Superoxide radical-mediated alteration of synaptosome membrane structure and high-affinity gamma-[14C]aminobutyric acid uptake

Chronic and acute regulation of Na+/Cl- -dependent neurotransmitter transporters: drugs, substrates, presynaptic receptors, and signaling systems

Na+/Cl- -dependent neurotransmitter transporters, which constitute a gene superfamily, are crucial for limiting neurotransmitter activity. Thus, it is critical to understand their regulation. This review focuses primarily on the norepinephrine transporter, the dopamine transporter, the serotonin transporter, and the gamma-aminobutyric acid transporter GAT1. Chronic administration of drugs that alter neurotransmitter release or inhibit transporter activity can produce persistent compensatory changes in brain transporter number and activity. However, regulation has not been universally observed. Transient alterations in norepinephrine transporter, dopamine transporter, serotonin transporter, and GAT1 function and/or number occur in response to more acute manipulations, including membrane potential changes, substrate exposure, ethanol exposure, and presynaptic receptor activation/inhibition. In many cases, acute regulation has been shown to result from a rapid redistribution of the transporter between the cell surface and intracellular sites. Second messenger systems involved in this rapid regulation include protein kinases and phosphatases, of which protein kinase C has been the best characterized. These signaling systems share the common characteristic of altering maximal transport velocity and/or cell surface expression, consistent with regulation of transporter trafficking. Although less well characterized, arachidonic acid, reactive oxygen species, and nitric oxide also alter transporter function. In addition to post-translational modifications, cytoskeleton interactions and transporter oligomerization regulate transporter activity and trafficking. Furthermore, promoter regions involved in transporter transcriptional regulation have begun to be identified. Together, these findings suggest that Na+/Cl- -dependent neurotransmitter transporters are regulated both long-term and in a more dynamic manner, thereby providing several distinct mechanisms for altering synaptic neurotransmitter concentrations and neurotransmission.

Differential effects of methamphetamine on Na(+)/Cl(-)-dependent transporters

It has been demonstrated that methamphetamine (METH) administration affects Na(+)/Cl(-)-dependent transporters; for example, METH treatment rapidly and reversibly decreases dopamine (DA) and serotonin (5HT) transporter function in rat striatum in vivo, as assessed in synaptosomes prepared from METH-treated rats. Because acute effects of METH on other transporters within this family have been less studied, the responses of norepinephrine (NE) and gamma-aminobutyric acid (GABA) transporters to METH administration(s) were determined. Both single and multiple METH administrations inhibited hippocampal NE uptake 1 h after METH treatment(s). In contrast, striatal GABA uptake was not affected by either treatment paradigm. The effects observed after both single and multiple METH administrations on NE transporters were attributable to increases in K(m,) with no changes in V(max); effects that were eliminated by repeated washing of the synaptosomes. These 'washout' data suggest that residual METH introduced by the in vivo subcutaneous injection(s) directly reduced NE transporter activity in the in vitro assay and that, unlike DA and 5HT transporters, METH did not indirectly alter NE transporter function. Taken together, these data demonstrate differences in the responses of NE, GABA, DA, and 5HT transporters to METH treatment.

Oxygen radicals differentially affect Na+/Cl(-)-dependent transporters

Incubation with the oxygen radical-generating enzyme, xanthine oxidase, dramatically reduced striatal dopamine transporter activity, but was unexpectedly without effect on rat hippocampal norepinephrine uptake. To determine whether environmental differences between the striatum and hippocampus contributed to this lack of oxidative effect on norepinephrine transporters, synaptosomal gamma-aminobutyric acid (GABA) uptake was assessed in both regions. Xanthine oxidase similarly decreased [3H]GABA uptake in both the striatum and hippocampus, supporting the conclusion that environmental differences did not account for the lack of effect on norepinephrine transport. These data suggest that norepinephrine transporters are less vulnerable than other Na+/Cl(-)-dependent transporters to oxidative inactivation.

Why tryptophan hydroxylase is difficult to purify: a reactive oxygen-derived species-mediated phenomenon that may be implicated in human pathology

1. Attempts and apparently successful procedures to obtain reasonable quantities of electrophoretically homogenous mammalian brain-derived tryptophan hydroxylase, (TPH), have been described, starting in the early 1970s. This work has been carried out with the primary objective to obtain specific antisera to this enzyme to map out serotonergic pathways in the nervous system. 2. By using a multitude of techniques, antisera have indeed been fabricated and employed. However, it is doubtful if pure, native TPH has ever been produced. Indeed, there is strong evidence that more than one isoform of TPH exists in the rat brain. Thus, these antisera are probably directed against TPH-derived polypeptides and not the holoenzyme(s). 3. The difficulty in the purification of TPH lies not only in its subjectivity to proteolysis, but more importantly in its probable capacity to produce superoxide leading to hydrogen perioxide formation. This, in turn, may undergo Fenton chemistry with iron at the active site of the protein to produce hydroxyl radicals that directly attack and destroy the enzyme molecule. Evidence for such a mechanism is presented together with possible protocols that might be used to produce pure stable holo TPH(s). 4. It is hypothesized that similar oxidative events may take place in vivo under certain conditions leading to pathological results. Strategies to block these events are suggested.

