The endogenous amine 1-methyl-1,2,3,4- tetrahydroisoquinoline prevents the inhibition of complex I of the respiratory chain produced by MPP(+)

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.

Bacopa Protects against Neurotoxicity Induced by MPP+ and Methamphetamine

The neurotoxins methamphetamine (METH) and 1-methyl-4-phenylpyridinium (MPP⁺) damage catecholamine neurons. Although sharing the same mechanism to enter within these neurons, METH neurotoxicity mostly depends on oxidative species, while MPP⁺ toxicity depends on the inhibition of mitochondrial activity. This explains why only a few compounds protect against both neurotoxins. Identifying a final common pathway that is shared by these neurotoxins is key to prompting novel remedies for spontaneous neurodegeneration. In the present study we assessed whether natural extracts from Bacopa monnieri (BM) may provide a dual protection against METH- and MPP⁺-induced cell damage as measured by light and electron microscopy. The protection induced by BM against catecholamine cell death and degeneration was dose-dependently related to the suppression of reactive oxygen species (ROS) formation and mitochondrial alterations. These were measured by light and electron microscopy with MitoTracker Red and Green as well as by the ultrastructural morphometry of specific mitochondrial structures. In fact, BM suppresses the damage of mitochondrial crests and matrix dilution and increases the amount of healthy and total mitochondria. The present data provide evidence for a natural compound, which protects catecholamine cells independently by the type of experimental toxicity. This may be useful to counteract spontaneous degenerations of catecholamine cells.

Preventative effects of 1-methyl-1,2,3,4-tetrahydroisoquinoline derivatives (N-functional group loading) on MPTP-induced parkinsonism in mice

1,2,3,4-Tetrahydroisoquinoline (TIQ) is endogenously present in human brain, and some of its derivatives are thought to contribute to the induction of Parkinson's disease (PD)-like signs in rodents and primates. In contrast, the endogenous TIQ derivative 1-methyl-TIQ (1-MeTIQ) is reported to be neuroprotective. In the present study, we compared the effects of artificially modified 1-MeTIQ derivatives (loading an N-propyl, N-propenyl, N-propargyl, or N-butynyl group) on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD-like signs in mice. In a behavioral study, MPTP-induced bradykinesia was significantly decreased by all compounds. However, only 1-Me-N-propargyl-TIQ showed an inhibitory effect by blocking the MPTP-induced reduction in striatal dopamine content and the number of nigral tyrosine hydroxylase-positive cells. Western blot analysis showed that 1-Me-N-propargyl-TIQ and 1-Me-N-butynyl-TIQ potently prevented the MPTP-induced decrease in dopamine transporter expression, whereas 1-MeTIQ and 1-Me-N-propyl-TIQ did not. These results suggest that although loading an N-propargyl group on 1-MeTIQ clearly enhanced neuroprotective effects, other N-functional groups showed distinct pharmacological properties characteristic of their functional groups. Thus, the number of bonds and length of the N-functional group may contribute to the observed differences in effect.

The Impact of the Combined Administration of 1MeTIQ and MK-801 on Cell Viability, Oxidative Stress Markers, and Glutamate Release in the Rat Hippocampus

MK-801, as an N-methyl-D-aspartate (NMDA) receptor inhibitor, causes elevation in glutamate release, which may lead to an increase in excitotoxicity, oxidative stress and, consequently, cell death. 1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) shows antioxidant activity. The aim of the present study was to evaluate the effect of combined treatment with 1MeTIQ and MK-801 on cell viability, antioxidant enzyme activity, and glutamate release in the rat hippocampus. Cytotoxicity was measured using lactate dehydrogenase leakage assay (LDH) and the methyl tetrazolium (MTT) assay; antioxidant enzyme activity (glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT)) were measured by ELISA kits. The release of glutamate in the rat hippocampus was measured using in vivo microdialysis methodology. An in vitro study showed that MK-801 induced cell death in a concentration-dependent manner and that 1MeTIQ partially reduced this adverse effect of MK-801. An ex vivo study indicated that MK-801 produced an increase in antioxidant enzyme activity (GPx, GR, and SOD), whereas coadministration of MK-801 and 1MeTIQ restored the activity of these enzymes to the control level. An in vivo microdialysis study demonstrated that combined treatment with both drugs decreased the release of glutamate in the rat hippocampus. The above results revealed that 1MeTIQ shows limited neuroprotective activity under conditions of glutamate-induced neurotoxicity.

