This lecture covers the classification, metabolism, and predominant sites of action of neurotransmitters such as GABA, glycine, ATP, adenosine, and neuropeptides. It delves into the role of neurotransmitters in sleep regulation, addiction, and the opioid crisis, as well as the study of addictive behaviors in animal models. Additionally, it explores the impact of soluble gases like nitric oxide on long-term potentiation (LTP) and neuroprotective mechanisms.