Dopamine transporter mediated release of dopamine: role of chloride

Target-directed evolution of novel modulators of the dopamine transporter in Lobelia cardinalis hairy root cultures

The dopamine transporter (DAT) is targeted in substance use disorders (SUDs), and "non-classical"" DAT inhibitors with low abuse potential are therapeutic candidates. Lobinaline, from Lobelia cardinalis, is an atypical DAT inhibitor lead. Chemical synthesis of lobinaline is challenging; thus, "target-directed evolution" was used for lead optimization. A target protein is expressed in plant cells, and a mutant cell population is selected under conditions where target protein functional inhibition confers a survival advantage. Surviving mutants are "mined" for the targeted activity. Applied to a mutant L. cardinalis cell population expressing the human DAT, we identified 20 mutants overproducing DAT inhibitors. Microanalysis prioritized novel lobinaline derivatives, and we first investigated the more water-soluble lobinaline N-oxide. It inhibited rat synaptosomal [3H]DA uptake with an IC₅₀ similar to lobinaline. Against repeated DA microinjections into the rat striatum, lobinaline produced transient DA clearance reductions. In contrast, lobinaline N-oxide prolongingly increased DA peak amplitudes, particularly in the ventral striatum. Lobinaline N-oxide also produced complex changes in post-peak DA clearance inconsistent with simple DAT inhibition. This unusual DAT interaction may prove therapeutically useful for treating SUDs. This study demonstrates the value of target-directed evolution of plant cells for optimizing lead compounds difficult to synthesize chemically.

Chloride requirement for monoamine transporters

This review focuses on the Cl(-) requirement for dopamine, serotonin, and norepinephrine (DA, 5-HT, and NE) transport and induced current via the transporters for these transmitters, DAT, SERT, and NET. Indirect evidence exists for the passage of Cl(-) ions through monoamine transporters; however, direct evidence is sparse. An unanswered question is why in some preparations, notably native neurons, it appears that Cl(-) ions carry the current through DAT, whereas in heterologous expression systems Na(+) ions carry the current often referred to as the uncoupled current. It is suggested that different functional states in monoamine transporters represent conformational states that carry dominantly Cl(-) or Na(+). Structures of monoamine transporters contribute enormously to structure-function relationships; however, thus far no structural features support the functionally relevant ionic currents that are known to exist in monoamine transporters.

Sertraline inhibits pre-synaptic Na⁺ channel-mediated responses in hippocampus-isolated nerve endings

In the present study, a possible sertraline action on cerebral pre-synaptic Na(+) channels was investigated. For this purpose, the effect of sertraline on responses induced by the Na(+) channel opener, veratridine, namely the increase in Na(+) and in neurotransmitter release in hippocampus-isolated nerve endings was investigated. Results show that sertraline in the low μM range (1.5-25 μM) progressively inhibits the rise in Na(+) and the release of pre-loaded [(3) H]Glu as well as the release of endogenous 5-HT, Glu and GABA (detected by HPLC) induced by veratridine depolarization either under external Ca(2+) -free conditions or in the presence of external Ca(2+) . In addition, under non-depolarized conditions, sertraline (25 μM) increased the external concentration of 5-HT at expense of its internal concentration, and unchanged the external and internal concentrations of the amino acid neurotransmitters and of the 5-HT main metabolite, 5-HIAA. This result is consistent with the sertraline inhibitory action of the serotonin transporter. However, sertraline is unlikely to inhibit pre-synaptic Na(+) channels permeability by increasing external 5-HT. Because 5-HT in a wide concentration range (1-1000 μM) did not change the veratridine-induced increase in Na(+) . In summary, present findings demonstrate that besides the inhibition of 5-HT reuptake, sertraline is an effective inhibitor of pre-synaptic Na(+) channels controlling neurotransmitter release.

