Acetylcholine release from fetal tissue homotopically grafted to the motoneuron-depleted lumbar spinal cord. An in vivo microdialysis study in the awake rat

Neuromuscular Plasticity in a Mouse Neurotoxic Model of Spinal Motoneuronal Loss

Despite the relevant research efforts, the causes of amyotrophic lateral sclerosis (ALS) are still unknown and no effective cure is available. Many authors suggest that ALS is a multi-system disease caused by a network failure instead of a cell-autonomous pathology restricted to motoneurons. Although motoneuronal loss is the critical hallmark of ALS given their specific vulnerability, other cell populations, including muscle and glial cells, are involved in disease onset and progression, but unraveling their specific role and crosstalk requires further investigation. In particular, little is known about the plastic changes of the degenerating motor system. These spontaneous compensatory processes are unable to halt the disease progression, but their elucidation and possible use as a therapeutic target represents an important aim of ALS research. Genetic animal models of disease represent useful tools to validate proven hypotheses or to test potential therapies, and the conception of novel hypotheses about ALS causes or the study of pathogenic mechanisms may be advantaged by the use of relatively simple in vivo models recapitulating specific aspects of the disease, thus avoiding the inclusion of too many confounding factors in an experimental setting. Here, we used a neurotoxic model of spinal motoneuron depletion induced by injection of cholera toxin-B saporin in the gastrocnemius muscle to investigate the possible occurrence of compensatory changes in both the muscle and spinal cord. The results showed that, following the lesion, the skeletal muscle became atrophic and displayed electromyographic activity similar to that observed in ALS patients. Moreover, the changes in muscle fiber morphology were different from that observed in ALS models, thus suggesting that some muscular effects of disease may be primary effects instead of being simply caused by denervation. Notably, we found plastic changes in the surviving motoneurons that can produce a functional restoration probably similar to the compensatory changes occurring in disease. These changes could be at least partially driven by glutamatergic signaling, and astrocytes contacting the surviving motoneurons may support this process.

Cholinergic receptor alterations in the cerebral cortex of spinal cord injured rat

Many areas of the cerebral cortex process sensory information or coordinate motor output necessary for control of movement. Disturbances in cortical cholinergic system can affect locomotor coordination. Spinal cord injury causes severe motor impairment and disturbances in cholinergic signalling can aggravate the situation. Considering the impact of cortical cholinergic firing in locomotion, we focussed the study in understanding the cholinergic alterations in cerebral cortex during spinal cord injury. The gene expression of key enzymes in cholinergic pathway - acetylcholine esterase and choline acetyl transferase showed significant upregulation in the cerebral cortex of spinal cord injured group compared to control with the fold increase in expression of acetylcholine esterase prominently higher than cholineacetyl transferase. The decreased muscarinic receptor density and reduced immunostaining of muscarinic receptor subtypes along with down regulated gene expression of muscarinic M1 and M3 receptor subtypes accounts for dysfunction of metabotropic acetylcholine receptors in spinal cord injury group. Ionotropic acetylcholine receptor alterations were evident from the decreased gene expression of alpha 7 nicotinic receptors and reduced immunostaining of alpha 7 nicotinic receptors in confocal imaging. Our data pin points the disturbances in cortical cholinergic function due to spinal cord injury; which can augment the locomotor deficits. This can be taken into account while devising a proper therapeutic approach to manage spinal cord injury.

The Therapeutic Effectiveness of Delayed Fetal Spinal Cord Tissue Transplantation on Respiratory Function Following Mid-Cervical Spinal Cord Injury

Respiratory impairment due to damage of the spinal respiratory motoneurons and interruption of the descending drives from brainstem premotor neurons to spinal respiratory motoneurons is the leading cause of morbidity and mortality following cervical spinal cord injury. The present study was designed to evaluate the therapeutic effectiveness of delayed transplantation of fetal spinal cord (FSC) tissue on respiratory function in rats with mid-cervical spinal cord injury. Embryonic day-14 rat FSC tissue was transplanted into a C4 spinal cord hemilesion cavity in adult male rats at 1 week postinjury. The histological results showed that FSC-derived grafts can survive, fill the lesion cavity, and differentiate into neurons and astrocytes at 8 weeks post-transplantation. Some FSC-derived graft neurons exhibited specific neurochemical markers of neurotransmitter (e.g., serotonin, noradrenalin, or acetylcholine). Moreover, a robust expression of glutamatergic and γ-aminobutyric acid-ergic fibers was observed within FSC-derived grafts. Retrograde tracing results indicated that there was a connection between FSC-derived grafts and host phrenic nucleus. Neurophysiological recording of the phrenic nerve demonstrated that phrenic burst amplitude ipsilateral to the lesion was significantly greater in injured animals that received FSC transplantation than in those that received buffer transplantation under high respiratory drives. These results suggest that delayed FSC transplantation may have the potential to repair the injured spinal cord and promote respiratory functional recovery after mid-cervical spinal cord injury.