The Protective Effect of Repeated 1MeTIQ Administration on the Lactacystin-Induced Impairment of Dopamine Release and Decline in TH Level in the Rat Brain

Parkinson's disease (PD) is a neurodegenerative disorder of the central nervous system (CNS) caused by a progressive loss of nigrostriatal dopaminergic neurons. Dysfunction of the ubiquitin-proteasome system (UPS) plays an important role in the pathogenesis of PD. Intranigral administration of the UPS inhibitor lactacystin is used to obtain a valuable animal model for investigating putative neuroprotective treatments for PD. 1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) is an endogenous amine that displays neuroprotective properties. This compound acts as a reversible monoamine oxidase (MAO) inhibitor and a natural free radical scavenger. In the present experiment, we investigated the effect of acute and chronic treatment with 1MeTIQ on locomotor activity and the release of dopamine as well as its metabolites in the striatum of unilaterally lactacystin-lesioned and sham-operated rats using in vivo microdialysis. Additionally, changes in the level of tyrosine hydroxylase (TH) in the substantia nigra were measured. Unilateral lactacystin injection into the substantia nigra caused significant impairment of dopamine release (approx. 45%) and a marked decline in the TH level. These effects were completely antagonized by multiple treatments with 1MeTIQ. The results obtained from the in vivo microdialysis study as well as from the ex vivo experiments suggest that multiple administration of 1MeTIQ protects dopaminergic neurons against the lactacystin-induced decline in TH concentration in the substantia nigra and prevents disturbances of dopamine release in the striatum. We have demonstrated that 1MeTIQ is capable of maintaining the physiological functions of the striatal dopamine neurons damaged by unilateral lactacystin lesion.

Multiple Administration of Endogenous Amines TIQ and 1MeTIQ Protects Against a 6-OHDA-Induced Essential Fall of Dopamine Release in the Rat Striatum: In Vivo Microdialysis Study

Parkinson’s disease (PD) represents one of the neurodegenerative disorders which are caused by degeneration of dopaminergic neurons in the nigrostriatal pathway. Different toxins, e.g., 6-hydroxydopamine (6-OHDA), are used to model PD in animals. 6-OHDA is a neurotoxin which damages catecholaminergic neurons via production of oxygen radicals. Tetrahydroisoquinolines (TIQs) are endogenous amines which are present in the mammalian brain. Some of them, like TIQ and 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), demonstrate neuroprotective properties. These compounds act as reversible MAO inhibitors and this way block free radical formation. To continue our previous experiments, we evaluated the effect of acute and chronic treatment with TIQ and 1MeTIQ on locomotor/exploratory activity and the release of dopamine as well as its metabolite 3-methoxytyramine (3-MT) in the striatum of unilaterally 6-OHDA-lesioned and sham-operated rats using in vivo microdialysis methodology. Additionally, the changes in the concentration of tyrosine hydroxylase in the substantia nigra were measured. A unilateral 6-OHDA lesion in the substantia nigra produces a strong reduction in the release of dopamine (approx. 70%) and 3-MT (approx. 50%) in the rat striatum. This effect was completely inhibited by multiple administration of TIQ and 1MeTIQ. The results obtained from the in vivo microdialysis study suggest that multiple treatment with both endogenous amines, TIQ and 1MeTIQ, protects dopaminergic neurons against a 6-OHDA-induced deficit of dopamine release. Furthermore, these amines were able to maintain physiological functions of striatal dopamine neurons damaged by a unilateral 6-OHDA lesion.

Neuroprotective Effect of the Endogenous Amine 1MeTIQ in an Animal Model of Parkinson's Disease

Parkinson’s disease (PD) is a neurodegenerative disorder that is hallmarked by pathological changes associated with the death of dopaminergic neurons, particularly in the extrapyramidal system (substantia nigra pars compacta, striatum) of the brain. Although the causes of slow neuronal death in PD are unknown, both genetic and environmental factors are likely involved. Endogenous isoquinolines, such as 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ), present in the human brain have been previously reported to participate in the pathogenesis of PD. The chronic administration of 1BnTIQ induced parkinsonism in primates, and this effect might be associated with idiopathic PD. However, another endogenous derivative of tetrahydroisoquinoline, 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), displays clear neuroprotective properties in the brain. In the present study, we investigated the neuroprotective effects of 1MeTIQ (25 and 50 mg/kg) in an animal model of PD after the chronic administration of 1BnTIQ (25 mg/kg). Behavioral analyses demonstrate that both acute and repeated treatment with 1MeTIQ completely antagonized 1BnTIQ-induced changes in rat locomotor activity. Neurochemical experiments indicate that 1MeTIQ co-administered with 1BnTIQ completely antagonized 1BnTIQ-induced reduction in the dopamine (DA) concentration in rat brain structures. In conclusion, the results demonstrate that 1MeTIQ possesses important neuroprotective properties in the animal model of PD and that the rats did not develop tolerance after its chronic administration.

