N-methyl-(R)-salsolinol and its relevance to Parkinson's disease

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.

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.

Increased levels of monoamine-derived potential neurotoxins in fetal rat brain exposed to ethanol

Pregnant SD rats were exposed to ethanol (25 % (v/v) ethanol at 1.0, 2.0 or 4.0 g/kg body weight from GD8 to GD20) to assess whether ethanol-derived acetaldehyde could interact with endogenous monoamine to generate tetrahydroisoquinoline or tetrahydro-beta-carboline in the fetuses. The fetal brain concentration of acetaldehyde increased remarkably after ethanol administration (2.6 times, 5.3 times and 7.8 times as compared to saline control in 1.0, 2.0 and 4.0 g/kg ethanol-treated groups, respectively) detected by HPLC with 2,4-dinitrophenylhydrazine derivatization. Compared to control, ethanol exposure induced the formation of 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol, Sal), N-methyl-salsolinol (NMSal) and 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline (6-OH-MTHBC) in fetal rat brains. Determined by HPLC with electrochemical detector, the levels of dopamine and 5-hydroxytryptamine in whole fetal brain were not remarkably altered by ethanol treatment, while the levels of homovanillic acid and 5-hydroxyindole acetic acid in high dose (4.0 g/kg) of ethanol-treated rats were significantly decreased compared to that in the control animals. 4.0 g/kg ethanol administration inhibited the activity of mitochondrial monoamine oxidase (51.3 % as compared to control) and reduced the activity of respiratory chain complex I (61.2 % as compared to control). These results suggested that ethanol-induced alteration of monoamine metabolism and the accumulation of dopamine-derived catechol isoquinolines and 5-hydroxytryptamine-derived tetrahydro-beta-carbolines may play roles in the developmental dysfuction of monoaminergic neuronal systems.

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.

Role of salsolinol in the regulation of pituitary prolactin and peripheral dopamine release

(R)-Salsolinol (SAL), a dopamine (DA)-related tetrahydroisoquinoline, has been found in extracts of the neuro-intermediate lobes (NIL) of pituitary glands and in the median eminence of the hypothalamus obtained from intact male rats and from ovariectomized and lactating female rats. Moreover, analysis of SAL concentrations in NIL revealed parallel increases with plasma prolactin (PRL) in lactating rats exposed to a brief (10 min) suckling stimulus after 4-h separation. SAL is sufficiently potent in vivo to account for the massive discharge of PRL that occurs after physiological stimuli (i.e. suckling). At the same time, it was without effect on the secretion of other pituitary hormones. It has been also shown that another isoquinoline derivative, 1-methyldihydroisoquinoline (1MeDIQ), which is a structural analogue of SAL, can dose-dependently inhibit the in-vivo PRL-releasing effect of SAL. Moreover, 1MeDIQ can inhibit the elevation of plasma PRL induced by physiological stimuli, for example suckling, or in different stressful situations also. 1MeDIQ also has a psycho-stimulant action, which is fairly similar to the effect of amphetamine, i.e. it induces an increase in plasma catecholamine concentrations. It is clear from these data that this newly discovered endogenous compound could be involved in regulation of pituitary PRL secretion. It has also been observed that SAL is present in peripheral, sympathetically innervated organs, for example the atrium, spleen, liver, ovaries, vas deferens, and salivary gland. Furthermore, SAL treatment of rats results in dose-dependent and time-dependent depletion of the DA content of the organs listed above without having any effect on the concentration of norepinephrine. More importantly, this effect of SAL can be completely prevented by amphetamine and by 1MeDIQ pretreatment. It is clear there is a mutual interaction between SAL, 1MeDIQ, and amphetamine or alcohol, not only on PRL release; their interaction with catecholamine "synthesis/metabolism" of sympathetic nerve terminals is also obvious.

The formation of catechol isoquinolines in PC12 cells exposed to manganese

Chronic exposure to manganese causes parkinsonian symptoms and has been implicated as an environmental factor in the pathogenesis of Parkinson's disease (PD). Here we show that manganese inhibits the proliferation of PC12 cells and induces apoptosis through the formation of catechol isoquinolines. Manganese induces the production of 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol, Sal) and N-methyl-salsolinol (NMSal) in PC12 cells, and increases the levels of malondialdehyde (MDA) in a dose-dependent manner. The data indicates that the formation of catechol isoquinolines due to oxidative stress induced by MnCl(2) may be a mechanism by which manganese causes degeneration of dopaminergic neurons.

