Intra-striatal infusion of D-amphetamine induces hydroxyl radical formation: inhibition by MK-801 pretreatment

The neurotoxic mechanisms of amphetamine: Step by step for striatal dopamine depletion

Amphetamine (AMPH) is a commonly abused psychostimulant that induces neuronal cell death/degeneration in humans and experimental animals. Although multiple neurotoxic mechanisms of AMPH have been intensively investigated, the interplay between these mechanisms has remained elusive. In this study, we used a rat model of AMPH-induced long-lasting striatal dopamine (DA) depletion and identified mechanisms of neurotoxicity, energy failure, excitotoxicity, and oxidative stress. Pretreatment with nicotinamide (NAM, a co-factor for the electron transport chain) blocked AMPH-induced free radical formation, energy failure, and striatal DA decrease. Also, MK-801 (a NMDA receptor antagonist) blocked AMPH-induced free radical formation and striatal DA but not energy failure decrease, indicating excitotoxicity may occur before free radical formation and after energy failure. Thus, these results show that during AMPH intoxication, energy failure, excitotoxicity, and free radical formation are orchestrated consecutively to mediate the depletion of striatal DA.

Protein phosphatases but not reactive oxygen species play functional role in acute amphetamine-mediated dopamine release

Drug abuse-induced neurodegeneration can be triggered by elevated production of reactive oxygen species (ROS). Involvement of oxidative stress in acute amphetamine (AMPH)-mediated dopamine (DA) release, however, has not been completely understood yet. In order to elucidate the dopaminergic response of PC12 cells to a single dose of 10 μM AMPH, ROS production was measured as related to the extracellular DA level. Due to the spontaneous oxidation of peroxide-sensitive fluorophore 2',7'-dichlorofluorescin diacetate (DCFH-DA) to 2',7'-dichlorofluorescein (DCF), the increase in fluorescence could not be unambiguously attributed to AMPH-triggered ROS production. Based on Amplex Red fluorescence, no ROS production was detected after acute AMPH application. Our data strongly suggest that ROS development was not the main triggering factor for immediate DA release after acute AMPH treatment. On the other hand, AMPH-induced elevation of DA levels in rat brain striatal slices was quenched by the water soluble antioxidant, N-acetylcysteine (NAC) at 10 mM. In this study, we also investigated the contribution of protein phosphatases to the AMPH-induced rat brain striatal dopaminergic response. The experimental protocol, double AMPH challenge was applied for screening the effect of NAC and cantharidin on AMPH-mediated DA release. Here we show that AMPH-mediated DA release increased nearly twofold in striatal rat brain slices pretreated for 30 min with 1000 μM cantharidin, a selective PP1 and PP2A inhibitor. These findings prove the lack of ROS inhibitory action on protein phosphatase activity in acute AMPH-mediated DA efflux.

Neurotoxicity of substituted amphetamines: Molecular and cellular mechanisms

The amphetamines, including amphetamine (AMPH), methamphetamine (METH) and 3,4-methylenedioxymethamphetamine (MDMA), are among abused drugs in the US and throughout the world. Their abuse is associated with severe neurologic and psychiatric adverse events including the development of psychotic states. These neuropsychiatric complications might, in part, be related to drug-induced neurotoxic effects, which include damage to dopaminergic and serotonergic terminals, neuronal apoptosis, as well as activated astroglial and microglial cells in the brain. The purpose of the present review is to summarize the toxic effects of AMPH, METH and MDMA. The paper also presents some of the factors that are thought to underlie this toxicity. These include oxidative stress, hyperthermia, excitotoxicity and various apoptotic pathways. Better understanding of the cellular and molecular mechanisms involved in their toxicity should help to generate modern therapeutic approaches to prevent or attenuate the long-term consequences of amphetamine use disorders in humans.

