Regulation of the dopamine transporter - Addiction Research ...

Schmitt & Reith DAT Regulation
DAT-lacking animals. In particular, cocaine’s interaction
with the serotonin transporter (SERT)
has been hypothesized to contribute to continued
self-administration in DAT-knockout animals. 7,8
Nevertheless, convincing preclinical evidence implicating
specific activity at the DAT in cocaine dependence
comes from the observation that the reinforcing
effect of cocaine is lost in transgenic mice
expressing a triple point–mutated DAT with preserved
(albeit somewhat reduced) substrate translocation
but little appreciable affinity for cocaine. 9
It is conceivable that elevated basal dopaminergic
tone in these mice (a result of their reducedfunction
DAT isoform) causes adaptive changes,
altering the response to cocaine; however, knockdown
mutant mice with a ∼90% reduction in
DAT expression—but with functionally unmodified
DATs and elevated basal dopamine—still exhibit robust,
wild-type–like preference for cocaine, indicating
that interaction with the DAT is necessary for
the reinforcing effects of cocaine in animals that
carry the DAT. 10 Moreover, a recent investigation
by Thomsen et al. showed that a different strain
of DAT –/– (DAT knockout) mice generally fail to
acquire intravenous cocaine self-administration behavior.
11 Importantly, in the minority of mice that
do initially show self-administration, this behavior
is quickly extinguished as the response requirement
(the “work” required to obtain a dose of cocaine) is
increased. In contrast, both wild-type and SERT –/–
mice maintain consistent self-administration, even
when the response requirement for a single dose of
intravenous cocaine is increased 50- to 100-fold. Unlike
cocaine, other dopaminergic compounds (i.e.,
the dopamine D1-like receptor agonist SKF82958)
and food rewards are comparably administered by
DAT –/– and wild-type mice, indicating that elimination
of the DAT does not globally hinder behavioral
reinforcement. These data strongly suggest that cocaine
is not a reliable reinforcer in the absence of the
DAT, despite some intersubject differences in initial
responses among DAT –/– mice. 11
The DAT governs both the duration and magnitude
of dopaminergic neurotransmission by actively
translocating dopamine from the extracellular space
into presynaptic neurons. Extracellular dopamine is
also subject to enzymatic catabolism—thus terminating
its signaling potential—however, studies of
DAT-knockout mice 6,12 have demonstrated that the
DAT is the chief arbiter of dopaminergic signal-
ing flux (for review, see Gainetdinov and Caron 3 ).
Dopaminergic transmission is volume transmission
with an extrasynaptic predominance of dopamine
receptors and transporters; the function of the DAT
is not to remove dopamine from the synapse but
rather to regulate dopamine’s action at extrasynaptic
dopamine receptors (see Rice and Cragg 13 for
review and highly recommended, updated cartoon
of a dopamine synapse). The overall rate of de novo
DAT protein synthesis is a relatively slow process—
the half-life of striatal transporter proteins, for example,
is approximately 2–3 days. 14,15 Hence, it is
not surprising that cells use an assortment of posttranslational
regulatory strategies to rapidly alter
DAT function—without requiring de novo protein
synthesis or transcriptional-level modifications—to
dynamically shift the tone of dopaminergic activity
in response to various endogenous and exogenous
stimuli. There are two conceivable manners
in which a cell could modulate DAT function “on
the fly” (on a time scale of seconds to minutes) in
response to changing environmental conditions—
either via direct modification of single-transporter
parameters, such as intrinsic substrate permeation
efficacy, or by affecting the trafficking of the transporter,
redistributing DATs between the plasma
membrane and intracellular endosomal compartments.
4 Experimental evidence has demonstrated
that both processes may play some role in acute DAT
regulation, but that redistribution of transporters
to and from the cell surface is the predominant
mechanism. 16,17
This regulatory method—rapid insertion or removal
of plasmalemmal transporter proteins—may
seem like a cumbersome way of acutely controlling
neurotransmitter signaling; however, it makes logistical
sense when we consider the kinetics of the
substrate translocation cycle. That is, transporters
must undergo a conformational shift between the
discrete phases of extracellular substrate binding
(outward facing) and cytosolic substrate release (inward
facing), limiting the potential maximum uptake
velocity to around one substrate molecule per
second per transporter. 4 Ion channels, by contrast,
can achieve flux velocities on the order of millions of
ions per second per channel. 18 Because they offer a
far wider dynamic range in “substrate” flux rate, ion
channels are more amenable to regulation strategies
involving posttranslational modification at the individual
protein level than transporters, such as the
Ann. N.Y. Acad. Sci. 1187 (2010) 316–340 c○ 2010 New York Academy of Sciences. 317