Effect of nomifensine on the toxicity of 6-hydroxydopamine for mesotelencephalic dopamine neurons

Ethanolic extract of fructus Alpinia oxyphylla protects against 6-hydroxydopamine-induced damage of PC12 cells in vitro and dopaminergic neurons in zebrafish

In an attempt to understand the neuroprotective effect of Fructus Alpinia oxyphylla (AOE) and to elucidate its underlying mechanism of action, the ethanolic extract of AOE was investigated using zebrafish and PC12 cell models. AOE prevented and restored 6-hydroxydopamine (6-OHDA)-induced dopaminergic (DA) neuron degeneration and attenuated a deficit of locomotor activity in a zebrafish (Danio rerio) model of Parkinson's disease (PD). Treatment with AOE increased the viability of 6-OHDA-treated PC12 cells in vitro in a dose-dependent manner by attenuating cellular apoptosis. However, protocatechuic acid (PCA) and chrysin, two known polyphenol components of AOE, could not reproduce the neuroprotective activity of AOE in the PD zebrafish or PC12 cell models. A mechanistic study found that the protective effect of AOE against 6-OHDA-induced neuronal injury involved anti-inflammatory action (down-regulation of gene expression of IL-1β and TNF-α) and anti-oxidative action (inhibition of NO production and iNOS expression in PC12 cells). Moreover, the PI3K-AKT pathway might be part of the mechanism of neuroprotection of AOE. The results of this research are expected to provide a scientific rationale for the use of AOE in the treatment of PD. However, it is important that the active components that contribute to the neuroprotective action of AOE are identified and characterized.

Manganese superoxide dismutase protects against 6-hydroxydopamine injury in mouse brains

Dopaminergic neurons of the substantia nigra are susceptible to toxin-based insults. Intrastriatal injection of 6-hydroxydopamine results in selective toxicity to these neurons. A mechanistic role for reactive oxygen species is supported by observations that antioxidants confer protection from 6-hydroxydopamine. Although cell culture studies have suggested extracellular or nonmitochondrial mechanisms in 6-hydroxydopamine toxicity, the compartmentalization of oxidative injury mechanisms is incompletely defined in vivo. Transgenic mice overexpressing mitochondrial manganese superoxide dismutase or extracellular superoxide dismutase received unilateral intrastriatal injections of 6-hydroxydopamine. Mice that overexpress manganese superoxide dismutase showed significantly smaller striatal lesions than littermate controls. There were no differences in nonspecific striatal injury associated with contralateral vehicle injection. Manganese superoxide dismutase overexpression also protected against loss of neuronal cell bodies in the substantia nigra. In contrast, mice overexpressing extracellular superoxide dismutase showed no protection from 6-hydroxydopamine toxicity in either brain region. Protection of the nigrostriatal system by overexpression of manganese superoxide dismutase supports a role for mitochondrially derived superoxide in 6-hydroxydopamine toxicity. Mitochondrial oxidative stress appears to be a common mechanism among diverse models of Parkinson disease, whether involving toxins, mutated genes, or cybrid cells containing patient mitochondria. Antioxidant therapies that target this subcellular compartment may prove promising.

Expression of dopamine and vesicular monoamine transporters and differential vulnerability of mesostriatal dopaminergic neurons

Numerous studies suggest that the dopamine transporter (DAT), responsible for dopamine reuptake, may act as a vulnerability factor in the pathogenesis of Parkinson's disease (PD) and the vesicular monoamine transporter (VMAT2), responsible for its vesicular storage, as a neuroprotective factor. However, the relevance of each on the differential vulnerability of midbrain DA cells remains unknown. Here we studied the relationship between the expression pattern (mRNA and protein) of both transporters and the differential vulnerability of midbrain DA cells in a model of PD (intracerebroventricular injection of 6-OHDA in rats) and in monkey and human midbrain. Our results revealed that the expression patterns for VMAT2 mRNA and protein and DAT mRNA are similar, with the highest levels in the rostromedial region of substantia nigra (SNrm), followed by the caudoventral region of SN (SNcv), the ventral tegmental area and pigmented parabrabraquial nucleus (VTA/PBP), and finally the linear and interfascicular nuclei (Li/IF). In contrast, the expression of DAT protein in rats, monkeys, and humans followed a caudoventrolateral-to-rostrodorsomedial decreasing gradient (SNcv > SNrm > VTA/PBP > Li/IF), matching the degeneration profile observed after intracerebroventricular injection of 6-OHDA and in PD. In addition, DAT blockade made all midbrain DA cells equally resistant to 6-OHDA. These data indicate that DAT protein levels, but not DAT mRNA levels, are closely related to the differential vulnerability of midbrain DA cells and that this relationship is unaffected by the relative levels of VMAT2. Furthermore, the difference between DAT mRNA and protein profiles suggests internuclear differences in its posttransductional regulation.

Fluoxetine at anorectic doses does not have properties of a dopamine uptake inhibitor

Although fluoxetine is a highly selective inhibitor of serotonin uptake in vitro and in vivo, some investigators have suggested that dopamine uptake inhibition may contribute to anorectic actions of fluoxetine. The present experiments were done to determine fluoxetine's effects in some animal protocols in which dopamine uptake inhibitors have characteristic actions. Mazindol prevented the depletion of striatal dopamine and its metabolites by amphetamine in iprindole-pretreated rats, but fluoxetine had no effect. Mazindol prevented the depletion of striatal dopamine and its metabolites by 6-hydroxydopamine injected intracerebroventricularly into rats, but fluoxetine had no effect. Mazindol enhanced the elevation of 3,4-dihydroxyphenylacetic acid concentration in rat brain after spiperone injection, but fluoxetine did not cause that effect. Fluoxetine did not mimic amfonelic acid in antagonizing the retention of alpha-methyl-m-tyramine invant striatum after the injection of alpha-methyl-m-tyrosine. These results show that fluoxetine, at doses that are effective in blocking the serotonin uptake carrier and causing anorexia, does not block the dopamine uptake carrier.

Relationship between sensorimotor neglect and the specificity, degree and locus of mesotelencephalic dopaminergic cell loss following 6-hydroxydopamine

The involvement of cell groups within the dopaminergic mesotelencephalic system in the development of the sensorimotor neglect syndrome was re-evaluated in two ways. Firstly, dopaminergic specificity of the neglect was further established by studying the relationship between nomifensine protection of dopamine cells against 6-hydroxydopamine damage and the degree of neglect which resulted. The sensorimotor neglect syndrome which developed following injection of 6-hydroxydopamine was diminished by concomitant treatment with nomifensine in parallel with the degree of protection afforded the dopaminergic cells. Non-specific damage produced by 6-hydroxydopamine was unaltered by nomifensine. Secondly, the role in sensorimotor neglect of both total cell damage, and damage to regional sub-classes of dopaminergic cells was considered. It was found that the extent of the resulting neglect was correlated with the overall damage to the substantia nigra and ventral tegmental area, rather than to any individual region within this dopaminergic system. There was a threshold, involving destruction of approximately one third of the system, below which no neglect syndrome developed. Certain regions, including the ventral tegmental area (VTA), showed a higher partial correlation with the extent of neglect than other regions. While specific lesioning of the A8 or A10 dopaminergic neurons is probably insufficient to produce a neglect syndrome, damage to these areas potentiates the severity of the neglect produced by nigrostriatal lesions. It appears that the involvement of the individual subclasses of the mesotelencephalic dopaminergic neurons in the neglect syndrome is more widespread than previously thought.