Conditional RAC1 knockout in motor neurons restores H-reflex rate-dependent depression after spinal cord injury.

A major complication with spinal cord injury (SCI) is the development of spasticity, a clinical symptom of hyperexcitability within the spinal H-reflex pathway. We have previously demonstrated a common structural motif of dendritic spine dysgenesis associated with hyperexcitability disorders after injury or disease insults to the CNS. Here, we used an adeno-associated viral (AAV)-mediated Cre-Lox system to knockout Rac1 protein expression in motor neurons after SCI. Three weeks after AAV9-Cre delivery into the soleus/gastrocnemius of Rac1-"floxed" adult mice to retrogradely infect spinal alpha-motor neurons, we observed significant restoration of RDD and reduced H-reflex excitability in SCI animals. Additionally, viral-mediated Rac1 knockdown reduced presence of dendritic spine dysgenesis on motor neurons. In control SCI animals without Rac1 knockout, we continued to observe abnormal dendritic spine morphology associated with hyperexcitability disorder, including an increase in mature, mushroom dendritic spines, and an increase in overall spine length and spine head size. Taken together, our results demonstrate that viral-mediated disruption of Rac1 expression in ventral horn motor neurons can mitigate dendritic spine morphological correlates of neuronal hyperexcitability, and reverse hyperreflexia associated with spasticity after SCI. Finally, our findings provide evidence of a putative mechanistic relationship between motor neuron dendritic spine dysgenesis and SCI-induced spasticity.

Mice with a deletion of the major central myelin protein exhibit hypersensitivity to noxious thermal stimuli: involvement of central sensitization.

Null mutations in the gene encoding the major myelin protein of the central nervous system, proteolipid protein 1 (PLP1), cause an X-linked form of spastic paraplegia (SPG2) associated with axonal degeneration. While motor symptoms are the best known manifestations of this condition, its somatosensory disturbances have been described but poorly characterized. We carried out a longitudinal study in an animal model of SPG2 - mice carrying a deletion of the Plp1 gene (Plp-null mice). Plp-null mice exhibited severe early-onset thermal hyperalgesia, in the absence of thermal allodynia. We first performed an electrophysiological testing which showed an early decrease in peripheral and spinal conduction velocities in Plp null mice. Such as the abnormal sensitive behaviors, this slowing of nerve conduction was observed before the development of myelin abnormalities at the spinal level, from 3months of age, and without major morphological defects in the sciatic nerve. To understand the link between a decrease in nerve velocity and an increased response to thermal stimuli before the appearance of myelin abnormalities, we focused our attention on the dorsal horn of the spinal cord, the site of integration of somatosensory information. Immunohistochemical studies revealed an early-onset activation of astrocytes and microglia that worsened with age, associated later in age with perturbation of the expression of the sensory neuropeptides calcitonin-gene-related peptide and galanin. Taken together, these results represent complementary data supporting the hypothesis that Plp-null mice suffer from ganglionopathy associated with late onset central demyelination but with few peripheral nerve alterations, induced by the glial-cell-mediated sensitization of the spinal cord. The mechanism suggested here could underlie pain experiments in other leukodystrophies as well as in other non-genetic demyelinating diseases such as multiple sclerosis.

Enhancing proprioceptive input to motoneurons differentially affects expression of neurotrophin 3 and brain-derived neurotrophic factor in rat hoffmann-reflex circuitry.

The importance of neurotrophin 3 (NT-3) for motor control prompted us to ask the question whether direct electrical stimulation of low-threshold muscle afferents, strengthening the proprioceptive signaling, could effectively increase the endogenous pool of this neurotrophin and its receptor TrkC in the Hoffmann-reflex (H-reflex) circuitry. The effects were compared with those of brain-derived neurotrophic factor (BDNF) and its TrkB receptor. Continuous bursts of stimuli were delivered unilaterally for seven days, 80 min daily, by means of a cuff-electrode implanted over the tibial nerve in awake rats. The H-reflex was recorded in the soleus muscle to control the strength of stimulation. Stimulation aimed at activation of Ia fibers produced a strong increase of NT-3 protein, measured with ELISA, in the lumbar L3-6 segments of the spinal cord and in the soleus muscle. This stimulation exerted much weaker effect on BDNF protein level which slightly increased only in L3-6 segments of the spinal cord. Increased protein level of NT-3 and BDNF corresponded to the changes of NT-3 mRNA and BDNF mRNA expression in L3-6 segments but not in the soleus muscle. We disclosed tissue-specificity of TrkC mRNA and TrkB mRNA responses. In the spinal cord TrkC and TrkB transcripts tended to decrease, whereas in the soleus muscle TrkB mRNA decreased and TrkC mRNA expression strongly increased, suggesting that stimulation of Ia fibers leads to sensitization of the soleus muscle to NT-3 signaling. The possibility of increasing NT-3/TrkC signaling in the neuromuscular system, with minor effects on BDNF/TrkB signaling, by means of low-threshold electrical stimulation of peripheral nerves, which in humans might be applied in non-invasive way, offers an attractive therapeutic tool.

Serotonin 1A receptors alter expression of movement representations.

