Introduction: Following birth asphyxia, extracellular CNS levels of glutamate or other related excitatory amino acids rise and cause overstimulation of glutamate receptors. This leads to a rise in intracellular calcium which triggers a cascade of events ultimately causing neuronal cell death. The AMPA glutamate receptor has been cloned and consists of four subunits (GluR A-D) that can be expressed in alternatively spliced isoforms termed flip and flop. AMPA receptors are assembled from several subunits. The GluR-B subunit, whether expressed alone or in combination with other subunits, forms ion channels that are calcium impermeable. This unique property of the AMPA receptor places it in a pivotal position with regard to hypoxic-ischemic brain damage.

Hypothesis: During early ovine CNS development, an excess number of calcium permeable AMPA receptor subunits relative to calcium impermeable subunits renders the immature brain more vulnerable to hypoxic-ischemic insults.

Methods: Sheep brains from fetal sheep at 0.8 term and term, and lambs at 2 days, 10 days, and adult were serially sectioned. Using in situ hybridization with 35S labeled probes to all four AMPA receptor subunits and both flip and flop isoforms, the distribution and density of mRNA coding for all AMPA receptor subunits was characterized, and then the ratio of calcium permeable to impermeable AMPA receptor subunits at specific brain regions of interest was determined. A large value of this ratio is associated with greater calcium permeability.

Results: In the dentate gyrus and the CA1 region of the hippocampus, the ratio of calcium permeable to impermeable AMPA receptor subunits was high prenatally and then declined into adulthood.

Conclusion: During early ovine CNS development, certain brain regions, such as the dentate gyrus and the CA1 region of the hippocampus, express more calcium permeable AMPA receptor subunits relative to calcium impermeable subunits, and thus are more vulnerable to hypoxic-ischemic insults.Funded by AG08843, AHA-FL, CMN, and The Bryan W. Robinson Neurological Foundation.