Effects of alpha-phenyl-N-tert-butyl nitrone and N-acetylcysteine on hydroxyl radical formation and dopamine depletion in the rat striatum produced by d-amphetamine

Previous studies have shown that treatment with free radical scavengers attenuated the D-amphetamine (AMPH) neurotoxicity. But several of these agents also prevent AMPH-induced elevation of body temperature in the rat. Thus, further studies are needed to determine if blockade of the production of free radical or hypothermia are related to the neuroprotective mechanism of the free radical scavengers for AMPH neurotoxicity. In the present study, we examined the effects of the free radical scavengers alpha-phenyl-N-tert-butyl nitrone (PBN) and N-acetylcysteine (NAC) on long-term depletion of striatal dopamine (DA) and lipid peroxidation formation and on hyperthermia induced by AMPH. We also determined their effects on acute hydroxyl radical formation after direct intrastriatal infusion of AMPH. The results showed that both significantly attenuated long-term DA depletion and lipid peroxidation formation in the rat striatum at the dose range that did not block hyperthermia induced by AMPH. These agents also completely inhibited the production of hydroxyl radical after AMPH infusion into the striatum. Our results suggest that free radical scavengers such as PBN and NAC could protect against AMPH-induced oxidative stress and DAergic terminal toxicity via their free radical removing property independent of lowering the core body temperature of rats, and imply that supplement with antioxidants is a potential strategy in the treatment of AMPH neurotoxicity.

Acute and chronic continuous methamphetamine have different long-term behavioral and neurochemical consequences

We compared two different methamphetamine dosing regimens and found distinct long-term behavioral and neurochemical changes. Adult rats were treated with 1-day methamphetamine injection (3x5 mg/kg s.c., 3 h apart) or 7-day methamphetamine minipump (20 mg/kg/day s.c.). The minipump regimen models the sustained methamphetamine plasma levels in some human bingers whereas the 1-day regimen models a naive user overdose. On withdrawal days 7 and 28, rats were acutely challenged with cocaine to test for behavioral sensitization and subsequently sacrificed for caudate and accumbens dopamine tissue content. Other rats were analyzed on withdrawal days 3, 7 or 28 using voltammetry in caudate slices. On withdrawal days 7 and 28, the methamphetamine injection but not the minipump rats showed behavioral cross-sensitization to cocaine. There was no change in baseline dopamine release, reuptake or sensitivity to quinpirole in any treatment group on either withdrawal day. However, consistent with the behavioral sensitization, cocaine had a greater effect in potentiating dopamine release and in blocking dopamine reuptake in methamphetamine injection versus saline irrespective of withdrawal day. The minipump group showed tolerance to the dopamine releasing effect of cocaine on withdrawal day 28 and had lower dopamine tissue content in the caudate versus the methamphetamine injection group. Dopamine turnover as measured by the DOPAC/dopamine ratio tended to be higher in the minipump-treated rats. These data suggest that the behavioral cross-sensitization seen in the methamphetamine injection rats could be in part due to the increased potency of cocaine in blocking dopamine reuptake and in increasing dopamine release. The decreased potency of cocaine in the caudate slices from the minipump-treated group may be related to decreased dopamine tissue content.

The non-peptidic delta opioid receptor agonist TAN-67 enhances dopamine efflux in the nucleus accumbens of freely moving rats via a mechanism that involves both glutamate and free radicals

