Increased corticofugal plasticity after unilateral cortical lesions combined with neutralization of the IN-1 antigen in adult rats.

If damage to the central nervous system (CNS) occurs early in life, extensive rearrangements of the remaining fiber systems as well as regeneration of lesioned fibers take place. In the rat or hamster, newly grown projections have been described only if the lesion occurred within the first two weeks postnatally. This decreasing growth ability correlates with CNS maturation and the progression of myelination. Myelin contains the potent neurite growth inhibitors NI-35/250 that are crucially involved in the failure of long-distance regeneration and the lack of compensatory structural plasticity after adult CNS lesions. In this study, we show that extensive remodeling occurs well after the termination of the growth permissive period in the adult rat if we neutralize the inhibitory properties of myelin with the monoclonal antibody IN-1. After ablation of one motor cortex and treatment with the antibody IN-1, we observed that the remaining corticospinal tract (CST) from the spared hemisphere sprouted into the denervated, contralateral red nucleus and pons. In the pons, these fibers terminated in a typical somatotopic pattern. For comparison with neonatal plasticity, we performed the same lesion in two-day-old rats (no antibody). This lesion led as well to sprouting of the remaining CST into denervated brainstem nuclei, resulting in a bilateral corticofugal projection. Our results show that neutralization of myelin-associated neurite-growth inhibitors after CNS lesions leads to a structural remodeling of the spared corticofugal fibers in adult rats, a process normally restricted to a short postnatal period.     

Improving axonal growth and functional recovery after experimental spinal cord injury by neutralizing myelin associated inhibitors

Injuries of the spinal cord often result in an irretrievable loss of motor and sensory functions of all body parts situated below the lesion site. Functional recovery is restricted mainly due to the limited regeneration and plasticity of injured axons in the adult central nervous system. Over the last few years different experimental approaches have led to axonal growth and functional benefits in animal models. This review focuses on the effects of the neutralization of myelin-associated neurite growth inhibitors, in particular Nogo-A, using the monoclonal antibody IN-1. Acute mAb IN-1 treatment of adult CNS lesioned rats results in extensive plastic changes of neuronal connections and regenerative fiber growth. In two different lesion paradigms (i.e. pyramidal tract lesion and incomplete spinal cord lesion in adult rats), the mAb IN-1-treated animals always showed a higher degree of recovery in various behavioral tests. These observations, together with electrophysiological results, suggest that neuronal CNS circuits of mAb IN-1-treated animals can be rearranged, and that sprouting and regenerating axons form functionally meaningful connections.     

Corticostriatal plasticity is restricted by myelin-associated neurite growth inhibitors in the adult rat.

After unilateral cortical lesions in neonatal rats, the spared unablated hemisphere is known to demonstrate remarkable neuroanatomical plasticity in corticofugal connectivity. This same type of structural plasticity is not seen after similar lesions in adult rats. One possibility for the lack of such a plastic response in the adult central nervous system may be the presence of myelin-associated neurite growth inhibitory proteins NI-35/NI-250. These proteins have previously been found to play a crucial role in preventing axotomized fibers from regenerating after adult rat spinal cord lesions. The aim of this study was to determine if blocking these inhibitory proteins by the application of the specific monoclonal antibody IN-1 would enhance corticostriatal plasticity from the spared hemisphere after unilateral cortical lesions in adult rats. Six- to 8-week-old Lewis rats underwent unilateral aspiration lesion of the sensorimotor cortex. Animals were immediately treated with either monoclonal antibody IN-1 or a control antibody released from hybridoma cells in Millipore filter capsules. After a survival period of 12 weeks, the opposite sensorimotor cortex was stereotaxically injected with the anterograde tracer biotinylated dextran amine, and biotinylated dextran amine-positive corticostriatal fibers were analyzed. The monoclonal antibody IN-1-treated animals showed an increase in corticostriatal fibers in the dorsolateral striatum contralateral to the injection site compared with control antibody-treated animals or normal controls, indicating a specific sprouting response in the deafferented zone. These results support the idea that through blockade of myelin-associated neurite inhibitory proteins, lesion-induced corticofugal plasticity is possible even in the adult central nervous system.     

