Brain Advance Access published online on February 25, 2008
Brain, doi:10.1093/brain/awn024
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
BDNF increases homotypic olivocerebellar reinnervation and associated fine motor and cognitive skill
1Université Pierre et Marie Curie-Paris 6, Unité Mixte de Recherche (UMR) 7102–Neurobiologie des Processus Adaptatifs (NPA); Centre National de la Recherche Scientifique (CNRS), UMR 7102–NPA, F-75005 Paris, France, 2School of Veterinary and Biomedical Sciences, James Cook University, Australia, 3Assistance Publique, Hôpitaux de Paris, Hôpital Charles Foix, UEF, F-94200, Ivry sur Seine, France and 4Developmental Neuroplasticity Laboratory, School of Anatomy and Human Biology, University of Western Australia, Australia
Correspondence to:
R.M. Sherrard, Labo DVSN UMR7102 NPA, Case 14, Université Pierre et Marie Curie, 9 quai St Bernard, 75005 Paris, France E-mail: rachel.sherrard{at}snv.jussieu.fr
 |
Summary |
|---|
 Top Summary IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Recovery of complex neural function after injury to the adult CNS is limited by minimal spontaneous axonal regeneration and/or sprouting from remaining pathways. In contrast, the developing CNS displays spontaneous reorganization following lesion, in which uninjured axons can develop new projections to appropriate target neurons and provide partial recovery of complex behaviours. Similar pathways can be induced in the mature CNS, providing models to optimize post-injury recovery of complex neural functions. After unilateral transection of a developing olivocerebellar path (pedunculotomy), remaining inferior olivary axons topographically reinnervate the denervated hemicerebellum and compensate functional deficits. Brain-derived neurotrophic factor (BDNF) partly recreates such reinnervation in the mature cerebellum. However the function of this incomplete reinnervation and any unwanted behavioural effects of BDNF remain unknown. We measured olivocerebellar reinnervation and tested rotarod and navigation skills in Wistar rats treated with BDNF/vehicle and pedunculotomized on day 3 (Px3; with reinnervation) or 11 (Px11; without spontaneous reinnervation). BDNF treatment did not affect motor or spatial behaviour in normal (control) animals. Px11-BDNF animals equalled controls on the rotarod, outperforming Px11-vehicle animals. Moreover, Px3-BDNF and Px11-BDNF animals achieved spatial learning and memory tasks as well as controls, with Px11-BDNF animals showing better spatial orientation than Px11-vehicle counterparts. BDNF slightly increased olivocerebellar reinnervation in Px3 animals and induced sparse (22% Purkinje cells) yet widespread reinnervation in Px11 animals. As reinnervation correlated with spatial function, these data imply that after injury even a small amount of reinnervation that is homotypic to correct target neurons compensates deficits in appropriate complex motor and spatial skills. As there was no effect in control animals, BDNF effectively induces this axon collateralisation without interfering with normal neuronal circuits.
Key Words: climbing fibres; gait; reinnervation; spatial function
Abbreviations: BDNF, brain-derived neurotrophic factor; CF, climbing fibre; CNS, central nervous system; LTD, long-term depression; PC, Purkinje cell; Px, pedunculotomy; SC, spinal cord
Received November 13, 2007. Revised January 16, 2008. Accepted January 25, 2008.
|
Introduction |
|---|
|
TopSummaryIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The adult mammalian CNS has limited repair after injury due to intrinsic neuronal properties (Neumann and Woolf, 1999
) and inhibitory extracellular molecules (Fawcett, 2006) which prevent effective axonal regeneration. However, remaining uninjured axons can develop a few collaterals in grey matter that reinnervate denervated neurons (spinal cord = SC, Bareyre et al., 2004; hippocampus, Deller and Frotscher, 1997; cerebellum, Rossi et al., 1991); and this response is increased by neurotrophins (SC, Zhou and Shine, 2003; Vavrek et al., 2006). While neurotrophic factor treatment has reached clinical trials for neurodegenerative disease (Kordower et al., 2006; Price et al., 2007), neurotrophin application to neurotrauma remains preclinical, principally because their capacity to repair neural circuits and restore complex functions is unknown. In addition, denervation-induced axon collaterals often synapse heterotypically on available post-synaptic sites creating incorrect neural circuits (SC, Vavrek et al., 2006; vision, Finlay et al., 1979) that only provide ‘adaptive’ function: e.g. in the injured SC locomotor recovery depends on inappropriate muscles approximating the required movement (Ballermann et al., 2006). To recover skilled actions (Smith et al., 2007; Willson et al., 2007), axon collaterals must reinnervate the same type of neuron, i.e. homotypically (cerebellum, Angaut et al., 1985; hippocampus, Deller and Frotscher 1997; red nucleus, Smith et al., 2007). Although neurotrophins promote axonal growth through white matter (Lu et al., 2004), their capacity to generate homotypic, topographically accurate collaterals through that milieu, which integrate into remaining neural circuits and remediate complex behaviours, e.g. fine motor skills or learning, is unknown.
Axon collateral sprouting through white matter tracts to re-form projections with appropriate afferent-target connections can occur in the injured neonatal CNS (corticospinal, Hicks and D’Amato, 1970; corticorubral, Naus et al., 1987; vision, Spear, 1995; cerebellum, Zagrebelsky et al., 1997). The new circuits compensate motor (SC, Weber and Stelzner, 1977; cerebellum, Dixon et al., 2005) and cognitive (Levine et al., 1987; Willson et al., 2007) deficits, in proportion to the specificity with which they recreate the original circuit (vision, Finlay et al., 1979; cerebellum, Gramsbergen and Ijkema-Paassen, 1982; Willson et al., 2007). Thus recreating developmental plasticity in the mature CNS may improve recovery following injury.
We studied the behavioural sequelae of recreating developmental plasticity in the relatively mature system, using the rat olivocerebellar projection as a model of axonal injury. In the adult, olivocerebellar axons enter the cerebellum via the contralateral inferior peduncle and terminate on Purkinje cells (PCs) as climbing fibres (CFs) organized with precise parasagittal topography (Buisseret-Delmas and Angaut, 1993; Sugihara et al., 2001). This path regulates motor learning (Apps and Lee, 2002) and spatial cognition (Meignin et al., 1999; Rondi-Reig and Burguière, 2005). After unilateral CF transection (pedunculotomy) early in development, the contralateral (axotomized) inferior olive degenerates and new axons, arising from the remaining inferior olive, reinnervate PCs of the denervated hemicerebellum and partly recreate the olivo–cortico–nuclear circuit (Sugihara et al., 2003; Fig. 1A). Dense reinnervation of the medial hemicerebellum (Angaut et al., 1985) compensates motor deficits (Dixon et al., 2005), whereas sparse reinnervation to the lateral hemicerebellum (Sugihara et al., 2003) partially restores spatial learning (Willson et al., 2007). However, these behavioural improvements may not depend on CF reinnervation, since there is extensive neuronal plasticity throughout the CNS of young animals.
|
Olivocerebellar reinnervation can be created in juvenile and young adult animals by injection into the denervated hemicerebellum of a factor involved in olivocerebellar development, brain-derived neurotrophic factor (BDNF: Sherrard and Bower, 2001
; Dixon and Sherrard, 2006). However it is less dense than neonatal reinnervation and occurs predominantly in the vermis (Dixon and Sherrard, 2006), raising a question about its behavioural efficacy. Moreover, BDNF is a potent neuromodulator (Lang et al., 2006) that may induce unwanted effects on adjacent circuits and alter normal cerebellar function. We studied motor and spatial ability in animals with BDNF-induced reinnervation. We show that BDNF does not affect normal animals and that increasing homotypic olivocerebellar reinnervation commensurately improves appropriate complex neural functions, specifically fine motor and cognitive skills, thus suggesting that neurotrophins could improve recovery following neurotrauma. |
Materials and Methods |
|---|
|
TopSummaryIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Olivocerebellar axonal transection
Wistar rat pups (176 from 18 litters) were used. Experiments were performed under license from James Cook University (A732) and University of Western Australian (AEC 04/100/359) in accordance with regulations of the NH&MRC of Australia and the NIH. The birthdate was designated P0 and pups were allocated to two experimental groups: (1) pups lesioned on P3, after which transcommissural olivocerebellar reinnervation occurs spontaneously; and (2) juveniles lesioned on P11 in which transcommissural reinnervation does not develop spontaneously but is induced by BDNF (Sherrard and Bower, 2001
). After diethyl ether anaesthesia (BDH, Poole UK), pups underwent unilateral transection of the left inferior cerebellar peduncle at P3 (Px3) or P11 (Px11). The skin over the neck was incised longitudinally and the muscles retracted to expose the atlanto-occipital membrane. A capsulotomy knife (MSP, 3 mm blade) was inserted parallel to the brainstem into the fourth ventricle and rotated to the left to cut the left inferior cerebellar peduncle (Bower and Waddington, 1981; Dixon et al., 2005). Some pups in each litter underwent a sham operation leaving the peduncle intact. After recovery from the anaesthetic, animals were returned to the dam. Food and water were provided ad libitum.
