A novel enzyme enantio-selectively synthesizes (R)salsolinol, a precursor of a dopaminergic neurotoxin, N-methyl(R)salsolinol

Quantitative Determination of Endogenous Tetrahydroisoquinolines, Potential Parkinson's Disease Biomarkers, in Mammals

Current diagnostic options for Parkinson's disease are very limited and primarily based on characteristic clinical symptoms. Thus, there are urgent needs for reliable biomarkers that enable us to diagnose the disease in the early stages, differentiate it from other atypical Parkinsonian syndromes, monitor its progression, increase knowledge of its pathogenesis, and improve the development of potent therapies. A promising group of potential biomarkers are endogenous tetrahydroisoquinoline metabolites, which are thought to contribute to the multifactorial etiology of Parkinson's disease. The aim of this critical review is to highlight trends and limitations of available traditional and modern analytical techniques for sample pretreatment (extraction and derivatization procedures) and quantitative determination of tetrahydroisoquinoline derivatives in various types of mammalian fluids and tissues (urine, plasma, cerebrospinal fluid, brain tissue, liver tissue). Particular attention is paid to the most sensitive and specific analytical techniques, involving immunochemistry and gas or liquid chromatography coupled with mass spectrometric, fluorescence, or electrochemical detection. The review also includes a discussion of other relevant agents proposed and tested in Parkinson's disease.

Salsolinol Induces Parkinson's Disease Through Activating NLRP3-Dependent Pyroptosis and the Neuroprotective Effect of Acteoside

Endogenous neurotoxin 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroiso-quinoline (Salsolinol, SAL) is a dopamine metabolite that is toxic to dopaminergic neurons in vitro and in vivo, and is involved in the pathogenesis of Parkinson's disease (PD). However, the molecular mechanism by which SAL induces neurotoxicity in PD remains challenging for future investigations. This study found that SAL induced neurotoxicity in SH-SY5Y cells and mice. RNA sequencing (RNAseq) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis were used to detect differentially expressed genes in SAL-treated SH-SY5Y cells. We found that NLR family pyrin domain-containing 3 (NLRP3)-dependent pyroptosis was enriched by SAL, which was validated by in vitro and in vivo SAL models. Further, NLRP3 inflammasome-related genes (ASC, NLRP3, active caspase 1, IL-1β, and IL-18) were increased at the mRNA and protein level. Acteoside mitigates SAL-induced neurotoxicity by inhibiting NLRP3 inflammasome-related pyroptosis in in vitro and in vivo PD models. In summary, the present study suggests for the first time that NLRP3-dependent pyroptosis plays a role in the pathogenesis of SAL-induced PD, and acteoside mitigates SAL-induced pyroptosis-dependent neurotoxicity in in vitro and in vivo PD models. The present results demonstrated a new mechanism whereby SAL mediates neurotoxicity by activating NLRP3-dependent pyroptosis, further highlighting SAL-induced pyroptosis-dependent neurotoxicity as a potential therapeutic target in PD.

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.

Stereoselectivity in the Membrane Transport of Phenylethylamine Derivatives by Human Monoamine Transporters and Organic Cation Transporters 1, 2, and 3

Stereoselectivity is well known and very pronounced in drug metabolism and receptor binding. However, much less is known about stereoselectivity in drug membrane transport. Here, we characterized the stereoselective cell uptake of chiral phenylethylamine derivatives by human monoamine transporters (NET, DAT, and SERT) and organic cation transporters (OCT1, OCT2, and OCT3). Stereoselectivity differed extensively between closely related transporters. High-affinity monoamine transporters (MATs) showed up to 2.4-fold stereoselective uptake of norepinephrine and epinephrine as well as of numerous analogs. While NET and DAT preferentially transported (S)-norepinephrine, SERT preferred the (R)-enantiomer. In contrast, NET and DAT showed higher transport for (R)-epinephrine and SERT for (S)-epinephrine. Generally, MAT stereoselectivity was lower than expected from their high affinity to several catecholamines and from the high stereoselectivity of some inhibitors used as antidepressants. Additionally, the OCTs differed strongly in their stereoselectivity. While OCT1 showed almost no stereoselective uptake, OCT2 was characterized by a roughly 2-fold preference for most (R)-enantiomers of the phenylethylamines. In contrast, OCT3 transported norphenylephrine and phenylephrine with 3.9-fold and 3.3-fold preference for their (R)-enantiomers, respectively, while the para-hydroxylated octopamine and synephrine showed no stereoselective OCT3 transport. Altogether, our data demonstrate that stereoselectivity is highly transporter-to-substrate specific and highly diverse even between homologous transporters.

