Functional mapping of neural pathways in rodent brain in vivo using manganese-enhanced three-dimensional magnetic resonance imaging

This work presents three-dimensional MRI studies of rodent brain in vivo after focal and systemic administration of MnCl2. Particular emphasis is paid to the morphology and dynamics of Mn2+-induced MRI signal enhancements, and the physiological mechanisms underlying cerebral Mn2+ uptake and distribution. It turns out that intravitreal and intrahippocampal injections of MnCl2 emerge as useful tools for a delineation of major axonal connections in the intact central nervous system. Subcutaneous administrations may be exploited to highlight regions involved in fundamental brain functions such as the olfactory bulb, inferior colliculus, cerebellum and hippocampal formation. Specific insights into the processes supporting cerebral Mn2+ accumulation may be obtained by intraventricular MnCl2 injection as well as by pharmacologic modulation of, for example, hippocampal function. Taken together, Mn2+-enhanced MRI opens new ways for mapping functioning pathways in animal brain in vivo with applications ranging from assessments of transgenic animals to follow-up studies of animal models of human brain disorders.     

Is sprouting the result of a persistent neonatal connection?

The present study aims to test the hypothesis that early neonatal connections might account for certain sprouting phenomena in the central nervous system. Fink-Heimer's method for degenerating axons, electron microscopy and autoradiography were used to trace the neonatal corticofugal projections from the sensorimotor cortex (SMC) of albino rats. The results showed that neonatal corticofugal projections to the superior colliculus, red nucleus and spinal cord were unilateral and could not explain the bilateral corticofugal projections to these areas soon after neonatal SMC lesions [8, 10, 12, 13]. The presence of a neonatal contralateral corticopontine projection is also not sufficient to account for the presence of a contralateral corticopontine projection seen after neonatal unilateral SMC lesion, but might account for the contralateral corticopontine projection seen after neonatal unilateral cerebellar hemispherectomy [11].     

磁共振DTI技术和锰离子示踪方法活体定位鼠脑神经纤维

本文应用磁共振(MR)DTI的方法清晰准确描绘出鼠脑白质内主要神经纤维的位置、方向和形态,包括胼胝体、前联合、内囊、外囊、视束、大脑脚、锥体束等。皮质内注射MnCl2后MR成像,可见皮质脊髓束、皮质丘脑束、皮质纹状体束和纹状体黑质通路被示踪,可重复性地表现为特定部位的MR3D-FSPGR图像和T1W像高信号改变,相应区域T2WI为低信号。LFB染色各纤维束位置、形态与DTI-DEC张量图和MnCl2示踪MR图像结果相一致。因此,磁共振DTI技术和锰离子示踪的方法都能够对鼠脑主要神经纤维和投射通路进行定位,能够作为活体示踪脑内神经纤维束的手段。     

An anterograde HRP-WGA study of aberrant corticorubral projections following neonatal lesions of the rat sensorimotor cortex

Summary Anterograde transport of horseradish peroxidase — wheat germ agglutinin (HRP-WGA) was used to examine the effect of unilateral neonatal ablation of the sensorimotor cortex on the remaining corticofugal projections to the midbrain in the rat. In unlesioned animals, the sensorimotor cortical efferents to the midbrain were entirely ipsilateral, terminal labeling being evident in the red nucleus, the midbrain reticular formation, the periaqueductal gray, the intermediate gray layer of the superior colliculus, the nucleus parafascicularis prerubralis and the perilemniscal area. Corticorubral fibers were seen to reach the midbrain through the thalamus or the cerebral peduncle. In the red nucleus, terminal labeling was essentially restricted to the parvocellular region. In neonatally lesioned adults, aberrant corticofugal fibers crossed the midline to terminate in the contralateral red nucleus, the midbrain reticular formation, the periaqueductal gray, the nucleus parafascicularis prerubralis and the intermediate gray layer of the superior colliculus. The aberrant projections maintained the topographic specificity of the normal ipsilateral projections. This was most evident in the corticorubral projection, where the aberrant contralateral fibers terminated in the parvocellular area of the red nucleus.Abbreviations 3V Third ventricle - Aq Cerebral aqueduct - CP Cerebral peduncle - DG Deep gray layer of the superior colliculus - fr Fasciculus retroflexus - HRP-WGA Horseradish peroxidase — wheat germ agglutinin - IG Intermediate gray layer of the superior colliculus - mc Magnocellular red nucleus - pc Parvocellular red nucleus - RF Reticular formation - SG Superficial gray layer of the superior colliculus - SMC Sensorimotor cortex     

