Light and electron microscopic distribution of the AMPA receptor subunit, GluR2, in the spinal cord of control and G86R mutant superoxide dismutase transgenic mice.

Excitotoxicity has been hypothesized to contribute to amyotrophic lateral sclerosis (ALS) neurodegeneration. The similar pattern of vulnerability in the spinal cord of mutant superoxide dismutase (SOD-1) transgenic mice and mice treated with excitotoxins supports a role for excitotoxicity in the mechanism of degeneration. The distribution of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) class of glutamate receptors (GluRs) with different calcium permeabilities has been proposed as an explanation for this differential vulnerability. GluR2 appears to be the dominant determinant of calcium permeability for AMPA receptors; thus, it is critical for their contribution to excitotoxic mechanisms. In this study, we investigate the distribution of GluR2 immunoreactivity in the spinal cord of control and SOD-1 transgenic mice. GluR2 immunoreactivity is present equally within vulnerable neurons (i.e., motor neurons and calretinin-immunoreactive neurons) as well as nonvulnerable neurons (i.e., calbindin-immunoreactive neurons and dorsal horn neurons). In addition, postembedding immunoelectron microscopy reveals that GluR2 is present in synapses of dorsal and ventral horn neurons and that the percentage of labeled synapses and numbers of immunogold particles per synapse do not vary between these spinal cord regions. Comparing control mice with SOD-1 transgenic mice, at both the light and the electron microscopic levels, the distribution and intensity of GluR2-immunoreactivity do not appear to be altered. These results suggest that the cellular and synaptic distribution of GluR2 is not a determinant of the selective vulnerability observed in SOD-1 transgenic mice or in ALS patients.