Regular ArticlePost-lesion transcommissural olivocerebellar reinnervation improves motor function following unilateral pedunculotomy in the neonatal rat
Introduction
The adult central nervous system has a limited capacity for axonal regeneration after injury due to a combination of intrinsic neuronal properties (Rossi et al., 1997) and inhibitory influences in the neural microenvironment (Rhodes et al., 2003, Schwab, 1996). Furthermore, what little regeneration that can be induced does not always improve function at the behavioural level (Dunlop et al., 1997). By comparison, after lesions in the developing central nervous system, there is some axonal re-growth and associated functional benefit (Nicholls and Saunders, 1996). But more frequently, alternative pathways develop in the immature nervous system, which appear to replace those that have been destroyed (Angaut et al., 1985, Naus et al., 1984). Unfortunately, these pathways are sometimes maladaptive, although the relationship between any dysfunction and anatomical abnormality remains poorly defined.
The rat olivocerebellar projection is one example in which the generation of an alternative pathway occurs following injury during early development and therefore provides a model for studying any functional compensation provided by developmental neural plasticity. In the normal adult animal, olivocerebellar axons enter the cerebellum via the contralateral inferior cerebellar peduncle and terminate as climbing fibres (CFs) on Purkinje cells (PCs) located within a narrow parasagittal band (Sugihara et al., 2001). This path regulates motor learning (Llinas et al., 1975) and synchronises repetitive movements such as gait (Petrosini et al., 1990, Rondi-Reig et al., 1997). During development, CF axons enter the cerebellar anlage on embryonic day 17 (E17) forming immature synapses on PCs by birth (Chedotal and Sotelo, 1993). By post-natal day 2 (P2), PC responses to CF stimulation have begun (Mariani and Changeux, 1981) through temporary perisomatic synapses (Altman, 1972, Mason et al., 1990) arising from several olivocerebellar axons (Mariani and Changeux, 1981). From P7–P10, CFs translocate to their permanent sites on PC primary dendrites (Altman, 1972, Mason et al., 1990) and subsequently regress the supernumerary terminals to leave the adult configuration of only one CF per PC by P15 (Mariani and Changeux, 1981). Following unilateral CF transection between birth and P10 the contralateral (axotomised), inferior olive degenerates and new axons, arising from the remaining ipsilateral inferior olive, grow into the denervated hemicerebellum (Angaut et al., 1982, Zagrebelsky et al., 1997). These reinnervating transcommissural olivocerebellar axons synapse on Purkinje and deep cerebellar neurons in a sagittally orientated topography that partly recreates the normal olivo-cortico-nuclear circuit (Sherrard et al., 1986, Sugihara et al., 2003, Zagrebelsky et al., 1997). However, the behavioural consequences of this reinnervation remain unknown.
The present study was undertaken to assess motor function associated with reinnervating olivocerebellar axons for several reasons. First, the reinnervation is incomplete as not all PCs are reinnervated (Angaut et al., 1982) and the percentage of reinnervated PCs decreases in more lateral regions (Sugihara et al., 2003), thus there may be insufficient reinnervation to affect motor control. Second, the topography of this pathway is less precise than in the normal pathway, with each reinnervating CF synapsing on many more PCs than normal (Sugihara et al., 2003), so behavioural benefit cannot be presumed. Third, such alternate paths, including CFs, can be induced in the more mature central nervous system by augmenting growth-promotion (Coumans et al., 2001, Rossi et al., 1995, Sherrard and Bower, 2001) or decreasing growth-inhibition (Bregman et al., 1995, Buffo et al., 2000) within the microenvironment. Therefore, an examination of the behavioural effects of the transcommissural CF reinnervation, which occurs during development, is necessary in order to characterise the olivocerebellar pathway as a model for extending developmental plasticity into the increasingly mature brain.
Section snippets
Animals
Experiments were performed on Wistar rats under licence from James Cook University Animal Experimentation Ethics Committee (Number A567), which is in accordance with the “Code of practice for the care and use of animals for scientific purposes” of the National Health and Medical Research Council of Australia. Animals were housed in a 12 h light/dark cycle with food and water ad libitum.
Transection of olivocerebellar axons
Eight litters of rat pups were divided into 2 experimental groups: one pedunculotomised (Px) on post-natal day
Results
Although transcommissural olivocerebellar axons partly recreate the olivo-cortico-nuclear circuit (Sugihara et al., 2003), any functional benefit of this reinnervation on the animals' motor ability was not known. In addition, the effect of perturbed CF–PC interaction that is associated with a process of denervation and reinnervation on the maturation of motor skills was also unknown. Therefore, we compared basic motor control and coordination between sham-operated controls (n = 19), animals
Discussion
In this study, we examined the behavioural sequelae of post-lesion reinnervation to the denervated hemicerebellum by transcommissural olivocerebellar axons. Our data show that this transcommissural reinnervation correlates with normal dynamic postural adjustment and a broad-based gait which can be correctly synchronised at moderate speeds. Our data also reveal that in the absence of this reinnervation the maturation of postural reflexes is delayed, complex locomotor skills are impaired and gait
Conclusions
This study extends previous work on the transcommissural olivocerebellar projection (Angaut et al., 1985, Sugihara et al., 2003, Zagrebelsky et al., 1997) by adding that the anatomical compensation continues to the behavioural level. Furthermore, the association of transcommissural CF reinnervation with improvement in tasks controlled specifically by the olivocerebellar path indicates that this reinnervation is genuinely compensatory and that the motor performance in the Px3 group does not
Acknowledgments
This work was supported by University funds to AJB and a JCU MRG grant (# 72/02) to RMS. We thank Ms Catriona McElnea for assisting with sectioning and Ms Joan Hendrikz, University Statistical Advisor for advice with the statistics. Also, we are grateful to the C and V Ramaciotti Foundation who provided the fluorescence microscope for analysis of retrograde tracing.
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