Topography of projections from the primary and non-primary auditory cortical areas to the medial geniculate body and thalamic reticular nucleus in the rat

The functional significance of parallel and redundant information processing by multiple cortical auditory fields remains elusive. A possible function is that they may exert distinct corticofugal modulations on thalamic information processing through their parallel connections with the medial geniculate body and thalamic reticular nucleus. To reveal the anatomical framework for this function, we examined corticothalamic projections of tonotopically comparable subfields in the primary and non-primary areas in the rat auditory cortex. Biocytin was injected in and around cortical area Te1 after determining best frequency at the injection site on the basis of epicortical field potentials evoked by pure tones. The rostral part of area Te1 (primary auditory area) and area temporal cortex, area 2, dorsal (Te2D) (posterodorsal auditory area) dorsal to the caudal end of area Te1, which both exhibited high best frequencies, projected to the ventral zone of the ventral division of the medial geniculate body. The caudal end of area Te1 (auditory area) and the rostroventral part of area Te1 (a part of anterior auditory field), which both exhibited low best frequencies, projected to the dorsal zone of the ventral division of the medial geniculate body. In contrast to the similar topography in the projections to the ventral division of the medial geniculate body, collateral projections to the thalamic reticular nucleus terminated in the opposite dorsal and ventral zones of the lateral and middle tiers of the nucleus in each pair of the tonotopically comparable cortical subfields. In addition, the projections of the non-primary cortical subfields further arborized in the medial tier of the thalamic reticular nucleus. The results suggest that tonotopically comparable primary and non-primary subfields in the auditory cortex provide corticofugal excitatory effects to the same part of the ventral division of the medial geniculate body. On the other hand, corticofugal inhibition via the thalamic reticular nucleus may operate in different parts of the ventral division of the medial geniculate body or different thalamic nuclei. The primary and non-primary cortical auditory areas are presumed to subserve distinct gating functions for auditory attention.     

Individual corticorubral neurons project bilaterally during postnatal development and following early contralateral cortical lesions

The corticorubral projections in adult cats are primarily uncrossed. However, early in development and after early unilateral lesions of the sensorimotor cortex, crossed corticorubral projections are also observed. The present study was performed to disclose (1) whether the crossed projections originate from neuronal subpopulations different from those producing uncrossed ones and (2) how the neurons that give rise to the crossed projections in the lesioned animals are related to those occurring in normal development. We injected fluorescent latex microspheres into the red nucleus of two groups of animals: (1) intact kittens at postnatal week 3 and (2) kittens that had received unilateral ablation of the cerebral cortex at this stage and were then allowed to survive for at least 4 weeks. Red fluorescing microspheres were injected on one side and green ones on the other. In both normal and lesioned kittens, a number of cells in the cortex were labeled as a result of the contralateral as well as the ipsilateral injections, and no difference in size or distribution was found between the cells labeled from contralateral and ipsilateral injections. More than half of the cells labeled from contralateral injections were double-labeled in both groups of animals. These results indicate that individual corticorubral cells project bilaterally in normal development as well as following unilateral lesions of the cortex. With respect to the cells producing crossed projections, they were similar in both laminar and regional distributions between the intact and lesioned animal, suggesting that the crossed projections arise from the same neuronal subpopulation before and after cortical lesions. This view was supported by sequential injections of the tracers, which indicated that cells normally projecting contralaterally maintained the crossed projection after the lesions. Taking into account our previous observations that growth and proliferation of crossed corticorubral axons took place in the red nucleus (Murakami et al. 1991a), it is likely that growth and proliferation of the axons in denervated targets play a major role in lesion-induced establishment of aberrant projections.     

