Mouse barrel cortex viewed as Dirichlet domains.

Barrels are patterned groups of neurons in rodent somatosensory cortex that correspond one to one with the animal's facial whiskers. Dirichlet domains are a class of convex polygon found frequently in nature, often arising by nucleation from center points. Analytic and graphical methods were devised to verify the hypothesis that Dirichlet domains accurately describe the adult barrel fields of normal mice. We found that normal barrel fields and abnormal barrel fields caused by supernumerary whiskers or lesions to the whisker pad are closely approximated by this mathematical formalism. This implies that each developing cortical barrel organizes about a center point. Experiments in neonatal animals (Senft and Woolsey, 1991a) demonstrate foci in the thalamocortical afferent (TCA) distributions. These results support an hypothesis in which TCAs are the nucleating agents causing barrels to organize as Dirichlet domains. This is made possible because TCA terminals from each barreloid (a whisker-related group of cells in the ventrobasal complex of the thalamus) initially colonize somatosensory cortex with an approximately "Gaussian" distribution. These peaked groups of related TCAs behave as Dirichlet domain centers. They generate barrel structures competitively, in animals with normal or with perturbed whisker patterns, via statistical epigenetic interactions within and between distinct TCA Gaussians associated with separate whiskers. This leads to selective axon outgrowth and pruning of single TCA branches, regulated by the TCA population, and creates beneath each Gaussian the dense knot of related TCA arbors typical of the barrel cortex. Similar parcellation of neuronal processes into contending subgroups having spatially coherent actions could lead to nucleation of other geometric patterns as Dirichlet domains elsewhere in the brain.     

Differential Expression of Acetylcholinesterase in the Developing Barrel Cortex of Three Rodent Species

Acetylcholinesterase (AChE) is transiently expressed in severalimmature axon systems. Its presence in developing thalamocorticalafferents has led to the use of enzyme histochemistry to visualizethis axon system in rats. Because of the spatiotemporal distributionof the enzyme in the rat neocortex. it has been suggested thatAChE plays a role in the establishment of thalamocortical connectivity. We show here that AChE is distributed in a pattern that is markedlydifferent in SI cortex of rats as compared to that of mice andhamsters. In rat pups, AChE-rich patches are distributed ina vibrissa-related array in the SI cortex soon after birth,whereas regions of cortex that lie between individual patches,and between rows of patches, are impoverished in the enzyme.In contrast. sections through flattened cortices from PND3 andolder mice and hamsters reveal lightly stained, AChE-positivespots in the center of barrel cores, while barrel walls remaindevoid of AChE; septae that divide individual barrels are denselyenzyme positive. Differences in laminar localization of theenzyme for all three species are also visible. In the thalamus of postnatal rats, both the ventral posteriormedial (VPM) and ventral posterior lateral (VPL) nuclei expressAChE, correlating with the presence of enzyme-containing patchesthroughout the barrelfield cortex. In the other two rodents,however, the enzyme is present in VPL but not in VPM, despitethe fact that in these species the cortical barrels associatedwith both thalamic nuclei have very little of the enzyme. Thus,the relationship between the distribution of AChE in nucleiof the thalamic ventrobasal complex and the presence of AChEin the terminals of their cortical axons in the barrelfieldis not consistent across different rodent species. Our results call for caution in the use of AChE histochemistryas a universal marker for immature thalamocortical axons, andchallenge the generality of currently hypothesized roles forthis transiently expressed enzyme during the development ofthe rodent thalamocortical projection.     

Embryonic and postnatal development of the layer I-directed ("matrix") thalamocortical system in the rat

Inputs to the layer I apical dendritic tufts of pyramidal cells are crucial in "top-down" interactions in the cerebral cortex. A large population of thalamocortical cells, the "matrix" (M-type) cells, provides a direct robust input to layer I that is anatomically and functionally different from the thalamocortical input to layer VI. The developmental timecourse of M-type axons is examined here in rats aged E (embryonic day) 16 to P (postnatal day) 30. Anterograde techniques were used to label axons arising from 2 thalamic nuclei mainly made up of M-type cells, the Posterior and the Ventromedial. The primary growth cones of M-type axons rapidly reached the subplate of dorsally situated cortical areas. After this, interstitial branches would sprout from these axons under more lateral cortical regions to invade the overlying cortical plate forming secondary arbors. Moreover, retrograde labeling of M-type cell somata in the thalamus after tracer deposits confined to layer I revealed that large numbers of axons from multiple thalamic nuclei had already converged in a given spot of layer I by P3. Because of early ingrowth in such large numbers, interactions of M-type axons may significantly influence the early development of cortical circuits.     

