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

Regulation of the Dopamine and Vesicular Monoamine Transporters: Pharmacological Targets and Implications for Disease

Dopamine (DA) plays a well recognized role in a variety of physiologic functions such as movement, cognition, mood, and reward. Consequently, many human disorders are due, in part, to dysfunctional dopaminergic systems, including Parkinson's disease, attention deficit hyperactivity disorder, and substance abuse. Drugs that modify the DA system are clinically effective in treating symptoms of these diseases or are involved in their manifestation, implicating DA in their etiology. DA signaling and distribution are primarily modulated by the DA transporter (DAT) and by vesicular monoamine transporter (VMAT)-2, which transport DA into presynaptic terminals and synaptic vesicles, respectively. These transporters are regulated by complex processes such as phosphorylation, protein-protein interactions, and changes in intracellular localization. This review provides an overview of 1) the current understanding of DAT and VMAT2 neurobiology, including discussion of studies ranging from those conducted in vitro to those involving human subjects; 2) the role of these transporters in disease and how these transporters are affected by disease; and 3) and how selected drugs alter the function and expression of these transporters. Understanding the regulatory processes and the pathologic consequences of DAT and VMAT2 dysfunction underlies the evolution of therapeutic development for the treatment of DA-related disorders.

Seizure-induced oxidative stress in rat brain regions: blockade by nNOS inhibition

Free radicals have been implicated in the pathogenesis of various neurological disorders including epilepsy. Experimental seizures are often accompanied by the generation of free radicals that cause lipid peroxidation (LPO), which may subsequently cause neurodegeneration observed in certain types of human epilepsy. We recently reported a trigger role for nitric oxide (NO) derived by activation of neuronal isoform of nitric oxide synthase (nNOS) and that the action of conventional antiepileptic drugs (AEDs) was potentiated by inhibition of nNOS. In the present study, we extend our observations to understand the significance of blockade of the nNOS pathway on seizure-induced oxidative stress. Increased NO and LPO levels was observed at the time that corresponded to the onset of generalized seizures in rat brain regions following administration of GABA(A) receptor antagonist, picrotoxin (PCT). Treatment with the selective nNOS inhibitor, 7-nitroindazole (7-NI), decreased NO and LPO levels. The AEDs, diazepam and phenobarbitone also prevented seizure-induced increase in NO and LPO levels. Seizures resulted in a significant increase in the activity of antioxidant enzymes, superoxide dismutase in the frontal cortex and hippocampus. On the other hand, the activity of glutathione peroxidase was decreased in the hippocampus and midbrain. Whereas treatment with 7-NI could minimize the effects of PCT, the AEDs per se did not have any significant impact on the activity of the antioxidant enzymes, though co-treatment with 7-NI and AEDs could significantly decrease seizure-induced alterations in antioxidant enzyme activities. These observations suggest that the AEDs may not have a significant role in modulating the activities of antioxidant enzymes and that their ability to decrease LPO is realized more likely by their ability to prevent free radical formation. In conclusion, the present study demonstrates that NO contributes to LPO observed following seizures induced by PCT. The study also provides evidence for the ability of the AEDs to inhibit seizure-induced increase in LPO levels, the effect being enhanced by co-treatment with 7-NI suggesting that 7-NI and the AEDs together could prevent the neurotoxic cascade induced by oxidative stress.

Temperature and 3,4-methylenedioxymethamphetamine alter human serotonin transporter-mediated dopamine uptake

Although studies have suggested that dopamine can be transported by serotonin transporters (SERTs), such activity has not been characterized at the cloned SERTs. Dopamine and serotonin uptake by human SERT expressed in HEK-293 cells was compared at 37 and 40 degrees C. Elevated temperature was found to alter serotonin transport, but had no significant effect on dopamine transport. These effects led to a 10-fold increase in the serotonin:dopamine transport ratio reflecting an increased preference of SERTs for dopamine as opposed to serotonin at the higher temperature. The effects of 3,4-methylenedioxymethamphetamine (MDMA) on SERT-mediated dopamine transport were also evaluated by pre-incubating SERT-expressing cells with MDMA. The presence of intracellular MDMA caused a decrease in [3H]dopamine uptake but had no effect on [3H]serotonin transport suggesting that intracellular MDMA may be capable of inhibiting transporter function.