Intracerebellar BDNF treatment
To increase (Px3) or induce (Px11) reinnervation, animals were treated with recombinant human BDNF (r-metHu BDNF, Amgen Inc., CA, USA) in 0.1% bovine serum albumin in phosphate buffer. The dose, 3.5 nmol/ml of cerebellar tissue, is optimal for inducing transcommissural reinnervation (Sherrard and Bower, 2001). The amount of BDNF for each age group is calculated from the left hemicerebellar volume, which has been measured during development (Heinsen, 1977). Vehicle contained cytochrome C (Sigma-Aldrich), which has a similar charge and molecular weight as BDNF (Caleo et al., 2003). Under ether anaesthesia, a craniotomy was performed to expose the left hemicerebellum and 1 µl containing the appropriate concentration of BDNF/vehicle was injected in multiple (12–14) aliquots (Sherrard and Bower, 2001). After Px3, BDNF/vehicle was injected into the left lateral hemicerebellum from the lobulus simplex rostrally to crus II caudally, 72 h after lesion (P6; Fig. 1A) when reinnervating axons first reach this region (Zagrebelsky et al., 1997). In Px11 pups, BDNF/vehicle was injected in the left medial/intermediate hemicerebellum (lobules VI-VIII) up to 1.5 mm from the midline at 24 h post-lesion (P12; Fig. 1B; Sherrard and Bower, 2001). Sham-operated controls were also injected, to control for the effect of an injection and/or BDNF on normal behaviour. The incision was sutured and all animals were allowed to recover and mature to P30 after which motor and spatial functions were assessed.
Basic motor skills
Three motor tests were used to identify motor dysfunction that could affect the spatial learning. From P30 to P35, all animals underwent daily tests (3x/day for 6 days) assessing simple locomotor skills (Petrosini et al., 1990; Dixon et al., 2005): to cross a narrow bridge (60 x 3 cm, 60 cm above foam), ascend a ladder (17 steps, 25° tilt) and progress along a wire (140 x 0.3 cm, 50 cm above foam) to an escape platform (Dixon et al., 2005). For each test, the time taken and success or failure to complete the task within a 3 min session were noted.
Complex motor synchronization
The rotarod was used to test the animals’ ability to correctly match stepping frequency and stride length to the speed of rotation, which relies in part on an intact CF path (Chen et al., 1995; Rondi-Reig et al., 1997). The rotarod is a horizontal cylinder (50 cm long and 5 cm diameter), which rotates about its long axis at 10, 20 or 30 revolutions per minute (rpm). The rat was placed on the rotating cylinder so it had to walk forward, synchronizing its gait to the rotation speed, to maintain its position on top of the rod. At each speed, there were 3 trials/day for 9 days with an inter-trial interval of 3 min (Dixon et al., 2005). For each trial, the error latency (falling or clinging to the rod) or the upper limit of 180 s was recorded. This limit was chosen because animals that reach this time can walk for much longer (Auvray et al., 1989).
Spatial training and tests
The water maze (Morris, 1984) is a circular pool (120 cm in diameter) filled with water (21°C). A clear Plexiglas escape platform (15 cm in diameter) was positioned in one (northeast) quadrant and submerged 2 cm below the water surface in the hidden platform test. The animal's starting position was randomly selected from one of the four entry points (N, S, E and W) and the rat was released facing the pool wall. The maze was in a room with numerous extra-maze cues (Rondi-Reig et al., 2002) and white noise. Three versions of the spatial task were carried out in the following order: hidden platform training over three sessions (10 trials/session), probe test over two trials (removal of escape platform) and a retrieval test 7 days after the probe test (four trials).
For hidden platform training, the animal was given 120 s to find the escape platform (Willson et al., 2007). The measured variables in each trial were: (i) escape latency (ii) total quadrants crossed (iii) percentage of direct swims, defined as a swim path that did not deviate outside a 20 cm wide corridor from the rat's entry to the platform (Day et al., 1999; Willson et al., 2007) and (iv) success to locate the goal. From this data, a search score was calculated to quantify the swim trajectory (Burguière et al., 2005). The probe test was conducted 2 h after the last hidden platform training trial in which the escape platform was removed and the rat given 60 s to search the platform's former (‘test’) location. The time spent in the test quadrant and the same measures as training were taken. A learned spatial position was recorded if the swim time in the test quadrant was greater than chance (i.e. 1:4 = 15 s). Seven days after the probe test, animals underwent a retrieval test (the hidden platform was returned to its original position) and the same protocol as the hidden platform was used.
Swimming assessment
Although cerebellar lesions have less effect on swimming than on terrestrial locomotion (Federico et al., 2006), animals underwent four training sessions (5 trials/session) in a visible platform test, to ensure that motor dysfunction did not impair the animal orientating and swimming to the platform. The escape platform was 2 cm above the waterline in the south-west quadrant with a flag attached (proximal cue). Measured variables were escape latency, total quadrants crossed and search score. Since prior motor training and swimming experience can mask water maze abnormalities (Cain et al., 1996), these tests were made after the spatial learning tests.
Olivocerebellar axonal tracing
After behavioural testing, the presence or absence of CF reinnervation was revealed using retrograde or anterograde tracing. After motor testing Fast Blue (2% in distilled water, Illing, Germany) was injected into the left cerebellar hemisphere (Sherrard and Bower, 2001). After spatial learning tests animals were injected in the left inferior olive with Fluoro-Emerald (4% in distilled water; dextran-FITC conjugate; 10 000 MW, Molecular Probes, OR, USA) to reveal reinnervating CFs, as described previously (Dixon and Sherrard, 2006).
Histology
Seven days after injections, animals were reanaesthetized with Lethobarb (365 mg/kg) and transcardially perfused with heparinized saline (5 units/ml) and 4% paraformaldehyde in phosphate buffer (pH 7.4; Dixon and Sherrard, 2006). The cerebellum and brainstem were dissected free, post-fixed and cryoprotected in 30% buffered sucrose. Three parallel sets of serial coronal or sagittal cerebellar sections were cut at 30 and 40 µm, respectively. Two parallel sets of serial 30 µm coronal sections of the brainstem were also taken.
To confirm complete pedunculotomy, one set of coronal brainstem and cerebellar sections was stained with 0.5% methylene blue. We only retained animals with: (i) total degeneration of the right inferior olive (Angaut et al., 1985; Sherrard et al., 1986) (ii) separation of the left hemicerebellum from the brainstem at the level of the inferior cerebellar peduncle (Sherrard et al., 1986) and (iii) residual left deep cerebellar nuclei to provide cerebellar output (Altman and Bayer, 1997). Forty animals were excluded due to either incomplete pedunculotomy, degeneration of the left deep cerebellar nuclei or inadvertent brainstem damage, e.g. to the vestibular nuclei.