Oxygen radicals diminish dopamine transporter function in rat striatum

Incubation of striatal synaptosomes with the oxygen radical generating enzyme, xanthine oxidase, decreased [3H]dopamine uptake: an effect attributable to a decreased Vmax. Concurrent incubation with the superoxide radical scavenger, superoxide dismutase, abolished the xanthine oxidase-induced decrease. These results indicate that, like methamphetamine administration in vivo, reactive oxygen species diminish dopamine transporter function in vitro. The significance of these findings to mechanisms responsible for effects of methamphetamine is discussed.

Effects of iron-induced lipid peroxidation and of acidosis on choline uptake by synaptosomes

The effects of iron-induced lipid peroxidation and of lactic acidosis on [3H]choline uptake were investigated on crude synaptosomes prepared from rat cerebral cortices. Fe(2+)-induced lipid peroxidation as evidenced from the production of thiobarbituric acid reactives substances (TBARS) was correlated with a decrease in high-affinity choline uptake (HACU). Trolox C, a free radical scavenger, prevented both Fe(2+)-induced TBARS production and decrease in HACU. Lactic acidosis (pH 6.0 for 30 or 60 min) increased the TBARS production with concomitant decrease in HACU (-48%, -78%, respectively). The acidosis dependent decrease was not reversible following pH 7.4 readjustment after 60 min acidosis. It was not prevented by trolox C, although trolox C inhibited the acidosis-induced production of TBARS. The results suggest that the contribution of acidosis to peroxidative damages is probably of less importance in comparison to other cytotoxic mechanisms.

Prevention by Ginkgo biloba extract (EGb 761) and trolox C of the decrease in synaptosomal dopamine or serotonin uptake following incubation

Prolonged incubation of synaptosomes in Krebs-Ringer oxygenated medium in the presence of ascorbic acid (10(-4) M) led, after 20 min, to a decrease in [3H]dopamine (DA) (synaptosomes prepared from the striatum) and [3H]serotonin (5HT) (synaptosomes prepared from the cortex) uptake. The decrease was progressive and uptake was virtually abolished after a 60 min incubation period. A concentration-dependent (from 5 x 10(-6) M) role of ascorbic acid in the decrease of [3H]DA or [3H]5HT uptake was demonstrated. This decrease was potentiated by Fe2+ ions and prevented by the ferrous chelating agent desferrioxamine. Thus, the progressive decrease in synaptosomal uptake of either [3H]DA or [3H]5HT could depend on the generation of free radicals by the association of ascorbic acid with Fe2+ ions. The decrease in synaptosomal uptake was prevented, in a concentration-dependent manner, by the Ginkgo biloba extract EGb 761 (4-16 micrograms/mL) and the vitamin E analog trolox C (10(-4) M). The terpenic fraction of EGb 761, Bn 52063 (up to 0.5 microgram/mL), did not prevent the reduction of [3H]amine uptake. In contrast, the flavonoidic fraction, Cp 202, was effective (from 1 microgram/mL) and its efficacy was shared by the flavonoid quercetin (from 0.1 microgram/mL). The prolongation of the ability of synaptosomes to take up [3H]amine elicited by EGb 761, in particular its flavonoidic fraction, as well as by trolox C could be due to their free radical scavenger properties.

Free radicals involvement in neurological porphyrias and lead poisoning

Porphyrias are inherited and acquired diseases of erythroid or hepatic origin, in which there are defects in specific enzymes of the heme biosynthetic pathway. In patients with intermittent acute porphyria and lead poisoning the erythrocytic activities of superoxide dismutase and glutathione peroxidase are reported to be increased. Our studies demonstrated that d-aminolevulinic acid, a heme precursor accumulated in both diseases, undergoes enolization at pH less than 7.0 before it autoxidizes. The autoxidation of d-aminolevulinic acid, in the presence or absence of oxyhemoglobin has been proposed as a source of oxy and carbon-centred radicals in the cells of intermittent acute porphyria and saturnism carriers. Thus, the increased levels of antioxidant enzymes can be viewed as an intracellular response against the deleterious effects of these extremely reactive species.