Structural plasticity of the adult CNS. Negative control by neurite growth inhibitory signals

The neuronal network of the adult central nervous system (CNS) retains a limited capacity for growth and structural change. This structural plasticity has been best studied in the context of lesion-induced growth and repair. More recently, structural changes underlying functional plasticity occurring under specific physiological conditions have also been documented, in particular in the cortex and the hippocampus. Areas known for their adult plastic potential retain high levels of the growth associated protein GAP-43, suggesting a persistence of important components of the intracellular growth machinery throughout life. Interestingly, a pronounced negative correlation exists between the levels of GAP-43 and myelination in the adult CNS. Because CNS myelin contains potent neurite growth inhibitory membrane proteins, neurite growth, sprouting and plasticity were investigated in the spinal cord and brain in areas where oligodendrocyte development and myelin formation was experimentally prevented, or in the presence of an inhibitor neutralizing antibody (mAB-IN-1). In all areas, lesion-induced or spontaneous sprouting was enhanced, in parallel with persistent high levels of GAP-43. Thus, spontaneous sprouting of side branches occurred from retinal axons in the optic nerve in the absence of myelin, and target-deprived retinal axons showed increased sprouting and innervation of the contralateral optic tectum in the presence of mAB IN-1. In experimentally myelin-free spinal cords collaterals from intact dorsal roots grew over long distances to innervate deafferented target regions following the section of three dorsal roots. Similarly, the corticospinal tract sprouted across the the midline and re-established a dense plexus of fibres on the contralateral side of the spinal cord following section of one corticospinal tract in juvenile rats. Following bilateral dorsal hemisection of the spinal cord including both corticospinal tracts in young and adult rats, long distance regeneration of corticospinal fibres leading to significant functional improvements of locomotion and certain reflexes was induced by the neurite growth inhibitor neutralizing antibody IN-1.     

Regenerating corticospinal fibers in the Marmoset (Callitrix jacchus) after spinal cord lesion and treatment with the anti-Nogo-A antibody IN-1

Neutralizing the myelin-associated growth inhibitor Nogo-A in adult spinal cord-injured rats can promote regeneration of injured and compensatory sprouting of uninjured axons. Nogo-A is present in humans, making its neutralization a possible novel treatment option for paraplegic patients. In this study we examined the effects of an extensively used anti-Nogo-A antibody (mAb IN-1) on the regenerative capabilities of lesioned corticospinal tract (CST) axons in a primate, the Marmoset monkey. Unilateral thoracic lesions of the CST were performed in six adult Marmosets, followed by the application of mAb IN-1 into the cerebrospinal fluid circulation by a graft of hybridoma cells. A unilateral injection of biotin dextran amine into the motor cortex was performed to analyse sprouting and regeneration of the lesioned axons. In the control antibody-treated animal CST fibers stopped rostral to the lesion site and often showed retraction bulbs. In contrast, in four out of five mAb IN-1-treated animals fine labeled neurites had grown into, through and around the lesion site. Thus, this study provides first anatomical evidence that in primates, the neutralization of the myelin-associated inhibitor Nogo-A results in increased regenerative sprouting and growth of lesioned spinal cord axons.     

Delayed treatment with monoclonal antibody IN-1 1 week after stroke results in recovery of function and corticorubral plasticity in adult rats.

Neuronal death due to ischemic stroke results in permanent deficits in sensory, language, and motor functions. The growth-restrictive environment of the adult central nervous system (CNS) is an obstacle to functional recovery after stroke and other CNS injuries. In this regard, Nogo-A is a potent neurite growth-inhibitory protein known to restrict neuronal plasticity in adults. Previously, we have found that treatment with monoclonal antibody (mAb) IN-1 to neutralize Nogo-A immediately after stroke enhanced motor cortico-efferent plasticity and recovery of skilled forelimb function in rats. However, immediate treatment for stroke is often not clinically feasible. Thus, the present study was undertaken to determine whether cortico-efferent plasticity and functional recovery would occur if treatment with mAb IN-1 was delayed 1 week after stroke. Adult rats were trained on a forelimb-reaching task, and the middle cerebral artery was occluded to induce focal cerebral ischemia to the forelimb sensorimotor cortex. After 1 week, animals received mAb IN-1 treatment, control antibody, or no treatment, and were tested for 9 more weeks. To assess cortico-efferent plasticity, the sensorimotor cortex opposite the stroke lesion was injected with an anterograde neuroanatomical tracer. Behavioral analysis demonstrated a recovery of skilled forelimb function, and anatomical studies revealed neuroplasticity at the level of the red nucleus in animals treated with mAb IN-1, thus demonstrating the efficacy of this treatment even if administered 1 week after stroke.     

