ESTRADIOL INCREASES DENDRITIC SPINE DENSITY BY REDUCING GABA NEUROTRANSMISSION IN HIPPOCAMPAL-NEURONS

We have previously shown that estradiol causes a twofold rise in dendr itic 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 s piny pyramidal neurons may be indirect. We therefore examined the poss ibility that estradiol affects spine density by regulating inhibition in cultured hippocampal interneurons. Immunocytochemically, estrogen r eceptors were found to be co-localized with glutamate decarboxylase (G AD)-positive neurons (similar to 21% of total neurons in the culture), Exposure of cultures to estradiol for 1 d caused a marked decrease (u p to 80%) in the GAD content of the interneurons, measured both by imm unohistochemistry and Western blotting, Also, the number of GAD-positi ve neurons in the cultures decreased to 12% of the total cell populati on. 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 e xpressed a dose-dependent decrease in GABA immunostaining that mimicke d that seen with estradiol, MA-treated cultures displayed a significan t 50% increase in dendritic spine density over controls, similar to th at produced by estradiol, These results indicate that estradiol decrea ses GABAergic inhibition in the hippocampus, which appears to effectiv ely increase the excitatory drive on pyramidal cells, and thus may pro vide a mechanism for formation of new dendritic spines.