Enantiomeric composition of urinary salsolinol in parkinsonian patients after Madopar

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: 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.

Salsolinol, an endogenous compound triggers a two-phase opposing action in the central nervous system

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), an endogenous compound present in the brain, was suspected of participation in the etiopathogenesis of Parkinson’s disease, the most common serious movement disorder worldwide. In this study, we evaluated the effect of different (50, 100, and 500 µM) concentrations of salsolinol on markers of glutamate-induced apoptotic and neurotoxic cell damage, such as caspase-3 activity, lactate dehydrogenase (LDH) release, and the loss of mitochondrial membrane potential. Biochemical data were complemented with the cellular analysis, including Hoechst 33342 and calcein AM staining, to visualize apoptotic DNA-fragmentation and to assess cell survival, respectively. The assessment of all investigated parameters was performed in primary cultures of rat or mouse hippocampal and striatum cells. Our study showed that salsolinol had biphasic effects, namely, at lower concentrations (50 and 100 µM), it demonstrated a distinct neuroprotective activity, whereas in the highest one (500 µM) caused neurotoxic effect. Salsolinol in concentrations of 50 and 100 µM significantly antagonized the pro-apoptotic and neurotoxic effects caused by 1 mM glutamate. Salsolinol diminished the number of bright fragmented nuclei with condensed chromatin and increased cell survival in Hoechst 33342 and calcein AM staining in hippocampal cultures. Additionally, in the low 50 µM concentration, it produced a significant inhibition of glutamate-induced loss of membrane mitochondrial potential. Only the highest concentration of salsolinol (500 µM) enhanced the glutamate excitotoxicity. Ex vivo studies indicated that both acute and chronic administration of salsolinol did not affect the dopamine metabolism, its striatal concentration or α-synuclein and tyrosine hydroxylase protein level in the rat substantia nigra and striatum. Summarizing, the present studies exclude possibility that salsolinol under physiological conditions could be an endogenous factor involved in the neurogenerative processes; conversely, it can exert a protective action on nerve cells in the brain. These findings may have important implications for the development of the new strategies to treat or prevent neural degeneration.

TRPC1 protects dopaminergic SH-SY5Y cells from MPP+, salsolinol, and N-methyl-(R)-salsolinol-induced cytotoxicity

Neurotoxins and alterations in Ca2+ homeostasis have been associated with Parkinson's disease (PD), but the role of store-operated Ca2+ entry channels is not well understood. Previous studies have shown the neurotoxicity of salsolinol and 1-methyl-4-phenylpyridinium ion on SH-SY5Y cells and cytoprotection induced by transient receptor potential protein 1 (TRPC1). In the present study, N-methyl-(R)-salsolinol was tested for its cellular toxicity and effects on TRPC1 expression. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays, DAPI (4',6-diamidino-2-phenylindole), fluorescein isothiocyanate-Annexin-V/propidium iodide, western blot analysis, and JC-1 labeling revealed that the three indicated drugs could induce caspase-dependent, mitochondrial-mediated apoptosis. Exposure of SH-SY5Y cells to the indicated drugs resulted in a significant decrease in thapsigargin-mediated Ca2+ influx and TRPC1 expression. Immunocytochemistry experiments revealed that neurotoxins treatment induced TRPC1 translocation to the cytoplasm. Taken together, our results indicate that treatment with neurotoxins may alter Ca2+ homeostasis and induce mitochondrial-mediated caspase-dependent cytotoxicity, an important characteristic of PD.

Regional distribution of tetrahydroisoquinoline derivatives in rodent, human, and Parkinson's disease brain

Several members of the tetrahydroisoquinoline (TIQ) family of monoamine alkaloids can be formed from dopamine or its oxidized metabolites and may be involved in the pathogenesis of monoaminergic cell death in Parkinson's disease (PD). Using enantiomeric-selective high-performance liquid chromatography with electrochemical detection and liquid chromatography with tandem mass spectroscopy, the regional concentrations of several TIQ derivatives, including salsolinols, were determined in mouse, rat, normal human, and PD brain. TIQ derivatives were detected in all regions subjected to analysis. In general, salsolinols were present at higher concentrations than TIQ and its benzyl and methyl derivatives, especially in human brain. Moreover, salsolinols were concentrated in areas with increased dopamine synthesis and turnover such as the ventral midbrain and striatum, respectively. A possible consequence of nigrostriatal dopaminergic cell death, significantly lower levels of (R)salsolinol, (S)salsolinol, N-methyl-(R)salsolinol and N-methyl-(S)salsolinol were found in the caudate nuclei of PD in comparison with normal human brain. Our data support the hypothesis of endogenous synthesis of salsolinols and provide evidence for their accumulation in catecholaminergic neurons.