Quantification of olivocerebellar reinnervation
Depending on the tracer injected, a second set of sections was used to visualize either retrogradely Fast-Blue-labelled olivary neurons or Fluoro-Emerald-filled reinnervating CF arbors and CF terminals labelled by vesicular glutamate transporter VGLUT2 immunohistochemistry (Hioki et al., 2003; Miyazaki et al., 2003). To label VGLUT2, sections were washed in phosphate-buffered saline containing 0.25% Triton-X100 (T-PBS) followed by blocking solution 0.2% gelatin in T-PBS (T-PBS-G) for 1 h. Sections were incubated overnight with guinea pig polyclonal anti-VGLUT2 (Euromedex, 1 : 3000) in T-PBS-G. The VGLUT2 was revealed for 2 h with Cy3-conjugated donkey anti-guinea pig (Beckman, 1 : 200) in T-PBS-G. After washes in T-PBS, sections were mounted in Mowiol.
In animals injected with Fast Blue, CF reinnervation was quantified by counting all the retrogradely labelled neurons in the left inferior olive. In animals injected with Fluoro-Emerald and processed for VGLUT2 immunohistochemistry, the cerebellum of pedunculotomized animals was divided into a series of parasagittal zones (500 µm wide) extending from the midline to the left lateral hemicerebellum. Within each zone, the amount of VGLUT2 positive CF reinnervation was scored in each lobule using an arbitrary scale i.e. 1 = few strands, 2 = one-fourth CF-filled lobule, 3 = half lobule, 4 = three-fourth lobule and 5 = completely CF-filled lobule (Fig. 5A). Lobule scores in each 500 µm zone were summed to generate a reinnervation value for each cerebellar cortical zone and functional region. The functional regions were designated medial (0–1500 µm), intermediate (1500–2500 µm) and lateral hemicerebellum (>2500 µm; Sugihara et al., 2003) according to the olivo–cortico–nuclear zones (Voogd and Glickstein, 1998). To compare the density of CF reinnervation between experimental groups, we used the reinnervation values of all animals in each group for each zone to obtain an average score for each parasagittal zone.
|
filled lobule etc until 5 = completely CF-filled lobule. Asterisks indicate regions with reinnervation. WM = white matter, GCL = granule cell layer, ML = molecular layer. Bar = 100 µm. (B) An unfolded cerebellum (adapted from Buisseret-Delmas and Angaut, 1993In addition, a parallel series of animals were used to quantify the percentage of reinnervated PCs in the areas treated with BDNF: the hemisphere in Px3 groups and vermis in the Px11 group. Experimental animals were anaesthetized with isoflurane (Baxter S.A., Maurepas, France), decapitated and cerebellar slices (300 µm) were prepared from pedunculotomized and control hemicerebella from animals aged 24–36 days using standard procedures (Llano et al., 1991
) as described in Sugihara et al. (2003). Whole-cell patch-clamp recordings were made from visually identified PCs, and CF currents were elicited by stimulation in the granular layer. Patch pipettes were filled with a solution containing (in mM) Cs D-gluconate, 120; biocytin, 13; HEPES, 10; BAPTA, 10; TEA Cl, 3; Na2ATP, 2; Mg ATP, 2; NaGTP, 0.2; pH 7.3, 290–300 mOsm. Chemicals for the internal solutions were from Sigma (St Quentin Fallavier, France). CF currents were identified by their all-or-none character and by the demonstration of paired-pulse depression. PCs were considered to be non-innervated if no CF current could be elicited by stimulation at several different stimulation locations and intensities.
Data analyses
Motor tests
For each basic motor test, performance plateaued by day 5, thus the three trials on day 5 were averaged to calculate each animal's mean time for inter-group comparisons. For the rotarod, at each speed the three trials on each day were averaged to obtain each animal's mean performance for intra-group comparisons and those from the last (9th) day were used for inter-group analyses.
Spatial/swim tests
For each test the data for all trials within one session were averaged to calculate the mean performance (escape latency/quadrants/search score) of each animal, which was then used to calculate the mean group performance during each session.
Correlation of reinnervation and behaviour
To correlate behaviour to reinnervation, each animal's motor performance and their escape latency/quadrants crossed/search score from the last session of the spatial tests (visible platform, hidden platform and retention) were converted to a score depending on how far performance differed from control animals (Willson et al., 2007). Thus a behavioural score was obtained for each animal in each test. The reinnervation value in each cortical zone was normalized to a theoretical value if all the lobules were CF-filled (5 x number of lobules) and a reinnervation score was calculated for each functional region. This reinnervation score was correlated to the animal's behavioural score.
Statistics
Transformed (logged) motor and water maze data revealed homogeneity of variance and inter-group comparisons were analysed using analysis of variance (ANOVA) and either Bonferroni's or Dunnets T3 (if normality was not attained) post hoc tests. Intra-group comparisons were made by repeated measures ANOVA. For the probe trials, the time spent in the ‘test’ quadrant was compared to that predicted by chance with t-tests. The percentage success of a task, frequency of direct swims and percentage PC innervation were assessed using the 
2 test. Pearson's correlation was used to test the relation between reinnervation and behavioural scores. All values were stated as mean ± SEM and 
= 0.05.
|
Results |
|---|
|
TopSummaryIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Minor effects of BDNF treatment on simple locomotor tasks
We examined whether BDNF treatment in Px3 and Px11 animals affected their ability to perform simple locomotor tasks. Compared to age-matched vehicle-treated animals, BDNF treatment did not change the performance of sham-operated controls nor significantly improve the time taken by either pedunculotomized group in the bridge, ladder or wire tests. The only exception was that the Px3-BDNF animals performed better than Px3-vehicle on the ladder [F(5, 31) = 6.9, P < 0.05]. Vehicle-treated groups essentially replicated previous findings: Px3 and Px11 animals were slower on the bridge and ladder tests and Px11-vehicle animals were also slower on the wire (Fig. 2A, B and C; Willson et al., 2007
).
|
BDNF improves synchronization of gait to rotarod speed in Px11-lesioned animals
As a more sensitive test of CF function (Rondi-Reig et al., 1997
), some animals (Control-vehicle, n = 6; Px11-vehicle, n = 10; Px11-BDNF, n = 6) had their gait tested on the rotarod. We did not examine a separate set of BDNF-treated controls because, with an upper time limit of 180 s, they had no opportunity to walk for longer than vehicle-treated controls who reach the maximum time (Dixon et al., 2005, this study, Fig. 2D–G). There were no inter-group differences at a slow speed (10 rpm). At 20 rpm, the Px11-BDNF group walked for longer than the Px11-vehicle group [F(3, 27) = 11.26, P < 0.05] and were similar to controls. Furthermore at 30 rpm, when Px11-vehicle animals were worse than controls [F(3,24) = 25.3, P < 0.01], the Px11-BDNF group showed an intermediate performance not significantly different from control or from Px11-vehicle (Fig. 2D).
We also examined how the animals learned this task, because CFs are required for motor learning and the same cellular mechanisms are thought to underlie the learning of both motor and spatial tasks (Ito, 2001; Burguière et al., 2005). Learning was examined at 10 rpm, since skills learned at one speed are transferred to the next (Rondi-Reig et al., 1997). The Px11-BDNF group displayed learning at 10 rpm improving over the first 2 days [F(8, 53) = 3.82, P < 0.01: Fig. 2E–G]. This contrasts with controls, which learned within the first day, and Px11-vehicle animals which did not demonstrate any learning. These data suggest that BDNF-treatment in juvenile rats allows learning in the rotarod task and better matching of gait with rotarod speed, compared to rats without reinnervation. This is consistent with BDNF inducing transcommissural olivocerebellar reinnervation (as previously described: Sherrard and Bower, 2001).