Properties of 3-methyl-TIQ and 3-methyl-N-propargyl-TIQ for preventing MPTP-induced parkinsonism-like symptoms in mice

BACKGROUND

Selegiline, a therapeutic drug for Parkinson's disease (PD), structurally resembles the endogenous parkinsonism-related compound 1,2,3,4-tetrahydroisoquinoline (TIQ). In the present study, we evaluated the effects of 3-methyl-TIQ (3-MeTIQ) and 3-methyl-N-propargyl-TIQ (3-Me-N-proTIQ), selegiline mimetic TIQ derivatives, for preventing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism-like symptoms in mice.

METHODS

We evaluated the preventative effects of 3-MeTIQ and 3-Me-N-proTIQ on MPTP-induced bradykinesia and depletion of striatal dopamine (DA) and nigral tyrosine hydroxylase (TH)-positive cells.

RESULTS

MPTP-induced bradykinesia was not different when mice were pretreated with 3-MeTIQ, except for the high-dose group. However, pretreatment with 3-Me-N-proTIQ significantly prevented the appearance of this akinesic status. MPTP-induced striatal DA and 3,4-dehydroxyphenylacetic acid reduction were significantly prevented by pretreatment with 3-Me-N-proTIQ, but not 3-MeTIQ, in a dose-dependent manner. On the other hand, levels of serotonin and its metabolite, 5-hydroxyindole acetic acid, in the striatum were increased following treatment with 3-MeTIQ. In addition, the MPTP-induced decrease in TH-positive cells in the substantia nigra was significantly reduced by pretreatment with 3-Me-N-proTIQ, but not 3-MeTIQ.

CONCLUSIONS

These results suggest that not only does 3-Me-N-proTIQ have potential as a candidate compound for disease-modifying therapy for PD, but also the N-propargyl functional group plays an important role in neuroprotection.

1MeTIQ provides protection against Aβ-induced reduction of surface expression of synaptic proteins and inhibits H₂O₂-induced oxidative stress in primary hippocampal neurons

Alzheimer's disease (AD) is associated with increased brain levels of β-amyloid (Aβ) peptides, which readily self-aggregate into fibrils and oligomers that have particularly deleterious properties toward synapses of excitatory glutamatergic neurons. Here, we examined the neuroprotective effects of 1-methyl-1,2,3,4,-tetrahydroisoquinoline (1MeTIQ) against Aβ-induced loss of synaptic proteins in cultured primary hippocampal neurons. Exposure of mature primary hippocampal neurons to 10 μM synthetic Aβ1-40 over 72 h resulted in ~60 % reduction in the surface expression of NR1 subunit of the NMDA receptor (NMDAR), PSD-95, and synaptophysin, without causing neuronal death. Concomitant treatment with 500 μM of 1MeTIQ, a low-affinity NMDAR antagonist significantly ameliorated the loss of synaptic protein markers. The neuroprotective properties of 1MeTIQ were compared with those of MK-801, which at 0.5 μM concentration also prevented Aβ1-40-induced loss of synaptic proteins in primary neuronal cultures. Furthermore, we provide novel evidence demonstrating effectiveness of 1MeTIQ in reducing the level of reactive oxygen species (ROS) in primary neuronal culture system. As oxidative stress contributes importantly to neurodegeneration in AD, 1MeTIQ may provide a dual neuroproctective effect in AD both as a NMDARs antagonist and ROS formation inhibitor. 1MeTIQ occurs endogenously at low concentrations in the brain and its synthetic form readily penetrates the blood-brain barrier after the systemic administration. Our results highlight a possibility of the application of 1MeTIQ as a neuroprotective agent in AD-related neurodegeneration.