Neuroplasticity and Repair in Rodent Neurotoxic Models of Spinal Motoneuron Disease

Retrogradely transported toxins are widely used to set up protocols for selective lesioning of the nervous system. These methods could be collectively named "molecular neurosurgery" because they are able to destroy specific types of neurons by using targeted neurotoxins. Lectins such as ricin, volkensin, or modeccin and neuropeptide- or antibody-conjugated saporin represent the most effective toxins used for neuronal lesioning. Some of these specific neurotoxins could be used to induce selective depletion of spinal motoneurons. In this review, we extensively describe two rodent models of motoneuron degeneration induced by volkensin or cholera toxin-B saporin. In particular, we focus on the possible experimental use of these models to mimic neurodegenerative diseases, to dissect the molecular mechanisms of neuroplastic changes underlying the spontaneous functional recovery after motoneuron death, and finally to test different strategies of neural repair. The potential clinical applications of these approaches are also discussed.

Novel Mechanisms of Spinal Cord Plasticity in a Mouse Model of Motoneuron Disease

A hopeful spinal cord repairing strategy involves the activation of neural precursor cells. Unfortunately, their ability to generate neurons after injury appears limited. Another process promoting functional recovery is synaptic plasticity. We have previously studied some mechanisms of spinal plasticity involving BDNF, Shh, Notch-1, Numb, and Noggin, by using a mouse model of motoneuron depletion induced by cholera toxin-B saporin. TDP-43 is a nuclear RNA/DNA binding protein involved in amyotrophic lateral sclerosis. Interestingly, TDP-43 could be localized at the synapse and affect synaptic strength. Here, we would like to deepen the investigation of this model of spinal plasticity. After lesion, we observed a glial reaction and an activity-dependent modification of Shh, Noggin, and Numb proteins. By using multivariate regression models, we found that Shh and Noggin could affect motor performance and that these proteins could be associated with both TDP-43 and Numb. Our data suggest that TDP-43 is likely an important regulator of synaptic plasticity, probably in collaboration with other proteins involved in both neurogenesis and synaptic plasticity. Moreover, given the rapidly increasing knowledge about spinal cord plasticity, we believe that further efforts to achieve spinal cord repair by stimulating the intrinsic potential of spinal cord will produce interesting results.

Noggin and Sonic hedgehog are involved in compensatory changes within the motoneuron-depleted mouse spinal cord

Sonic hedgehog and Noggin are morphogenetic factors involved in neural induction and ventralization of the neural tube, but recent findings suggest that they could participate in regeneration and functional recovery after injury. Here, in order to verify if these mechanisms could occur in the spinal cord and involve synaptic plasticity, we measured the expression levels of Sonic hedgehog, Noggin, Choline Acetyltransferase, Synapsin-I and Glutamate receptor subunits (GluR1, GluR2, GluR4), in a motoneuron-depleted mouse spinal cord lesion model obtained by intramuscular injection of Cholera toxin-B saporin. The lesion caused differential expression changes of the analyzed proteins. Moreover, motor performance was found correlated with Sonic hedgehog and Noggin expression in lesioned animals. The results also suggest that Sonic hedgehog could collaborate in modulating synaptic plasticity. Together, these findings confirm that the injured mammalian spinal cord has intrinsic potential for repair and that some proteins classically involved in development, such as Sonic hedgehog and Noggin could have important roles in regeneration and functional restoration, by mechanisms including synaptic plasticity.

Involvement of brain-derived neurotrophic factor and sonic hedgehog in the spinal cord plasticity after neurotoxic partial removal of lumbar motoneurons

