Distribution and morphology of corticospinal tract neurons in reeler mouse cortex by the retrograde HRP method

Reeler, a recessive mutation in mice., causes cytoarchitectqnic abnormalities to the cerebral and cerebellar cortices. In normal controls corticospinal (CS) tract neurons retrogradely labelled after HRP injection into the lumbar cord were situated only in layer V (the inner pyramidal layer). In the reeler, by contrast, the labelled CS neurons were scattered diffusely throughout all levels of the corresponding cortical area. In addition to the malpositioning of the somata, the labelled CS neurons in the cortex of the reeler could be divided into two major classes according to their dendritic pattern: typical pyramidal neurons and atypical ones. The typical pyramidal neuron had an apical dendrite projecting from the superior pole of the soma and ascending toward the pia mater and several basal and basolateral dendrites projecting from the inferior pole of the soma. The atypical neurons consisted of six types: (l)inverted, (2)tumbled, (3)bipolar, (4) V-shaped, (5) hook-shaped, and (6) superficial polymorphic. The typical pyramidal neurons in the reeler tended to be situated relatively deep in the cortex and the atypical neurons tended to lie relatively superficially in the cortex. The axons of both the typical pyramidal neurons and the atypical ones in the reeler usually extended from the lower surface of the soma or one of the descending dendrites as in the normal control.     

Thalamic connectivity of the primary motor cortex of normal and reeler mutant mice

Reeler, an autosomal recessive mutation in mice, causes cytoarchitectonic abnormalities of the cerebral cortex, which are characterized by malposition of neurons. Retrograde and anterograde transport of horseradish peroxidase (HRP) was employed to examine the reciprocal connectivity between the hindlimb area of the primary motor cortex (MI) and thalamus of normal and reeler mutant mice. In the normal mouse, most of the cells labelled after HRP injection into the hindlimb area of MI were located in the ventrolateral nucleus, the lateral division of the ventrobasal nucleus, the central lateral, paracentral and central intralaminar nuclei, and the medial division of the posterior complex. HRP reaction product anterogradely transported was also observed in the same nuclei and in the thalamic reticular nucleus. In the reeler mutant mouse, retrogradely labelled neurons and anterogradely labelled terminals were again found in the nuclei referred to above, and the distribution pattern and morphology of HRP-filled neurons were also similar to those of normal controls. The present results suggest therefore that the normal reciprocal connectivity between MI (hindlimb representation) and thalamus is preserved in the reeler mouse. That is to say, dislocated cortical neurons appropriately project to their target nuclei of the thalamus, and conversely, thalamic neurons send their axons precisely to their target cortical areas of the radially disorganized cortex.     

Observations on the brainstem-spinal descending systems of normal and reeler mutant mice by the retrograde HRP method

Brainstem neurons which project to the lumbar spinal level were identified in both reeler mutant mice and normal controls (Balb/c mice) by the retrograde horseradish peroxidase (HRP) technique. In normal controls after HRP injection into the lumbar cord, retrogradely labelled neurons were observed in (1) the lateral vestibular nucleus, (2) the pontine and medullary reticular formations including the nucleus centralis caudalis pontis, nucleus gigantocellularis, nucleus paragigantocellularis, nucleus raphe magnus et pallidus, and nucleus centralis medullae oblongatae pars ventralis et dorsalis, and (3) the dorsal column nuclei, i.e., the nucleus gracilis and nucleus cuneatus medialis. In reeler mutant mice, labelled neurons were again seen in the nuclei referred to above, and their cellular type and distribution patterns within the corresponding nuclei were similar to those of the normal controls. These observations suggest that (1) the brainstem nuclei of reeler mutant mice which project to the lumbar spinal cord are cytoarchitecturally normal, (2) the reeler genetic locus (rl) does not affect the nonlaminated structures in the brainstem, at least those referred to above, and (3) the motor dysfunctions observed in the reeler, such as action tremor, dystonic posture, and reeling ataxic gait, are not attributable to the brainstem-spinal descending systems.     

