Pericytes impair capillary blood flow and motor function after chronic spinal cord injury.

Blood vessels in the central nervous system (CNS) are controlled by neuronal activity. For example, widespread vessel constriction (vessel tone) is induced by brainstem neurons that release the monoamines serotonin and noradrenaline, and local vessel dilation is induced by glutamatergic neuron activity. Here we examined how vessel tone adapts to the loss of neuron-derived monoamines after spinal cord injury (SCI) in rats. We find that, months after the imposition of SCI, the spinal cord below the site of injury is in a chronic state of hypoxia owing to paradoxical excess activity of monoamine receptors (5-HT1) on pericytes, despite the absence of monoamines. This monoamine-receptor activity causes pericytes to locally constrict capillaries, which reduces blood flow to ischemic levels. Receptor activation in the absence of monoamines results from the production of trace amines (such as tryptamine) by pericytes that ectopically express the enzyme aromatic L-amino acid decarboxylase (AADC), which synthesizes trace amines directly from dietary amino acids (such as tryptophan). Inhibition of monoamine receptors or of AADC, or even an increase in inhaled oxygen, produces substantial relief from hypoxia and improves motoneuron and locomotor function after SCI.

Decrease of mRNA Editing after Spinal Cord Injury is Caused by Down-regulation of ADAR2 that is Triggered by Inflammatory Response.

We recently showed that spinal cord injury (SCI) leads to a decrease in mRNA editing of serotonin receptor 2C (5-HT2CR) contributing to post-SCI spasticity. Here we study post-SCI mRNA editing and global gene expression using massively parallel sequencing. Evidence is presented that the decrease in 5-HT2CR editing is caused by down-regulation of adenosine deaminase ADAR2 and that editing of at least one other ADAR2 target, potassium channel Kv1.1, is decreased after SCI. Bayesian network analysis of genome-wide transcriptome data indicates that down-regulation of ADAR2 (1) is triggered by persistent inflammatory response to SCI that is associated with activation of microglia and (2) results in changes in neuronal gene expression that are likely to contribute both to post-SCI restoration of neuronal excitability and muscle spasms. These findings have broad implications for other diseases of the Central Nervous System and could open new avenues for developing efficacious antispastic treatments.

Recovery of motoneuron and locomotor function after spinal cord injury depends on constitutive activity in 5-HT2C receptors.

Muscle paralysis after spinal cord injury is partly caused by a loss of brainstem-derived serotonin (5-HT), which normally maintains motoneuron excitability by regulating crucial persistent calcium currents. Here we examine how over time motoneurons compensate for lost 5-HT to regain excitability. We find that, months after a spinal transection in rats, changes in post-transcriptional editing of 5-HT2C receptor mRNA lead to increased expression of 5-HT2C receptor isoforms that are spontaneously active (constitutively active) without 5-HT. Such constitutive receptor activity restores large persistent calcium currents in motoneurons in the absence of 5-HT. We show that this helps motoneurons recover their ability to produce sustained muscle contractions and ultimately enables recovery of motor functions such as locomotion. However, without regulation from the brain, these sustained contractions can also cause debilitating muscle spasms. Accordingly, blocking constitutively active 5-HT2C receptors with SB206553 or cyproheptadine, in both rats and humans, largely eliminates these calcium currents and muscle spasms, providing a new rationale for antispastic drug therapy.

Spastic tail muscles recover from myofiber atrophy and myosin heavy chain transformations in chronic spinal rats.

Without intervention after spinal cord injury (SCI), paralyzed skeletal muscles undergo myofiber atrophy and slow-to-fast myofiber type transformations. We hypothesized that chronic spasticity-associated neuromuscular activity after SCI would promote recovery from such deleterious changes. We examined segmental tail muscles of chronic spinal rats with long-standing tail spasticity (7 mo after sacral spinal cord transection; older chronic spinals), chronic spinal rats that experienced less spasticity early after injury (young chronic spinals), and rats without spasticity after transection and bilateral deafferentation (spinal isolated). These were compared with tail muscles of age-matched normal rats. Using immunohistochemistry, we observed myofiber distributions of 15.9 +/- 3.5% type I, 18.7 +/- 10.7% type IIA, 60.8 +/- 12.6% type IID(X), and 2.3 +/- 1.3% type IIB (means +/- SD) in young normals, which were not different in older normals. Young chronic spinals demonstrated transformations toward faster myofiber types with decreased type I and increased type IID(X) paralleled by atrophy of all myofiber types compared with young normals. Spinal isolated rats also demonstrated decreased type I myofiber proportions and increased type II myofiber proportions, and severe myofiber atrophy. After 4 mo of complete spasticity (older chronic spinals), myofiber type transformations were reversed, with no significant differences in type I, IIA, IID(X), or IIB proportions compared with age-matched normals. Moreover, after this prolonged spasticity, type I, IIA, and IIB myofibers recovered from atrophy, and type IID(X) myofibers partially recovered. Our results indicate that early after transection or after long-term spinal isolation, relatively inactive tail myofibers atrophy and transform toward faster myofiber types. However, long-term spasticity apparently produces neuromuscular activity that promotes recovery of myofiber types and myofiber sizes.

Tail muscles become slow but fatigable in chronic sacral spinal rats with spasticity.

Paralyzed skeletal muscle sometimes becomes faster and more fatigable after spinal cord injury (SCI) because of reduced activity. However, in some cases, pronounced muscle activity in the form of spasticity (hyperreflexia and hypertonus) occurs after long-term SCI. We hypothesized that this spastic activity may be associated with a reversal back to a slower, less fatigable muscle. In adult rats, a sacral (S2) spinal cord transection was performed, affecting only tail musculature and resulting in chronic tail spasticity beginning 2 wk later and lasting indefinitely. At 8 mo after injury, we examined the contractile properties of the segmental tail muscle in anesthetized spastic rats and in age-matched normal rats. The segmental tail muscle has only a few motor units (<12), which were easily detected with graded nerve stimulation, revealing two clear motor unit twitch durations. The dominant faster unit twitches peaked at 15 ms and ended within 50 ms, whereas the slower unit twitches only peaked at 30-50 ms. With chronic injury, this slow twitch component increased, resulting in a large overall increase (>150%) in the fraction of the peak muscle twitch force remaining at 50 ms. With injury, the peak muscle twitch (evoked with supramaximal stimulation) also increased in its time to peak (+48.9%) and half-rise time (+150.0%), and decreased in its maximum rise (-35.0%) and decay rates (-40.1%). Likewise, after a tetanic stimulation, the tetanus half-fall time increased by 53.8%. Therefore the slow portion of the muscle was enhanced in spastic muscles. Consistent with slowing, posttetanic potentiation was 9.2% lower and the stimulation frequency required to produce half-maximal tetanus decreased 39.0% in chronic spinals. Interestingly, in spastic muscles compared with normal, whole muscle twitch force was 81.1% higher, whereas tetanic force production was 38.1% lower. Hence the twitch-to-tetanus ratio increased 104.0%. Inconsistent with overall slowing, whole spastic muscles were 61.5% more fatigable than normal muscles. Thus contrary to the classical slow-to-fast conversion that is seen after SCI without spasticity, SCI with spasticity is associated with a mixed effect, including a preservation/enhancement of slow properties, but a loss of fatigue resistance.