A comparison of pattern formation by thalamocortical and serotonergic afferents in the rat barrel field cortex.

In the present study we compare the formation of vibrissa-related patterns by thalamocortical afferents from the ventrobasal (VB) nucleus to that by raphe-cortical, serotonergic afferents from the raphe nuclei. In opposite hemispheres of the same brain, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) labeling of VB afferents and serotonin (5-HT) immunohistochemistry reveal that the pattern of vibrissa-specific patches is first exhibited by thalamocortical axons in barrel field cortex. Shortly thereafter, 5-HT axons form patches in the same locations as the dense concentrations of VB afferents. To assess a possible role of 5-HT afferents in the specification of barrel field cortex, neonatal rat pups were administered p-chloroamphetamine (PCA), a selective 5-HT neurotoxin. The formation of vibrissa-related patterns by thalamocortical and serotonergic afferents was compared in normal and PCA-treated rat pups at various developmental stages. PCA treatment led to a significant decrease in the number of serotonergic axons in barrel field cortex. Despite this decrease, VB afferents segregated in patches over individual barrels. However, a delay in the emergence of the thalamocortical pattern was noted in toxin-treated animals. We conclude that PCA treatment does not prevent formation of a vibrissa-specific pattern by thalamocortical afferents, and discuss the possibility that the 5-HT axons may play a trophic role in the maturation of VB afferents.     

Morphogenesis of callosal arbors in the parietal cortex of hamsters

The morphogenesis of callosal axons originating in the parietal cortex was studied by anterograde labeling with Phaseolus lectin or biocytin injected in postnatal (P) hamsters aged 7-25 days. Some labeled fibers were serially reconstructed. At P7, some callosal fibers extended as far as the contralateral rhinal fissure, with simple arbors located in the homotopic region of the opposite cortical gray matter, and two or three unbranched sprouts along their trajectory. From P7 to P13, the homotopic arbors became more complex, with branches focused predominantly, but not exclusively, in the supra- and infragranular layers of the homotopic region. Simultaneously, the lateral extension of the trunk axon in the white matter became shorter, finally disappearing by P25. Arbors in the gray matter were either bilaminar (layers 2/3 and 5) or supragranular. A heterotopic projection to the lateral cortex was consistently seen at all ages; the heterotopic arbors follow a similar sequence of events to that seen in homotopic regions. These observations document that callosal axons undergo regressive tangential remodeling during the first postnatal month, as the lateral extension of the trunk fiber gets eliminated. Radially, however, significant arborization occurs in layer-specific locations. The protracted period of morphogenesis suggests a correspondingly long plastic period for this system of cortical fibers.     

RORβ induces barrel-like neuronal clusters in the developing neocortex

Neurons in layer IV of the rodent whisker somatosensory cortex are tangentially organized in periodic clusters called barrels, each of which is innervated by thalamocortical axons transmitting sensory information from a single principal whisker, together forming a somatotopic map of the whisker pad. Proper thalamocortical innervation is critical for barrel formation during development, but the molecular mechanisms controlling layer IV neuron clustering are unknown. Here, we investigate the role in this mapping of the nuclear orphan receptor RORβ, which is expressed in neurons in layer IV during corticogenesis. We find that RORβ protein expression specifically increases in the whisker barrel cortex during barrel formation and that in vivo overexpression of RORβ is sufficient to induce periodic barrel-like clustering of cortical neurons. Remarkably, this clustering can be induced as early as E18, prior to innervation by thalamocortical afferents and whisker derived-input. At later developmental stages, these ectopic neuronal clusters are specifically innervated by thalamocortical axons, demonstrated by anterograde labeling from the thalamus and by expression of thalamocortical-specific synaptic markers. Together, these data indicate that RORβ expression levels control cytoarchitectural patterning of neocortical neurons during development, a critical process for the topographical mapping of whisker input onto the cortical surface.     