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.

Contrasting effects of catecholic and O-methylated tetrahydroisoquinolines on hydroxyl radical production

Tetrahydroisoquinolines (TIQs) are intraneuronal, catecholamine-derived alkaloids that have been implicated in the etiology of Parkinson's disease and in alcohol related disorders. The in vitro production of the cytotoxic hydroxyl radical (*OH) was recorded during the autoxidation of salsolinol (SAL) and salsolinol-1-carboxylic acid (SAL-1C), but not when these two catecholic TIQs were oxidized by tyrosinase. Significantly higher levels of the radical were produced when these catecholic TIQs were incubated with *OH generating complexes, or with chelated iron. In contrast, mono-O-methylated TIQs such as salsoline (SLN) and salsoline-1-carboxylic acid (SLN-1C) did not generate *OH during autoxidation or when incubated with chelated iron or tyrosinase. Radical production by *OH-generating complexes was reduced in the presence of O-methylated TIQs. The neurotoxicity of TIQs may result from their propensity to autoxidize and generate reactive quinoids and ensuing oxygen radicals. The functional significance of the replacement of a hydroxyl group attached to C-7 of SAL or SAL-1C with a methoxyl group remains to be determined. This single structural modification may prevent mono-O-methylated TIQs from participating in catalytic redox cycling reactions that would otherwise augment *OH production. If true, then O-methylation and other cellular mechanisms that circumvent the autoxidation of catecholamine-derived TIQs may reduce the likelihood of these substances forming cytotoxic quinoids and influencing endogenous *OH-generating reactions.

R- and S-salsolinol are not increased in cerebrospinal fluid of Parkinsonian patients

Various investigators address an augmented synthesis of tetrahydroisoquinolines, such as salsolinol (SAL), or an increased N-methylation of these compounds as putative pathophysiologic mechanisms in Parkinson's disease (PD). Objectives of this study were (1) the evaluation of a putative elevation of enantiomers (R-, S-) of SAL and (2) the investigation of relations between these metabolic precursors of neurotoxic N-methylated-SAL (NMSAL) and dopamine in cerebrospinal fluid of untreated de-novo Parkinsonian patients and age- and sex-matched healthy controls. Levels of R- and S-SAL and dopamine did not significantly (R-SAL: P = 0.75, S-SAL: P = 0.69, dopamine: P = 0.46) differ and dopamine did not correlate to R-SAL and S-SAL in both groups. We conclude, that central accumulation of R-NMSAL, which is neurotoxic to dopaminergic nigrostriatal neurons, is not due to elevated synthesis of R-SAL and/or S-SAL in PD.

Salsolinol, catecholamine metabolites, and visual hallucinations in L-dopa treated patients with Parkinson's disease

We could quantify the tetrahydroisoquinoline derivative salsolinol in urine of patients with Parkinson's disease and normal control subjects by means of high performance liquid chromatography and electrochemical detection. Urine levels of salsolinol were positively related to the homovanillic acid/3-O-methyl-dopa ratio in the cerebrospinal fluid that reflects dopamine metabolism. In the patient group with visual hallucinations, mean salsolinol level was significantly increased to almost the 3-fold of those found in patients without hallucinations. Since the daily L-dopa doses of both patient groups were nearly identical this result is not due to different L-dopa medications. Additionally, either high values of the main serotonin metabolite, 5-hydroxyindole acetic acid (HIAA) or the L-dopa/3-O-methyl-dopa ratio were found in cerebrospinal fluid of patients with hallucinations. The enhanced salsolinol levels in patients with visual hallucinations seem to be due to an overloaded dopaminergic pathway with an imbalance between dopaminergic and serotonergic systems. Thus, salsolinol appears as a predictor for hallucinosis in Parkinson's disease.

