A simple method for combined fluorescence in situ hybridization and immunocytochemistry

By combining in situ hybridization (ISH) and immunocytochemistry (IC), microscopic topological localization of mRNAs and proteins can be determined. Although this technique can be applied to a variety of tissues, it is particularly important for use on neuronal cells which are morphologically complex and in which specific mRNAs and proteins are located in distinct subcellular domains such as dendrites and dendritic spines. One common technical problem for combined ISH and IC is that the signal for immunocytochemical localization of proteins often becomes much weaker after conducting ISH. In this manuscript, we report a simplified but robust protocol that allows immunocytochemical localization of proteins after ISH. In this protocol, we fix cultured cortical or hippocampal neurons with 4% paraformaldehyde (PFA), rinse briefly in PBS, and then further fix the cells with C methanol. Our method has several major advantages over previously described ones in that (1) it is simple, as it is just consecutive routine fixation procedures, (2) it does not require any special alteration to the fixation procedures such as changes in salt concentration, and (3) it can be used with antibodies that are compatible with either methanol (MeOH-) or PFA-fixed target proteins. To our best knowledge, we are the first to employ this fixation method for fluorescence ISH + IC.