The activation of the delta-opioid receptors in the nucleus accumbens is known to induce a large and rapid increase of accumbal dopamine efflux. (+/-)-TAN-67 (2-methyl-4a(alpha)-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a(alpha)-octahydro-quinolino[2,3,3,-g]isoquinoline) is a centrally acting non-peptidic delta opioid receptor agent which has recently become available. Interestingly, the (+) enantiomer of TAN-67 induces hyperalgesia in contrast to the (-) enantiomer of TAN-67 that produces profound antinociceptive effects in mice; the latter effects are mediated through delta-1 receptor stimulation. Using the microdialysis technique, the ability of the enantiomers of TAN-67 to alter the release of accumbal dopamine in vivo was analyzed. Like the 25-min infusion of the selective delta-1 opioid receptor agonist (D-[Pen2,5]-enkephalin) DPDPE (50 nM) and the delta-2 opioid receptor agonist deltorphin II (50 nM), the 25-min infusion of both (-)-TAN-67 (25 and 50 nM) and (+)-TAN-67 (25 and 50 nM) into the nucleus accumbens produced a similar transient dose-dependent increase in the accumbal extracellular dopamine level. Naloxone (1 mg/kg i.p., given 25 min prior to the drugs), namely a treatment that is known to inhibit the increase of dopamine induced by DPDPE and deltorphin II, did not affect the transient increase in the accumbal dopamine level produced by infusion of the enantiomers of TAN-67. The DPDPE and deltorphin II-induced increase in accumbal dopamine level, but not that of (-)-TAN-67 and (+)-TAN-67, was eliminated by subsequently perfused tetrodotoxin (2 microM) into the nucleus accumbens. The increase in accumbal dopamine level produced by an infusion of (-)-TAN-67 and (+)-TAN-67 was not altered by a Ca2+-free Ringer's solution. The (-)-TAN-67 and (+)-TAN-67-induced accumbal dopamine efflux was strongly prevented by reserpine (5 mg/kg i.p., given 24 h earlier) or alpha-methyl-para-tyrosine (250 mg/kg i.p., given 2 h earlier). The effects of the enantiomers of TAN-67 on the accumbal dopamine were nullified by combined treatment with reserpine and alpha-methyl-para-tyrosine. The (-)-TAN-induced dopamine efflux was significantly reduced by the N-methyl-D-aspartate (NMDA) receptor antagonists ifenprodil (20 mg/kg i.p., 20 min before) and MK-801 (0.5 mg/kg i.p., 20 min before), respectively. The effects of (-)-TAN-67 on the dopamine efflux were also inhibited by the free radical scavenger N-2-mercaptopropionyl glycine (100 mg/kg i.p., 20 min before). These results show that both enantiomers of TAN-67 enhance the release of reserpine sensitive, vesicular dopamine and alpha-methyl-p-tyrosine sensitive, cytosolic dopamine from dopaminergic nerve terminals in the nucleus accumbens in a way that is independent of neural activity; activation of delta opioid receptors plays no role in these events. All together, the results suggest that (-)-TAN-67 can generate a burst of free radicals that in turn trigger a release of glutamate that ultimately via activation of NMDA receptors enhances the release of dopamine from dopaminergic nerve terminals in the nucleus accumbens.

Comparative Effects of NO-Synthase Inhibitor and NMDA Antagonist on Generation of Nitric Oxide and Release of Amino Acids and Acetylcholine in the Rat Brain Elicited by Amphetamine Neurotoxicity

The aim of this study was to clarify the role of nitric oxide (NO) and lipid peroxidation (LPO) processes as well as the contribution of various neurotransmitters in pathophysiological mechanisms of neurotoxicity induced by amphetamine (AMPH). NO level was determined directly in brain tissues using electron paramagnetic resonance spectroscopy technique. The content of the products of lipid peroxidation (LPO) was measured spectrophotometrically as thiobarbituric acid reactive species (TBARS). The output of neurotransmitter amino acids (glutamate, aspartate, and GABA) and acetylcholine (ACH) was monitored in nucleus accumbens (NAc) by push-pull technique with HPLC detection. Repeated, systemic application of AMPH elevated striatal and cortical NO generation and LPO production. Moreover, administration of AMPH led to a marked and long-lasting increase of ACH release. Surprisingly, while glutamate output was not affected, aspartate release was enhanced 30 to 50 min after each AMPH injection. The release rate of GABA was also elevated. The selective NO-synthase inhibitor 7-nitroindazole (7-NI) was highly effective in abating the rise in the neurotransmitter release induced by the AMPH. The NOS inhibitor also abolished the increase of NO generation produced by AMPH, but did not influence the intensity of LPO elicited by the AMPH administration. Pretreatment with the noncompetitive NMDA receptor antagonist dizocilpine (MK-801) completely prevented increase of NO generation and TBARS formation induced by multiple doses of AMPH. Dizocilpine also abolished the effect of the psychostimulant drug on the release of neurotransmitters ACH, glutamate, aspartate, and GABA in the NAc. Our findings suggest a key role of NO in AMPH-induced transmitter release, but not in the formation of LPO products. It appears that AMPH enhances release of ACH and neurotransmitter amino acids through increased NO synthesis and induces neurotoxicity via NO and also by NO-independent LPO.

