Ampyrone

Ampyrone is a metabolite of aminopyrine with analgesic, anti-inflammatory, and antipyretic properties. While the parent drug, aminopyrine, has been discouraged due to the risk of agranulocytosis, ampyrone itself has significantly lower toxicity. It is used as a reagent for biochemical reactions producing peroxides or phenols. Ampyrone stimulates liver microsomes and is also used to measure extracellular water.
By applying an ampyrone solution, scientists can make skin temporarily transparent ; and e.g. see directly into the brain : "This opens a literal window to peek into the brain's development.. Not only can we image the structures of these neurons, but we can also image the neural activity over time in an animal model. In the future, this approach could enable us to look at how these circuits form during the development of an animal."