Linalool acts as a fast and reversible anesthetic in Hydra
The ability to make transgenic Hydra lines has opened the door for quantitative in vivo studies of Hydra regeneration and physiology. These studies commonly include excision, grafting and transplantation experiments along with high-resolution imaging of live animals, which can be challenging due to the animals response to touch and light stimuli. While various anesthetics have been used in Hydra studies over the years, they tend to be toxic over the course of a few hours or their long-term effects on animal health have not been studied. Here we show that the monoterpenoid linalool is a useful anesthetic for Hydra. Linalool is easy to use, non-toxic, fast acting, and reversible. It has no detectable long-term effects on cell viability or cell proliferation. We demonstrate that the same animal can be immobilized in linalool multiple times at intervals of several hours for repeated imaging over 2-3 days. This uniquely allows for in vivo imaging of dynamic processes such as head regeneration. We further directly compare linalool to currently used anesthetics and show its superior performance. Because linalool, which is frequently utilized in perfumes and cosmetic products, is also non-hazardous to humans, it will be a useful tool for Hydra research in both research and teaching contexts.