. In these animals, there was a significant reduction in the mean number of labeled cells in the infragranular layers, and labeled cells were distributed symmetrically around the injection site. The same injections of CTB also labeled thalamocortical neurons in the ventrobasal thalamus. Analysis of the distribution of these cells revealed that, in both control and experimental animals, the vast majority of labeled cells were restricted to a homologous (i.e., corresponding to the injected cortical barrel) thalamic barreloid. These findings indicate that manipulations of sensory experience alter patterns of intracortical, but not thalamocortical, connections.

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

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

Anatomical correlates of representational map reorganization induced by partial vibrissectomy in the barrel cortex of adult mice

We examined the potential for changes in cortical connectivity to accompany long-term plastic changes in functional cortical representations of mystacial vibrissae. Plasticity in the barrel cortex of young adult mice was evoked by vibrissectomy that spared row C of whiskers. We found that 2-deoxyglucose brain mapping causes a progressive expansion of cortical representation of the spared vibrissae. Two months after vibrissectomy, when the width of the cortical map of the spared row of vibrissae doubled, living cortical slices of the barrel cortex were injected with fluorescent dextrans. The injections were centered on spared, deprived and control vibrissal columns. The injections labeled three intracortical projection systems: (i) local connections from one vibrissal column to neighboring columns; (ii) long-range projections running in the septa and walls of the barrels and spanning several barrels; and (iii) very-long-range fibers running horizontally in the lower part of layer V. The local, short-range projection system was analysed following small injections into the centers of columns in layers III and IV. We found that injections into spared barrels labeled axons extending for significantly greater distances in all layers (except layer V), and labeled cell bodies situated significantly further, than after injections into deprived or control barrels. Also, the total axonal density labeled by injections into the spared barrel was higher by 70% than for the deprived or control barrels. Alterations of topographical maps in adult somatosensory cortex may occur immediately after functional denervation, but may also increase with time, as in the case of our experimental situation. Our results indicate that persistent, long-term plastic change can remodel connectivity in the barrel cortex.

Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal

Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation.

Sensitive period for lesion-induced reorganization of intracortical projections within the vibrissae representation of rat's primary somatosensory cortex

Previous experiments from this laboratory demonstrated that intracortical connections in lamina IV of the rat primary somatosensory cortex (SI) are most dense outside the patches of cytochrome oxidase (CO) staining that correspond to the mystacial vibrissae. This pattern of intracortical connections becomes apparent on postnatal day 4 (P-4), at least 2 days after the appearance of the vibrissae-related pattern of thalamocortical afferents. Transection of the infraorbital nerve (ION) on the day of birth (P-0) disrupts both the CO and intracortical projection patterns. This series of experiments was undertaken to determine whether the patterning of either thalamocortical afferents or intracortical projections defines the end of the period over which peripheral damage can alter intracortical projections in lamina IV of SI. The infraorbital nerve (ION) was transected in different cohorts of rats on P-1 through P-5, and animals were allowed to survive > or =45 days, at which time biotinylated dextran amine (BDA) injections were made into the SI. After 7 days, animals were killed, and alternate cortical sections were processed for the demonstration of BDA or CO. Transection of the ION on P-1 or P-2 altered the patterning of both CO and intracortical connections in the SI. In contrast, cutting the ION on P-3 left the pattern of CO densities in the SI intact, but significantly altered the patterning of intracortical connections. Transection of the nerve on P-5 resulted in qualitatively and quantitatively normal patterns of both CO densities and BDA-labelled intracortical projections. These results indicate that the establishment of a stable barrel pattern in layer IV of the SI is not sufficient for normal adult patterning of intracortical projections in this lamina. However, once the mature pattern of intracortical projections in layer IV is established, ION lesions can no longer alter it.

BDNF and trkB mRNA expression in neurons of the neonatal mouse barrel field cortex: normal development and plasticity after cauterizing facial vibrissae

