Secondary injury in the spinal cord, which
results in axonal degeneration, scar and cavity formation and cell death,
occurs around the site of the initial trauma and is a primary cause for
the lack of axonal regeneration observed after spinal cord injury (SCI).
The immune response after SCI is under investigation as a potential
mediator of secondary injury. Treatment of SCI with 810 nm light
suppresses the immune response and improves axonal regeneration.
We hypothesize that these beneficial
effects observed in the injured spinal cord are accompanied by alterations
in gene expression within the spinal cord, particularly of those genes
involved in secondary injury and the immune response. To test this
hypothesis, a dorsal hemisection at vertebral level T9 was performed. The
injured spinal cord from rat was then exposed to laser light (810nm,
150mW, 2,997 seconds, 0.3cm2 spot area, 1589 J/cm2) and spinal cord
samples, including the injury site, were harvested at 6 and 48 hours and 4
days post-injury. Total RNA was extracted and purified from the lesioned
spinal cord and cDNA copies were either labeled with [32P] for microarray
analysis or amplified and analyzed with a polymerase chain reaction (PCR).
Microarray results revealed a suppression
of genes involved in the immune response and excitotoxic cell death at 6
hours post-injury, as well as cell proliferation and scar formation at 48
hours post-injury in the light treated group. Analysis of the PCR products
revealed that light treatment resulted in a significant suppression of
expression of genes that normally peak between 6 and 24 hours post-injury,
including the pro-inflammatory cytokine interleukin 6 (IL6), the chemokine
monocyte chemoattractant protein 1 (MCP-1) and inducible nitric oxide
synthase (iNOS; p<0.05). Genes expressed earlier than 6 hours post-injury,
such as IL1b, tumor necrosis factor a (TNFa) and macrophage inflammatory
protein 1a (MIP-1a) were not affected by light treatment.
Although the precise role some of these
genes play in axonal regeneration after spinal cord injury is currently
unclear, these data demonstrate that light therapy has an
anti-inflammatory effect on the injured spinal cord, and may reduce
secondary injury, thus providing a possible mechanism by which light
therapy may result in axonal regeneration.