BDNF treatment does not alter swimming ability in the cued water maze
Since prior sensorimotor training (e.g. the rotarod) can mask subtle water maze abnormalities (Cain et al., 1996), we used a different set of animals for spatial learning tasks (Control-vehicle, n = 6; Control-BDNF, n = 3; Px3-vehicle, n = 4; Px3-BDNF, n = 6; Px11-vehicle, n = 7; Px11-BDNF, n = 10). We tested all groups in the visible platform task as this controlled for any effect of motor impairment or BDNF on the animal's ability to orientate to and reach a target platform. All groups, irrespective of lesion or BDNF, had similar escape latencies and total quadrants crossed (Fig. 3A and B). Moreover, escape latency correlated with search score (i.e. swimming trajectories) indicating that all animals swam at a similar speed. The same correlation value (R2 = 0.79, P < 0.01 for both; Fig. 3C) was found for vehicle and BDNF-treated groups, revealing similar search strategies for both groups and confirming that BDNF did not affect how the animal behaved in the maze.
|
BDNF-treated lesioned animals show improved acquisition of a spatial task
The hidden platform water maze was used to assess whether BDNF-induced reinnervation improved spatial learning. In addition to confirming previous results for vehicle-injected groups (Willson et al., 2007
), with Px3-vehicle animals reaching control performance and Px11-vehicle animals performing poorly in the maze, our present study revealed that BDNF facilitated spatial learning performance in pedunculotomized animals without significantly altering the results in control animals. The BDNF-treated control and Px3 groups performed in the maze equally to their vehicle treated counterparts. They improved their performance in the maze between all sessions with decreased escape latencies (P < 0.05; Fig. 4A), fewer quadrants crossed (P < 0.01; Fig. 4B) and better search strategies (P < 0.01; Fig. 4C). To ensure that the BDNF groups had reached their optimal performance within the testing period, we analysed individual training trials within each session (10 trials per session). The Px3-BDNF group plateaued in their performance by the last two trials of session 2 (trials 19 and 20) while the control-BDNF group reached a plateau at the mid-point of session 3 (trials 22–28). Also, both BDNF- and vehicle-treated Px11 groups improved all parameters of their maze performance until session 2 and thereafter remained unchanged (P < 0.05 for escape latencies, quadrants crossed and search scores). Therefore, because the control-BDNF animals continued to improve between sessions 2 and 3, in session 3 they were better than BDNF-treated Px11 animals [escape latency F(5, 66) = 6.7, P < 0.01; quadrants crossed F(5, 66) = 6.5, P < 0.05; search score F(5, 66) = 5.7, P < 0.05]. Nevertheless, the Px11-BDNF group had the same performance as vehicle-treated controls and both Px3 groups during training sessions 2 and 3 (Fig. 4A–D), whereas the Px11-vehicle animals remained significantly impaired (P < 0.05 for escape latency and quadrants crossed in both sessions 2 and 3).
|
The effect of BDNF on spatial function was further demonstrated in the number of direct swims to the platform. Animals in both BDNF-treated pedunculotomized groups made more direct swims to the hidden platform compared to their vehicle-treated counterparts (
2 test: Px3, P < 0.05; Px11, P < 0.01) and equalled the control (control-vehicle) group (Table 1). In addition, the Px11-BDNF animals also made as many direct swims as both Px3 groups (Table 1).
|
To discriminate whether the better motor performance (e.g. on the rotarod) of the BDNF-treated Px11 animals contributed to their improved spatial function, we made three further analyses. First, we tested for correlations between the measured variables in the visible and hidden platform mazes (Cain et al., 1996
). No correlations were observed for any group between these two versions of the water maze, but only existed for each group within each test. Second, we re-compared our groups using the time taken in the basic motor tests as a covariate of the escape latency in the hidden platform (Martin et al., 2003). For the ladder there was no covariate effect, although for the bridge and wire tests there was a significant effect of the covariate, i.e. better motor function [RANCOVA covariate: bridge, F(1, 5) = 8.75, P < 0.01; wire, F(1, 5) = 5.23, P < 0.05] but the differences between groups for escape latency persisted [RANCOVA group: bridge, F(1, 5) = 3.19, P < 0.01; wire, F(1, 5) = 2.48, P < 0.05]. Third, we compared Px11 BDNF- and vehicle-treated groups using performance in the visible platform (cued maze) as covariate for the hidden platform results (Martin et al., 2003) and found significant effects of both covariate [RANCOVA covariate: F(1, 2) = 5.7, P < 0.05] and group [RANCOVA group, F(1, 2) = 4.3, P < 0.05]. These data indicate that motor ability does affect the performance in the hidden platform test. However, inter-group differences remained when the motor effect was taken into account, indicating true differences in spatial learning.
BDNF treatment of lesioned-animals facilitates spatial memory
A probe test, in which the escape platform is removed, evaluates how accurately animals learned the spatial task. All groups showed similar spatial bias to the test quadrant (
32% of test time, 19/60 s). In addition, the number of quadrant entries, total quadrants crossed and direct swims were not significantly different between any groups.
A retrieval test assesses the animals’ memory of previously learned strategies. In this test, the platform was replaced into its original location and performance was compared to the last hidden platform training session. The intervening week between the probe and retrieval test did not impair escape latency, total quadrants crossed or search strategies for any group. However, BDNF-treated animals remembered better, as the number of direct swims was similar for all BDNF-treated groups (Table 1), whereas vehicle-treated lesioned animals made fewer direct trajectories than their controls (Px3, P < 0.01; Px11, P < 0.01).
Olivocerebellar reinnervation and its relation to behaviour
To examine the presence or absence of reinnervation we used retrograde or anterograde tracing with VGLUT2 immunohistochemistry. Animals tested in simple motor tests and rotarod received Fast Blue into the left cerebellar hemisphere. Retrogradely labelled olivary neurons were observed only in the left olive of the Px11-BDNF group (9–31 neurons), confirming transcommissural olivocerebellar reinnervation and the lack of aberrant sprouting in intact animals (Sherrard and Bower, 2001; Dixon and Sherrard, 2006).
In animals tested for simple motor tests and spatial function, anterograde tracing revealed the normal crossed path in control animals and the expected reinnervation with normal morphology (data not shown) in Px3 and Px11-BDNF groups (Dixon and Sherrard, 2006; Letellier et al., 2007). However, the present study provides new detail on the distribution and density of this reinnervation. VGLUT2-positive reinnervating CFs were distributed in the left hemicerebellum up to 8 mm from the midline, predominately in vermal lobules III–VIII and in the hemisphere from lobulus simplex to copula pyramidis (Fig. 5B). In Px3 animals, VGLUT2 labelling appeared denser in the intermediate left hemicerebellum of BDNF- compared to vehicle-treated animals, although quantitatively the difference was not significant (P > 0.05). In contrast, in Px11 animals BDNF-induced reinnervation was less extensive and much less dense than in either vehicle or BDNF-treated Px3 groups (P < 0.05).
In addition, to assess the effect of BDNF on CF–PC reinnervation at a cellular level, we counted the percentage of PCs from which CF-induced currents could be recorded in those cerebellar regions which had been injected with BDNF or vehicle. BDNF did not alter the percentage of PC innervation in control animals, nor in the left hemisphere of Px3 animals (Fig. 6A). BDNF induced reinnervation to only 22% PCs in Px11 animals, which is less than in either Px3 group (Fig. 6B). Furthermore, none of the pedunculotomized groups was completely reinnervated. As there was no reinnervation in Px11-vehicle animals, we did not record from this group.
|
To examine whether olivocerebellar reinnervation was related to an animal's behaviour, we correlated reinnervation scores for each functional region of the left hemicerebellum (medial, intermediate or lateral) and behaviour scores for each motor and water maze test. For the basic motor tests, reinnervation (either by VGLUT2 mapping or retrograde olivary analysis) did not correlate to motor score in any group, which is consistent with previous findings (Willson et al., 2007
). In addition, there was no correlation between the number of retrogradely labelled olivary neurons (9, 9, 17, 20, 24 and 31) and rotarod score at any speed (10 rpm, R2 = 0; 20 rpm, R2 = –0.46 and 30 rpm, R2 = –0.40). In contrast, in the hidden platform and retention tests, the total quadrants crossed and escape latency correlated with reinnervation in the intermediate and lateral hemicerebellum of Px3 groups (n = 5 animals, hidden platform: R2 = 0.72, P < 0.001; retention test: R2 = 0.65, P < 0.01; Fig. 5C). Furthermore, when all groups are considered, the reinnervation score of the left hemicerebellum directly correlated with the escape latencies in the hidden platform task (R2 = 0.40, P < 0.05). These results indicate that increasing olivocerebellar reinnervation aids spatial learning. |
Discussion |
|---|
|
TopSummaryIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
We used the olivocerebellar path as a model of axonal injury to examine the effect of BDNF-induced collateral reinnervation on complex motor and cognitive functions. We observed that BDNF promoted transcommissural CF reinnervation in juvenile (Px11) animals and this was associated with enhanced CF-related functions: rotarod motor skills (Rondi-Reig et al., 1997
), spatial learning (Dahhaoui et al., 1992; Meignin et al., 1999; Rondi-Reig et al., 2002) and memory (Gasbarri et al., 2003). We also show that sparse reinnervation, when it is homotypic, compensates deficits in complex and cognitive tasks, and that BDNF creates this effect without overtly disturbing undamaged paths.