Corticospinal tract neurons are radially malpositioned in the sensory-motor cortex of the shaking rat Kawasaki

Shaking rat Kawasaki (SRK) is an autosomal recessive mutant rat that exhibits tremor, dystonia, and ataxia and that is characterized by abnormal lamination of the cerebral and cerebellar cortices and the hippocampus. To examine whether or not layer V neurons in the mutant neocortex are malpositioned in accordance with the aberrant laminar cytoarchitecture, horseradish peroxidase (HRP) was injected into the lumbar spinal cord of SRK mutant and normal control rats to label cortical pyramids projecting through the corticospinal tract (CST). HRP-labeled CST neurons of both normal and SRK rats were found mainly in the hindlimb area of the sensory-motor cortex, indicating a normal tangential distribution of labeled CST neurons in the SRK mutant. In the radial axis, however, labeled CST neurons were spread throughout all layers of the mutant cortex, whereas those in normal rats were restricted to layer V. In the mutant, most labeled CST neurons located in the inner third of the cortex had a typical pyramidal form with an upright apical dendrite, but many of those located near the pial surface displayed abnormal shapes and could be subdivided into inverted pyramidal, horizontal, and bipolar neurons on the basis of their dendritic morphology. The abnormal distribution pattern of labeled CST neurons in the mutant was quantified using a standardized measure of their depth distribution, where 0% = the level of the white matter and 100% = the pial surface. The mean value for the SRK cortex of 47.0% was significantly greater than the figure of 40.5% for normal rats (P < 0.01, Student's t test), indicating a spread of CST neurons toward the pial surface in SRK, but even more striking was the size of the standard deviation: 30.4 in SRK compared with 7.1 in controls. The distribution pattern of CST neurons of the SRK rat was also statistically identical with that of the reeler mouse, which is a well-known mutant that also exhibits an abnormal lamination pattern in the cerebral cortex. These results indicate that neuronal components of the neocortex of the SRK mutant are intermingled along the radial axis, but not in the tangential axis, and provide further evidence for a strong similarity between this spontaneous rat mutation and the reeler malformation. J. Comp. Neurol. 383:370-380, 1997. © 1997 Wiley-Liss, Inc.     

Covariation of distributions of callosal cell bodies and callosal axon terminals in layer III of cat primary auditory cortex

Primary auditory cortex in the cat is both the source and target of callosal fibers. Injection of horseradish peroxidase (HRP) in the high frequency representation of AI in one hemisphere retrogradely labels callosal cell bodies and anterogradely labels callosal axon terminals in AI of the opposite hemisphere. In tissue sections cut through layer III parallel to the cortical surface, elongated patches composed of dense aggregates of callosal cell bodies and callosal axon terminals alternate with regions containing lower concentrations of these elements. Labeling in AI is most dense in regions corresponding to the frequency representation of the injected site. In layer III of the densely labeled region, patches of high concentrations of labeled callosal axon terminals correspond with high concentrations of labeled callosal cell bodies. On the other hand, little correspondence is apparent between the distributions of the two elements in layer III in the sorrounding area of lighter labeling. Layers V and VI contain relatively few labeled callosal axon terminals and cell bodies, and our data do not suggest whether the two distributions covary in these layers.     

Distribution of corticospinal motor fibres within the cervical spinal cord with special reference to the phrenic nucleus: a WGA-HRP anterograde transport study in the cat

The anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) was used to demonstrate the corticospinal fibres which originate in the motor cortex and project to the cervical spinal cord, specifically to the phrenic nucleus, in the cat. Following injections of WGA-HRP into the pericruciate cortex large numbers of fibres were labelled in the contralateral lateral and ventral funiculi and fewer fibres were labelled in the ipsilateral and ventral funiculi. Labelled corticospinal motor fibres entered the gray matter laterally in laminae V and VI and terminated within these two laminae as well as throughout the entire extent of lamina VII. A few labelled fibres were present in medial lamina VIII and also in lamina IX where they were in close association with the phrenic motoneuron pool. Labelling was present in the gray matter at both sides, with a stronger labelling contralaterally. Labelled axons were also seen crossing from each side of the gray matter to the other side. The results suggest that in the cat the corticospinal motor fibres have a wider distribution in the spinal gray matter than has been previously shown, and that corticospinal motor axons may be in direct contact with phrenic motoneurons.     