Orthograde axonal transport studies of projections from the zona incerta and pretectum to the basilar pontine nuclei in the rat

This study employed orthogradely transported axonal tracers to demonstrate, in the rat, projections that reach the basilar pontine nuclei from the zona incerta or pretectal nuclei. Except for the most rostral levels, all subdivisions of the zona incerta give rise to substantive basilar pontine projections. Although some topographic differences exist among the temination patterns of various subdivisions, no clear somatotopically organized scheme is apparent. Most incertopontine axons descend to the basilar pons in association with fibers of the medial lemniscus or crus cerebri and reach ipsilateral ventral and medial pontine gray regions. A sparse number of terminals are evident in the contralateral medial pontine gray. The anterior pretectal axons also descend with the medial lemniscus and crus cerebri to enter exclusively the ipsilateral basilar pons where they terminate most densely in ventral and medial regions. Dual orthograde labeling experiments indicate that some pretectal terminal fields in the pontine gray are shared with incertopontine projections and with afferents from the dorsal column nuclei. This potential convergence of basilar pontine afferent projections is significant in light of (1) the known somatosensory input to the zona incerta and pretectum and (2), the fractured somatotopy of peripheral cutaneous inputs that arrive in the cerebellar cortex via mossy fibers. The present studies also employed electron microscopy to identify synaptic boutons formed by incerto- and pretectopontine axons, and they proved to be remarkably similar. Each is a medium to small-sized bouton that contains spheroidal synaptic vesicles and forms asymmetric membrane specializations. Most incerto- and pretectopontine boutons participate in glomerular synaptic complexes that include a single, centrally located bouton contacted on its perimeter by several types of dendritic profiles including shafts and spine-like appendages. A relatively small number of labeled boutons of either type contacts single, isolated dendritic elements in the neuropil. Taken together, these findings suggest that some basilar pontine neurons might receive convergent inputs from the zona incerta and pretectum as well as other somatosensory related systems such as the dorsal column nuclei and sensorimotor cortex. © 1995 Wiley-Liss, Inc.     

Oligodendrocyte- and myelin-associated inhibitors of neurite outgrowth: their involvement in the lack of CNS regeneration.

Until now central nervous system (CNS) neurites have been thought to have little capacity for regeneration following a lesion. When allowed to grow into peripheral nervous system (PNS) grafts, however, lesioned CNS axons are known to regenerate. Recently, an inhibitory substrate effect of CNS myelin and oligodendrocytes has been discovered which could be directly involved in the lack of regeneration. In culture, neurite growth cones were shown to specifically arrest their movement when contacting oligodendrocyte processes. The inhibitory components were characterized as two proteins of 35 and 250 kDa. A specific monoclonal antibody was generated (IN-1) that could neutralize these inhibitory effects. The role of the inhibitors in CNS regeneration was investigated in young rats receiving lesions of the corticospinal tract and implanted with a source of IN-1 mAB or control mAB. Results showed clear regeneration to over 10 mm in 2-5 weeks in IN-1 mAB-treated animals, while no fibers were detected further than 1 mm caudal to the lesion in controls. A similar, highly significant enhancement of regeneration was also found for the cholinergic septohippocampal pathway and for the optic nerve. These results show that lesioned CNS neurons can regenerate in CNS tissue when specific myelin components are neutralized, thus demonstrating that these inhibitory components play a crucial role in the lack of CNS regeneration.     

