Origins and collateralization of corticospinal, corticopontine, corticorubral and corticostriatal tracts: a multiple retrograde fluorescent tracing study

How deep is the brain? The shallow brain hypothesis

Deep learning and predictive coding architectures commonly assume that inference in neural networks is hierarchical. However, largely neglected in deep learning and predictive coding architectures is the neurobiological evidence that all hierarchical cortical areas, higher or lower, project to and receive signals directly from subcortical areas. Given these neuroanatomical facts, today's dominance of cortico-centric, hierarchical architectures in deep learning and predictive coding networks is highly questionable; such architectures are likely to be missing essential computational principles the brain uses. In this Perspective, we present the shallow brain hypothesis: hierarchical cortical processing is integrated with a massively parallel process to which subcortical areas substantially contribute. This shallow architecture exploits the computational capacity of cortical microcircuits and thalamo-cortical loops that are not included in typical hierarchical deep learning and predictive coding networks. We argue that the shallow brain architecture provides several critical benefits over deep hierarchical structures and a more complete depiction of how mammalian brains achieve fast and flexible computational capabilities.

Motor cortex is required for flexible but not automatic motor sequences

How motor cortex contributes to motor sequence execution is much debated, with studies supporting disparate views. Here we probe the degree to which motor cortex's engagement depends on task demands, specifically whether its role differs for highly practiced, or 'automatic', sequences versus flexible sequences informed by external events. To test this, we trained rats to generate three-element motor sequences either by overtraining them on a single sequence or by having them follow instructive visual cues. Lesioning motor cortex revealed that it is necessary for flexible cue-driven motor sequences but dispensable for single automatic behaviors trained in isolation. However, when an automatic motor sequence was practiced alongside the flexible task, it became motor cortex-dependent, suggesting that subcortical consolidation of an automatic motor sequence is delayed or prevented when the same sequence is produced also in a flexible context. A simple neural network model recapitulated these results and explained the underlying circuit mechanisms. Our results critically delineate the role of motor cortex in motor sequence execution, describing the condition under which it is engaged and the functions it fulfills, thus reconciling seemingly conflicting views about motor cortex's role in motor sequence generation.

Motor cortex projections to red and pontine nuclei have distinct roles during movement in the mouse

Motor control largely depends on the deep layer 5 (L5) pyramidal neurons that project to subcortical structures. However, it is largely unknown if these neurons are functionally segregated with distinct roles in movement performance. Here, we analyzed mouse motor cortex L5 pyramidal neurons projecting to the red and pontine nuclei during movement preparation and execution. Using photometry to analyze the calcium activity of L5 pyramidal neurons projecting to the red nucleus and pons, we reveal that both types of neurons activate with different temporal dynamics. Optogenetic inhibition of either kind of projection differentially affects forelimb movement onset and execution in a lever press task, but only the activity of corticopontine neurons is significantly correlated with trial-by-trial variations in reaction time. The results indicate that cortical neurons projecting to the red and pontine nuclei contribute differently to sensorimotor integration, suggesting that L5 output neurons are functionally compartmentalized generating, in parallel, different downstream information.

Different subtypes of motor cortex pyramidal tract neurons projects to red and pontine nuclei

Introduction

Pyramidal tract neurons (PTNs) are fundamental elements for motor control. However, it is largely unknown if PTNs are segregated into different subtypes with distinct characteristics.

Methods

Using anatomical and electrophysiological tools, we analyzed in mice motor cortex PTNs projecting to red and pontine midbrain nuclei, which are important hubs connecting cerebral cortex and cerebellum playing a critical role in the regulation of movement.

Results

We reveal that the vast majority of M1 neurons projecting to the red and pontine nuclei constitutes different populations. Corticopontine neurons have higher conduction velocities and morphologically, a most homogeneous dendritic and spine distributions along cortical layers.

Discussion

The results indicate that cortical neurons projecting to the red and pontine nuclei constitute distinct anatomical and functional pathways which may contribute differently to sensorimotor integration.

The corticospinal tract primarily modulates sensory inputs in the mouse lumbar cord

It is generally assumed that the main function of the corticospinal tract (CST) is to convey motor commands to bulbar or spinal motoneurons. Yet the CST has also been shown to modulate sensory signals at their entry point in the spinal cord through primary afferent depolarization (PAD). By sequentially investigating different routes of corticofugal pathways through electrophysiological recordings and an intersectional viral strategy, we here demonstrate that motor and sensory modulation commands in mice belong to segregated paths within the CST. Sensory modulation is executed exclusively by the CST via a population of lumbar interneurons located in the deep dorsal horn. In contrast, the cortex conveys the motor command via a relay in the upper spinal cord or supraspinal motor centers. At lumbar level, the main role of the CST is thus the modulation of sensory inputs, which is an essential component of the selective tuning of sensory feedback used to ensure well-coordinated and skilled movement.

Role of DSCAM in the Development of Neural Control of Movement and Locomotion

Locomotion results in an alternance of flexor and extensor muscles between left and right limbs generated by motoneurons that are controlled by the spinal interneuronal circuit. This spinal locomotor circuit is modulated by sensory afferents, which relay proprioceptive and cutaneous inputs that inform the spatial position of limbs in space and potential contacts with our environment respectively, but also by supraspinal descending commands of the brain that allow us to navigate in complex environments, avoid obstacles, chase prey, or flee predators. Although signaling pathways are important in the establishment and maintenance of motor circuits, the role of DSCAM, a cell adherence molecule associated with Down syndrome, has only recently been investigated in the context of motor control and locomotion in the rodent. DSCAM is known to be involved in lamination and delamination, synaptic targeting, axonal guidance, dendritic and cell tiling, axonal fasciculation and branching, programmed cell death, and synaptogenesis, all of which can impact the establishment of motor circuits during development, but also their maintenance through adulthood. We discuss herein how DSCAM is important for proper motor coordination, especially for breathing and locomotion.

Routes of the thalamus through the history of neuroanatomy

The most distant roots of neuroanatomy trace back to antiquity, with the first human dissections, but no document which would identify the thalamus as a brain structure has reached us. Claudius Galenus (Galen) gave to the thalamus the name 'thalamus nervorum opticorum', but later on, other names were used (e.g., anchae, or buttocks-like). In 1543, Andreas Vesalius provided the first quality illustrations of the thalamus. During the 19th century, tissue staining techniques and ablative studies contributed to the breakdown of the thalamus into subregions and nuclei. The next step was taken using radiomarkers to identify connections in the absence of lesions. Anterograde and retrograde tracing methods arose in the late 1960s, supporting extension, revision, or confirmation of previously established knowledge. The use of the first viral tracers introduced a new methodological breakthrough in the mid-1970s. Another important step was supported by advances in neuroimaging of the thalamus in the 21th century. The current review follows the history of the thalamus through these technical revolutions from Antiquity to the present day.

