Regulation of the dopamine transporter - Addiction Research ...

Schmitt & Reith DAT Regulation
methamphetamine users typically take upward of
100 mg (a “point”) per dose session—for those
that administer it via inhalation (as smoked “crystal
meth” or “ice”), this can be even higher—often
taking multiple subsequent doses in a “binge” pattern
(every few hours); chronic methamphetamine
addicts can easily consume doses on the order of
grams per day. 145 Intranasal administration of a 50mg
dose of d-methamphetamine results in an average
plasma concentration of 758 nM in human
subjects. 148 Sequential 100-mg doses could thus result
in prolonged plasma drug concentrations ranging
into the micromolar levels shown in animal
and in vitro studies to elicit profound dopamine
release, cytotoxicity, and long-lasting downregulation
of DAT function. Methamphetamine addicts
that consume gram quantities per day can
achieve steady-state plasma concentrations exceeding
13 �M. 74 Hence, it is likely that the transient reduction
in plasmalemmal DAT expression observed
after a single high-level dose of methamphetamine
and the prolonged reduction seen in chronic
high-dose methamphetamine users are two entirely
different phenomena: the former representing a
neuroadaptive feedback mechanism and the latter
representing accumulated apoptotic oxidative injury
to dopaminergic neurons.
Conclusion
Neuronal DATs are the primary regulators of
extracellular dopamine in the brain and are thus responsible
for both the duration and the magnitude
of interneuronal dopaminergic signals. Biogenic
amine transporter proteins, such as the DAT, are
highly dynamic entities that rapidly cycle between
the plasma membrane and early endosomal compartments,
probably in specialized vesicles roughly
similar to synaptic vesicles in size (e.g., Ref. 19). By
altering the pattern of DAT trafficking, cells can adjust
the number of active (surface localized) DATs
in real time, allowing for condition-specific adaptive
changes in dopaminergic signaling. DAT trafficking
is regulated by several intracellular signaling cascades
involving protein kinases and phosphatases.
Activation or inhibition of PKC, PI3K, and members
of the MAPK family can trigger transporter
membrane redistribution; however, the identity of
the kinase(s) ultimately responsible for direct phosphorylation
of the DAT is currently unknown. With
the recent evidence that syntaxin 1A and other scaffolding
proteins interact with the DAT, it is certainly
conceivable that central signaling kinases, such as
PKC, affect DAT activity via DAT-associated scaffolding
or cytoskeletal proteins that more directly
control the transporter’s trafficking fate. In addition,
ligands for certain presynaptic GPCRs found
on dopaminergic neurons (such as dopamine D2like
autoreceptors and �-opioid receptors) can also
acutely regulate DAT surface presence, probably by
modulating the activity of these cell signal transduction
pathways.
Finally, DAT trafficking is acutely regulated by ligands
that directly interact with the DAT itself (substrates
and inhibitors). Acute exposure to high doses
of substrates (including several amphetaminergic
compounds and dopamine itself) elicits a biphasic
effect on DAT trafficking: initially, a rapid yet
transient (beginning within seconds and lasting for
several minutes) increase in surface DAT occurs,
followed by significant transporter endocytosis that
begins after approximately 15 min of substrate exposure.
In direct contrast with the internalizing effects
of substrates, cocaine-like DAT inhibitors appear
to induce upregulation of surface DAT levels. Upregulation
of surface DAT concentration might be
responsible for much of the anhedonic, depressionlike
symptoms that chronic cocaine addicts experience
during withdrawal—doubtless triggering
cravings for more cocaine. The DAT-internalizing
effect of substrates, such as d-amphetamine, might
thus be useful in ameliorating symptoms of cocaine
withdrawal. In limited preclinical trials,
d-amphetamine has shown efficacy in reducing cocaine
consumption and craving. 43 Amphetaminergic
prodrugs would be desirable for this purpose,
because their requisite metabolic activation after
oral administration and slow onset of action would
prevent the possibility of parenteral abuse, a clear
concern in psychostimulant-addicted patients. The
atypical psychostimulant compound modafinil also
shows preclinical promise as a potential therapeutic
for stimulant addiction. Modafinil decreases cocaine
craving during withdrawal 149 and attenuates
the effect of concomitantly administered cocaine 150
or methamphetamine, 151 warranting investigation
into its effects on DAT trafficking. There is also evidence
that withdrawal from psychostimulant use
is associated with deficits in not only dopaminergic
but also serotonergic function. Moreover, agents
Ann. N.Y. Acad. Sci. 1187 (2010) 316–340 c○ 2010 New York Academy of Sciences. 333