Isoquinoline neurotoxins in the brain and Parkinson's disease

Parkinson's disease is thought to be caused by some unknown endogenous or exogenous factors interacting with genetic dispositions. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is an exogenous neurotoxin producing parkinsonism in humans, monkeys and various animals as the result of monoamine oxidase type B (MAO-B)-catalyzed conversion of it to the 1-methyl-4-phenyl-pyridinium ion (MPP+), which selectively kills the nigrostriatal dopaminergic neurons. Various isoquinoline derivatives were found in the brain of patients with Parkinson's disease. Isoquinoline derivatives have neurochemical properties similar to those of MPTP and they are considered to be the endogenous neurotoxins which cause Parkinson's disease. Among them, tetrahydroisoquinoline (TIQ), 1-benzyl-TIQ, and (R)-1,2-dimethyl-5,6-dihydroxy-TIQ [(R)-N-methyl-salsolinol)] have the most potent neurotoxicity. TIQs, like MPTP, may be activated via N-methylation by N-methyltransferase and oxidation by MAO. TIQs as well as MPP+ inhibit complex I of the electron transport system in mitochondria, thereby reducing ATP formation and producing oxygen radicals. Although the properties of TIQs are similar to those of MPTP, the neurotoxicity of TIQs is weaker than that of MPTP. Since Parkinson's disease is a slowly progressing neurodegenerative disease, long term neurotoxic effects of IQs remain to be further examined in primates.

Increase in salsolinol level in the cerebrospinal fluid of parkinsonian patients is related to dementia: advantage of a new high-performance liquid chromatography methodology

The study was carried out on the lumbar cerebrospinal fluid (CSF) samples taken from nonparkinsonian, early parkinsonian, and advanced parkinsonian patients. Some patients showed dementia, and some were treated with L-dopa. In the samples, salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) was assayed with a newly developed sensitive high-performance liquid chromatography (HPLC) method; 3-O-methyldopa (3-O-MD) and homovanillic acid (HVA) were also assayed by HPLC. CSF salsolinol concentrations were significantly enhanced in patients with signs of dementia, regardless of the degree of parkinsonism, and were not affected by L-dopa treatment; HVA and, particularly, 3-O-MD levels were elevated in patients receiving L-dopa. The strong association of CSF salsolinol level with dementia, but not with L-dopa treatment suggests that salsolinol does not originate from L-dopa metabolism, and that its elevation is an indicator of neurodegenerative processes resulting in damage to brain areas mediating cognitive functions. We found no correlation between the advancement of parkinsonism and the concentrations of 3-O-MD and HVA.

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.

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.

Determination of the (R)- and (S)-enantiomers of salsolinol and N-methylsalsolinol by use of a chiral high-performance liquid chromatographic column

A new method for the quantitative determination of the enantiomers of salsolinol and N-methylsalsolinol, biologically important alkaloids, is reported. The enantiomers were completely separated without derivatization, using a cyclodextrin-modified silica gel column with an HPLC-electrochemical detection system. The HPLC conditions were examined for the best resolution. The method was sensitive enough to detect salsolinol and N-methylsalsolinol at a concentration of less than 0.1 pmol per injection. In the product of the Pictet-Spengler reaction of acetaldehyde with dopamine or epinine, almost equimolar (R)- and (S)-enantiomers of salsolinol and N-methylsalsolinol were detected. Preliminary results indicate that the (R)-enantiomer of both isoquinoline derivatives predominate in the human brain.

Determination of total dopamine, R- and S-salsolinol in human plasma by cyclodextrin bonded-phase liquid chromatography with electrochemical detection

A reliable and sensitive high-performance liquid chromatographic (HPLC) method is presented for the determination of total (free and conjugated) plasma dopamine and the enantiomers R- and S-salsolinol. Plasma is purified on two cartridges, containing primary and secondary amines and phenylboronic acid. Dopamine, R- and S-salsolinol are then separated by HPLC using a beta-cyclodextrin-OH phase column. The eluate is monitored electrochemically, without further purification nor derivatization. The method is suited for routine analysis. It allows the detection of total (free and conjugated) dopamine and R- and S-salsolinol in human plasma in concentrations as low as 0.02 ng/ml plasma. The sensitivity is sufficient to measure the naturally occurring levels of salsolinol.