Autotoxicity, methylation and a road to the prevention of Parkinson's disease

Xenobiotic enzymes normally protect against toxins but on occasion can convert protoxins into toxins. N-methylated pyridines (such as the N-methyl-4 phenylpyridinium ion (MPP+)) are well-established dopaminergic toxins. The enzyme nicotinamide N-methyltransferase (NNMT) can covert otherwise harmless pyridines such as 4-phenylpyridine into MPP+ like compounds. This enzyme has recently been shown to be present in the human brain, which is a necessity for neurotoxicity, as charged compounds such as MPP+ cannot cross the blood brain barrier. Moreover, it is present in increased concentration in the brain of patients with Parkinson's disease (PD). This would increase MPP+ like compounds at the same time as decreasing intraneuronal nicotinamide, a neuroprotectant at several levels, thus creating a "multiple hit", as additionally complex 1 of the mitochondrial complex would also be poisoned and starved of its major substrate, nicotinamide adenine dinucleotide (NAD). Thus, PD may be a disease of autointoxication. Xenobiotic enzyme inhibitors of NNMT, with or without dietary modification, would be a novel way to attempt primary prevention of PD.

Parkinson's disease: the first common neurological disease due to auto-intoxication?

Parkinson's disease may be a disease of autointoxication. N-methylated pyridines (e.g. MPP+) are well-established dopaminergic toxins, and the xenobiotic enzyme nicotinamide N-methyltransferase (NNMT) can convert pyridines such as 4-phenylpyridine into MPP+, using S-adenosyl methionine (SAM) as the methyl donor. NNMT has recently been shown to be present in the human brain, a necessity for neurotoxicity, because charged compounds cannot cross the blood-brain barrier. Moreover, it is present in increased concentration in parkinsonian brain. This increase may be part genetic predisposition, and part induction, by excessive exposure to its substrates (particularly nicotinamide) or stress. Elevated enzymic activity would increase MPP+-like compounds such as N-methyl nicotinamide at the same time as decreasing intraneuronal nicotinamide, a neuroprotectant at several levels, creating multiple hits, because Complex 1 would be poisoned and be starved of its major substrate NADH. Developing xenobiotic enzyme inhibitors of NNMT for individuals, or dietary modification for the whole population, could be an important change in thinking on primary and secondary prevention.

Binding site of salsolinol: its properties in different regions of the brain and the pituitary gland of the rat

It has been recently shown that salsolinol (SAL) is present in the hypothalamic neuroendocrine dopaminergic (NEDA) system and appears to be a selective and potent stimulator of prolactin (PRL) secretion in the rat. Furthermore, the lack of interference of SAL with 3H-spiperone binding in the striatum and the anterior lobe (AL) of the pituitary gland has been also demonstrated. These data clearly indicate that SAL does not act at the dopamine (DA) D(2) receptors, and suggest that SAL supposedly has a binding site through which the secretion of PRL may be affected. Therefore, binding of 3H-SAL to different regions of the central nervous system (CNS) has been investigated. Specific and saturable binding has been detected in the striatum, cortex, median eminence and in the hypothalamus as well as in the AL and the neuro-intermediate lobe (NIL) of the pituitary gland. K(D) values of the bindings were in the nanomolar range in all tissue tested. 3H-SAL displacing activity of several agonists and antagonists of known DA receptors have also been tested. It has been found that DA and in a lesser extent, apomorphine could displace 3H-SAL, but other DA receptor specific ligands have not been able to affect it. Furthermore, several pharmacologically active compounds, selected on the basis of their influence on DA synthesis, transport mechanisms and signal transduction, have also been tested. Neither mazindol (a selective DA transporter inhibitor) nor clonidine (an alpha(2)-adrenoreceptor agonist) could alter SAL binding. At the same time, L-dopa, carbidopa, benserazide and alpha-methyldopa were able to displace 3H-SAL. The possible changes in SAL binding due to physiological and pharmacological stimuli, like suckling stimulus and reserpine pretreatment (that blocks vesicular monoamine transport in DA terminals), respectively, have also been investigated. In the NIL of the pituitary gland and in the median eminence of the hypothalamus the binding decreased following 10 min of suckling stimulus compared to the binding detected in the same tissues obtained from mothers separated from their pups for 4h and not allowed to be suckled. At the same time, there were no changes in the binding at the AL and striatum. Following reserpine pretreatment that has completely prevented PRL releasing effect of SAL, the binding was significantly augmented. These results support our assumption that SAL should have specific binding sites through which it can affect PRL secretion. Furthermore, it clearly suggests that it may regulate DAergic neurotransmission of NEDA neurons by an altered intracellular or intraterminal synthesis and/or distribution of hypophysiotropic DA.

