Variability in the distribution of callosal projection neurons in the adult rat parietal cortex

Previous reports have shown that the barrel field area of the parietal cortex of the adult rat contains relatively few callosal projection neurons, even though callosal projection neurons are abundant in this cortical region in the neonatal rat. Furthermore, it has been shown that many of the callosal neurons which seem to disappear as the animal matures do not die, but project to ipsilateral cortical areas. These findings rely on the ability of retrograde transport techniques which utilize injections of horseradish peroxidase (HRP) or of fluorescent dyes into one hemisphere. We now show that several technical modifications of the HRP technique yield a wider distribution of HRP-containing neurons in the contralateral barrel field area of the adult rat than previously reported. These include implants of HRP pellets into transected axons of the corpus callosum, the addition of DMSO and nonidet P40 to Sigma VI HRP, wheat germ agglutinin HRP and the use of tetramethyl benzidine as the chromogen in the reaction procedure. Our findings have implications for transport studies in general and for the development of the cortical barrel field in particular.     

Dopamine terminals synapse on callosal projection neurons in the rat prefrontal cortex

Dopamine (DA) afferents to the prefrontal cortex (PFC) play an important role in the cognitive functions subserved by this cortical area. Within the PFC, DA terminals synapse onto the distal dendrites of both local circuit neurons and pyramidal projection cells. We have previously demonstrated in the rat PFC that some of the dendrites and spines postsynaptic to DA terminals arise from pyramidal neurons that project to the nucleus accumbens. However, it is not known whether the pyramidal cells that give rise to callosal intercortical connections of the PFC also receive DA synaptic input. To address this question, retrograde tract tracing using an attenuated strain of pseudorabies virus (PRV-Bartha) was combined with immunocytochemistry for tyrosine hydroxylase (TH) to identify DA terminals in the PFC. Thirty-six to 40 hours following injection of PRV into the contralateral PFC, numerous callosal projection neurons were extensively labeled throughout their dendritic trees, with no evidence of PRV trans-synaptic passage. In tissue prepared for electron microscopy, labeling for PRV was distributed throughout pyramidal cell somata and extended into distal dendrites and dendritic spines. Some PRV-labeled dendrites and spines received symmetric synaptic input from terminals containing peroxidase labeling for TH. These results demonstrate that DA terminals synapse onto the distal dendrites of callosally projecting PFC neurons and suggest substrates through which DA may modulate interhemispheric cortical communication.     

Effects of prenatal exposure to ethanol on callosal projection neurons in rat somatosensory cortex

The distribution and density of callosal projection neurons in the somatosensory cortex of mature rats was altered by prenatal exposure to ethanol. The density of callosal neurons was significantly greater in ethanol-treated rats than in controls. Ethanol exposure also altered the laminar distribution of callosal projection neurons. Whereas in control rats the cell bodies of callosal projection neurons were in layers II/III and V, in ethanol-treated rats most of these neurons were distributed in layers V and VI. Many of the ectopic neurons were generated toward the end of cortical neuronogenesis (i.e., on gestational day 20). This contrasts with controls wherein co-generated cohorts were distributed in layer II/III. Thus, the connectional phenotype of the callosal projection neurons is retained regardless of its laminar residence. These ethanol-induced abnormalities apparently result from defects in neuronal migration and axonal pruning.     

The distribution of the callosal projection to the occipital visual cortex in rats and mice

The principal finding in this study is that the callosal projection to the occipital cortex in rats and mice follows a complex and highly reproducible pattern which has not previously been described in detail. In some regions, the callosal projection is associated with well defined cytoarchitectonic boundaries such as the border between areas 17 and 18a. However, extrastriate cortex lateral to area 17 receives callosal inputs which are not related to previously defined cytoarchitectonic boundaries. Following intraocular injections of [³H]fucose, transneuronal label occupies area 17 and mainly the posterior part of area 18a. A region in posterolateral area 18a which is ‘subdivided’ into callosal and sparsely callosal regions appears to receive an input from the lateral geniculate nucleus, based on transneuronal autoradiography. Comparison of the distribution of callosal axons and transneuronal label suggests that regions of murid cortex similar to areas 18, 19 and lateral suprasylvian cortex in cats may be located posteriorly in area 18a.     