Regeneration and Sprouting of Chronically Injured Corticospinal Tract Fibers in Adult Rats Promoted by NT-3 and the mAb IN-1, Which Neutralizes Myelin-Associated Neurite Growth Inhibitors

Myelin-associated inhibitors of neurite growth play an important role in the regenerative failure after injury in the adult mammalian CNS. The application of the mAb IN-1, which efficiently neutralizes the NI-250/35 inhibitory proteins, alone or in combination with neurotrophin-3 (NT-3), has been shown to promote axonal regeneration when applied in acute injury models. To test whether IN-1 application can induce axonal growth also in a chronic injury model, we treated rats with IN-1 and NT-3 starting 2 or 8 weeks after injury. Rats underwent bilateral dorsal hemisection of the spinal cord at the age of 5–6 weeks. Regeneration of corticospinal (CST) fibers into the caudal spinal cord was observed in three of eight of those animals with a 2-week delay between lesion and treatment. CST fibers regenerated for 2–11.4 mm. In the control group sprouting occurred rostral to the lesion but no long-distance regeneration occurred. In animals where treatment started at 8 weeks after injury the longest fibers observed grew up to 2 mm into the caudal spinal cord. The results show that transected corticospinal axons retain the ability to regenerate at least for a few weeks after injury. Functional analysis of these animals showed a slight improvement of functional recovery.     

Corticobulbar projections of the marsupial phalanger (Trichosurus vulpecula). I. Projections to the pons and medulla oblongata

A total of 27 adult phalangers was employed to investigate the pattern of neocortical projections to the pontine and medullary portions of the brain stem. Lesions restricted to neocortical areas rostral to the orbital sulcus resulted in fiber degeneration which distributed mainly to midline and medial areas of the pontine and medullary reticular formation. The greatest amount of fiber degeneration was located within the superior central nucleus, the nucleus of the pontine raphe, the nucleus pontis centralis oralis and the nucleus pontis centralis caudalis. However, a few degenerating fibers were present within the nucleus gigantocellularis and the magnocellular portion of the medullary raphe. In contrast, lesions which were located just caudal to the orbital sulcus resulted in fiber degeneration chiefly within the more lateral parvocellular reticular formation and within the subnucleus dorsalis of the nucleus medullae oblongatae centralis. In such cases, additional degenerating fibers were present within the dorsal column nuclei and within more medial areas of the reticular formation. In those brains with ventral parietal ablations, degenerating fibers were present within the chief sensory and spinal nuclei of the trigeminal complex and the closely adjacent reticular formation. All of the above neocortical lesions resulted in fiber degeneration within the basilar pontine gray. In those specimens subjected to caudal (striate and peristriate) or ventrocaudal (temporal) lesions, degenerating fibers were present within the basilar pontine gray, but not within other areas of the pons or the medulla oblongata.     

A qualitative electron microscopic study of the corticopontine projections after neonatal cerebellar hemispherectomy

The present study shows that 3–5 days following leions of the dentate and interposed nuclei in normal adult rats degenerating axons and axon terminals can be detected in the contralateral pontine gray. The degenerating axon terminals form Gray's type I axo-dendritic contacts with fine intermediate dendrites measuring between 0.8–2.4 μm. The present study also investigates, by electron microscopy, the synaptic rearrangement of the sensorimotor corticopontine projections following neonatal left cerebellar hemispherectomy¹⁹. Following neonatal left cerebellar hemispherectomy, the right sensorimotor and adjacent cortex (SMC) presents a very dense ipsilateral and a modest amount of contralateral corticopontine projections in contrast with a predominantly ipsilateral corticopontine projection seen in the normal adult rat. As with the ipsilateral corticopontine projection seen in the normal adult animal, the bilateral corticopontine projections seen in the experimental animals form contacts with dendrites suggestive of Gray's type I synapses. While the corticopontine projections in normal control animals form synapses with fine dendrites measuring 0.2–1.2 μm the corticopontine projections in the experimental animals form synaptic relations with fine dendrites and with intermediate dendrites measuring 0.2–2.4 μm. As the normal cerebellopontine fibers from the dentate and interposed nuclei also form axo-dendritic synapses on fine and intermediate dendrites and the contacts formed are also of Gray's type I synapses, it is possible that some of the newly formed corticopontine fibers in the experimental animals might have replaced the cerebellopontine fibers synapsing on intermediate dendrites. Synaptic rearrangement appears to take place as suggested by the presence of synaptic complexes in which one axon terminal contacts two of more dendrites or two or more axon terminals contact one dendrite. Such complexes are frequently seen to undergo degeneration following the right SMC lesion in the experimental animals. Other complex synaptic structures are also present in both the right and left pontine gray in the experimental animals. They are not seen to undergo degeneration following the right SMC lesions. Occasional features of neuronal reaction could still be seen both sides of the pontine gray for as long as 3–6 months after the neonatal cerebellar lesions.     