Characterization of the in vivo action of (R)-salsolinol, an endogenous metabolite of alcohol, on serotonin and dopamine metabolism: a microdialysis study

Using a microdialysis-HPLC technique in conscious rats, we examined the action of (R)-1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, (R)-salsolinol (R-Sal), a possible endogenous metabolite of alcohol, on serotonin (5-HT) and dopamine (DA) metabolism in four regions of the brain: the striatum, the substantia nigra, the hippocampus and the hypothalamus. Following 1 mM R-Sal perfusion, the dialysate level of 5-HT in the striatum markedly increased from non-detectable levels to 4259.2 +/- 617.5 nM, while DA increased from 3.4 +/- 0.9 nM to 206.0 +/- 56.5 nM. This increase was one order of magnitude larger in 5-HT than in DA. Conversely, the output of 5-hydroxyindoleacetic acid decreased markedly to non-detectable levels, while 3,4-dihydroxyphenylacetic acid and homovanillic acid outputs decreased below 40% of basal levels. These effects were dose-related to R-Sal (1 microM to 1 mM) and were confirmed also in 3 other brain regions. The R-Sal-induced responses in the striatum were observed even after pretreatment of 2 microM tetrodotoxin, a blocker of nerve-firing activity, via the dialysis membrane. The repetitive perfusion with 1 mM R-Sal into the striatum induced the reproducible response of 5-HT and DA. Furthermore, the potencies of 1 mM R-Sal to increase the output of 5-HT and DA were approximately 783.0-fold and 2.6-fold stronger, respectively, than those of the same dose of methamphetamine. The results suggest that R-Sal acts to stimulate a release of monoamines, 5-HT preferentially, with inhibition of monoamine oxidase and catechol-O-methyltransferase activities.

Naturally-occurring isoquinolines perturb monamine metabolism in the brain: studied by in vivo microdialysis

Naturally occurring isoquinolines affected the monoamine metabolism in the rat striatum, as proved by in vivo microdialysis technique. By analysis of monoamines and their metabolites in the dialysate, dopamine-derived 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines were found to inhibit monoamine oxidase and catechol-O-methyltransferase activity. 1-Methyl- and 2-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline were found to inhibit activity of type A monoamine oxidase most markedly. To compare the structure-activity relationship, corresponding isoquinolines without a catechol structure were also examined. The inhibition by catechol isoquinolines was more manifest than those without a catechol structure. Among latter isoquinolines, N-methyl-isoquinolinium ion was the most potent inhibitor of monoamine oxidase. In addition, catechol isoquinolines increased monoamine levels in the brain. The number and the site of the methyl group are essentially required for the inhibition of monoamine oxidase and a catechol structure for that of catechol-O-methyl-transferase. These results are discussed in relation to possible involvement of these isoquinolines to the clinical features of some neuro-psychiatric diseases, such as alcoholism or in L-DOPA therapy.

N-methylation of dopamine-derived 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, (R)-salsolinol, in rat brains: in vivo microdialysis study

N-Methylation of (R)-1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [(R)-salsolinol] derived from dopamine was proved by in vivo microdialysis study in the rat brain. The striatum was perfused with (R)-salsolinol and N-methylated compound was identified in the dialysate using HPLC and electrochemical detection with multichanneled electrodes. N-Methylation of (R)-salsolinol was confirmed in three other regions of the brain, the substantia nigra, hypothalamus, and hippocampus. In the substantia nigra, the amount of N-methylated (R)-salsolinol was significantly larger than in the other three regions. These results indicate that around dopaminergic neurons, particularly in the substantia nigra, (R)-salsolinol was methylated into N-methyl-(R)-salsolinol, which has a chemical structure similar to that of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the selective dopaminergic neurotoxin. N-Methylation of tetrahydroisoquinolines and beta-carbolines have already been proven to increase their toxicity to dopaminergic neurons and N-methylation might be an essential step for these alkaloids to increase their toxicity. On the other hand, after perfusion of (R)-salsolinol, release of dopamine and 5-hydroxytryptamine was observed and inhibition of monoamine oxidase was indicated. (R)-Salsolinol and its derivatives may be candidates for being dopaminergic neurotoxins.

