Enzymatic oxidation of the dopaminergic neurotoxin, 1(R), 2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, into 1,2(N)-dimethyl-6,7-dihydroxyisoquinolinium ion

Neuroprotective Activity of Enantiomers of Salsolinol and N-Methyl-(R)-salsolinol: In Vitro and In Silico Studies

Salsolinol (1-methyl-1,2,3,4-tetrahydroisoquinoline-6,7-diol) is a close structural analogue of dopamine with an asymmetric center at the C1 position, and its presence in vivo, both in humans and rodents, has already been proven. Yet, given the fact that salsolinol colocalizes with dopamine-rich regions and was first detected in the urine of Parkinson's disease patients, its direct role in the process of neurodegeneration has been proposed. Here, we report that R and S enantiomers of salsolinol, which we purified from commercially available racemic mixture by means of high-performance liquid chromatography, exhibited neuroprotective properties (at the concentration of 50 μM) toward the human dopaminergic SH-SY5Y neuroblastoma cell line. Furthermore, within the study, we observed no toxic effect of N-methyl-(R)-salsolinol on SH-SY5Y neuroblastoma cells up to the concentration of 750 μM, either. Additionally, our molecular docking analysis showed that enantiomers of salsolinol should exhibit a distinct ability to interact with dopamine D2 receptors. Thus, we postulate that our results highlight the need to acknowledge salsolinol as an active dopamine metabolite and to further explore the neuroregulatory role of enantiomers of salsolinol.

Salsolinol Protects SH-SY5Y Cells Against MPP+ Damage and Increases Enteric S100-Immunoreactivity in Wistar Rats

A dopamine derivative, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, known as salsolinol (SAL), has increasingly gained attention since its first detection in the urine of Parkinson's disease patients treated with levodopa, and has been proposed as a possible neurotoxic contributor to the disease. Yet, so far, the neurobiological role of SAL remains unclear. Thus, the main aims of our study were to compare the neurotoxic potential of SAL with MPP⁺ (1-methyl-4-phenylpyridinium ion) in vitro, and to examine intestinal and metabolic alterations following intraperitoneal SAL administration in vivo. In vitro, SH-SY5Y neuroblastoma cell line was monitored following MPP⁺ and SAL treatment. In vivo, Wistar rats were subjected to SAL administration by either osmotic intraperitoneal mini-pumps or a single intraperitoneal injection, and after two weeks, biochemical and morphological parameters were assessed. SH-SY5Y cells treated with MPP⁺ (1000 μM) and SAL (50 µM) showed increase in cell viability and fluorescence intensity in comparison with the cells treated with MPP⁺ alone. In vivo, we predominantly observed decreased collagen content in the submucosal layer, decreased neuronal density with comparable ganglionic area in the jejunal myenteric plexus, and increased glial S100 expression in both enteric plexuses, yet with no obvious signs of inflammation. Besides, glucose and triglycerides levels were lower after single SAL-treatment (200 mg/kg), and low- to high-density lipoprotein (LDL/HDL) ratio and aspartate to alanine aminotransferases (AST/ALT) ratio levels were higher after continuous SAL-treatment (200 mg/kg in total over 2 weeks). Low doses of SAL were non-toxic and exhibited pronounced neuroprotective properties against MPP⁺ in SH-SY5Y cell line, which supports the use of SAL as a reference compound for in vitro studies. In vivo results give insight into our understanding of gastrointestinal remodeling following intraperitoneal SAL administration, and might represent morphological correlates of a microglial-related enteric neurodegeneration and dopaminergic dysregulation.

Behavioral consequences of the downstream products of ethanol metabolism involved in alcohol use disorder

Research concerning Alcohol Use Disorder (AUD) has previously focused primarily on either the behavioral or chemical consequences experienced following ethanol intake, but these areas of research have rarely been considered in tandem. Compared with other drugs of abuse, ethanol has been shown to have a unique metabolic pathway once it enters the body, which leads to the formation of downstream metabolites which can go on to form biologically active products. These metabolites can mediate a variety of behavioral responses that are commonly observed with AUD, such as ethanol intake, reinforcement, and vulnerability to relapse. The following review considers the preclinical and chemical research implicating these downstream products in AUD and proposes a chemobehavioral model of AUD.