Quality rather than quantity: BDNF-induced CF reinnervation ameliorates relevant functional deficits
This study provides new insights into neurotrophin-related repair in the injured CNS. Transcommissural olivocerebellar reinnervation that takes place in the presence of BDNF was associated with improved CF-mediated behaviours: complex motor and spatial skills. This may not seem surprising because CF reinnervation ameliorates motor and spatial deficits in neonatal animals (Dixon et al., 2005; Willson et al., 2007) and BDNF induces this reinnervation in young adult animals (Dixon and Sherrard, 2006; Letellier et al., 2007). However in the injured neonatal brain before P10, there is widespread anatomical plasticity (Weber and Stelzner, 1977; Gramsbergen and Ijkema-Paassen, 1982; Sherrard et al., 1986; Naus et al., 1987) whose contribution to behavioural improvement is unknown; but which can no longer contribute to the observed recovery in juvenile Px11-BDNF animals.
Our data suggest that it is specificity rather than amount of reinnervation that is important. First, the Px11 BDNF-treated animals showed remarkable recovery in both motor skill and spatial function despite sparse transcommissural reinnervation. Although reinnervation was light, with only 20% PCs responding to CF stimulation, it was homotypic with normal arbors developing on reinnervated PCs (this study; Dixon and Sherrard, 2006; Letellier et al., 2007). Also, reinnervation extended into the left hemisphere (Fig. 5), where spatial function is largely regulated (Joyal et al., 1996), which is consistent with the improved spatial function we observed. Because our retrograde tracing only assessed olivocerebellar reinnervation to the injected left hemisphere, the lack of correlation between olivary labelling and motor skill in Px11-BDNF animals is consistent with reinnervating CFs showing relatively normal topography, although on the wrong side of the midline (Sherrard et al., 1986; Zagrebelsky et al., 1997; Sherrard and Bower, 2001). This topography would allow PCs of the medial hemicerebellum that normally inter-connect with spinal circuits (Bosco and Poppele, 2003; Cerminara et al., 2003), to receive CFs from appropriate regions of the inferior olive. As each inferior olive receives sensory information from both sides of the body (Atkins and Apps, 1997; Voogd et al., 2003), reinnervated PCs could mediate bilateral sensorimotor integration. Similar benefits of specific reinnervation have also been demonstrated in the spinal cord when spared descending axons are induced to sprout across the midline (Smith et al., 2007). In contrast to most collaterals that synapse heterotypically on propriospinal interneurons (Coumans et al., 2001; Zhou and Shine, 2003; Zhou et al., 2003; Ballermann and Fouad, 2006; Chen et al., 2006; Vavrek et al., 2006) and activate alternate locomotor patterns (Ballermann et al., 2006), when collaterals reach appropriate target neurons (e.g. in the red nucleus or lateral spinal grey) fine motor skills are regained (Smith et al., 2007).
However, such dramatic behavioural improvement was not observed in the Px3-BDNF group when compared to their vehicle-treated group. In these animals the small behavioural improvement compared to vehicle-treated animals is entirely consistent with the small increase in VGLUT2 density, unchanged percentage of PC reinnervation and correlation between CF distribution and learning scores. Furthermore, despite having much greater reinnervation density than the Px11-BDNF animals, there were no differences in learning scores or percentage of correct spatial orientation (i.e. direct swims) between Px3 and Px11-BDNF groups.
These data indicate that even sparse reinnervation, which is homotypic and topographically correct, promotes significant behavioural improvement (c.f. Px11-BDNF versus Px11-vehicle motor and spatial functions), but then further increasing reinnervation density provides little extra advantage (c.f. Px3-BDNF versus Px3-vehicle spatial functions). From this it may be hypothesized that reinnervation may only need to reach a threshold level in order to engender functional benefit.
Climbing fibres, not BDNF, underlie functional recovery
In addition to inducing CF reinnervation, BDNF itself may have altered the behavioural outcomes by modulating neuronal activity and/or inducing sprouting in other paths damaged during pedunculotomy: e.g. spino-/cuneo-cerebellar mossy fibres (Bosco and Poppele, 2003; Cerminara et al., 2003), periolivary serotoninergic input (Kitzman and Bishop, 1994) and some cerebello-vestibular efferents. Consideration of the non-specific actions of BDNF is important given clinical trials of neurotrophins (Price et al., 2007) and their requirement for intrathecal delivery (Egleton and Davis, 2005) either by infusion (animals, Kobayashi et al., 1997; Coumans et al., 2001; Lu et al., 2004; humans, Nagano et al., 2005) or tissue genetic engineering (animals, Lu et al., 2001; Zhou et al., 2003; Ruitenberg et al., 2005; humans, Visted et al., 2001; Alvarez et al., 2006; Gasmi et al., 2007).
Our data indicate that the major effect of BDNF was to induce CF reinnervation, and that modulation of residual neural circuits alone could not account for the functional improvements observed. There are several lines of support. First, control-BDNF animals did not develop aberrant transcommissural olivocerebellar axons and were functionally the same as control-vehicle animals, confirming that BDNF does not alter normal neural circuits (cerebellum, Sherrard and Bower, 2001; SC, Zhou et al., 2003; Vavrek et al., 2006).
Second, coordinated limb action is required to match walking to rotarod speed and select appropriate movements to swim to a platform. Such skilled movements require groups of PCs to fire synchronously in response to olivary activation to select appropriate muscle groups that perform the movement (Welsh et al., 1995; Lang et al., 2006). While CF–PC reinnervation re-establishes olivo-PC synchrony, therefore facilitating skilled actions, it is difficult to see how BDNF alone or mossy fibre/serotoninergic reinnervation could produce such synchronizing function.
Third, in addition to synchronous firing of PC groups, motor and spatial functions also require correct patterns of Purkinje cell activity (Dahhaoui et al., 1992; Le Marec et al., 1997; Meignin et al., 1999; Martin et al., 2003). CF-deprived PCs have altered firing rates (Montarolo et al., 1982; Batini et al., 1985) which change activity in their target deep cerebellar nuclear neurons (Batini et al., 1985) and disrupt sensorimotor processing (Hoebeek et al., 2005). Reinnervating CFs, which induce normal post-synaptic currents in their target PCs (Sugihara et al., 2003; Letellier et al., 2007), should normalize PC firing and deep cerebellar nuclear outflow and hence improve sensorimotor regulation. In contrast, since BDNF is excitatory to PCs (Kafitz et al., 1999; Carter et al., 2002; Sadakata et al., 2007), any injected BDNF that was still active at the time of behavioural testing would increase PC firing and exacerbate the effects of CF denervation. Likewise, serotonin inhibits deep cerebellar nuclear activity (Kitzman and Bishop, 1994); hence BDNF-induced serotoninergic reinnervation would also exacerbate the effects of CF loss. Neither of these is compatible with the observed functional improvement.