Thalamic and callosal connections of the rat auditory cortex

This study was designed to assess the relative distributions of two entrinsic afferent fiber systems in the rat auditory cortex as indicated by the patterns of specific lesion-induced degeneration evident in Fink-Heimer preparations. The auditory cortex consists of cytoarchitectural areas 41, 20 and 36. Lesions were made in the medial geniculate body (MGB) or the corpus callosum in some rats, while in other rats, lesions were made in both the MGB and the corpus callosum. Following the thalamic lesions, degenerating terminals occur throughout the auditory region of cortex, principally in layer IV and deep layer III, but also in layer VI and in the superficial part of layer I. With the exception of the band of degenerationin layer I, the density of the thalamic degeneration is uneven, such as that patches of increased density of degeneration are seperated by regions with few degenerating terminals. Following lesions of the corpus callosum, degenerating callosal terminals are also evident thoughout the auditory region of cortex and they occur in deep layer I through layer III, superficial layer V and layer VI. The dennsity of the degenerating callosal terminals is not uniform throughout most of area 41, to the extent that there are radially-oriented bands of increased density which appears within the continuous callosal projection. Following the double lesions, degenerating terminals throughout the auditory region are distributed homogenoously within all cortical layers with the exception of deep layer Vwhish is relatively free of degeneration. The results indicate that all regions within the rat auditory cortex are subject to both thalamic and callosal influence, although the input is not completely uniform, for the zones in layers IV and VI which have decreased thalamic input appear to have increased callosal input.     

A qualitative and quantitative light microscopic study of the inferior olivary complex of normal, reeler, and weaver mutant mice

In the normal mouse (+/+; +/rl) cerebellar Purkinje cells (PCs) are aligned in a monolayer and provide the main targets for incoming olivocerebellar climbing fibers (CF). In the neurological mutants, homozygous reeler (rl/rl), homozygous weaver (wv/wv) and heterozygous weaver (wv/+), cerebellar abnormalities exist in which many PCs are either missing or displaced. Therefore, it is of interest of determine if the inferior olivary complex (IO) in these mutants is also abnormal. This report concerns results obtained from a light microscopic study of the inferior olivary complex. Counts of IO cells revealed apparent differences in the IO in homozygous reeler when compared to normal littermates. Whereas in the normal mouse there are approximately 37,000 IO cells and clearly defined olivary subdivisions, the IO of the homozygous reeler has a 22.6% reduction in IO cells (mean = 28,770) and indistinct borders between the major olivary subdivisions. With regard to the heterozygous and homozygous weaver, surprisingly the IO morphology and cell numbers are similar to that of the wildtype mouse even though the animals have only 86% (wv/+, mean = 158,155) and 72% (wv/wv, mean = 131,882), respectively, of the normal numbers of PCs (+/+, mean = 183,857). Purkinje cell counts revealed that the midline vermal region is the most affected area in the cerebellum in wv/+ and wv/wv whereas counts in the lateral hemisphere are near normal. The PC/IO ratio in the homozygous weaver is approximately 3:1 as compared to 5:1 in the wildtype mouse. Recent electrophysiological findings in wv/wv indicate that PCs are multiply innervated by CFs. Since a transient phase of multiple innervation is normal in the immature rat, the situation in the adult homozygous weaver may represent a retention of this immature state. A factor which may play a role in this is the loss of parallel fiber (PF)-PC synapses resulting from massive postnatal granule cell death. An hypothesis suggesting an intrinsic PC time-dependent mutant gene effect is presented to account for the differences in the loss of Purkinje cells between wv/wv and wv/+ and between different regions of the cerebellum.     

Distribution of visual callosal neurons in normal and strabismic cats

It has been suggested that synchronous activation of cortical loci in the two cerebral hemispheres during development leads to the stabilization of juvenile callosal connections in some areas of the visual cortex. One way in which loci in opposite hemispheres can be synchronously activated is if they receive signals generated by the same stimulus viewed through different eyes. These ideas lead to the prediction that shifts in the cortical representation of the visual field caused by misalignment of the visual axes (strabismus) should change the width of the callosal zone in the striate cortex. We tested this prediction by using quantitative techniques to compare the tangential distribution of callosal neurons in the striate cortex of strabismic cats to that in normally reared cats. Animals were rendered strabismic surgically at 8–10 days of age and were allowed to survive a minimum of 18 weeks, at which time multiple intracortical injections of the tracer horseradish peroxidase (HRP) were used to reveal the distribution of callosally projecting cells in the contralateral striate cortex. HRP-labeled cells were counted in coronal sections, and data from four animals with divergent strabismus (exotropia) and four with convergent strabismus (esotropia) were compared to those from four normally reared animals. Although our data from strabismic cats do not differ markedly from those reported previously, we find that the distribution of callosal cells in the striate cortex of these cats does not differ significantly from that in our normally reared control cats. These results do not bear out the prediction that surgically shifting the visual axes leads to stabilization of juvenile callosal axons in anomalous places within the striate cortex. © 1996 Wiley-Liss, Inc.     

