Biomedical Engineering Reference
In-Depth Information
other results suggesting that cocaine locks the DAT in an outward facing conformation. The bind-
ing models have been substantiated by extensive mutagenesis of the proposed interacting residues.
Specii cally, the buried nature of the cocaine/cocaine analogue-binding site has been validated by
trapping the radiolabeled cocaine analogue [
3
H]CFT in the transporter either through cross-linking
of engineered cysteines or with an engineered Zn
2+
-
binding site situated extracellular to the predicted
common binding pocket.
In support of the proposed binding site for cocaine and benztropines, photoafi nity labeling studies in
DAT with, e.g., the cocaine analogue [
125
I]RTI-82 and the benztropine analogue [
125
I]MFZ 2-24 have
identii ed the possible major binding domains to be located in TM1 and 6, respectively. Although
this technique does not permit detailed insight into the precise nature of the binding site, the results
suggest that the binding sites are positioned in the same location as predicted by the binding models
described earlier. In the SERT the most convincing data regarding the binding site for cocaine and
cocaine analogues show that a combined mutation of Tyr95 in TM1 and Ile172 in TM3 markedly
decrease the afi nity for cocaine and RTI-55, suggesting that these residues possess a direct interaction
very similar in the two transporters.
14.3.2 A
MPHETAMINE
AND
O
THER
N
ONENDOGENOUS
S
UBSTRATES
Several nonendogenous compounds are substrates of the biogenic amine transporters and are used
either as medication, drugs of abuse, or biochemical tools. Amphetamine and derivatives thereof,
e.g., metamphetamine,
p
-chloroamphetamine, and 3,4-methylenedioxymetamphetamine (MDMA or
ecstacy) are a class of psychostimulants that are transported by DAT, NET, and SERT (Figure 14.4).
Methamphetamine preferentially acts on the DAT and NET while
p
-chloroamphetamine and MDMA
have higher specii city for the SERT. This is supported by analyses of mice dei cient in either DAT
or SERT, i.e., DAT knockout mice are hyperactive and do not respond to amphetamine, while SERT
dei cient mice display locomotor insensitivity to MDMA. Interestingly, amphetamines do not only
increase the synaptic concentration of dopamine DAT by competing with dopamine for uptake via
DAT but also by promoting reversal of transport resulting in efl ux of dopamine via the transporter.
This efl ux dramatically increases the levels of extracellular dopamine and is believed to be of major
importance for the psychostimulatory properties of amphetamines. Increasing evidence supports
that this efl ux is not just the result of “facilitated exchange,” but also might involve a channel mode
of the transporter. Furthermore, recent studies suggest that the efl ux is dependent on binding to the
DAT C-term inus of Ca
2+
/calmodulin-dependent protein kinase
) that in turn facilitates
phosphorylation of one or more serines situated in the distal N-terminus of the transporter.
As for the inhibitors, the binding sites for amphetamine and MDMA have also been investigated
by molecular docking models suggesting an overlap of binding sites with dopamine.
α
(CaMKII
α
14.3.3 A
NTIDEPRESSANTS
The biogenic amine transporters are also targets for medicines used against depression and anxiety.
The selective serotonin-reuptake inhibitors (SSRIs), such as citalopram, l uoxetin, paroxetin, and
sertraline, are, as implicated by their name, potent and selective inhibitors of the SERT (Figure 14.4).
Another class of antidepressants includes the so-called serotonin-norepinephrine reuptake inhibi-
tors (SNRIs) or “dual action” antidepressants such as venlafaxine and duloxetine that are active at
both SERT and NET (Figure 14.4). Finally, the classical though still often used tricyclic antidepres-
sants (TCAs) are potent inhibitors of NET and/or SERT with imipramine and amitriptyline being
approximately 10 times more potent on SERT, while desipramine is relatively a selective inhibitor
of the NET (see also Chapter 18). Interestingly, the antiobesity drug sibutramine exerts its action via
combined inhibition also of NET and SERT. Conceivably, this effect is achieved through a combina-
tion of an anorectic effect due increased extracellular serotonin levels and increased thermogenesis
due to increased norepinephrine levels.