Development of the central somatosensory system is profoundly modulated by the sensory periphery. Cauterization of facial whiskers alters the segregation pattern of barrels in rodents only during a few days just after birth (critical period). Although a molecular basis of the segregation of barrel neurons and the critical period for the anatomical plasticity observed in layer IV barrel neuron is not clear yet, the accumulating evidence suggests that neurotrophins modulate synaptic connections including central nervous system. In this study, we showed by in situ hybridization that mouse barrel side neurons express brain-derived neurotrophic factor (BDNF) mRNA and both catalytic and non-catalytic forms of trkB mRNA. Cautery of row C vibrissae on the right side of the face within 24 h after birth (post natal day 0, PND0) reduced the expression of BDNF and trkB mRNA from the division region between the contralateral row C barrels at PND7. The vibrissae in row A, C, and E were cauterized at PND0 followed by quantitative RT-PCR for BDNF and trkB mRNA with total RNA isolated from the barrel region at PND7. The result showed that BDNF, but not trkB, mRNA was increased several-fold in the contralateral barrel region. These data suggest that the expression of BDNF mRNA is differentially regulated between injured barrels and actively innervated barrels. The differential expression of the mRNA encoding neurotrophins and their receptors may be important in regulating the injury-dependent re-segregation of barrels.

Activity blockade does not prevent the construction of olfactory glomeruli in the moth Manduca sexta

During metamorphic development, the arrival at the olfactory (antennal) lobe of olfactory receptor axons initiates the process of glomerulus formation. The glomeruli are discrete spheroidal regions of neuropil that are the sites of synaptic interactions among receptor neurons and their target antennal-lobe neurons. The process of glomerulus formation begins as groups of receptor axons form protoglomeruli. These dense clusters of terminal branches mostly are discrete entities from the time they can be recognized, although a few branches from neighboring protoglomeruli overlap laterally. A previous study by Schweitzer et al. [Schweitzer E. S., Sanes J. R. and Hildebrand J. G. (1976) Ontogeny of electroantennogram responses in the moth, Manduca sexta. J. Insect Physiol. 22, 955-960] has shown that odor-induced activity in the receptor neurons can be detected first in recordings from the axons in the antennal nerve only in the last few days of metamorphic development and thus could not influence the process of glomerulus formation. In this study, we have tested directly the possibility that an earlier presence of spontaneous activity in either the receptor axons or the antennal-lobe neurons could affect the process. Tetrodotoxin, a Na(+)-channel blocker, was injected into the hemolymph prior to the onset of glomerulus formation to block any spontaneous Na(+)-dependent activity. Subsequent intracellular recordings from antennal-lobe neurons revealed no spike activity. Comparison with vehicle-injected control animals at stages during and after glomerulus formation revealed no differences in the localization of receptor-axon terminal branches in the glomeruli, in the border of glial cells that forms around each glomerulus, or in the morphology of the tufted glomerular arbors of one of the antennal-lobe neurons. We conclude that: (1) the process of glomerulus formation is largely independent of activity; and (2) glomeruli as modular units of the CNS more closely resemble cortical barrels than cortical columns, both in their ontogeny and in the lack of an obvious effect of activity on the morphology of the neurons arborizing within them.

Development and plasticity of local intracortical projections within the vibrissae representation of the rat primary somatosensory cortex

Labelling with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Di-A) was used to assess the development of projections within the primary somatosensory cortex (SI) of rats aged between postnatal day 2 and 8 (P-2 and P-8). 1,1'-Dioctadecyl-3,3,3,"3'-tetramethylindocarbocyanine perchlorate (Di-I) was used in these same animals to label thalamocortical afferents. Particular attention was paid to the emergence of lamina IV intracortical projections that form a pattern complementary to vibrissae-related thalamocortical afferents. A vibrissae-related pattern of Di-A-labelled cells and fibers that was restricted largely to the septa regions was not apparent in rats killed on P-2, but it was visible in animals killed on P-4 and later ages. Tracing with biotinylated dextran amine (BDA) was used to assess intra-SI projections of adult rats that sustained transection of the infraorbital nerve (ION) on P-0 or P-7 or implantation of a tetrodotoxin (TTX)-impregnated polymer chip over the cortex on P-0. Rats that sustained ION transection on P-7 or that had TTX implants demonstrated normal patterns of projections within SI. The patterns of labelling in the supra- and infragranular layers of the cortices of the rats that sustained ION transection on P-0 were generally similar to those in the other groups evaluated. However, in lamina IV, there was no organization that could be related to the distribution of the vibrissae. These results indicate that the vibrissae-related pattern of intracortical projections within SI develops shortly after birth and that two manipulations that alter cortical activity, but not the patterning of thalamocortical afferents (application of TTX and transection of the ION after thalamocortical afferent patterns are established), have no significant effect on it. However, a manipulation that alters thalamocortical development (transection of the ION on P-0) profoundly affects the patterning of intracortical connections.