A change in injured corticospinal tract originating from the premotor cortex to the primary motor cortex in a patient with intracerebral hemorrhage

Many studies have attempted to elucidate the motor recovery mechanism of stroke,but the majority of these studies focus on cerebral infarct and relatively little is known about the motor recovery mechanism of intracerebral hemorrhage.In this study,we report on a patient with intracerebral hemorrhage who displayed a change in injured corticospinal tract originating from the premotor cortex to the primary motor cortex on diffusion tensor imaging.An 86-year-old woman presented with complete paralysis of the right extremities following spontaneous intracerebral hemorrhage in the left frontoparietal cortex.The patient showed motor recovery,to the extent of being able to extend affected fingers against gravity and to walk independently on even ground at 5 months after onset.Diffusion tensor imaging showed that the left corticospinal tract originated from the premotor cortex at 1 month after intracerebral hemorrhage and from the left primary motor cortex and premotor cortex at 5 months after intracerebral hemorrhage.The change of injured corticospinal tract originating from the premotor cortex to the primary motor cortex suggests motor recovery of intracerebral hemorrhage.     

Postnatal development of corticospinal axon terminal morphology in the cat

The corticospinal system undergoes important postnatal development, leading to the mature topography and specificity of connections. The purpose of this study was to determine the time-course of development of corticospinal axonal branching and varicosity density within the cervical gray matter. Corticospinal neurons were labeled after small injections of the anterograde tracer biotinylated dextran amine into the primary motor cortex of cats. Tracer injection and transport times were adjusted to examine labeling at 25, 35, 55, and 75 days and in adults. We measured the numbers and lengths of nonreconstructed terminal and preterminal branches and the numbers and locations of axon varicosities. We found significant age-dependent increases in all morphologic measures. At 25 days, corticospinal axon branching was sparse, with only a few scattered varicosities. By day 35, the mean number of branches, varicosities per branch, and varicosity density increased. Several morphologic measures did not increase between day 35 and 55, but further changes occurred between 55 days and maturity. Beginning around day 55, there was extensive development of small terminal axon branches with high densities of varicosities. We also found, by using spatial point analysis, that there was an age-dependent increase in varicosity clustering. Our results show for the first time that terminal and preterminal corticospinal axon branches increase in complexity during a protracted early postnatal period. This developmental period extended beyond the early postnatal period of activity-dependent refinement of the topography of terminations. Comparison with the time-course of maturation of the cortical motor representation revealed development of substantial, albeit incomplete, branching and varicosity density of CS axons before cortical motor circuits effectively drive their spinal targets.     

The relationship of corpus callosum connections to electrical stimulation maps of motor, supplementary motor, and the frontal eye fields in owl monkeys

Microstimulation and anatomical techniques were combined to reveal the organization and interhemispheric connections of motor cortex in owl monkeys. Movements of body parts were elicited with low levels of electrical stimulation delivered with microelectrodes over a large region of precentral cortex. Movements were produced from three physiologically defined cortical regions. The largest region, the primary motor field, M-I, occupied a 4-6-mm strip of cortex immediately rostral to area 3a. M-I represented body movements from tail to mouth in a grossly somatotopic mediolateral cortical sequence. Specific movements were usually represented at more than one location, and often at as many as six or seven separate locations within M-I. Although movements related to adjoining joints typically were elicited from adjacent cortical sites, movements of nonadjacent joints also were produced by stimulation of adjacent sites. Thus, both sites producing wrist movements and sites producing shoulder movements were found next to sites producing digit movements. Movements of digits of the forepaw were evoked at several locations including a location rostral to or within cortex representing the face. Overall, the somatotopic organization did not completely correspond to previous concepts of M-I in that it was neither a single topographic representation, nor two serial or mirror symmetric representations, nor a "nesting about joints" representation. Instead, M-I is more adequately described as a mosaic of regions, each representing movements of a restricted part of the body, with multiple representations of movements that tend to be somatotopically related. A second pattern of representation of body movements, the supplementary motor area (SMA), adjoined the rostromedial border of M-I. SMA represented the body from tail to face in a caudorostral cortical sequence, with the most rostral portion related to eye movements. Movements elicited by near-threshold levels of current were often restricted to a single muscle or joint, as in M-I, and the same movement was sometimes multiply represented. Typically, more intense stimulating currents were required for evoking movements in SMA than in M-I. A third motor region, the frontal eye field (FEF), bordered the representation of eyelids and face in M-I. Eye movements elicited from this cortex consisted of rapid horizontal and downward deviation of gaze into the contralateral visual hemifield.     