Serotonin has a myriad of central functions involving mood, appetite, sleep, and memory and while its release within the spinal cord is particularly important for generating movement, the corresponding role on cortical movement representations (motor maps) is unknown. Using adult rats we determined that pharmacological depletion of serotonin (5-HT) via intracerebroventricular administration of 5,7 dihydroxytryptamine resulted in altered movements of the forelimb in a skilled reaching task as well as higher movement thresholds and smaller maps derived using high-resolution intracortical microstimulation (ICMS). We ruled out the possibility that reduced spinal cord excitability could account for the serotonin depletion-induced changes as we observed an enhanced Hoffman reflex (H-reflex), indicating a hyperexcitable spinal cord. Motor maps derived in 5-HT1A receptor knock-out mice also showed higher movement thresholds and smaller maps compared with wild-type controls. Direct cortical application of the 5-HT1A/7 agonist 8-OH-DPAT lowered movement thresholds in vivo and increased map size in 5-HT-depleted rats. In rats, electrical stimulation of the dorsal raphe lowered movement thresholds and this effect could be blocked by direct cortical application of the 5-HT1A antagonist WAY-100135, indicating that serotonin is primarily acting through the 5-HT1A receptor. Next we developed a novel in vitro ICMS preparation that allowed us to track layer V pyramidal cell excitability. Bath application of WAY-100135 raised the ICMS current intensity to induce action potential firing whereas the agonist 8-OH-DPAT had the opposite effect. Together our results demonstrate that serotonin, acting through 5-HT1A receptors, plays an excitatory role in forelimb motor map expression.

The extracellular matrix glycoprotein tenascin-C is beneficial for spinal cord regeneration.

Tenascin-C (TNC), a major component of the extracellular matrix, is strongly upregulated after injuries of the central nervous system (CNS) but its role in tissue repair is not understood. Both regeneration promoting and inhibiting roles of TNC have been proposed considering its abilities to both support and restrict neurite outgrowth in vitro. Here, we show that spontaneous recovery of locomotor functions after spinal cord injury is impaired in adult TNC-deficient (TNC(-/-)) mice in comparison to wild-type (TNC(+/+)) mice. The impaired recovery was associated with attenuated excitability of the plantar Hoffmann reflex (H-reflex), reduced glutamatergic input, reduced sprouting of monaminergic axons in the lumbar spinal cord and enhanced post-traumatic degeneration of corticospinal axons. The degeneration of corticospinal axons in TNC(-/-) mice was normalized to TNC(+/+) levels by application of the alternatively spliced TNC fibronectin type III homologous domain D (fnD). Finally, overexpression of TNC-fnD via adeno-associated virus in wild-type mice improved locomotor recovery, increased monaminergic axons sprouting, and reduced lesion scar volume after spinal cord injury. The functional efficacy of the viral-mediated TNC indicates a potentially useful approach for treatment of spinal cord injury.

Better functional outcome of compression spinal cord injury in mice is associated with enhanced H-reflex responses.

Alterations in spinal reflexes and functional improvements occur after incomplete spinal cord injury but the relationship between these phenomena is not understood. Here we show that spontaneous functional recovery after compression injury of the spinal cord at low-thoracic level (Th10-12) in C57BL/6J mice is associated with a progressively increasing, over 3 months, excitability of the plantar H-reflex. The stimulation rate-sensitive H-reflex depression, already strongly reduced at 1 week after injury, when compared with non-injured mice, decreased further during the observation time period. Twelve weeks after injury, the degree of motor recovery estimated by single-frame motion analysis in individual animals correlated positively with their H-reflex responses at 2-Hz stimulation. Functional recovery and reflex alterations were accompanied by an increase in glycine/GABAergic and glutamatergic terminals around motoneuron cell bodies between 6 and 12 weeks after injury. Enhanced H-reflex responses at frequencies between 0.1 and 5 Hz were also observed in mice deficient in the extracellular matrix glycoprotein tenascin-R and the adhesion molecule close homolog of L1, mice previously shown to have better motor recovery after spinal cord injury than wild-type littermates. These results indicate that better functional outcome of compression spinal cord injury in mice is associated with alterations of the monosynaptic reflex pathway which facilitate motoneuron recruitment. Our observations support the view that plasticity of spinal circuitries underlies specific aspects of motor recovery and demonstrate the usefulness of H-reflex analyses in studies on spinal cord injury in mice.

Proprioceptive sensory neuropathy in mice with a mutation in the cytoplasmic Dynein heavy chain 1 gene.

Mice heterozygous for the radiation-induced Sprawling (Swl) mutation display an early-onset sensory neuropathy with muscle spindle deficiency. The lack of an H reflex despite normal motor nerve function in the hindlimbs of these mutants strongly suggests defective proprioception. Immunohistochemical analyses reveal that proprioceptive sensory neurons are severely compromised in the lumbar dorsal root ganglia of newborn Swl/+ mice, whereas motor neuron numbers remain unaltered even in aged animals. We have used positional cloning to identify a nine base-pair deletion in the cytoplasmic dynein heavy chain 1 gene (Dync1h1) in this mutant. Furthermore, we demonstrate that Loa/+ mice, which have previously been shown to carry a missense point mutation in Dync1h1 that results in late-onset motor neuron loss, also present with a severe, early-onset proprioceptive sensory neuropathy. Interestingly, in contrast to the Loa mutation, the Swl mutation does not delay disease progression in a motor neuron disease mouse model overexpressing a human mutant superoxide dismutase (SOD1(G93A)) transgene. Together, we provide in vivo evidence that distinct mutations in cytoplasmic dynein can either result in a pure sensory neuropathy or in a sensory neuropathy with motor neuron involvement.

Clinical and electrodiagnostic findings, nerve biopsy and blood group markers in a family with hereditary neuropathy with liability to pressure palsies.

Clinical, electrophysiologic and biopsy findings as well as studies of blood group markers in a family with hereditary neuropathy with liability to pressure palsies (HNPP) are reported. There was an autosomal dominant trait without genetic linkage between the HNPP gene and blood group markers controlled by chromosome 1. Reduced motor and sensory nerve conduction velocity was found in clinically affected and unaffected nerves. Characteristic morphological changes in sural nerve biopsy including tomaculous swelling were present.