Intrathecal infusion of the nitric oxide synthase inhibitor N-methyl L-arginine after experimental spinal cord injury in guinea pigs

The Dark Side of an Essential Amino Acid: L-Arginine in Spinal Cord Injury

L-arginine is a semi-essential amino acid involved in a variety of physiological processes in the central nervous system (CNS). It is essential in the survival and functionality of neuronal cells. Nonetheless, L-arginine also has a dark side; it potentiates neuroinflammation and nitric oxide (NO) production, leading to secondary damage. Therefore, modulating the L-arginine metabolism is challenging because both detrimental and beneficial effects are dependent on this semi-essential amino acid. After spinal cord injury (SCI), L-arginine plays a crucial role in trauma-induced neuroinflammation and regenerative processes via the two key enzymes: nitric oxide synthase (NOS) and arginase (ARG). Studies on L-arginine metabolism using ARG and NOS inhibitors highlighted the conflicting role of this semi-essential amino acid. Similarly, L-arginine supplementation resulted in both negative and positive outcomes after SCI. However, new data indicate that arginine depletion substantially improves spinal cord regeneration after injury. Here, we review the challenging characteristics of L-arginine metabolism as a therapeutic target after SCI.

Extensive scarring induced by chronic intrathecal tubing augmented cord tissue damage and worsened functional recovery after rat spinal cord injury

Intrathecal infusion has been widely used to directly deliver drugs or neurotrophins to a lesion site following spinal cord injury. Evidence shows that intrathecal infusion is efficient for 7 days but is markedly reduced after 14 days, due to time dependent occlusion. In addition, extensive fibrotic scarring is commonly observed with intrathecal infusion. These anomalies need to be clearly elucidated in histology. In the present study, all adult Long-Evans rats received a 25 mm contusion injury on spinal cord T10 produced using the NYU impactor device. Immediately after injury, catheter tubing with an outer diameter of 0.38 mm was inserted through a small dural opening at L3 into the subdural space with the tubing tip positioned near the injury site. The tubing was connected to an Alzet mini pump, which was filled with saline solution and was placed subcutaneously. Injured rats without tubing served as control. Rats were behaviorally tested for 6 weeks using the BBB locomotor rating scale and histologically assessed for tissue scarring. Six weeks later, we found that the intrathecal tubing caused extensive scarring and inflammation, related to neutrophils, macrophages and plasma cells. The tubing's tip was occluded by scar tissue and inflammatory cells. The scar tissue surrounding the tubing consists of 20-70 layers of fibroblasts and densely compacted collagen fibers, seriously compressing and damaging the cord tissue. BBB scores of rats with intrathecal tubing were significantly lower than control rats (p<0.01) from 2 weeks after injury, implying serious impairment of functional recovery caused by the scarring.

Role of peroxynitrite in secondary oxidative damage after spinal cord injury

Peroxynitrite (PON, ONOO(-)), formed by nitric oxide synthase-generated nitric oxide radical ( NO) and superoxide radical (O(2) (-)), is a crucial player in post-traumatic oxidative damage. In the present study, we determined the spatial and temporal characteristics of PON-derived oxidative damage after a moderate contusion injury in rats. Our results showed that 3-nitrotyrosine (3-NT), a specific marker for PON, rapidly accumulated at early time points (1 and 3 h) and a significant increase compared with sham rats was sustained to 1 week after injury. Additionally, there was a coincident and maintained increase in the levels of protein oxidation-related protein carbonyl and lipid peroxidation-derived 4-hydroxynonenal (4-HNE). The peak increases of 3-NT and 4-HNE were observed at 24 h post-injury. In our immunohistochemical results, the co-localization of 3-NT and 4-HNE results indicates that PON is involved in lipid peroxidative as well as protein nitrative damage. One of the consequences of oxidative damage is an exacerbation of intracellular calcium overload, which activates the cysteine protease calpain leading to the degradation of several cellular targets including cytoskeletal protein (alpha-spectrin). Western blot analysis of alpha-spectrin breakdown products showed that the 145-kDa fragments of alpha-spectrin, which are specifically generated by calpain, were significantly increased as soon as 1 h following injury although the peak increase did not occur until 72 h post-injury. The later activation of calpain is most likely linked to PON-mediated secondary oxidative impairment of calcium homeostasis. Scavengers of PON, or its derived free radical species, may provide an improved antioxidant neuroprotective approach for the treatment of post-traumatic oxidative damage in the injured spinal cord.

Improved functional outcome after spinal cord injury in iNOS-deficient mice

STUDY DESIGN

Functional outcome was evaluated following experimental compression-type spinal cord injury (SCI) in wild-type mice and knockout mice, lacking the inducible nitric oxide synthase (iNOS) gene.

OBJECTIVES

To evaluate the role of the nitric oxide generating enzyme iNOS in SCI.