Monosynaptic Retrograde Tracing From Prelimbic Neuron Subpopulations Projecting to Either Nucleus Accumbens Core or Rostromedial Tegmental Nucleus

The prelimbic (PL) region of the medial prefrontal cortex (mPFC) has been implicated in both driving and suppressing motivated behaviors, including cocaine-seeking in rats. These seemingly opposing functions may be mediated by different efferent targets of PL projections, such as the nucleus accumbens (NAc) core and rostromedial tegmental nucleus (RMTg), which have contrasting roles in reward-seeking behaviors. We sought to characterize the anatomical connectivity differences between PL neurons projecting to NAc core and RMTg. We used conventional retrograde tracers to reveal distinct subpopulations of PL neurons projecting to NAc core vs. RMTg in rats, with very little overlap. To examine potential differences in input specificity for these two PL subpopulations, we then used Cre-dependent rabies virus (EnvA-RV-EGFP) as a monosynaptic retrograde tracer and targeted specific PL neurons via injections of retrograde CAV2-Cre in either NAc core or RMTg. We observed a similar catalog of cortical, thalamic, and limbic afferents for both NAc- and RMTg-projecting populations, with the primary source of afferent information arising from neighboring prefrontal neurons in ipsilateral PL and infralimbic cortex (IL). However, when the two subpopulations were directly compared, we found that RMTg-projecting PL neurons received a greater proportion of input from ipsilateral PL and IL, whereas NAc-projecting PL neurons received a greater proportion of input from most other cortical areas, mediodorsal thalamic nucleus, and several other subcortical areas. NAc-projecting PL neurons also received a greater proportion of contralateral cortical input. Our findings reveal that PL subpopulations differ not only in their efferent target but also in the input specificity from afferent structures. These differences in connectivity are likely to be critical to functional differences of PL subpopulations.

Laminar Origin of Corticostriatal Projections to the Motor Putamen in the Macaque Brain

In the macaque brain, projections from distant, interconnected cortical areas converge in specific zones of the striatum. For example, specific zones of the motor putamen are targets of projections from frontal motor, inferior parietal, and ventrolateral prefrontal hand-related areas and thus are integral part of the so-called "lateral grasping network." In the present study, we analyzed the laminar distribution of corticostriatal neurons projecting to different parts of the motor putamen. Retrograde neural tracers were injected in different parts of the putamen in 3 Macaca mulatta (one male) and the laminar distribution of the labeled corticostriatal neurons was analyzed quantitatively. In frontal motor areas and frontal operculum, where most labeled cells were located, almost everywhere the proportion of corticostriatal labeled neurons in layers III and/or VI was comparable or even stronger than in layer V. Furthermore, within these regions, the laminar distribution pattern of corticostriatal labeled neurons largely varied independently from their density and from the projecting area/sector, but likely according to the target striatal zone. Accordingly, the present data show that cortical areas may project in different ways to different striatal zones, which can be targets of specific combinations of signals originating from the various cortical layers of the areas of a given network. These observations extend current models of corticostriatal interactions, suggesting more complex modes of information processing in the basal ganglia for different motor and nonmotor functions and opening new questions on the architecture of the corticostriatal circuitry.SIGNIFICANCE STATEMENT Projections from the ipsilateral cerebral cortex are the major source of input to the striatum. Previous studies have provided evidence for distinct zones of the putamen specified by converging projections from specific sets of interconnected cortical areas. The present study shows that the distribution of corticostriatal neurons in the various layers of the primary motor and premotor areas varies depending on the target striatal zone. Accordingly, different striatal zones collect specific combinations of signals from the various cortical layers of their input areas, possibly differing in terms of coding, timing, and direction of information flow (e.g., feed-forward, or feed-back).

Neuroanatomical tract-tracing techniques that did go viral

Neuroanatomical tracing methods remain fundamental for elucidating the complexity of brain circuits. During the past decades, the technical arsenal at our disposal has been greatly enriched, with a steady supply of fresh arrivals. This paper provides a landscape view of classical and modern tools for tract-tracing purposes. Focus is placed on methods that have gone viral, i.e., became most widespread used and fully reliable. To keep an historical perspective, we start by reviewing one-dimensional, standalone transport-tracing tools; these including today's two most favorite anterograde neuroanatomical tracers such as Phaseolus vulgaris-leucoagglutinin and biotinylated dextran amine. Next, emphasis is placed on several classical tools widely used for retrograde neuroanatomical tracing purposes, where Fluoro-Gold in our opinion represents the best example. Furthermore, it is worth noting that multi-dimensional paradigms can be designed by combining different tracers or by applying a given tracer together with detecting one or more neurochemical substances, as illustrated here with several examples. Finally, it is without any doubt that we are currently witnessing the unstoppable and spectacular rise of modern molecular-genetic techniques based on the use of modified viruses as delivery vehicles for genetic material, therefore, pushing the tract-tracing field forward into a new era. In summary, here, we aim to provide neuroscientists with the advice and background required when facing a choice on which neuroanatomical tracer-or combination thereof-might be best suited for addressing a given experimental design.

Operant conditioning paradigm for juxtacellular recordings in functionally identified cortical neurons during motor execution in head-fixed rats

BACKGROUND

Understanding the configuration of neural circuits and the specific role of distinct cortical neuron types involved in behavior, requires the study of structure-function and connectivity relationships with single cell resolution in awake behaving animals. Despite head-fixed behaving rats have been used for in vivo measuring of neuronal activity, it is a concern that head fixation could change the performance of behavioral task.

NEW METHOD

We describe the procedures for efficiently training Wistar rats to develop a behavioral task, involving planning and execution of a qualified movement in response to a visual cue under head-fixed conditions. The behavioral and movement performance in freely moving vs head-fixed conditions was analyzed.

RESULTS

The best behavioral performance was obtained in the rats that were trained first in freely moving conditions and then placed in a head-restrained condition compared with the animals which first were habituated to head-restriction and then learned the task. Moreover, head restriction did not alter the movement performance. Stable juxtacellular recordings from sensorimotor cortex neurons were obtained while the rats were performing forelimb movements. Biocytin electroporation and retrograde tracer injections, permits identify the hodology of individual long-range projecting neurons.

COMPARISON WITH EXISTING METHODS

Our method shows no difference in the behavioral performance of head fixed and freely moving conditions. Also includes a computer aided design of a discrete and ergonomic head-post allowing enough stability to perform juxtacellular recording and labeling of cortical neurons.

CONCLUSIONS

Our method is suitable for the in vivo characterization of neuronal circuits and their long-range connectivity.

Mouse corticospinal system comprises different functional neuronal ensembles depending on their hodology

BACKGROUND

Movement performance depends on the synaptic interactions generated by coherent parallel sensorimotor cortical outputs to different downstream targets. The major outputs of the neocortex to subcortical structures are driven by pyramidal tract neurons (PTNs) located in layer 5B. One of the main targets of PTNs is the spinal cord through the corticospinal (CS) system, which is formed by a complex collection of distinct CS circuits. However, little is known about intracortical synaptic interactions that originate CS commands and how different populations of CS neurons are functionally organized. To further understand the functional organization of the CS system, we analyzed the activity of unambiguously identified CS neurons projecting to different zones of the same spinal cord segment using two-photon calcium imaging and retrograde neuronal tracers.

RESULTS

Sensorimotor cortex slices obtained from transgenic mice expressing GCaMP6 funder the Thy1 promoter were used to analyze the spontaneous calcium transients in layer 5 pyramidal neurons. Distinct subgroups of CS neurons projecting to dorsal horn and ventral areas of the same segment show more synchronous activity between them than with other subgroups.

CONCLUSIONS

The results indicate that CS neurons projecting to different spinal cord zones segregated into functional ensembles depending on their hodology, suggesting that a modular organization of CS outputs controls sensorimotor behaviors in a coordinated manner.

