Early development of the representation of the body surface in SI cortex barrel field in neonatal rats as demonstrated with peanut agglutinin binding: evidence for differential development within the rattunculus

Lower jaw-to-forepaw rapid and delayed reorganization in the rat forepaw barrel subfield in primary somatosensory cortex

We used the forepaw barrel subfield (FBS), that normally receives input from the forepaw skin surface, in rat primary somatosensory cortex as a model system to study rapid and delayed lower jaw-to-forepaw cortical reorganization. Single and multi-unit recording from FBS neurons was used to examine the FBS for the presence of "new" lower jaw input following deafferentations that include forelimb amputation, brachial plexus nerve cut, and brachial plexus anesthesia. The major findings are as follows: (1) immediately following forelimb deafferentations, new input from the lower jaw becomes expressed in the anterior FBS; (2) 7-27 weeks after forelimb amputation, new input from the lower jaw is expressed in both anterior and posterior FBS; (3) evoked response latencies recorded in the deafferented FBS following electrical stimulation of the lower jaw skin surface are significantly longer in both rapid and delayed deafferents compared to control latencies for input from the forepaw to reach the FBS or for input from lower jaw to reach the LJBSF; (4) the longer latencies suggest that an additional relay site is imposed along the somatosensory pathway for lower jaw input to access the deafferented FBS. We conclude that different sources of input and different mechanisms underlie rapid and delayed reorganization in the FBS and suggest that these findings are relevant, as an initial step, for developing a rodent animal model to investigate phantom limb phenomena.

Terminal Arbors of Callosal Axons Undergo Plastic Changes in Early-Amputated Rats

Sensory information is processed in specific brain regions, and shared between the cerebral hemispheres by axons that cross the midline through the corpus callosum. However, sensory deprivation usually causes sensory losses and/or functional changes. This is the case of people who suffered limb amputation and show changes of body map organization within the somatosensory cortex (S1) of the deafferented cerebral hemisphere (contralateral to the amputated limb), as well as in the afferented hemisphere (ipsilateral to the amputated limb). Although several studies have approached these functional changes, the possible finer morphological alterations, such as those occurring in callosal axons, still remain unknown. The present work combined histochemistry, single-axon tracing and 3D microscopy to analyze the fine morphological changes that occur in callosal axons of the forepaw representation in early amputated rats. We showed that the forepaw representation in S1 was reduced in the deafferented hemisphere and expanded in the afferented side. Accordingly, after amputation, callosal axons originating from the deafferented cortex undergo an expansion of their terminal arbors with increased number of terminal boutons within the homotopic representation at the afferented cerebral hemisphere. Similar microscale structural changes may underpin the macroscale morphological and functional phenomena that characterize limb amputation in humans.

Bilateral enucleation at birth modifies calcium spike amplitude, but not frequency, in neurons of the somatosensory thalamus and cortex: Implications for developmental cross-modal plasticity

Bilateral eye enucleation at birth (BE) leads to an expansion of the primary somatosensory cortex (S1) in rat pups. Although increased growth of the somatosensory thalamo-cortical afferents (STCAs) in part explains S1 expansion, timing mechanisms governing S1 formation are also involved. In this work, we begin the search of a developmental clock by intending to document the existence of putative clock neurons in the somatosensory thalamus (VPM) and S1 based upon changes of spontaneous spike amplitude; a biophysical property sensitive to circadian regulation; the latter known to be shifted by enucleation. In addition, we also evaluated whether STCAs growth rate and segregation timing were modified, as parameters the clock might time. We found that spontaneous spike amplitude transiently, but significantly, increased or decreased in VPM and S1 neurons of BE rat pups, respectively, as compared to their control counterparts. The growth rate and segregation timing of STCAs was, however, unaffected by BE. These results support the existence of a developmental clock that ticks differently in the VPM and S1 after BE. This observation, together with the fact that STCAs growth rate and segregation timing is unchanged, suggests that S1 expansion in BE rats may in part be controlled at the cortical level.