Finally, the cerebellar (procedural) component of spatial navigation involves learning the motor patterns required to make a direct path to a specific target and linking them to inputs from the environment (Leggio et al., 1999; Burguière et al., 2005). These processes involve PC long-term depression (LTD; Burguière et al., 2005), which is generated by synaptic activity at the PC of both its CF and parallel fibre afferents (Ito, 2001). Since BDNF generally prevents the generation of LTD in CNS neurons (hippocampus, Aicardi et al., 2004; visual cortex, Kinoshita et al., 1999), any residual activity of injected BDNF would impair spatial learning, which does require LTD (Burguière et al., 2005). In fact, even though BDNF-induced mossy/serotoninergic fibre reinnervation may facilitate parallel fibre-PC synapses and therefore LTD, our covariate analyses demonstrate that improved spatial navigation is related to CF reinnervation rather than BDNF treatment (Px3-vehicle, Px3 and Px11 BDNF groups). This suggests that reinnervating CFs enable PC LTD to provide the link between learned motor patterns and environmental cues. This view is supported by improvement in lesioned BDNF-treated groups of spatial knowledge retention, which also involves CF function (Dahhaoui et al., 1992).
In summary, despite its potent neuromodulatory effects and potential to induce sprouting of non-CF afferents, our data suggest that BDNF promotes functional recovery by inducing CF reinnervation that can provide an anatomical substrate for motor and spatial learning.
Olivocerebellar reinnervation: relevance to the injured CNS?
As discussed earlier, our data reveal that it is the specificity of CF reinnervation that BDNF induces, which is most important for recovery of spatial cognition and skilled motor functions. When combined with recent similar findings in the corticospinal tract (Smith et al., 2007), it may be suggested that homotypic (re)innervation is a generic requirement applicable to other neural systems and therefore potentially to human neurological dysfunction. Importantly, our data also suggest a therapeutic threshold for reinnervation, since even in a task as complex as spatial cognition (1) sparse input (Px11-BDNF), when specific, correlates with significant improvement, while (2) further increasing it (Px3-BDNF) provides minimal additional benefit. Since an ‘innervation threshold’ is well known to precede symptom onset in Parkinson's disease (Conley and Kirchner, 1999), our data imply the same principle can be applied to reinnervation, so that ‘less is more’, with its smaller risk of (re)connectivity errors. A threshold of reinnervation, rapidly improving peptide delivery to the CNS (Jain, 2007; de Boer and Gaillard, 2007) and the efficacy of a single post-lesion, albeit invasive, treatment broadens our understanding of recovery from neural injury/dysfunction. Moreover, our study adds that a system-relevant growth-factor (BDNF) has this effect without also altering adjacent normal circuits, as indicated by the normal behaviour and olivocerebellar structure in control-BDNF animals. Finally, as BDNF induces similar reinnervation in young adult rats (Dixon and Sherrard, 2006), re-creating developmental plasticity focuses effective treatment to the demography most commonly involved in neural injury; young adult males (National Hospital Morbidity Study, Australia; NINDS Public Liaison Office).
|
Acknowledgements |
|---|
We would like to thank Prof. Adrian Bower for helpful advice about this study and insightful critiques on the manuscript. Also we thank Kelly Gifford and Marie-Claude Malidor for assistance with tissue sectioning and Amgen Inc. for the generous donation of BDNF. This work was supported by university funds from James Cook University and Notre Dame (Australia) to R.M.S., Université Pierre et Marie Curie and CNRS funds to J.M., a Graduate Research Scheme grant to M.L.W., and l’Institut pour la Recherche sur la Moelle Épinière et l’Encéphale to A.M.L. M.L.W. was supported by an Australian Postgraduate Award and la Fondation pour la Recherche Médicale. The international collaboration was supported by a PICS grant from the CNRS to A.M.L.
|
References |
|---|
|
TopSummaryIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Aicardi G, Argilli E, Cappello S, Santi S, Riccio M, Thoenen H, et al. Induction of long-term potentiation and depression is reflected by corresponding changes in secretion of endogenous brain-derived neurotrophic factor. Proc Natl Acad Sci USA (2004) 101:15788–92.
Altman J, Bayer SA. The development of the cerebellar system: in relation to its evolution, structure and functions (1997) Boca Raton: CRC Press.
Alvarez XA, Cacabelos R, Laredo M, Couceiro V, Sampedro C, Varela M, et al. A 24-week, double-blind, placebo-controlled study of three dosages of Cerebrolysin in patients with mild to moderate Alzheimer's disease. Eur J Neurol (2006) 13:43–54.[CrossRef][Web of Science][Medline]
Angaut P, Alvarado-Mallart RM, Sotelo C. Compensatory climbing fibre innervation after unilateral pedunculotomy in the newborn rat: origin and topographic organisation. J Comp Neurol (1985) 236:161–78.[CrossRef][Medline]
Apps R, Lee S. Central regulation of cerebellar climbing fibre input during motor learning. J Physiol (2002) 541:301–17.
Atkins MJ, Apps R. Somatotopical organisation within the climbing fibre projection to the paramedian lobule and copula pyramidis of the rat cerebellum. J Comp Neurol (1997) 389:249–63.[CrossRef][Web of Science][Medline]
Auvray N, Caston J, Reber A, Stelz T. Role of the cerebellum in the ontogenesis of the equilibrium behavior in the young rat: a behavioral study. Brain Res (1989) 505:291–301.[CrossRef][Medline]
Ballermann M, Fouad K. Spontaneous locomotor recovery in spinal cord injured rats is accompanied by anatomical plasticity of reticulospinal fibers. Eur J Neurosci (2006) 23:1988–96.[CrossRef][Web of Science][Medline]
Ballermann M, Tse ADY, Misiaszek JE, Fouad K. Adaptations in the walking pattern of spinal cord injured rats. J Neurotrauma (2006) 23:897–907.[CrossRef][Medline]
Bareyre FM, Kerschensteiner M, Raineteau O, Mettenleiter TC, Weinmann O, Schwab ME. The injured spinal cord spontaneously forms a new intraspinal circuit in adult rats. Nat Neurosci (2004) 7:269–77.[CrossRef][Web of Science][Medline]
Batini C, Billard JM, Daniel H. Long term modification of cerebellar inhibition after inferior olive degeneration. Exp Brain Res (1985) 59:404–9.[Web of Science][Medline]
de Boer AG, Gaillard PJ. Drug targeting to the brain. Ann Rev Pharmacol Toxicol (2007) 47:323–55.[CrossRef][Web of Science][Medline]
Bosco G, Poppele RE. Modulation of dorsal spinocerebellar responses to limb movement. II. Effect of sensory input. J Neurophysiol (2003) 90:3372–83.
Bower AJ, Waddington G. A simple operative technique for chronically severing the cerebellar peduncles in neonatal rats. J Neurosci Methods (1981) 4:181–8.[CrossRef][Web of Science][Medline]
Buisseret-Delmas C, Angaut P. The cerebellar olivo-corticonuclear connections in the rat. Prog Neurobiol (1993) 40:63–87.[CrossRef][Web of Science][Medline]
Burguière E, Arleo A, Hojjati M, Elgersma Y, De Zeeuw CI, Berthoz A, et al. Spatial navigation impairment in mice lacking cerebellar LTD: a motor adaptation deficit? Nat Neurosci (2005) 8:1292–4.[CrossRef][Medline]
Cain DP, Saucier D, Hall J, Hargreaves EL, Boon F. Detailed behavioral analysis of water maze acquisition under APV or CNQX: contribution of sensorimotor disturbances to drug-induced acquisition deficits. Behav Neurosci (1996) 110:86–102.[CrossRef][Web of Science][Medline]
Caleo M, Medini P, von Bartheld CS, Maffei L. Provision of brain-derived neurotrophic factor via anterograde transport from the eye preserves the physiological responses of axotomized geniculate neurons. J Neurosci (2003) 23:297–6.
Carter AR, Chen C, Schwartz PM, Segal RA. Brain-derived neurotrophic factor modulates cerebellar plasticity and synaptic ultrastructure. J Neurosci (2002) 22:1316–27.
