Antibodies raised against aldehyde-fixed antigens improve sensitivity for postembedding electron microscopy

•Antibodies raised against glutaraldehyde-fixed antigen improve immunogold labeling.•Conventional antibodies have poor affinity for aldehyde fixed protein.•Immunolabeling with aldehyde-protein antibodies show low background levels.•Immunization with fixed antigen does not reduce the specificity of the antibodies.•Different types of antibodies may be needed for electron microscopy. Antibodies are one of the most important tools in biological research. High specificity and sensitivity of antibodies are crucial to obtain reliable results. Tissue fixed with glutaraldehyde (GA) is commonly used in electron microscopical investigations. The fixation and embedding routine in preparation of tissue for post-embedding electron microscopy (EM) will mask and structurally alter epitopes, making antibody-antigen interaction inefficient, with low labeling intensities. One of the main factors in this regard is the use of GA as fixative. To alleviate these technical challenges, we immunized rabbits with antigen pre-fixed with GA. We hypothesized that the resulting antibodies would have stronger affinity to antigens that have been conformationally changed and denatured by GA, the way they are in fixed tissue. An initial screening with western blotting (WB) showed results consistent with our hypothesis. In-house antibodies raised against GA-fixed SNARE proteins SNAP-25 and VAMP2, binds more strongly to fixed proteins compared to non-fixed proteins, while the pattern is opposite with the commercially available antibodies raised against non-fixed antigens (standard antibodies). Quantitative post-embedding EM of hippocampal synapses gave higher labeling intensities with anti-GA-SNAP-25 and anti-GA-VAMP2 compared to standard antibodies. Importantly, light microscopy (LM) and EM with our antibodies revealed stronger labeling of GA-fixed than formaldehyde (FH) treated brains. Our results highlight the experimental potential of raising antibodies against GA-treated antigen to improve sensitivity of the antibodies for postembedding immunogold EM.