Neurotoxicity and Underlying Mechanisms of Endogenous Neurotoxins

Endogenous and exogenous neurotoxins are important factors leading to neurodegenerative diseases. In the 1980s, the discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) contributes to Parkinson's disease (PD) symptoms led to new research investigations on neurotoxins. An abnormal metabolism of endogenous substances, such as condensation of bioamines with endogenous aldehydes, dopamine (DA) oxidation, and kynurenine pathway, can produce endogenous neurotoxins. Neurotoxins may damage the nervous system by inhibiting mitochondrial activity, increasing oxidative stress, increasing neuroinflammation, and up-regulating proteins related to cell death. This paper reviews the biological synthesis of various known endogenous neurotoxins and their toxic mechanisms.

The Mechanism of Action of Salsolinol in Brain: Implications in Parkinson's Disease

1-Methyl-1,2,3,4-tetrahydroisoquinoline-6,7-diol, commonly known as salsolinol, is a compound derived from dopamine. It was first discovered in 1973 and has gained attention for its role in Parkinson's disease. Salsolinol and its derivatives were claimed to play a role in the pathogenesis of Parkinson's disease as a neurotoxin that induces apoptosis of dopaminergic neurons due to its structural similarity to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its ability to induce Parkinsonism. In this article, we discussed the biosynthesis, distribution and blood-brain barrier permeability of salsolinol. The roles of salsolinol in a healthy brain, particularly the interactions with enzymes, hormone and catecholamine, were reviewed. Finally, we discussed the involvement of salsolinol and its derivatives in the pathogenesis of Parkinson's disease.

Salsolinol: an Unintelligible and Double-Faced Molecule-Lessons Learned from In Vivo and In Vitro Experiments

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is a tetrahydroisoquinoline derivative whose presence in humans was first detected in the urine of Parkinsonian patients on l-DOPA (l-dihydroxyphenylalanine) medication. Thus far, multiple hypotheses regarding its physiological/pathophysiological roles have been proposed, especially related to Parkinson’s disease or alcohol addiction. The aim of this review was to outline studies related to salsolinol, with special focus on in vivo and in vitro experimental models. To begin with, the chemical structure of salsolinol together with its biochemical implications and the role in neurotransmission are discussed. Numerous experimental studies are summarized in tables and the most relevant ones are stressed. Finally, the ability of salsolinol to cross the blood–brain barrier and its possible double-faced neurobiological potential are reviewed.

Occurrence and distribution of salsolinol-like compound, 1-acetyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (ADTIQ) in parkinsonian brains

Parkinson's disease (PD) arises from the loss of dopaminergic neurons in the substantia nigra. 1-Methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) is well known to cause Parkinsonism in humans with neurotoxicity specific for dopaminergic neurons. The experience with MPTP supports the hypothesis that endogenous or xenobiotic neurotoxins are involved in the pathogenesis of PD in humans. In our study, 1-acetyl-6, 7-dihydroxy-1, 2, 3, 4-tetrahydro-isoquinoline (ADTIQ), a novel compound, was found in frozen human brain tissues. The formation of ADTIQ was demonstrated using dopamine and methylglyoxal under physiological conditions. Methylglyoxal is a by-product of glycolysis. ADTIQ and its precursors, dopamine and methylglyoxal, were detected in different regions of frozen human brains such as the substantia nigra, caudate nucleus, putamen, frontal cortex, and the cerebellum. A significant difference in ADTIQ levels between control and Parkinson's patients was found; for instance, the ADTIQ level in putamen of PD patients was 0.76 ± 0.27 nmol/g compared to 0.10 ± 0.01 nmol/g in control. Our results might indicate that ADTIQ is possibly related to Parkinson's disease.

Revisiting the controversial role of salsolinol in the neurobiological effects of ethanol: old and new vistas

The possible involvement of salsolinol (Sal), an endogenous condensation product of ACD (the first metabolite of ethanol) and dopamine, in the neurochemical basis underlying ethanol action has been repeatedly suggested although it has not been unequivocally established, still being a controversial matter of debate. The main goal of this review is to evaluate the presumed contribution of Sal to ethanol effects summarizing the reported data since the discovery in the 1970s of Sal formation in vitro during ethanol metabolism until the more recent studies characterizing its behavioral and neurochemical effects. Towards this end, we first analyze the production and detection of Sal, in different brain areas, in basal conditions and after alcohol consumption, highlighting its presence in regions especially relevant in regulating ethanol-drinking behaviour and the importance of the newly developed methods to differentiate both enantiomers of Sal which could help to explain some previous negative findings. Afterwards, we review the behavioral and neurochemical studies. Finally, we present and discuss the previous and current enunciated mechanisms of action of Sal in the CNS.