Motor cortex is required for learning but not for executing a motor skill

Motor cortex is widely believed to underlie the acquisition and execution of motor skills, but its contributions to these processes are not fully understood. One reason is that studies on motor skills often conflate motor cortex's established role in dexterous control with roles in learning and producing task-specific motor sequences. To dissociate these aspects, we developed a motor task for rats that trains spatiotemporally precise movement patterns without requirements for dexterity. Remarkably, motor cortex lesions had no discernible effect on the acquired skills, which were expressed in their distinct pre-lesion forms on the very first day of post-lesion training. Motor cortex lesions prior to training, however, rendered rats unable to acquire the stereotyped motor sequences required for the task. These results suggest a remarkable capacity of subcortical motor circuits to execute learned skills and a previously unappreciated role for motor cortex in "tutoring" these circuits during learning.

Cell-specific activity-dependent fractionation of layer 2/3→5B excitatory signaling in mouse auditory cortex

Auditory cortex (AC) layer 5B (L5B) contains both corticocollicular neurons, a type of pyramidal-tract neuron projecting to the inferior colliculus, and corticocallosal neurons, a type of intratelencephalic neuron projecting to contralateral AC. Although it is known that these neuronal types have distinct roles in auditory processing and different response properties to sound, the synaptic and intrinsic mechanisms shaping their input-output functions remain less understood. Here, we recorded in brain slices of mouse AC from retrogradely labeled corticocollicular and neighboring corticocallosal neurons in L5B. Corticocollicular neurons had, on average, lower input resistance, greater hyperpolarization-activated current (Ih), depolarized resting membrane potential, faster action potentials, initial spike doublets, and less spike-frequency adaptation. In paired recordings between single L2/3 and labeled L5B neurons, the probabilities of connection, amplitude, latency, rise time, and decay time constant of the unitary EPSC were not different for L2/3→corticocollicular and L2/3→corticocallosal connections. However, short trains of unitary EPSCs showed no synaptic depression in L2/3→corticocollicular connections, but substantial depression in L2/3→corticocallosal connections. Synaptic potentials in L2/3→corticocollicular connections decayed faster and showed less temporal summation, consistent with increased Ih in corticocollicular neurons, whereas synaptic potentials in L2/3→corticocallosal connections showed more temporal summation. Extracellular L2/3 stimulation at two different rates resulted in spiking in L5B neurons; for corticocallosal neurons the spike rate was frequency dependent, but for corticocollicular neurons it was not. Together, these findings identify cell-specific intrinsic and synaptic mechanisms that divide intracortical synaptic excitation from L2/3 to L5B into two functionally distinct pathways with different input-output functions.

Electrophysiological properties of genetically identified subtypes of layer 5 neocortical pyramidal neurons: Ca²⁺ dependence and differential modulation by norepinephrine

We studied neocortical pyramidal neurons from two lines of bacterial artificial chromosome mice (etv1 and glt; Gene Expression Nervous System Atlas: GENSAT project), each of which expresses enhanced green fluorescent protein (EGFP) in a different subpopulation of layer 5 pyramidal neurons. In barrel cortex, etv1 and glt pyramidal cells were previously reported to differ in terms of their laminar distribution, morphology, thalamic inputs, cellular targets, and receptive field size. In this study, we measured the laminar distribution of etv1 and glt cells. On average, glt cells were located more deeply; however, the distributions of etv1 and glt cells extensively overlap in layer 5. To test whether these two cell types differed in electrophysiological properties that influence firing behavior, we prepared acute brain slices from 2-4-wk-old mice, where EGFP-positive cells in somatosensory cortex were identified under epifluorescence and then studied using whole cell current- or voltage-clamp recordings. We studied the details of action potential parameters and repetitive firing, characterized by the larger slow afterhyperpolarizations (AHPs) in etv1 neurons and larger medium AHPs (mAHPS) in glt cells, and compared currents underlying the mAHP and slow AHP (sAHP) in etv1 and glt neurons. Etv1 cells exhibited lower dV/dt for spike polarization and repolarization and reduced direct current (DC) gain (lower f-I slope) for repetitive firing than glt cells. Most importantly, we found that 1) differences in the expression of Ca(2+)-dependent K(+) conductances (small-conductance calcium-activated potassium channels and sAHP channels) determine major functional differences between etv1 and glt cells, and 2) there is differential modulation of etv1 and glt neurons by norepinephrine.

Neurophotonics applications to motor cortex research

Neurophotonics methods offer powerful ways to access neuronal signals and circuits. We highlight recent advances and current themes in this area, emphasizing tools for mapping, monitoring, and manipulating excitatory projection neurons and their synaptic circuits in mouse motor cortex.

Brain-wide map of efferent projections from rat barrel cortex

The somatotopically organized whisker barrel field of the rat primary somatosensory (S1) cortex is a commonly used model system for anatomical and physiological investigations of sensory processing. The neural connections of the barrel cortex have been extensively mapped. But most investigations have focused on connections to limited regions of the brain, and overviews in the literature of the connections across the brain thus build on a range of material from different laboratories, presented in numerous publications. Furthermore, given the limitations of the conventional journal article format, analyses and interpretations are hampered by lack of access to the underlying experimental data. New opportunities for analyses have emerged with the recent release of an online resource of experimental data consisting of collections of high-resolution images from 6 experiments in which anterograde tracers were injected in S1 whisker or forelimb representations. Building on this material, we have conducted a detailed analysis of the brain wide distribution of the efferent projections of the rat barrel cortex. We compare our findings with the available literature and reports accumulated in the Brain Architecture Management System (BAMS₂) database. We report well-known and less known intracortical and subcortical projections of the barrel cortex, as well as distinct differences between S1 whisker and forelimb related projections. Our results correspond well with recently published overviews, but provide additional information about relative differences among S1 projection targets. Our approach demonstrates how collections of shared experimental image data are suitable for brain-wide analysis and interpretation of connectivity mapping data.

Amygdala afferents monosynaptically innervate corticospinal neurons in rat medial prefrontal cortex

The amygdala provides the medial prefrontal cortex (mPFC; areas 25, 32, and 24b) with salient emotional information. This study investigated the synaptic connectivity of identified amygdalocortical boutons (ACBs; labeled anterogradely following injections of Phaseolus vulgaris leucoagglutinin into the basolateral nucleus of the amygdala), with the dendritic processes of identified layer 5 corticospinal neurons in the rat mPFC. The corticospinal (CS) neurons in the mPFC had been retrogradely labeled with rhodamine fluorescent latex microspheres and subsequently intracellularly filled with biotinylated lucifer yellow to visualize their basal and apical dendrites. Two main classes of mPFC CS neurons were identified. Type 1 cells had apical dendrites bearing numerous dendritic spines with radiate basal dendritic arbors. Type 2 cells possessed apical dendrites with greatly reduced spine densities and a broad range of basal dendritic tree morphologies. Identified ACBs made asymmetric synaptic junctions with labeled dendritic spines and the labeled apical and basal dendritic shafts of identified CS neurons. On average, eight ACBs were closely associated with the labeled basal dendritic arbors of type 1 CS neurons and five ACBs with type 2 CS basal dendrites. The mean Scholl distance of ACBs from CS somata (for both types 1 and 2 cells) was 66 μm-coinciding with a region containing the highest length density of CS neuron basal dendrites. These results indicate that neurons in the BLA can monosynaptically influence CS neurons in the mPFC that project to autonomic regions of the thoracic spinal cord and probably to other additional subcortical target regions, such as the lateral hypothalamus.

