Differences in dopamine efflux induced by MPP+ and beta-carbolinium in the striatum of conscious rats

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

9-Methyl-beta-carboline has restorative effects in an animal model of Parkinson's disease

In a previous study, a primary culture of midbrain cells was exposed to 9-methyl-beta-carboline for 48 h, which caused an increase in the number of tyrosine hydroxylase-positive cells. Quantitative RT-PCR revealed increased transcription of genes participating in the maturation of dopaminergic neurons. These in vitro findings prompted us to investigate the restorative actions of 9-methyl-beta-carboline in vivo. The compound was delivered for 14 days into the left cerebral ventricle of rats pretreated with the neurotoxin 1-methyl-4-phenyl-pyridinium ion (MPP+) for 28 days applying a dose which lowered dopamine by approximately 50%. Interestingly, 9-methyl-beta-carboline reversed the dopamine-lowering effect of the neurotoxin in the left striatum. Stereological counts of tyrosine hydroxylase-immunoreactive cells in the substantia nigra revealed that the neurotoxin caused a decrease in the number of those cells. However, when treated subsequently with 9-methyl-beta-carboline, the number reached normal values. In search of an explanation for the restorative activity, we analyzed the complexes that compose the respiratory chain in striatal mitochondria by 2-dimension gel electrophoresis followed by MALDI-TOF peptide mass fingerprinting.We found no changes in the overall composition of the complexes. However, the activity of complex I was increased by approximately 80% in mitochondria from rats treated with MPP+ and 9-methyl-beta-carboline compared to MPP+ and saline and to sham-operated rats, as determined by measurements of nicotinamide adenine dinucleotide dehydrogenase activity. Microarray technology and single RT-PCR revealed the induction of neurotrophins: brain-derived neurotrophic factor, conserved dopamine neurotrophic factor, cerebellin 1 precursor protein, and ciliary neurotrophic factor. Selected western blots yielded consistent results. The findings demonstrate restorative effects of 9-methyl-beta-carboline in an animal model of Parkinson's disease that improve the effectiveness of the respiratory chain and promote the transcription and expression of neurotrophin-related genes.

The organic cation transporter-3 is a pivotal modulator of neurodegeneration in the nigrostriatal dopaminergic pathway

Toxic organic cations can damage nigrostriatal dopaminergic pathways as seen in most parkinsonian syndromes and in some cases of illicit drug exposure. Here, we show that the organic cation transporter 3 (Oct3) is expressed in nondopaminergic cells adjacent to both the soma and terminals of midbrain dopaminergic neurons. We hypothesized that Oct3 contributes to the dopaminergic damage by bidirectionally regulating the local bioavailability of toxic species. Consistent with this view, Oct3 deletion and pharmacological inhibition hampers the release of the toxic organic cation 1-methyl-4-phenylpyridinium from astrocytes and protects against 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced dopaminergic neurodegeneration in mice. Furthermore, Oct3 deletion impairs the removal of the excess extracellular dopamine induced by methamphetamine and enhances striatal dopaminergic terminal damage caused by this psychostimulant. These results may have far-reaching implications for our understanding of the mechanism of cell death in a wide range of neurodegenerative diseases and may open new avenues for neuroprotective intervention.

N-methylation underlying Parkinson's disease

The discovery of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) leads to the hypothesis that Parkinson's disease (PD) is maybe initiated or precipitated by environmental or endogenous toxins by the mechanism similar to that of MPTP in genetically-predisposed individuals. Endogenous analogs of MPTP, such as beta-carbolines (betaCs) and tetrahydroisoquinolines, have been proposed as possible causative candidates causing PD and are bioactivated into potential neurotoxins by N-methylation enzyme(s). These N-methylated betaCs and tetrahydroisoquinoline have been higher cerebrospinal levels in parkinsonian patients than age-matched controls. Thus, there is a hypotheses to influence the pathogenesis of PD, that is, the excess enzyme activity to activate neurotoxins, such as N-methyltransferase, might be higher in PDs. Indeed, simple betaCs, via N-methylation steps, induced bradykinesia with the decreased dopamine contents in the striatum and midbrain in C57/BL mice. In younger (65 years old) PD patients, the excretion amount of N(1)-methyl-nicotinamaide was significantly higher than that in younger controls. The protein amount of nicotinamide N-methyltransferase (NNMT) was also significantly higher in younger PD patients than that in younger controls. These findings described here would indicate that the excess N-methylation ability for azaheterocyclic amines, such as betaCs, before the onset had been implicated in PD pathogenesis. On the other hand, the contribution of aberrant cytochrome P450 or aldehyde oxidase activity acting on the pyridine ring, that could act as detoxification routes of endogenous neurotoxins, would be small in the etiology of PD.