Effect of (R)-salsolinol and N-methyl-(R)-salsolinol on the balance impairment between dopamine and acetylcholine in rat brain: involvement in pathogenesis of Parkinson disease

BACKGROUND

Parkinson disease (PD), a progressive neurodegenerative disease, affects at least 1% of population above the age of 65. Although the specific etiology of PD remains unclear, recently the endogenous neurotoxins such as (R)-salsolinol [(R)-Sal] and N-methyl-(R)-salsolinol [(R)-NMSal] have been thought to play a major role in PD. Much interest is focused on the degeneration of dopamine neurons induced by these neurotoxins. However, little literature is available on the impact of endogenous neurotoxins on the balance between dopamine (DA) and acetylcholine (ACh).

METHODS

After injection of (R)-Sal or (R)-NMSal into the rat brain striatum, the concentrations of DA and its metabolites were detected by HPLC with electrochemical detection. We assessed the influence of neurotoxins on acetylcholinesterase (AChE) activity and developed a microdialysis-electrochemical device to measure ACh concentrations with enzyme-modified electrodes.

RESULTS

(R)-Sal and (R)-NMSal led to concentration-dependent decreases in the activity of AChE. ACh concentrations in striatum treated with (R)-Sal or (R)-NMSal were increased to 131.7% and 239.8% of control, respectively. As to the dopaminergic system, (R)-NMSal caused a significant decrease in DA concentrations and (R)-Sal reduced the concentrations of DA metabolites in the striatum.

CONCLUSIONS

(R)-Sal and (R)-NMSal exerted a considerable effect on the balance between DA and ACh by impairing the cholinergic system as well as the dopaminergic system. It is likely that the disruption of balance between DA and ACh plays a critical role in the pathogenesis of neurotoxin-induced PD.

Type A monoamine oxidase is the target of an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, leading to apoptosis in SH-SY5Y cells

Mitochondrial monoamine oxidase (MAO) has been considered to be involved in neuronal degeneration either by increased oxidative stress or protection with the inhibitors of type B MAO (MAO-B). In this paper, the role of type A MAO (MAO-A) in apoptosis was studied using human neuroblastoma SH-SY5Y cells, where only MAO-A is expressed. An endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, an MAO-A inhibitor, reduced membrane potential, DeltaPsim, in isolated mitochondria, and induced apoptosis in the cells, which 5-hydroxytryptamine, an MAO-A substrate, prevented. In contrast, beta-phenylethylamine, an MAO-B substrate, did not suppress the DeltaPsim decline by N-methyl(R)salsolinol. The binding of N-methyl(R)salsolinol to mitochondria was inhibited by clorgyline, a MOA-A inhibitor, but not by (-)deprenyl, an MAO-B inhibitor. RNA interference targeting MAO-A significantly reduced the binding of N-methyl(R)salsolinol with simultaneous reduction in the MAO activity. To examine the intervention of MAO-B in the apoptotic process, human MAO-B was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even although the activity and protein of MAO increased markedly. These results demonstrate a novel function of MAO-A in the binding of neurotoxins and the induction of apoptosis, which may account for neuronal cell death in neurodegenerative disorders, including Parkinson's disease.

Dopamine-Derived Salsolinol Derivatives as Endogenous Monoamine Oxidase Inhibitors: Occurrence, Metabolism and Function in Human Brains

Salsolinol, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, is an endogenous catechol isoquinoline detected in humans by M. Sandler. In human brain, a series of catechol isoquinolines were identified as the condensation products of dopamine or other monoamines with aldehydes or keto-acids. Recently selective occurrence of the (R)enantiomers of salsolinol derivatives was confirmed in human brain, and they are synthesized by enzymes in situ, but not by the non-enzymatic Pictet-Spengler reaction. A (R)salsolinol synthase catalyzes the enantio-specific synthesis of (R)salsolinol from dopamine and acetaldehyde, and (R)salsolinol N-methyltransferase synthesizes N-methyl(R)salsolinol, which is further oxidized into 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion by non-enzymatic and enzymatic oxidation. The step-wise reactions, N-methylation and oxidation, induce the specified distribution of the N-methylated and oxidized derivatives in the human nigro-striatum, suggesting that these derivatives may be involved in the function of dopamine neurons under physiological and pathological conditions. As shown by in vivo and in vitro experiments, salsolinol derivatives affect the levels of monoamine neurotransmitters though the inhibition of enzymes related in the metabolism of catechol- and indoleamines. In addition, the selective neurotoxicity of N-methyl(R)salsolinol to dopamine neurons was confirmed by preparation of an animal model of Parkinson's disease in rats. The involvement of N-methyl(R)salsolinol in the pathogenesis of Parkinson's disease was further indicated by the increase in the N-methyl(R)salsolinol levels in the cerebrospinal fluid and that in the activity of its synthesizing enzyme, a neural (R)salsolinol N-methyltransferase, in the lymphocytes prepared from parkinsonian patients. N-methyl(R)salsolinol induces apoptosis in dopamine neurons, which is mediated by death signal transduction in mitochondria. In addition, salsolinol was found to function as a signal transmitter for the prolactin release in the neuro-intermediate lobe of the brain. These results are discussed in relation to role of dopamine-derived endogenous salsolinol derivatives as the regulators of neurotransmission, dopaminergic neurotoxins and neuro-hormonal transmitters in the human brain.

