This protocol combines in situ hybridization techniques with immunocytochemistry (ICC). Such a protocol generally allows you to identify cells that have both a specific species of mRNA expressed and a separate specific protein. This combination of techniques is particularly powerful for a system like neuropeptide Y (NPY). NPY has a broad distribution in the mammalian central nervous system (CNS) and probably has multiple functions. One way to understand those functions is to assess changes in neuronal activity of NPYergic cells using an antibody directed to the protein Fos. Fos is the product of the early immediate gene c-fos and is typically found in neurons after treatments that result in their activation. As such, it has become a broadly used and accepted marker for increased neuronal activity after a specific treatment.