Enantio-selective occurrence of (S)-tetrahydropapaveroline in human brain

Linking Ethanol-Addictive Behaviors With Brain Catecholamines: Release Pattern Matters

Using a variety of animal models that simulate key features of the alcohol use disorder (AUD), remarkable progress has been made in identifying neurochemical targets that may contribute to the development of alcohol addiction. In this search, the dopamine (DA) and norepinephrine (NE) systems have been long thought to play a leading role in comparison with other brain systems. However, just recent development and application of optogenetic approaches into the alcohol research field provided opportunity to identify neuronal circuits and specific patterns of neurotransmission that govern the key components of ethanol-addictive behaviors. This critical review summarizes earlier findings, which initially disclosed catecholamine substrates of ethanol actions in the brain and shows how the latest methodologies help us to reveal the significance of DA and NE release changes. Specifically, we focused on recent optogenetic investigations aimed to reveal cause-effect relationships between ethanol-drinking (seeking and taking) behaviors and catecholamine dynamics in distinct brain pathways. These studies gain the knowledge that is needed for the better understanding addiction mechanisms and, therefore, for development of more effective AUD treatments. Based on the reviewed findings, new messages for researches were indicated, which may have broad applications beyond the field of alcohol addiction.

Neurotoxicity and Underlying Mechanisms of Endogenous Neurotoxins

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

Aldehyde adducts inhibit 3,4-dihydroxyphenylacetaldehyde-induced α-synuclein aggregation and toxicity: Implication for Parkinson neuroprotective therapy

3,4-Dihydroxyphenylacetaldehyde (DOPAL), the monoamine oxidase (MAO) metabolite of dopamine, plays a role in pathogenesis of Parkinson disease, inducing α-synuclein aggregation. DOPAL generates discrete α-synuclein aggregates. Inhibiting this aggregation could provide therapy for slowing Parkinson disease progression. Primary and secondary amines form adducts with aldehydes. Rasagiline and aminoindan contain these amine groups. DOPAL-induced α-synuclein aggregates were resolved in the presence and absence of rasagiline or aminoindan using quantitative Western blotting. DOPAL levels in incubation mixtures, containing increased rasagiline or aminoindan concentrations, were determined by high pressure liquid chromatography (HPLC). Schiff base adducts between DOPAL and rasagiline or aminoindan were determined using mass spectrometry. A neuroprotective effect of rasagiline and aminoindan against DOPAL-induced toxicity was demonstrated using PC-12 cells. Rasagiline and aminoindan significantly reduced aggregation of α-synuclein of all sizes in test tube and PC-12 cells experiments. Dimethylaminoindan did not reduce aggregation. DOPAL levels in incubation mixtures were reduced with increasing rasagiline or aminoindan concentrations but not with dimethylaminoindan. Schiff base adducts between DOPAL and either rasagiline or aminoindan were demonstrated by mass spectrometry. A neuroprotective effect against DOPAL-induced toxicity in PC-12 cells was demonstrated for both rasagiline and aminoindan. Inhibiting DOPAL-induced α-synuclein aggregation through amine adducts provides a therapeutic approach for slowing Parkinson disease progression.

Endogenous Isoquinoline Alkaloids Agonists of Acid-Sensing Ion Channel Type 3

Acid-sensing ion channels (ASICs) ASIC3 expressed mainly in peripheral sensory neurons play an important role in pain perception and inflammation development. In response to acidic stimuli, they can generate a unique biphasic current. At physiological pH 7.4, human ASIC3 isoform (hASIC3) is desensitized and able to generate only a sustained current. We found endogenous isoquinoline alkaloids (EIAs), which restore hASIC3 from desensitization and recover the transient component of the current. Similarly, rat ASIC3 isoform (rASIC3) can also be restored from desensitization (at pH < 7.0) by EIAs with the same potency. At physiological pH and above, EIAs at high concentrations were able to effectively activate hASIC3 and rASIC3. Thus, we found first endogenous agonists of ASIC3 channels that could both activate and prevent or reverse desensitization of the channel. The decrease of EIA levels could be suggested as a novel therapeutic strategy for treatment of pain and inflammation.