Nonregenerative axonal growth within the mature mammalian brain: ultrastructural identification of sympathohippocampal sprouts

Damage to septohippocampal neurons in the adult rat results in sprouting of sympathetic axons into the denervated hippocampal formation. However, the distribution of sympathohippocampal fibers has only been assessed with light microscopic techniques, and it is not known if the sprouted fibers leave the blood vessels, along which they migrate into the hippocampal formation, to enter the hippocampal neuropil and, if they do, whether they form synaptic contacts with central neurons. Using the tetramethylbenzidine technique to visualize anterogradely transported wheat germ agglutinin-horseradish peroxidase conjugate, we identified sprouted sympathetic fibers in the hippocampal formation at both the light and electron microscopic level in albino rats receiving medial septal lesions. The majority of labeled fibers were observed within the regions immediately above and below the granule cell layer. Although most of the labeled sprouts were observed in association with intraparenchymal blood vessels, where they were usually apposed to the basal lamina, approximately a third of the labeled profiles were present within the neuropil with no obvious vascular relationships. Most of the profiles were identified as unmyelinated axons or vesicle-filled varicosities. Many of the latter structures contained small dense-cored vesicles, but in our sample none of the labeled profiles were observed to form membrane specializations with adjacent structures, and many were partly surrounded by presumed astrocytic processes. These results document the invasion of the CNS by sprouting axons of peripheral origin indicating that axonal elongation from uninjured neurons can occur within the mature mammalian CNS under certain circumstances. In addition, the presence of significant numbers of sympathetic fibers within the hippocampal neuropil indicates that they may be in a strategic position to influence hippocampal function.     

Behavioral recovery and anatomical plasticity in adult rats after cortical lesion and treatment with monoclonal antibody IN-1

We have previously reported that monoclonal antibody (mAb) IN-1 treatment after ischemic infarct in adult rats results in significant recovery of skilled forelimb use. Such recovery was correlated with axonal outgrowth from the intact, opposite motor cortex into deafferented subcortical motor areas. In the present study, we investigated the effects of mAb IN-1 treatment after adult sensorimotor cortex (SMC) aspiration lesion on behavioral recovery and neuroanatomical plasticity in the corticospinal tract. Adult rats underwent unilateral SMC aspiration lesion and treatment with either mAb IN-1 or a control Ab, or no treatment. Animals were then tested over a 6-week period in the skilled forelimb use task and the skilled ladder rung walking task. We found that animals treated with mAb IN-1 after SMC lesion fully recovered the use of forelimb reaching, but showed no improvement in digit grasping as tested in the skilled forelimb use task. The mAb IN-1 treatment group was also significantly improved as compared to control groups in the skilled ladder rung walking test. Furthermore, neuroanatomical tracing revealed a significant increase in the corticospinal projections into the deafferented motor areas of the spinal cord after mAb IN-1 treatment. These results indicate that treatment with mAb IN-1 after cortical aspiration lesion induces remodeling of motor pathways resulting in recovery in only certain behavioral tasks, suggesting that the cause of brain damage influences behavioral recovery after mAb IN-1 treatment.     

PlexinA2 limits recovery from corticospinal axotomy by mediating oligodendrocyte-derived Sema6A growth inhibition

Axonal growth from both intact and severed fibers is limited after adult mammalian CNS injury. Myelin proteins contribute to inhibition of axonal growth. Semaphorin6A protein inhibits the extension of developing axons and is highly expressed in adult oligodendrocytes. This expression pattern suggests that a developmental axon guidance cue contributes to the restriction of adult CNS growth. Here, we assessed the role of a Sema6A receptor, PlexinA2, in recovery from adult trauma. Adult sensory neuron inhibition by Sema6A requires PlexinA2, with complete protection in PlexinA2-/- cultures. Mice lacking another myelin inhibitor receptor, NgR1, are known to exhibit greater axonal sprouting and functional recovery after lesions of the corticospinal tract at the medullary pyramid, so we investigated PlexinA2 in this lesion. Without injury, the corticofugal projection into the cervical spinal cord is normal in adult PlexinA2 null mice. After unilateral pyramidotomy, unlesioned PlexinA2-/- corticospinal fibers sprout across the midline to innervate the contralateral gray matter of the spinal cord to a significantly greater extent than do fibers in wild type mice. Sprouted fibers display frequent synaptophysin-positive synaptic puncta. The increased axonal growth in PlexinA2-/- mice after injury is accompanied by improved behavioral recovery in a pellet retrieval task using the impaired forelimb, and in a tape removal task. Thus, PlexinA2, as a receptor for oligodendrocyte-derived Sema6A and for secreted class 3 Semaphorins, plays a role in limiting adult axon growth and recovery after trauma.     