L-Deprenyl prevents the cell hypoxia induced by dopaminergic neurotoxins, MPP(+) and beta-carbolinium: a microdialysis study in rats

N-Methyl-4-phenylpyridinium (MPP(+)) and 2,9-di-methyl-norharmanium (2,9-Me2NH(+)), which is a beta-carbolinium proposed as an endogenous MPP(+)-like toxin underlying Parkinson's disease, are strong mitochondrial toxins. We have measured the extracellular lactate levels as a marker for the in vivo cell hypoxia in the striatum of freely moving rats. The perfusions with MPP(+) and 2,9-Me2NH(+) increased extracellular lactate levels in a dose-dependent manner. These increases in lactate levels were significantly prevented by the co-perfusion with 10 microM L-deprenyl, a selective monoamine oxidase (MAO)-B inhibitor, but not by pargyline, a non-specific MAO inhibitor. The increase in extracellular lactate levels was considered to be the reflection of the cell damage resulted from the impairment of mitochondrial function. The present results suggested that L-deprenyl would rescue nerve cells from these toxins through the direct influence on the mitochondrial electron transport.

Structural significance of azaheterocyclic amines related to Parkinson's disease for dopamine transporter

We have evaluated the neuronal uptake of 12 neutral and quaternary azaheterocyclic amines that are possible candidates for idiopathic Parkinson's disease via dopamine transporter of striatal synaptosomes. The double-reciprocal plots for dopamine transporter obtained from Wistar rat and C57BL/6 mouse synaptosomes with N-methyl-4-phenylpyridinium cation (MPP+) as a substrate were identical to each other. Neutral beta-carbolines and tetrahydroisoquinolines were unfavorable substrates for dopamine transporter. The quarternization of these compounds strikingly increased the affinity for dopamine transporter with 2-10 times greater Km and 10 times smaller Vmax values than MPP+. Although catechol tetrahydroisoquinolines were weak substrates, their quarternization reduced their original properties as substrates for dopamine transporter. These results provide both topographic and electrogenic information of azaheterocyclic amines for the dopamine transporter-mediated influx. The intramolecular distance between the N-atom and the centroid of the benzene ring could be an important factor for the recognition of binding site of dopamine transporter, and an adequate net charge similar to dopamine would be further required for translocation into the cells.

Deprenyl decreases an endogenous parkinsonism-inducing compound, 1-benzyl-1,2,3,4-tetrahydroisoquinoline in mice: in vivo and in vitro studies

We examined the effect of deprenyl, a promising drug for the therapy of Parkinson's disease on the formation of a parkinsonism-inducing compound, 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ). The 1BnTIQ content was significantly decreased in the brain of deprenyl-treated mouse in vivo, and deprenyl also inhibited 1BnTIQ formation from phenethylamine by a mouse brain homogenate supernatant in vitro. In vivo, the content of a parkinsonism-preventing compound, 1-methyl-1,2,3, 4-tetrahydroisoquinoline (1MeTIQ) was slightly increased in mice injected with deprenyl. The marked decrease of the ratio of 1BnTIQ to 1MeTIQ might play a role in the clinical effect of deprenyl.

Octreotide dependency and headache: a case report

We report on headache induced by a somatostatin octapeptide analog (octreotide) used for the treatment of acromegaly. This "rebound" headache has severe tension-type characteristics and occurs every 6-8 h. It resolves dramatically within minutes with octreotide administration. This is the first report of headache developing under treatment with octreotide.