Semaphorin 5B is a novel inhibitory cue for corticofugal axons

Neuronal connectivity is generated by the precise guidance of neuronal growth cones in response to the spatiotemporal distribution of molecular guidance cues in the developing embryo. Here we show that the class 5 semaphorin, Semaphorin 5B, is expressed in regions of the cortex and subcortex flanking the projection of and avoided by descending cortical axons, suggesting a role as a repulsive guidance cue in the formation of the internal capsule. Axons from cortical explants cultured in vitro with Semaphorin 5B-expressing cells exhibited characteristic avoidance behaviors. In organotypic slices, ectopic Semaphorin 5B expression along the presumptive internal capsule was sufficient to cause cortical axons to avoid their normal trajectory, resulting in either stalling at the boundary of Semaphorin 5B or turning into inappropriate areas of the cortex. In contrast, thalamocortical axons were not inhibited either in vitro or in slice culture by ectopic Semaphorin 5B. To further examine the function of Semaphorin 5B in situ, we knocked down its expression in the ventricular zone (VZ) at the corticostriatal angle. We found that labeled cortical fibers aberrantly navigated into the VZ where Semaphorin 5B expression was reduced. We propose that Semaphorin 5B functions to prevent corticofugal axons from abnormally projecting into germinal regions as they project to their subcortical targets.     

Emergence of connectivity in the embryonic rat parietal cortex.

In order to understand how cortical circuitry is put together, we examined the emergence of corticofugal projection cells and the arrival of subcortical afferents in the presumptive parietal cortex of the embryonic rat cerebrum. Afferent and efferent projections were selectively labeled by applications of the lipophilic tracers DiI and DiA in aldehyde-fixed brains of 12-18-d-old rat embryos (E12-E18; gestation: 21 d). On E12 and E13, the neocortical anlage consists of a ventricular zone and a preplate, with no extracortical connections. By E14, just prior to the appearance of the cortical plate, polymorphic cells located in the ventrolateral preplate of the telencephalic vesicle send out the first group of corticofugal axons toward the ganglionic eminence. Shortly thereafter, the cortical plate emerges as a dense band of radially oriented cells that also contribute to the corticofugal projection. By E15, axons of the early cortical projection cells cascade through the striatal primordium, the future site of the internal capsule. At the time of cortical plate formation and initial corticofugal axon outgrowth, ascending corticopetal axon systems have not yet arrived in the neocortex. Double-labeling experiments in which one dye is placed in the neocortex and the other in the ipsilateral dorsal thalamus reveal that cortical efferents encounter the first ascending wave of thalamofugal axons at the level of the striatum. Collectively, these two axonal systems bridge the necortex and the diencephalon. Upon their arrival in the neocortex on E16, thalamic axons follow a ventrolateral to dorsomedial course within the intermediate zone. Thalamic axons are the first subcortical afferent system to arrive in the neocortex. Other ascending afferent systems arising from the midbrain tegmentum enter the neocortex after E17. Comparison of thalamocortical and tegmentocortical projections in two halves of the same brain and across various embryonic ages clearly reveals that the two projection systems differ in their trajectories as well as in their time of arrival. Present observations challenge the view that the precocious arrival of subcortical axons provides the impetus for cortical maturation, and suggest that cortical plate differentiation and the initial organization of corticofugal projection patterns occur independent of ascending pathways.     

Thalamic ablations and neocortical development: alterations in thalamic and callosal connectivity.

Corticofugal pathways (callosal, intracortical, and subcortical) have initial axon outgrowth to many areas where no adult connections will persist. Corticofugal projections also demonstrate considerable reorganization after early damage. At the level of gross projections from specific thalamic nuclei to cortical cytoarchitectonic areas, early thalamocortical projections appear to show greater specificity for their targets than do corticofugal projections, and their potential for reorganization after early damage is not known. In this article, we explore the nature of the reorganization shown by the thalamocortical system after early thalamic lesions, and contrast it with reorganization of the origin of contralateral visual callosal projections in the same animals. Hamster pups were given electrolytic lesions in the posterior thalamus on the day of birth, damaging principally either the ventrobasal (somatosensory) or the dorsal lateral geniculate (visual) nucleus. After 30 d of age, HRP was implanted in either the somatosensory or the visual cortex, matching the area of implant with the intended thalamic lesion. The thalamus was reconstructed to determine the remaining nuclei, and the distribution of retrogradely labeled cells was plotted. For animals with HRP implants in visual cortex, the location of callosally projecting cells from the contralateral cortex was charted. These animals were compared to a group of normal adult animals with HRP implants approximately matched for size and location. In seven of eight adult animals with neonatal thalamic lesions, the remaining thalamus did not reorganize to innervate the thalamically denervated cortex. In contrast, the callosal projections from the contralateral visual cortex showed a wider tangential origin in the experimental animals compared to the controls. This expanded callosal projection included cells from temporal cortex, a projection not seen in normal animals. Thus, thalamocortical and callosal projection systems differ in both the magnitude and the nature of their reorganization after early damage.     

