The document discusses neuromodulators and their role in the brain and association with drugs of abuse. It notes that neuromodulators like acetylcholine, serotonin, and dopamine are released by small groups of neurons and project widely in the brain to alter responses. Drugs of abuse are often associated with these neuromodulatory systems. The document also discusses how neuromodulators can alter ion channel properties and neuronal responses through phosphorylation, as well as their uneven distribution across dendrites leading to changes in response to different inputs.