Dopamine-derived endogenous N-methyl-(R)-salsolinol: its role in Parkinson's disease

A dopamine-derived alkaloid, N-methyl-(R)-salsolinol [NM(R)Sal], enantioselectively occurs in human brains and accumulates in the nigrostriatal system. It increases in the cerebrospinal fluid (CSF) of parkinsonian patients and the activity of a neutral (R)-salsolinol [(R)Sal] N-methyltransferase, a key enzyme in the biosynthesis of this toxin, increases in the lymphocytes from parkinsonian patients, suggesting its involvement in the pathogenesis of Parkinson's disease (PD). The studies of animal and cellular models of PD proved that this isoquinoline is selectively cytotoxic to dopamine neurons. Using human dopaminergic SH-SY5Y cells, NM(R)Sal induces apoptosis by the activation of the apoptotic cascade initiated in mitochondria. In this article, we review the recent advance in proving our hypothesis that the dopamine-derived neurotoxin causes the selective depletion of dopamine neurons in PD.

Involvement of endogenous N-methyl(R)salsolinol in Parkinson's disease: induction of apoptosis and protection by (-)deprenyl

An endogenous dopamine-derived N-methyl(R)salsolinol has been suggested to be involved in the pathogenesis of Parkinson's disease. In Parkinson's disease, the level of N-methyl(R)salsolinol increased in cerebrospinal fluid and the high activity of a synthesizing enzyme, (R)salsolinol N-methyltransferase, was detected in lymphocytes. This isoquinoline induced apoptotic DNA damage in human dopaminergic neuroblastoma SH-SY5Y cells. Among catechol isoquinolines, only N-methylsalsolinol induced apoptosis in the cells, and the scavengers of hydroxyl radicals and antioxidants suppressed DNA damage, suggesting that reactive oxygen species initiate apoptosis. The isoquinoline activated caspase-3 like proteases and a caspase-3 inhibitor protected the cells from DNA damage. (-)Deprenyl, but neither clorgyline nor pargyline, prevented apoptotic cell death. The mechanism of the protection was due to stabilization of mitochondrial membrane potential reduced by the toxin. In Parkinson's disease apoptosis may be induced in dopamine neurons by this endogenous neurotoxin, and (-)deprenyl may protect them from apoptotic death process.

Apoptosis induced by an endogenous neurotoxin, N-methyl(R)salsolinol, in dopamine neurons

A dopamine-derived neurotoxin, 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2, 3,4-tetrahydroisoquinoline [N-methyl(R)salsolinol] was found to cause parkinsonian in rats and to deplete selectively dopamine neurons in the substantia nigra after infusion in the striatum. This isoquinoline occurs enantio-specifically in the nigra-striatum of human brains. The biosynthesis from dopamine is catalyzed by two enzymes, (R)salsolinol synthase and (R)salsolinol N-methyltransferase. The isoquinoline increases in the cerebrospinal fluid from parkinsonian patients, and the increase is ascribed to high activity of its synthesizing neutral (R)salsolinol N-methyltransferase, as shown by analyses in lymphocytes. The cell death caused by this neurotoxin in dopaminergic human neuroblastoma SH-SY5Y cells proved to be apoptotic. Apoptosis by this neurotoxin is mediated by intracellular sequential process, loss of mitochondrial membrane potential, activation of caspases and DNA fragmentation. These results are discussed in relation to the role of apoptosis in neurodegenerative diseases and the involvement of the endogenous toxin in the pathogenesis of Parkinson's disease.