Amphetamine neurotoxicity: accomplishments and remaining challenges

In addition to the social, cultural and indirect medical complications of amphetamine analog abuse, this class of drugs is also known to have the potential to damage brain monoaminergic cells directly. Using methamphetamine as a prototype, this article provides a brief review of the history of amphetamine neurotoxicity research and the progress that has been made toward defining its characteristics. Remaining challenges for this line of investigation are outlined, and suggested avenues for addressing these challenges are provided.

The effect of phosphorylation on amphetamine-mediated outward transport

Amphetamine elicits its locomotor-activating and drug-reinforcing effects by releasing the catecholamines dopamine and norepinephrine into the synapse. Amphetamine is a substrate of the plasmalemmal transporters for both dopamine and norepinephrine. As such, it binds to the transporters in conjunction with Na+ and Cl-, facilitating a conformational change leading the transporter to face inward. The subsequent binding of intracellular catecholamine results in an outward transport and release of the catecholamine into the synapse. Both inward and outward transport through the catecholamine transporters are regulated by protein kinases, particularly protein kinase C, but the effect of the enzyme on the two processes appears to be asymmetric. The purpose of this review is to discuss the evidence showing that protein kinase C activation facilitates outward transport through the catecholamine plasmalemmal transporters which may mediate amphetamine action in intact tissue.

Dopamine transporter-dependent induction of C-Fos in HEK cells

The psychostimulants cocaine and amphetamine increase expression of the immediate early gene (IEG) c-fos indirectly, via D1 dopamine receptor activation. To determine whether dopamine transporter substrates and inhibitors can affect c-Fos expression directly, we investigated their effects on c-Fos protein and c-fos mRNA in HEK-293 (HEK) cells transfected with the human dopamine transporter (hDAT). In untransfected HEK cells, methylphenidate and cocaine produced a small but statistically significant increase in c-Fos, whereas dopamine and amphetamine did not. In hDAT cells, DAT substrates (dopamine, amphetamine) increased c-Fos immunoreactivity 6- and 3-fold (respectively). The DAT inhibitors cocaine, methylphenidate, and bupropion also increased c-Fos approximately 3-fold in hDAT cells. If coincubated with dopamine, the inhibitors attenuated dopamine-induced c-Fos in hDAT cells. The magnitude of c-fos mRNA induction by substrates and inhibitors paralleled induction of c-Fos protein immunoreactivity. The results indicate that substrates or inhibitors of the DAT can trigger induction of IEG expression in the absence of D1 dopamine receptor. For substrates, IEG induction is DAT-dependent, but for certain DAT inhibitors the cellular response can be elicited in the absence of the DAT in HEK cells. Oxidative stress may partly, but not fully, account for the DA-induced c-Fos induction as an inhibitor of oxidative stress Trolox C, attenuated DA-induced c-Fos induction. Protein kinase C (PKC) may also partially account for c-Fos induction as a specific inhibitor of PKC Bisindolylmaleimide I (BIS) attenuated DA-induced c-Fos by 50%. DAT substrate and inhibitor effects on IEGs, other fos-related antigens, and possible mechanisms that contribute to c-Fos induction warrant investigation in presynaptic neurons as a potential contribution to the long-term effects of psychostimulants.

