Postembedding immunogold labeling was performed as described previously (Petralia and Wenthold 1999 (link); Petralia et al. 2010 (link); Yao et al. 2015 (link)). Briefly, following cryoprotection and embedding, rat brain sections were processed and embedded in Lowicryl HM-20 resin using a Leica AFS freeze-substitution instrument. After blocking, the sections were incubated with primary antibody followed by 10-nm gold-conjugated secondary antibody. Following thorough washes, the sections were stained with uranyl acetate and lead citrate. All electron micrographs were stored in their original formats. For micrographs presented for figures, their levels, brightness and contrast were minimally and evenly adjusted in Adobe Photoshop. Control sections omitting the primary antibody showed only rare gold labeling.