The dopamine system is implicated in the control of locomotion, cognit
ion, and endocrine function. The relative contribution of the various
dopamine-related components is not well established mainly because dru
gs that target the dopaminergic system often lack selectivity. The in
vivo gene inactivation procedure, or knockout, enables the creation of
new strains of mice lacking a specific gene. This technique has been
applied recently to inactivate the expression of the plasma membrane d
opamine transporter. Here we summarize the main findings obtained with
these transgenic mice carrying this ''genetic defect,'' leading to a
better understanding of the relative contribution of the dopamine tran
sporter regarding locomotor activity, regulation of the expression of
peptides under the control of dopaminergic activity, and responses to
various drugs targeting the dopamine system, Our results establish not
only the central importance of the transporter as the key element con
trolling-dopamine levels in the brain, but also its role as an obligat
ory target for the behavioral and biochemical action of amphetamine an
d cocaine. In addition, the genetically altered mice offer a unique mo
del to test the specificity and selectivity of dopamine transporter-ac
ting drugs and may provide important new concepts related to the clini
cal and social implications of conditions such as Parkinson's disease,
schizophrenia, and drug addiction.