Estradiol increases dendritic spine density by reducing GABA neurotransmission in hippocampal neurons

We have previously shown that estradiol causes a twofold rise in dendritic spine density in cultured rat hippocampal neurons, as it does in vivo. More recently, estrogen receptors have been localized to aspiny inhibitory hippocampal interneurons, indicating that their effect on spiny pyramidal neurons may be indirect. We therefore examined the possibility that estradiol affects spine density by regulating inhibition in cultured hippocampal interneurons. Immunocytochemically, estrogen receptors were found to be co-localized with glutamate decarboxylase (GAD)-positive neurons (approximately 21% of total neurons in the culture). Exposure of cultures to estradiol for 1 d caused a marked decrease (up to 80%) in the GAD content of the interneurons, measured both by immunohistochemistry and Western blotting. Also, the number of GAD-positive neurons in the cultures decreased to 12% of the total cell population. Moreover, GABAergic miniature IPSCs were reduced in both size and frequency by estradiol, whereas miniature EPSCs increased in frequency. We then mimicked the proposed effects of estradiol by blocking GABA synthesis with mercaptopropionic acid (MA). Cultures treated with MA expressed a dose-dependent decrease in GABA immunostaining that mimicked that seen with estradiol. MA-treated cultures displayed a significant 50% increase in dendritic spine density over controls, similar to that produced by estradiol. These results indicate that estradiol decreases GABAergic inhibition in the hippocampus, which appears to effectively increase the excitatory drive on pyramidal cells, and thus may provide a mechanism for formation of new dendritic spines.