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.

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.

Asymmetric Biocatalytic Synthesis of 1-Aryltetrahydro-β-carbolines Enabled by "Substrate Walking"

Stereoselective catalysts for the Pictet-Spengler reaction of tryptamines and aldehydes may allow a simple and fast approach to chiral 1-substituted tetrahydro-β-carbolines. Although biocatalysts have previously been employed for the Pictet-Spengler reaction, not a single one accepts benzaldehyde and its substituted derivatives. To address this challenge, a combination of substrate walking and transfer of beneficial mutations between different wild-type backbones was used to develop a strictosidine synthase from Rauvolfia serpentina (RsSTR) into a suitable enzyme for the asymmetric Pictet-Spengler condensation of tryptamine and benzaldehyde derivatives. The double variant RsSTR V176L/V208A accepted various ortho-, meta- and para-substituted benzaldehydes and produced the corresponding chiral 1-aryl-tetrahydro-β-carbolines with up to 99 % enantiomeric excess.

Pictet-Spenglerases in alkaloid biosynthesis: Future applications in biocatalysis

Pictet-Spenglerases provide a key role in the biosynthesis of many biologically active alkaloids. There is increasing use of these biocatalysts as an alternative to traditional organic synthetic methods as they provide stereoselective and regioselective control under mild conditions. Products from these enzymes also contain privileged drug scaffolds (such as tetrahydroisoquinoline or β-carboline moieties), so there is interest in the characterization and use of these enzymes as versatile biocatalysts to synthesize analogs of the corresponding natural products for drug discovery. This review discusses all known Pictet-Spenglerase enzymes and their applications as biocatalysts.

Asymmetric Synthesis of (R)-1-Alkyl-Substituted Tetrahydro-ß-carbolines Catalyzed by Strictosidine Synthases

Stereoselective methods for the synthesis of tetrahydro-ß-carbolines are of significant interest due to the broad spectrum of biological activity of the target molecules. In the plant kingdom, strictosidine synthases catalyze the C-C coupling through a Pictet-Spengler reaction of tryptamine and secologanin to exclusively form the (S)-configured tetrahydro-ß-carboline (S)-strictosidine. Investigating the biocatalytic Pictet-Spengler reaction of tryptamine with small-molecular-weight aliphatic aldehydes revealed that the strictosidine synthases give unexpectedly access to the (R)-configured product. Developing an efficient expression method for the enzyme allowed the preparative transformation of various aldehydes, giving the products with up to >98 % ee. With this tool in hand, a chemoenzymatic two-step synthesis of (R)-harmicine was achieved, giving (R)-harmicine in 67 % overall yield in optically pure form.

Isolation and Sequencing of Salsolinol Synthase, an Enzyme Catalyzing Salsolinol Biosynthesis

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), a derivate of dopamine, is suspected to be the most probable neurotoxin in the degeneration of dopaminergic neurons. Numerous hypotheses regarding its pathophysiological roles have been raised, especially related to Parkinson's disease and alcohol addiction. In the mammalian brain, salsolinol may be enzymatically synthesized by salsolinol synthase from dopamine and acetaldehyde. However, the direct evidence of its biosynthesis was still missing. In this study, we purified salsolinol synthase from rat brain by a systematical procedure involving acid precipitation, ultrafiltration, and hydrophilic interaction chromatography. The molecular weight of salsolinol synthase determined by MALDI-TOF MS is 8622.29 Da, comprising 77 amino acids (MQIFVKTLTG KTITLEVEPS DTIKNVKAKI QDKEGIPPDQ QRLIFAGKQL EDGRTLSDYN IQKKSTLHLV LRLRVDY). Homology analysis showed that the enzyme is a ubiquitin-like protein, with a difference of four amino acids, which suggests it is a novel protein. After it was overexpressed in eukaryotic cells, the production of salsolinol was significantly increased as compared with control, confirming the catalytic function of this enzyme. To our knowledge, it is the first systematic purification and sequencing of salsolinol synthase. Together, this work reveals a formerly anonymous protein and urges further exploration of its possible prognostic value and implications in Parkinson's disease and other related disorders.