Preventative effects of 1,3-dimethyl- and 1,3-dimethyl-N-propargyl-1,2,3,4-tetrahydroisoquinoline on MPTP-induced Parkinson's disease-like symptoms in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is well known as an exogenous dopaminergic neurotoxin that induces Parkinson's disease-like symptoms. In addition, 1,2,3,4-tetrahydroisoquinoline (TIQ) derivatives have been investigated as endogenous MPTP mimetic compounds that structurally resemble selegiline, a commercially available drug for treating Parkinson's disease. In the present study, we examined the ability of 1,3-dimethyl-TIQ (1,3-diMeTIQ) and 1,3-dimethyl-N-propargyl-TIQ (1,3-diMe-N-proTIQ) to prevent MPTP-induced Parkinson's disease-like symptoms in mice and to prevent 1-methyl-4-phenylpyridinium ion (MPP+, an active metabolite of MPTP)-induced cytotoxicity in vitro, including its structural stereoselectivity. Repeated administration of MPTP induced bradykinesia, a symptom of behavioral abnormality; this was prevented by both 1,3-diMeTIQ and 1,3-diMe-N-proTIQ pretreatments. Pretreatment with 1,3-diMeTIQ did not prevent the MPTP-induced decrease in dopamine content in the striatum or the decrease in the number of tyrosine hydroxylase-positive cells in the substantia nigra. On the other hand, 1,3-diMe-N-proTIQ prevented these Parkinson's disease-like symptoms; in particular, the trans-isomer of this agent showed potent protective effects. However, the ability of the trans-1,3-diMe-N-proTIQ isomer to prevent MPP+-induced PC12 cell death was weaker than that of its cis-isomer. Thus, stereoisomers of 1,3-diMe-N-proTIQ exhibit different effects; cis-1,3-diMe-N-proTIQ inhibits MPP+-induced cytotoxicity while trans-1,3-diMe-N-proTIQ exhibits neuroprotective effects primarily through MPTP-related biological events in mice. These results also indicate the possibility of utilizing, at least in part, the stereoselective efficacy of 1,3-diMe-N-proTIQ against MPTP and/or MPP+-induced adverse states.

Parkinsonism-preventing activity of 1-methyl-1,2,3,4-tetrahydroisoquinoline derivatives in C57BL mouse in vivo

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), an endogenous parkinsonism-preventing substance, and its 5-, 6-, and 7-hydroxylated derivatives are reported to show in vitro neuroprotective activity against toxicity due to salsolinol in SH-SY5Y human neuroblastoma cells. In the present study, we tested the parkinsonism-preventing potential of these derivatives by means of the pole test in C57BL mice in vivo, and measured brain dopamine contents by liquid chromatography-tandem mass spectrometry. Parkinsonism was induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydroisoquinoline(MPTP), and pretreatment with any of the 1MeTIQ derivatives prevented its induction. 6-Hydroxy-1MeTIQ showed the greatest preventive activity. The amount of dopamine in the brain was reduced by MPTP treatment, and this reduction was suppressed by pretreatment with 1MeTIQ derivatives. These hydroxy-1MeTIQ derivatives may have potential for the treatment of Parkinson's disease as well as 1MeTIQ itself.

Synthesis and neurotoxicity of tetrahydroisoquinoline derivatives for studying Parkinson's disease

Parkinson's disease involves the progressive degeneration of dopaminergic neurons in the substantia nigra. However, the etiology of the disease remains to be elucidated. Endogenous amines, such as 1,2,3,4-tetrahydroisoquinoline (TIQ) derivatives present in the mammalian brain, are known to participate in the pathogenesis of Parkinson's disease. These endogenous neurotoxins have been extensively studied because of their structural resemblance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), an agent widely used for generating animal models of Parkinson's disease-like symptoms. Investigations of the synthesis and pharmacological properties of TIQ derivatives are expected to contribute to the development of new therapeutic agents for treating Parkinson's disease. In the present study, we describe more efficient synthesis methods for TIQ derivatives via Pummerer-type cyclization of the substrate N-acyl sulfoxide. Furthermore, the modified Pummerer reaction provided a convenient and efficient method for synthesizing various TIQs. TIQ and its derivative, 1-benzyl-TIQ, can induce parkinsonism in primates and rodents. On the other hand, one TIQ derivative, 1-methyl-TIQ, has been shown to prevent MPTP, TIQ, and 1-benzyl-TIQ induced behavioral abnormalities. Therefore, TIQ derivatives are considered to play an important role in both the onset and prevention of Parkinson's disease. In this article, we focus on the synthesis and pharmacological aspects of 1,2,3,4-tetrahydroisoquinoline derivatives in Parkinson's disease.

