In vivo hydroxylation of the neurotoxin, 1-methyl-4-phenylpyridinium, and the effect of monoamine oxidase inhibitors: electrospray-MS analysis of intra-striatal microdialysates

MAO-B inhibitors: multiple roles in the therapy of neurodegenerative disorders?

Monoamine oxidases play a central role in catecholamine catabolism in the central nervous system. The biochemical and pharmacological properties of inhibitors of the monoamine oxidase type B are reviewed. The evidence for biochemical activities distinct from their ability to inhibit MAO-B is discussed, including possible antioxidative and antiapoptotic activities of these agents. The significance of these properties for the pharmacological management of Parkinson's disease and the evidence for a neuroprotective effect of one such agent (selegiline) is also discussed.

Ocular microdialysis: a continuous sampling technique to study pharmacokinetics and pharmacodynamics in the eye

The unique anatomy and physiology of the eye present many challenges to the successful development and delivery of ophthalmic drugs. Any therapeutic strategy developed to control the progression of anterior and posterior segment diseases requires continuous monitoring of effective drug concentrations in the relevant ocular tissues and fluids. Ocular microdialysis has gained popularity in recent years due to its ability to continuously monitor drug concentrations and substantially reduce the number of animals needed. The intrusive nature of ocular microdialysis experimentation has restricted these studies to animal models. This review article intends to highlight various aspects of ocular microdialysis and its relevance in examining the disposition of drugs in the anterior and posterior segments.

Liquid chromatographic-electrospray mass spectrometric determination of 1-methyl-4-phenylpyridine (MPP+) in discrete regions of murine brain

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is widely used as a neurotoxin in several models of Parkinson's disease in mice. MPTP is metabolized to 1-methyl-4-phenylpyridinium (MPP(+)), which is a mitochondrial toxicant of central dopamine (DA) neurons. There are species, strain, and age differences in sensitivity to MPTP. Simultaneous measurement of the MPTP active metabolite MPP(+) and dopamine (DA) in the brain would be helpful in mechanistic studies of this neurotoxin. The objective of this study was to develop a liquid chromatography-mass spectrometry (LC/MS) method for analysis of MPTP and MPP(+) in brain tissue and correlate these in the same sample with changes in DA measured via HPLC coupled with electrochemical detection. Twenty-five C57BL/6J7 8-week old female mice were used in the study. Mice were given a single subcutaneous injection of MPTP (20 mg/kg) and were sacrificed 1, 2, 4, or 8 h later. Zero time control mice received an injection of 0.9% normal saline (10 ml/kg) and were killed 1 h later. Brains were rapidly harvested and quickly frozen, and microdissected brain regions were placed in 0.1 M phosphate-citric acid buffer containing 20% methanol (pH 2.5). A new LC/MS method was successfully developed that utilized selected reaction monitoring (SRM) of MPP(+) m/z 170→127, 170→128, and 170→154 fragmentation for quantitation and area ratios (m/z 127)/(m/z 128) and (m/z 154)/(128) for identity confirmation. A similar SRM strategy from m/z 174 was unable to detect any significant levels of MPTP down to 0.4 ppb. According to this method, MPP(+) was detected in the nucleus accumbens (NA) and the striatum (ST), with the levels in the NA being 3-times higher than those in the ST. The advantage of this approach is that the tissue buffer used in this procedure allowed concurrent measurement of striatal DA, thus enabling direct correlation between accumulation of tissue MPP(+) and depletion of DA concentrations in discrete regions of the brain.

High-performance liquid chromatography/tandem mass spectrometry assay for the determination of 1-methyl-4-phenyl pyridinium (MPP+) in brain tissue homogenates

A high-throughput liquid chromatography/tandem mass spectrometry method has been developed for the quantitative assessment of 1-methyl-4-phenylpyridinium (MPP+) in brain tissue samples. This separation is based on reversed phase chromatography using formic acid and acetonitrile as the mobile phase. Using gradient separation conditions, MPP+ was resolved within 5 min and detected using tandem mass spectrometry in the positive ion electrospray mode. The limit of detection for MPP+ was found to be 1 fmol on column with a signal to noise ratio of 3:1. The assay has been used routinely in our laboratory for the measurement of MPP+ levels in brain tissue from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice, and can be used to distinguish neuroprotective efficacy and monoamine oxidase inhibition.

Development of a method for sample preparation for subsequent identification and measurement of 1,2,3,4-tetrahydroisoquinolines and other potentially neurotoxic compounds by high-performance liquid chromatography with ultraviolet and fluorescence detection in blood plasma of Parkinson's disease patients

We developed sample preparation methods for the detection of various biogenic phenylethylamine derivatives such as 3,4-dihydroxyphenylalanine, and their cyclisation products with aldehydes, i.e., 1,2,3,4-tetrahydroisoquinoline derivatives in blood samples. 1,2,3,4-Tetrahydroisoquinolines are considered to play an essential role as neurotoxic compounds in the pathomechanism of Parkinson's disease. We used reversed-phase high-performance liquid chromatography with ultraviolet and fluorescence detection for separation and identification. Ultrafiltration, protein precipitation and solid-phase extraction were investigated for purification of blood samples and enrichment of various compounds with a wide range of hydrophilicity and hydrophobicity. Protein precipitation by methanol and perchloric acid is a fast method to separate the analytes from the plasma matrix. A higher yield of the analytes is attained with prior addition of an alkylsulfonic acid giving a fine-grained precipitate. With the addition of ion pairing compounds into the sample it is possible to enrich not only lipophilic compounds such as norharman, tryptamine and melatonin, but also hydrophilic ones such as 3,4-dihydroxyphenylalanine by reversed-phase solid-phase extraction. Ultrafiltration is not useful as a screening method.

Local striatal infusion of MPP+ does not result in increased hydroxylation after systemic administration of 4-hydroxybenzoate

In vivo bilateral microdialysis in the rat striatum was used to investigate hydroxyl radical formation under basal conditions and after intrastriatal administration of the neurotoxin, 1-methyl-4-phenylpyridinium (MPP+). After a short equilibration period, 4-hydroxybenzoate (4HBZ), which scavenges hydroxyl radicals to produce 3,4-dihydroxybenzoate (34DHB), was injected intraperitoneally 15 min before infusion of MPP+. To evaluate the enzymatic contribution to hydroxyl radical formation, two other series of microdialyses were performed following administration of monoamine oxidase B inhibitors, either 1-deprenyl (selegiline) or MDL 72,974A [(E)-2-(4-fluorophenethyl)-3-fluoroallylamine hydrochloride]. Microdialysate samples were analyzed by high-performance liquid chromatography for catecholamines, 3,4-dihydroxyphenylacetate (DOPAC), homovanillate (HVA), along with the hydroxyl radical adduct, 34DHB and its precursor, 4HBZ. MPP+ administration resulted in a massive release of dopamine along with a decrease in DOPAC and HVA in all three groups. A striking effect in all three groups was noted in which MPP+ resulted in a decrease in interstitial 4HBZ to < 50% of the non-MPP+ -treated side. In absolute terms, the amount of 34DHB produced was low but similar in all three groups, even after unilateral MPP+ infusion. When 34DHB was normalized to 4HBZ release to account for differences in precursor availability, there were no significant differences in the 34DHB/4HBZ ratios either with or without MAO inhibitor treatment or after local MPP+ infusion. Systemic 4HBZ administration appears to result predominantly in intra-cellular sampling of hydroxyl radicals which produces different results from local infusion of trapping agents such as salicylate.