Mechanisms underlying striatal vulnerability to 3-nitropropionic acid

The striatum is a cerebral structure particularly susceptible to the metabolic challenge exerted by 3-nitropropionic acid (3-NPA), a toxin that inhibits the respiratory chain at complex II. The striatum, which receives the nerve endings of the nigro-striatal pathway, concentrates the largest amount of 3,4-dihydroxyphenylethylamine or dopamine (DA) in the brain. DA is metabolized to 3,4-dihydroxyphenylacetic acid (DOPAC) by monoamine oxidase (MAO), an enzyme that contains a redox-active disulfide in the active site. In striatum isolated nerve endings exposed to 3-NPA in vitro, DA increased and DOPAC decreased already after 10 min, and after 2 h also an increase in reactive oxygen species and DA-quinone products formation was detected. These 3-NPA-induced effects resulted in an increase in DA release after 2 h. In striatum homogenates from animals presenting motor disturbances in response to 3-NPA in vivo, the DA metabolites homovanillic acid and DOPAC were increased. It is concluded that in the striatum nerve endings where DA is particularly concentrated, the increase in reactive oxygen species induced by 3-NPA, oxidizes DA generating DA-quinones. These DA-quinones may form adducts with the active site of MAO type A reducing its activity. The DA not metabolized to DOPAC is both, used to unchain generation of more of the harmful DA-oxidation products and released to the external medium, where is metabolized by the non-neuronal enzymes MAO type B and catechol-O-methyltransferase.

Characterization of the participation of sodium channels on the rise in Na+ induced by 4-aminopyridine (4-AP) in synaptosomes

The participation of voltage-sensitive Na+ channels (VSSC) on the changes on internal (i) Na+, K+, Ca2+, and on DA, Glu, and GABA release caused by different concentrations of 4-AP was investigated in striatum synaptosomes. TTX, which abolished the increase in Na(i) (as determined with SBFI), induced by 0.1 mM 4-AP only inhibited by 30% the rise in Na(i) induced by 1 mM 4-AP. One millimolar 4-AP markedly decreased the fluorescence of the K+ indicator dye PBFI but 0.1 mM 4-AP did not. Like 1 mM 4-AP, ouabain decreased PBFI fluorescence and increased a considerable fraction of Na(i) in a TTX-insensitive manner. In contrast with the different TTX sensitivity of the rise in Na(i) induced by 0.1 and 1 mM 4-AP, the rise in Ca(i) (as determined with fura-2) induced by the two concentrations of 4-AP was markedly inhibited by TTX, as well as by omega-agatoxin in combination with omega-conotoxin GVIA, indicating that only the TTX-sensitive fraction of the rise in Na(i) induced by 4-AP is linked with the activation of presynaptic Ca2+ channels. It is concluded that the TTX-sensitive fraction of neurotransmitter release evoked by 4-AP is released by exocytosis, and the TTX insensitive fraction involves reversal of the neurotransmitters transporters. This contrasts with the exocytosis evoked by high K+ that is unchanged by TTX and with the neurotransmitter-transporter-mediated release evoked by veratridine, which is highly TTX sensitive and does not require activation of Ca2+ channels.

Characterization of vinpocetine effects on DA and DOPAC release in striatal isolated nerve endings

The effect of vinpocetine, a nootropic drug with anti-ischemic potential, on the release of DA and its main metabolite, DOPAC, was investigated in striatum isolated nerve endings under resting and depolarized conditions. Vinpocetine does not modify the baseline release of DA or the exocytotic release of DA evoked by high K(+), but inhibits the release of DA evoked by veratridine reversal of the DA transporter. In addition to these results, which confirm the vinpocetine selective blockade of voltage-sensitive presynaptic Na(+) channels (VSSC) previously reported [Neurochem. Res. 24 (1999) 1585], vinpocetine increases DOPAC release either under resting, veratridine or high K(+) depolarized conditions. This latter effect, which does not involve VSSC, was characterized. The parallel determination of the released and retained catecholamine concentrations revealed that vinpocetine increases DOPAC release at the expense of internal DA in a dose-dependent manner (low microM range). In contrast to vinpocetine, the selective MAO-A inhibitor, clorgyline, increases DA and decreases DOPAC formation. The combined action of vinpocetine and clorgyline does not indicate, however, that the activation of MAO is the mechanism responsible for the increase in DOPAC caused by vinpocetine. Reserpine, although more potent and efficient than vinpocetine, qualitatively exerts the same pattern of changes on DA and DOPAC concentrations. It is concluded that, in addition to the inhibition of presynaptic VSSC permeability, which selectively inhibits the transporter-mediated release of all neurotransmitters, vinpocetine increases DOPAC by impairing the vesicular storage of DA. Our results indicate that the cytoplasm extravesicular DA is metabolized by MAO to DOPAC. Most of the DOPAC formed is exported to the extracellular medium.

