Dopamine reuptake transporter

The various stimulants have no obvious chemical relationships and do not share primary neurochemical effects, despite their similar behavioral effects. Cocaines chemical strucmre does not resemble that of caffeine, nicotine, or amphetamine. Cocaine binds to the dopamine reuptake transporter in the central nervous system, effectively inhibiting dopamine reuptake. It has similar effects on the transporters that mediate norepinephrine and serotonin reuptake. As discussed later in this chapter in the section on neurochemical actions mediating stimulant reward, dopamine is very important in the reward system of the brain the increase of dopamine associated with use of cocaine probably accounts for the high dependence potential of the drug. 

Atomoxetine (Straterra , originally tomoxetine or tomoxetin, 3) was first described and synthesized by chemists at Eli Lilly in the late 1970s and was one of the few compounds that was known to display meaningful selectivity for the norepinephrine reuptake transporter (NET) versus the serotonin reuptake transporter (SERT) and the dopamine reuptake transporter (DAT) (Barnett, 1986 Molloy and Schmiegel, 1997). Atomoxetine was one of several structurally related and commercially successful monoamine reuptake inhibitors that were developed by Lilly for the treatment of various psychiatric disorders (Eig. 17.4). Fluoxetine (43) and duloxetine (44) have both gained approval in the United States as Prozac and Cymbalta , respectively, and nisoxetine (45) is widely used as a tool in biology. 

A useful model of the action of these two drugs in the reward centers of the CNS is shown in Figure 32-1. Cocaine reduces reuptake of dopamine into the neuron by inhibiting the dopamine reuptake transporter. Amphetamine causes the intracellular release of dopamine within the terminal and reverses the transporter direction so that dopamine is released into the synapse by reverse transport rather than ordinary exocytosis. In addition, amphetamine inhibits intracellular MAO metabolism of dopamine. Note that both drugs result in an increase in the concentration of dopamine in the synapse. 

A model for the action of cocaine and amphetamine at a dopaminergic synapse in the central nervous system. Cocaine (right side) blocks the dopamine reuptake transporter (DAT). Amphetamine (left side) has several effects. It enters the nerve ending via reverse transport by the DAT and displaces dopamine (DA) from vesicles by altering their pH. It also inhibits dopamine metabolism by MAO in the nerve ending. The increased intraneuronal dopamine causes reversal of the DAT and dopamine floods into the synapse. 

Amphetamine acts on both transmitter transporters - the plasmalemmal one, which is the site of action of cocaine, and the vesicular transporter, which is targeted by reserpine. It is imported into the cell by the plasmalemmal transporter. This will result in inhibited reuptake of the physiological transmitter, not so much apparently by direct competition (as is the case with cocaine) but by subsequent endocytosis of the receptor. This is clearly shown in Figure 10.16. In the experiment shown, the dopamine reuptake transporter was recombinantly expressed in cultured cells and visualized by immimofluorescence . Initially, the fluorescence is confined to the surface of the cells expressing the transporter (Figure 10.16b, left panel). After exposure of the cells to amphetamine, the stain gradually disappears from the surface and is translocated into the cell interior, indicating endocytosis of the transporter". Simultaneously, the... 

Figure 10.16. Endocytosis of dopamine reuptake transporters triggered by amphetamine, a Application of amphetamine reduces the transmembrane current that is a measure of dopamine reuptake. Shown are several independent experiments, b Detection of the transporter by immunofluorescence and confocal microscopy. In the absence of amphetamine, the transporter is located at the cell surfaces. Amphetamine induces translocation of the fluorescence to intracellular compartments.

The affinity data for the SSRIs show that the SSRIs, as a group, are very potent and selective inhibitors for SERT compared with their affinity for NE and dopamine reuptake transporters (Fig. 21.6) and are more potent inhibitors of 5-HT reuptake than are the tertiary amine TCAs, with the exoeption of clomipramine. None of the SSRIs has substantial effeot on the NET or dopamine transporter. Of the SSRIs, sertraline exhibits the most potent inhibition of dopamine reuptake transporter, although it is still 100 times less potent in terms of inhibiting the dopamine versus the SERT. Therefore, the plasma oonoentration of sertraline would have to be increased by as much as 100 times to inhibit the dopamine reuptake transporter. When drugs are this selective for the reuptake transporters, differences in potency become olinioally irrelevant, because the plasma concentration oan be dose-adjusted to achieve inhibition of the desired transporter without affecting the other transporters. Clomipramine displays less affinity for SERT than oitalopram, fluvoxamine, paroxetine, or sertraline does and is more potent than fluoxetine. In terms of the ability to inhibit the NET, the SSRIs are two to three times less potent than the SNRI TCA, desipramine. 