Cerminara NL, Makarabhirom K, Rawson JA. Somatosensory properties of cuneocerebellar neurones in the main cuneate nucleus of the rat. Cerebellum (2003) 2:131–45.[CrossRef][Medline]
Chen C, Kano M, Abeliovich A, Chen L, Bao S, Kim JJ, et al. Impaired motor coordination correlates with persistent multiple climbing fiber innervation in PKC gamma mutant mice. Cell (1995) 83:1233–42.[CrossRef][Web of Science][Medline]
Chen Q, Zhou L, Shine HD. Expression of Neurotrophin-3 promotes axonal plasticity in the acute but not chronic injured spinal cord. J Neurotrauma (2006) 23:1254–60.[CrossRef][Medline]
Conley SC, Kirchner JT. Parkinson's disease–the shaking palsy: underlying factors, diagnostic considerations, and clinical course. Postgrad Med (1999) 106:39–52.[Medline]
Coumans JV, Lin TT, Dal HN, MacArthur L, McAtee M, Nash C, et al. Axonal regeneration and functional recovery after complete spinal cord transection in rats by delayed treatment with transplants and neurotrophins. J Neurosci (2001) 21:9334–44.
Dahhaoui M, Stelz T, Caston J. Effects of lesion of the inferior olivary complex by 3-acetylpyridine on learning and memory in the rat. J Comp Physiol A (1992) 171:657–64.[Medline]
Day LB, Weisend M, Sutherland RJ, Schallert T. The hippocampus is not necessary for a place response but may be necessary for pliancy. Behav Neurosci (1999) 193:914–24.
Deller T, Frotscher M. Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion. Prog Neurobiol (1997) 53:687–727.[CrossRef][Web of Science][Medline]
Dixon KJ, Hilber W, Speare S, Willson ML, Bower AJ, Sherrard RM. Post-lesion transcommissural olivocerebellar reinnervation improves motor function following unilateral pedunculotomy in the neonatal rat. Exp Neurol (2005) 196:254–65.[CrossRef][Medline]
Dixon KJ, Sherrard RM. Brain-derived neurotrophic factor induces post-lesion transcommissural growth of olivary axons that develop normal climbing fibers on mature Purkinje cells. Exp Neurol (2006) 202:44–56.[CrossRef][Web of Science][Medline]
Egleton RD, Davis TP. Development of neuropeptide drugs that cross the blood-brain barrier. NeuroRx (2005) 2:44–53.
Fawcett JW. Overcoming inhibition in the damaged spinal cord. J Neurotrauma (2006) 23:371–83.[CrossRef][Web of Science][Medline]
Federico F, Leggio MG, Neri P, Mandolesi L, Petrosini L. NMDA receptor activity in learning spatial procedural strategies II. The influence of cerebellar lesions. Brain Res Bull (2006) 70:356–67.[CrossRef][Web of Science][Medline]
Finlay BL, Wilson KG, Schneider GE. Anomalous ipsilateral retinotectal projections in Syrian hamsters with early lesions: topography and functional capacity. J Comp Neurol (1979) 183:721–40.[CrossRef][Web of Science][Medline]
Gasbarri A, Pompili A, Pacitti C, Cicirata F. Comparative effects of lesions to the ponto-cerebellar and olivo-cerebellar pathways on motor and spatial learning in the rat. Neuroscience (2003) 116:1131–40.[CrossRef][Medline]
Gasmi M, Brandon EP, Herzog CD, Wilson A, Bishop KM, Hofer EK, et al. AAV2-mediated delivery of human neurturin to the rat nigrostriatal system: Long-term efficacy and tolerability of CERE-120 for Parkinson's disease. Neurobiol Dis (2007) 27:67–76.[CrossRef][Medline]
Gramsbergen A, Ijkema-Paassen J. CNS plasticity after hemicerebellectomy in the young rat. Quantitative relations between aberrant and normal cerebello-rubral projections. Neurosci Lett (1982) 33:129–34.[CrossRef][Medline]
Heinsen H. Quantitative anatomical studies on the postnatal development of the cerebellum of the albino rat. Anat Embryol (1977) 151:201–18.[CrossRef][Medline]
Hicks SP, D’Amato CJ. Motor-sensory and visual behaviour after hemispherectomy in newborn and mature rats. Exp Neurol (1970) 29:416–38.[CrossRef][Medline]
Hioki H, Fujiyama F, Taki K, Tomioka R, Furuta T, Tamamaki N, et al. Differential distribution of vesicular glutamate transporters in the rat cerebellar cortex. Neuroscience (2003) 117:1–6.[CrossRef][Web of Science][Medline]
Hoebeek FE, Stahl JS, van Alphen AM, Schonewille M, Luo C, Rutterman M, et al. Increased noise level of Purkinje cell activities minimizes impact of their modulation during sensorimotor control. Neuron (2005) 45:953–65.[CrossRef][Web of Science][Medline]
Ito M. Cerebellar long-term depression: characterization, signal transduction, and functional roles. Physiol Rev (2001) 81:1143–95.
Jain KK. Nanotechnology-based drug delivery to the central nervous system. Neurodegen Dis (2007) 4:287–91.[CrossRef]
Joyal CC, Meyer C, Jacquart G, Mahler P, Caston J, Lalonde R. Effects of midline and lateral cerebellar lesions on motor coordination and spatial orientation. Brain Res (1996) 739:1–11.[CrossRef][Web of Science][Medline]
Kafitz KW, Rose CR, Thoenen H, Konnerth A. Neurotrophin-evoked rapid excitation through TrkB receptors. Nature (1999) 401:918–21.[CrossRef][Medline]
Kinoshita S, Yasuda H, Taniguchi N, Katoh-Semba R, Hatanaka H, Tsumoto T. Brain-derived neurotrophic factor prevents low-frequency inputs from inducing long-term depression in the developing visual cortex. J Neurosci (1999) 19:2122–30.
Kitzman PH, Bishop GA. The origin of serotoninergic afferents to the cat's cerebellar nuclei. J Comp Neurol (1994) 340:541–50.[CrossRef][Medline]
Kobayashi NR, Fan DP, Giehl KM, Bedard AM, Wiegand SJ, Tetzlaff W. BDNF and NT-4/5 prevent atrophy of rat rubrospinal neurons after cervical axotomy, stimulate GAP-43 and Talpha1-tubulin mRNA expression, and promote axonal regeneration. J Neurosci (1997) 17:9583–95.
Kordower JH, Herzog CD, Dass B, Bakay RAE, Stansell J 3rd, Gasmi M, et al. Delivery of neurturin by AAV2 (cere-120)-mediated gene transfer provides structural and functional neuroprotection and neurorestoration in MTPT-treated monkeys. Ann Neurol (2006) 60:706–15.[CrossRef][Medline]
Lang EJ, Sugihara I, Llinas R. Olivocerebellar modulation of motor cortex ability to generate vibrissal movements in rat. J Physiol (2006) 571:101–20.
Le Marec N, Dahhaoui M, Stelz T, Bakalian A, Delhaye-Bouchaud N, Caston J, et al. Effect of cerebellar granule cell depletion on spatial learning and memory and in an avoidance conditioning task: studies in postnatally X-irradiated rats. Dev Brain Res (1997) 99:20–8.[CrossRef][Medline]
Leggio MG, Neri P, Graziano A, Mandolesi L, Molinari M, Petrosini L. Cerebellar contribution to spatial event processing: characterization of procedural learning. Exp Brain Res (1999) 127:1–11.[CrossRef][Web of Science][Medline]
Letellier M, Bailly Y, Demais V, Sherrard RM, Mariani J, Lohof AM. Reinnervation of late post-natal Purkinje cells by climbing fibres: neosynaptogenesis without transient multi-innervation. J Neurosci (2007) 27:5373–83.
Levine SC, Huttenlocher P, Banich MT, Duda EE. Factors affecting cognitive functioning of hemiplegic children. Dev Med Child Neurol (1987) 29:27–35.[Web of Science][Medline]
Llano I, Marty A, Armstrong CM, Konnerth A. Synaptic- and agonist-induced excitatory currents of Purkinje cells in rat cerebellar slices. J Physiol (1991) 434:183–213.