Simultaneous action of MK-801 (dizclopine) on dopamine, glutamate, aspartate and GABA release from striatum isolated nerve endings

The simultaneous effect of MK-801 on the baseline- and depolarization (20 microM veratridine or 30 mM high K+)-evoked release of endogenous dopamine, glutamate (Glu), aspartate (Asp), and GABA is investigated in the same preparation of rat striatum isolated nerve endings. MK-801, in the microM range, selectively increases the baseline and high K+ depolarization-evoked release of dopamine, without causing any effect on the baseline or on the high K+-evoked release of Glu, Asp and GABA. In addition to this selective action on dopamine release, MK-801 inhibits the veratridine depolarization-evoked release of all the neurotransmitters tested, including dopamine. In SBFI and fura-2 preloaded striatal synaptosomes, MK-801 inhibits the elevation of internal Na+ (Na(i)) and the elevation of internal Ca2+ (Ca(i)) induced by veratridine depolarization. The elevation of Ca(i) induced by high K+ depolarization is unchanged by MK-801. This study reveals two separate MK-801 actions. (1) The voltage-independent action, which increases dopamine release selectively, and might contribute to the effects of MK-801 on motor coordination. (2) The voltage-dependent action, which inhibits all the veratridine-evoked responses including the evoked release of the excitatory amino acids (which are particularly concentrated in striatum nerve endings), and might contribute to the anticonvulsant and neuroprotective effects of MK-801.

Chronic verapamil modifies striatal and frontal cortex dopamine levels

In an attempt to elucidate if a change in dopamine (DA) levels was involved in the antimanic action of verapamil reported in various clinical studies, monoamine concentrations in three brain regions (striatum, frontal cortex and hippocampus) obtained from verapamil-treated rats (10 mg/kg i.p. per day for 21 days) were quantified by HPLC coupled to electrochemical detection, and compared with monoamine concentrations in haloperidol-treated animals (5 mg/kg i.p. per day for 21 days). We have found that verapamil and haloperidol, when injected for 3 weeks to rats sacrificed 2 h after the last injection, decreased the striatal DA concentration to a similar extent. This decrease was not observed in short-term (one injection 2 h before sacrifice) verapamil- or haloperidol-treated rats. Moreover, after such a single injection of verapamil the striatal DA concentration was even increased. The striatal concentration of 3,4-dihydroxyphenylacetic acid (DOPAC) was increased about two-fold by haloperidol, but not by verapamil. This haloperidol-induced increase in striatal DOPAC was similar after one injection and after 21 days of haloperidol administration. Neither verapamil nor haloperidol modified the concentrations of homovanillic acid (HVA) or 3-methoxytyramine (3-MT) in the striatum. In the frontal cortex, chronic verapamil increased the concentrations of DA two-fold, and chronic haloperidol increased the concentration of DOPAC two-fold. The other DA metabolites, namely HVA and 3-MT were not significantly changed. The concentration of serotonin (5-HT) and its main metabolite, 5-hydroxyindoleacetic acid (5-HIAA), in control, verapamil- and haloperidol-treated rats were similar in the three brain regions studied. We conclude that DA autoreceptors are implicated in verapamil's effects on frontal cortex and striatum DA levels; and that the presumed antimanic action exerted by verapamil is due to its long-term effect on these receptors.

