Time-dependent effects of levodopa on regional brain dopamine metabolism and lipid peroxidation

Effect of Chronic N-Acetyl Cysteine Administration on Oxidative Status in the Presence and Absence of Induced Oxidative Stress in Rat Striatum

Antioxidants have possible therapeutic value in neurodegenerative disorders, although they may have pro-oxidant effects under certain conditions. Glutathione (GSH) is a key free radical scavenger. N-acetylcysteine (NAC) bolsters GSH and intracellular cysteine and also has effective free radical scavenger properties. The effects of chronic NAC administration (50 mg/kg/day, 500 mg/kg/day, 1500 mg/kg/day x 21 days) on cellular markers of oxidative status was studied in striatum of healthy male Sprague-Dawley rats as well as in animals with apparent striatal oxidative stress following chronic haloperidol treatment (1.5 mg/kg/day x 3 weeks). In non-haloperidol treated animals, NAC 50 and 500 mg/kg did not affect oxidative status, although NAC 1,500 mg/kg significantly increased striatal superoxide levels, decreased lipid peroxidation and increased consumption of reduced glutathione (GSH). Haloperidol alone evoked a significant increase in superoxide and lipid peroxidation. All NAC doses blocked haloperidol induced increases in superoxide levels, while NAC 500 mg/kg and 1,500 mg/kg prevented haloperidol-associated lipid peroxidation levels and also increased the GSSG/GSH ratio. NAC may protect against conditions of striatal oxidative stress, although possible pro-oxidative actions at high doses in otherwise healthy individuals, e.g. to offset worsening of neurodegenerative illness, should be viewed with caution.

Enhanced cortical dopamine output and antipsychotic-like effect of raclopride with adjunctive low-dose L-dopa

BACKGROUND

Clozapine shows superior efficacy in schizophrenia and enhances prefrontal dopamine (DA) output like other atypical, but not typical, antipsychotic drugs (APDs). Clinical data also suggest an improved effect of typical APDs in schizophrenia by adjunctive treatment with low doses of 3,4-dihydroxyphenylalanine (L-dopa), but experimental support is scarce, and the underlying mechanisms are poorly understood.

METHODS

Antipsychotic efficacy of the D2 antagonist raclopride with or without adjunctive treatment with a low dose of L-dopa was assessed with the conditioned avoidance response paradigm. Extrapyramidal side effects were scored by the catalepsy test. Finally, in vivo microdialysis was used to measure DA efflux in the prefrontal cortex and the nucleus accumbens.

RESULTS

A low dose of L-dopa (3 mg/kg) combined with benserazide, an inhibitor of peripheral DOPA decarboxylase, significantly enhanced the antipsychotic-like effect of raclopride without any associated catalepsy. L-dopa/benserazide alone had no effect. In contrast to raclopride alone, combined L-dopa/raclopride also produced a much larger increase in DA output in the prefrontal cortex than in the nucleus accumbens.

CONCLUSIONS

These data support the clinical observation that low-dose L-dopa might improve the effect of typical APDs in schizophrenia and indicate that increased prefrontal DA output per se enhances the antipsychotic effect of typical APDs.

Neuroprotection by Pergolide Against Levodopa-Induced Cytotoxicity of Neural Stem Cells

Neural stem cells (NSCs) are currently considered very hopeful candidates for cell replacement therapy in neurodegenerative pathologies such as Parkinson's disease (PD), but like embryonic neural tissue transplantation, levodopa medication may still be required to improve symptoms even after cell transplantation. The issues of whether levodopa induces cytotoxicity and apoptosis of NSCs following transplantation, as well as the means to prevent these processes from occurring remain to be elucidated. In this study, the possible cytotoxicity of levodopa at different doses on C17.2 neural stem cells and subsequent neuroprotection by pergolide were investigated. The cell viability was determined by the MTT assay. Cell proliferation was assayed by BrdU labeling, while apoptosis was detected by Annexin-V-FLUOS staining and flow cytometry. Levels of p53, Bax, Bcl-2, NFkB, cytochrome c, caspase-3 as well as cleavage of caspase-3 were measured by western blotting. We found levodopa induced a concentration- and time-dependent decrease in cell viability and proliferation. Apoptotic cells were observed at different stages, specifically 12 and 24 h following exposure to levodopa (200 microM). Elevated p53, Bax, cytochrome c, caspase-3 and active fragments of caspase-3 protein were observed in the cells exposed to levodopa. These alterations were partly inhibited by pergolide, a dopamine receptor agonist, while Bcl-2 and NFkB p65 levels remained constant at the various time-points in all the groups examined. These observations indicate that levodopa at high concentrations (> or = 200 microM) was neurotoxic to C17.2 neural stem cells via inhibition of DNA synthesis and cell proliferation. Activation of the mitochondria-dependent pathway and caspase-3 protease may contribute to the mechanism by which levodopa induces apoptosis. Pergolide, an anti-Parkinson drug, has a neuroprotective effect and partly blocks levodopa-induced cytotoxicity.