Changes to the somatosensory barrel cortex in C57BL/6J mice at early adulthood (56 days post-natal) following prenatal alcohol exposure

Children with Fetal Alcohol Spectrum Disorder (FASD) have impaired sensory processing skills as a result of neurodevelopmental anomalies. The somatosensory barrel field of rodent brain is a readily accessible model for studying the effects of alcohol exposure. Within the barrel field, the posterior medial barrel subfield (PMBSF) receives sensory inputs from the large vibrissae on the contralateral face. This study reports on the consequence of prenatal exposure to alcohol on the somatosensory cortices of mice later in life. Two control groups, a sucrose and a non-treated control, were also examined. At postnatal day (PND) 56 the cerebral hemisphere of mice from each group were processed for cytochrome oxidase reactivity. In contrast to previous studies, there were no significant differences in the mean areas of: (I) the PMBSF enclosure, (II) the PMBSF barrels, (III) the individual PMBSF barrels and (IV) the septal portion of the PMBSF in the alcohol group compared to the controls. However barrel sizes in rows D and E in the alcohol group were significantly reduced, indicating an alcohol-induced damage on the barrel development and which may reduce the amount of the cortex devoted to processing somatosensory input- a common defect seen in children with FASD.

Hypergravity within a critical period impacts on the maturation of somatosensory cortical maps and their potential for use-dependent plasticity in the adult

We investigated experience-dependent plasticity of somatosensory maps in rat S1 cortex during early development. We analyzed both short- and long-term effects of exposure to 2G hypergravity (HG) during the first 3 postnatal weeks on forepaw representations. We also examined the potential of adult somatosensory maps for experience-dependent plasticity after early HG rearing. At postnatal day 22, HG was found to induce an enlargement of cortical zones driven by nail displacements and a contraction of skin sectors of the forepaw map. In these remaining zones serving the skin, neurons displayed expanded glabrous skin receptive fields (RFs). HG also induced a bias in the directional sensitivity of neuronal responses to nail displacement. HG-induced map changes were still found after 16 wk of housing in normogravity (NG). However, the glabrous skin RFs recorded in HG rats decreased to values similar to that of NG rats, as early as the end of the first week of housing in NG. Moreover, the expansion of the glabrous skin area and decrease in RF size normally induced in adults by an enriched environment (EE) did not occur in the HG rats, even after 16 wk of EE housing in NG. Our findings reveal that early postnatal experience critically and durably shapes S1 forepaw maps and limits their potential to be modified by novel experience in adulthood.

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.

Early postnatal alcohol exposure reduced the size of vibrissal barrel field in rat somatosensory cortex (SI) but did not disrupt barrel field organization

Prenatal alcohol exposure (PAE) has been shown to alter the somatosensory cortex in both human and animal studies. In rodents, PAE reduced the size, but not the pattern of the posteromedial barrel subfield (PMBSF) associated with the representation of the whiskers, in newborn, juvenile, and adult rats. However, the PMBSF is not present at birth, but rather first appears in the middle of the first postnatal week during the brain-growth spurt period. These findings raise questions whether early postnatal alcohol exposure might disrupt both barrel field pattern and size, questions that were investigated in the present study. Newborn Sprague-Dawley rats were assigned into alcohol (Alc), nutritional gastric control (GC), and suckle control (SC) groups on postnatal day 4 (P4). Rat pups in Alc and GC were artificially fed with alcohol and maltose-dextrin dissolved in milk, respectively, via an implant gastrostomy tube, from P4 to P9. Pups in the Alc group received alcohol (6.0 g/kg) in milk, while the GC controls received isocaloric equivalent maltose-dextrin dissolved in milk. Pups in the SC group remained with their mothers and breast fed throughout the experimental period. On P10, pups in each group were weighed, sacrificed, and their brains removed and weighed. Cortical hemispheres were separated, weighed, flattened, sectioned tangentially, stained with cytochrome oxidase, and PMBSF measured. The sizes of barrels and the interbarrel septal region within PMBSF, as well as body and brain weights were compared between the three groups. The sizes of PMSBF barrel and septal areas were significantly smaller (P<.01) in Alc group compared to controls, while the PMBSF barrel pattern remained unaltered. Body, whole-brain, forebrain, and hemisphere weights were significantly reduced (P<.01) in Alc pups compared to control groups. GC and SC groups did not differ significantly in all dependent variables, except body weight at P9 and P10 (P<.01). These results suggest that postnatal alcohol exposure, like prenatal exposure, significantly influenced the size of the barrel field, but not barrel field pattern formation, indicating that barrel field pattern formation consolidated prior to P4. These results are important for understanding sensorimotor deficits reported in children suffering from fetal alcohol spectrum disorder (FASD).

