MPP+ enhances potassium-evoked striatal dopamine release through a omega-conotoxin-insensitive, tetrodotoxin- and nimodipine-sensitive calcium-dependent mechanism

Melatonin inhibits 6-hydroxydopamine production in the brain to protect against experimental parkinsonism in rodents

We tested the hypothesis that melatonin regulates formation of 6-hydroxydopamine (6-OHDA) in the brain and thereby protects animals from dopaminergic neurotoxicity and the development of parkinsonism in animals. Employing a ferrous-ascorbate-dopamine (FAD) hydroxyl radical ((*)OH) generating system, in the present study we demonstrate a dose-dependent attenuation of 6-OHDA generation by melatonin in vitro. Intra-median forebrain bundle infusion of FAD caused significant depletion of striatal dopamine (DA), which was blocked by melatonin. Per-oral administration of l-3,4-dihydroxyphenylalanine (L-DOPA) for 7 days caused a dose-dependent increase in the formation of 6-OHDA in the mouse striatum, which was increased synergistically by the systemic administration of the parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on the 7th day of L-DOPA treatment. Melatonin treatment significantly attenuated both the L-DOPA and MPTP-induced increases in the levels of striatal 6-OHDA, and protected against striatal DA depletion caused by the neurotoxin. These observations suggest a novel mode of melatonin-induced dopaminergic neuroprotection in two models of Parkinson's disease, and suggest the possible therapeutic use of this well-known antioxidant indoleamine neurohormone in parkinsonism.

The Parkinsonian Neurotoxin 1-Methyl-4-Phenylpyridinium (MPP+) Mediates Release of l-3,4-Dihydroxyphenylalanine (l-DOPA) and Inhibition of l-DOPA Decarboxylase in the Rat Striatum: A Microdialysis Study

Reactive oxygen species (ROS) and reactive nitrogen species (RNS), particularly peroxynitrite, have been implicated as key participants in the dopaminergic neurotoxicity of 1-methyl-4-phenylpyridinium (MPP(+)). However, on the basis of available information, it is not clear whether the MPP(+)-induced overproduction of ROS and RNS occurs in the intraneuronal and/or extracellular compartment. Early steps in the neurotoxic mechanism evoked by MPP(+) include a profound dopaminergic energy impairment, which mediates a massive release of dopamine (DA), glutathione (GSH), and cysteine (CySH). In the event that MPP(+) mediates extracellular generation of ROS (such as superoxide and/or hydroxyl radicals) and/or peroxynitrite, released DA, GSH, and CySH should be oxidized forming thioethers of DA and disulfides. Using microdialysis experiments in which MPP(+) was perfused into the striatum of awake rats, the present study was unable to detect the presence of such biomarkers of extracellular ROS and/or RNS generation. However, MPP(+) induced a transient, concentration-dependent rise of extracellular l-3,4-dihydroxyphenylalanine (l-DOPA), identified on the basis of dialysate analysis using several HPLC methods and its conversion to DA by purified l-DOPA decarboxylase (DDC). Methamphetamine (30 mg/kg, i.p.) similarly caused a significant but transient rise of l-DOPA in the rat striatum. Antioxidants such as salicylate and mannitol had no effect on the MPP(+)-mediated elevation of extracellular l-DOPA, suggesting that it is not formed by nonenzymatic hydroxylation of l-tyrosine by ROS or RNS. Rather, in vivo, but not in vitro, MPP(+) caused rapid inhibition of DDC, which appears to result in intraneuronal accumulation and subsequent release of l-DOPA. Because l-DOPA can mediate l-glutamate release, as well as be an excitotoxin, the possibility is raised that l-DOPA may play a role in the dopaminergic neurotoxicity of MPP(+).

Calcium overload enhances hydroxyl radical generation by 1-methyl-4 phenylpyridinium ion (MPP+) in rat striatum

We examined whether ouabain-induced Ca(2+) overload increases hydroxyl radical (*OH) generation by 1-methyl-4-phenylpyridinium ion (MPP(+)) in rat striatum. These elevations seem to induce lipid peroxidation of striatum of rats, as detected by increases in non-enzymatic formation of 2,3-dihydroxybenzoic acid (DHBA) levels. Ouabain enhanced MPP(+)-induced *OH formation trapped as DHBA. Moreover, when iron (II) was administered to MPP(+) then ouabain (100 micro M)-pretreated animals, a marked elevation in the level of DHBA was observed, as compared with the iron (II)-only-treated animals. These results suggests that Ca(2+) overload might enhance *OH generation by MPP(+) in rat striatum.

