Ross University School of Veterinary Medicine 1 article published in JoVE Neuroscience Rapid In Situ Hybridization using Oligonucleotide Probes on Paraformaldehyde-prefixed Brain of Rats with Serotonin Syndrome Ibrahim M. Shokry1, John J. Callanan1, John Sousa2, Rui Tao2 1Ross University School of Veterinary Medicine, 2Charles E. Schmidt College of Medicine, Florida Atlantic University This protocol describes a rapid and simplified in situ hybridization method ideal forparaformaldehyde-prefixed brain, thus reducing the need for prolonged complex steps while using fresh frozen tissues. The method is validated using the identification of the serotonin 5-HT2A receptor gene htr2a in rats.