Prenatal alcohol exposure delays the development of the cortical barrel field in neonatal rats

In-utero alcohol exposure produces sensorimotor developmental abnormalities that often persist into adulthood. The rodent cortical barrel field associated with the representation of the body surface was used as our model system to examine the effect of prenatal alcohol exposure (PAE) on early somatosensory cortical development. In this study, pregnant female rats were intragastrically gavaged daily with high doses of alcohol (6 gm/kg body weight) throughout the first 20 days of pregnancy. Blood alcohol levels were measured in the pregnant dams on gestational days 13 (G13) and G20. The ethanol treated group (EtOH) was compared to the normal control chowfed (CF) group, nutritionally matched pairfed (PF) group, and cross-foster (XF) group. Cortical barrel development was examined in pups across all treatment groups from G25, corresponding to postnatal day 2 (P2), to G32 corresponding to P9. The EtOH and control group pups were weighed, anesthetized, and perfused. Brains were removed and weighed with, and without cerebellum and olfactory bulbs, and neocortex was removed and weighed. Cortices were then flattened, sectioned tangentially, and stained with a metabolic marker, cytochrome oxidase (CO) to reveal the barrel field. Progression of barrel development was distinguished into three categories: (a) absent, (b) cloudy barrel-like pattern, and (c) well-formed barrels with intervening septae. The major findings are: (1) PAE delayed barrel field development by one or more days, (2) the barrel field first appeared as a cloudy pattern that gave way on subsequent days to an adult-like pattern with clearly demarcated intervening septal regions, (3) the barrel field developed differentially in a lateral-to-medial gradient in both alcohol and control groups, (4) PAE delayed birth by one or more days in 53% of the pups, (5) regardless of whether pups were born on G23 (normal expected birth date for non-alcohol controls) or as in the case for the alcohol-delayed pups born as late as G27, the barrel field was never present at birth suggesting the importance of postnatal experience on barrel field development, and (6) PAE did not disrupt the normal barrel field pattern, although both total body and brain weights were compromised. These findings suggest that PAE delays the development of the somatosensory cortex (SI); such delays may interfere with timing and formation of cortical circuits. It is unknown whether other nuclei along the somatosensory pathway undergo similar delays in development or if PAE selectively disrupts cortical circuitry.

Genetic analysis of barrel field size in the first somatosensory area (SI) in inbred and recombinant inbred strains of mice

We measured the combined area of posterior medial barrel subfield (PMBSF) and anterior lateral barrel subfield (ALBSF) areas in four common inbred strains (C3H/HeJ, A /J, C57BL /6J, DBA/2J), B6D2F1, and ten recombinant inbred (RI) strains generated from C57BL/6J and DBA/2J progenitors (BXD) as an initial attempt to examine the genetic influences underlying natural variation in barrel field size in adult mice. These two subfields are associated with the representation of the whisker pad and sinus hairs on the contralateral face. Using cytochrome oxidase labeling to visualize the barrel field, we measured the size of the combined subfields in each mouse strain. We also measured body weight and brain weight in each strain. We report that DBA/2J mice have a larger combined PMBSF/ALBSF area (6.15 +/- 0.10 mm(2), n = 7) than C57BL /6J (5.48 +/- 0.13 mm(2), n = 10), C3H/HeJ (5.37 +/- 0.16 mm(2), n = 10), and A/J mice (5.04 +/- 0.09 mm(2), n = 15), despite the fact that DBA/2J mice have smaller average brain and body sizes. This finding may reflect dissociation between systems that control brain size with those that regulate barrel field area. In addition, BXD strains (average n = 4) and parental strains showed considerable and continuous variation in PMBSF/ALBSF area, suggesting that this trait is polygenic. Furthermore, brain, body, and cortex weights have heritable differences between inbred strains and among BXD strains. PMBSF/ALBSF pattern appears similar among inbred and BXD strains, suggesting that somatosensory patterning reflects a common plan of organization. This data is an important first step in the quantitative genetic analysis of the parcellation of neocortex into diverse cytoarchitectonic zones that vary widely within and between species, and in identifying the genetic factors underlying barrel field size using quantitative trait locus (QTL) analyses.

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.

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.