Adaptive Modifications of Maternal Hypothalamic-Pituitary-Adrenal Axis Activity during Lactation and Salsolinol as a New Player in this Phenomenon

Both basal and stress-induced secretory activities of the hypothalamic-pituitary-adrenal (HPA) axis are distinctly modified in lactating females. On the one hand, it aims to meet the physiological demands of the mother, and on the other hand, the appropriate and stable plasma cortisol level is one of the essential factors for the proper offspring development. Specific adaptations of HPA axis activity to lactation have been extensively studied in several animal species and humans, providing interesting data on the HPA axis plasticity mechanism. However, most of the data related to this phenomenon are derived from studies in rats. The purpose of this review is to highlight these adaptations, with a particular emphasis on stress reaction and differences that occur between species. Existing data on breastfeeding women are also included in several aspects. Finally, data from the experiments in sheep are presented, indicating a new regulatory factor of the HPA axis-salsolinol-which typical role was revealed in lactation. It is suggested that this dopamine derivative is involved in both maintaining basal and suppressing stress-induced HPA axis activities in lactating dams.

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.

Manipulation of the Precursor Supply in Yeast Significantly Enhances the Accumulation of Prenylated β-Carbolines

The tryptophan derivative 1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (MTCA) is present in many plants and foods including fermentation products of the baker's yeast Saccharomyces cerevisiae. MTCA is formed from tryptophan and acetaldehyde via a Pictet-Spengler reaction. In this study, up to 9 mg/L of MTCA were detected as a mixture of (1S,3S) and (1R,3S) isomers in a ratio of 2.2:1 in Saccharomyces cerevisiae cultures. To the best of our knowledge, this is the first report on the presence of MTCA in laboratory baker's yeast cultures. Expression of three fungal tryptophan prenyltransferase genes, fgaPT2, 5-dmats, and 7-dmats in S. cerevisiae resulted in the formation of MTCA derivatives with prenyl moieties at different positions of the indole ring. Expression of these genes in dimethylallyl diphosphate and tryptophan overproducing strains led to generation of up to 400 mg/L of prenylated MTCAs as mixtures of (1S,3S) and (1R,3S) diastereomers in ratios similar to that of unprenylated MTCA. The structures of the described substances including their stereochemistry were unequivocally elucidated by mass spectrometry as well as one- and two-dimensional NMR spectroscopy. The results of this study provide a convenient system for the production of high amounts of designed prenylated MTCAs in S. cerevisiae. Furthermore, our work can be considered as an excellent example for the construction of more complex molecules by introducing just one key gene.

Racemic Salsolinol and its Enantiomers Act as Agonists of the μ-Opioid Receptor by Activating the Gi Protein-Adenylate Cyclase Pathway

Background: Several studies have shown that the ethanol-derived metabolite salsolinol (SAL) can activate the mesolimbic system, suggesting that SAL is the active molecule mediating the rewarding effects of ethanol. In vitro and in vivo studies suggest that SAL exerts its action on neuron excitability through a mechanism involving opioid neurotransmission. However, there is no direct pharmacologic evidence showing that SAL activates opioid receptors.

Methods: The ability of racemic (R/S)-SAL, and its stereoisomers (R)-SAL and (S)-SAL, to activate the μ-opioid receptor was tested in cell-based (light-emitting) receptor assays. To further characterizing the interaction of SAL stereoisomers with the μ-opioid receptor, a molecular docking study was performed using the crystal structure of the μ-opioid receptor.

