Dopamine transporter proteins

Gelernter J, Kranzler HR, Satel SL, et al Genetic association between dopamine transporter protein alleles and cocaine-induced paranoia. Neuropsychopharmacology 11 195-200, 1994... 

Cocaine (8), from Erythroxylum coca Lam., besides causing euphoria by inhibiting the dopamine transport protein (DAT) responsible for its recreational and illegal use, exerts a local anesthetic activity through blocking sodium channels and is still used as a probe for this target. 

Altropane and ioflupane bind L-dopamine transporter protein. They are being developed, respectively, as potential contrast agent for radioimaging and imaging... 

Gelemter, J., H. R. Kranzler, S. L. Satel, and P. A. Rao. 1994. "Genetic Association Between Dopamine Transporter Protein Alleles and Cocaine-Induced Paranoia." Neuropsychopftamtacology 11 195-200. 

Interestingly, the breeding of a strain of knockout mice devoid of the gene for synthesis of the dopamine transporter protein results in hyperactive animals. This is probably because their neurons cannot remove released dopamine from synapses and consequently dopamine remains in the synapse 100 times longer. The animals attempt to compensate for this defect in their brain by downregulating the entire dopamine system, including (1) decreased synthesis of dopamine within the neurons and (2) reducing the number of postsynaptic receptor sites. Eventually, because of the absence of the dopamine transporter protein in these animals, they lose their sensitivity to cocaine. 

Chang MY, Lee SH, Kim JH, Lee KH, Kim YS, et al. 2001. Protein kinase C-mediated functional regulation of dopamine transporter is not achieved by direct phosphorylation of the dopamine transporter protein. J. Neurochem. 77 754-61... 

Figure 12.11. Radioligands for dopamine neurons. The se include [ F] -labeled dopa and nonhydro-lyzable, long-acting phenyltropane analogs of cocaine, which bind selectively to the dopamine transporter proteins and are highly specific markers of dopamine neurpns. These agents are useful for imaging with C F] for PET and for SPECT.

Dopamine is a neurotransmitter of importance in Parkinson s disease, Alzheimer s disease and some types of drug abuse. Because the dopamine transporter proteins are embedded in cell membranes, it is very difficult to obtain their structure as they cannot be easily crystallized. 

Levodopa, a dopamine precursor, is the most effective agent for PD. Patients experience a 40% to 50% improvement in motor function. It is absorbed in the small intestine and peaks in the plasma in 30 to 120 minutes. A stomach with excess acid, food, or anticholinergic medications will delay gastric emptying time and decrease the amount of levodopa absorbed. Antacids decrease stomach acidity and improve levodopa absorption. Levodopa requires active transport by a large, neutral amino acid transporter protein from the small intestine into the plasma and from the plasma across the blood-brain barrier into the brain (Fig. 29-2). Levodopa competes with other amino acids, such as those contained in food, for this transport mechanism. Thus, in advanced disease, adjusting the timing of protein-rich meals in relationship to levodopa doses may be helpful. Levodopa also binds to iron supplements and administration of these should be spaced by at least 2 hours from the levodopa dose.1,8,16,25... 

Many neurotransmitters are inactivated by a combination of enzymic and non-enzymic methods. The monoamines - dopamine, noradrenaline and serotonin (5-HT) - are actively transported back from the synaptic cleft into the cytoplasm of the presynaptic neuron. This process utilises specialised proteins called transporters, or carriers. The monoamine binds to the transporter and is then carried across the plasma membrane it is thus transported back into the cellular cytoplasm. A number of psychotropic drugs selectively or non-selectively inhibit this reuptake process. They compete with the monoamines for the available binding sites on the transporter, so slowing the removal of the neurotransmitter from the synaptic cleft. The overall result is prolonged stimulation of the receptor. The tricyclic antidepressant imipramine inhibits the transport of both noradrenaline and 5-HT. While the selective noradrenaline reuptake inhibitor reboxetine and the selective serotonin reuptake inhibitor fluoxetine block the noradrenaline transporter (NAT) and serotonin transporter (SERT), respectively. Cocaine non-selectively blocks both the NAT and dopamine transporter (DAT) whereas the smoking cessation facilitator and antidepressant bupropion is a more selective DAT inhibitor. 

Cysteine string protein (CSP) Cytochrome b561 Peripheral membrane protein that is paimitoylated on >10 cysteines. May have a role in Ca2+ sensitivity of exocytosis. Electron-transport protein required for intravesicular monooxygenases in subsets of secretory vesicles. Required for dopamine- -hydroxylase and peptide amidase activity. 

C-termini and a large glycosylated extracellular loop between transmembrane domains 3 and 4. The proteins show the most homology in their transmembrane spanning domains, particularly domains 1, 2, and 4-8, which may be involved in moving the transmitter across the membrane. The transporters are substrates for PKC-dependent phosphorylation, which reduces their activity. The dopamine transporter is phosphorylated on the extreme end of the N-terminal tail, and consensus phosphorylation sites for various other kinases are present in the intracellular loops and domains [20-22] (Fig. 12-4). The dopamine and norepinephrine transporters form functional homo-oligomers, although it is not known if this is required for transport activity, and the transporters also interact with many other membrane proteins that may control their cell-surface expression or other properties. 

Niznik HB, Tyndale RF, Sallee FR, Gonzalez FJ, Hardwick JP, et al. 1990. The dopamine transporter and cytochrome P450IID1 (debrisoquine 4-hydroxylase) in brain resolution and identification of two distinct [3H] GBR-12935 binding proteins. Arch Biochem Biophys 276 424-432. 

Fig. 9.8 Examples of rule-of-three compliant molecules that have biological activity better than 10 nM. Under each molecule, the following information is included molecule name, MW, ClogP, the biological activity type, value and target. Target names are as follows D3 and D4 - dopaminergic receptor types 2 and 3 AChE and BChE - acetyl- and butyryl-choline esterases PRa and PRb - progesterone receptor types A and B H] and H3, histamine receptor types 1 and 3 5-HT2a, 5-HT2b, 5-HT2c, 5-HT3, 5-HT4 - serotonin receptor subtypes 2A, 2B, 2C, and types 3 and 4 DAT, NET, 5-HTT - dopamine, norepinephrine and serotonin transporter proteins /X], /x.2, S, ki, ks - opioid receptor types mu-1, mu-2, delta, kappa-1 and kappa-3 5a-Rl and 5o -R2 - 5-alpha-reductase isozymes 1 and 2 Flt-1-fms-like tyrosine kinase receptor.

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]. 

NET, SLC6A2, the norepinephrine transporter, is a member of the SLC family, as are similar transporters responsible for the reuptake of dopamine (DAT, SLC6A3) and 5-HT (serotonin, SERT, SLC6A4) into the neurons that release these transmitters. These transport proteins are found in peripheral tissues and in the CNS wherever neurons utilizing these transmitters are located. 

HTxR, serotonin receptor CB1R, cannabinoid-1 DAT, dopamine transporter GABA, y-aminobutyric acid Kir3 channels, G protein-coupled inwardly rectifying potassium channels LSD, lysergic acid diethylamide i -OR, H-opioid receptor nAChR, nicotinic acetylcholine receptor NET, norepinephrine transporter NMDAR, N -methyl-D-aspartate receptor SERT, serotonin transporter VMAT, vesicular monoamine transporter indicates data not available. 

Li R, Peng N, Li XP, Le WD. 2006. (—)-Epigallocatechin gallate regulates dopamine transporter internalization via protein kinase C-dependent pathway. Brain Res 1097 85-89. 

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. 

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