Amygdaloid pathway to the trigeminal motor nucleus via the pontine reticular formation in the rat

The connections of the amygdala with the trigeminal motor nucleus were studied by light and electron microscopy. Horseradish peroxidase (HRP) experiments showed that the pontine reticular formation, ventromedial to the spinal trigeminal nucleus at the level rostral to the genu of the facial nerve, receives fibers from the central nucleus of the amygdala ipsilaterally and sends fibers to the trigeminal motor nucleus contralaterally. Electron microscopic observations were carried out on the pontine reticular formation after electrolytic lesions in the central nucleus of the amygdala and HRP injections into the contralateral trigeminal motor nucleus were made on the same animal. These experiments using the combined degeneration and HRP technique clearly demonstrated that degenerating amygdaloid fibers made synaptic contacts with retrogradely labeled neurons.     

Sprouting and regeneration after pyramidotomy and blockade of the myelin-associated neurite growth inhibitors NI 35/250 in adult rats

After a selective unilateral lesion of the corticospinal tract (CST) at the level of the brainstem (pyramidotomy) and neutralization of the myelin associated neurite growth inhibitors NI-35/250 with the monoclonal antibody (mAb) IN-1, we had previously observed a strong behavioural recovery in parallel with an enhanced structural plasticity of the lesioned as well as the unlesioned CST. The present study focuses on the regenerative response of the cut CST axons at the lesion site in these adult rats. The results show a strong enhancement of regenerative sprouting of CST fibres by treatment with the mAb IN-1. Successful elongation of these sprouts through the pyramidal decussation and into the cervical spinal cord was also dependent on the presence of this antibody. In the spinal cord, regenerating fibres were rarely found in the position of the former CST; most of the fibres were distributed seemingly randomly over the entire lateral extent of the spinal cord.     

Plasticity of catecholaminergic terminals in rat paraventicular hypothalamic nucleus after 6-Hydroxydopamine lesion: an emphasis on bouton sizes and synaptic frequency

Catecholaminergic (CA) nerve terminals in the paraventicular hypothalamic nucleus (PVN) of adult rats were studied at 4, 21, 56 and 180 days after a single injection of 6-hydroxydopamine (6-OHDA) neurotoxin into the right lateral ventricle of the brain. We previously described and quantified the extent of CA terminal sprouting in the PVN after 6-OHDA lesions. For this communication we studied parameters, specifically the bouton sizes and the synaptic frequencies of CA terminals during the renewal process, and evaluated how changes of these parameters are ralated to axonal sprouting. The CA boutons were identifiable in the electron microscope by exhibiting small granular vesicle (SGVs) after central administration of 5-hydroxydopamine (5-OHDA) marker. The marked CA boutons were measured and further categorized according to whether or not they were associated with distinct synaptic specializations at various post-lesion stages. The average sizes of CA boutons were strikingly similar in their diameters (1.0 υm) for both control and experimental tissues. However, CA boutons larger than 2.1 υm were rare and seen more often in the experimental tissues with 6-OHDA lesion and were sustained up to 180 days after lesions. Catecholaminergic profiles with ultrastructural features of growth cones were also seen in the PVN following the 6-OHDA lesions, indicating that there is growth activity in the PVN after 6-OHDA lesion. There were 33% of CA boutons in the PVN from the control tissues that appeared to have contacts. Four days after lesions with a significant reduction of CA terminals, the synaptic frequency was seen on 31% of surviving CA terminals; this synaptic frequency (31%) is close to that observed before lesioning. Of the CA boutons, 42% possessed synaptic contacts at both 21 and 56 days postlesion. At these stages there is still limited regeneration of CA terminals. At 180 days afler lesions with a significant CA terminal restoration, the synaptic frequency of CA boutons had dropped to 36%, a value closer to the control level. The following conclusions can be drawn: (1) the synaptic frequency of CA terminals 4 days afler lesions resembles that of controls, suggesting that synaptic contacts found at this short survival period are those CA terminals escaped from 6-OHDA destruction, not from regenerating fibers; (2) a higher synaptic frequency was exhibited by CA terminals at 21 and 56 days after lesions; by 180 days after the lesion; a subsequent decrease of synaptic frequency was observed on CA terminals, indicating synaptic plasticity of CA terminals; and (3) CA stumps in the PNV are able to extend to establish their synaptic contacts at a frequency close to control levels by 180 days after 6-OHDA lesion, suggesting that regenerative fibers may be functional.     