Neurochemical changes and neurotoxic effects of an acute treatment with sydnocarb, a novel psychostimulant: comparison with D-amphetamine

Sydnocarb [(phenylisopropyl)N-phenylcarbamoylsydnonimine; SYD] was introduced to clinical practice in Russia as a psychostimulant drug used for the treatment of asthenia and apathy, which accompany schizophrenia and manic depression. It has been described as a psychostimulant with addiction liability and toxicity less than amphetamine (AMPH). The precise cellular mechanisms by which sydnocarb elicits its psychostimulant effect are still unclear. At present its neurochemical and neurotoxic effects are compared to those of AMPH in the striatum, the main input structure of the basal ganglia. The expression of c-fos protein in striatal neurons was much more increased after a single injection of D-AMPH (5 mg/kg) than after an equimolar concentration of SYD (23.8 mg/kg) in both the anterior and the posterior part of the striatum. Using in situ hybridization on striatal slices, we observed that AMPH increased the striatal levels of preprodynorphin (PPDYN) mRNAs in both parts of the striatum, while SYD did not affect basal levels of PPDYN mRNAs. Furthermore, AMPH and SYD increased striatal preprotachykinin (PPT-A) and preproenkephalin (PPE) mRNA levels. The effects of AMPH and SYD on PPT-A-mRNA levels were similar. A differential effect of AMPH and SYD was observed only on the PPE-mRNA levels measured in the anterior striatum where SYD increased these levels more than AMPH. The acute neurotoxicity of these two psychostimulants was analyzed by measuring their effects on the parameters of oxidative stress, such as nitric oxide (NO) generation, as well as specific indices of lipid peroxidation (i.e., thiobarbituric acid reactive substances; TBARS), while, on the other hand, the alpha-tocopherol level was taken as an index of antioxidant defense processes. Measuring generation of NO directly by electron paramagnetic resonance, it was observed that AMPH shows a more pronounced increase in comparison to SYD, in the striatum and in cortex. TBARS levels in the striatum and cortex were significantly less enhanced than AMPH after a single injection of SYD. Similarly, the alpha-tocopherol level was decreased only by AMPH in the striatum, and neither AMPH nor SYD had any effect in the cortex. Results show that a single injection of a high dose of AMPH is able to induce several neurotoxic effects. The study also demonstrates that SYD has mild neurochemical effects as well as fewer neurotoxic properties than AMPH.

Dizocilpine inhibits amphetamine-induced formation of nitric oxide and amphetamine-induced release of amino acids and acetylcholine in the rat brain

Glutamate receptor activation participates in mediation of neurotoxic effects in the striatum induced by the psychomotor stimulant amphetamine. The effects of the non-competitive NMDA receptor antagonist dizocilpine (MK-801) on amphetamine-induced toxicity and formation of nitric oxide (NO) in both striatum and cortex and on induced transmitter release in the nucleus accumbens were investigated. Repeated, systemic application of amphetamine elevated striatal and cortical lipid peroxidation and NO production. Moreover, amphetamine caused an immediate release of acetylcholine and aspartate and a delayed release of GABA in the nucleus accumbens. Surprisingly, glutamate release was not affected. Dizocilpine abolished the amphetamine-induced lipid peroxidation and NO production in striatum and cortex and diminished the elevation of neurotransmitter release. These findings suggest that amphetamine evokes neurotoxic effects in both striatal and cortical brain areas that are prevented by inhibiting NMDA receptor activation. The amphetamine-induced acetylcholine, aspartate and GABA release in the nucleus accumbens is also mediated through NMDA receptor-dependent mechanisms. Interestingly, the enhanced aspartate release might contribute to NMDA receptor activation in the nucleus accumbens, while glutamate does not seem to mediate amphetamine-evoked transmitter release in this striatal brain area.

