DREADD agonist 21 is primarily used in neuroscience research to selectively activate designer receptors exclusively activated by designer drugs (DREADDs). This chemical enables precise control over specific neuronal populations, allowing researchers to study brain function, neural circuits, and behaviors in a highly targeted manner. It is particularly valuable in understanding the role of specific neurons in conditions such as anxiety, depression, addiction, and neurodegenerative diseases. By modulating neuronal activity in a controlled way, it helps elucidate the underlying mechanisms of brain disorders and aids in the development of potential therapeutic strategies. Its application is limited to experimental settings and is not used in clinical treatments.