Effects of monoamine oxidase A inhibition on barrel formation in the mouse somatosensory cortex: determination of a sensitive developmental period

Genetic inactivation of monoamine oxidase A (MAOA) in C3H/HeJ mice causes a complete absence of barrels in the somatosensory cortex, and similar alterations are caused by pharmacological inhibition of MAOA in wild type mice. To determine when and how MAOA inhibition affects the development of the barrel field, the MAOA inhibitor clorgyline was administered to mice of the outbred strain OF1 for various time periods between embryonic day 15 (E15) and postnatal day 7 (P7), and the barrel fields were analyzed with cytochrome oxidase and Nissl stains in P10 and adult mice. High-pressure liquid chromatography measures of brain serotonin (5-HT) showed three- to eightfold increases during the periods of clorgyline administration. Perinatal mortality was increased and weight gain was slowed between P3 and P6. Clorgyline treatments from E15 to P7 or from P0 to P7 disrupted the formation of barrels in the anterior snout representation and in parts of the posteromedial barrel subfield (PMBSF). Treatments from P0 to P4 caused similar although less severe barrel field alterations. Clorgyline treatments only during embryonic life or starting on P4 caused no detectable abnormalities. In cases with barrel field alterations, a rostral-to-caudal gradient of changes was noted: Rostral barrels of the PMBSF were most frequently fused and displayed an increased size tangentially. Thus, MAOA inhibition resulting in increased brain levels of 5-HT affects barrel development during the entire first postnatal week, with a sensitive period between P0 and P4. The rostral-to-caudal gradient of changes in the barrel field parallels known developmental gradients in the sensory periphery and in the maturation thalamocortical afferents. The observed barrel fusions could correspond to a default in the initial segregation of thalamic fibers or to a continued, exuberant growth of these fibers that overrides the tangential domain that is normally devoted to individual whiskers.

Lectin-binding properties of oligodendrocyte lineage cells aid in defining functionally important surface proteins

Oligodendrocytes are the myelin-forming cells of the central nervous system. They develop from migratory and proliferative precursor cells, which differentiate to mature myelinating cells. As a first step toward investigating the expression of cell surface glycoproteins by oligodendrocyte lineage cells, we tested 14 different lectins for their binding to oligodendrocyte lineage cells. Peanut agglutinin (PNA) was the only lectin used that showed a differentiation stage-dependent binding to oligodendrocytes. PNA-binding molecules are specifically expressed by oligodendrocyte precursor cells, downregulated with differentiation, and reexpressed by mature oligodendrocytes. It was additionally observed that PNA stimulates the proliferation of oligodendrocyte precursor cells. PNA may therefore be a useful tool for isolating and characterizing important cell surface glycoproteins expressed by oligodendrocyte lineage cells.

Patterning and specification of the cerebral cortex

Regionalization of the cerebral cortex occurs during development by the formation of anatomically and functionally discrete areas of the brain. Descriptive evidence based on expression of molecules and structural features suggests that an early parcelation of the cerebral wall may occur during fetal development. Experimental strategies using tissue transplants and cell culture models have explored the nature of the timing of areal specification. New signaling systems displaying the sensitivity of precursor cells to environmental cues that define the fate of neurons destined for specific areas of the cortex have been discovered. Studies in the field now suggest mechanisms of regulating cell phenotype in the cortex that are common to all parts of the neuraxis.

Differential synaptic vesicle protein expression in the barrel field of developing cortex

The distribution of four proteins associated with synaptic vesicles, SV2, synaptophysin, synapsin I, and rab3a, was investigated during postnatal development of the posteromedial barrel subfield (PMBSF) in the rat somatosensory cortex. A distinct progression in the appearance of the different synaptic vesicle proteins within the PMBSF was observed. SV2, synapsin I, and synaptophysin revealed the organization of the barrel field in the neonate. This early demarcation of the cortical representation of the vibrissal array coincides with the earliest known age for the emergence of the cytoarchitectonic organization of this region. In contrast, rab3a did not delimit the barrels until the end of the 1st postnatal week, coincident with the known onset of adult-like physiological activity and the loss of plasticity in afferents to this region. In addition, the appearance of the different synaptic vesicle proteins occurred earlier within the PMBSF than in the adjacent extra-barrel regions of the cortex. These results show that the molecular differentiation of synaptic fields across the cortex is not a homogeneous and synchronous process in terms of synaptic vesicle protein expression. Because these proteins act together in mature synapses to ensure the regulated release of neurotransmitters, our results suggest that this temporo-spatial asynchrony may underlie different potentials for synaptic activity and thus contribute to the development of cortical maps.