MK801 suppresses the L-DOPA-induced increase of glutamate in striatum of hemi-Parkinson rats

In vivo microdialysis in freely moving rats was used to investigate the influence of the indirect dopamine receptor agonist levodopa (L-DOPA), alone and combined with the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK801), on extracellular glutamate levels in the striatum of intact and 6-hydroxydopamine-lesioned rats. L-DOPA (25 mg/kg i.p. after benserazide 10 mg/kg i.p.) increased extracellular glutamate levels in the striatum of both intact and dopamine-depleted rats. A prior injection of MK801 (0.1 and 1.0 mg/kg i.p.) did not alter the L-DOPA-induced glutamate release in the striatum of intact rats. In contrast, the L-DOPA-induced increase in glutamate in the striatum of 6-hydroxydopamine-lesioned rats was suppressed by MK801 (0.1 mg/kg i.p.). The data presented here suggest that NMDA receptors do not play a role in the L-DOPA-induced increase in striatal glutamate in intact rats but are involved in the glutamate release in the dopamine-depleted striatum. The suppression of this increase by prior administration of MK801 could represent a neuroprotective effect.

Amphetamine-induced toxicity in dopamine terminals in CD-1 and C57BL/6J mice: complex roles for oxygen-based species and temperature regulation

In order to examine differential strain susceptibility to neurotoxic effects of amphetamine and to assess the potential role of superoxide radicals in amphetamine-induced dopaminergic damage, the drug was injected to mice with different levels of copper/zinc superoxide dismutase (Cu/Zn SOD) enzyme. Administration of amphetamine (10 mg/kg, i.p., given every 2 h, a total of four times) to wild-type CD-1 and C57BL/6J mice caused significant decreases in dopamine and 3,4-dihydroxyphenylacetic acid levels, in [(125)I]RTI-121-labeled dopamine transporters as well as a significant depletion in the concentration of dopamine transporter and vesicular monoamine transporter 2 proteins. The amphetamine-induced toxic effects were less prominent in CD-1 mice, which have much higher levels of Cu/Zn SOD activity (0.69 units/mg of protein) in their striata than C57BL/6J animals (0.007 units/mg of protein). Transgenic mice on CD-1 and C57BL/6J background, which had striatal levels of Cu/Zn SOD 2.57 and 1.67 units/mg of protein, respectively, showed significant protection against all the toxic effects of amphetamine. The attenuation of toxicity observed in transgenic mice was not caused by differences in amphetamine accumulation in wild-type and mutant animals. However, CD-1-SOD transgenic mice showed marked hypothermia to amphetamine whereas C57-SOD transgenic mice did not show a consistent thermic response to the drug. The data obtained demonstrate distinctions in the neurotoxic profile of amphetamine in CD-1 and C57BL/6J mice, which show some differences in Cu/Zn SOD activity and in their thermic responses to amphetamine administration. Thus, these observations provide evidence for possible complex interactions between thermoregulation and free radical load in the long-term neurotoxic effects of this illicit drug of abuse.

Methamphetamine neurotoxicity: necrotic and apoptotic mechanisms and relevance to human abuse and treatment

Research into methamphetamine-induced neurotoxicity has experienced a resurgence in recent years. This is due to (1) greater understanding of the mechanisms underlying methamphetamine neurotoxicity, (2) its usefulness as a model for Parkinson's disease and (3) an increased abuse of the substance, especially in the American Mid-West and Japan. It is suggested that the commonly used experimental one-day methamphetamine dosing regimen better models the acute overdose pathologies seen in humans, whereas chronic models are needed to accurately model human long-term abuse. Further, we suggest that these two dosing regimens will result in quite different neurochemical, neuropathological and behavioral outcomes. The relative importance of the dopamine transporter and vesicular monoamine transporter knockout is discussed and insights into oxidative mechanisms are described from observations of nNOS knockout and SOD overexpression. This review not only describes the neuropathologies associated with methamphetamine in rodents, non-human primates and human abusers, but also focuses on the more recent literature associated with reactive oxygen and nitrogen species and their contribution to neuronal death via necrosis and/or apoptosis. The effect of methamphetamine on the mitochondrial membrane potential and electron transport chain and subsequent apoptotic cascades are also emphasized. Finally, we describe potential treatments for methamphetamine abusers with reference to the time after withdrawal. We suggest that potential treatments can be divided into three categories; (1) the prevention of neurotoxicity if recidivism occurs, (2) amelioration of apoptotic cascades that may occur even in the withdrawal period and (3) treatment of the atypical depression associated with withdrawal.