Determination of dopamine and dopamine-derived (R)-/(S)-salsolinol and norsalsolinol in various human brain areas using solid-phase extraction and gas chromatography/mass spectrometry

Using a solid-phase extraction procedure and a gas chromatographic-mass spectrometric (GC/MS) method the levels of dopamine and the levels of dopamine-derived salsolinol (SAL) and norsalsolinol (NorSAL) were determined in human brain areas involved in the etiology of alcoholism, parkinsonism and other diseases. The possibility that biosynthesis of salsolinol occurs through a stereospecific enzymatic reaction was considered. Using a two-step derivatization with N-methyl-N-trimethylsilyltrifluoracetamide (MSTFA) and the chiral reagent (R)-(-)-2-phenylbutyryl chloride, baseline separated peaks of (R)- and (S)-SAL were obtained. Both enantiomers were found in human brain samples with no correlations between levels of salsolinol and dopamine. These findings do not support the hypothesis that only an enantio-selective synthesis of (R)-SAL by a putative salsolinol synthase is responsible for the in vivo formation. In our opinion, non-enzymatic formation of salsolinol via the Pictet-Spengler reaction reveals both salsolinol enantiomers and an additional enzymatic synthesis of only (R)-SAL explains the enantiomer ratio (R)-/(S)-SAL of approximately 2.

Cell death of dopamine neurons in aging and Parkinson's disease

Dopamine neurons in the substantia nigra of human brain are selectively vulnerable and the number decline by aging at 5-10% per decade. Enzymatic and non-enzymatic oxidation of dopamine generates reactive oxygen species, which induces apoptotic cell death in dopamine neurons. Parkinson's disease (PD) is also caused by selective cell death of dopamine neurons in this brain region. The pathogenesis of Parkinson's disease remains to be an enigma, but it was found that an endogenous MPTP-like neurotoxin, 1(R), 2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [N-methyl(R)salsolinol, NM(R)Sal], may be one of the pathogenic agents of PD. NM(R)Sal increases in cerebrospinal fluid from untreated parkinsonian patients, and two enzymes, a (R)salsolinol synthase and a neutral N-methyltransferase, synthesize this neurotoxin in the nigro-striatum. The activity of a neutral N-methyltransferase is significantly higher in lymphocytes from parkinsonian patients than in control. The mechanism of cell death by this toxin was proved to be by the induction of apoptosis, by use of dopaminergic SH-SY5Y cells. The apoptosis was suppressed by anti-oxidants, suggesting that the generation of reactive oxygen species may initiate cellular death process. These results indicate that in aging and PD oxidative stress induces degeneration of dopamine neurons, and the antioxidant therapy may delay the decline of dopamine neurons in the brain.

A systematic regional study of dopamine and dopamine-derived salsolinol and norsalsolinol levels in human brain areas

Dopamine and the dopamine-derived tetrahydroisoquinoline alkaloids salsolinol and norsalsolinol were measured by high-performance liquid chromatography with electrochemical detection in 15 regions of the human brain. The regional distribution of dopamine in 32 brains was similar to previous reports with highest concentrations in the basal ganglia, especially in the striatum, followed by the substantia nigra and the hypothalamus. Significant amounts of salsolinol and norsalsolinol were only found in these dopamine-rich areas, whereas in the other regions no alkaloids were detected. These findings suggest that the concentration of the substrate dopamine may determine the alkaloid level during in vivo formation.

An endogenous MPTP-like dopaminergic neurotoxin, N-methyl(R)salsolinol, in the cerebrospinal fluid decreases with progression of Parkinson's disease

There have been an increasing number of evidences indicating that dopamine-derived N-methyl(R)salsolinol is an endogenous MPTP-like neurotoxin to cause Parkinson's disease. In the cerebrospinal fluid from newly diagnosed untreated patients with Parkinson's disease, the level of this toxin was found to increase significantly, compared to control and a disease control, multiple system atrophy. The effects of the disease duration and the medication on the level of N-methyl(R)salsolinol were studied from the same patients. After about a 2-year period, the level was significantly reduced. The depletion of dopamine neurons by the disease progression may account for the reduction of the neurotoxin level, whereas L-DOPA therapy did not seem to affect the level of this toxin, even though the enhanced dopamine turnover. The results suggest that N-methyl(R)salsolinol level in the cerebrospinal fluid may indicate remaining dopamine neurons in the parkinsonian brain.