Corticofugal modulation of the midbrain frequency map in the bat auditory system

The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.     

Cortico-cortical efferents of primary motor and somatosensory regions of the cerebral cortex in Macaca fascicularis

The cortical efferents of the primary somatic sensory and motor areas were traced autoradiographically in 13 monkeys (Macaco fascicularis). Precentral fibers were traced bilaterally to the first and second somatic sensorimotor areas (M_I,II, S_I,II), to the rostral and caudal half of area 5 as well as into the ventral bank of the sulcus cingularis within area 24. The projections to area 5 and the cingulate gyrus appeared to arise only from precentral areas representing trunk and limbs. Furthermore, ipsilateral connections were demonstrated to the caudal parts of area 6 on the lateral aspect of the hemisphere.The existence of ipsilateral postcentral projections to the primary and secondary sensorimotor cortex was confirmed whereas a bilateral projection was found to S_II (all cases), S_I (all but the ‘arm’ cases) and m_I (only ‘face’ cases). Bilateral connections were also traced to the opercular-insular cortex. The distribution of fibers of the postcentral trunk and limb regions to the superior parietal lobulus appeared to be restricted to the ipsilateral and rostral parts of area 5.Apart from the well-known somatotopic and homotopic organization of S_I and M_I efferents, heterotopic intra- and interhemispheric projections could be demonstrated. It was suggested that cortical regions representing proximal as well as distal body regions might be transcallosally interconnected. The relationship between primary sensorimotor regions and the cingulate, opercular and insular cortex was discussed, and the implications of the predominant precentral input to area 5 were considered.     

When should we expect prenatally damaged human brains to have abnormal neuronal connections?

Abnormal function in prenatally damaged brains may occur as the result of loss of neuronal an/or glial elements, abnormal morphology of neurons, or abnormal and inappropriate connections. By abnormal connection I mean projections from a given nucleus to a nucleus or cortical region in the brain which does not normally receive such a projection. In the present paper it is proposed that focal brain damage, i.e. that damage involving circumscribed regions of the developing brain on one side of the body, is more likely to induce abnormal connections, than diffuse brain damage, affecting most of the brain bilaterally. This proposition is based on the available evidence from animal studies, albeit scanty, and is presented as a working hypothesis for future studies of human congenital brain damage.     

Organization of cortical afferent and efferent pathways in the white matter of the rat visual system.

Fibers forming reciprocal connections between the dorsal lateral geniculate nucleus and primary visual cortex run in separate tracts in the white matter. The corticofugal fibers are organized into bundles which project through the coronal plane at an oblique angle. We have examined the organization of corticofugal fiber tracts within the white matter of the rat using cortical slices which encompassed the predicted trajectory of these fiber bundles. Field potentials were evoked in layer VI of visual cortex by focal stimulation of subcortical white matter using microbipolar electrodes. Two major responses were elicited: a short-latency and a longer-latency response. The short-latency response was elicited in the superficial strata of white matter with proximal stimulation sites and was obtained in deeper strata for more distant, lateral sites. The longer-latency response was associated with superficial strata in white matter at both proximal and distant stimulation sites. Based on the electrophysiological properties and the white matter location for eliciting these responses, it is likely that the short-latency response is due to antidromic activation of corticogeniculate fibers, whereas the longer-latency response probably arises from orthodromic activation of geniculocortical fibers. These findings provide an electrophysiological demonstration that cortical afferent and efferent pathways are segregated within the white matter and that they can be selectively activated by focal stimulation. The fact that the fiber bundle model successfully predicted the trajectory of corticofugal fibers provides additional support for this model of white matter organization. A double labeling technique which combined orthograde axonal transport and neuronal degeneration was used to examine the topographic arrangement of corticofugal fibers in the white matter.(ABSTRACT TRUNCATED AT 250 WORDS)     