Prevention of chemically induced synaptosomal changes

The ability of altered environmental conditions to modulate some properties of synaptosomes has been studied. Incubation conditions used included the presence of methyl mercury or an organochlorine insecticide: chlordecone. Other adverse chemical conditions during incubation were the absence of calcium salts from the incubation medium or the addition of agents bringing about enhanced oxidative conditions. Synaptosomal parameters studied were the cytosolic level of free, ionic calcium, [Ca2+]i, the extent of depolarization-induced uptake of radioactive calcium, and the permeability of the limiting membrane. In addition, peroxidative activity was estimated by quantitation of thiobarbituric acid-reactive material. All these facets of synaptosomal function were responsive to the presence of these potentially deleterious changes in the incubation medium. While the response of [Ca2+]i was potentially in either direction, all adverse conditions increased synaptosomal permeability as evaluated by leakage of fura-2 into the extracellular compartment. Pretreatment with ganglioside GM1 in some situations or alpha-tocopherol in others could either wholly or partially prevent the onset of such altered synaptosomal characteristics.

Iron-induced lipid peroxidation and inhibition of dopamine synthesis in striatum synaptosomes

Crude striatum synaptosomes (P2 fraction) from Fischer 344 female rats were incubated in the presence of ADP-chelated Fe3 (0.5-50 microM) and ascorbate (250 microM). Intrasynaptosomal conversion of tyrosine to dopamine (DA) was measured by 14CO2 evolution from L-[1-14C]tyrosine in the absence of added cofactors and DOPA decarboxylase. Malondialdehyde (MDA) was measured as an index of lipid peroxidation. A concentration-dependent inhibition of DA synthesis by ADP-Fe3./ascorbate was found with 50% inhibition occurring at 2.5 microM Fe3 concentration. This was accompanied by marked accumulation of MDA. Ascorbate or ADP alone did not affect DA synthesis and ADP-Fe3 in the absence of exogenous ascorbate was effective only above 25 microM. Exogenously added MDA did not inhibit DA synthesis. Purified synaptosomes were isolated from peroxidized and control P2 actions using sucrose gradients. Membrane microviscosity of the purified synaptosomes was assessed by nitroxyl spin labels of stearic acid using electron paramagnetic resonance techniques. There was a significant increase in membrane microviscosity as a result of ADP-Fe3./ascorbate induced peroxidation. Maleimide nitroxide spin-label binding to protein sulfhydryls was significantly modified by peroxidation of striatum synaptosomes. The weakly immobilized component of the sulfhydryl spin-label (w) was drastically decreased whereas the strongly immobilized component (s) was modified less, thus leading to a marked reduction of w/s ratio. The exposure of striatum synaptosomes to the peroxidizing system resulted in a significant increase in total iron and in a 25% decrease in protein sulfhydryl content. It is concluded that iron-induced damage to the DA synthetic system is mediated by alterations of the structural properties of nerve ending membranes.

High-affinity transport of gamma-aminobutyric acid, glycine, taurine, L-aspartic acid, and L-glutamic acid in synaptosomal (P2) tissue: a kinetic and substrate specificity analysis

In a cortical P2 fraction, [14C]gamma-aminobutyric acid ([14C]GABA), [14C]glycine, [14C]taurine, and [14C]glutamic and [14C]aspartic acids are transported by four separate high-affinity transport systems with L-glutamic acid and L-aspartic acid transported by a common system. GABA transport in cortical synaptosomal tissue occurs by one high-affinity system, with no second, low-affinity, transport system detectable. Only one high-affinity system is observed for the transport of aspartic/glutamic acids; as with GABA transport, no low-affinity transport is detectable. In the uptake of taurine and glycine (cerebral cortex and pons-medulla-spinal cord) both high- and low-affinity transport processes could be detected. The high-affinity GABA and high-affinity taurine transport classes exhibit some overlap, with the GABA transport system being more specific and having a much higher Vmax value. High-affinity GABA transport exhibits no overlap with either the high-affinity glycine or the high-affinity aspartic/glutamic acid transport class, and in fact they demonstrate somewhat negative correlations in inhibition profiles. The inhibition profiles of high-affinity cortical glycine transport and those of high-affinity cortical taurine and aspartic/glutamic acid transport also show no significant positive relationship. The inhibition profiles of high-affinity glycine transport in the cerebral cortex and in the pons-medulla-spinal cord show a significant positive correlation with each other; however, high-affinity glycine uptake in the pons-medulla-spinal cord is more specific than that in the cerebral cortex. The inhibition profile of high-affinity taurine transport exhibits a nonsignificant negative correlation with that of the aspartic/glutamic acid transport class.