The role of L1 in axon pathfinding and fasciculation

The neural cell adhesion molecule L1 has been found to play important roles in axon growth and fasciculation. Our main objective was to determine the role of L1 during the development of connections between thalamus and cortex. We find that thalamocortical and corticothalamic axons in mice lacking L1 are hyperfasciculated, a subset of thalamocortical axons make pathfinding errors and thalamocortical axon growth cones are abnormally long in the subplate. These defects occur despite formation of six cortical layers and formation of topographically appropriate thalamocortical connections. The loss of L1 is accompanied by loss of expression of ankyrin-B, an intracellular L1 binding partner, suggesting that L1 is involved in the regulation of Ank2 stability. We postulate that the pathfinding errors, growth cone abnormalities and hyperfasciculation of axons following loss of L1 reflect both a shift in binding partners among axons and different substrates and a loss of appropriate interactions with the cytoskeleton.     

Critical period plasticity of axonal arbors of layer 2/3 pyramidal neurons in rat somatosensory cortex: layer-specific reduction of projections into deprived cortical columns

We examined the effect of whisker trimming during early postnatal development on the morphology of axonal arbors in rat somatosensory cortex. Axonal arbors from populations of layer 2/3 pyramidal neurons in the D2 column were labeled by lentivirus-mediated expression of green fluorescent protein. Axonal projection patterns were compared between untrimmed control animals and animals with all whiskers in A-, B-, and C-rows trimmed (D- and E-rows left intact) from postnatal days 7 to 15 (termed from here on DE-pairing). Control animals had approximately symmetrical horizontal projections toward C- and E-row columns in both supra- and infragranular layers. Following DE-pairing, the density of axons in supragranular layers projecting from the labeled neurons in the D2 column was higher in E- than in C-row columns. This asymmetry resulted primarily from a reduction in projection density toward the deprived C-row columns. In contrast, no change was observed in infragranular layers. The results indicate that DE-pairing during early postnatal development results in reduced axonal projection from nondeprived into deprived columns and that cortical neurons are capable of structural rearrangements at subsets of their axonal arbors.     

VPM and PoM nuclei of the rat somatosensory thalamus: intrinsic neuronal properties and corticothalamic feedback

Sensory information originating in individual whisker follicles ascends through focused projections to the brainstem, then to the ventral posteromedial nucleus (VPM) of the thalamus, and finally into barrels of the primary somatosensory cortex (S1). By contrast, the posteromedial complex (PoM) of the thalamus receives more diffuse sensory projections from the brainstem and projects to the interbarrel septa of S1. Both VPM and PoM receive abundant corticothalamic projections from S1. Using a thalamocortical slice preparation, we characterized differences in intrinsic neuronal properties and in responses to corticothalamic feedback in neurons of VPM and PoM. Due to the plane of the slice, the majority of our observed responses came from activation of layer VI because most or all of the layer V axons terminating in PoM are cut. We found that VPM neurons exhibit higher firing rates than PoM neurons when stimulated with injected current. Stimulation of corticothalamic fibers evoked monosynaptic excitation, disynaptic inhibition, or a combination of the two in both nuclei. A few differences in the feedback responses emerged: purely excitatory postsynaptic potentials (EPSPs) in VPM were smaller and facilitated more than those in PoM, and only the EPSPs in VPM had a strong NMDA component. For both nuclei, some of the feedback responses were purely disynaptic inhibitory postsynaptic potentials (IPSPs) from the thalamic reticular nucleus (TRN). This was due to EPSP failures within VPM and PoM combined with greater reliability of S1-originating synapses onto TRN. These findings suggest that despite the exclusively excitatory nature of corticothalamic fibers, activation of cortex can trigger excitation or inhibition in thalamic relay neurons.     

Developmental history of the subplate and developing white matter in the murine neocortex. Neuronal organization and relationship with the main afferent systems at embryonic and perinatal stages