The basilar pontine gray of the opossum (Didelphis virginiana). II. Experimental determination of neocortical input

The pattern of neocortical input into the basilar pons of the opossum was determined by employing the Nauta-Gygax technique ('54) on the brains of animals previously subjected to neocortical lesions. The results indicate that every neocortical area projects to some portion of the basilar pons in this form. Degenerating fibers resulting from more rostral cortical lesions (frontal and preorbital areas) terminated profusely within the medial and ventral nuclei and, to a lesser extent, in the smaller dorsal nucleus. Fascicles from more caudal areas (striate and peristriate cortices) ended abundantly in the lateral nuclear group, and in the lateral part of the ventral nucleus. Every neocortical region studied projected to some portion of the ventral pontine nucleus. Degenerating fibers terminated to some extent within each nucleus of the basilar pons as a result of lesions in midcortical regions (paramarginal, postorbital and parietal cortices). A few fibers of frontal, orbital and parietal origin terminated in the contralateral basilar pontine gray.     

Presence of crossed corticorubral fibers and increase of crossed projections after unilateral lesions of the cerebral cortex of the kitten: a demonstration using anterograde transport of Phaseolus vulgaris leucoagglutinin

Brain lesions made during early developmental stages produce more prominent remodeling of synaptic organization than those made in adults. This difference in the extent of neuronal or synaptic plasticity between immature and mature animals may be due to difference in the capacity for axonal elongation. Alternatively, it could be due to the prevention of retraction of exuberant projections present only in the early developmental stages. Aberrant crossed corticorubral projections seen after neonatal hemispherectomy have been ascribed to collateral sprouting. To determine whether these results from the prevention of retraction of crossed fibers, we studied the corticorubral pathway in normal kittens and compared it with that observed after unilateral cortical lesion, using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L). One to two weeks after injection of PHA-L, many immunocytochemically labelled fibers were observed in the red nucleus (RN) ipsilateral to the cortical injection. Although very few, labelled fibers were also seen in the RN contralateral to the injection in normal kittens. By contrast, many labelled fibers were seen in the RN contralateral to the injection in lesioned animals. Many growth-cone like axonal endings were also observed. The abundant crossed corticorubral fibers seen in lesioned animals may be ascribed to the increase in the number of fibers crossing the midline towards the contralateral RN or they could be due to increased branching of pre-existing crossed fibers.     

Myelin-associated inhibitors of neurite growth

CNS myelin and oligondendrocyte membranes contain two minor proteins with strong inhibitory effects on growing neurites (neurite growth inhibitors NI-35 and NI-250). Monoclonal antibodies (IN-1, IN-2) were obtained that neutralize this activity in a variety of culture assays including adult rat optic nerve explants, which are invaded by growing neurites under the influence of these antibodies. In vivo, corticospinal tract lesions in young rats are known to be followed by abortive sprouting, not exceeding 1 mm of elongation. In contrast, the presence of antibody IN-1 led to regrowth of corticospinal axons over more than 5 mm within 2-3 weeks. In development, a negative guidance or channeling function may be associated with these inhibitors for late growing CNS tracts. In fact, application of antibodies or absence of oligodendrocytes during the first postnatal week led to severe anatomical disturbance of the developing rat corticospinal tract. Additional, e.g., stabilizing functions for these inhibitors in the adult CNS remain to be investigated.