Endogenous synthesis of N-methylsalsolinol, an analogue of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, in rat brain during in vivo microdialysis with salsolinol, as demonstrated by gas chromatography-mass spectrometry

N-Methylsalsolinol, an analogue of 1,2,3,6-tetrahydropyridine, is present in the brains of patients with Parkinson's disease. To determine the metabolic pathway for the synthesis of N-Methylsalsolinol in the brain, salsolinol was perfused through the striatum or the substantia nigra of the rat brain by in vivo microdialysis. N-Methylsalsolinol was detected in the brain dialysate samples during microdialysis with salsolinol using gas chromatography-mass spectrometry with selected-ion monitoring. These results demonstrate that endogenous N-methylation of salsolinol into N-methylsalsolinol occurs in the brain in vivo.

Determination of free salsolinol concentrations in human urine using gas chromatography-mass spectrometry

The urine concentrations of free salsolinol were determined in six healthy volunteers, using a gas chromatographic-mass spectrometric method with electron-capture negative-ion chemical ionization after derivatization with pentafluoropropionyl anhydride. The sensitivity of this method allows the quantification of salsolinol concentrations of 0.55 pmol/ml. The synthesis of [2H4]salsolinol from dopamine and [2H4]acetaldehyde via a Pictet-Spengler condensation is described; [2H4]salsolinol was used as the internal standard for salsolinol quantification. The urine concentrations of free salsolinol ranged from ca. 1 to 6 pmol/ml.

Presence of 2-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline and 1,2-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, novel endogenous amines, in parkinsonian and normal human brains

2-Methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline and 1,2-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline were identified for the first time as novel endogenous amines in parkinsonian and normal human brains by gas chromatography-mass spectrometry. It is of interest that these tetrahydroisoquinolines are analogues of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) which produces Parkinson's disease.

Urinary elimination of salsolinol enantiomers in alcoholics

The analysis of the urine from 6 chronic alcoholics showed that (R)- and (S)-salsolinol were detectable in 4 subjects, the R enantiomer was only found in one and that both enantiomers were under the limit of detection in another one. 1,2-Dehydrosalsolinol was present in the urine of all of them. There was no correlation between the presence of alcohol in blood upon admission to the hospital and that of either salsolinol enantiomer in urine In a previous study (Strolin Benedetti et al., 1989 b), both salsolinol enantiomers were found in the urine of 3 out of 6 healthy subjects, possibly in relation with regular intake of alcoholic beverages. The content in (R)- and (S)-salsolinol was determined in the same 3 subjects after deprivation of alcohol for 24 h. Under these conditions, only (R)-salsolinol was detected in urine and this also after ingestion of 50 g of alcohol (500 ml Chianti). The possible involvement of the non-physiological enantiomer of salsolinol in alcohol addiction deserves further study.

Biosynthesis of salsolinol, a tetrahydroisoquinoline alkaloid, in healthy subjects

The R enantiomer of salsolinol was detected in the urine of two out of six healthy subjects, whereas 1,2-dehydrosalsolinol was present in the urine of all the subjects. (S)-salsolinol was never detected. Administration of Madopar for 7 days resulted in the presence of large amounts of (R)- and (S)-salsolinol in the urine of five out of the six subjects, the urinary excretion of 1,2-dehydrosalsolinol being generally not markedly increased. The presence of 1,2-dehydrosalsolinol in urine suggests that the biosynthesis of salsolinol in healthy volunteers should occur by condensation of dopamine with pyruvic acid, in keeping with Hahn's hypothesis. The absence of salsolinol in the urine of one subject after Madopar administration seems to indicate that the biological system(s) involved in the reduction of the C = N bond in 1,2-dehydrosalsolinol can be missing or not, or poorly, functional in some individuals, and suggests that there is no alternative pathway for the formation of salsolinol in healthy volunteers.