Chapter 2: Transporters, Receptors, and Enzymes as Targets of Psychopharmacological Drug Action

The chapter introduces neuroscience-based nomenclature as a contemporary framework for classifying drugs according to their mechanisms of action rather than their clinical applications. The vast majority of psychotropic agents operate through one of three primary mechanisms: inhibition of neurotransmitter reuptake transporters, modulation of G-protein-coupled receptors, or inhibition of neurotransmitter-metabolizing enzymes. Plasma membrane transporters for monoamine neurotransmitters, including serotonin transporter, norepinephrine transporter, and dopamine transporter, represent critical therapeutic targets for conditions ranging from major depression to attention deficit hyperactivity disorder. These transporters function as active transport systems powered by sodium and chloride gradients maintained by the sodium potassium ATPase, allowing psychotropic medications to enhance synaptic neurotransmitter concentrations by blocking reuptake. Within the presynaptic terminal, vesicular transporters sequester monoamines in storage compartments using a proton gradient generated by the vesicular proton pump. The second major drug target class comprises G-protein-coupled receptors, seven-transmembrane proteins that mediate diverse cellular responses through intracellular signaling cascades. Drug-receptor interactions occupy a functional spectrum from full agonists producing maximal signal transduction to inverse agonists that suppress receptor signaling below baseline activity levels, with partial agonists occupying intermediate positions. This framework permits precise characterization of drug effects and predicts how medications will modulate neural activity under varying neurotransmitter concentrations. A smaller proportion of psychotropic drugs achieve therapeutic efficacy by inhibiting enzymes involved in neurotransmitter catabolism or in intracellular signaling cascades. Additionally, the chapter addresses how hepatic and gastrointestinal cytochrome P450 enzymes metabolize psychotropic drugs, emphasizing that genetic polymorphisms in these metabolizing enzymes create individual variation in drug metabolism rates. Pharmacogenomic assessment enables clinicians to predict metabolism phenotypes and adjust dosing accordingly, optimizing therapeutic outcomes while minimizing adverse effects.