Development and selective neurodegeneration in cell cultures from different hippocampal regions

Previous studies have shown that pyramidal neurons in hippocampal regions CA1 and CA3 are selectively vulnerable in several neurodegenerative disorders and that a subpopulation of pyramidal neurons in cell cultures of embryonic hippocampus are sensitive to glutamate neurotoxicity. In order to determine whether the patterns of cell loss seen in situ correlate with intrinsic differences in neuronal sensitivities to glutamate-induced denegeration acquired during development, we characterized cultures established from different regions of postnatal rat hippocampus and then examined neuronal sensitivity to glutamate. Tissue corresponding to the dentate gyrus (DG) and regions CA1, CA2, and CA3 of Ammon's horn was removed by microdissection from transverse hippocampal slices and was used to establish cultures of dissociated cells. Cultures from all 4 regions contained 3 major morphological classes of neurons; pyramidal-like, bipolar and stellate. Pyramidal-like neurons comprised the majority of neurons in all cultures; these neurons extended one long and branching axon, and one or more short dendrites. Immunocytochemistry showed that all neurons possessed high levels of glutamate-like and γ-aminobutyric acid (GABA)-like immunoreactivity when grown in isolation. In contrast, when bipolar and pyramidal neurons were cultured in contact with glial cells, glutamate and GABA immunoreactivity were selectively reduced in the bipolar and pyramidal cells, respectively, suggesting that cell interactions influence neurotransmitter phenotype. Subpopulations of hippocampal neurons from each hippocampal region were vulnerable to glutamate-induced neurotoxicity. Bipolar and stellate cells were resistant to glutamate, while pyramidal-like neurons showed varying degrees of sensitivity to glutamate depending upon which region they were taken from. Experiments with specific glutamate receptor agonists and antagonists demonstrated that both non N-methyl-d-spartic acid (NMDA) receptors and NMDA receptors mediated glutamate-induced degeneration. There were clear differences in the vulnerability of the pyramidal-like neuron populations in cultures from the different hippocampal regions. The rank order of the vulnerability of pyramidal-like neurons to glutamate-induced neurodegeneration between regions in culture was: DG