In labs, brains are chemically preserved with a chemical called formaldehyde which “fixes” the tissue and minimizes degradation of the sample. Scientists can then slice parts of the brain and add antibodies, which are other proteins that specifically bind to the proteins of interest in the brain. They can then quantify where and in what quantities specific proteins are expressed in different regions of the brain. Recently some efforts have been made to preserve the brain at very low temperatures in liquid nitrogen. This is called cryonics. Some people hope that we will be able to “revive” brains from these frozen samples, but this is highly debated.
Learn more about the cryonics debate here and here.