Results: This study shows that SAL activates the μ-opioid receptor by the classical G protein-adenylate cyclase pathway with an half-maximal effective concentration (EC₅₀) of 2 × 10⁻⁵ M. The agonist action of SAL was fully blocked by the μ-opioid antagonist naltrexone. The EC₅₀ for the purified stereoisomers (R)-SAL and (S)-SAL were 6 × 10⁻⁴ M and 9 × 10⁻⁶ M respectively. It was found that the action of racemic SAL on the μ-opioid receptor did not promote the recruitment of β-arrestin. Molecular docking studies showed that the interaction of (R)- and (S)-SAL with the μ-opioid receptor is similar to that predicted for the agonist morphine.

Conclusions: It is shown that (R)-SAL and (S)-SAL are agonists of the μ-opioid receptor. (S)-SAL is a more potent agonist than the (R)-SAL stereoisomer. In silico analysis predicts a morphine-like interaction between (R)- and (S)-SAL with the μ-opioid receptor. These results suggest that an opioid action of SAL or its enantiomers is involved in the rewarding effects of ethanol.

From Ethanol to Salsolinol: Role of Ethanol Metabolites in the Effects of Ethanol

In spite of the global reputation of ethanol as the psychopharmacologically active ingredient of alcoholic drinks, the neurobiological basis of the central effects of ethanol still presents some dark sides due to a number of unanswered questions related to both its precise mechanism of action and its metabolism. Accordingly, ethanol represents the interesting example of a compound whose actions cannot be explained as simply due to the involvement of a single receptor/neurotransmitter, a scenario further complicated by the robust evidence that two main metabolites, acetaldehyde and salsolinol, exert many effects similar to those of their parent compound. The present review recapitulates, in a perspective manner, the major and most recent advances that in the last decades boosted a significant growth in the understanding on the role of ethanol metabolism, in particular, in the neurobiological basis of its central effects.

(R)-Salsolinol, a product of ethanol metabolism, stereospecifically induces behavioral sensitization and leads to excessive alcohol intake

Ethanol is oxidized in the brain to acetaldehyde, which can condense with dopamine to generate (R/S)-salsolinol [(RS)-SAL]. Racemic salsolinol [(RS)-SAL] is self-infused by rats into the posterior ventral tegmental area (VTA) at significantly lower concentrations than those of acetaldehyde, suggesting that (RS)-SAL is a most active product of ethanol metabolism. Early studies showed that repeated intraperitoneal or intra-VTA administration of (RS)-SAL (10 mg/kg) induced conditioned place preference, led to locomotor sensitization and increased voluntary ethanol consumption. In the present study, we separated the (R)- and (S)-enantiomers from a commercial (RS)-SAL using a high-performance liquid chromatography with electrochemical detection system fitted with a β-cyclodextrin-modified column. We injected (R)-SAL or (S)-SAL (30 pmol/1.0 μl) into the VTA of naïve UChB rats bred as alcohol drinkers to study whether one or both SAL enantiomers are responsible for the motivated behavioral effects, sensitization and increase in voluntary ethanol intake. The present results show that repeated administration of (R)-SAL leads to (1) conditioned place preference; (2) locomotor sensitization; and (3) marked increases in binge-like ethanol intake. Conversely, (S)-SAL did not influence any of these parameters. Overall, data indicate that (R)-SAL stereospecifically induces motivational effects, behavioral sensitization and increases ethanol intake.

Oxidative modification of human ceruloplasmin induced by a catechol neurotoxin, salsolinol

Salsolinol (SAL), a compound derived from dopamine metabolism, is the most probable neurotoxin involved in the pathogenesis of Parkinson's disease (PD). In this study, we investigated the modification and inactivation of human ceruloplasmin (hCP) induced by SAL. Incubation of hCP with SAL increased the protein aggregation and enzyme inactivation in a dose-dependent manner. Reactive oxygen species scavengers and copper chelators inhibited the SAL-mediated hCP modification and inactivation. The formation of dityrosine was detected in SAL-mediated hCP aggregates. Amino acid analysis post the exposure of hCP to SAL revealed that aspartate, histidine, lysine, threonine and tyrosine residues were particularly sensitive. Since hCP is a major copper transport protein, oxidative damage of hCP by SAL may induce perturbation of the copper transport system, which subsequently leads to deleterious conditions in cells. This study of the mechanism by which ceruloplasmin is modified by salsolinol may provide an explanation for the deterioration of organs under neurodegenerative disorders such as PD.

