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

Cortical reintegration after facial allotransplantation

Neural plasticity of the brain or its ability to reorganize following injury has likely coincided with the successful clinical correction of severe deformity by facial transplantation since 2005. In this study, we present the cortical reintegration outcomes following syngeneic hemifacial vascularized composite allograft (VCA) in a small animal model. Specifically, changes in the topographic organization and unit response properties of the rodent whisker-barrel somatosensory system were assessed following hemifacial VCA. Clear differences emerged in the barrel-cortex system when comparing naïve and hemiface transplanted animals. Neurons in the somatosensory cortex of transplanted rats had decreased sensitivity albeit increased directional sensitivity compared with naïve rats and evoked responses in transplanted animals were more temporally dispersed. In addition, receptive fields were often topographically mismatched with the indication that the mismatched topography reorganized within adjacent barrel (same row-arc bias following hemifacial transplant). These results suggest subcortical changes in the thalamus and/or brainstem play a role in hemifacial transplantation cortical plasticity and demonstrate the discrete and robust data that can be derived from this clinically relevant small animal VCA model for use in optimizing postsurgical outcomes.NEW & NOTEWORTHY Robust rodent hemifacial transplant model was used to record functional changes in somatosensory cortex after transplantation. Neurons in the somatosensory cortex of face transplant recipients had decreased sensitivity to stimulation of whiskers with increased directional sensitivity vs. naive rats. Transplant recipient cortical unit response was more dispersed in temporary vs. naive rats. Despite histological similarities to naive cortices, transplant recipient cortices had a mix of topographically appropriate and inappropriate whiskered at barrel cortex relationships.

Regenerative Failure Following Rat Neonatal Chorda Tympani Transection is Associated with Geniculate Ganglion Cell Loss and Terminal Field Plasticity in the Nucleus of the Solitary Tract

Neural insult during development results in recovery outcomes that vary dependent upon the system under investigation. Nerve regeneration does not occur if the rat gustatory chorda tympani nerve is sectioned (CTX) during neonatal (≤P10) development. It is unclear how chorda tympani soma and terminal fields are affected after neonatal CTX. The current study determined the impact of neonatal CTX on chorda tympani neurons and brainstem gustatory terminal fields. To assess terminal field volume in the nucleus of the solitary tract (NTS), rats received CTX at P5 or P10 followed by chorda tympani label, or glossopharyngeal (GL) and greater superficial petrosal (GSP) label as adults. In another group of animals, terminal field volumes and numbers of chorda tympani neurons in the geniculate ganglion (GG) were determined by labeling the chorda tympani with DiI at the time of CTX in neonatal (P5) and adult (P50) rats. There was a greater loss of chorda tympani neurons following P5 CTX compared to adult denervation. Chorda tympani terminal field volume was dramatically reduced 50 days after P5 or P10 CTX. Lack of nerve regeneration after neonatal CTX is not caused by ganglion cell death alone, as approximately 30% of chorda tympani neurons survived into adulthood. Although the total field volume of intact gustatory nerves was not altered, the GSP volume and GSP-GL overlap increased in the dorsal NTS after CTX at P5, but not P10, demonstrating age-dependent plasticity. Our findings indicate that the developing gustatory system is highly plastic and simultaneously vulnerable to injury.

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.

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.

Modeling early cortical serotonergic deficits in autism

Autism is a developmental brain disorder characterized by deficits in social interaction, language and behavior. Brain imaging studies demonstrate increased cerebral cortical volumes and micro- and macro-scopic neuroanatomic changes in children with this disorder. Alterations in forebrain serotonergic function may underlie the neuroanatomic and behavioral features of autism. Serotonin is involved in neuronal growth and plasticity and these actions are likely mediated via serotonergic and glutamatergic receptors. Few animal models of autism have been described that replicate both etiology and pathophysiology. We report here on a selective serotonin (5-HT) depletion model of this disorder in neonatal mice that mimics neurochemical and structural changes in cortex and, in addition, displays a behavioral phenotype consistent with autism. Newborn male and female mice were depleted of forebrain 5-HT with injections of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), into the bilateral medial forebrain bundle (mfb). Behavioral testing of these animals as adults revealed alterations in social, sensory and stereotypic behaviors. Lesioned mice showed significantly increased cortical width. Serotonin immunocytochemistry showed a dramatic long-lasting depletion of 5-HT containing fibers in cerebral cortex until postnatal day (PND) 60. Autoradiographic binding to high affinity 5-HT transporters was significantly but transiently reduced in cerebral cortex of 5,7-DHT-depleted mice. AMPA glutamate receptor binding was decreased at PND 15. We hypothesize that increased cerebral cortical volume and sensorimotor, cognitive and social deficits observed in both 5-HT-depleted animals and in individuals with autism, may be the result of deficiencies in timely axonal pruning to key cerebral cortical areas.