Nomifensine is a substituted phenyipiperidine (an aminophenyitetrahydroisoquinoline) structurally related to sertraline that was marketed as a stimulatory antidepressant in the mid-1970s but later withdrawn because of a high incidence of hemolytic anemia. Nomifensine inhibits the NE and dopamine reuptake transporters. It displays high affinity for NET (human pK, = 7.8), moderate affinity for dopamine transporter (pK, = 6.6), and a low affinity for SERT (5-HT NE ratio, 65). 

Mazindol (Sanorex ) is a phenyl-substituted imidazobenzoisoindole that inhibits both NE and dopamine reuptake transporters. It exhibits high affinity for the NET (rat pKj = 9.3), good affinity for dopamine transporter (rat pKj = 7.8), and a 5-HT NE ratio of 224. Dopamine reuptake inhibitors suppress appetite thus, mazindol is approved to be marketed as an appetite suppressant. 

Sibutramine is available as the hydrochloride salt and is prepared as the racemate. The (R)-enantiomer of the mono- and didemethylated metabolite appear to show an increased affinity towards the dopamine reuptake transporter, which led to the question as to whether they may be useful for the treatment of ADHD. 

The dopamine transporter has been a target for developing pharmacotherapies for a number of CNS disorders including ADHD, stimulant abuse, depression and Parkinson s disease. Several excellent reviews in this area have been recently published [28-30]. The dopamine reuptake inhibitor methylphenidate has been successfully used for decades in the management of ADHD in children and adolescents. It remains a first-line treatment along with amphetamine for this disorder [31,32]. 

Slow-onset, long duration dopamine reuptake inhibitors with reduced potential for substance abuse have been suggested as therapies for psychostimulant addiction [33-35]. A series of slow-onset, long duration N-alkyl analogues of methylphenidate were recently reported to have enhanced selectivity for the dopamine transporter [34]. A representative compound is 13, an RR/SS diastereomer (DAT K, = 16nM, SERT K = 5900 nM, NET K-, = 840 nM). In a locomotor activity assay in mice, 13 has a slow onset of activity (20-30 min) with peak activity occurring between 90 and 120 min. In contrast, both methylphenidate and cocaine are active within 10 min and reach peak activity within 30 min. 

After more than a decade of use, bupropion (24) is considered a safe and effective antidepressant, suitable for use as first-line treatment. In addition, it is approved for smoking cessation and seasonal affective disorder. It is also prescribed off-label to treat the sexual dysfunction induced by SSRIs. Bupropion is often referred to as an atypical antidepressant and has much lower affinity for the monoamine transporters compared with other monoamine reuptake inhibitors. The mechanism of action of bupropion is still uncertain but may be related to inhibition of dopamine and norepinephrine reuptake transporters as a result of active metabolites [71,72]. In a recently reported clinical trial, bupropion extended release (XL) had a sexual tolerability profile significantly better than that of escitalopram with similar re-... 

Triple reuptake inhibitors (TRIs), which inhibit reuptake at all three transporters, have attracted considerable interest in recent years [77]. The involvement of dopamine reuptake in the etiology of depression and other CNS disorders has been recognized [29,30]. As a result, TRIs have been proposed to offer a faster onset of action and improved efficacy for depression over currently prescribed single or dual action monoamine reuptake inhibitors. Historically, the mesocorticolimbic dopamine pathway is thought to mediate the anhedonia and lack of motivation observed in depressed patients [78,79]. In addition, methylphenidate, both immediate release and extended release formula, has been found to be effective as an augmenting agent in treatment-resistant depression [4]. Furthermore, clinical studies using the combination of bupropion and an SSRI or SNRI have showed improved efficacy for the treatment of MDD in patients refractory to the treatment with SSRIs, SNRIs, or bupropion alone [5,80,81]. 

