Glutamate, the major excitatory neurotransmitter
of the central nervous system (CNS) is released
excessively after injury. Soon after trauma to the
spinal cord, extracellular glutamate levels rise within
and around the injury site (McAdoo et al. 1999),
and it is known to produce direct damage to the
cord, and indirect damage from production of reactive
oxygen and nitrogen species and from alterations
in microcirculatory function and secondary
ischemia (Dumont et al. 2001). The resultant influx
of Ca2+ into neurons causes neuronal death by necrosis
or apoptosis through a process known as excitotoxic
cell death (Xu et al. 2005). Glutamate, however,
must first bind to receptor proteins that also
act as potassium and calcium gates before influx of
these ions into the neurons. Neurons and oligodendrocytes
are particularly vulnerable to glutamate
excitotoxicity because they express a full complement
of glutamate receptors. Excitotoxic injury to
oligodendrocytes and neurons results in demyelination
of axons and loss of neurons around the injury
site, leading to a drastic reduction or complete halt
of axonal transmission (conduction block), thereby
enhancing the disconnect between the brain and
spinal segments below the level of injury, and thus
contributing to motor and sensory deficits.
Consequently, glutamate excitotoxicity markedly
exacerbate the functional problems encountered
after SCI. Researchers have studied drugs that block
glutamate receptors in the hope of preventing excess
potassium and calcium from entering and destroying
the neuron (Lea and Faden 2003) or inhibiting
the injurious interaction between excitotoxicity and
inflammation (Yune et al. 2007).