1-Methyl-1,2,3,4-tetrahydroisoquinoline enhances the anticonvulsant action of carbamazepine and valproate in the mouse maximal electroshock seizure model

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTHIQ - an endogenous parkinsonism-preventing substance) administered intraperitoneally at a dose of 20 mg/kg considerably elevated the threshold for electroconvulsions in mice from 6.4 to 8.4 mA (P < 0.05). In contrast, the agent administered at 5 and 10 mg/kg had no significant impact on the electroconvulsive threshold in mice. Moreover, 1-MeTHIQ (at a subthreshold dose of 10 mg/kg) potentiated the anticonvulsant action of valproate (VPA) against maximal electroshock (MES)-induced seizures in mice, reducing its median effective dose (ED50) from 232 to 170 mg/kg (P < 0.001). Similarly, 1-MeTHIQ (at 10 mg/kg) enhanced the antielectroshock activity of carbamazepine (CBZ) in mice, decreasing its ED50 from 10.8 to 7.8 mg/kg (P < 0.05). In contrast, 1-MeTHIQ (at 10 mg/kg) did not affect the anticonvulsant action of phenytoin and phenobarbital against MES-induced seizures in mice. The evaluation of acute neurotoxic effects of the studied antiepileptic drugs (AEDs) in combination with 1-MeTHIQ, as regards motor coordination impairment in the chimney test, revealed no significant changes in median toxic doses (TD50) of conventional AEDs after systemic administration of 1-MeTHIQ (up to 10 mg/kg). Pharmacokinetic characterization of interactions between 1-MeTHIQ (10 mg/kg) and VPA (170 mg/kg) or CBZ (7.8 mg/kg) revealed no significant changes in total brain concentrations of CBZ and VPA, indicating that the observed enhancement of antiseizure effects of CBZ and VPA by 1-MeTHIQ was pharmacodynamic in nature. Based on our preclinical study, it may be concluded that 1-MeTHIQ exerts the anticonvulsant effects increasing the threshold for electroconvulsions and potentiating the antiseizure action of CBZ and VPA against maximal electroshock. The antiseizure properties of 1-MeTHIQ (an endogenous parkinsonism-preventing substance) and its exact physiological role in the brain need extensive examination in further neuropharmacological studies.

Neuroprotective effect of 1-methyl-1,2,3,4-tetrahydroisoquinoline on cultured rat mesencephalic neurons in the presence or absence of various neurotoxins

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) is an endogenous brain amine and its content in parkinsonian brain is decreased compared with that in control brain. There is some evidence that 1MeTIQ protects dopaminergic neurons against dysfunction such as that seen in Parkinson's disease. In this study, we examined the neuroprotective effect of 1MeTIQ against four dopaminergic neurotoxins, 1-methyl-4-phenylpyridinuim ion, 6-hydroxydopamine, rotenone, and l-benzyl-1,2,3,4-tetrahydroisoquinoline, in cultured rat mesencephalic neurons. 1MeTIQ exerted neuroprotective action against all these toxins. Furthermore, (R)-1MeTIQ was neuroprotective, while (S)-1MeTIQ had little effect, indicating that the effect is stereoselective. The protective action of 1MeTIQ was most effective in mesencephalic neurons, especially in tyrosine hydroxylase-positive neurons. 1MeTIQ showed no affinity for dopamine receptors and did not influence the inhibition of mitochondrial respiratory complex I by rotenone, 1-methyl-4-phenylpyridinuim ion, or 1-benzyl-1,2,3,4-tetrahydroisoquinoline. These results raise the possibility that 1MeTIQ indirectly acts as an anti-oxidant such as the induction of anti-oxidative enzymes, because all these four neurotoxins can burden oxidative stress in common. This is the first report to confirm a protective effect of 1MeTIQ at the cultured neuron level, and it may have potential as a lead compound for the development of new agents to treat Parkinson's disease.

Neuroprotective or neurotoxic activity of 1-methyl-1,2,3,4-tetrahydroisoquinoline and related compounds

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) 1 and various 5- or 6,7-substituted analogues were synthesized and assayed for neurotoxicity towards SH-SY5Y cells. Among mono-substituted derivatives of 1, hydroxyl substitution decreased the toxicity, while methoxyl substitution increased it. Disubstituted derivatives of 1, 5a and 5b, showed the opposite tendency. Hydroxy-1MeTIQ derivatives were tested for neuroprotective activity, and 3b and 4b exhibited greater efficacy than 1. We suggest that hydroxy-1MeTIQ derivatives, especially 4b, may have potential for the treatment of Parkinson's disease.