Heterogeneous spine loss in layer 5 cortical neurons after spinal cord injury

A large thoracic spinal cord injury disconnects the hindlimb (HL) sensory-motor cortex from its target, the lumbar spinal cord. The fate of the synaptic structures of the axotomized cortical neurons is not well studied. We evaluated the density of spines on axotomized corticospinal neurons at 3, 7, and 21 days after the injury in adult mice expressing yellow fluorescence protein in a subset of layer 5 neurons. Spine density of the dendritic segment proximal to the soma (in layer 5) declined as early as 3 days after injury, far preceding the onset of somatic atrophy. In the distal segment (in layer 2/3), spine loss was slower and less severe than in the proximal segment. Axotomy of corticospinal axons in the brainstem (pyramidotomy) induced a comparable reduction of spine density, demonstrating that the loss is not restricted to the neurons axotomized in the thoracic spinal cord. Surprisingly, in both forms of injury, the spine density of putative non-axotomized layer 5 neurons was reduced as well. The spine loss may reflect fast rearrangements of cortical circuits after axotomy, for example, by a disconnection of HL cortical neurons from synaptic inputs that no longer provide useful information.

Wiring of divergent networks in the central auditory system

Divergent axonal projections are found throughout the central auditory system. Here, we evaluate these branched projections in terms of their types, distribution, and putative physiological roles. In general, three patterns of axon collateralization are found: intricate local branching, long-distance collaterals, and branched axons (BAs) involved in feedback-control loops. Local collaterals in the auditory cortex may be involved in local processing and modulation of neuronal firing, while long-range collaterals are optimized for wide-dissemination of information. Rarely do axons branch to both ascending and descending targets. Branched projections to two or more widely separated nuclei or areas are numerically sparse but widespread. Finally, branching to contralateral targets is evident at multiple levels of the auditory pathway and may enhance binaural computations for sound localization. These patterns of axonal branching are comparable to those observed in other modalities. We conclude that the operations served by BAs are area- and nucleus-specific and may complement the divergent unbranched projections of local neuronal populations.

Genetic deletion of paired immunoglobulin-like receptor B does not promote axonal plasticity or functional recovery after traumatic brain injury

The rewiring of neural networks is a fundamental step in recovering behavioral functions after brain injury. However, there is limited potential for axonal plasticity in the adult CNS. The myelin-associated proteins Nogo, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp) are known to inhibit axonal plasticity, and thus targeting the inhibitory pathways they participate in is a potential means of promoting plasticity and functional recovery. Each of Nogo, MAG, and OMgp interacts with both the Nogo receptor (NgR) and paired immunoglobulin-like receptor B (PirB). Here, we determined whether blocking PirB activity enhances axonal reorganization and functional recovery after cortical injury. We found that axons of the contralesional corticospinal tract sprouted into the denervated side of the cervical spinal cord after unilateral injury of the motor cortex. The extent to which this axonal reorganization occurred was far greater in mice lesioned during early postnatal days than in mice lesioned at an age when myelin had begun to form. This suggests that myelin-associated proteins might limit axonal remodeling in vivo. However, the number of sprouting fibers within either the corticospinal or corticorubral tract was not enhanced in PirB(-/-) mice. Blocking PirB signaling also failed to enhance functional recovery with three motor tests. Our results suggest that blocking the function of PirB is not sufficient to promote axonal reorganization or functional recovery after cortical injury.

Branched thalamic afferents: what are the messages that they relay to the cortex?

Many of the axons that carry messages to the thalamus for relay to the cerebral cortex are branched in a pattern long known from Golgi preparations. They send one branch to the thalamus and the other to motor centers of the brainstem or spinal cord. Because the thalamic branches necessarily carry copies of the motor instructions their messages have the properties of efference copies. That is, they can be regarded as providing reliable information about impending instructions contributing to movements that will produce changes in inputs to receptors, thus allowing neural centers to compensate for these changes of input. We consider how a sensory pathway like the medial lemniscus, the spinothalamic tract or the optic tract can also be seen to act as a pathway for an efference copy. The direct connections that ascending and cortical inputs to the thalamus also establish to motor outputs create sensorimotor relationships that provide cortex with a model of activity in lower circuits and link the sensory and the motor sides of behavior more tightly than can be expected from motor outputs with a single, central origin. These transthalamic connectional patterns differ from classical models of separate neural pathways for carrying efference copies of actions generated at higher levels, and introduce some different functional possibilities.

Axon regeneration through scaffold into distal spinal cord after transection

We employed Fast Blue (FB) axonal tracing to determine the origin of regenerating axons after thoracic spinal cord transection injury in rats. Schwann cell (SC)-loaded, biodegradable, poly(lactic-co-glycolic acid) (PLGA) scaffolds were implanted after transection. Scaffolds loaded with solubilized basement membrane preparation (without SCs) were used for negative controls, and nontransected cords were positive controls. One or 2 months after injury and scaffold implantation, FB was injected 0-15 mm caudal or about 5 mm rostral to the scaffold. One week later, tissue was harvested and the scaffold and cord sectioned longitudinally (30 microm) on a cryostat. Trans-scaffold labeling of neuron cell bodies was identified with confocal microscopy in all cell-transplanted groups. Large (30-50 microm diameter) neuron cell bodies were predominantly labeled in the ventral horn region. Most labeled neurons were seen 1-10 mm rostral to the scaffold, although some neurons were also labeled in the cervical cord. Axonal growth occurred bidirectionally after cord transection, and axons regenerated up to 14 mm beyond the PLGA scaffolds and into distal cord. The extent of FB labeling was negatively correlated with distance from the injection site to the scaffold. Electron microscopy showed myelinated axons in the transverse sections of the implanted scaffold 2 months after implantation. The pattern of myelination, with extracellular collagen and basal lamina, was characteristic of SC myelination. Our results show that FB labeling is an effective way to measure the origin of regenerating axons.

Motor cortex electrical stimulation applied to patients with complex regional pain syndrome

Motor cortex stimulation (MCS) is useful to treat patients with neuropathic pain syndromes, unresponsive to medical treatment. Complex regional pain syndrome (CRPS) is a segmentary disease treated successfully by spinal cord stimulation (SCS). However, CRPS often affects large body segments difficult to cover by SCS. This study analyzed the MCS efficacy in patients with CRPS affecting them. Five patients with CRPS of different etiologies underwent a small craniotomy for unilateral 20-grid-contact implantation on MC, guided by craniometric landmarks. Neurophysiological and clinical tests were performed to identify the contacts position and the best analgesic responses to MCS. The grid was replaced by a definitive 4-contacts-electrode connected to an internalized system. Pain was evaluated by international scales. Changes in sympathetic symptoms, including temperature, perspiration, color and swelling were evaluated. Pre-operative and post-operative monthly evaluations were performed during one year. A double-blind maneuver was introduced assigning two groups. One had stimulators turned OFF from day 30-60 and the other from day 60-90. Four patients showed important decrease in pain, sensory and sympathetic changes during the therapeutic trial, while one patient did not have any improvement and was rejected for implantation. VAS and McGill pain scales diminished significantly (p<0.01) throughout the follow-up, accompanied by disappearance of the sensory (allodynea and hyperalgesia) and sympathetic signs. MCS is effective not only to treat pain, but also improve the sympathetic changes in CPRS. Mechanism of action is actually unclear, but seems to involve sensory input at the level of the spinal cord.