Glutamate receptor blockade at cortical synapses disrupts development of thalamocortical and columnar organization in somatosensory cortex

The segregation of thalamocortical inputs into eye-specific stripes in the developing cat or monkey visual cortex is prevented by manipulations that perturb or abolish neural activity in the visual pathway. Such findings show that proper development of the functional organization of visual cortex is dependent on normal patterns of neural activity. The generalisation of this conclusion to other sensory cortices has been questioned by findings that the segregation of thalamocortical afferents into a somatotopic barrel pattern in developing rodent primary somatosensory cortex (S1) is not prevented by activity blockade. We show that a temporary block of N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in rat S1 during the critical period for barrel development disrupts the topographic refinement of thalamocortical connectivity and columnar organization. These effects are evident well after the blockade is ineffective and thus may be permanent. Our findings show that neural activity and specifically the activation of postsynaptic cortical neurons has a prominent role in establishing the primary sensory map in S1, as well as the topographic organization of higher order synaptic connections.

Widespread activation of barrel cortex by small numbers of neonatally spared whiskers

Activity-dependent plasticity in rodent whisker barrel cortex was examined by means of high-resolution 2-deoxyglucose (2-DG) with immunohistochemical double labeling. Hamsters with all but one, two, or four follicles ablated on postnatal day 7 received 2-DG injections as adults. Autoradiograms of follicle-ablated animals showed heavy activation of the entire barrel field during normal behavior, despite the missing whiskers. The intensity of 2-DG labeling was significantly reduced if the whiskers spared after follicle ablation were trimmed prior to the 2-DG injection, demonstrating that the widespread activation was driven by the spared whiskers. This widespread metabolic activation of the adult barrel field after neonatal follicle ablation was in sharp contrast to the somatotopically appropriate 2-DG labeling in barrel fields of normal adults subject to acute trimming of most whiskers, but was similar to that seen in normal adult animals with all whiskers intact. The results demonstrate large-scale plasticity of barrel circuitry following neonatal sensory deprivation, and provide a powerful functional anatomical setting to investigate underlying mechanisms.

Patterning of local intracortical projections within the vibrissae representation of rat primary somatosensory cortex

Anterograde and retrograde tracing with biotinylated dextran amine and Phaseolus vulgaris leukoagglutinin was used to assess projection patterns within the vibrissae representation of the rat's primary somatosensory cortex (S-I). Large and small injections of either tracer into the center of the vibrissae representation yielded dense anterograde and retrograde labelling throughout much of the tangential extent of the vibrissae representation within S-I. In all layers, the pattern and extent of retrograde and anterograde label was in rough congruence. The organization of this labelling varied across cortical layers. In layers II and III, labelled fibers extended away from injection sites in all directions and yielded a uniform pattern, which decreased in density with increasing distance from the tracer injection. There was a tendency for labelling to be more extensive along the representation of the row of vibrissae follicles that included the injection site than across rows. There was also a tendency for anterograde labelling to be more extensive in the direction of the representation of follicles more rostral on the face than that injected. In lamina IV, both labelled fibers and cells were restricted for the most part to the septa regions between the barrels. However, a small number of retrogradely labelled neurons were also located in the barrels (approximately one-ninth of the number found in the septa). The pattern observed in laminae II-III was repeated in layers V and VI. In these laminae, there was no evidence of a pattern of intracortical connections related to the vibrissae representation in overlying lamina IV.

Early development of the somatotopic map and barrel patterning in rat somatosensory cortex

Several lines of evidence implicate a crucial role for thalamic afferents from the ventroposterior nucleus (VP) in the development of barrels and their characteristic pattern in the primary somatosensory cortex (S1) of rodents. We sought to determine the stage in development when VP thalamocortical afferents are first distributed in a periphery-related pattern and the sequence of events that culminate in a mature pattern. Using acetylcholinesterase (AChE) histochemistry, an early marker for VP thalamocortical afferents, and the anterograde axon tracer DiI, we show that VP thalamocortical afferents become distributed into a periphery-related pattern earlier than was previously reported, including their parcellation into a barrel-related pattern that mirrors the distribution of sensory hairs on the face. The earliest periphery-related patterning observed is transiently present in the deep cortical layers prior to the emergence of layer 4, the layer in which barrels later develop. AChE histochemistry reveals a clear sequence of maturation of the barrel pattern in the distribution of VP afferents: An initially patternless distribution of AChE-reactive afferents is followed by their distribution in a nascent trigeminal representation, from which rows subsequently emerge; barrel-related clusters of afferents then emerge from the rows. This process begins before birth, and the transition from row-related to barrel-related distributions of VP afferents is evident during the first postnatal day (P0). This demonstration of a periphery-related pattern in developing rat S1 precedes by about 2 days that revealed by any other marker reported to delineate barrels. These findings confirm that VP thalamocortical afferents are the first barrel component to have a periphery-related pattern and support the hypothesis that thalamocortical afferents provide to immature S1 the patterning information that initiates the formation of barrels and their characteristic array. Furthermore because these findings show an earlier onset for barrel formation than was previously realized, they necessitate a reevaluation of conclusions drawn from experiments examining developmental plasticity in barrel patterning.