Adult mammals could spontaneously achieve a partial sensory-motor recovery after spinal cord injury, by mechanisms including synaptic plasticity. We previously showed that this recovery is associated to the expression of synapsin-I, and that sonic hedgehog and Notch-1 could be also involved in plasticity. The role of brain-derived neurotrophic factor and glutamate receptors in regulating synaptic efficacy has been explored in the last decade but, although these mechanisms are now well-defined in the brain, the molecular mechanisms underlying the so called "spinal learning" are still less clear. Here, we measured the expression levels of choline acetyltransferase, synapsin-I, sonic hedgehog, Notch-1, glutamate receptor subunits (GluR1, GluR2, GluR4, NMDAR1) and brain-derived neurotrophic factor, in a motoneuron-depleted mouse spinal lesion model obtained by intramuscular injection of cholera toxin-B saporin. The lesion caused the down-regulation of the majority of analysed proteins. Moreover, we found that in lesioned but not in control spinal tissue, synapsin-I expression is associated to that of both brain-derived neurotrophic factor and sonic hedgehog, whereas GluR2 expression is linked to that of Shh. These results suggest that brain-derived neurotrophic factor and sonic hedgehog could collaborate in modulating synaptic plasticity after the removal of motoneurons, by a mechanism involving both pre- and post-synaptic processes. Interestingly, the involvement of sonic hedgehog showed here is novel, and offers new routes to address spinal cord plasticity and repair.

The role of embryonic motoneuron transplants to restore the lost motor function of the injured spinal cord

Spinal cord injury or disease result in the loss of critical numbers of spinal motoneurons and consequentially, in severe functional impairment. The most successful way to replace missing motoneurons is the use of embryonic postmitotic motoneuron grafts. This method may also at least partially restore integrity of the injured spinal cord. It has been shown that grafted motoneurons survive, differentiate and integrate into the host cord and many of them are able to reinnervate the denervated muscles. If grafted motoneurons are provided with a conduit (e.g. reimplanted ventral root) the grafted cells are able to extend their axons along the entire length of the peripheral nerves and reach the hind or forelimb muscles and to restore limb locomotion patterns. Grafted motoneurons show excellent survival in motoneuron-depleted adult host cords, but the developing spinal cord appears to provide an unfavourable environment for these motoneurons as they do not survive in immature cords. The long term survival and maturation of the grafted neurons depend on the availability of a nerve conduit and one or more target muscles, independently of whether these are ectopic nerve-muscle implants or limb muscles in their original site. Thus, grafted and host motoneurons induce functional recovery in the denervated limb muscles when their axons can grow into an avulsed and reimplanted ventral root and then reach the limb muscles. Following segmental loss of motoneurons induced by partial spinal cord injury, motoneuron-enriched embryonic grafts can be placed into the gap-like hemisection cavity in the cervical spinal cord. Such transplants induce the regeneration of great numbers of host motoneurons possibly by the bridging effect of the grafts. In this case, the regenerating host motoneurons reinnervate their original target muscles while the small graft plays a minimal role in the reinnervation of muscles. These results suggest that reconstruction of the injured spinal cord using an embryonic motoneuron-enriched spinal cord graft is a feasible way to achieve improvement after severe functional motor deficits of the spinal cord.

Selective lesion of the developing central noradrenergic system: short- and long-term effects and reinnervation by noradrenergic-rich tissue grafts

The possibility to selectively remove noradrenergic neurons in the locus coeruleus/subcoeruleus (LC/SubC) complex by the immunotoxin anti-dopamine-beta-hydroxylase (DBH)-saporin has offered a powerful tool to study the functional role of this projection system. In the present study, the anatomical consequences of selective lesions of the LC/SubC on descending noradrenergic projections during early postnatal development have been investigated following bilateral intraventricular injections of anti-DBH-saporin or 6-hydroxydopamine to immature (4 day old) rats. Administration of increasing doses (0.25-1.0 microg) of the immunotoxin produced, about 5 weeks later, a dose-dependent loss of DBH-immunoreactive neurons in the LC/SubC complex (approximately 45-90%) paralleled by a similar reduction of noradrenergic innervation in the terminal territories in the lumbar spinal cord. Even at the highest dose used (1.0 microg) the immunotoxin did not produce any detectable effects on dopaminergic, adrenergic, serotonergic or cholinergic neuronal populations, which, by contrast, were markedly reduced after administration of 6-hydroxydopamine. The approximately 90% noradrenergic depletion induced by 0.5 and 1.0 microg of anti-DBH-saporin remained virtually unchanged at 40 weeks post-lesion. Conversely, the approximately 45% reduction of spinal innervation density estimated at 5 weeks in animals injected with the lowest dose (0.25 microg) of the immunotoxin was seen recovered up to near-normal levels at 40 weeks, possibly as a result of the intrinsic plasticity of the developing noradrenergic system. A similar reinnervation in the lumbar spinal cord was also seen promoted by grafts of fetal LC tissue implanted at the postnatal day 8 (i.e. 4 days after the lesion with 0.5 microg of anti-DBH-saporin). In these animals, the number of surviving neurons in the grafts and the magnitude of the reinnervation, with fibers extending in both the grey and white matter for considerable distances, were seen higher than those reported in previous studies using adult recipients. This would suggest that the functional interactions between the grafted tissue and the host may recapitulate the events normally occurring during the ontogenesis of the coeruleo-spinal projection system, and can therefore be developmentally regulated. Thus, the neonatal anti-DBH-saporin lesion model, with the possibility to produce graded noradrenergic depletions, holds promises as a most valuable tool to address issues of compensatory reinnervation and functional recovery in the severed CNS as well as to elucidate the mechanisms governing long-distance axon growth from transplanted neural precursors.