Characterization of glycine release mediated by glycine transporter 1 stably expressed in HEK-293 cells

We constructed a cell line which stably expresses glycine transporter 1 (GlyT1) proteins. The cell line showed significant [14C]glycine uptake and could keep steep glycine concentration gradient between intracellular and extracellular space (in > out). Using this cell line, we investigated glycine release mediated by this transporter. The [14C]glycine release was enhanced by extracellular glycine and sarcosine, a selective inhibitor of the transporter, in a dose-dependent manner. In addition, the replacement of extracellular Na+ with Li+ or extracellular Cl- with acetate- markedly increased the release. Furthermore, we investigated the effects of extracellular Ca2+ and K+. The removal of these ions also showed enhancement of the release. These results suggest that glycine transporter 1 protein, which might be involved in the NMDA receptor neurotransmission, can release glycine into the extracellular space in the vicinity of synapses, and that the release might be influenced by the extracellular substrate concentration and ion composition in the synaptic cleft.

omega-Conotoxin GVIA and desipramine insensitive norepinephrine efflux from cardiac sympathetic nerve terminal

Using dialysis technique, prominent accumulation of norepinephrine (NE) in the myocardial interstitial space was observed under local administration of a monoamine oxidase inhibitor (pargyline, 10 mM), and a vesicle uptake inhibitor (reserpine, 10 microM). Accumulation of NE in the myocardial interstitial space was associated with a reduction of dihydroxyphenylglycol levels. Pretreatment with omega-conotoxin GVIA (10 microM) or desipramine (10 microM) did not suppress this response. A brisk increase in dialysate NE levels might occur as a consequence of exocytotic or neurotransporter insensitive efflux of NE.

Heterogeneity of electrically evoked dopamine release and reuptake in substantia nigra, ventral tegmental area, and striatum

Somatodendritic dopamine (DA) released in substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) may mediate extrasynaptic neuronal signaling. The concentration of extracellular DA ([DA]o) attained during somatodendritic activation will be governed by the density of release sites and properties of DA uptake. We evaluated these factors in SNc, VTA, and dorsal striatum with carbon-fiber microelectrodes and fast-scan cyclic voltammetry to monitor [DA]o during local electrical stimulation (10 Hz, 5 s) in guinea pig brain slices. Stimulated DA efflux was site specific, with significantly higher [DA]o in caudal (0.48 +/- 0.03 microM, mean +/- SE) than rostral SNc (0.16 +/- 0.01 microM), averaged over their mediolateral extents, and higher [DA]o in VTA (0.74 +/- 0.07 microM) than in medial (0.43 +/- 0.04 microM) or lateral SNc (0.29 +/- 0.05 microM), averaged rostrocaudally. Throughout SNc, evoked [DA]o correlated positively (r = 0.91) with the density of tyrosine-hydroxylase-immunoreactive cells. Modulation of evoked [DA]o by uptake was also site specific. The selective DA uptake inhibitor GBR 12909 significantly increased evoked [DA]o in caudal SNc (to 185 +/- 27%) and striatum (408 +/- 24%), but had no effect in rostral SNc or VTA. Conversely, the norepinephrine (NE) uptake inhibitor desipramine did not alter stimulated [DA]o in caudal SNc or striatum, but caused significant enhancement in rostral SNc (196 +/- 17%) and VTA (126 +/- 12%). Paroxetine, a selective 5-hydroxytryptamine uptake inhibitor had little effect in any region tested. Site-specific sensitivity to desipramine mandated evaluation of dopamine-beta-hydroxylase immunoreactivity (D beta H-ir) in midbrain. The density of filaments positive for D beta H-ir was greater in rostral SNc and VTA than in caudal SNc, suggesting DA clearance via the NE transporter in these regions. Importantly, D beta H-ir was most dense in sections rostral to SNc where no catecholamine signal was detected and no enhancement was observed with desipramine, indicating a lack of NE contribution to evoked release in any region examined. Taken together, these data confirmed that evoked somatodendritic [DA]o depends on DA cell density and on local uptake properties. Uptake was less efficient in SNc and VTA than in striatum. Moreover, enhancement of stimulated [DA]o by GBR 12909 demonstrated that evoked release from dendrites is not via reversal of the DA transporter. Lastly, the heterogeneous patterns of DA uptake within SNc and VTA were consistent with the pattern of degeneration in Parkinson's disease; less vulnerable DA cells, e.g., those in VTA, have less DA uptake than the more vulnerable cells of caudal SNc.