Alterations in brainstem and cortical organization of rats sustaining prenatal vibrissa follicle lesions

Vibrissa follicles were cauterized in late fetal or newborn rats to determine whether the relationships between brainstem and cortical changes observed after neonatal peripheral damage would also be obtained when vibrissa follicles were cauterized earlier in development. Vibrissa follicles were cauterized between embryonic day 15 (E-15) and the day of birth (P-0). The vibrissa-related representation in the brainstem was examined with cytochrome oxidase histochemistry, and that in the cortex was evaluated with either serotonin immunocytochemistry or anterograde labeling with Di-I when animals reached 6-8 days of age. There was a significant relationship between the ages at which lesions were carried out and the extent to which the representations of undamaged vibrissa follicles were altered in the brainstem and cortex. Peripheral lesions carried out between E-15 and E-18 resulted in significant increases in the cross-sectional areas of the patches corresponding to the undamaged vibrissa follicles in both the brainstem and cortex. Lesions at later ages resulted in significant increases only in the cortex. In some animals that sustained peripheral damage on E-20 and all of those that received lesions on P-0, there were aggregates of labeling in cortex that had no counterpart in the brainstem. Prenatal, but not postnatal, vibrissa follicle damage also reduced the overall dimensions of the cortical region devoted to the representation of these receptor organelles. Finally, there was a strong negative correlation between the magnitude of peripheral lesions (i.e., the number of vibrissa follicles ablated) and the extent of the reorganization in the brainstem and cortex.

Loss of glutamate immunoreactivity from mouse first somatosensory (SI) cortex following neonatal vibrissal lesion

Whisker follicles were surgically ablated (lesioned) on two entire rows (B and C) of the left snout of two groups of Swiss mice, in the first 2 days after birth (neonatally lesioned) with the animals being allowed to survive for 4 weeks. In the second group at 8 weeks of age (adults), the whisker follicles of rows B and C were similarly lesioned and a survival period of 3 days allowed. Glutamate-immunoreactivity (Glu-IR) was examined in tangential sections of the first somatosensory (SI) 'barrel' cortex of these two groups (at which time it was also confirmed that the follicles had not regrown). In the neonatally lesioned mice, barrel rows B and C were more poorly defined in the right (experimental) hemisphere sections and a semi-quantitative study showed that there was a decrease in Glu-IR cell number (up to 41%) in rows B and C of the right hemisphere compared to the spared barrel rows (A and D). The loss appears to occur over almost the entire area of each deprived barrel rather than being confined to the sides or hollows. In contrast to neonatally lesioned animals, the barrels of the adult-lesioned mice appeared intact and visually similar in both the experimental (right) and control hemispheres (left). The only notable change in Glu-IR observed in the adult-lesioned animals was a decrease of 38% in the number of Glu-IR cells in the sides between the two deafferented rows of barrels (B and C), compared to the cell number between the unaffected barrels (D and E), a change also seen in the neonatally lesioned mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Postsynaptic control of plasticity in developing somatosensory cortex

The rearrangement of synaptic connections during normal and deprived development is though to be controlled by correlations in afferent impulse activity. A favoured model is based on post-synaptic detection of synchronously active afferents; synapses are stabilized when pre- and postsynaptic activity is correlated and weakened or eliminated when their activity is uncorrelated. Most evidence for this model comes from demonstrations that correlated afferent input is necessary for the segregation of eye-dominant inputs in the developing vertebrate visual system and that critical period plasticity of ocular dominance columns in cat visual cortex is disrupted by blockade of postsynaptic transmission. We tested whether the developmental plasticity of somatosensory columns, known as 'barrels', in rodent primary somatosensory cortex (S1) is similar to that of ocular dominance columns. We report here that the selective disruption of postsynaptic activation in rat S1 by application of a glutamate receptor antagonist inhibits rearrangements in the somatotopic patterning of thalamocortical afferents induced by manipulations of the sensory periphery during the critical period. These findings show that postsynaptic activation has a prominent role in critical period plasticity in S1 cortex.

