Modulation of neuronal activity by neurotransmitters is a fundamental mecha
nism of brain function. Perturbations of neurotransmission in disease state
s can be targeted therapeutically through the administration of psychotropi
c drugs. This article reviews three mechanisms that explain the therapeutic
and adverse effects of psychotropic agents. First, the location and projec
tion of neurons determines their behavioral affiliation. Second, neurotrans
mitter receptors lead to changes in the electrical properties (i.e., membra
ne potential) or the biochemical properties (i.e., protein function) of neu
rons. Third, intracellular signaling pathways lead to immediate or delayed,
longer-term effects on neuronal function. These three principles and their
clinical implications are reviewed for the glutamatergic, GABAergic, choli
nergic, serotonergic, noradrenergic, and dopaminergic systems.