Expression of cell fate determinants and plastic changes after neurotoxic lesion of adult mice spinal cord by cholera toxin-B saporin

Recent studies have attempted to repair the damaged spinal cord (SC) by stimulating neurogenesis or neuroplasticity. Sonic hedgehog (Shh), Notch-1 and Numb are involved in the stem cell functioning. Additionally, Notch-1 has a role as modulator of synaptic plasticity. However, little is known about the role of these proteins in the adult SC after removal of motoneurons. In this study, we have injected cholera toxin-B saporin into the gastrocnemius muscle to induce a depletion of motoneurons within the lumbar SC of adult mice, and analysed the expression of choline acetyltransferase (ChAT), Synapsin-I, Shh, Notch-1 and Numb proteins. The functional outcome of the lesion was monitored by grid walk and rotarod tasks. The neurotoxin lesion determined a motoneuron depletion and a transient decrease of ChAT, Synapsin-I, Shh and Numb levels in the lumbar SC. ChAT was associated with Synapsin-I expression and motor performance at 1 week but not 1 month after lesion, suggesting that the recovery of locomotion could depend on synaptic plasticity, at least in an early phase. Shh and Notch-1 were associated with Synapsin-I levels, suggesting a role in modulating synaptic plasticity. Numb expression also appeared reduced after lesion and linked to motor performance. Moreover, unlike other lesion models, we observed glial reaction but no evidence of cell proliferation within the depleted SC. Given the mentioned roles of Shh, Notch-1 and Numb, we believe that an in vivo manipulation of their signalling after lesion could represent a suitable way to improve functional recovery by modulating synaptic plasticity and/or neurogenesis.

Long-term survival and development of fetal ventral spinal grafts into the motoneuron-depleted rat spinal cord: role of donor age

Fetal spinal cord (SC) tissue grafts can survive and develop into the lesioned SC, but no conclusive data are available concerning the long-term fate of transplanted material and the relation between the graft fate and the donor embryo age. Here, pre-labelled suspensions of ventral SC from E12 or E17 rat fetuses were grafted to the lumbar SC of adult rats with motoneuron depletion induced by perinatal injection of volkensin. E12 and E17 are presumably the stages when motoneuron development starts and terminates, respectively. Four or 10months post-grafting, SCs were analyzed to check the graft survival rate and to follow the differentiation and spatial distributions of grafted cells. Neurotoxic lesion produced a 61% motoneuronal loss in the lumbar SC. In transplanted animals, all E12 fetal grafts survived until the observed time-points and developed various mature cell phenotypes. Many motoneuron-like labelled cells were found within the graft area or adjacent to it. Conversely, none of the E17 fetal grafts survived, since no graft-derived elements with neuronal morphology were found either in the site of graft placement or adjacent to it. The present findings indicate that spinal neuroblasts can survive for a long time and develop within the motoneuron-depleted SC, and that the donor embryo age is crucial for successful engraftment.

Microdialysis in central nervous system disorders and their treatment

Central nervous system (CNS) insults elevate endogenous toxins and alter levels of indicators of metabolic disorder. These contribute to neurotrauma, neurodegenerative diseases and chronic pain and are possible targets for pharmaceutical treatment. Microdialysis samples substances in the extracellular space for chemical analysis. It has demonstrated that toxic levels of glutamate are released and that toxic levels of the reactive species O(2)(-), H(2)O(2), HO. NO and HOONO are generated upon CNS injury. Agent administration by microdialysis can also help elucidate mechanisms of damage and protection, and to identify targets for clinical application. Microdialysis sampling indicates that circuits descending from the brain to the spinal cord transmit and modulate pain signals by releasing neurotransmitter amines and amino acids. Efforts are under way to develop microdialysis into a technique for intensive care monitoring and predicting outcomes of brain insults. Finally, microdialysis sampling has demonstrated in vivo elevation of glial cell line-derived neurotrophic factor following grafting of primed fetal human neural stem cells into brain-injured rats, the first in vivo demonstration of the release of a neurotrophic factor by grafted stem cells. This increased release correlated with significantly improved spatial learning and memory.