Cell-type specific properties of pyramidal neurons in neocortex underlying a layout that is modifiable depending on the cortical area

To understand sensory representation in cortex, it is crucial to identify its constituent cellular components based on cell-type-specific criteria. With the identification of cell types, an important question can be addressed: to what degree does the cellular properties of neurons depend on cortical location? We tested this question using pyramidal neurons in layer 5 (L5) because of their role in providing major cortical output to subcortical targets. Recently developed transgenic mice with cell-type-specific enhanced green fluorescent protein labeling of neuronal subtypes allow reliable identification of 2 cortical cell types in L5 throughout the entire neocortex. A comprehensive investigation of anatomical and functional properties of these 2 cell types in visual and somatosensory cortex demonstrates that, with important exceptions, most properties appear to be cell-type-specific rather than dependent on cortical area. This result suggests that although cortical output neurons share a basic layout throughout the sensory cortex, fine differences in properties are tuned to the cortical area in which neurons reside.

Delayed transplantation with exogenous neurotrophin administration enhances plasticity of corticofugal projections after spinal cord injury

Functional deficits following spinal cord injury (SCI) result from a disruption of corticofugal projections at the lesion site. Not only direct regeneration of the severed axons but also anatomical re-organization of spared corticofugal pathways can reestablish connections between the supraspinal and spinal motor centers. We have previously shown that delayed transplantation of fetal spinal cord tissue and neurotrophin administration by two weeks after SCI supported recovery of forelimb function in adult rats. The current study determined whether the same intervention enhances plasticity of corticofugal fibers at the midbrain and spinal cord level. Anterograde tracing of the left corticorubral fibers revealed that the animals with transplants and neurotrophins (BDNF or NT-3) increased the extent of the traced fibers crossing to the right red nucleus (RN), of which the axons are spared by a right cervical overhemisection lesion. More neurons in the left motor cortex were recruited by the treatment to establish connections with the right RN. The right corticorubral projections also increased the density of midline crossing fibers to the axotomized left RN in response to transplants and neurotrophins. Transplants plus NT-3, but not BDNF, significantly increased the amount of spared corticospinal fibers in the left dorsolateral funiculus at the spinal level both rostral and caudal to the lesion. These results suggest that corticofugal projections retain the capacity until at least two weeks after injury to undergo extensive reorganization along the entire neuraxis in response to transplants and neurotrophins. Targeting anatomical plasticity of corticofugal projections may be a promising strategy to enhance functional recovery following incomplete SCI.

Region-specific sprouting of crossed corticofugal fibers after unilateral cortical lesions in adult mice

Long considered to be limited to early development or restricted adult brain regions in mammals, axonal sprouting of spared axons into denervated brain areas now appears more widespread in the adult mammalian brain. However, its extent and mechanisms remain poorly understood. In this study, we show that robust sprouting of corticofugal axons occurs in the dorsolateral striatum but not the red nucleus of adult mice after unilateral lesions of the sensorimotor cortex induced either by mechanical removal or by thermocoagulation of pial blood vessels. These results show that local factors are critical for axonal sprouting in adult brain. They also extend previous findings in rats to a species readily amenable to genetic analysis in order to elucidate the mechanisms of this effect.

Prefrontal cortex in the rat: projections to subcortical autonomic, motor, and limbic centers

This paper describes the quantitative areal and laminar distribution of identified neuron populations projecting from areas of prefrontal cortex (PFC) to subcortical autonomic, motor, and limbic sites in the rat. Injections of the retrograde pathway tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) were made into dorsal/ventral striatum (DS/VS), basolateral amygdala (BLA), mediodorsal thalamus (MD), lateral hypothalamus (LH), mediolateral septum, dorsolateral periaqueductal gray, dorsal raphe, ventral tegmental area, parabrachial nucleus, nucleus tractus solitarius, rostral/caudal ventrolateral medulla, or thoracic spinal cord (SC). High-resolution flat-map density distributions of retrogradely labelled neurons indicated that specific PFC regions were differentially involved in the projections studied, with medial (m)PFC divided into dorsal and ventral sectors. The percentages that WGA-HRP retrogradely labelled neurons composed of the projection neurons in individual layers of infralimbic (IL; area 25) prelimbic (PL; area 32), and dorsal anterior cingulate (ACd; area 24b) cortices were calculated. Among layer 5 pyramidal cells, approximately 27.4% in IL/PL/ACd cortices projected to LH, 22.9% in IL/ventral PL to VS, 18.3% in ACd/dorsal PL to DS, and 8.1% in areas IL/PL to BLA; and 37% of layer 6 pyramidal cells in IL/PL/ACd projected to MD. Data for other projection pathways are given. Multiple dual retrograde fluorescent tracing studies indicated that moderate populations (<9%) of layer 5 mPFC neurons projected to LH/VS, LH/SC, or VS/BLA. The data provide new quantitative information concerning the density and distribution of neurons involved in identified projection pathways from defined areas of the rat PFC to specific subcortical targets involved in dynamic goal-directed behavior.

Electrical stimulation of the primary somatosensory cortex inhibits spinal dorsal horn neuron activity

Cortical stimulation has been demonstrated to alleviate certain pain conditions. The aim of this study was to determine the responses of the spinal cord dorsal horn neurons to stimulation of the primary somatosensory cortex (SSC). We hypothesized that direct stimulation of the SSC will inhibit the activity of spinal dorsal horn neurons by activating the descending inhibitory system. Thirty-four wide dynamic range spinal dorsal horn neurons were recorded in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields while a stepwise electrical stimulation (300 Hz, 0.1 ms, at 10, 20, and 30 V) was applied in the SSC through a bipolar tungsten electrode. The responses to brush at control, 10 V, 20 V, 30 V, and recovery were 16.0 +/- 2.3, 15.8 +/- 2.2, 14.6 +/- 1.8, 14.8 +/- 2.0, and 17.0 +/- 2.2 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, 30 V, and recovery were 44.7 +/- 5.5, 37.0 +/- 5.6, 29.5 +/- 4.8, 31.6 +/- 5.2, and 43.2 +/- 5.7 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, 30 V, and recovery were 58.1 +/- 7.0, 42.9 +/- 5.5, 34.8 +/- 3.9, 34.6 +/- 4.4, and 52.6 +/- 6.0 spikes/s, respectively. Significant decreases of the dorsal horn neuronal responses to pressure and pinch were observed during SSC stimulation. It is concluded that electrical stimulation of the SSC produces transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting innocuous stimuli.