Diversity of dendritic morphology and entorhinal cortex synaptic effectiveness in mouse CA2 pyramidal neurons

Excitatory synaptic inputs from specific brain regions are often targeted to distinct dendritic arbors on hippocampal pyramidal neurons. Recent work has suggested that CA2 pyramidal neurons respond robustly and preferentially to excitatory input into the stratum lacunosum moleculare (SLM), with a relatively modest response to Schaffer collateral excitatory input into stratum radiatum (SR) in acute mouse hippocampal slices, but the extent to which this difference may be explained by morphology is unknown. In an effort to replicate these findings and to better understand the role of dendritic morphology in shaping responses from proximal and distal synaptic sites, we measured excitatory postsynaptic currents and action potentials in CA2 pyramidal cells in response to SR and SLM stimulation and subsequently analyzed confocal images of the filled cells. We found that, in contrast to previous reports, SR stimulation evoked substantial responses in all recorded CA2 pyramidal cells. Strikingly, however, we found that not all neurons responded to SLM stimulation, and in those neurons that did, responses evoked by SLM and SR were comparable in size and effectiveness in inducing action potentials. In a comprehensive morphometric analysis of CA2 pyramidal cell apical dendrites, we found that the neurons that were unresponsive to SLM stimulation were the same ones that lacked substantial apical dendritic arborization in the SLM. Neurons responsive to both SR and SLM stimulation had roughly equal amounts of dendritic branching in each layer. Remarkably, our study in mouse CA2 generally replicates the work characterizing the diversity of CA2 pyramidal cells in the guinea pig hippocampus. We conclude, then, that like in guinea pig, mouse CA2 pyramidal cells have a diverse apical dendrite morphology that is likely to be reflective of both the amount and source of excitatory input into CA2 from the entorhinal cortex and CA3.     

Events governing organization of postmigratory neurons: Studies on brain development in normal and reeler mice

The purpose of the present work is to examine some of the mechanisms responsible for the early architectonic differentiation of the central nervous system, as well as for the abnormal development which occurs in certain hereditary malformations. In order to approach these questions, the embryonic development of the cerebral cortex, the cerebellum, the inferior olivary complex and the facial nerve nucleus has been studied in normal and reeler mutant mice, using morphological methods.The adult reeler phenotype is characterized not only by extreme laminar abnormalities of cell positioning in the telencephalic and cerebellar cortices, but also by relatively less extreme, though distinct abnormal architectonics in non-cortical structures such as the inferior olive and the facial nerve nucleus. Study of the embryonic development of these structures reveals that neurons are generated at the normal time and migrate along normal pathways. Moreover, the processes of directional axonal growth, differentiation of class specific features of neurons and glia, and synaptogenesis appear similar in both genotypes and are probably not directly affected by the reeler mutation. However, in all instances, the early architectonic organization achieved by reeler cortical, Purkinje, olivary or facial neurons at the end of their migration is consistently less regular than in normal embryos. In addition, these anomalies become amplified during the later developmental period.This evidence for the early appearance of abnormalities in reeler embryos indicates that the disposition of neurons at maturity cannot be exclusively regarded as secondary to the maturation of cells, neurites and connections, but is contingent upon a specific mechanism. One may infer that the presence of a normal allele at the reeler locus is necessary for the normal completion of this histogenetic step, which consequently is submitted to genetic control.Although the factor(s) responsible for the stable configuration of the early architectonics is unknown, various hypotheses are considered. Several lines of evidence are presented which argue against a major role being played by diffusible factors, mesodermal components and afferent fiber systems. Two mechanisms are considered particularly worth evaluating: (1) a diminution of relative adhesivity between neurons and radial glial fibers at the end of migration, and (2) a stabilization of neuronal configuration by selective recognition-adhesion among postmigratory neurons. The reeler gene could, directly or indirectly, affect these cell-cell interactions.A better definition of the mechanisms responsible for the early architectonic patterning is central to our understanding of brain development in normal as well as in pathological states.     