Lu P, Blesch A, Tuszynski MH. Neurotrophism without neurotropism: BDNF promotes survival but not growth of lesioned corticospinal neurons. J Comp Neurol (2001) 436:456–70.[CrossRef][Web of Science][Medline]
Lu P, Yang H, Jones LL, Filbin MT, Tuszynski MH. Combinatorial therapy with neurotrophins and cAMP promotes axonal regeneration beyond sites of spinal cord injury. J Neurosci (2004) 24:6402–9.
Martin LA, Goldowitz D, Mittleman G. The cerebellum and spatial ability: dissection of motor and cognitive components with a mouse model system. Eur J Neurosci (2003) 18:2002–10.[CrossRef][Web of Science][Medline]
Meignin C, Hilber P, Caston J. Influence of stimulation of the olivocerebellar pathway by harmaline on spatial learning in the rat. Brain Res (1999) 824:277–83.[CrossRef][Medline]
Miyazaki T, Fukaya M, Shimizu H, Watanabe M. Subtype switching of vesicular glutamate transporters at parallel-fibre-Purkinje cell synapses in developing mouse cerebellum. Eur J Neurosci (2003) 17:2563–72.[CrossRef][Web of Science][Medline]
Montarolo PG, Palestini M, Strata P. The inhibitory effect of the olivocerebellar input on the cerebellar Purkinje cells in the rat. J Physiol (1982) 332:187–202.
Morris R. Developments of a water-maze procedure for studying spatial learning in the rat. J Neurosci Methods (1984) 11:47–60.[CrossRef][Web of Science][Medline]
Nagano I, Shiote M, Murakami T, Kamada H, Hamakawa Y, Matsubara E, et al. Beneficial effects of intrathecal IGF-1 administration in patients with amyotrophic lateral sclerosis. Neurol Res (2005) 27:768–72.[CrossRef][Medline]
Naus CG, Flumerfelt BA, Hrycyshyn AW. Ultrastructural study of remodeled rubral afferents following neonatal lesions in the rat. J Comp Neurol (1987) 259:131–9.[CrossRef][Medline]
Neumann S, Woolf CJ. Regeneration of dorsal column fibers into and beyond the lesion site following adult spinal cord Injury. Neuron (1999) 23:83–91.[CrossRef][Web of Science][Medline]
Petrosini L, Molinari M, Gremoli T. Hemicerebellectomy and motor behaviour in rats. I Development of motor function after neonatal lesion. Exp Brain Res (1990) 82:472–82.[Medline]
Petrosini L, Molinari M, Dell’Anna ME. Cerebellar contribution to spatial event processing: Morris water maze and T-maze. Eur J Neurosci (1996) 8:1882–96.[CrossRef][Medline]
Price RD, Milne SA, Sharkey J, Matsuoka N. Advances in small molecules promoting neurotrophic function. Pharm Ther (2007) 115:292–306.[CrossRef][Medline]
Rondi-Reig L, Burguière E. Is the cerebellum ready for navigation? Prog Brain Res (2005) 148:212.
Rondi-Reig L, Delhaye-Bouchaud N, Mariani J, Caston J. Role of the inferior olivary complex in motor skills and motor learning in the adult rat. Neuroscience (1997) 77:955–63.[CrossRef][Medline]
Rondi-Reig L, Le Marec N, Caston J, Mariani J. The role of climbing and parallel fibers inputs to cerebellar cortex in navigation. Behav Brain Res (2002) 132:11–8.[CrossRef][Medline]
Rossi F, Wiklund L, van der Want JJL, Strata P. Reinnervation of cerebellar Purkinje cells by climbing fibres surviving a subtotal lesion of the inferior olive in the adult rat. I. Development of new collateral branches and terminal plexuses. J Comp Neurol (1991) 308:513–35.[CrossRef][Web of Science][Medline]
Ruitenberg MJ, Levison DB, Lee SV, Verhaagen J, Harvey AR, Plant GW. NT-3 expression from engineered olfactory ensheathing glia promotes spinal sparing and regeneration. Brain (2005) 128:839–53.
Sadakata T, Takegawa W, Mizoguchi A, Washida M, Katoh-Semba R, Shutoh F, et al. Impaired cerebellar development and function in mice lacking CAPS2, a protein involved in neurotrophin release. J Neurosci (2007) 27:2472–82.
Sherrard RM, Bower AJ. BDNF and NT3 extend the critical period for developmental climbing fibre plasticity. Neuroreport (2001) 12:2871–4.[CrossRef][Web of Science][Medline]
Sherrard RM, Bower AJ, Payne JN. Innervation of the adult rat cerebellar hemisphere by fibres from the ipsilateral inferior olive following unilateral neonatal pedunculotomy: an autoradiographic and retrograde fluorescent double-labelling study. Exp Brain Res (1986) 62:411–21.[CrossRef][Web of Science][Medline]
Smith JM, Lunga P, Story D, Harris N, Le Belle J, James MF, et al. Inosine promotes recovery of skilled motor function in a model of focal brain injury. Brain (2007) 130:915–25.
Spear PD. Plasticity following neonatal visual cortex damage in cats. Can J Physiol Pharmacol (1995) 73:1389–97.[Web of Science][Medline]
Sugihara I, Lohof AM, Letellier M, Mariani J, Sherrard RM. Post-lesion transcommissural growth of olivary climbing fibres creates functional synaptic microzones. Eur J Neurosci (2003) 18:3027–36.[CrossRef][Web of Science][Medline]
Sugihara I, Wu HS, Shinoda Y. The entire trajectories of single olivocerebellar axons in the cerebellar cortex and their contribution to cerebellar compartmentalisation. J Neurosci (2001) 21:7715–23.
Vavrek R, Girgis J, Tetzlaf W, Hiebert GW, Fouad K. BDNF promotes connections of corticospinal neurons onto spared descending interneurons in spinal cord injured rats. Brain (2006) 129:1534–45.
Visted T, Bjerkvig R, Enger PO. Cell encapsulation technology as a therapeutic strategy for CNS malignancies. Neurooncology (2001) 3:201–10.[Abstract]
Voogd J, Glickstein M. The anatomy of the cerebellum. Trends Neurosci (1998) 21:370–5.[CrossRef][Web of Science][Medline]
Voogd J, Pardoe J, Ruigrok TJH, Apps R. The distribution of climbing and mossy fiber collateral branches from the copula pyramidis and the paramedian lobule: congruence of climbing fiber cortical zones and the pattern of zebrin banding within the rat cerebellum. J Neurosci (2003) 23:4645–56.
Weber ED, Stelzner DJ. Behavioral effects of spinal cord transection in the developing rat. Brain Res (1977) 125:241–55.[CrossRef][Web of Science][Medline]
Welsh JP, Lang EJ, Sugihara I, Llinas R. Dynamic organization of motor control within the olivocerebellar system. Nature (1995) 374:453–7.[CrossRef][Medline]
Willson ML, Bower AJ, Sherrard RM. Developmental neural plasticity and its cognitive benefits: olivocerebellar reinnervation compensates spatial function in the cerebellum. Eur J Neurosci (2007) 25:1475–83.[CrossRef][Medline]
Zagrebelsky M, Strata P, Hawkes R, Rossi F. Reestablishment of the olivocerebellar projection map by compensatory transcommmisural reinnervation following unilateral transection of the inferior cerebellar peduncle in the newborn rat. J Comp Neurol (1997) 379:283–99.[CrossRef][Web of Science][Medline]
Zhou L, Baumgartner BJ, Hill-Felberg SJ, McGowen LR, Shine DH. Neurotrophin-3 expressed in situ induces axonal plasticity in the adult spinal cord. J Neurosci (2003) 23:1424–31.
Zhou L, Shine DH. Neurotrophic factors expressed in both cortex and spinal cord induce axonal plasticity after spinal cord injury. J Neurosci Res (2003) 74:221–6.[CrossRef][Medline]
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||