黄芪对脑缺血再灌注损伤c-fos表达和细胞凋亡的影响

目的探讨黄芪注射液对大鼠局灶性脑缺血再灌注损伤后的保护作用。方法采用线栓法阻断大鼠一侧大脑中动脉(MCA)血流2h,再灌注24h制成局灶性脑缺血再灌注损伤模型。将24只Wistar雄性大鼠随机分成假手术组、缺血再灌注组、黄芪组。造模前1h时黄芪组给与黄芪200mg/kg腹腔注射。假手术组、缺血再灌注组给予等剂量生理盐水。再灌注24h后断头取脑、切片,进行HE染色、c-fos免疫组化染色和细胞凋亡检测。结果缺血2h再灌注24h后,黄芪组和缺血再灌注组大鼠缺血侧皮层可检测到细胞凋亡细胞,黄芪组凋亡细胞数明显少于缺血再灌注组,假手术组未见凋亡细胞;黄芪组和缺血再灌注组大鼠缺血侧皮层c-fos阳性细胞数均高于假手术组,与缺血再灌注组相比,黄芪组大鼠缺血侧皮层c-fos表达降低。结论黄芪可抑制缺血再灌注损伤后缺血侧皮质的细胞凋亡。     

Axotomized,adult basal forebrain neurons can innervate fetal frontal cortex grafts: A double fluorescent tracer study in the rat

Summary The ability of axonal regeneration of identified adult basal forebrain (BFB) neurons was examined after homotopic grafting of fetal neocortical tissue to a lesion cavity in the frontal neocortex. Using a four step experimental procedure, adult rats first received an injection of the fluorescent dye Fluoro-Gold (FG) into the sensorimotor cortex in order to label those neurons with projections to the area by retrograde axonal transport. After one week the injection area was removed by aspiration, leaving a cavity in the neocortex. One week later a block of fetal (E14) frontal cortical tissue was placed in the cavity. The animals were then allowed to survive for 6 weeks before a second fluorescent tracer, Nuclear Yellow (NY), was injected into the transplant. The animals were sacrificed 24 h later and analyzed by fluorescence microscopy. Both single labeled, FG and NY containing neurons and double labeled neurons containing both tracers were found in the BFB. The results demonstrate that adult BFB neurons can reestablish cortical projections into fetal cortical grafts (double labeled neurons), and they suggest that other BFB neurons, not initially innervating the lesioned cortical area, have sprouted into the transplant (NY labeled neurons).     

Plasticity of neuronal connections in developing brains of mammals.

Although mature nervous systems show substantial malleability following various surgical or environmental manipulations, developing brains show far more prominent plasticity, particularly in terms of morphological features. Neuronal circuits, for example, can be dramatically rewired following neonatal but not adult brain lesions. It remains unknown why neuronal circuits in developing brains show such remarkable plasticity. A number of anatomical and physiological studies suggest that there are transient projections in developing brains and they are eliminated by cell death and/or collateral elimination as development proceeds. This raises a possibility that aberrant projections observed following various surgical or environmental manipulations such as partial denervation, results from retention or stabilization of transient projections. However, evidence suggests that cell death does not play an important role in developmental fine-tuning of neuronal projections. Furthermore, although the elimination of axon collaterals takes place, individual neurons appear to elaborate axonal arbors in appropriate target areas, resulting in a net increase in the size of axonal arbor emerging from individual neurons. In accord with these observations, the number of synapses appear to increase during the period when axonal elimination proceeds. Taken together, reinforcement of appropriate projections rather than elimination of excessive connections plays a major role in developmental specification of neuronal connections. Appearance of aberrant projections after partial denervation may not be a consequence of disordered axonal growth, since they form topographic maps which precisely mirrors those for normal projections. They may be induced due to reinforcement of pre-existing neuronal connections rather than to construction of novel pathways. Observations of axonal morphology in denervated areas indicate that lesion-induced enlargement of projections is due to transformation of axonal morphology, from simple and poorly branched to multiply branched. Perhaps such simple and poorly branched axons in inappropriate target areas may represent ones in the course of elimination but they may serve as a source of sprouting when denervated. In other words, after total elimination of axons any surgical or environmental manipulation cannot induce enlargement of projections. The mechanisms underlying such modifiability of neuronal connections remains unclarified but possible participation of an activity-dependent competitive mechanism is discussed.     