2-DG uptake patterns related to single vibrissae during exploratory behaviors in the hamster trigeminal system

Stimulation of one or several whiskers activates discrete foci throughout the trigeminal (V) neuraxis. These foci contribute to patterns, corresponding to the patterns of vibrissae, that have been directly related to aggregates of cells and axon terminals in the "barrel" cortex. Here, we combine high-resolution, 2-deoxyglucose (2DG) mapping and cytochrome oxidase (CO) staining to determine whether the known pattern of V primary afferent projections is sufficient to deduce the functional activation of their targets during exploratory behavior. Four adult hamsters had all of their large mystacial vibrissae trimmed acutely, except for C3 on the left, and B2 and D4 on the right; in two others, the left C3 and right A1 and E4 whiskers were spared. After fasting overnight, 2DG was injected and the animals behaved freely in the dark for 45 minutes. The brainstem, thalamus, and cortices were sectioned, then processed for both CO staining and 2DG autoradiography. Image-processing microscopy was used to separate the autoradiographic silver grains from the histochemical staining. CO patches were patterned in a whisker-like fashion in the full rostrocaudal extent of V nucleus principalis and in caudal portions of spinal V subnuclei interpolaris and caudalis, but absent in subnucleus oralis. 2DG silver grains were densest above those CO patches in the pattern corresponding to the active whiskers. There were no consistent 2DG foci in subnuclei oralis or rostral caudalis. In these same cases, prominent 2DG labeling was restricted to the appropriate barrels in the contralateral cortex. Only one case, however, displayed a clear and appropriate region of heightened 2DG uptake in contralateral ventroposteromedial thalamus (VPM) and the adjacent part of the reticular thalamic nucleus. Patterns of increased glucose utilization with single whisker stimulation are well matched to the CO patterns that mirror distributions of neurons associated with a vibrissa in the V brainstem complex, thalamus, and cortex. Single whiskers are represented by relatively homogeneous longitudinal columns of 2DG labeling in the V brainstem nuclei. The columns are not continuous through the axial extent of the V brainstem complex; rather, they occur separately within principalis, interpolaris, and caudalis. While whisker columns were consistently labeled in interpolaris and caudalis in all animals, the labeling was increasingly variable in principalis, barrel cortex, and VPM, respectively. This suggests that the behaving animal can and does significantly modulate activity in this major, synaptically secure pathway.

Development and remodeling of cerebral blood vessels and their flow in postnatal mice observed with in vivo videomicroscopy

Changes of blood vessels in the mouse somatosensory (barrel) cortex were assessed from birth (P0) to adulthood. Surface vessel anatomy and flow were observed directly with videomicroscopy through closed cranial windows and with intravascular fluorescent tracers. Histology was used to determine the internal capillary density. At birth, arterioles had numerous anastomoses with each other, pial capillaries formed a dense surface plexus, and pial venules and veins were relatively small and irregular. Morphological changes over the next 2 weeks included (a) fewer arteriolar anastomoses, (b) formation and growth of venules, (c) more uniform diameters of all types of vascular segments, (d) increase in intraparenchymal capillary length density (Lv), and (e) decreases in superficial capillary density and diameters. A simple morphological test showed that wall shear rates at arteriolar branch points were matched on average in neonates and adults. Flow characteristics in single vessels were evaluated. In arterioles of like diameters, (a) Vmax, (b) peak wall shear rates, and (c) peak flows were similar at all ages; (d) velocity was very high in occasional arteriovenous (AV) shunts in newborns; and (e) flow in arteriolar anastomoses was slow and variable. Although flow was heterogeneous in all types of vessel, the marked similarities in newborn and adult mice of average peak velocities and calculated wall shear rates in arterioles of the same size suggest that blood flow regulates in part the remodeling of blood vessels during development (Rovainen et al., 1992). The rodent barrel cortex undergoes major neuronal and vascular development, functional differentiation, and remodeling during the first weeks after birth. It provides special opportunities for testing how blood vessels grow and adapt to supply the local metabolic requirements of neural modules in the brain.