A meta-analysis of the response to chronic L-dopa in patients with schizophrenia: therapeutic and heuristic implications

RATIONALE

While it is generally believed that administration of the dopamine precursor levodopa ( L-dopa) exacerbates symptoms of schizophrenia, numerous reports suggest that adjunctive L-dopa may be beneficial. This body of literature has not been critically reviewed.

OBJECTIVES

On the basis of published studies, to determine whether L-dopa administered concomitantly with antipsychotic drugs provides a beneficial response in patients with schizophrenia.

METHODS

This review examined 30 studies involving 716 patients. Due to wide methodological variability and limited statistical information, only five studies encompassing 160 patients could be included in a meta-analysis. The others were evaluated qualitatively.

RESULTS

When L-dopa was added to antipsychotic drugs, the overall improvement was moderate ( d=0.71) and highly significant ( P<0.0001). There were 16 other studies in which L-dopa was added to antipsychotic drugs, but which did not meet criteria for inclusion in the meta-analysis. In these, worsening occurred in less than 20% of patients; the percentage of improved patients varied widely but had a central tendency around 50%.

CONCLUSIONS

. In patients already on antipsychotic drugs, the addition of L-dopa can be beneficial. Dopamine agonists merit further consideration as adjuncts to antipsychotic drugs in the treatment of schizophrenia.

Deficits in a tricarboxylic acid cycle enzyme in brains from patients with Parkinson's disease

Parkinson's disease (PD) is associated with mitochondrial dysfunction, specifically a deficiency of complex I of the electron transport chain. Most, although not all, studies indicate that this deficiency is limited to brain regions with neurodegeneration. The current studies tested for deficiencies in other mitochondrial components in PD brain in a neuropathologically unaffected region where the abnormality cannot be attributed to secondary effects of neurodegeneration. The activity of a key (and arguably rate-limiting) tricarboxylic acid cycle enzyme, the alpha-ketoglutarate dehydrogenase complex (KGDHC), was measured in the cerebellum of patients with PD. Activity in 19 PD brains was 50.5% of that in 18 controls matched for age, sex, post-mortem interval, and method of preservation (P<0.0019). The protein subunits of KGDHC were present in normal amounts in PD brains, indicating a relatively discrete abnormality in the enzyme. The activities of another mitochondrial enzyme, glutamate dehydrogenase (GDH), were normal in PD brains. These results demonstrate that specific reductions in KGDHC occur even in pathologically unaffected areas in PD, where the decline is unlikely to be a non-specific result of neurodegeneration. Reductions in the activity of this enzyme, if widespread in the brain, may predispose vulnerable regions to further damage.

Regular exercise improves cognitive function and decreases oxidative damage in rat brain

The biochemical mechanisms by which regular exercise significantly benefits health and well being, including improved cognitive function, are not well understood. Four-week-old (young) and 14-month-old (middle aged) Wistar rats were randomly assigned to young control and young exercised, middle-aged control and middle-aged exercised groups. Exercise groups were exposed to a swimming regime of 1 h a day, 5 days a week for 9 weeks. The passive avoidance test showed that middle-aged exercised rats had significantly (P<0.05) better short- (24 h) and long-term (72 h) memory than aged-matched control rats. Conditioned pole-jumping avoidance learning was improved markedly in both age groups by exercise. Brain thiobarbituric acid-reactive substances and 8-hydroxy-2'deoxyguanosine content in the DNA did not change significantly, while the protein carbonyl levels decreased significantly (P<0.05) in both exercised groups. This decrease was accompanied by an increase in the chymotrypsin-like activity of proteasome complex in the exercised groups, whereas trypsin-like activity did not differ significantly between all groups. The DT-diaphorase activity increased significantly (P<0.05) in the brain of young exercised animals. These data show that swimming training improves some cognitive functions in rats, with parallel attenuation of the accumulation of oxidatively damaged proteins.