Effects of tectal slits on the anomalous corticotectal fibres

Summary Following cortical lesion and a mid-saggital cut through the tecturn in newborn albino rats, the remaining intact cortex was seen to project bilaterally to the superior colliculus with anomalous fibres running across the mid-line to the contralateral superior colliculus. In animals lesioned at day 5 postnatally and later, the anomalous fibres were mostly deflected by the scar tissue resulting from the tectal injury. In animals lesioned within the first 2 days after birth, a good proportion of the anomalous fibres coursed through the scar tissue. In all animals studied except one the pattern of terminations of the anomalous fibres was in agreement with the observations of Leong and Lund (1973).     

Cortical networks generating movement-related EEG rhythms in Alzheimer's disease: an EEG coherence study

Patients with mild Alzheimer's disease (AD) present with abnormally strong values of frontal and ipsilateral central sensorimotor rhythms. The authors tested 2 working hypotheses of the related electroencephalographic (EEG) coherence: disconnection, defined as a sign of a reduced coordination within the frontoparietal and interhemispheric networks, and cooperation, defined as a reflection of the reorganization of the brain sensorimotor networks. Results showed that, compared with healthy controls, patients with mild AD had an unreactive and abnormally low interhemispheric EEG coherence and an unreactive and abnormally high frontoparietal EEG coherence. These findings support the hypothesis of an impaired mechanism of sensorimotor cortical coupling (disconnection) in mild AD.     

大鼠皮质脊髓束的磁共振与DiI示踪对照研究

目的:验证Mn2 增强磁共振技术与DiI荧光示踪技术对于大鼠皮质脊髓束的追踪效果,并对两种方法进行比较评价。方法:采用Wistar大鼠30只,随机分为MnCl2组和DiI组各15只,分别经感觉运动区皮层进行MnCl2(0.8mol/L)注射和5%DiI涂布。MnCl2组,于术后24h行MR扫描;DiI组动物存活10d后,灌杀、切片、观察。结果:两种方法在颈髓以上节段对皮质脊髓束的示踪结果一致。结论:Mn2 增强磁共振技术与DiI荧光示踪技术对皮质脊髓束的示踪效果好,二者结合应用,对临床影像学神经束路追踪研究具有较大的应用价值。     

Manganese enhancement in non-CNS organs

Manganese-enhanced magnetic resonance imaging (MEMRI) is a novel imaging technique capable of monitoring calcium influx, in vivo. Manganese (Mn2+) ions, similar to calcium ions (Ca2+), are taken up by activated cells where their paramagnetic properties afford signal enhancement in T(1)-weighted MRI methodologies. In this study we have assessed Mn2+ distribution in mice using magnetization-prepared rapid gradient echo (MP-RAGE) based MRI, by measuring changes in T(1)-effective relaxation times (T(1)-eff), effective R(1)-relaxation rates (R(1)-eff) and signal intensity (SI) profiles over time. The manganese concentration in the tissue was also determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). Our results show a strong positive correlation between infused dose of MnCl2 and the tissue manganese concentration. Furthermore, we demonstrate a linear relationship between R(1)-eff and tissue manganese concentration and tissue-specific Mn2+ distribution in murine tissues following dose-dependent Mn2+ administration. This data provides an optimized MnCl2 dose regimen for an MP-RAGE based sequence protocol for specific target organs and presents a potential 3D MRI technique for in vivo imaging of Ca2+ entry during Ca2+-dependent processes in a wide range of tissues.     

