The expression ratio of GAD65 in GABAergic neurons was increased specifically in layers receiving afferents from the entorhinal cortex (EC), i.e., SR/SLM of the CA1 region and molecular layer of the dentate gyrus (DG), after repeated ECS administration, whereas the expression ratio of GAD67 in GABAergic neurons was decreased in several layers by the same treatment. In the CA3 region, the ND of GAD65(+) neurons was decreased in the stratum oriens and SR/SLM after single ECS. In the CA1 region, the ND of GAD65(+) neurons was increased in the strata radiatum/lacunosum-moleculare (SR/SLM) by repeated ECS. On the other hand, the ND of GAD65(+) neurons was changed in a layer-specific manner. GAD67-positive (GAD67(+)) neurons were also rarely affected by ECS.

The numerical density (ND) of entire population of GABAergic neurons (expressing GAD65 and/or GAD67) was seldom altered by the administration of ECS. In this stereology-based study, we identified GABAergic neurons by immunostaining for two isoforms of glutamic acid decarboxylase (GAD), GAD65, and GAD67 and estimated the expression changes induced by single or repeated electroconvulsive shock (ECS an animal model of ECT). The goal of our study was to investigate the influences of electroconvulsive therapy (ECT), one of the most effective treatments for depression, on the GABAergic system in the hippocampus. There is an increasing body of evidence suggesting that GABAergic dysfunction is involved in various psychiatric disorders.

Hippocampus stereology