Parkinson's disease, L-DOPA, and endogenous morphine: a revisit

Clinical observations stemming from widespread employment of restorative L-3,4-dihydroxyphenylalanine (L-DOPA) therapy for management of dyskinesia in Parkinson's Disease (PD) patients implicate a regulatory role for endogenous morphine in central nervous system dopamine neurotransmission. Reciprocally, it appears that restorative L-DOPA administration has provided us with a compelling in vivo pharmacological model for targeting peripheral sites involved in endogenous morphine expression in human subjects. The biological activities underlying endogenous morphine expression and its interaction with its major precursor dopamine strongly suggest that endogenous morphine systems are reciprocally dysregulated in PD. These critical issues are examined from historical and current perspectives within our short review.

Endogenous morphine-like compound immunoreactivity increases in parkinsonism

Morphine is endogenously synthesized in the central nervous system and endogenous dopamine is thought to be necessary for endogenous morphine formation. As Parkinson's disease results from the loss of dopamine and is associated with central pain, we considered how endogenous morphine is regulated in the untreated and l-DOPA-treated parkinsonian brain. However, as the cellular origin and overall distribution of endogenous morphine remains obscure in the pathological adult brain, we first characterized the distribution of endogenous morphine-like compound immunoreactive cells in the rat striatum. We then studied changes in the endogenous morphine-like compound immunoreactivity of medium spiny neurons in normal, Parkinson's disease-like and l-DOPA-treated Parkinson's disease-like conditions in experimental (rat and monkey) and human Parkinson's disease. Our results reveal an unexpected dramatic upregulation of neuronal endogenous morphine-like compound immunoreactivity and levels in experimental and human Parkinson's disease, only partially normalized by l-DOPA treatment. Our data suggest that endogenous morphine formation is more complex than originally proposed and that the parkinsonian brain experiences a dramatic upregulation of endogenous morphine immunoreactivity. The functional consequences of such endogenous morphine upregulation are as yet unknown, but based upon the current knowledge of morphine signalling, we hypothesize that it is involved in fatigue, depression and pain symptoms experienced by patients with Parkinson's disease.

Neurotoxic effects of tetrahydroisoquinolines and underlying mechanisms

Tetrahydropapaveroline (THP), a neurotoxic tetrahydroisoquinoline alkaloid formed by condensation between dopamine and dopaldehyde, has been speculated to cause Parkinson's disease and also to contribute to alcohol dependence. Having two catechol moieties, THP may readily undergo oxidation to form an o-quinone intermediate with concomitant production of reactive oxygen species, which can cause neuronal cell death and DNA damage. This review will deal with the current knowledge of neurotoxic effects of this endogenous alkaloid and underlying biochemical mechanisms.

Urinary excretion of morphine and biosynthetic precursors in mice

It has been firmly established that humans excrete a small but steady amount of the isoquinoline alkaloid morphine in their urine. It is unclear whether it is of dietary or endogenous origin. There is no doubt that a simple isoquinoline alkaloid, tetrahydropapaveroline (THP), is found in human and rodent brain as well as in human urine. This suggests a potential biogenetic relationship between both alkaloids. Unlabeled THP or [1,3,4-D(3)]-THP was injected intraperitoneally into mice and the urine was analyzed. This potential precursor was extensively metabolized (96%). Among the metabolites found was the phenol-coupled product salutaridine, the known morphine precursor in the opium poppy plant. Synthetic [7D]-salutaridinol, the biosynthetic reduction product of salutaridine, injected intraperitoneally into live animals led to the formation of [7D]-thebaine, which was excreted in urine. [N-CD(3)]-thebaine was also administered and yielded [N-CD(3)]-morphine and the congeners [N-CD(3)]-codeine and [N-CD(3)]-oripavine in urine. These results show for the first time that live animals have the biosynthetic capability to convert a normal constituent of rodents, THP, to morphine. Morphine and its precursors are normally not found in tissues or organs, presumably due to metabolic breakdown. Hence, only that portion of the isoquinoline alkaloids excreted in urine unmetabolized can be detected. Analysis of urine by high resolution-mass spectrometry proved to be a powerful method for tracking endogenous morphine and its biosynthetic precursors.