Estrogen reduces acute striatal dopamine responses in vivo to the neurotoxin MPP+ in female, but not male rats

The effects of in vivo estrogen treatment upon MPP(+)-induced dopamine (DA) release were determined using in vivo microdialysis in female and male rats. Ovariectomized female rats were implanted or not with an estrogen pellet (0.1 mg, 17beta estradiol) and subjected to microdialysis 6 days later. After baseline DA release was determined, 5 mM MPP(+) was infused through the microdialysis probe for one 20-min interval. Perfusion resumed with normal medium for the duration of the experiment. A significant attenuation of MPP(+)-induced DA release was obtained in estrogen-treated females. One week later, striatal DA and dihydroxyphenylacetic acid (DOPAC) concentrations were determined for the lesioned and non-lesioned striata of each animal. MPP(+) infusion significantly decreased striatal DA concentrations, however, there was no effect of estrogen treatment on striatal DA depletion. This experiment was repeated using orchidectomized male rats treated with 0, 0.1, or 5 mg estradiol. In contrast to the females, no differences in MPP(+)-induced DA release were seen among these males, and there was no significant effect of the varying estrogen treatments on striatal DA or DOPAC concentrations. These results demonstrate that in vivo estrogen treatment attenuates MPP(+)-induced striatal DA release in gonadectomized female, but not male, rats.

MPTP-induced deficits in motor activity: neuroprotective effects of the spintrapping agent, alpha-phenyl-tert-butyl-nitrone (PBN)

In Experiment 1, groups of mice were administered either saline or MPTP (2 x 30 mg/kg, s.c., separated by a 24-hr interval) 30 min after being injected either PBN (15, 50 or 150 mg/kg, s.c., low, medium and high doses, respectively) or L-Deprenyl (0.25 or 10.0 mg/kg, s.c., low and high doses, respectively), the reference compound used, or saline. Tests of spontaneous motor activity 14 days later indicated that the MPTP-induced hypokinesia for locomotion and rearing was alleviated by prior administration with PBN (50 or 150 mg/kg) or L-Deprenyl (10.0 mg/kg); lower doses of PBN (15 mg/kg) and L-Deprenyl (0.25 mg/kg) did not affect the MPTP-induced deficits. Dopamine (DA) concentrations in the striatum confirmed a more severe loss of DA in the MPTP, PBN (15) + MPTP and Deprenyl(0.25) + MPTP groups than in the control group. Significant protection of DA was observed in the PBN(50) + MPTP, PBN(150) + MPTP and Deprenyl(10) + MPTP groups that did not exhibit an hypokinetic behaviour. In Experiment 2, the effects of repeated treatment with PBN (50 mg/kg, s.c. over 12 days), post-MPTP, were studied in aged (15-month-old) and young (3-month-old) mice. Subchronic administration of PBN increased substantially the motor activity of old and young mice that had received MPTP. Aged control (saline) mice showed an activity deficit compared to young control mice; this deficit was abolished by repeated PBN treatment. The results suggest that moderate-to-high doses of PBN whether injected in a single dose prior to MPTP or subchronically following MPTP injections may afford protective effects against both the functional changes and DA-loss caused by MPTP treatment, possibly through an antioxidant mechanism.

Role of quinones in toxicology

Quinones represent a class of toxicological intermediates which can create a variety of hazardous effects in vivo, including acute cytotoxicity, immunotoxicity, and carcinogenesis. The mechanisms by which quinones cause these effects can be quite complex. Quinones are Michael acceptors, and cellular damage can occur through alkylation of crucial cellular proteins and/or DNA. Alternatively, quinones are highly redox active molecules which can redox cycle with their semiquinone radicals, leading to formation of reactive oxygen species (ROS), including superoxide, hydrogen peroxide, and ultimately the hydroxyl radical. Production of ROS can cause severe oxidative stress within cells through the formation of oxidized cellular macromolecules, including lipids, proteins, and DNA. Formation of oxidatively damaged bases such as 8-oxodeoxyguanosine has been associated with aging and carcinogenesis. Furthermore, ROS can activate a number of signaling pathways, including protein kinase C and RAS. This review explores the varied cytotoxic effects of quinones using specific examples, including quinones produced from benzene, polycyclic aromatic hydrocarbons, estrogens, and catecholamines. The evidence strongly suggests that the numerous mechanisms of quinone toxicity (i.e., alkylation vs oxidative stress) can be correlated with the known pathology of the parent compound(s).