Comparison of the distribution of parvalbumin-immunoreactive and other synapses onto the somata of callosal projection neurons in mouse visual and somatosensory cortex

The distribution of synapses made by parvalbumin-immunoreactive (pv-ir) and nonimmunoreactive terminals was determined for the cell bodies of callosal projection neurons in the somatosensory and visual areas of mouse cerebral cortex. Callosal neurons were labeled by the retrograde transport of horseradish peroxidase applied to the contralateral hemisphere. The surface areas of somata belonging to callosal cells in somatosensory cortex ranged from 230 to 243 μm² in size and received roughly one-third of their synapses from pv-ir terminals. Visual cortex, in contrast, contained two populations of callosal cell bodies: relatively large ones ranging in size from 255 to 279 μm² that received 3–9% of their synapses from pv-ir terminals and smaller cell bodies that both in size (232–237 μm²) and in the proportion of synapses received from pv-ir terminals resemble the callosal cells examined in somatosensory cortex. That different functional areas of the cortex have populations of callosal cells similar in size, and displaying similar patterns of somatic synapses, supports the notion that a common plan of synaptic connectivity characterizes different functional areas. Results in visual cortex indicate that functional areas contain, in addition, area-specific patterns of synapses. J. Comp. Neurol. 379:198–210, 1997. © 1997 Wiley-Liss, Inc.     

Synapses of extrinsic and intrinsic origin made by callosal projection neurons in mouse visual cortex

Neurons in areas 17/18a and 17/18b of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of labeled neurons (intrinsic terminals) were identified in the border regions of area 17 with areas 18a and 18b, and their distribution and synaptic connectivity were determined. Also examined were the synaptic connections of extrinsic callosal axon terminals labeled by lesion-induced degeneration consequent to the severing of callosal fibers. A postlesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by horseradish peroxidase. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where, with rare exception, they formed asymmetrical synapses. Layers II and III contained the highest concentrations of intrinsic and extrinsic callosal axon terminals. Analyses of serial thin sections through layers II and III in both areas 17/18a and 17/18b yielded similar results: 97% of the intrinsic (1,412 total sample) and of the extrinsic (414 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus, the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal (97%) contrasts with the proportion of asymmetrical axospinous synapses that occurs in the surrounding neuropil where about 64% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of callosal projection neurons in mouse somatosensory cortex, and lends additional weight to the hypothesis that axonal pathways are highly selective for the types of elements with which they synapse.     

The contribution of late-generated neurons to the callosal projection in the rat: A study with prenatal x-irradiation

Studies utilizing horseradish peroxidase tracing methods have suggested that there are species differences in the relative contribution of the different neocortical layers to the callosal projection. The present investigation utilized x-irradiation at different gestational ages to eliminate the late-generated neurons of the rat neocortex. The caudorostral gradient of reduction in the neuronal population of the supragranular layers is closely correlated with the gradient of reduction in the size of the corpus callosum. Furthermore, the callosal projection is absent in anteroposterior cortical segments in which the development of the supragranular layers was prevented without a reduction of the number of neurons in the infragranular layers of the neocortex. These results indicate that late-generated neurons residing primarily in the supragranular layers are essential for the formation of the corpus callosum.     

Ontogenetic change in the distribution of callosal projection neurons in the postcentral gyrus of the fetal rhesus monkey

In the postcentral gyrus of the mature rhesus monkey the distribution of callosal projection neurons is discontinuous. The density of callosal projection neurons, which are mainly located in the supragranular layers, varies both within and across cytoarchitectonic areas (Killackey et al., '83). In the present study, we investigated the ontogeny of corpus callosum projections of the postcentral gyrus in five fetal rhesus monkeys, ranging in age from embryonic day (E) 108 to E 133. Multiple large injections of horseradish peroxidase that involved the underlying white matter were made into the postcentral gyrus of one hemisphere and the distribution of labeled neurons in the ipsilateral thalamus and the other hemisphere was determined. The pattern of thalamic label indicated that the tracer was effectively transported from all portions of the postcentral gyrus. We found that the areal distribution pattern of labeled callosal projection neurons varied at the different fetal ages. At early fetal ages (E 108, E 111, and E 119) callosal projection neurons were continuously distributed throughout the postcentral gyrus. As in the adult animal, the vast majority of labeled callosal projection neurons were found in the supragranular layers, although a few labeled cells were located in the infragranular layers. From the earliest age, there was regional variation in the width of the band of labeled supragranular callosal projection neurons. The difference between the precentral and postcentral gyrus was most obvious, but there was also a difference between anterior and posterior portions of the postcentral gyrus. The first indication of some discontinuity in the distribution of callosal projection neurons was noted at E 126. By E 133, approximately 1 month before birth, the distribution of callosal projection neurons appeared remarkably mature. On E 119 aggregations of anterograde label could be detected in restricted portions of the posterior postcentral gyrus beneath the cortical layers. By E 133 anterograde label was found within the cortical layers (most densely in layer IV) in these regions of the postcentral gyrus. Thus, the emergence of the discrete pattern of callosal projection neurons appears to be temporally correlated with the ingrowth of callosal afferents. On the basis of these observations, as well as those of others (discussed in the text), we propose that the ontogenetic changes in the distribution of callosal projection neurons reflect the unique strategy employed by cortical projection neurons in establishing their patterns of connectivity. It is hypothesized that this strategy may involve multiple processes.     