Differential regulation of the growth-associated proteins GAP-43 and superior cervical ganglion 10 in response to lesions of the cortex and substantia nigra in the adult rat

Investigation of the elements underlying synapse replacement after brain injury is essential for predicting the neural compensation that can be achieved after various types of damage. The growth-associated proteins superior cervical ganglion-10 and growth-associated protein-43 have previously been linked with structural changes in the corticostriatal system in response to unilateral deafferentation. To examine the regulation of this response, unilateral cortical aspiration lesion was carried out in combination with ipsilateral 6-hydroxydopamine lesion of the substantia nigra, and the time course of the contralateral cortical molecular response was followed. Unilateral cortical aspiration lesion in rats corresponds with an upregulation of superior cervical ganglion-10 mRNA at 3 and 10 days post-lesion, and protein, sustained from three to at least 27 days following lesion. With the addition of substantia nigra lesion, the response shifts to an upregulation of growth-associated protein-43 mRNA at 3 and 10 days post-lesion, and protein after 10 days. Nigral lesion alone does not alter contralateral expression of either gene. Likewise, motor function assessment using the rotorod test revealed no significant long-term deficits in animals that sustained only nigrostriatal damage, but cortical lesion was associated with a temporary deficit which was sustained when nigrostriatal input was also removed. Growth-associated protein-43 and superior cervical ganglion-10, two presynaptic genes that are postulated to play roles in lesion-induced sprouting, are differentially upregulated in corticostriatal neurons after cortical versus combined cortical/nigral lesions. The shift in contralateral gene response from superior cervical ganglion-10 to growth-associated protein-43 upregulation and associated behavioral deficit following combined cortical and nigral denervation suggest that nigrostriatal afferents regulate cortical lesion-induced gene expression and ultimate functional outcome.

Functional circuits mediating sensorimotor integration: Quantitative comparisons of projections from rodent barrel cortex to primary motor cortex, neostriatum, superior colliculus, and the pons

Motor performance depends on somatosensory feedback, and consistent with this finding, primary somatosensory (SI) cortex projects to several regions involved in motor control. Although the pathways mediating sensorimotor integration are known, few studies have compared their projection patterns. Therefore, in each animal, we injected two anterograde tracers into SI barrel cortex and compared the relative density and spatial extent of the labeled projections to the primary motor (MI) cortex, neostriatum, superior colliculus, and basal pons. Quantitative analysis revealed that these projections terminated most extensively in the neostriatum, to a lesser extent in MI cortex, and innervated the least amount of neuropil in the superior colliculus and pontine nuclei. Tracer overlap in the pontine nuclei was significantly higher than in the other three brains regions, and was strongly correlated with overlap in the superior colliculus, presumably because some projections to these two brain regions represent collaterals of the same neurons. The density of labeled varicosities was highest in the pons and lowest in MI. As a proportion of total labeling, densely packed clusters of labeled terminals were most prevalent in the pons, less prevalent in neostriatum and superior colliculus, and least prevalent in MI cortex. These results are consistent with physiological evidence indicating strong coherence between SI barrel cortex and the cerebellum during whisking behavior.

Spinal dorsal horn neuron response to mechanical stimuli is decreased by electrical stimulation of the primary motor cortex

Motor cortex stimulation (MCS) has been used clinically as a tool for the control for central post-stroke pain and neuropathic facial pain. The underlying mechanisms involved in the antinociceptive effect of MCS are not clearly understood. We hypothesize that the antinociceptive effect is through the modulation of the spinal dorsal horn neuron activity. Thirty-two wide dynamic range spinal dorsal horn neurons were recorded, in response to graded mechanical stimulation (brush, pressure, and pinch) at their respective receptive fields, while a stepwise electrical stimulation was applied simultaneously in the motor cortex. The responses to brush at control, 10 V, 20 V, and 30 V, and recovery were 11.5+/-1.6, 12.1+/-2.6, 11.1+/-2.2, 10.5+/-2.1, and 13.2+/-2.5 spikes/s, respectively. The responses to pressure at control, 10 V, 20 V, and 30 V, and recovery were 33.2+/-6.1, 22.9+/-5.3, 20.5+/-5.0, 17.3+/-3.8, and 27.0+/-4.0 spikes/s, respectively. The responses to pinch at control, 10 V, 20 V, and 30 V, and recovery were 37.2+/-6.4, 26.3+/-4.7, 25.9+/-4.7, 22.5+/-4.3, and 35.0+/-6.2 spikes/s, respectively. It is concluded that, in the rat, electrical stimulation of the motor cortex produces significant transient inhibition of the responses of spinal cord dorsal horn neurons to higher intensity mechanical stimuli without affecting their response to an innocuous stimulus.

Target-specific differences in somatodendritic morphology of layer V pyramidal neurons in rat motor cortex

Dendritic geometry has been shown to be a critical determinant of information processing and neuronal computation. However, it is not known whether cortical projection neurons that target different subcortical nuclei have distinct dendritic morphologies. In this study, fast blue retrograde tracing in combination with intracellular Lucifer yellow injection and diaminobenzidine (DAB) photoconversion in fixed slices was used to study the morphological features of corticospinal, corticostriatal, and corticothalamic neurons in layer V of rat motor cortex. Marked differences in the distribution of soma, somal size, and dendritic profiles were found among the three groups of pyramidal neurons. Corticospinal neurons were large, were located in deep layer V, and had the most expansive dendritic fields. The apical dendrites of corticospinal pyramidal neurons were thick, spiny, and branched. In contrast, nearly all corticostriatal neurons were small cells located in superficial layer V. Their apical dendritic shafts were significantly more slender, though spiny like those of corticospinal neurons. Corticothalamic neurons, which were located in superficial layer V and in layer VI, had small or medium-sized soma, slender apical dendritic shafts, and dendrites that were largely spine free. This study indicates that, in layer V of rat motor cortex, each population of projection neurons has a unique somatodendritic morphology and suggests that distinct modes of cortical information processing are operative in corticospinal, corticostriatal, and corticothalamic neurons.

Functional recovery and neuroanatomical plasticity following middle cerebral artery occlusion and IN-1 antibody treatment in the adult rat

Stroke is a prevalent and devastating disorder, and no treatment is currently available to restore lost neuronal function after stroke occurs. One unique therapy that may improve functional recovery after stroke is blockade of the neurite inhibitory protein Nogo-A with the monoclonal antibody IN-1, through enhancement of neuroanatomical plasticity from uninjured areas of the central nervous system. In the present study, we combined IN-1 treatment with an ischemic lesion (permanent middle cerebral artery occlusion) to determine the effect of Nogo-A neutralization on cortical plasticity and functional recovery. We report here that, following ischemic stroke and treatment with IN-1, adult rats demonstrated functional recovery on a forelimb-reaching task and new cortico-efferent projections from the opposite, unlesioned hemisphere. These results support the efficacy of Nogo-A blockade as a treatment for ischemic stroke and implicate plasticity from the unlesioned hemisphere as a mechanism for recovery.

Neuronal plasticity and formation of new synaptic contacts follow pyramidal lesions and neutralization of Nogo-A: a light and electron microscopic study in the pontine nuclei of adult rats

Regeneration and compensatory sprouting are limited after lesions in the mature mammalian central nervous system in contrast to the developing central nervous system (CNS). After neutralization of the growth inhibitor Nogo-A, however, massive sprouting and rearrangements of fiber connections occurred after unilateral pyramidal tract lesions in adult rats: Corticofugal fibers from the lesioned side crossed the midline of the brainstem and innervated the contralateral basilar pontine nuclei. To determine whether these newly sprouted fibers formed synaptic contacts, we analyzed the corticofugal fibers in the basilar pontine nuclei contralateral to the lesion by light and electron microscopy 2 weeks after pyramidotomy and treatment with the Nogo-A-inhibiting monoclonal antibody IN-1 (mAb IN-1). The mAb IN-1, but not a control antibody, led to structural changes in the basilar pons ipsilateral and contralateral to the lesion site. Fibers sprouted across the pontine midline and terminated topographically. They established asymmetric synaptic contacts with the characteristics of normal corticopontine terminals. These results show that adult CNS fibers are able to sprout and to form new synaptic contacts after a lesion when a growth-permissive microenvironment is provided.