MPTP) and sodium benzoate induce a strong downregulation in the expression of tyrosine hydroxylase mRNA and in the tyrosine hydroxylase-positive cells in the ventral diencephalon. These chemicals also decreased the expression of the dopamine transporter (DAT), a membrane transport protein involved in dopamine reuptake, that is, a specific marker of dopaminergic neurons [9]. 

Atomoxetine is a selective inhibitor of norepinephrine presynaptic reuptake transporters that has been shown to increase extracellular norepinephrine and dopamine concentrations in the prefrontal cortex in rats (Bymaster et al. 2002), which may account for its clinical efficacy in the treatment of ADHD symptomatology. However, atomoxetine does not appear to affect dopamine levels in the striatum or nucleus accumbens and consequently is not thought to carry the abuse potential associated with stimulant medications. 

FIGURE 12-3. When (/-amphetamine binds to the presynaptic dopamine transporter on the dopamine neuron, it not only blocks dopamine reuptake but actually causes dopamine release. There may be a preference or selectivity for cortical over striatal dopamine presynaptic terminals by (/-amphetamine, so lower doses may have preferential effects on attention rather than on motor activity. Methylphenidate has a similar action, which is not quite as rapid but longer-lasting in many patients. 

FIGURE 12-4. The enantiomer of -amphetamine is /-amphetamine, which has no preference between the norepinephrine and the dopamine transporters. Thus, it will target both the norepinephrine reuptake site (shown here), as well as the dopamine reuptake site (shown in Fig. 12-2). -Amphetamine is selective for the dopamine transporter. 

Cocaine (Fig. 13—3) has two major properties it is both a local anesthetic and an inhibitor of monoamine transporters, especially dopamine (Fig. 13—4). Cocaine s local anesthetic properties are still used in medicine, especially by ear, nose, and throat specialists (otolaryngologists). Freud himself exploited this property of cocaine to help dull the pain of his tongue cancer. He may have also exploited the second property of the drug, which is to produce euphoria, reduce fatigue, and create a sense of mental acuity due to inhibition of dopamine reuptake at the dopamine transporter. Cocaine also has similar but less important actions at the norepinephrine and the serotonin transporters (Fig. 13—3). Cocaine may do more than merely block the transporter—it may actually release dopamine (or norepinephrine or serotonin) by reversing neurotransmitter out of the presynaptic neuron via the monoamine transporters (Fig. 13—4). 

FIGURE 23.7 Dopamine (DA) is synthesized within neuronal terminals from the precursor tyrosine by the sequential actions of the enzymes tyrosine hydroxylase, producing the intermediary L-dihydroxyphenylalanine (Dopa), and aromatic L-amino acid decarboxylase. In the terminal, dopamine is transported into storage vesicles by a transporter protein (T) associated with the vesicular membrane. Release, triggered by depolarization and entry of Ca2+, allows dopamine to act on postsynaptic dopamine receptors (DAR). Several distinct types of dopamine receptors are present in the brain, and the differential actions of dopamine on postsynaptic targets bearing different types of dopamine receptors have important implications for the function of neural circuits. The actions of dopamine are terminated by the sequential actions of the enzymes catechol-O-methyl-transferase (COMT) and monoamine oxidase (MAO), or by reuptake of dopamine into the terminal. 

Amphetamine s primary effects (increased wakefulness, appetite suppression, and increased locomotor activity) are thought to be mediated by the release of norepinephrine from noradrenergic neurons in the CNS (36). However, research points to the role of plasma transport inhibition of dopamine, norepinephrine, and serotonin as well as inhibition of the vesicular monoamine transporter (138). Wisor et al. (139) summarize evidence that dopamine reuptake inhibition produces a greater alerting effect than norepinephrine transport blockade. 

Again, there are various dmgs with different ranges and specificities. Cocaine has a particularly broad spectrum, affecting the reuptake of norepinephrine, dopamine, and serotonin alike. The effect of cocaine can be quantitatively studied in mice by observing their excitement in response to being placed into a new environment, which is measured simply as the distance travelled within their new home over time. In experiments with transgenic mice, the reuptake transporters for both dopamine and serotonin had to be deleted in order to abolish the increase in excitement induced by cocaine. 

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