Salsolinol: a potential modulator of the activity of the hypothalamic-pituitary-adrenal axis in nursing and postweaning sheep

The most well-known physiological action of salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline) is the stimulation of prolactin secretion, especially during lactation. In addition, our recent work demonstrated that salsolinol inhibits the stress-induced activity of the hypothalamic-pituitary-adrenal (HPA) axis in lactating sheep. Here, we investigated whether salsolinol regulates the basal activity of the HPA axis in lactating sheep and whether its inhibitory action on the stress-induced activity of the HPA axis is present during the postweaning period. The first experiment was performed during the fifth week of lactation, in which unstressed sheep received an intracerebroventricular infusion of an antagonistic analogue of salsolinol, 1-MeDIQ (1-methyl-3,4-dihydroisoquinoline). Simultaneously, the infundibular nucleus and/or median eminence was perfused using the push-pull method. Sheep that received 1-MeDIQ infusion showed significantly higher concentration of plasma ACTH during the second, third, and fourth hour (P < 0.001, P < 0.01, and P < 0.001, respectively) and cortisol during the third and fourth hour (P < 0.001 and P < 0.01, respectively) than did sheep that received control infusion. There was no significant difference in the mean perfusate corticotropin-releasing hormone concentration between the 1-MeDIQ and control treatments. In the second experiment, sheep received an intracerebroventricular infusion of salsolinol during the ninth week of lactation and 48 h after lamb weaning. A comparison between the control groups in the first and second experiments revealed that sheep after weaning (ninth week of lactation) had significantly higher mean ACTH (P < 0.001) and cortisol (P < 0.001) concentrations during the first 2 h of the experiment than the nursing females (fifth week of lactation) had. Salsolinol significantly reduced the increased concentrations of ACTH and cortisol (P < 0.01) in sheep after lamb weaning. However, there was no difference in the expression of proopiomelanocortin messenger RNA within the anterior pituitary between the control and salsolinol-treated groups. In conclusion, salsolinol regulates the basal activity of the HPA axis in lactating sheep. In addition, the HPA axis of postweaning females is more sensitive to stressors associated with the experimental procedures, and salsolinol attenuates ACTH and cortisol release in this phenomenon.

A newly discovered neurotoxin ADTIQ associated with hyperglycemia and Parkinson's disease

BACKGROUND

Diabetes is associated with an increased risk of Parkinson's disease (PD). Number of studies have suggested that methylglyoxal (MGO) induced by diabetes is related to PD. However, very little is known about its molecular mechanism. On other hand, 1-acetyl-6, 7- dihydroxyl-1, 2, 3, 4- Tetrahydroisoquinoline(ADTIQ) is a dopamine (DA)-derived tetrahydroisoquinoline (TIQ), a novel endogenous neurotoxins, which was first discovered in frozen Parkinson's disease human brain tissue. While ADTIQ precursor methylglyoxal was also found in diabetic patients related to the glucose metabolism and diabetic patients.

METHODS

LC-MS/MS, 1H NMR and infrared spectroscopy identified the structure of ADTIQ. The Annexin V-FITC/PI, MTT and western blot analysis were used to measure the neurotoxicity of ADTIQ. The levels of ADTIQ and methylglyoxal were detected by LC-MS/MS.

RESULTS

Here we report the chemical synthesis of ADTIQ, demonstrate its biosynthesis in SH-SY5Y neuroblastoma cell line and investigate its role in the pathogenesis of PD. In addition, a significant increase in the level of ADTIQ was detected in the brains of transgenic mice expressing mutant forms (A53T or A30P) of α-synuclein. ADTIQ also reduced the cell viability and induced mitochondrial apoptosis in dopaminergic cells, suggesting that ADTIQ acts as an endogenous neurotoxin and potentially involved in the pathogenesis of PD. Methylglyoxal, a major byproduct of glucose metabolism and abnormalities in glucose metabolism could influence the levels of ADTIQ. Consistent with the hypothesis, increased levels of ADTIQ and methylglyoxal were detected in the striatum of diabetic rats and SH-SY5Y cells cultured in the presence of high glucose concentrations.