Dopamine, morphine, and nitric oxide: an evolutionary signaling triad

Morphine biosynthesis in relatively simple and complex integrated animal systems has been demonstrated. Key enzymes in the biosynthetic pathway have also been identified, that is, CYP2D6 and COMT. Endogenous morphine appears to exert highly selective actions via novel mu opiate receptor subtypes, that is, mu3,-4, which are coupled to constitutive nitric oxide release, exerting general yet specific down regulatory actions in various animal tissues. The pivotal role of dopamine as a chemical intermediate in the morphine biosynthetic pathway in plants establishes a functional basis for its expansion into an essential role as the progenitor catecholamine signaling molecule underlying neural and neuroendocrine transmission across diverse animal phyla. In invertebrate neural systems, dopamine serves as the preeminent catecholamine signaling molecule, with the emergence and limited utilization of norepinephrine in newly defined adaptational chemical circuits required by a rapidly expanding set of physiological demands, that is, motor and motivational networks. In vertebrates epinephrine, emerges as the major end of the catecholamine synthetic pathway consistent with a newly incorporated regulatory modification. Given the striking similarities between the enzymatic steps in the morphine biosynthetic pathway and those driving the evolutionary adaptation of catecholamine chemical species to accommodate an expansion of interactive but distinct signaling systems, it is our overall contention that the evolutionary emergence of catecholamine systems required conservation and selective "retrofit" of specific enzyme activities, that is, COMT, drawn from cellular morphine expression. Our compelling hypothesis promises to initiate the reexamination of clinical studies, adding new information and treatment modalities in biomedicine.

Nrf2-mediated heme oxygenase-1 induction confers adaptive survival response to tetrahydropapaveroline-induced oxidative PC12 cell death

Tetrahydropapaveroline (THP), a dopaminergic isoquinoline neurotoxin, has been reported to contribute to neurodegeneration in parkinsonism. As THP bears two catechol moieties, it undergoes autooxidation or enzymatic oxidation to produce reactive oxygen species (ROS), which may contribute to the THP-induced cell death. Although ROS are cytotoxic, the initial accumulation of ROS may provoke a survival response. In this study, treatment of PC12 cells with THP increased expression of heme oxygenase-1 (HO-1) as an adaptive survival response. Furthermore, THP-induced cytotoxicity was attenuated by the HO-1 inducer (SnCl2) and exacerbated by the HO-1 inhibitor (ZnPP). To elucidate the molecular mechanisms underlying THP-mediated HO-1 expression, we examined the possible involvement of NF-E2-related factor 2 (Nrf2), which plays an important role in the transcriptional regulation of detoxifying/antioxidant genes. THP treatment elevated nuclear translocation of Nrf2 and subsequent binding to antioxidant response element (ARE). PC12 cells transfected with dominant-negative Nrf2 exhibited increased cytotoxicity and decreased HO-1 expression after THP treatment. Moreover, U0126 and LY294002, which are pharmacologic inhibitors of extracellular signal-regulated kinase1/2 and phosphoinositide 3-kinase, respectively, attenuated HO-1 expression as well as Nrf2-ARE binding activity. Taken together, these findings suggest that HO-1 induction via Nrf2 activation may confer a cellular adaptive response against THP-mediated cell death.