Adaptative response of antioxidant enzymes in different areas of rat brain after repeated d-amphetamine administration

d-Amphetamine has been shown to be a potential brain neurotoxic agent, particularly to dopaminergic neurones. Reactive oxygen species indirectly generated by this drug have been indicated as an important factor in the appearance of neuronal damage but little is known about the adaptations of brain antioxidant systems to its chronic administration. In this study, the activities of several antioxidant enzymes in different areas of rat brain were measured after repeated administration of d-amphetamine sulphate (sc, 20 mg/kg/day, for 14 days), namely glutathione-S-transferase (GST), glutathione peroxidase (GPx), glutathione reductase (GRed), catalase, and superoxide dismutase (SOD). When compared to a pair-fed control group, d-amphetamine treatment enhanced the activity of GST in hypothalamus to 167%, GPx in striatum to 127%, in nucleus accumbens to 192%, and in medial prefrontal cortex to 139%, GRed in hypothalamus to 139%, as well as catalase in medial prefrontal cortex to 153%. However, the same comparison revealed a decrease in the activity of GRed in medial pre-frontal cortex by 35%. Food restriction itself reduced GRed activity by 49% and enhanced catalase activity to 271% in nucleus accumbens. The modifications observed for the measured antioxidant enzymes reveal that oxidative stress probably plays a role in the deleterious effects of this drug in CNS and that, in general, the brain areas studied underwent adaptations which provided protection against the continuous administration of the drug.

Amphetamine increases glutamate efflux in the rat ventral tegmental area by a mechanism involving glutamate transporters and reactive oxygen species

We have shown that amphetamine produces a delayed and sustained increase in glutamate levels in the ventral tegmental area, a region containing dopamine cell bodies important in acute and chronic effects of amphetamine administration. The present study characterized the mechanism underlying amphetamine-induced glutamate efflux. It was abolished by the glutamate uptake inhibitor dihydrokainate, but unaffected by perfusion with a low Ca(2+)/high Mg(2+) solution, implicating glutamate transporters. Because reactive oxygen species inhibit glutamate uptake, we examined the effect of amphetamine on hydroxyl radical formation by perfusing with D-phenylalanine (5 mM) and monitoring p-tyrosine production. Although no increase in hydroxyl radical formation was detected, D-phenylalanine completely prevented the amphetamine-induced increase in glutamate efflux, as did systemic injection of another trapping agent, alpha-phenyl-N-tert-butyl nitrone (60 mg/kg). Thus, amphetamine-induced glutamate efflux may involve reactive oxygen species. In other studies, we found that repeated coadministration of alpha-phenyl-N-tert-butyl nitrone with amphetamine attenuated the development of behavioral sensitization. This supports prior results indicating that the increase in glutamate efflux produced by each amphetamine injection in a chronic regimen is important in triggering drug-induced adaptations in ventral tegmental area dopamine neurons, and that such adaptations may in part represent a response to metabolic and oxidative stress

The novel non-competitive N-methyl-D-aspartate antagonist gacyclidine blocks the glutamate-induced release of hydroxyl radicals in the striatum under conditions in which dizocilpine does not. A microdialysis study in rats

Gacyclidine, a new neuroprotectant aimed to non-competitively antagonize N-methyl-D-aspartate (NMDA) receptors, and dizocilpine blocked the hydroxyl radical response to toxic amounts of glutamate, perfused through a microdialysis implanted in the striatum of conscious rats. Furthermore, the hydroxyl radical response resulting from the infusion of lower doses of glutamate, which could not be inhibited by the same amounts of dizocilpine nor by acute alcohol exposure, still remained sensitive to gacyclidine inhibition. Thus, oxidative stress resulting from a glutamate discharge involves the activation of both NMDA receptors, and of non-NMDA mechanism(s) which, with moderate glutamate levels, were still antagonized by gacyclidine. Enhanced blockage of toxic hydroxyl radicals might explain the different and possibly higher neuroprotective property of gacyclidine as compared with other non-competitive NMDA antagonists.