CONCLUSIONS

Increased levels of ADTIQ could be related with Hyperglycemia and death of dopaminergic neurons.

GENERAL SIGNIFICANCE

The increased levels of ADTIQ could be a reason of dopamine neuron dysfunction in diabetes. Therefore, ADTIQ may play a key role in increasing the risk for PD in patients with diabetes.

Chronic salsolinol administration prevents the behavioral and neurochemical effects of L-DOPA in rats

1-Methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) is a well-known endogenous compound that has been proposed as a factor involved in the pathogenesis of Parkinson’s disease. In the present study, we investigated the impact of acute and chronic salsolinol (100 mg.kg i.p.) administration on l-DOPA-induced locomotor hyperactivity and neurochemical changes (the dopamine level and its metabolism in rat brain structures). Moreover, using the in vivo microdialysis technique, we measured the effect of acute and chronic salsolinol injection on l-DOPA-induced dopamine release in the rat striatum. The behavioral data demonstrated that both acute and chronic salsolinol administration antagonized l-DOPA-mediated hyperactivity. An ex vivo neurochemical experiment indicated that chronic but not acute salsolinol administration partially inhibited the l-DOPA-induced increases in the concentration of dopamine and all of its metabolites in dopaminergic structures. Additionally, the in vivo dopamine release data obtained from the microdialysis experiments clearly indicated that the differences in the effect of salsolinol on the activities of l-DOPA depended on the mode of salsolinol treatment. Acute injection of salsolinol enhanced the l-DOPA-induced elevation of dopamine release (by ~1200 %; P < 0.01), whereas chronic administration of salsolinol completely blocked the l-DOPA-induced elevation of dopamine release in the rat striatum. These data demonstrated that chronic administration of salsolinol significantly impaired the response of dopaminergic neurons to l-DOPA administration. In conclusion, we propose that an elevated salsolinol level in parkinsonian patients may represent a serious risk factor of the clinical efficacy of l-DOPA therapy.

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.

Salsolinol, free of isosalsolinol, exerts ethanol-like motivational/sensitization effects leading to increases in ethanol intake

Salsolinol is formed non-enzymatically when ethanol-derived acetaldehyde binds to dopamine, yielding 2 distinct products, i.e., salsolinol and isosalsolinol. Early animal studies, revealing that salsolinol promotes alcohol consumption and recent evidence that animals will readily self-administer salsolinol into the posterior ventral tegmental area (p-VTA) together with the finding that salsolinol is able to induce conditioned place preference and to increase locomotor activity, have outlined a role of salsolinol in the behavioral and neurobiological actions of ethanol. Until recently, the only commercially available salsolinol was a mixture containing 85% salsolinol and 10-15% isosalsolinol. The possibility thus exists that either salsolinol or isosalsolinol explains the reinforcing properties of ethanol. We report here that a newly available salsolinol is free of isosalsolinol. Thus, salsolinol, free of isosalsolinol, was injected intracerebrally (30 pmol/0.2 μL, into the ventral tegmental area [VTA]) or intraperitoneally (i.p.) (10 mg/kg) to naïve rats bred as alcohol drinkers to study salsolinol's motivational effects and its role on voluntary ethanol intake. Salsolinol produced conditioned place preference and increased locomotor activity, whether injected intra-VTA or intraperitoneally. Following systemic (i.p.) administration of 10 mg/kg salsolinol, this molecule was detected in vivo by microdialysis of neostriatum, reaching an estimated concentration of 100 nM in the dialyzate. These results indicate that systemically administered salsolinol is able to cross the blood-brain barrier (BBB). Repeated administration of salsolinol sensitized rats to the locomotor activity and led to increases in voluntary ethanol consumption, which was prevented by intra-VTA pretreatment with naltrexone.