Transient expression of NADPH diaphorase activity in the mouse whisker to barrel field pathway

Development of the topographic map of the somatosensory cortex of rodents appears to depend on fine-tuned patterns of neuronal activity. Nitric oxide (NO) has been described as a potent messenger in the modulation of neural activity associated with synaptic plasticity. To evaluate the role of NO in the murine somatosensory pathway, we investigated NO synthase activity by NADPH diaphorase histochemistry at crucial developmental stages. At birth, NADPH diaphorase activity was detected in the cortical plate of the developing somatosensory cortex. At day 3, diffuse NADPH diaphorase activity increased within the emerging layer 4 in the future barrel field hollows. This staining was most intense at day 6 in the barrel field hollows and became undetectable by the end of the second postnatal week. The appearance of the diffuse NADPH diaphorase staining pattern was also observed in a similar time course and topography in the ascending relays of the somatosensory cortex, specifically in the barreloids within the ventrobasal nucleus of the thalamus and the barrelettes of the trigeminal nucleus of the brainstem. Lesioning the C row of whiskers at day 1 (i.e. during the critical period of barrel formation) led to fused C barrels of diffuse NADPH diaphorase activity in the barrel fields. In addition, highly NADPH diaphorase activity-positive individual cells present in the deeper layers of the somatosensory cortex at days 0 and 3 became visible in the upper layers at day 6 and remained until day 15. In layer 4, these cells were predominantly localized in the septa at day 6 and 9. No positive individual cells were detected in barrelettes or barreloids at any age. We conclude that NADPH diaphorase activity is present during experience-dependent consolidation of synaptic contacts in the somatosensory pathway.

Regulation of alpha7 nicotinic acetylcholine receptors in the developing rat somatosensory cortex by thalamocortical afferents

Distributions of alpha7 nicotinic acetylcholine receptor (nAChR) mRNA and [125]alpha-bungarotoxin (alpha-BTX) binding sites in the developing rat somatosensory cortex were characterized in relation to acetylcholinesterase (AChE) histochemical staining of thalamocortical terminals to investigate the role of this receptor in cortical development. Using quantitative in situ hybridization and receptor autoradiography, elevated levels of mRNA and binding-site expression were first detected at post-natal day 1 (P1) in deep and superficial layers, just beneath the AChE-stained thalamocortical terminals. Onset of expression occurred approximately 1 d after ingrowth of AChE-stained thalamocortical afferents. By P5, mRNA and binding-site expression exhibited a disjunctive, barrel-like pattern in layer IV and, more clearly, in layer VI. The mRNA and binding-site expressions peaked at approximately 1 week postnatal and then declined to adult levels. Unilateral electrolytic or cytochemical lesions placed in the thalamic ventrobasal complex at P0 (just as thalamocortical afferents are innervating the cortex) and at P6 (when the somatotopic map is well established) resulted in a marked reduction of alpha7 nAChR mRNA and [125]alpha-BTX binding-site levels in layers IV and VI, indicating their regulation by thalamocortical afferents. With P6 lesions, this reduction was observed as early as 6 hr postlesion. These results suggest that alpha7 nAChRs are localized primarily on cortical cells in rat somatosensory cortex and provide further evidence for thalamocortical influence on cortical ontogeny. These data also suggest a role for cholinergic systems during a critical period of cortical synaptogenesis.

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

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

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.

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.

Expression of the extracellular matrix glycoprotein tenascin in the somatosensory cortex of the mouse during postnatal development: an immunocytochemical and in situ hybridization analysis

Layer 4 of the rodent somatosensory cortex contains the barrel field which is the cortical representation of the whisker pad located on the contralateral side of the face. Each barrel within the barrel field is related one to one to its corresponding whisker both anatomically and physiologically. The astrocyte-derived extracellular matrix glycoprotein tenascin has been shown by immunocytochemistry to delineate the boundaries between barrels during their formation until the end of the second postnatal week. The present study describes the anatomical localization of tenascin mRNA expressing cells in the somatosensory cortex of the mouse from birth to postnatal day 15. During this time, a general down-regulation of tenascin-specific message was observed as a function of the state of maturation, with layers 5 and 6 down-regulating the message earlier than layers 1 and 2/3. Tenascin (as detected by immunocytochemistry) also revealed this gradual down-regulation with maturation. Layer 4 of the somatosensory cortex was different in that, with the onset of formation of barrel field boundaries at postnatal day 3, tenascin protein and mRNA were down-regulated more in layer 4 than in the upper and the lower layers of the somatosensory cortex and, interestingly, not in layer 4 of adjacent cortical areas. At postnatal day 6 tenascin immunoreactivity was most clearly distinguished in the barrel field boundaries while tenascin-specific mRNA was no longer detectable in layer 4. Down-regulation of tenascin message was also seen at P6 at the level of the enlarged barrel corresponding to an early postnatal lesioned row of whiskers. At postnatal day 15, tenascin protein and mRNA were no longer detectable in the somatosensory cortex. Distribution of glial fibrillary acidic protein immunoreactivity did not reveal any preferential accumulation of GFAP-positive radial glial processes in barrel field hollows versus barrel field boundaries at any stage.