Oxidation of N-methyl(R)salsolinol: involvement to neurotoxicity and neuroprotection by endogenous catechol isoquinolines

1(R), 2(N)-Dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, N-methyl(R)salsolinol, is a potent dopaminergic neurotoxin to induce parkinsonism in rats. The cytotoxicity of N-methyl(R)salsolinol proved to be ascribed to its oxidation into cytotoxic 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion with generation of hydroxyl radical. The isoquinolinium ion caused massive necrosis in the striatum, whereas N-methyl(R)salsolinol depleted selectively dopaminergic neurons in the substantia nigra without necrotic tissue reaction. N-Methyl(R)salsolinol induced DNA damage to human neuroblastoma SH-SY5Y cells, which could be prevented by anti-oxidants and cycloheximide. These results suggest that oxidative stress through oxidation of N-methyl(R)salsolinol induces apoptotic cell death. On the other hand, (R)salsolinol proved to scavenge hydroxyl radical produced by oxidation of dopamine. The neurotoxicity and neuroprotection of catechol isoquinolines may be ascribed to their oxidation and scavenging of radicals.

(R)salsolinol N-methyltransferase activity increases in parkinsonian lymphocytes

Recently, an endogenous catechol isoquinoline, 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [N-methyl(R)salsolinol], was proved to be a neurotoxin specific for dopamine neurons by in vivo and in vitro experiments. This N-methyl(R)salsolinol was found to increase significantly in the cerebrospinal fluid of untreated parkinsonian patients, suggesting its possible involvement in the pathogenesis of Parkinson's disease. To clarify the mechanism of the increase, the activity of enzymes related to the metabolism of the neurotoxin was examined in lymphocytes prepared from parkinsonian patients and controls. In patients with Parkinson's disease, the activity of a neutral N-methyltransferase, measured by using (R)salsolinol as a substrate, was found to increase significantly (100.2 +/- 81.8 pmol/min/mg of protein) in comparison with that in controls (18.9 +/- 15.0 pmol/min/mg of protein). The distribution of the activity was bimodal in the parkinsonian patients, whereas it was singular in controls. The activity of other related enzymes, an alkaline N-methyltransferase and N-methyl(R)salsolinol oxidase, in parkinsonian lymphocytes was the same as in controls. Increase of the neutral N-methyltransferase may be an endogenous factor in the pathogenesis of Parkinson's disease.

DNA strand scission and PC12 cell death induced by salsolinol and copper

The naturally occurring neurotoxin, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol; SAL), has been speculated to contribute to Parkinson's disease and neuropathology of chronic alcoholism. In the present study, we found the capability of SAL to cause DNA cleavage in the presence of Cu(ll). Incubation of SAL and CuCl2 with calf thymus DNA caused strand breaks. Likewise, SAL in combination with Cu(ll) mediated the strand scission in øX174 RFI supercoiled DNA in a time-related manner. Neither Cu(ll) nor the catechol alone induced any appreciable DNA cleavage. The reaction of SAL with Cu(ll) was accompanied by the reduction of Cu(ll) to Cu(I). Furthermore, SAL induced cell death in cultured PC12 cells, which was exacerbated by Cu(ll). From these data, it seems likely that SAL undergoes redox cycling catalyzed by Cu(ll) to generate reactive species which may be responsible for the neurotoxic action of this catechol isoquinoline.

A neutral N-methyltransferase activity in the striatum determines the level of an endogenous MPP+-like neurotoxin, 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion, in the substantia nigra of human brains

An endogenous MPTP-like dopaminergic neurotoxin, N-methyl(R)salsolinol, increases in the parkinsonian cerebrospinal fluid and accumulates in the human nigro-striatum. An N-methyltransferase specific for (R)salsolinol was found in human brain with optimal pH at 7.0 and 8.5. The correlation of the enzyme activity with the level of N-methyl(R)salsolinol and its oxidation product, 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion was examined in the brain regions. Neutral N-methyltransferase activity in the striatum was found to correlate with the level of the endogenous MPP+-like isoquinolinium ion in the substantia nigra (P < 0.001). Considering that this neutral N-methyltransferase activity increases in parkinsonian lymphocytes, the enzyme may be an endogenous factor in the pathogenesis of Parkinson's disease.

