These results indicate that our immunogold labeling for the 1 (1(Gp; aa342-430)) and 2 (2(Gp; aa343-430)) subunits in hippocampal perisomatic synapses is probably due to specific antibody-protein interactions. many GABAAR subunit types and receive GABAergic inputs from distinct interneurons. Previous experiments revealed input-specific differences in 1 and 2 subunit densities in perisomatic synapses, suggesting distinct IPSC decay kinetics. However, IPSC decays evoked by axo-axonic, parvalbumin- or cholecystokinin-expressing basket cells were found to be similar. Using replica immunogold labeling, here we show that all CA1 Catharanthine sulfate PC somatic and AIS synapses contain the 1, 2, 1, 2, 3 and 2 subunits. In CA3 PCs, 90% of the perisomatic synapses are immunopositive for the 1 subunit and all synapses are positive for the remaining five subunits. Somatic synapses form unimodal distributions based on their immunoreactivity for these subunits. The 2 2 subunit densities in somatic synapses facing Cav2.1 (i.e. parvalbumin) or Cav2.2 (cholecystokinin) positive presynaptic active zones are comparable. We conclude that perisomatic synapses made by three distinct interneuron types have similar GABAA receptor subunit content. DOI: http://dx.doi.org/10.7554/eLife.18426.001 multiple comparisons of mean ranks for all groups. Significance was taken at p 0.05 (*), p 0.01 (**) or p 0.001 (***). Medians and lower (Q1) and upper quartiles (Q3) were used to describe distributions throughout the manuscript. Specificity of the immunoreactions Specificity of immunogold labeling for the 1, 2 and 2 subunits was verified by using two antibodies directed against different epitopes of the same protein. Similar labeling patterns were obtained with a rabbit anti-1 (1(Rb; aa1-9)) and a mouse anti-1 (1(Mo; aa28-43)) antibody, indicating the specificity of the reaction. Mirror replica labeling was used to assess the specificity of our 2 and 2 labeling, by using antibodies directed against an extracellular and an Catharanthine sulfate intracellular epitope. We observe similar gold particle labeling patterns with a rabbit anti-2 antibody (2(Rb; aa322-357)) on the P-face compared to that obtained with a guinea-pig anti-2 antibody (2(Gp; aa1-9)) on the E-face. Our 2 labeling on the E-face, obtained with a rabbit antibody (2(Rb, aa39-67)), was very similar to the immunogold labeling seen on the P-face with a rabbit anti-2 antibody (2(Rb, aa319-366)), recognizing an intracellular epitope (Figure 1ACD). We could not purchase anti- subunit antibodies raised against different epitopes, and therefore we could not test the specificity of the labeling using two antibodies. However, we performed SDS-FRL immunogold labeling for the 1 and 2 subunits in brain areas and nerve cells where the genes of these subunits are not expressed (e.g. cerebellar Purkinje cells, and medial habenula neurons), and observe very few gold particles labeling for 1 or 2 2 subunits in GABAergic synapses (zero or 1C3 gold particles RHEB / synapse; data not shown). These results indicate that our immunogold labeling for the 1 Catharanthine sulfate (1(Gp; aa342-430)) and 2 (2(Gp; aa343-430)) subunits in hippocampal perisomatic synapses is probably due to specific antibody-protein interactions. We observed a similar labeling pattern with our guinea-pig anti-3 antibody (3(Gp; aa344-429)) to that published by Kasugai et al. (2010) with a different antibody against the 3 subunit, the specificity of which was proven in 3-/- mice. Acknowledgements ZN is the recipient of a Hungarian Academy of Sciences Momentum Grant (Lendlet, LP2012-29) and a European Research Council Advanced Grant (293681). The financial support from these funding bodies is gratefully acknowledged. We would like to thank Drs. Peter Somogyi, Thomas Klausberger, Gabor Nyiri and Mark Eyre for their comments on the manuscript; Drs. Jean-Mark Fritschy and Werner Sieghart for kindly providing GABAAR-specific antibodies. We thank va Dobai for her excellent technical assistance. Funding Statement The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication. Funding Information This paper was supported by the following grants: European Research Council 293681 to Zoltan Nusser. Magyar Tudomnyos Akadmia LP2012-29 to Zoltan Nusser. Additional information Competing interests The authors declare that no competing interests exist. Author contributions KK-S, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article. ZN, Conception and design, Analysis and interpretation of data, Drafting or revising the article. Ethics Animal experimentation: All experiments were conducted in accordance with the Hungarian Act of Animal Care and Experimentation (1998, XXVIII, section 243/1998) and with the ethical guidelines of the Institute of Experimental Medicine Protection of Research Subjects Committee..