Properties of antidromically activated callosal neurons and neurons responsive to callosal input in rabbit binocular cortex

Extracellular spikes were recorded from the cell bodies of antidromically activated callosal axons in the binocular visual cortex of unanesthetized, unparalyzed rabbits. Callosal axons were stimulated near their terminals in the contralateral cortex. Recordings were also obtained from neurons which responded synaptically to contralateral cortical stimulation. The primary method for differentiating antidromic from synaptic activation was the test for collision of impulses. Additional tests provided further confirmation of antidromic activation. Units which sent an axon across the corpus callosum (callosal neurons) were thereby distinguished from units which responded synaptically to callosal input. Eighteen percent of units sampled sent an axon across the corpus callosum. The median conduction velocity of callosal axons was less than 2 m/sec. An additional 18% of units encountered were synaptically activated by contralateral cortical stimulation. Callosal neurons were found to differ from synaptically activated units in three distinct ways. Callosal neurons had very low spontaneous firing rates (median =< 1.0 spike/sec), responded with a single spike to contralateral cortical stimulation and never responded to diffuse flash illumination. In contrast, most synaptically activated units demonstrated high spontaneous firing rates (median = 10.2 spikes/sec), responded with a burst of spikes to contralateral cortical stimulation and were also driven by diffuse flash illumination.     

Synapses made by axons of callosal projection neurons in mouse somatosensory cortex: emphasis on intrinsic connections

This is one of a series of papers aimed at identifying the synaptic output patterns of the local and distant projections of subgroups of pyramidal neurons. The subgroups are defined by the target site to which their main axon projects. Pyramidal neurons in areas 1 and 40 of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of these neurons ("intrinsic" terminals) were identified in somatosensory areas 1 and 40, and their distribution and synaptic connectivity were examined. Also examined were the synaptic connections of "extrinsic" callosal axon terminals labeled by lesion induced degeneration consequent to the severing of callosal fibers. A post-lesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by HRP. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where they formed only asymmetrical synapses. Layers II and III contained the highest concentrations of both types of callosal axon terminal. Analyses of serial thin sections through layers II and III in both areas 1 and 40 yielded similar results: 97% of the extrinsic (277 total sample) and of the intrinsic (1215 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal contrasts with the proportion of asymmetrical, axospinous synapses that occur in the surrounding neuropil where only about 80% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of both extrinsic and intrinsic axonal pathways in the cortex (White and Keller, 1989: Cortical Circuits; Boston: Birkhauser): in all instances, axonal pathways are highly selective for the types of elements with which they synapse.     

Age-related changes in the distribution of visual callosal neurons following monocular enucleation in the rat

The distribution of visual callosal neurons was identified in tangential sections from flattened hemispheres of normal rats and animals unilaterally enucleated at various postnatal ages with the retrogradely transported fluorescent label, Fast Blue. Following enucleation on or after postnatal day 10, callosal neurons along the 17/18a border appear adult-like in their configuration. Enucleation prior to this age stabilizes callosal development in the hemisphere contralateral to the enucleated eye.     

A slice preparation preserving the callosal projection to contralateral visual cortex

Due to the curved path they follow, the visual callosal projections to areas OC1 and OC2 of the rat visual cortex have been inaccessible to studies using brain slices. In this paper we describe a new slice preparation in which a curved cutting blade was used to obtain slices in which callosal fibers projecting to OC1 or OC2 are preserved. Stimulation of the contralateral white matter resulted in EPSPs recorded in layer II/III and V cells of OC2 studied with intracellular recording. Current source density analysis of extracellular field potentials collected in OC1 and OC2 revealed laminar current sink patterns paralleling the laminar distribution of callosal terminations reported by Miller and Vogt (Dev. Brain Res., 14 (1984) 304-309). Exposure of slices to 2 mM kynurenic acid reversibly abolished current sinks in OC1 recorded in response to callosal stimulation indicating that glutamate receptors mediate the response of OC1 to callosal afferent activity. This new slicing technique can be readily adapted to study other systems in the nervous system in which neural processes follow curved trajectories.     

Time of origin of cortico-collicular projection neurons in the rat visual cortex.

The time of origin and the radial gradient of neurogenesis of cortico-collicular neurons have been studied in the rat visual area 17. We used a combined technique for the histochemical detection of the retrogradely transported horseradish peroxidase from the superior colliculus and the autoradiographic detection of the [3H]-thymidine administered during the gestational period. The cortico-collicular neurons of visual area 17 are located in layer V and are generated on gestational day (GD) 15 (59.78%), GD 16 (36.21%), and GD 17 (4.01%). This finding reveals that, for the cortico-collicular neuronal population, the birth date is well-correlated with the laminar position in the adult animal. To see whether the cortico-collicular neurons located at various radial levels of layer V are generated concurrently, or whether they follow an "inside-out" pattern of positioning, we divided layer V into three (upper, middle and lower) sublaminae. Most cortico-collicular neurons located in the lower two-thirds of layer V are generated on GD 15 (65%), whereas the neurons located in the upper third of the layer are generated both on GD 15 and GD 16 in almost equal proportions (52.53% and 44.39%, respectively).     