Effects of red nucleus ablation and exogenous neurotrophin-3 on corticospinal axon terminal distribution in the adult rat

Collateral sprouting of undamaged descending axons is one potential mechanism for recovery of function after incomplete spinal cord injury. In this study, we have investigated whether terminals of the intact corticospinal tract in the rat would sprout following ablation of a parallel descending pathway, the rubrospinal tract. No sprouting was detected after this injury alone. However, the combination of rubrospinal tract ablation with administration of 100ng neurotrophin-3 to neurons of the corticospinal tract resulted in marked increased density of corticospinal innervation in the superficial dorsal horn. There was no effect of administration of neurotrophin-3 alone and increase in axon density was not detected in the deep dorsal horn. These results imply that spontaneous sprouting of undamaged corticospinal axons does not occur following ablation of a parallel tract system, although collateral sprouting can be induced through a combination of the lesion plus exogenous growth factor. Induced change in corticospinal terminal density is detected in the superficial dorsal horn only, supporting the hypothesis that this is an area particularly supportive of circuit reorganisation.

Forward signaling mediated by ephrin-B3 prevents contralateral corticospinal axons from recrossing the spinal cord midline

To investigate Eph-ephrin bidirectional signaling, a series of mutations were generated in the ephrin-B3 locus. The absence of both forward and reverse signaling resulted in mice with mirror movements as typified by a hopping locomotion. The corticospinal tract was defective as axons failed to respect the midline boundary of the spinal cord and bilaterally innervated both contralateral and ipsilateral motor neuron populations. A second mutation that expresses a truncated ephrin-B3 protein lacking its cytoplasmic domain did not lead to hopping, indicating that reverse signaling is not required for corticospinal innervation. Ephrin-B3 is concentrated at the spinal cord midline, while one of its receptors, EphA4, is expressed in postnatal corticospinal neurons as their fibers pathfind down the contralateral spinal cord. Our data indicate ephrin-B3 functions as a midline-anchored repellent to stimulate forward signaling in EphA4-expressing axons.

Compensatory sprouting and impulse rerouting after unilateral pyramidal tract lesion in neonatal rats

After lesions of the developing mammalian CNS, structural plasticity and functional recovery are much more pronounced than in the mature CNS. We investigated the anatomical reorganization of the corticofugal projections rostral to a unilateral lesion of the corticospinal tract at the level of the medullary pyramid (pyramidotomy) and the contribution of this reorganization and other descending systems to functional recovery. Two-day-old (P2) and adult rats underwent a unilateral pyramidotomy. Three months later the corticofugal projections to the red nucleus and the pons were analyzed; a relatively large number of corticorubral and corticopontine fibers from the lesioned side had crossed the midline and established an additional contralateral innervation of the red nucleus and the pons. Such anatomical changes were not seen after adult lesions. Intracortical microstimulation of the primary motor cortex with EMG recordings of the elbow flexor muscles were used to investigate possible new functional connections from the motor cortex of the pyramidotomy side to the periphery. In rats lesioned as adults, stimulation of the motor cortex ipsilateral to the pyramidotomy never elicited EMG activity. In contrast, in P2 lesioned rats bilateral forelimb EMGs were found. EMG latencies were comparable for the ipsilateral and contralateral responses but were significantly longer than in unlesioned animals. Transient inactivation of both red nuclei with the GABA receptor agonist muscimol led to a complete loss of these bilateral movements. Movements and EMGs reappeared after wash-out of the drug. These results suggest an important role of the red nucleus in the reconnection of the cortex to the periphery after pyramidotomy.

Differential regulation of the growth-associated proteins, GAP-43 and SCG-10, in response to unilateral cortical ablation in adult rats

Synapse replacement after brain injury has been widely documented by anatomical studies in various parts of both the developing and adult nervous system. However, the molecular events that define the specificity of the empirically derived rules of reactive synaptogenesis in different regions of the adult brain remain unclear. In this study we examined the differential regulation of the lesion-induced response of the two growth-associated proteins, superior cervical ganglia-10 and growth-associated protein-43, after unilateral cortex ablation, and determined a hierarchical order for the lesion response from remaining afferent projection neurons originating from the contralateral cortex, ipsilateral thalamus and substantia nigra. We report that in response to unilateral cortex ablation both messenger RNA, by northern blot, and protein, by western blot, for superior cervical ganglia-10 but not growth-associated protein-43 was increased in the homologous area of the contralateral cortex but not the ipsilateral thalamus or substantia nigra. In addition, the specificity of the superior cervical ganglia-10 response, assessed by combined in situ hybridization and retrograde FluoroGold labeling of striatal afferent neurons, found that superior cervical ganglia-10 messenger RNA was increased prominently in layer V pyramidal neurons of the contralateral corticostriatal pathway but was unchanged in afferent projection neurons from the thalamus and substantia nigra. Furthermore, the increase in both superior cervical ganglia-10 messenger RNA and protein seen at three days postlesion in contralateral corticostriatal neurons coincides in time with the initiation of neurite outgrowth in the deafferented striatum by contralateral corticostriatal axons described in our previous ultrastructural study. However, if cortical input to the striatum was removed bilaterally the lesion-induced response for superior cervical ganglia-10 messenger RNA shifted secondarily to thalamostriatal neurons in the ipsilateral thalamus. These data provide evidence that superior cervical ganglia-10 and growth-associated protein-43 are differentially regulated in neurons of the contralateral corticostriatal pathway in response to unilateral cortex ablation and suggests that superior cervical ganglia-10 plays a role in the regulation of neurite outgrowth in the adult striatum after brain injury. However, the specific role that superior cervical ganglia-10 may play in reactive synaptogenesis remains unclear. In addition, our data suggest that a hierarchical order exists for the reinnervation of deafferented striatal neurons after unilateral cortex ablation with preference given to homologous axons from the contralateral cortex.

Afferent and efferent connections of the caudolateral neostriatum in the pigeon (Columba livia): a retro- and anterograde pathway tracing study

The avian caudolateral neostriatum (NCL) was first identified on the basis of its dense dopaminergic innervation. This fact and data from lesion studies have led to the notion that NCL might be the avian equivalent of prefrontal cortex (PFC). A key feature of the PFC is the ability to integrate information from all modalities needed for the generation of motor plans. By using antero- and retrograde pathway tracing techniques, we investigated the organization of sensory afferents to the NCL and the connections with limbic and somatomotor centers in the basal ganglia and archistriatum. Data from all tracing experiments were compared with the distribution of tyrosine-hydroxylase (TH)-immunoreactive fibers, serving as a marker of dopaminergic innervation. The results show that NCL is reciprocally connected with the secondary sensory areas of all modalities and with at least two parasensory areas. Retrograde tracing also demonstrated further afferents from the deep layers of the Wulst and from the frontolateral neostriatum as well as the sources of thalamic input. Efferents of NCL project onto parts of the avian basal ganglia considered to serve somatomotor or limbic functions. Projections to the archistriatum are mainly directed to the somatomotor part of the intermediate archistriatum. In addition, cells in caudal NCL were found to be connected with the ventral and posterior archistriatum, which are considered avian equivalents of mammalian amygdala. All afferents and projection neurons were confined to the plexus of densest TH innervation. Our results show that the NCL is positioned to amalgamate information from all modalities and to exert control over limbic and somatomotor areas. This organization might comprise the neural basis for such complex behaviours as working memory or spatial orientation.

