A PMHNP is reviewing neuropharmacology and considers why the onset of antidepressant action takes weeks despite immediate receptor binding. She recalls that most monoamine receptors are G-protein coupled receptors (GPCRs) that signal through intracellular second messenger cascades rather than producing immediate ion flux. A student asks why GPCR signaling produces slower but more sustained effects compared to ionotropic receptor signaling. Which of the following best describes GPCR signal transduction and its relevance to psychopharmacology?