Decrease of 1-methyl-1,2,3,4-tetrahydroisoquinoline synthesizing enzyme activity in the brain areas of aged rat

1-Methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ), an endogenous monoamine, which prevents the neurotoxic effect of 1-methyl-4-phenylpyridinium ion (MPP(+)) and other endogenous neurotoxins, has been described as being enzymatically formed in the brain by the 1-MeTIQ synthesizing enzyme (1-MeTIQse). In this paper, we report the brain's regional distribution of this enzyme in 3- and 24-month-old rats. The results show that the activity is spread throughout the brain, the highest activity being in the dopaminergic areas (striatum and substantia nigra) and in the cortex. During aging there was a 1-MeTIQse activity reduction ( approximately 50%) in the areas implicated in the ethyology of Parkinson disease (substantia nigra, striatum) and in the cerebral cortex.

Determination method of 1-methyl-1,2,3,4-tetrahydroisoquinoline, an endogenous parkinsonism-preventing substance, by radioimmunoassay

To develop a sensitive and simple assay method for 1-methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ), an endogenous parkinsonism-preventing substance, we designed two kinds of 1MeTIQ-bovine serum albumin (BSA) conjugates to recognize the methyl group at the 1 position of 1MeTIQ since this is the critical structural difference between 1MeTIQ and parkinsonism-inducing substances. These hapten antigens were synthesized from 1MeTIQ analogues and BSA. A specific antiserum against 1MeTIQ was obtained from a rabbit immunized with one of the hapten antigens. To utilize this antiserum for radioimmunoassay, detailed studies were carried out to establish optimum conditions. The antiserum recognized 1MeTIQ and showed little cross-reactivity with endogenous 1MeTIQ analogues and proteins. It was confirmed to be suitable for radioimmunoassay, and a standard curve was prepared in the range of 0.5 to 100 pmol of 1MeTIQ. This method was sensitive enough to measure endogenous 1MeTIQ in rat brain. This method should be applicable for evaluation of the progression or prognosis of Parkinson's disease (PD).

Stereoselective effect of (R)- and (S)-1-methyl-1,2,3,4-tetrahydroisoquinolines on a mouse model of Parkinson's disease

We carried out behavioral, pathological, and biochemical studies in order to determine whether the stereo-structure of 1-methyl-1,2,3,4-tetrahydroisoquinoline (1-MeTIQ) affects the onset of Parkinson's disease-like symptoms, which are induced by 1,2,3,4-tetrahydroisoquinoline (TIQ) in mice. Pretreatment with (R)-1-MeTIQ or its racemate (RS)-1-MeTIQ prevented the TIQ-induced bradykinesia. Pretreatment with a combination of L-DOPA and carbidopa significantly prevented subsequent TIQ-induced bradykinesia. Furthermore, the pathological study demonstrated that either (R)-1-MeTIQ or its racemate protected against TIQ-induced loss of tyrosine hydroxylase-positive cells of the substantia nigra pars compacta. (R)-1-MeTIQ and its racemate also prevented the TIQ-induced reduction in the levels of dopamine and its metabolites in the striatum. Serotonin and its metabolite were not affected by repeated administration of (RS)-1-MeTIQ or its derivatives. On the other hand, (S)-1-MeTIQ induced moderate but significant bradykinesia, whereas (R)-1-MeTIQ did not induce this behavioral abnormality at all. In addition, (S)-enantiomer prevented the onset of TIQ-induced bradykinesia, though to a lesser extent than did either (R)-enantiomer or its racemate. However, (S)-enantiomer did not prevent the loss of tyrosine hydroxylase-positive neurons in the substantia nigra pars compacta. We concluded that (R)-1-MeTIQ, and not (S)-enantiomer, plays a crucial role in protection against TIQ-induced parkinsonism, a fact which suggests that enantiomeric biochemical events such as 1-MeTIQ biosynthesis may participate in the pathogenesis of Parkinson's disease.

Biochemical and pathological study of endogenous 1-benzyl-1,2,3,4-tetrahydroisoquinoline-induced parkinsonism in the mouse

We administered 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1-BnTIQ; 80 mg/kg, i.p.), an endogenous neurotoxin known to cause bradykinesia, the Parkinson's disease-like symptom, in order to obtain biochemical and pathological evidence of behavioral abnormalities. Immunohistochemical analysis demonstrated that 1-BnTIQ did not decrease the number of tyrosine hydroxylase-positive cells in the substantia nigra. Biochemical analysis demonstrated significantly increased striatal dopamine (DA) content, while DA metabolites in the striatum remained at control levels. We concluded that the 1-BnTIQ-induced bradykinesia has a different mechanism of action than that underlying the MPTP-induced depletion of striatal DA neurons.