Study on the ability of 1,2,3,4-tetrahydropapaveroline to cause oxidative stress: Mechanisms and potential implications in relation to parkinson's disease

Tetrahydropapaveroline (THP) is a compound derived from dopamine monoamine oxidase-mediated metabolism, particularly present in the brain of parkinsonian patients receiving L-dopa therapy, and is capable of causing dopaminergic neurodegeneration. The aim of this work was to evaluate the potential of THP to cause oxidative stress on mitochondrial preparations and to gain insight into the molecular mechanisms responsible for its neurotoxicity. Our data show that THP autoxidation occurs with a continuous generation of hydroxyl radicals (*OH) and without the involvement of the Fenton reaction. The presence of ascorbate enhances this process by establishing a redox cycle, which regenerates THP from its quinolic forms. It has been shown that the production of *OH is not affected by the presence of either ferrous or ferric iron. Although THP does not affect lipid peroxidation, it is capable of reducing the high levels of thiobarbituric acid-reactive substances obtained in the presence of ascorbate and/or iron. However, THP autoxidation in the presence of ascorbate causes both an increase in protein carbonyl content and a reduction in protein-free thiol content. THP also increases protein carbonyl content when the autoxidation occurs in the presence of iron. The remarkable role played by ascorbate in the production of oxidative stress by THP autoxidation is of particular interest.

How human neuroblastoma cells make morphine

Recently, our laboratory demonstrated that human neuroblastoma cells (SH-SY5Y) are capable of synthesizing morphine, the major active metabolite of opium poppy. Now our experiments are further substantiated by extending the biochemical studies to the entire morphine pathway in this human cell line. L-[1,2,3-13C3]- and [ring-2',5',6'-2H3]dopa showed high isotopic enrichment and incorporation in both the isoquinoline and the benzyl moiety of the endogenous morphine. [2,2-2H2]Dopamine, however, was exclusively incorporated only into the isoquinoline moiety. Neither the trioxygenated (R,S)-[1,3-13C2]norcoclaurine, the precursor of morphine in the poppy plant, nor (R)-[1,3,4-2H3]norlaudanosoline showed incorporation into endogenous morphine. However, (S)-[1,3,4-2H3]norlaudanosoline furnished a good isotopic enrichment and the loss of a single deuterium atom at the C-9 position of the morphine molecule, indicating that the change of configuration from (S)- to (R)-reticuline occurs via the intermediacy of 1,2-dehydroreticuline. Additional feeding experiments with potential morphinan precursors demonstrated substantial incorporation of [7-2H]salutaridinol, but not 7-[7-2H]episalutaridinol, and [7-2H,N-C2H3]oripavine, and [6-2H]codeine into morphine. Human morphine biosynthesis involves at least 19 chemical steps. For the most part, it is a reflection of the biosynthesis in opium poppy; however, there is a fundamental difference in the formation of the key intermediate (S)-reticuline: it proceeds via the tetraoxygenated initial isoquinoline alkaloid (S)-norlaudanosoline, whereas the plant morphine biosynthesis proceeds via the trioxygenated (S)-norcoclaurine. Following the plant biosynthetic pathway, (S)-reticuline undergoes a change of configuration at C-1 during its transformation to salutaridinol and thebaine. From thebaine, there is a bifurcate pathway leading to morphine proceeding via codeine or oripavine, in both plants and mammals.

Potential roles of NF-kappaB and ERK1/2 in cytoprotection against oxidative cell death induced by tetrahydropapaveroline

Tetrahydropapaveroline (THP), a dopamine-derived tetrahydroisoquinoline catechol, has been suspected to be dopaminergic neurotoxin that elicits parkinsonism and neurobehavioral abnormalities associated with chronic alcoholism. THP has been detected in the brains of parkinsonian patients, and its urinary as well as brain level increases after l-3,4-dihydroxyphenylalanine treatment. Autoxidation or enzymatic oxidation of THP and subsequent generation of reactive oxygen species (ROS) may contribute to the degeneration of dopaminergic neurons induced by this tetrahydroisoquinoline alkaloid. In the present study, THP was found to elicit cytotoxicity in cultured rat pheochromocytoma (PC12) cells, which was completely blocked by reduced glutathione and N-acetyl-L-cysteine. THP-treated PC12 cells exhibited increased intracellular accumulation of ROS and underwent apoptosis as determined by poly(ADP-ribose)polymerase cleavage, an increased ratio of Bax to BclxL, terminal transferase-mediated dUTP nick end labeling, and nuclear fragmentation or condensation. THP treatment caused activation of the redox-sensitive transcription factor nuclear factor kappaB (NF-kappaB). Pretreatment of PC12 cells with NF-kappaB inhibitors, such as l-1-tosylamido-2-phenylethyl chloromethyl ketone and parthenolide, aggravated THP-induced cell death. THP treatment resulted in differential activation of mitogen-activated protein kinases as well as Akt/protein kinase B, thereby transmitting cell survival or death signals. In conclusion, THP induces apoptosis in PC12 cells by generating ROS. THP-mediated oxidative stress was accompanied by differential activation of intracellular signaling kinases and NF-kappaB.