A neurotoxin N-methyl(R)salsolinol induces apoptotic cell death in differentiated human dopaminergic neuroblastoma SH-SY5Y cells

In retinoic acid-differentiated SH-SY5Y cells an endogenous neurotoxin N-methyl(R)salsolinol induced apoptotic cell death. Using a single cell gel electrophoresis (comet) assay, DNA damage was quantitatively measured, and it was found to depend on the concentration of N-methyl(R)salsolinol and the incubation time up to 6 h. The differentiated cells were more sensitive to N-methyl(R)salsolinol than the undifferentiated cells. Radical scavengers protected the cells from DNA damage, indicating oxidative stress is involved in the apoptotic cell process. These results suggest that apoptosis induced by endogenous neurotoxins might be the mechanism of the cell death of dopamine neurons in Parkinson's disease.

A dopaminergic neurotoxin, 1(R), 2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, N-methyl(R)salsolinol, and its oxidation product, 1,2(N)-dimethyl-6,7-dihydroxyisoquinolinium ion, accumulate in the nigro-striatal system of the human brain

N-Methyl(R)salsolinol was found to be an endogenous dopaminergic neurotoxin inducing parkinsonism in rodents and to increase in the cerebrospinal fluid of parkinsonian patients. The amounts of N-methyl(R)salsolinol and related compounds in the human brain regions were quantitatively analyzed. Only the (R)-enantiomer of salsolinol derivatives were detected, which suggests their enzymatic synthesis in situ. In the nigro-striatal system, the concentration of N-methyl(+)salsolinol was higher than in the frontal cortex, and its oxidized catechol isoquinolinium ion was detected only in the substantia nigra significantly. The accumulation of these neurotoxins in the nigro-striatal region might account for selective cell death of dopamine neurons in the substantia nigra of Parkinson's disease.

N-methyl-(R)salsolinol as a dopaminergic neurotoxin: from an animal model to an early marker of Parkinson's disease

A dopamine-derived 1(R), 2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydrosioquinoline [N-methyl-(R)salsolinol] was found to occur enantioselectively in human brain. This isoquinoline induced parkinsonism in rat after injection in the striatum, and the behavioral, biochemical and pathological changes were very similar to those in Parkinson's disease. N-Methyl-(R)salsolinol depleted dopamine neurons in the rat substantia nigra without necrotic tissue reaction, which may be due to the apoptotic death process, as proved by its induction of DNA damage in dopaminergic neuroblastoma SH-SY5Y cells. N-Methyl-(R)salsolinol was found to increase significantly in the cerebrospinal fluid of parkinsonian patients. All these results suggest that N-methyl-(R)salsolinol may be an endogenous neurotoxin to cause Parkinson's disease and the enzymes involved in its biosynthesis and catabolism may be endogenous factors in the pathogenesis of this disease.

Cytotoxicity of endogenous isoquinolines to human dopaminergic neuroblastoma SH-SY5Y cells

Endogenous isoquinolines with and without catechol structure have been proposed to be neurotoxins specific for dopamine neurons. In this paper they were examined for the cytotoxicity of human dopaminergic neuroblastoma SH-SY5Y cells. The cytotoxicity was quantitatively determined using Alamar Blue assay, by which the reduction-oxidation potency in the living cells can be measured spectrometrically. 1,2-Dimethyl-6,7-dihydroxyisoquinolinium ion [1,2-DMDHIQ+], an oxidation product of a parkinsonism-inducing isoquinoline, 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahroisoquinoline [N-methyl-(R)salsolinol, NM(R)Sal] was found to be the most potent toxin among isoquinolines examined. In general, catechol isoquinolines were more toxic than isoquinolines without catechol structure. With and without catechol structure, the oxidized isoquinolinium ion having methyl groups at C-1 and N-2 positions proved to be more cytotoxic than the simple isoquinolines. The involvement of 1,2-DMDHIQ+ to the neurotoxicity of NM(R)Sal was suggested and discussed.

A dopaminergic neurotoxin, (R)-N-methylsalsolinol, increases in Parkinsonian cerebrospinal fluid

The concentration of (R)-N-methylsalsolinol, which is a dopamine-derived neurotoxin selective to dopamine neurons and induces parkinsonism in rats, was found to be increased significantly in the cerebrospinal fluid of untreated patients with Parkinson's disease. The enantio-specific occurrence of (R)-N-methylsalsolinol in cerebrospinal fluid suggests its enzymatic synthesis in the human brain. The individual differences in the activities of the enzymes determining the metabolism of (R)-N-methylsalsolinol in the brain might be involved in the pathogenesis of Parkinson's disease.

