Regulation of the dopamine transporter - Addiction Research ...

DAT Regulation Schmitt & Reith
that they are bona fide neuromodulators. �-
Phenethylamine (�-PEA), p-tyramine, and DMT
act as full agonists at the trace amine receptor
TAAR1, triggering the production cAMP. 106
Interestingly, like �-PEA, the DAT substrates damphetamine
and d-methamphetamine are also
full agonists at TAAR1. 74 In HEK cells expressing
hDAT and TAAR1, exposure to �-PEA causes a
rapid reduction in [ 3 H]dopamine uptake in both
HEK cells and mouse synaptosomes—importantly,
this effect does not appear to be due to generalized
DAT substrate activity of �-PEA, because the effect
is absent in synaptosomes prepared from TAAR1knockout
mice. 107 Later analysis of transfected HEK
cells demonstrated that TAAR1 and dopamine D2
receptors may have opposing effects on DAT regulation:
cells expressing DAT and D2 receptors show the
expected increase in [ 3 H]dopamine uptake, whereas
cells expressing DAT and TAAR1 show a decrease
in [ 3 H]dopamine uptake under identical conditions.
108 However, the relevance of this finding is
difficult to interpret, because dopamine was used as
both the D2R and TAAR1 agonist, but as discussed in
the foregoing, radiolabeled [ 3 H]dopamine used to
measure uptake would also probably activate receptors.
Because the neuromodulatory roles of these
once-mysterious trace amines are just now beginning
to be uncovered, further investigations into
the effects of TAAR1 receptor activation on transporter
trafficking are clearly needed. Subtle modulation
of monoaminergic neurotransmission by
TAAR1 agonists has, for example, been speculated
to underlie the anxiolytic effects of small doses of
d-amphetamine and DMT in humans. 109
Downregulation of DAT activity by κ-opioid
receptors
Interest in the effects of �-opioid receptor activation
on DAT function stems from initial observations
that �-opioid agonists attenuate the dramatic
rise in extracellular dopamine levels after cocaine
administration. In the rat nucleus accumbens, for
example, a single high-dose injection of cocaine
(20 mg/kg) causes a 10-fold increase in extracellular
dopamine concentration—pretreatment with
the �-agonist U50488 decreases this effect by more
than 50%. 110 Using voltammetry and in vivo microdialysis,
Thompson et al. demonstrated that the
DATmodulatoryeffectsof�-opioid agonists differ
between acute and chronic agonist administra-
tion. 111 That is, whereas acute administration of the
�-agonist U69593 increases dopamine uptake, repeated
administration results in a decrease in uptake
with a concomitant decrease in surface DAT
labeling by the tropane radioligand [ 125 I]CIT (2�carbomethoxy-3�-(4-iodophenyl)tropane)
but no
loss in total DAT protein. Importantly, the effect
of repeated cocaine treatment was opposite that of
U69593 (a significant acceleration of dopamine uptake,
indicating an upregulation of surface DATs),
but the effect of both drugs in combination was not
significantly different from control. 111 The mechanism
underlying �-opioid–induced DAT regulation
is unknown, but it is hypothesized that activation
of PI3K by �-receptor agonists plays a role. 16 The
DAT-modulating effects of �-opioids prompt consideration
of their receptors as potential targets for
novel cocaine addiction therapeutics. For example,
�-opioid receptor agonists attenuate the acute reinforcing
effects of cocaine, and repeated �-opioid
agonist treatment may oppose some of the alterations
in dopaminergic transmission observed after
withdrawal from chronic cocaine. 111
Possible implications of DAT regulation to
the putative toxicity of several substituted
amphetamines
Historically, neurotransmitter transporters, such as
the DAT, were regarded as relatively static entities:
“molecular vacuums” expressed at a constant level
at presynaptic terminals. 16 This preconceived notion
has changed dramatically over the last decade,
because many studies have indicated that—akin to
classically defined transmembrane receptors—the
activity and membrane trafficking of transporter
proteins is rapidly and differentially regulated by
interaction with chemically distinct ligands. In fact,
current research suggests that DAT trafficking is
the primary mechanism by which cells modulate
extracellular dopaminergic tone. That is, dynamic
change in the rate of dopamine uptake is achieved
by shifting the distribution of DAT proteins between
the plasmalemmal and endosomal compartments,
rapidly altering the number of DATs at the cell surface.
As discussed earlier, the most robust example
of ligand-mediated transporter redistribution is the
internalization response triggered by amphetaminergic
substrates. Long before it was known that
transporter proteins undergo dynamic trafficking
328 Ann. N.Y. Acad. Sci. 1187 (2010) 316–340 c○ 2010 New York Academy of Sciences.