Differential topography of the bilateral cortical projections to the whisker and forepaw regions in rat motor cortex

Whisker and forelimb movements in rats have distinct behavioral functions that suggest differences in the neural connections of the brain regions that control their movements. To test this hypothesis, retrograde tracing methods were used to characterize the bilateral distribution of the cortical neurons that project to the whisker and forelimb regions in primary motor (MI) cortex. Tracer injections in each MI region revealed labeled neurons in more than a dozen cortical areas, but most labeling was concentrated in the sensorimotor areas. Cortical projections to the MI forepaw region originated primarily from the primary somatosensory (SI) cortex in the ipsilateral hemisphere. In contrast, most projections to the MI whisker region originated from the MI whisker region in the contralateral hemisphere. Tracer injections in the MI whisker region also revealed a higher proportion of labeled neurons in the claustrum and in the posterior parietal cortex. Injections of different tracers into the MI whisker and forepaw regions of some rats revealed a topographic organization of neuronal labeling in several sensorimotor regions. Collectively, these findings indicate that the MI whisker and forepaw regions receive different sets of cortical inputs. Whereas the MI whisker region is most strongly influenced by callosal projections, presumably to mediate bilateral coordination of the whiskers, the MI forepaw region is influenced mainly by ipsilateral SI inputs that convey somatosensory feedback.     

The effect of cortical lesion on systemic penicillin epilepsy in rats.

There is a certain recovery of function following brain damage, due to neuronal plasticity. The experiments were performed in order to investigate the effects of cortical lesion on seizural activity in rats induced by systemic application of penicillin. The sensorimotor cortex was unilaterally removed in the lesioned animals, while the control animals were only sham operated or non-operated (before implantation of the electrodes). Seizural activity was recorded by means of electroencephalograms before and after penicillin treatment (1,000,000 I.U./kg, i.p). Testing of penicillin started at least 30 days after cortical lesion. Seizural activity was characterized by spike and wave complexes accompanied by vigilance reduction and sometimes by mild myoclonic jerks in both control and lesioned animals. The early period (about 2 h after penicillin administration) with appearance of the spike-wave discharges with relative increase of the mean total electroencephalogram powers as well as the succeeding period 2.5-5.5 h after penicillin administration) with maximum number of spike-wave discharges did not differ in the electroencephalogram of the control and lesioned animals. The late period of penicillin effect (from 6-11 h after penicillin administration) with frequent spike-wave discharges and still large mean total electroencephalogram powers was observed only in lesioned animals. It is concluded that a cortical lesion destabilizes the brain function in the rat model of epilepsy induced by parenteral administration of penicillin.     

Projections from auditory cortex to cochlear nucleus: A comparative analysis of rat and mouse

Mammalian hearing is a complex special sense that involves detection, localization, and identification of the auditory stimulus. The cerebral cortex may subserve higher auditory processes by providing direct modulatory cortical projections to the auditory brainstem. To support the hypothesis that corticofugal projections are a conserved feature in the mammalian brain, this article reviews features of the rat corticofugal pathway and presents new data supporting the presence of similar projections in the mouse. The mouse auditory cortex was localized with electrophysiological recording and neuronal tracers were injected into AI. The cochlear nucleus was dissected and examined for terminal fibers by light and electron microscopy. Bouton endings were found bilaterally forming synapses with dendrites of granule cells of the cochlear nucleus. This report provides evidence for direct auditory cortex projections to the cochlear nucleus in the mouse. The distribution of projections to the granule cell domain and the synapses onto granule cell dendrites are consistent with what has been reported for rats and guinea pigs. These findings suggest a general plan for corticofugal modulation of ascending auditory information in mammals. Corticobulbar inputs to the auditory brainstem likely provided a survival advantage by improving sound detection and identification, thus allowing the development of complex social behaviors and the navigation of varied environments.