Chorda tympani nerve transection at different developmental ages produces differential effects on taste bud volume and papillae morphology in the rat

Chorda tympani nerve transection (CTX) results in morphological changes to fungiform papillae and associated taste buds. When transection occurs during neonatal development in the rat, the effects on fungiform taste bud and papillae structure are markedly more severe than observed following a comparable surgery in the adult rat. The present study examined the potential "sensitive period" for morphological modifications to tongue epithelium following CTX. Rats received unilateral transection at 65, 30, 25, 20, 15, 10, or 5 days of age. With each descending age at the time of transection, the effects on the structural integrity of fungiform papillae were more severe. Significant losses in total number of taste buds and filiform-like papillae were observed when transection occurred 5-30 days of age. Significant reduction in the number of taste pores was indicated at every age of transection. Another group of rats received chorda tympani transection at 10, 25, or 65 days of age to determine if the time course of taste bud degeneration differed depending on the age of the rat at the time of transection. Taste bud volumes differed significantly from intact sides of the tongue at 2, 8, and 50 days post-transection after CTX at 65 days of age. Volume measurements did not differ 2 days post-transection after CTX at 10 or 25 days of age, but were significantly reduced at the other time points. Findings demonstrate a transitional period throughout development wherein fungiform papillae are highly dependent upon the chorda tympani for maintenance of morphological integrity.

Patterned activity via spinal dorsal quadrant inputs is necessary for the formation of organized somatosensory maps

The normal development of the somatosensory system requires intact sensory inputs from the periphery during a critical window of time early in development. Here we determined how the removal of only part of the ascending spinal inputs early in development affects the anatomical and neurophysiological development of the somatosensory system. We performed spinal overhemisections in rat pups at C3/C4 levels on the third day after birth. This procedure hemisects the spinal cord on one side and transects the dorsal funiculus on the other side. When the rats were 6-8 months old, the responsiveness and somatotopy of the primary somatosensory cortex (S1) contralateral to the hemisection were determined using standard multiunit mapping techniques. Sections of the flattened cortex were processed for cytochrome oxidase activity, Nissl substance, or myelin. We found that histologically apparent modules that are normally present in the regions of the forepaw and the hindpaw representations were absent, whereas the lateral barrel field representing the face was completely normal. The neurons in the forepaw regions of S1 either did not respond to the stimulation of the skin of any region of the body or responded to the stimulation of the upper arm afferents that enter the spinal cord rostral to the site of the lesion. The results show that a lack of normal sensory inputs via ascending pathways in the dorsal spinal cord during early development results in massive anatomical and neurophysiological abnormalities in the cortex. Intact crossed spinothalamic pathways are unable to support the normal development of the forepaw barrels.

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.

Early (E12) cortical progenitors can change their fate upon heterotopic transplantation

To help understand how the cortical map is set up during the early stages of corticogenesis, we have examined the developmental fate of embryonic day (E) 12 cortical progenitors in the rat. We have analysed the pattern of thalamic connections and cytoarchitectonic organization developed by progenitor cells removed at E12 from the presumptive parietal or occipital cortex and grafted into the parietal cortex of newborn hosts. Occipital progenitors grafted into the parietal cortex differentiated into neurons that developed reciprocal connections with the ventrobasal complex of the host thalamus. They could also form barrel-like structures, within which axons of the ventrobasal complex were distributed in dense patches. Some of these barrel-like structures were arranged in rows. Moreover, these progenitors failed to develop characteristic traits of occipital cortex cells as they did not establish connections with the dorsal lateral geniculate nucleus. We propose that cortical progenitors are not committed at E12 and, upon heterotopic transplantation, have the capacity to respond to local cues and to subsequently differentiate and maintain major phenotypic characteristics of neurons in their new environment. Only early progenitors are multipotent. By E13/E14, indeed, most cortical cells become irreversibly committed and upon heterotopic transplantation differentiate neurons with phenotypic characteristics of their cortical site of origin (Pinaudeau et al., 2000, Eur. J. Neurosci., 12, 2486-2496).

