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  • 1 University of Pécs Department of Laboratory Medicine, Faculty of Medicine H-7624 Pécs Hungary
  • 2 Aarhus University Hospital Centre for Psychiatric Research 8240 Risskov Denmark
  • 3 University of Pécs Szentágothai János Research Center, Neuroendocrinology Research Group H-7624 Pécs Hungary
  • 4 University of Pécs Central Electron Microscopic Laboratory, Faculty of Medicine H-7624 Pécs Hungary
  • 5 University of California Los Angeles Department of Neurobiology and Brain Research Institute Los Angeles CA 90095 USA
  • 6 Greater Los Angeles Healthcare System Research Service, Veterans Administration West Los Angeles, Los Angeles CA 90073 USA
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In rodent models for neuropsychiatric disorders reduced number of hippocampal interneurons have been reported, but the total number of GABAergic neurons in the normal rat hippocampus is yet unknown. We used in situ hybridization method to label the 65 isoform of glutamic acid decarboxylase (GAD65) and counted the number of GAD65 mRNA-expressing neurons along the entire septo-temporal axis of the hippocampus. We found that 2/3 of the interneurons were in Ammon’s horn (61,590) and 1/3 in the dentate gyrus (28,000). We observed the following numbers in Ammon’s horn: CA3 area 33,400, CA2 area 4,190, CA1 area 24,000 and in the dentate gyrus: 6,000 in the molecular and 9,000 in the granule cell layers and 13,000 in the hilus. GAD65 mRNA-expressing neurons were significantly more numerous in dorsal than in ventral hippocampus. The ratio between interneurons and principal cells was lowest in the granule cell layer (0.9%) and highest in hilus (21%). In Ammon’s horn this ratio was constant being 13% in CA3 and 8% in CA1-2 areas. In the entire hippocampal formation, the interneuron/principal cell ratio was 6%, with a significant difference between Ammon’s horn (9.5%) and the dentate gyrus (2.8%) including the hilus. Such low ratios could suggest that even a limited loss of GABAergic neurons in the hippocampus may have a considerable functional impact.

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