Inhibitors of mitochondrial respiration, iron (II), and hydroxyl radical evoke release and extracellular hydrolysis of glutathione in rat striatum and substantia nigra: potential implications to Parkinson's disease

In this investigation, microdialysis has been used to study the effects of 1-methyl-4-phenylpyridinium (MPP+), an inhibitor of mitochondrial complex I and alpha-ketoglutarate dehydrogenase and the active metabolite of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), on extracellular concentrations of glutathione (GSH) and cysteine (CySH) in the rat striatum and substantia nigra (SN). During perfusion of a neurotoxic concentration of MPP+ (2.5 mM) into the rat striatum or SN, extracellular concentrations of GSH and CySH remain at basal levels (both approximately 2 microM). However, when the perfusion is discontinued, a massive but transient release of GSH occurs, peaking at 5,000% of basal levels in the striatum and 2,000% of basal levels in the SN. The release of GSH is followed by a slightly delayed and smaller elevation of extracellular concentrations of CySH that can be blocked by the gamma-glutamyl transpeptidase (gamma-GT) inhibitor acivicin. Low-molecular-weight iron and extracellular hydroxyl radical (OH*) have been implicated as participants in the mechanism underlying the dopaminergic neurotoxicity of MPTP/MPP+. During perfusion of Fe2+ (OH*) into the rat striatum and SN, extracellular levels of GSH also remain at basal levels. When perfusions of Fe2+ are discontinued, a massive transient release of GSH occurs followed by a delayed, small, but progressive elevation of extracellular CySH level that again can be blocked by acivicin. Previous investigators have noted that extracellular concentrations of the excitatory/excitotoxic amino acid glutamate increase dramatically when perfusions of neurotoxic concentrations of MPP+ are discontinued. This observation and the fact that MPTP/MPP+ causes the loss of nigrostriatal GSH without corresponding increases of glutathione disulfide (GSSG) and the results of the present investigation suggest that the release and gamma-GT/dipeptidase-mediated hydrolysis of GSH to glutamate, glycine, and CySH may be important factors involved with the degeneration of dopamine neurons. It is interesting that a very early event in the pathogenesis of Parkinson's disease is a massive loss of GSH in the SN pars compacta that is not accompanied by corresponding increases of GSSG levels. Based on the results of this and prior investigations, a new hypothesis is proposed that might contribute to an understanding of the mechanisms that underlie the degeneration of dopamine neurons evoked by MPTP/MPP+, other agents that impair neuronal energy metabolism, and Parkinson's disease.

Species differences in the role of excitatory amino acids in experimental parkinsonism

The present review discusses species differences in relation to the effects produced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP); in particular, it focuses on recent evidence regarding the role of excitatory amino acids in experimental parkinsonism. The main aim of the review is to provide a phylogenetic perspective which may serve as a useful tool to study Parkinson's disease in rodents. Excitotoxicity might represent the final common pathway on which the actions of different neurotoxins, selectively directed towards nigrostriatal dompaminergic neurons, converge. This is clearly demonstrated in methamphetamine- and 6-dihydroxy-dopamine-induced parkinsonism. The role of excitotoxicity in the mechanism of action of MPTP is less clear. Although there are several species differences for MPTP it is possible to obtain in mice the same effects induced in MPTP-treated primates by combining acetaldehyde or diethyldithiocarbamate with MPTP administration. When mice are administered these combined treatments, the onset of experimental parkinsonism can be prevented using the same pharmacological agents (i.e. glutamate N-methyl-D-aspartate antagonists) that are effective in primates.