Alpha-synuclein overexpression induced mitochondrial damage by the generation of endogenous neurotoxins in PC12 cells

Alpha-synuclein is one of the important components of Lewy body which involved in neuropathology of Parkinson's disease (PD). The relationship between α-synuclein and cell death is still unclear. In the study, PC12 cell, stably over expressing α-synuclein model was used, and we investigated the level of intracellular oxidative stress, dopamine and endogenous neurotoxin. The results showed that the level of oxidative stress and intracytoplasmic dopamine (DA) was increased in cells over expressing α-synuclein compared with normal PC12 cells. Simultaneously, additional generation of endogenous neurotoxins 1-methyl-4-phenyl-1,2,3,4-tetrahydroisoquinoline (salsolinol) and 1(R),2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolin (NM-salsolinol) was detected and this phenomenon was exacerbated after exposed to H₂O₂ for 24 h, but mitigated when treated with dopamine synthesis inhibitors. The presence of endogenous neurotoxins exacerbated α-synuclein induced mitochondrial damage. These results suggest that the endogenous neurotoxins may become a bridge between α-synuclein and cell death.

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.

The possible involvement of salsolinol and hypothalamic prolactin in the central regulatory processes in ewes during lactation

Salsolinol, a dopamine-related compound and prolactin-producing cells were found in the ovine hypothalamus. This study was designed to test the hypothesis that salsolinol, acting from the CNS level, is able to stimulate pituitary prolactin release as well as prolactin mRNA expression in the anterior pituitary cells (AP) and in the mediobasal hypothalamus (MBH) in lactating ewes. The intracerebroventricular infusions of salsolinol in two doses, total of 50 ng or 5 μg, were performed in a series of five 10-min infusions at 20-min intervals. All infusions were made from 12:30 to 15:00 and the pre-infusion period was from 10:00 to 12.30 h. The prolactin concentration in plasma samples, collected every 10 min, was determined by radioimmunoassay; prolactin mRNA expression in AP and MBH tissues was determined by real-time PCR. The obtained results showed that salsolinol infused at the higher dose significantly (p < 0.001) increased plasma prolactin concentration in lactating ewes, when compared with the concentration noted before the infusion and with that in lactating controls. In lactating ewes, the relative levels of prolactin mRNA expression in the AP and MBH were up to twofold and fivefold higher respectively than in non-lactating ewes (p < 0.05). In our experimental design, salsolinol did not significantly affect the ongoing process of prolactin gene expression in these tissues. We conclude that in ewes, salsolinol may be involved, at least, in the process of stimulation of prolactin release during lactation and that hypothalamic prolactin plays an important role in the central mechanisms of adaptation to lactation.

The Pictet-Spengler reaction in nature and in organic chemistry

Alkaloids are an important class of natural products that are widely distributed in nature and produced by a large variety of organisms. They have a wide spectrum of biological activity and for many years were used in folk medicine. These days, alkaloids also have numerous applications in medicine as therapeutic agents. The importance of these natural products in inspiring drug discovery programs is proven and, therefore, their continued synthesis is of significant interest. The condensation discovered by Pictet and Spengler is the most important method for the synthesis of alkaloid scaffolds. The power of this synthesis method has been convincingly proven in the construction of stereochemicaly and structurally complex alkaloids.

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.

Neuromelanin selectively induces apoptosis in dopaminergic SH-SY5Y cells by deglutathionylation in mitochondria: involvement of the protein and melanin component

Parkinson's disease (PD) is characterized by selective depletion of nigral dopamine (DA) neurons containing neuromelanin (NM), suggesting the involvement of NM in the pathogenesis. This study reports induction of apoptosis by NM in SH-SY5Y cells, whereas protease-K-treated NM, synthesized DA- and cysteinyl dopamine melanin showed much less cytotoxicity. Cell death was mediated by mitochondria-mediated apoptotic pathway, namely collapse of mitochondrial membrane potential, release of cytochrome c, and activation of caspase 3, but Bcl-2 over-expression did not suppress apoptosis. NM increased sulfhydryl content in mitochondria, and a major part of it was identified as GSH, whereas dopamine melanin significantly reduced sulfhydryl levels. Western blot analysis for protein-bound GSH demonstrated that only NM reduced S-glutathionylated proteins in mitochondria and dissociated macromolecular structure of complex I. Reactive oxygen and nitrogen species were required for the deglutathionylation by NM, which antioxidants reduced significantly with prevention of apoptosis. These results suggest that NM may be related to cell death of DA neurons in PD and aging through regulation of mitochondrial redox state and S-glutathionylation, for which NM-associated protein is absolutely required. The novel function of NM is discussed in relation to the pathogenesis of PD.