Peroxynitrite inactivates the human dopamine transporter by modification of cysteine 342: potential mechanism of neurotoxicity in dopamine neurons

Peroxynitrite (ONOO(-)) has been implicated as a causative factor in dopamine neuronal damage resulting from exposure to methamphetamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and it may be involved in the etiology of Parkinson's Disease. ONOO(-) causes a concentration-dependent and irreversible reduction in dopamine uptake by EM4 cells stably expressing the human dopamine transporter (hDAT). The effect of ONOO(-) is manifested as a reduction in V(max). Cysteine, dithiothreitol, glutathione, and N-acetyl-cysteine, reagents that interact directly with ONOO(-), prevent this inhibition, whereas a scavenger of hydroxyl radical (dimethylsulfoxide), hydrogen peroxide (catalase), and superoxide (superoxide dismutase) did not. Dopamine in the extracellular medium protects the hDAT from ONOO(-), whereas intracellular dopamine does not. Parachloromercuribenzoic acid and 2-aminoethyl methanethiosulfonate (MTSEA), which share with ONOO(-) the ability to modify cysteine sulfhydryls, also inhibit hDAT function. ONOO(-) treatment lowers cysteine-specific labeling of the hDAT by MTSEA-biotin, suggesting that ONOO(-) reacts with one or more cysteines in hDAT. A mutant of hDAT (X7C) in which all intracellular and extracellular loop cysteines were mutated was resistant to inhibition by ONOO(-). Sensitivity to ONOO(-) was restored in mutants of hDAT in which reduced cysteines were present only in the first (C135) and third (C342) intracellular loops (CD-DAT), or in which C342 alone had been reintroduced into X7C (X7C-M342C). These results indicate that the hDAT is inhibited by ONOO(-) through oxidation of cysteine 342. Our studies also substantiate the possibility that drugs known to decrease DAT function in vivo (e.g., methamphetamine and MPTP) may exert their effects through ONOO(-)-mediated oxidative stress.

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.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

Differential effects of stimulants on monoaminergic transporters: pharmacological consequences and implications for neurotoxicity

Many psychostimulants alter plasmalemmal monoaminergic transporter function. Some, such as cocaine, prevent the reuptake of newly released dopamine, serotonin or norepinephrine into their associated neurons. Others, such as the amphetamines, facilitate release of these transmitters into the extraneuronal space by causing a reversal of function of these carrier proteins. An understanding of how psychostimulants regulate the function of not only plasmalemmal, but also vesicular monoamine transporter function is important to appreciate the pharmacological and sometimes neurotoxic consequences of administering these drugs, as well as the physiological regulation of these carrier proteins. Hence, this review will describe recent ex vivo studies investigating the rapid and differential affects of several stimulants on both plasmalemmal and vesicular monoamine transporter function.

The effects of methamphetamine on serotonin transporter activity: role of dopamine and hyperthermia

Multiple administrations of methamphetamine (METH) rapidly decreased serotonin (5HT) transporter (SERT) function in rat striatum and hippocampus. The purpose of this study was to identify the mechanisms/ factors contributing to this METH-induced decrease in SERT function. Multiple high-dose METH injections rapidly decreased 5HT uptake without altering binding of the 5HT transporter ligand paroxetine. Hyperthermia contributed to this deficit in transporter function in striatum and hippocampus, as prevention of METH-induced hyperthermia attenuated this decrease. A role for dopamine (DA) was suggested by findings that pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine, the D1 antagonist SCH-23390, or the D2 antagonist eticlopride attenuated the METH-induced decrease in striatal, but not hippocampal, SERT activity. These effects were independent of the ability of these DA-antagonizing drugs to prevent METH-induced hyperthermia. These results suggest that DA contributes to the decrease in SERT function caused by multiple METH injections in the striatum, but not hippocampus, and that hyperthermia facilitates these deficits in SERT function in both brain regions. In contrast, the response of SERT to a single administration of METH was DA and hyperthermia independent. These findings suggest that the mechanisms/ factors involved in decreasing SERT activity after a single administration of METH are distinct from that caused by multiple administrations.

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.