Antioxidant mechanism and protection of nigral neurons against MPP+ toxicity by deprenyl (selegiline)

The current research has demonstrated that MPP+ can induce lipid peroxidation in the nigrostriatal system of rat in vivo. Antioxidant agent U-78517F and .OH scavenger DMSO may protect against MPP+ toxicity through the inhibition of .OH radical-mediated oxidative injury in the substantia nigra. These findings indicate that the cytotoxic hydroxyl radical generated from dopamine oxidation in the iron-rich basal ganglia may contribute to the mechanism underlying the selective A9 melanized nigral degeneration in MPTP-Parkinsonism and possibly in idiopathic Parkinson's disease. In addition, the present studies also clearly demonstrate that deprenyl can substantially protect dopaminergic neurons against MPP+ toxicity in the substantia nigra zona compacta in vivo. The neuroprotective effect provided by deprenyl may not be the consequence of its inhibition of MAO-B activity or prevention of the uptake of MPP+ by dopaminergic neurons. A unique antioxidant property of deprenyl by suppressing .OH formation and associated oxidative injury induced by MPP+ may contribute to the apparent neuroprotective action. In perspective, this putative antioxidant effect of deprenyl may provide another mechanism to its overt neuroprotective effects against oxygen radical-mediated oxidative injury in some neurotoxic chemicals, such as 6-OHDA and DSP-4, and probably in Alzheimer's disease and senescent changes. Finally, based on the present data, a possible neuroprotective therapeutic window of deprenyl in the treatment of early Parkinson's disease has been proposed. It is suggested that deprenyl should be introduced as early as possible in de novo Parkinsonian patients to achieve its full neuroprotective effect on nigral degeneration. Moreover, a combination of early detection of individuals at risk of developing Parkinson's disease and early intervention of deprenyl and/or other centrally active antioxidants to these patients may provide a new preventive therapeutic strategy in the future, in addition to the current conventional levodopa treatment of Parkinson's disease.

In vivo generation of hydroxyl radicals and MPTP-induced dopaminergic toxicity in the basal ganglia

The in vivo generation of .OH free radicals in specific brain regions can be measured by intracerebral microdialysis perfusion of salicylate, avoiding many of the pitfalls inherent in systemic administration of salicylate. Direct infusion of salicylate into the brain can minimize the hepatic hydroxylation of salicylate and its contribution to brain levels of 2,5-DHBA. Levels of 2,5-DHBA detected in the brain dialysate may reflect the .OH adduct plus some enzymatic hydroxylation of salicylate in the brain. After minimizing the contribution of enzyme and/or blood-borne 2,5-DHBA, the present data demonstrate the validity of the use of 2,3-DHBA and apparently 2,5-DHBA as indices of .OH formation in the brain. Therefore, intracranial microdialysis of salicylic acid and measurement of 2,3-DHBA appears to be a useful .OH trapping procedure for monitoring the time course of .OH generation in the extracellular fluid of the brain. These results indicate that nonenzymatic and/or enzymatic oxidation of the dopamine released by MPTP analogues in the extracellular fluid may play a key role in the generation of .OH free radicals in the iron-rich basal ganglia. Moreover, a site-specific generation of cytotoxic .OH free radicals and quinone/semiquinone radicals in the striatum may cause the observed lipid peroxidation, calcium overload, and retrograde degeneration of nigrostriatal neurons. This free-radical-induced nigral injury can be suppressed by antioxidants (i.e., U-78517F, DMSO, and deprenyl) and possibly hypothermia as well. In the future, this in vivo detection of .OH generation may be useful in answering some of the fundamental questions concerning the relevance of oxidants and antioxidants in neurodegenerative disorders during aging. It could also pave the way for the research and development of novel neuroprotective antioxidants and strategies for the early or preventive treatment of neurodegenerative disorders, such as Parkinson's disease (Wu et al., this issue), amyotrophic lateral sclerosis, head trauma, and possibly Alzheimer's cognitive dysfunction as well. In conclusion, this in vivo free-radical trapping procedure provides evidence to support a current working hypothesis that a site-specific formation of cytotoxic .OH free radicals in the basal ganglia may be one of the neurotoxic mechanisms underlying nigrostriatal degeneration and Parkinsonism caused by the dopaminergic neurotoxin MPTP. Addendum added in proof: The controversy concerning possible neurotoxic and/or neuroprotective roles of NO. in cell cultures was discussed and debated at the symposium (Wink et al., this issue; Dawson et al., this issue; Lipton et al., this issue).(ABSTRACT TRUNCATED AT 400 WORDS)