Systemic administration of D-penicillamine prevents the locomotor activation after intra-VTA ethanol administration in rats

Although recently published studies seem to confirm the important role displayed by acetaldehyde (ACH), the main metabolite of ethanol, in the behavioral effects of ethanol, the origin of ACH is still a matter of debate. While some authors confer more importance to the central (brain metabolism) origin of ACH, others indicate that the hepatic origin could be more relevant. In this study we have addressed this topic using an experimental approach that combines local microinjections of ethanol into the ventral tegmental area (VTA) (which guarantees the brain origin of the ACH) to induce motor activation in rats together with systemic administration (i.p.) of several doses (0, 12.5, 25 and 50 mg/kg) of D-penicillamine (DP), a sequestering agent of ACH with contrasted efficiency to abolish the behavioral effects of the drug. Our results clearly show that DP prevented in a dose-dependent manner the motor activation induced by intra-VTA ethanol, being the 50 mg/kg dose the most efficient. DP per se did not affect the basal activity of the rats. In order to determine the specificity of the DP action, we also studied the effects of DP 50 mg/kg on the DAMGO-induced motor activation after the intra-VTA administration of this mu-opioid receptors agonist. DP did not significantly modify the motor activation induced by DAMGO thus confirming the specificity of the DP effects. Our results clearly suggest that the brain-derived ACH is necessary to manifest the activating effects resulting from ethanol administration.

A critical evaluation of influence of ethanol and diet on salsolinol enantiomers in humans and rats

BACKGROUND

(R/S)-Salsolinol (SAL), a condensation product of dopamine (DA) with acetaldehyde, has been speculated to have a role in the etiology of alcoholism. Earlier studies have shown the presence of SAL in biological fluids and postmortem brains from both alcoholics and nonalcoholics. However, the involvement of SAL in alcoholism has been controversial over several decades, since the reported SAL levels and their changes after ethanol exposure were not consistent, possibly due to inadequate analytical procedures and confounding factors such as diet and genetic predisposition. Using a newly developed mass spectrometric method to analyze SAL stereoisomers, we evaluated the contribution of ethanol, diet, and genetic background to SAL levels as well as its enantiomeric distribution.

METHODS

Simultaneous measurement of SAL enantiomers and DA were achieved by high performance liquid chromatography-tandem mass spectrometry (HPLC/MS/MS). Plasma samples were collected from human subjects before and after banana (a food rich in SAL) intake, and during ethanol infusion. Rat plasma and brain samples were collected at various time points after the administration of SAL or banana by gavage. The brain parts including nucleus accumbens (NAC) and striatum (STR) were obtained from alcohol-non-preferring (NP) or alcohol-preferring (P) rats as well as P-rats which had a free access to ethanol (P-EtOH).

RESULTS

Plasma SAL levels were increased significantly after banana intake in humans. Consistently, administration of banana to rats also resulted in a drastic increase of plasma SAL levels, whereas brain SAL levels remained unaltered. Acute ethanol infusion did not change SAL levels or R/S ratio in plasma from healthy humans. The levels of both SAL isomers and DA were significantly lower in the NAC of P rats in comparison to NP rats. The SAL levels in NAC of P rats remained unchanged after chronic free-choice ethanol drinking. There were decreasing trends of SAL in STR and DA in both brain regions. No changes in enantiomeric ratio were observed after acute or chronic ethanol exposure.

CONCLUSIONS

SAL from dietary sources is the major contributor to plasma SAL levels. No significant changes of SAL plasma levels or enantiomeric distribution after acute or chronic ethanol exposure suggest that SAL may not be a biomarker for ethanol drinking. Significantly lower SAL and DA levels observed in NAC of P rats may be associated with innate alcohol preference.