Dopamine-derived endogenous 1(R),2(N)-dimethyl-6,7-dihydroxy- 1,2,3,4-tetrahydroisoquinoline, N-methyl-(R)-salsolinol, induced parkinsonism in rat: biochemical, pathological and behavioral studies

Dopamine-derived 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol, Sal) and related compounds were examined for their selective neurotoxicity to dopamine neurons by injection into the rat striatum. Among salsolinol analogs examined, only N-methyl-(R)- salsolinol (NM(R)Sal) induced behavioral changes very similar to those in Parkinson's disease: hypokinesia, stiff tail, limb twitching at rest and postural abnormality. Biochemical analysis showed that after NM(R)Sal injection, NM(R)Sal itself and its oxidation product, 1-2-dimethyl-6,7-dihydroxyisoquinolinium ion (DMDHIQ+) accumulated in the striatum, and also in the substantia nigra definite amount of DMDHIQ+ was detected. Dopamine and noradrenaline were reduced in the striatum and more markedly in the substantia nigra, whereas serotonin and its metabolite were not affected. Morphological analysis revealed selective reduction of tyrosine hydroxylase (TH)-containing neurons in the substantia nigra after continuous NM(R)Sal administration in the striatum. These results demonstrate the selective cytotoxicity of NM(R)Sal to the dopamine neurons in the substantia nigra, and the possible involvement of this 6,7-dihydroxy-isoquinoline in the pathogenesis of Parkinson's disease is discussed.

Animal models of Parkinson's disease: an empirical comparison with the phenomenology of the disease in man

Animal models are an important aid in experimental medical science because they enable one to study the pathogenetic mechanisms and the therapeutic principles of treating the functional disturbances (symptoms) of human diseases. Once the causative mechanism is understood, animal models are also helpful in the development of therapeutic approaches exploiting this understanding. On the basis of experimental and clinical findings. Parkinson's disease (PD) became the first neurological disease to be treated palliatively by neurotransmitter replacement therapy. The pathological hallmark of PD is a specific degeneration of nigral and other pigmented brainstem nuclei, with a characteristic inclusion, the Lewy body, in remaining nerve cells. There is now a lot of evidence that degeneration of the dopaminergic nigral neurones and the resulting striatal dopamine-deficiency syndrome are responsible for its classic motor symptoms akinesia and bradykinesia. PD is one of many human diseases which do not appear to have spontaneously arisen in animals. The characteristic features of the disease can however be more or less faithfully imitated in animals through the administration of various neurotoxic agents and drugs disturbing the dopaminergic neurotransmission. The cause of chronic nigral cell death in PD and the underlying mechanisms remain elusive. The partial elucidation of the processes underlie the selective action of neurotoxic substances such as 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), has however revealed possible molecular mechanisms that give rise to neuronal death. Accordingly, hypotheses concerning the mechanisms of these neurotoxines have been related to the pathogenesis of nigral cell death in PD. The present contribution starts out by describing some of the clinical, pathological and neurochemical phenomena of PD. The currently most important animal models (e.g. the reserpine model, neuroleptic-induced catalepsy, tremor models, experimentally-induced degeneration of nigrostriatal dopaminergic neurons with 6-OHDA, methamphetamine, MPTP, MPP+, tetrahydroisoquinolines, beta-carbolines, and iron) critically reviewed next, and are compared with the characteristic features of the disease in man.

Stereospecific occurrence of a parkinsonism-inducing catechol isoquinoline, N-methyl(R)salsolinol, in the human intraventricular fluid

N-Methyl(R)salsolinol, an endogenous neurotoxin, has been proposed to be closely involved in the pathogenesis of Parkinson's disease. The selective toxicity to dopaminergic neurons was strictly limited for (R)-enantiomer of N-methylsalsolinol. Its precursor, (R)salsolinol was enzymatically synthesized from dopamine and acetaldehyde in human. However, it has never been examined whether a non-enzymatic reaction produces racemic salsolinol derivatives from dopamine especially in patients under L-DOPA therapy. To clarify the point, their contents were examined in intraventricular fluid from parkinsonian patients administrated with L-DOPA. Only (R)-enantiomer of N-methylsalsolinol and very low concentration of salsolinol could be detected. The results suggest that N-methyl(R)salsolinol synthesis may not depend on dopamine level, but on the activity of enzymes related to its synthesis and/or catabolism. The results are discussed in relation to pathogenesis Parkinson's disease.