Preliminary controlled trial of nimodipine in ultra-rapid cycling affective dysregulation

We report the initial results of the first controlled double-blind trial of nimodipine, a calcium channel antagonist, in the acute and prophylactic treatment of patients with treatment-refractory affective dysregulation. Active drug nimodipine (A) was substituted for placebo (B) in 12 patients. Patients were studied in a B-A-B design, with 3 of the 12 patients rechallenged with active drug in a B-A-B-A design (patients 9, 10, and 11). Five of the nine patients who completed the drug trial responded. One of three patients suffering from ultra-ultra-rapid (ultradian) cycling bipolar II disorder (patient 6) showed an essentially complete response; the other two ultradian patients (patients 4 and 9) showed evidence of a partial response on manic and depressive oscillations, one of which was confirmed in a B-A-B-A design. Only one of five less rapidly, but continuously cycling patients showed an excellent response (patient 10), and this was confirmed in a B-A-B-A design. The one patient who had recurrent brief depression (patient 11) showed a complete resolution of severe depressive recurrences, with response re-confirmed in an extended prophylactic trial with a B-A-B-A design. In the eight patients who completed self-ratings, nimodipine was associated with a significant reduction in the magnitude of mood fluctuations compared with the baseline placebo condition. Further clinical study of nimodipine, a calcium channel blocker with a unique profile of behavioral and anticonvulsant properties, appears warranted in patients with treatment-refractory affective illness characterized by recurrent brief depression and ultradian cycling.

Apparent antioxidant effect of l-deprenyl on hydroxyl radical formation and nigral injury elicited by MPP+ in vivo

Using a modified microdialysis procedure, we confirmed that intrastriatal administration of 1-methyl-4-phenylpyridinium ion (MPP+) induced a sustained overflow of dopamine accompanied by increased formation of hydroxyl free radicals (.OH) as reflected by salicylate hydroxylation. Pretreatment with l-deprenyl (selegiline 60 pmol, intrastriatal perfusion) significantly decreased the .OH formation elicited by MPP+ (75 nmol). There was a small decrease of dopamine efflux and an insignificant change of the ratio of 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine following l-deprenyl pretreatment. These in vivo findings support prior in vitro data that an unique antioxidant property of l-deprenyl may be independent of its inhibition of type B monoamine oxidase. In addition, intranigral co-administration of l-deprenyl (4.2 nmol) with MPP+ (4.2 nmol) completely protected nigral neurons from probable oxidative injuries induced by MPP+ (4.2 nmol), as reflected by a near 50% loss of striatal dopamine ipsilateral to the side of infusion of drug into the substantia nigra. This apparent neuroprotective effect of l-deprenyl on midbrain nigral neurons was also confirmed by histological findings. The present in vivo data clearly demonstrate that l-deprenyl can protect nigral neurons against dopamine neurotoxicity produced by MPP+, as suggested by an earlier in vitro study. Thus, l-deprenyl can preserve the function of MPP(+)-damaged nigral neurons perhaps by its apparent antioxidant property in addition to its blockade of the bioactivation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to toxic pyridinium metabolites by type B monoamine oxidase.

Intracranial microdialysis of salicylic acid to detect hydroxyl radical generation through dopamine autooxidation in the caudate nucleus: effects of MPP+

Ringer's solution containing salicylic acid (5 nmol/microliters/min) was infused directly through an intracranial microdialysis probe to detect the generation of hydroxyl radicals (.OH) reflected by the formation of dihydroxybenzoic acids (DHBA) in the caudate nucleus of anesthetized rats. Brain dialysate was assayed for dopamine, 2,3-, and 2,5-DHBA by a high-pressure liquid chromatography-electrochemical (HPLC-EC) procedure. 1-Methyl-4-phenylpyridinium ions (MPP+, 0 to 150 nmol) increased dose-dependently the release of dopamine and the formation of DHBA. A positive linear correlation between the release of dopamine and the formation of 2,3- or 2,5-DHBA was observed (R2 = .98). The present results demonstrate the validity of the use of not only 2,3-DHBA but also 2,5-DHBA as an in vivo index of oxidative damage generated by reactive .OH radicals. In conclusion, the present study demonstrates a novel use of intracranial microdialysis of salicylic acid to assess the oxidative damage elicited by .OH in living brain.