大鼠脑干前庭核团向大脑前庭皮层的投射研究

目的观察Wistar大鼠脑干前庭核团是否存在直接向大脑皮质的纤维投射。方法健康Wistar大鼠20只,随机分为实验组(10只)和对照组(10只)。实验组脑干前庭内、外侧核团注射绿色荧光标记的顺行示踪剂刀豆凝集素,对照组脑干前庭内、外侧核团注射生理盐水。5d后处死大鼠,行大脑连续冰冻切片,荧光显微镜下观察荧光细胞在大脑皮质的分布及形态。结果绿色荧光标记的神经元主要位于大脑皮质的前肢感觉区、后肢感觉区和本体感觉区。结论 Wistar大鼠脑干前庭核团存在直接向大脑皮质的纤维投射,部分和本体感觉区相重叠。     

Morphological characteristics of layer II projection neurons in the rat medial entorhinal cortex

The entorhinal cortex receives inputs from a variety of neocortical regions. Neurons in layer II of the entorhinal cortex originate one component of the perforant path which conveys this information to the dentate gyrus and hippocampus. The current study extends our previous work on the electroresponsive properties of layer II neurons of the medial entorhinal cortex in which we distinguished two categories of layer II neurons based on their electrophysiological attributes (Alonso and Klink [1993] J Neurophysiol 70:128–143). Here we report on the morphological features of layer II projection neurons, as revealed by in vitro intracellular injection of biocytin. We now report that the two electrophysiologically distinct types of neurons correspond to morphologically distinct types of cells. All neurons (65% of the total cells recorded) that developed sustained, subthreshold, sinusoidal membrane potential oscillations were found to have a stellate appearance. Neurons that did not exhibit oscillatory behavior had either a pyramidal-like (32%) or a horizontal cell morphology (3%). Stellate cells had multiple, thick, primary dendrites. Their widely diverging upper dendritic domain expanded mediolaterally over a distance of around 500 μm close to the pial surface. This mediolateral extent was more than double that of the pyramidal-like cells. Dendrites of stellate cells demonstrated long dendritic appendages, and their dendritic spines had a more complex morphology than those of nonstellates. The stellate cell axons emerged from a primary dendrite and were more than double the thickness (approximately 1.4 μm) of the axons of nonstellate cells. Recurrent axonal collaterization appeared more extensive in axons arising from stellate cells than from pyramidal-like cells. Hippocampus 1997;7:571–583. © 1997 Wiley-Liss, Inc.     

Development, specification, and diversity of callosal projection neurons

Callosal projection neurons (CPN) are a diverse population of neocortical projection neurons that connect the two hemispheres of the cerebral cortex via the corpus callosum. They play key roles in high-level associative connectivity, and have been implicated in cognitive syndromes of high-level associative dysfunction, such as autism spectrum disorders. CPN evolved relatively recently compared to other cortical neuron populations, and have undergone disproportionately large expansion from mouse to human. While much is known about the anatomical trajectory of developing CPN axons, and progress has been made in identifying cellular and molecular controls over midline crossing, only recently have molecular-genetic controls been identified that specify CPN populations, and help define CPN subpopulations. In this review, we discuss the development, diversity and evolution of CPN.     

Do callosal projection neurons reflect sex differences in axon number?

We have reported that female rats have more axons in the splenium of the corpus callosum than do male rats (12). To determine if the greater number of axons found in female rats might be reflected in a larger distribution of callosal projection neurons, horseradish peroxidase (HRP) was injected into the visual cortex of 55-65-day-old rats of both sexes that had been housed in a complex environment since weaning. The pattern of labeled neurons was examined in tangential sections in the cortex contralateral to the injection site, and three-dimensional reconstructions were quantified at the area 17/18a border and in area 18b. Male and female rats were found to have indistinguishable distributions of labeled callosal projection neurons. The present study failed to find an obvious difference in the distribution of projection neurons as the basis for the sex differences in axon number, but because of the limitations of tracing techniques, subtle differences cannot be excluded.     

Widespread callosal connections in infragranular visual cortex of the rat

Following multiple injections of HRP into the posterior cortex of one hemisphere of adult rats, dense and overlapping distributions of retrogradely labeled cells and anterogradely labeled terminations are observed throughout the depth of the cortex in the region of the border between the lateral portion of area 17 and area 18 in the opposite hemisphere. In contrast to previous studies of the visual callosal pathway, we also find large numbers of labeled callosal cells extending throughout areas 17 and 18 in cortical layers Vc and VIa.