Effect of transient cerebral ischemia on gamma-aminobutyric acidA receptor alpha 1-subunit-immunoreactive interneurons in the gerbil CA1 hippocampus.

Following transient cerebral ischemia, pyramidal cells within area CA1 of the hippocampus exhibit delayed neuronal death. While interneurons within this sector continue to survive long-term, there is evidence that some interneurons in area CA1 are vulnerable to damage. To determine the nature of vulnerability in a neurochemically heterogeneous population of interneurons throughout area CA1, we examined the labeling of gamma-aminobutyric acid (GABA)ergic interneurons with an antibody to the GABAA receptor alpha 1-subunit 1-35 days following cerebral ischemia in the Mongolian gerbil. Unlike some other GABA interneuron markers, this antibody labels both the dendrites and soma of interneurons, allowing dendritic structure to be examined. Three to four days following ischemia, the pyramidal cells in area CA1 had degenerated, and the alpha 1-subunit-positive interneurons in all layers of area CA1 had developed severely beaded dendrites. At longer survival times (21-35 days), the alpha 1-subunit-immunolabeled dendrites of these interneurons had a fragmented appearance. In contrast, interneurons bordering str. oriens and alveus typically exhibited normal dendritic morphology. Despite the pathologic changes, there was no evidence of interneuron loss in area CA1 up to 35 days post-ischemia. Normal interneuron morphology was also observed in area CA3 and dentate gyrus, regions where neither pyramidal neurons nor granule cells, respectively, die following 5 min of cerebral ischemia. To determine if the ischemia-induced changes in interneuron morphology could be prevented, diazepam was administered 30 and 90 min following ischemia. Diazepam produces long-term neuroprotection of area CA1 pyramidal neurons. In gerbils sacrificed 35 days after ischemia, diazepam markedly attenuated the dendritic beading of the area CA1 interneurons. In addition, the dendrites did not display the fragmented labeling by the alpha 1-subunit antibody. Thus, despite their long-term survival, CA1 hippocampal interneurons in the gerbil can express severe structural abnormalities after transient cerebral ischemia coincident with pyramidal cell degeneration, and the injury to the dendrites can be prevented by the neuroprotectant diazepam.