Physiological role of salsolinol: its hypophysiotrophic function in the regulation of pituitary prolactin secretion

We have recently observed that 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol) produced by hypothalamic neurons can selectively release prolactin from the anterior lobe (AL) of the pituitary gland. Moreover, high affinity binding sites for SAL have been detected in areas, like median eminence (ME) and the neuro-intermediate lobe (NIL) that are known terminal fields of the tuberoinfundibular DAergic (TIDA) and tuberohypophysial (THDA)/periventricular (PHDA) DAergic systems of the hypothalamus, respectively. However, the in situ biosynthesis and the mechanism of action of SAL are still enigmatic, these observations clearly suggest that sites other than the AL might be targets of SAL action. Based on our recent observations it may be relevant to postulate that an "autosynaptocrine" regulatory mechanism functioning at the level of the DAergic terminals localized in both the ME and NIL, may play a role in the hypophyseotrophic regulation of PRL secretion. Furthermore, SAL may be a key player in these processes. The complete and precise mapping of these intra-terminal mechanisms should help us to understand the tonic DAerg regulation of PRL secretion. Moreover, it may also give insight into the role of pre-synaptic processes that most likely have distinct and significant functional as well as pathological roles in other brain areas using DAergic neurotransmission, like striatonigral and mesolimbic systems.

Biochemical and pharmacological characterization of 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline: a biologically relevant neurotoxin?

Acute and long-term effects of exposure to reactive compounds as the result of environmental pollution, workplace conditions or dietary intake are suspected to be involved in the etiology of a variety of disorders, including neurodegenerative disorders such as Parkinson's disease. The recognition in 1970s that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxic by-product of illicit meperidine synthesis, elicits parkinsonian symptoms in primates, including man, prompted the search for naturally occurring analogs which might be involved in human disease. It has been suggested that one candidate, 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo), a potent dopaminergic neurotoxin, might be formed endogenously in humans following the administration of the hypnotic chloral hydrate or after the exposure to the industrial solvent trichloroethylene. Such spontaneous formation has, indeed, been recently reported. The biochemical and pharmacological characteristics of TaClo and related compounds are thus reviewed here, and their potential significance for human neurodegenerative disease discussed.

Salsolinol is a putative endogenous neuro-intermediate lobe prolactin-releasing factor

The isolation and identification of a prolactin-releasing factor (PRF) from the neuro-intermediate lobe of the pituitary gland has been pursued for over a decade. Using high-pressure liquid chromatography with electrochemical detection (HPLC-ECD) and gas chromatography/mass spectrometry (GC/MS) (R)-salsolinol (SAL) (a dopamine-related stereo-specific tetrahydroisoquinoline) was found to be present in neuro-intermediate lobe as well as median eminence extracts of male, intact-, and ovariectomized female rats. Moreover, analysis of SAL concentrations in neuro-intermediate lobe revealed parallel increases with plasma prolactin in lactating rats exposed to a brief (10 min) suckling stimulus following 4-h separation. SAL appears to be a selective and potent stimulator of prolactin secretion in vivo and it was without effect on the secretion of other pituitary hormones. We have also found that SAL can elevate prolactin release, although to a lesser extent, in pituitary cell cultures as well as in hypophysectomized rats bearing anterior lobe transplants under the kidney capsule. Lack of interference of SAL with [3H]-spiperone binding to AP homogenates indicates that SAL does not act at the dopamine D2 receptor. Moreover, [3H]-SAL binds specifically to homogenate of AL as well as neuro-intermediate lobe obtained from lactating rats. Taken together, our data clearly suggest that SAL is synthesized in situ and this compound can play a role in the regulation of pituitary prolactin secretion.

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.