Potential of visual cortex to develop an array of functional units unique to somatosensory cortex

The identification of specialized areas in the mammalian neocortex, such as the primary visual or somatosensory cortex, is based on distinctions in architectural and functional features. The extent to which certain features that distinguish neocortical areas in rats are prespecified or emerge as a result of epigenetic interactions was investigated. Late embryonic visual cortex transplanted to neonatal somatosensory cortex was later assayed for "barrels," anatomically identified functional units unique to somatosensory cortex, and for boundaries of glycoconjugated molecules associated with barrels. Barrels and boundaries form in transplanted visual cortex and are organized in an array that resembles the pattern in the normal barrelfield. These findings show that different regions of the developing neocortex have similar potentials to differentiate features that distinguish neocortical areas and contribute to their unique functional organizations.

The emergence of architectonic field structure and areal borders in developing monkey sensorimotor cortex

Adult monkey sensorimotor cortex consists of several structurally and functionally distinct areas. The developmental sequence through which the characteristic architectonic features and the borders of these areas become resolved was examined in a series of fetal, postnatal and adult monkeys by using Nissl staining, cytochrome oxidase and acetylcholinesterase histochemistry, and immunocytochemistry for GABA and the neuropeptides somatostatin, neuropeptide Y, substance P and cholecystokinin. At the youngest fetal age examined (E110), the pre- and postcentral gyri possess six clearly delineated cellular layers; populations of GABA- and neuropeptide-immunoreactive cells can be identified, but their somatic sensory cortex at E110 lacks areal cytoarchitectonic parcellation. Despite the apparent homogeneity in the cytoarchitecture of the somatic sensory cortex, incipient areal borders are revealed by staining for cytochrome oxidase and acetylcholinesterase activity, and by staining immunocytochemically for several neuropeptides. The motor cortex at E110 differs from that in adults by the presence of a prominent layer IV; a clear cytoarchitectonic border between areas 3a and 4 is detectable at E110, which is also revealed by chemoarchitectonic markers. With increasing age, the characteristic architectonic features gradually emerge and areal cytoarchitectonic borders appear, becoming adult-like by early postnatal ages. The gradual changes in cytoarchitecture are paralleled by redistributions of GABA- and neuropeptide-immunoreactive cells and fiber plexuses. The data demonstrate that the progressive refinement in cytoarchitectonic features and in the distributions of neurotransmitter- and peptide-containing cells occurs primarily during the latter third of gestation. The major changes are temporally coincident with the ingrowth of afferent axonal systems, suggesting that the establishment of connectivity may be capable of modulating finer details of structural or molecular phenotype, particularly intra-areal cytoarchitectonic features and neurotransmitter or peptide expression.

Thalamic axons confer a blueprint of the sensory periphery onto the developing rat somatosensory cortex

In order to study the role of afferents in the maturation of cortical axons projecting from the ventrobasal thalamic complex (VB) to the barrel field (SI) cortex were labeled with the carbocyanine dye DiI, in aldehyde-fixed embryonic and newborn rat brains. Our results reveal that the first few thalamic axons are in the cortical plate by embryonic day (E) 19. Between E19 and the day of birth (E21 = PND 0), layers V and VI differentiate from the lower part of the cortical plate. On PND 0, a plexus of growth-cone tipped thalamic axons is present within the cortical plate and a few VB fibers have reached the marginal zone. Increasing numbers of thalamic afferents invade and ramify within the cortical plate on PND 1 and, over the course of the next 24 h, form a vibrissa-specific pattern in the lower part of this zone, prior to the differentiation of layer IV into a distinct lamina. This periphery-related organization is exhibited by VB afferents earlier than reported for other afferents to the cortex, by glia- or neuron-associated extracellular elements or by the cytoarchitectonic specializations (barrels) of stellate cells. Our observations, in conjunction with the previous studies, demonstrate that thalamic afferents may have a pivotal role in determining the morphological specification of the primary somatosensory cortex.