The neuronal diversity of the subplate and developing white matter in the mouse was studied using a variety of neuronal markers. The subplate was first visible in lateral cortical areas at E13, coinciding with the emergence of the cortical plate. During prenatal development, this layer was formed by morphologically heterogeneous neurons, subsets of which were immunoreactive for GABA- and calcium-binding proteins. From E18 onwards, a few subplate cells also contained neuropeptides. Colocalization experiments demonstrated that the percentages of neurons immunoreactive for each antigen were similar to those described in adult neocortex. By E15, subplate cells had received synaptic contacts. Moreover, a second early-neuronal population was conspicuous from E13 in the lower intermediate zone: the intermediate-subventricular population. Unlike subplate cells, these neurons were morphologically uniform, smaller and horizontally oriented. Nevertheless, a few of these cells also appeared within the ventricular zone, with a perpendicular/ oblique orientation. Most of these cells were GABA-positive and showed calbindin immunoreactivity. At the electron microscopic level, no synaptic contacts were found in these neurons. Tracing studies using DiI showed that subplate neurons were the first to send axons outside the neocortex towards the ganglionic eminence at E13. At E14, subplate axons and ingrowing thalamic fibers met in the striate primordium. Subplate cells retained their projection to the thalamus during prenatal development. Thalamocortical axons reached the subplate at E15, and 1 day later began to invade the upper cortical layers. Early callosal axons, in contrast, did not run through the subplate to reach the contralateral hemisphere, nor did subplate cells send out callosal fibers. Callosal axons ran just above the subventricular zone, intermingled with the intermediate-subventricular neuronal population. We conclude that the subplate neuronal population has a chemical heterogeneity reminiscent of that of the adult cortex and is crucial to the establishment of thalamocortical relationships, whereas the intermediate-subventricular neurons constituted a particular GABAergic population, which includes resident cells and tangentially migrating postmitotic neurons spatially related to the development of callosal connections.     

Chronic cognitive deficits and long-term histopathological alterations following contusive brain injury in the immature rat

Although diffuse axonal injury is the primary pathology in pediatric brain trauma, the additional presence of focal contusions may contribute to the poor prognosis in brain-injured children younger than 4 years of age. Because existing models of pediatric brain trauma focus on diffuse brain injury, a model of contusive brain trauma was developed using postnatal day (PND) 11 and 17 rats, ages that are neurologically equivalent to a human infant and toddler, respectively. Closed head injury was modeled by subjecting the intact skull over the left parietal cortex of the immature rat to an impact with a metal-tipped indenter. Brain trauma on PND11 or PND17 led to significant spatial learning deficits at 28 days post-injury, compared to age-matched control rats (p < 0.05). Although both groups of rats sustained skull fractures on impact, the histopathologic response of the brain was distinctly age-dependent. At 3 days post-injury in PND11 rats, the cortex below the impact site was contused and hemorrhagic, and contained reactive astrocytes, while the subcortical white matter and thalamus contained injured (swollen) axons. At 14 and 28 days post-injury, the cortex, white matter, and hippocampus were substantially atrophied, and the lateral ventricle was enlarged. In contrast, in PND17 rats, the contused cortex observed at 3 days post-injury matured into a pronounced cavity lined with a glia limitans at 14 days; reactive astrocytes were present in both the hippocampus and thalamus up to 28 days post-injury. No evidence of traumatic axonal injury was observed in any region of the brain-injured PND17 rat. These data suggest that contusive brain trauma in the immature rat is associated with chronic cognitive deficits, but underscore the effect of the age-at-injury on behavioral and histopathologic outcomes.     

Early commitment of embryonic neocortical cells to develop area-specific thalamic connections

In this study we examined the thalamic connectivity developed by grafts of embryonic (E16) parietal or occipital cortex placed homo- or heterotopically into the neocortex of newborn rats. We also examined the cytoarchitectonic organization developed by the grafts. Our findings indicate that E16 parietal cortex grafted into the parietal cortex of newborn recipients develops reciprocal connections with the host thalamic ventrobasal complex (VB) but does not establish connections with the host dorsal lateral geniculate nucleus (DLG). When implanted into the occipital cortex, these grafts are still able to establish connections with the VB. In contrast, E16 occipital cortex grafted into the parietal cortex establishes only a few connections with the VB. These grafts are, however, able to develop a substantial system of connections with the host DLG. At 16 days of embryonic age, graft cells are committed to establish thalamic connections appropriate to their tangential locus of origin. In addition, our results show that E16 parietal or occipital cortical cells do not possess the capacity to differentiate and maintain barrel organization even though they are grafted into the terminal field of developing VB axons.     