Whisker-related neural patterns develop normally despite severe whisker defects in Msx2 knockout mice

In mice, whiskers on the snout form a highly specialized tactile organ with exquisitely patterned neural representations in the brain. Targeted deletion of the Msx2 gene leads to severe craniofacial defects, and stubby, curly whiskers. We examined the whisker pad histology, innervation, and whisker-related pattern formation along the trigeminal pathway in Msx2 -/- mice. Although the whiskers are severely deformed, whisker follicle structure, pattern and density of innervation, as well as central neural patterns in the brainstem, thalamus, and cortex appeared normal. We conclude that whisker-related neural patterns can form in the absence of normal whiskers, as long as whisker follicle innervation is intact.

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.

Gabaergic inhibition antagonizes adaptive adjustment of the owl's auditory space map during the initial phase of plasticity

We studied the influence of GABA-mediated inhibition on adaptive adjustment of the owl's auditory space map during the initial phase of plasticity. Plasticity of the auditory space map was induced by subjecting owls to a chronic prismatic displacement of the visual field. In the initial stages of plasticity, inhibition suppressed responses to behaviorally appropriate, newly functional excitatory inputs. As a result, adaptive changes in excitatory input were only partially expressed as postsynaptic spike activity. This masking effect of inhibition on map plasticity did not depend on the activity of NMDA receptors at the synapses that supported the newly learned responses. On the basis of these results, we propose that the pattern of feedforward inhibition is less dynamic than the pattern of feedforward excitation at the site of plasticity. As a result, initially in the adjustment process the preexisting pattern of feedforward GABAergic inhibition opposes changes in the auditory space map and tends to preserve the established response properties of the network. The implications of this novel role of inhibition for the functional plasticity of the brain are discussed.

Exploration of a novel environment leads to the expression of inducible transcription factors in barrel-related columns

Tactile information acquired through the vibrissae is of high behavioral relevance for rodents. Numerous physiological studies have shown adaptive plasticity of cortical receptive field properties due to stimulation and/or manipulation of the whiskers. However, the cellular mechanisms leading to these plastic processes remain largely unknown. Although genomic responses are anticipated to take place in this sequel, virtually no data so far exist for freely behaving animals concerning this issue. Thus, adult rats were placed overnight in an enriched environment and most of them were also subjected to clipping of different sets of whiskers. This type of stimulation led to a specific and statistically significant increase in the expression of the protein products of the inducible transcription factors c-Fos, JunB, inducible cyclic-AMP early repressor and Krox-24 (also frequently named Zif268 or Egr-1), but not c-Jun. The response was found in columns of the barrel cortex corresponding to the stimulated vibrissae; it displayed a layer-specific pattern. However, no induction of transcription factors was observed in the subcortical relay stations of the whisker-to-barrel pathway, i.e. the trigeminal nuclei and the ventrobasal complex. These results strongly suggest that a coordinated transcriptional response is initiated in the barrel cortex as a consequence of processing of novel environmental stimuli.

Single word reading in developmental stutterers and fluent speakers

Ten fluent speakers and nine developmental stutterers read isolated nouns aloud in a delayed reading paradigm. Cortical activation sequences were mapped with a whole-head magnetoencephalography system. The stutterers were mostly fluent in this task. Although the overt performance was essentially identical in the two groups, the cortical activation patterns showed clear differences, both in the evoked responses, time-locked to word presentation and mouth movement onset, and in task-related suppression of 20-Hz oscillations. Within the first 400 ms after seeing the word, processing in fluent speakers advanced from the left inferior frontal cortex (articulatory programming) to the left lateral central sulcus and dorsal premotor cortex (motor preparation). This sequence was reversed in the stutterers, who showed an early left motor cortex activation followed by a delayed left inferior frontal signal. Stutterers thus appeared to initiate motor programmes before preparation of the articulatory code. During speech production, the right motor/premotor cortex generated consistent evoked activation in fluent speakers but was silent in stutterers. On the other hand, suppression of motor cortical 20-Hz rhythm, reflecting task-related neuronal processing, occurred bilaterally in both groups. Moreover, the suppression was right-hemisphere dominant in stutterers, as opposed to left-hemisphere dominant in fluent speakers. Accordingly, the right frontal cortex of stutterers was highly active during speech production but did not generate synchronous time-locked responses. The speech-related 20-Hz suppression concentrated in the mouth area in fluent speakers, but was evident in both the hand and mouth areas in stutterers. These findings may reflect imprecise functional connectivity within the right frontal cortex and incomplete segregation between the adjacent hand and mouth motor representations in stutterers during speech production. A network including the left inferior frontal cortex and the right motor/premotor cortex, likely to be relevant in merging linguistic and affective prosody with articulation during fluent speech, thus appears to be partly dysfunctional in developmental stutterers.