Coherent corticomuscular oscillations originate from primary motor cortex: evidence from patients with early brain lesions

Coherent oscillations of neurons in the primary motor cortex (M1) have been shown to be involved in the corticospinal control of muscle activity. This interaction between M1 and muscle can be measured by the analysis of corticomuscular coherence in the beta-frequency range (beta-CMCoh; 14-30 Hz). Largely based on magnetoencephalographic (MEG) source-modeling data, it is widely assumed that beta-CMCoh reflects direct coupling between M1 and muscle. Deafferentation is capable of modulating beta-CMCoh, however, and therefore the influence of reafferent somatosensory signaling and corresponding neuronal activity in the somatosensory cortex (S1) has been unclear. We present transcranial magnetic stimulation (TMS) and MEG data from three adult patients suffering from congenital hemiparesis due to pre- and perinatally acquired lesions of the pyramidal tract. In these patients, interhemispheric reorganization had resulted in relocation of M1 to the contralesional hemisphere, ipsilateral to the paretic hand, whereas S1 had remained in the lesioned hemisphere. This topographic dichotomy allowed for an unequivocal topographic differentiation of M1 and S1 with MEG (which is not possible if M1 and S1 are directly adjacent within one hemisphere). In all patients, beta-CMCoh originated from the contralesional M1, in accordance with the TMS-evoked motor responses, and in contrast to the somatosensory evoked fields (SEFs) for which the sources (N20m) were localized in S1 of the lesioned hemisphere. These data provide direct evidence for the concept that beta-CMCoh reflects the motorcortical efferent drive from M1 to the spinal motoneuron pool and muscle. No evidence was found for a relevant contribution of neuronal activity in S1 to beta-CMCoh.     

Prenatal protein restriction alters synaptic mechanisms of callosal connections in the rat visual cortex

Mild prenatal protein malnutrition, induced by reduction of the casein content of the maternal diet from 25 to 8%, calorically compensated by the addition of excess carbohydrates, leads to so-called ‘‘hidden’’ malnutrition in the rat. This form of malnutrition results in normal body and brain weights of pups at birth, but in significant alterations of their central nervous system neurochemical profiles. Since severe forms of prenatal malnutrition induce morpho-functional deficits on callosal interhemispheric communication together with brain neurochemical disturbances, we evaluated, in rats born from mothers submitted to an 8% casein diet, the potassium-induced release of [¹H]-noradrenaline in visual cortex slices, as well as functional properties of callosal-cortical synapses by determining cerebral cortical excitability to callosal inputs and fatigability and temporal summation of transcallosal evoked responses. Rats born from mothers submitted to a 25% casein diet served as controls. At birth prenatally malnourished pups had significantly higher cortical percent net noradrenaline release (14.79±1.11) than controls (9.14±1.26). At 45–50 days of age, rehabilitated previously malnourished rats showed, when compared to controls : (i) significantly reduced percent net noradrenaline release in the visual cortex (4.50±0.52 vs 11.31±1.14); (ii) decreased cortical excitability to callosal inputs as revealed by significantly increased chronaxie (607.2±82.8 μs vs 351.3±47.7 μs); (iii) enhanced fatigability of transcallosal evoked responses as revealed by significantly decreased stimulus frequency required to fatigate the responses (4.9±0.8 Hz vs 9.2±1.3 Hz) ; and (iv) decreased ability of callosal-cortical synapses to perform temporal summation, as revealed by significantly reduced percent response increment to double-shock (54.2±6.2 vs 83.0±11.0, for a 3.2-ms interstimulus time interval). These changes, resulting from mild prenatal protein restriction, are discussed in relationship to developmental processes leading to the formation of synaptic contacts between callosal axons and their appropriate cortical target during perinatal age.