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.

Presence of reticuline in rat brain: a pathway for morphine biosynthesis

We demonstrate the presence of reticuline, an isoquinoline alkaloid that was purified and identified in the rat brain. This was achieved by high-performance liquid chromatography coupled with electrochemical detection. This material was finally identified by nano-electrospray ionization quadrupole time-of-flight tandem mass spectrometry. The expression of this tetrahydroisoquinoline alkaloid in rat brain is at 12.7+/-5.4 ng/g wet tissue. Furthermore, rat chow, rat small and large intestine and bacteria cultured from these tissues did not contain either morphine or reticuline, eliminating the possibility of contamination or an exogenous source of these compounds. This finding adds information which suggests that morphine biosynthesis may occur in rat neural tissues, and that its biosynthesis pathway may be similar to that reported in the poppy plant.

A re-evaluation of the role of tetrahydropapaveroline in ethanol consumption in rats

The role of tetrahydropapaveroline (THP), a condensation product of a dopaldehyde with dopamine, in the regulation of alcohol consumption was investigated. In the first experiment, rats received intraventricular injections of either racemic THP hydrobromide (0.65 or 1.3 microg/microl), R-(+)-THP (0.66 or 1.4 microg/microl), or an equal volume of vehicle. The lower doses of both (+/-)-THP and (+)-THP significantly increased volitional alcohol intake. For the racemic compound, the increase was significant at 7-13% concentrations. The R-(+)-enantiomer increased consumption at 4-11 and 15-20% concentrations of ethanol. The higher doses of both compounds did not significantly alter alcohol preference. A second experiment evaluated the chronic effect of THP delivered subcutaneously via osmotic minipump. Animals receiving THP (0.1, 0.5, 1.0, 2.0, and 4.0 mg/ml) did not differ in their alcohol intake, compared to vehicle-treated controls. Whether or not endogenously formed THP participates in the etiology of alcohol addiction remains unclear. Nonetheless, there are few known compounds that induce a preference for unsweetened alcohol solutions over water in laboratory animals.

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

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

Dopaminergic neurotoxins, 6,7-dihydroxy-1-(3', 4'-dihydroxybenzyl)-isoquinolines, cause different types of cell death in SH-SY5Y cells: apoptosis was induced by oxidized papaverolines and necrosis by reduced tetrahydropapaverolines

Dopamine-derived 6,7-dihydroxy-1-(3', 4'-dihydroxybenzyl)-isoquinolines, papaverolines and tetrahydropapaverolines, have been proposed to be neurotoxin candidates related to the pathogenesis of Parkinson's disease. In this paper, the cytotoxicity of papaverolines and their N-methyl derivatives was examined using human dopaminergic neuroblastoma SH-SY5Y cells as a model of dopamine neurons. Apoptotic and necrotic cell death were assessed by morphological observation of cells after staining with propidium iodide and Hoechst 33342. Papaveroline and N-methyl-papaveroline induced apoptosis in almost all the cells with typical features of condensed and fragmented nuclei. On the other hand, (R)- and (S)-tetrahydropapaveroline caused necrosis in cells. Tetrahydropapaverolines markedly reduced adenosine triphosphate (ATP) level, whereas papaverolines did not, suggesting that the types of cell death induced by these isoquinolines, necrosis and apoptosis, depend on ATP concentrations in the cells.