The effects of phosphorylation on the functional regulation of an expressed recombinant human dopamine transporter

Metabolic labeling experiments were performed using eukaryotic cells transfected with the human dopamine (DA) transporter cDNA. Autophosphorylation in the presence and absence of the transporter substrate DA, was analyzed. Dopamine transporter (DAT) was phosphorylated in the absence of DA and dephosphorylated in the presence of the substrate. The functional significance of this phenomenon was checked by incubating cells with phosphorylation promoting agents, all of which reduced substrate uptake and ligand binding significantly. It is shown that at least one site of phosphorylation on DAT is a serine residue. These experiments suggest that the state of phosphorylation of the DAT may play an important role in its biological function.

Oxidative stress and antioxidant therapy in Parkinson's disease

Parkinson's disease, known also as striatal dopamine deficiency syndrome, is a degenerative disorder of the central nervous system characterized by akinesia, muscular rigidity, tremor at rest, and postural abnormalities. In early stages of parkinsonism, there appears to be a compensatory increase in the number of dopamine receptors to accommodate the initial loss of dopamine neurons. As the disease progresses, the number of dopamine receptors decreases, apparently due to the concomitant degeneration of dopamine target sites on striatal neurons. The loss of dopaminergic neurons in Parkinson's disease results in enhanced metabolism of dopamine, augmenting the formation of H2O2, thus leading to generation of highly neurotoxic hydroxyl radicals (OH.). The generation of free radicals can also be produced by 6-hydroxydopamine or MPTP which destroys striatal dopaminergic neurons causing parkinsonism in experimental animals as well as human beings. Studies of the substantia nigra after death in Parkinson's disease have suggested the presence of oxidative stress and depletion of reduced glutathione; a high level of total iron with reduced level of ferritin; and deficiency of mitochondrial complex I. New approaches designed to attenuate the effects of oxidative stress and to provide neuroprotection of striatal dopaminergic neurons in Parkinson's disease include blocking dopamine transporter by mazindol, blocking NMDA receptors by dizocilpine maleate, enhancing the survival of neurons by giving brain-derived neurotrophic factors, providing antioxidants such as vitamin E, or inhibiting monoamine oxidase B (MAO-B) by selegiline. Among all of these experimental therapeutic refinements, the use of selegiline has been most successful in that it has been shown that selegiline may have a neurotrophic factor-like action rescuing striatal neurons and prolonging the survival of patients with Parkinson's disease.

Phylogenetic conservation of 4-aminobutyric acid (GABA) transporter isoforms. Cloning and pharmacological characterization of a GABA/beta-alanine transporter from Torpedo

A family of structurally related, Na+/Cl(-)-dependent plasma-membrane transporters catalyze the uptake of several neurotransmitters, osmolytes and other metabolites into cells. Four different members of this transporter family have been cloned from mammalian sources which all transport 4-aminobutyric acid (GABA) but differ in their pharmacological profiles and in their tissue distribution. We report on the cloning, sequencing and functional expression of a transporter for GABA and beta-alanine from the electric lobe of Torpedo. According to similarity of amino acid sequence (77% identity), pharmacological properties, and tissue distribution (nervous-system-specific), it appears to be the counterpart of the beta-alanine-sensitive GABA transporter, GAT-B/GAT-3/GAT4, previously cloned from rat and mouse. The identification of another GABA transporter isoform from Torpedo (after the recent characterization of a Torpedo GAT-1 transporter) indicates that GABA-transporter isoforms are phylogenetically ancient and arose before the divergence of vertebrates. Sequence comparison between isofunctional transporters from evolutionarily distant species aids in the identification of amino acid residues that are critical for functional specificity. The expression of transporters for GABA and beta-alanine raises questions regarding the unidentified physiological role of these amino acids in Torpedo electric lobe.