P2‐417: GAD65, GAD67 and GABAT immunoreactivity in human brain and GAD65 loss in Alzheimer's disease

Abstract

The GABAergic system is the main inhibitory neurotransmitter system in the vertebrate brain. Although it is well established that the GABAergic system is affected in neuropsychiatric disorders, in Alzheimer disease it has been considered to be relatively spared. To determine whether the GABAergic system is involved in AD we investigated the main enzymes of this system; glutamate decarboxylase 65 (GAD65), GAD67 and GABA transferase (GABAT), in human AD and control brain tissue by immunohistochemical and western blot methods. In normal human neocortex, hippocampus and cerebellum, GAD65 and GAD67 were found in neuropil granules, presumably synaptic boutons, and in a subset of small to midsized neurons. GAD65 mostly stained neuropil granules, while GAD67 stained neuronal cell bodies more prominently. GAD67 is thought to have mostly a cytoplasmic localization, while GAD65 is thought to be primarily localized in presynaptic terminals. GABAT predominantly labeled large intracellular granule in many types of neurons and glia cells. In addition to neurons astroglial cells were labeled by GAD67 and GABAT antibodies, most prominently in the white matter. GAD65 immunoreactivity of neurons and neuropil was severely and significantly reduced in Alzheimer's disease mid temporal cortex and hippocampus as determined by semiquantitative evaluation of fluorescence and light microscopic immunohistochemistry. This reduction in GAD65 protein levels in AD was also detected by Western blotting of cortex tissue. Our results suggest that in AD the GABAergic system is more severely affected then previously reported. This deficit may contribute to AD pathogenesis by loss of inhibitory activity.