Effects of serotonin on neurite outgrowth from thalamic neurons in vitro

Altering levels of serotonin in the primary somatosensory cortex during early postnatal life influences thalamocortical development. Recent in vivo experiments suggest that serotonin may have direct effects on the growth of thalamocortical axons, and the present study was undertaken to determine whether this amine influences process outgrowth from thalamic cells maintained in culture. Ventrobasal thalamic neurons were harvested from newborn rats and maintained in culture for eight days. At the end of this period, 0, 10, 25, 50 or 100 microM serotonin was added to the culture medium. After an additional six days, cultures were fixed and stained with neuron-specific enolase. Quantitative analysis of >500 cells from each condition indicated that 25 microM serotonin, but not the other concentrations of this amine, significantly increased the length of the primary (longest) process growing out from the cell body (P < 0.001), the total (summed) length of all processes (P < 0.0001), total neurites per cell (P < 0.05), number of branch points per cell (P < 0.01) and branch points on the primary neurite (P < 0.0005). These results demonstrate that exposing thalamic cells to serotonin increases process outgrowth from them in the absence of their cortical targets.

Infraorbital nerve transection alters serotonin transporter expression in sensory pathways in early postnatal rat development

The serotonin transporter MRNA has been found throughout the trigeminal sensory system late in gestation and during early postnatal development, a period known to be critical for maturation of the sensory circuitry. The purpose of the present study was to determine whether sensory denervation in newborn rat pups would alter either the density or pattern of expression of the 5-HT transporter (5-HTT) within the trigeminal system. We combined autoradiographic localization of 5-HT transporters and in situ hybridization techniques to visualize both the transporter protein and mRNA in thalamic sensory neurons and in the somatosensory cortex following unilateral infraorbital nerve transection at postnatal day 1. For comparative purposes, similar measurements were conducted in thalamic visual neurons as well as in the visual cortex. Lesion of the infraorbital nerve decreased the [3H]citalopram labelling of 5-HT transporters in the ventral basal and ventral medial areas of the thalamus contralateral to the lesion, while labelling of 5-HT transporters was decreased in both contralateral and ipsilateral sides of the lateral genicuate (visual thalamus). Citalopram labelling of 5-HT transporters was not significantly altered in somatosensory or in cingulate cortex, however a significant decrease was observed in the visual cortex. In contrast, there were no obvious changes in the intensity of the 5-HT mRNA hybridization signal in sensory or visual thalamic areas. Given that the serotonin transporter regulates extracellular concentrations of 5-HT, the present data suggest that altered peripheral innervation and thereby altered sensory inputs to the thalamus during fetal development can potentially influence 5-HT transporter densities and thus, may influence extracellular levels of 5-HT in thalamus and cortex during a critical period of synapse formation. In turn, modulation of 5-HT transporter levels may influence extracellular concentrations of 5-HT in thalamus and cortex during a critical period of synapse formation.

Cerebral metabolism following neonatal or adult hemineodecortication in cats: effect on oxidative capacity using cytochrome oxidase histochemistry

In order to determine the degree and extent of changes in cerebral oxidative capacity following cerebral hemineodecortication, adult cats which had undergone surgery early postnatally (mean age: 11.4 days) or during adulthood were studied using cytochrome oxidase histochemistry. A total of 18 animals were employed and 50 brain regions were quantified bilaterally using optical densitometry. Although many subcortical regions exhibiting extensive degenerative features revealed lower levels of cytochrome oxidase (C.O.) activity, this reduction was relatively unremarkable compared to intact controls. Nevertheless, it was interesting that this decrease (down to 66-89%) of normal was more pronounced in neonatal-lesioned cats, reaching significance in a number of ipsilateral thalamic nuclei, compared to adult-lesioned animals (91-100% of normal), suggesting a contribution of glial cells to the density of C.O. staining in the latter cats. Regions of the brain spared from degeneration exhibited a bilateral increase in C.O. activity which may reflect the demands for energy to support the anatomical reorganization which is prevalent in these animals. Surprisingly, such increases were more robust within spared regions of the adult-lesioned brain, reaching significance in four ipsilateral and nine contralateral areas with the density of the reaction attaining levels over 125% of control. This may indicate different demands for oxidative metabolism in the adult-lesioned cats. These results enhance our understanding of the mechanism(s) underlying the greater extent of functional sparing or recovery in cats sustaining injury to the cerebral cortex early vs. late in life. In addition, the findings complement our previous companion report on glucose metabolism supporting the concept of energy compartmentalization, which reflects the dynamic interaction between anatomical and functional changes in this age-at-lesion model of recovery.