Postnatal growth of corticospinal axons in the spinal cord of developing mice

The corticospinal tract (CST) plays an important role in the control of voluntary movements. Although the development of the CST has been studied extensively in other species, limited information is available on its development in mice. In the present study, the growth of corticospinal axons was characterized in developing mice using Phaseolus vulgaris leucoagglutinin (PHA-L). Our results indicate that the leading CST axons reach the 8th cervical segment at postnatal day (PD) 2, the 7th thoracic segment at PD4, the 13th thoracic segment at PD7, and the 5th lumbar segment at PD9. The arrival of corticospinal axons at the distal lumbar cord at PD9 was further confirmed by retrograde tracing using fast blue (FB). A waiting period of 2-3 days exists after the leading CST axons pass a particular segment before sending collaterals into the gray matter of that segment. The CST continues to increase in size in lower thoracic and lumbar areas up to PD14 when its adult appearance is achieved. In this study, the date of animal's sacrifice was used as the specific postnatal date to demonstrate the growth of the CST. This definition gives a more reliable indication of the exact location of the CST at a specific developmental time point since the CST continues to grow after tracer injections and since the dye is transported much faster than axonal growth. We suggest that these findings can be used as a template for studies on both normal and transgenic mice where some developmental significance is given to the CST.

Convergent prefrontal cortex and mamillary body projections to the medial pontine nuclei: a light and electron microscopic study in the rat

To study the convergence of medial prefrontal cortex and mamillary body projections to the medial pontine nuclei, light and electron microscopic, neuroanatomical, tract-tracing experiments were performed. Injections of horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), biotin conjugated to dextran (BD), or rhodamine conjugated to dextran (RD) were made individually or in combinations into the cerebral cortex, hypothalamus, or pons. In addition, injections of WGA-HRP into the medial prefrontal cortex and electrolytic lesions of the mamillary body were made to study the synaptology of afferent projections to the pontine nuclei. In the light microscopic studies, injections of WGA-HRP into the rostromedial pontine nuclei produced dense, retrograde labeling both in the dorsal peduncular area of the medial prefrontal cortex and in the medial mamillary nucleus, pars medialis. Injections of the anterograde tracers BD and RD into the medial prefrontal cortex and the medial mamillary nuclei, respectively, resulted in partially overlapping terminal fields in the rostromedial pontine nuclei. In the electron microscopic studies, injections of WGA-HRP into the dorsal peduncular area and electrolytic lesions of the mamillary body produced anterogradely labeled axon terminals and degenerating axon terminals that synapsed on the same dendrites or neuronal somata in the rostromedial pontine nuclei. The results demonstrate that the medial prefrontal cortex and the medial mamillary nuclei have partially overlapping projections to the rostromedial pontine nuclei and implicate precerebellar relay nuclei in the integration of limbic and/or autonomic functions mediated by convergent projections from the cerebral cortex and the hypothalamus.

Neonatal lesion of the rat's frontal cortex and subsequent transplantation of embryonic frontal cortex: evidence of appropriate synaptic integration of the graft neurons within the host thalamo-fronto-striate circuit

Previous observations in intact rats have indicated that axons from the ventrolateral thalamic nucleus (VL) establish direct axo-somatic or axo-dendritic contacts onto frontal cortical neurons projecting to the striatum. The embryonic frontal cortex was grafted into the damaged frontal cortex of newborn rats to study the capacity of homotopic transplants to restore the thalamo-fronto-striate pathway. Several months later, grafted neurons projecting to the striatum were identified by injecting a retrograde neurotracer (subunit b of the cholera toxin) into the ipsilateral caudate putamen. In the same animal, axons and terminations from the VL were labeled within the transplant with an anterograde neurotracer (Phaseolus vulgaris leuco-agglutinin) injected into the ipsilateral VL. The findings show that VL axons establish direct synaptic contacts onto grafted neurons projecting to the striatum. Although the synaptic contacts were scarce in the transplants, their organization was similar to that observed in intact rats. The contacts were axo-somatic or axo-dendritic. Our observations for the first time indicate that synaptic contacts are formed in cortical grafts and that fetal frontal cortex is susceptible to develop appropriate synaptic integration within the host thalamo-fronto-striate system.

Comparison of cortically and subcortically controlled motor systems: I. Morphology of intracellularly filled rubrospinal neurons in rat and turtle

The rat and turtle differ markedly in major structural features of the corticocerebellorubrospinal circuitry. Although both species have a well-developed cerebellorubrospinal system, they differ in that a direct cerebral cortical input to the red nucleus is present only in the rat. The aim of the present study was to compare features of the soma and dendritic morphology of rubrospinal neurons that receive cortical input, as in rats, with those that do not, as in turtles. Intracellular Lucifer Yellow injections of neurons retrogradely labeled with Fast Blue in the rat or activity-dependent sulforhodamine-labeled neurons in the turtle were used to fill rubrospinal neurons in 150-200-microm-thick fixed sections. Images of filled neurons were imported into a computer to analyze quantitatively soma and dendritic morphology. The results show that rubrospinal soma size is slightly larger in the rat than in the turtle. However, analysis of the dendritic morphology, including total dendritic length, length of primary, secondary, and tertiary dendritic branches, and a Scholl analysis of dendritic branch intersections across concentric rings, demonstrated no significant differences between the two species. These findings suggest that the basic dendritic morphology of rubrospinal neurons may have been established early in phylogeny, preceding the evolution of cortical inputs. Alternatively, similar dendritic morphologies may have arisen due to the presence of other synapses in the turtle that occupy the sites of the cortical input in the rat. This comparative approach provides insights into the information processing capabilities of cortically versus subcortically controlled motor systems.

Functional recovery and enhanced corticofugal plasticity after unilateral pyramidal tract lesion and blockade of myelin-associated neurite growth inhibitors in adult rats

After a lesion of the mature CNS, structural plasticity and functional recovery are very limited, in contrast to the developing CNS. The postnatal decrease in plasticity is correlated in time with the formation of myelin. To investigate the possible role of an important myelin-associated neurite growth inhibitor (NI-250; IN-1 antigen), one pyramidal tract of adult Lewis rats was lesioned (pyramidotomy), and the rats were treated with the antibody IN-1, a control antibody, or no antibody. Functional recovery was studied from postoperative day 14 until day 42 using a food pellet reaching task, rope climbing, and a grid walk paradigm. The corticofugal projections to the red nucleus and basilar pontine nuclei were analyzed after survival times of 2 and 16 weeks. Treatment with the monoclonal antibody IN-1 resulted in almost complete restoration of skilled forelimb use, whereas all the control groups showed severe and chronic impairments. This functional recovery was paralleled by sprouting of the corticorubral and the corticopontine fibers across the midline, thus establishing a bilateral, anatomically specific projection.