METHODS

The experimental animals were subjected to an extradural compression of the thoracic spinal cord. Functional outcome was studied during the first 2 weeks post-injury using a scoring system for assessment of hind limb motor function.

RESULTS

Injury resulted in initial paraplegia followed by gradual improvement of motor function in most cases. Mice lacking the iNOS gene (iNOS-/-) clearly tended to have a better functional outcome than wild-type mice. The difference was significant on day 14 after injury.

CONCLUSION

In accordance with a few earlier experimental studies, showing beneficial effects of pharmacological iNOS inhibition, the present report would indicate a destructive influence of iNOS following spinal cord trauma.

Increased production of tumor necrosis factor-alpha induces apoptosis after traumatic spinal cord injury in rats

We showed previously that, after spinal cord injury (SCI), tumor necrosis factor-alpha (TNF-alpha) may serve as an external signal, initiating apoptosis in neurons and oligodendrocytes. To further characterize the apoptotic cascade initiated by TNF-alpha after SCI, we examined the expression of TNF-alpha, inducible nitric oxide (NO) synthase (iNOS), and the level of NO after SCI. Western blots and reverse transcription polymerase chain reactions showed an early upregulation of TNF-alpha after injury. A peak TNF-alpha expression was observed within 1 h of injury. By 4 h after injury, the expression of iNOS and the level of NO were markedly increased in the injured spinal cord. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL)-positive cells were also first observed in the lesioned area 4 h after SCI. The largest number of TUNEL-positive cells was observed between 24-48 h after SCI. Injecting a neutralizing antibody against TNF-alpha into the lesion site after injury significantly reduced the expression of iNOS, the level of NO and the number of TUNEL-positive cells in the injured spinal cord. Injecting the NOS inhibitors, N(G)-monomethyl-L-arginine monoacetate and S-methylisothiourea sulfate, or an NO scavenger, carboxy-PTIO, into the lesion site also significantly reduced the level of NO and the degree of DNA laddering in the injured spinal cord. These data suggest that after SCI, apoptosis induced by TNF-alpha may be mediated in part by NO via upregulation of iNOS, induced in response to TNF-alpha.

Anaesthetic considerations for acute spinal cord injury

The recently published research data on the possible pathophysiology of acute spinal cord injury provide the basis of a number of exciting possibilities for its treatment. The present article reviews these lines of investigation. It focusses on methylprednisolone, which is the only effective proven therapy to limit secondary spinal cord injury known to date. In addition, the initial evaluation of patients with possible spinal cord trauma and airway management in patients with cervical spine injury are also discussed. Finally, the anaesthetic regimen in patients with these injuries is reviewed, showing that no anaesthetic agent or technique is superior to other anaesthetic methods.

Transitory expression of NADPH diaphorase (NOS) in axonal swellings after spinal cord injury

To investigate the sites of nitric oxide synthase (NOS) expression after a spinal cord (SC) injury, NADPH-d diaphorase histochemistry was performed in the SC of adult rats sacrificed at different times from 1 h to 90 days after both SC contusion or transection. NOS could first be seen 12 h after injury in axonal swellings (AS) (club shaped structures at the tip of damage axons, associated with tissue destruction). NOS expression reached a maximum 3 days after injury, and gradually disappeared after 7 days. Finally, AS collapsed leaving behind microcysts. NOS expression and the consequent production of nitric oxide could be involved in the pathophysiology of the secondary damage, and/or could reflect a failed attempt for axonal regeneration.

Quinolinic acid accumulation in injured spinal cord: time course, distribution, and species differences between rat and guinea pig

Experimental compression injury of the spinal cord in guinea pigs results in delayed neurologic deficits that continue to increase in severity for several days following trauma, coincident with inflammatory responses, including invasion of the lesion by mononuclear phagocytes and increased levels of the neurotoxin quinolinic acid (QUIN). Inflammatory responses and QUIN elevation also occur following spinal cord contusion in rats, but maximal neurologic deficits develop immediately. In this study, somatosensory evoked potentials (SEP) and tissue, serum, and cerebrospinal fluid levels of QUIN were measured in guinea pigs and rats following similar compression injuries of the thoracic spinal cord. SEP changes differed between the species, consistent with other neurological changes. In guinea pigs, increases in QUIN levels at the lesion site began at 1 day postinjury, achieved maximal elevation (100-fold) by 12 days, then declined, but remained above serum levels at 25 days postinjury. A similar increase occurred in adjacent areas of the spinal cord, with lower peak levels. In rats, tissue QUIN at the center of the lesion remained below serum levels at all times, increasing moderately (<10-fold) up to 7 days, then decreasing between 7 and 25 days. These data demonstrate differences in the time course and magnitude of QUIN accumulation and neurological deficit between guinea pig and rat, which may relate to differences in secondary pathological mechanisms. Such profound differences may affect the use of these species for evaluation of experimental therapy in this and other inflammatory conditions of the central nervous system.