SCH 23390 HCl is primarily used in research settings as a selective dopamine D1 receptor antagonist. It is widely applied in neuroscience studies to investigate the role of dopamine D1 receptors in various brain functions, including motor control, cognition, and reward mechanisms. Researchers use it to block D1 receptor activity, helping to understand the receptor's involvement in behaviors and neurological disorders such as Parkinson's disease, schizophrenia, and addiction. Additionally, it is utilized in pharmacological experiments to study the interaction between dopamine receptors and other neurotransmitter systems, providing insights into potential therapeutic targets for neurological and psychiatric conditions.