Inhibitory synaptogenesis in mouse somatosensory cortex

It is widely believed that inhibitory synapses are not present or present in only small numbers in the rodent cerebral cortex during the early postnatal period when the cortex is being innervated by thalamocortical fibers. Quantitative electron microscopy was carried out on the posteromedial barrel subfield of mouse somatosensory cortex from postnatal day 4 (P4) when thalamocortical innervation of the barrels is becoming established, through to sexual maturity (>P32), and in adulthood. Both asymmetrical (putatively excitatory) and symmetrical (putatively inhibitory) synapses were present in all layers from P4. The symmetrical synapses were immunoreactive for GABA at all ages. There was a progressive increase in both asymmetrical and symmetrical synapses up to P32, density in all layers increasing 16-fold, with the production of asymmetrical synapses leading and greatly outstripping that of symmetrical. From P32 to P120, the oldest age studied, synaptic numbers declined by 18% to 13 times the P4 level, but this affected predominantly layers II/III, IV and V, and mainly involved asymmetrical synapses. The relative percentage of asymmetrical to symmetrical synapses from P4 to P8 was 57%/43% but at P32 it was 89.5%/10.5% and in adulthood 85.4%/14.6%. These data indicate that inhibitory synaptogenesis in the rodent cortex begins earlier than previously thought, a basis for inhibition being present from the earliest period. Pruning of all synapses occurs well after thalamocortical innervation is established and inhibitory synapses are less affected by the pruning process.      

Role of thalamic axons in the expression of H-2Z1, a mouse somatosensory cortex specific marker.

In the H-2Z1 mouse line, postnatal expression of the lacZ containing transgene in the cerebral cortex is restricted to layer IV neurons of the somatosensory area. We have used H-2Z1 embryos in previous heterotopic transplantation experiments to investigate the chronology of determination of areal identity. From the onset of neurogenesis, the cortex was regionalized in domains fated to express or not the somatosensory area-specific transgene. Determination occured 1 day later. In the present study, we show that, in vivo, H-2Z1 expression coincides with invasion of the cortical plate by thalamic afferents. We therefore investigated the role of thalamic innervation in the onset of H-2Z1 expression. For this purpose, we examined the pattern of H-2Z1 expression in perinatal cortical explant, in reeler mutant and MaoA deficient mice, or in animals which had received neonatal lesions affecting the somatosensory cortex or the thalamocortical projection. We found that, around birth, a switch occurs in the control of H-2Z1 expression: whereas H-2Z1 expression developed autonomously in embryonic parietal cortex in the absence of thalamic fibers, a transient requirement for a thalamic axon derived signal was observed postnatally. This property has interesting implications for the plasticity of cortical areas in development and evolution.     

Layer-specific production of nitric oxide during cortical circuit formation in postnatal mouse brain

In the developing cerebral cortex, neuronal nitric oxide synthase (nNOS) is expressed abundantly, but temporarily. During the early postnatal stage, cortical neurons located in the multi-layered structure of the cortical plate start forming well-organized cortical circuits, but little is known about the molecular machinery for layer-specific circuit formation. To address the involvement of nitric oxide (NO), we utilized a new NO indicator (DAR-4M) and developed a protocol for the real-time imaging of NO produced in fresh cortical slices upon N-methyl-D-aspartic acid stimulation. At postnatal day 0 (P0), NO production was restricted to the deep layers (layers V and VI) of the somatosensory cortex where transient synapses are formed. At P10, the production of NO was expanded to layer IV where large numbers of thalamocortical axons form synapses. The pattern of NO production could correspond to active sites for synaptic formation. This study is the first clear demonstration of NO production in the postnatal mouse neocortex. The findings presented may reflect a function of NO in relation to the layer-specific development of neural circuits in the neocortex.     

Postnatal development of tyrosine hydroxylase- and dopamine transporter- immunoreactive axons in monkey rostral entorhinal cortex

Dopamine afferents from the mesencephalon appear to play a critical role in the normal development and cognitive functions of multiple areas of the primate cerebral cortex. In some regions, such as the prefrontal and motor cortices, dopamine innervation changes substantially during postnatal development. However, little is known about the postnatal maturation of dopamine afferents to the primate rostral entorhinal cortex, a periallocortical region that receives a dense dopamine innervation in adults. In this study, we used immunocytochemical techniques and antibodies against tyrosine hydroxylase and the dopamine transporter to examine the postnatal development of dopamine axons in the rostral subdivision of macaque monkey entorhinal cortex. Within animals, the axons labeled with each antibody did not differ in overall density or laminar distribution. Across development, the density of dopamine axons in layers I and VI did not change appreciably. In contrast, the density of labeled axons in layer III significantly increased by a factor of three between birth and 5-7 months of age. The timing of this change differs substantially from that observed in prefrontal cortex, where peak dopamine innervation occurs between 2 and 3 years of age. These findings, in concert with other data, suggest that developmental changes in the dopamine innervation of cortical regions may parallel the functional maturation of those areas.