Selegiline protects dopaminergic neurons in culture from toxic factor(s) present in the cerebrospinal fluid of patients with Parkinson's disease

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.

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-Benzyl-1,2,3,4-tetrahydroisoquinoline, a Parkinsonism-inducing endogenous toxin, increases alpha-synuclein expression and causes nuclear damage in human dopaminergic cells

1-Benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ), an endogenous neurotoxin, is known to cause parkinsonism in rodents and nonhuman primates. The levels of 1BnTIQ in cerebrospinal fluid of patients with Parkinson's disease (PD) were reported to be three times higher than those in control subjects. In the present study, we have evaluated the effects of 1BnTIQ on alpha-synuclein (alpha-syn) expression together with biochemical and morphological changes in human dopaminergic SH-SY5Y cells in culture. 1BnTIQ at lower concentrations (1-50 microM) increased alpha-syn protein expression in a time- and dose-dependent manner in these cells. There was also up-regulation of alpha-syn mRNA by 1BnTIQ. Inhibition of complex I by rotenone and depletion of glutathione by L-buthionine sulfoxamine also correlated with an increase in alpha-syn expression, suggesting that oxidative stress may cause an increase in alpha-syn levels in dopaminergic cells. Furthermore, 1BnTIQ significantly depleted glutathione levels. 1BnTIQ at higher concentrations (500 microM) increased reactive oxygen species levels, decreased ATP levels, and caused nuclear damage in the cells. The 1BnTIQ-induced alpha-syn up-regulation was inhibited by cotreatment with the antioxidants selegiline, coenzyme Q(10), and N-acetylcystein and the caspase inhibitor DEVD-CHO. Taken together, these results suggest that alpha-syn up-regulation and oxidative stress are contributing factors in 1BnTIQ-induced neurotoxicity in dopaminergic neurons in PD.

Neurotoxicity of an endogenous brain amine, 1-benzyl-1,2,3,4-tetrahydroisoquinoline, in organotypic slice co-culture of mesencephalon and striatum

Organotypic slice co-culture of the ventromedial portion of the mesencephalon and striatum was used to evaluate the neurotoxicity of 1-benzyl-1,2,3,4-tetrahydroisoquinoline, an endogenous brain amine related to Parkinson's disease. 1-Benzyl-1,2,3,4-tetrahydroisoquinoline is specifically increased in the cerebrospinal fluid of patients with Parkinson's disease and induces parkinsonian features in the monkey and mouse. Here, it decreased the dopamine content of the cultured mesencephalon in both dose- (10-100 microM) and time- (24 h to 7 days) dependent manners. This result suggests that the neurotoxicity of 1-benzyl-1,2,3,4-tetrahydroisoquinoline is correlated with the overall exposure (concentration multiplied by exposure time). Culture with 100 microM 1-benzyl-1,2,3,4-tetrahydroisoquinoline for 24 h irreversibly reduced the dopamine content. Furthermore, culture with 100 microM 1-benzyl-1,2,3,4-tetrahydroisoquinoline for 10 days caused morphological changes, including cell body shrinkage and distortion of dendritic morphology, in tyrosine hydroxylase-positive cells in the mesencephalon and reduced the number of cells by half. The increase in lactate dehydrogenase activity in the media produced by 1-benzyl-1,2,3,4-tetrahydroisoquinoline was significant in culture of the mesencephalon alone or its co-culture with striatum, but not in cultures of other brain regions. We suggest that 1-benzyl-1,2,3,4-tetrahydroisoquinoline is toxic to tyrosine hydroxylase-positive cells in the ventral mesencephalon and that it is correlated with the integral of the concentration by time of exposure. Thus a low concentration of 1-benzyl-1,2,3,4-tetrahydroisoquinoline may first induce a decrease in the dopamine content then shrinkage of the cell body, followed by the slow death of dopaminergic neurons over a long period. This is the first report that indicates 1-benzyl-1,2,3,4-tetrahydroisoquinoline exerts neurotoxicity at the cellular level, and reveals in part the character of its neurotoxicity.

[Tetrahydroisoquinoline derivatives as possible Parkinson's disease-inducing substances]

Parkinson's disease (PD) is believed to be induced by the interaction of genetic predisposition and environmental factors, and a type of neurotoxin is proposed to be one of the environmental factors. We designed and synthesized a molecule, 1-benzyl-1,2,3,4-tetrahydroisoquinoline (1BnTIQ) as a possible PD-eliciting neurotoxin and evaluated its characteristics relevant to PD. 1BnTIQ is an endogenous amine in the brain and the 1BnTIQ content increases in the patients with PD. Repeated administration of 1BnTIQ induced PD-like symptoms in monkeys and mice. 1BnTIQ was biosynthesized from 2-phenylethylamine and phenylacetaldehyde, which is a metabolite of 2-phenylethylamine, and used in in vivo and in vitro studies. 1BnTIQ inhibited [3H] dopamine uptake in HEK293 cells which stably express dopamine transporter. 1BnTIQ also inhibited NADH-ubiquinone oxidoreductase (complex I) in the mitochondrial respiratory chain. Next, we assessed 1BnTIQ neurotoxicity in the organotypic coculture of the ventromedial portion of the mesencephalon and striatum. 1BnTIQ decreased the dopamine content in the mesencephalon in both dose- and time-dependent manners and it irreversibly reduced the dopamine content. Furthermore, it caused morphological changes in tyrosine hydroxylase-positive cells in the mesencephalon and reduced the number of cells. 1-(3',4'-Dihydroxybenzyl)-1,2,3,4-tetrahydroisoquinoline (3'4'DHBnTIQ) is also an endogenous parkinsonism-inducing 1BnTIQ derivative. In vivo and in vitro studies revealed that 3'4'DHBnTIQ was O-methylated by soluble catechol-O-methyltransferase (COMT). The result that COMT inhibitor suppressed 3'4'DHBnTIQ neurotoxicity suggests that 3'4'DHBnTIQ is metabolically activated by COMT to exert toxic effects.

Biphasic effects of selegiline on striatal dopamine: lack of effect on methamphetamine-induced dopamine depletion

We tested the hypothesis that selegiline can attenuate dopamine depletion if administered following high doses of methamphetamine that cause neurotoxicity in the striatum. Methamphetamine produced decreases of 50% or greater in both striatal concentrations of dopamine and combined concentrations of homovanillic acid and DOPAC in mice. For animals not exposed to methamphetamine, chronic treatment with selegiline over 18 days caused biphasic effects on striatal dopamine content, with decreases, no effect, or increases observed for mice receiving treatment with 0.02, 0.2, and 2.0 mg/kg, respectively. Selegiline failed to modify methamphetamine-induced reductions in striatal dopamine content or combined concentrations of homovanillic acid and DOPAC. Significant increases in mortality following the onset of selegiline treatment (24 hours after the initial dose of methamphetamine) occurred in methamphetamine-treated mice that received saline or 2.0 mg/kg of selegiline, but not for mice treated with 0.02 or 0.2 mg/kg of selegiline. These results indicate that selegiline fails to attenuate dopamine depletion when administered chronically following exposure to methamphetamine, but may attenuate methamphetamine-induced mortality. In control animals that did not receive methamphetamine, low doses of selegiline produced decreases the concentration of striatal dopamine, while high dose treatment caused increases in striatal dopamine content.

Neurorescuing effects of the GAPDH ligand CGP 3466B

(-)-Deprenyl, used for the treatment of Parkinson's disease, was reported to possess neurorescuing/antiapoptotic effects independent of its MAO-B inhibiting properties. It is metabolized to (-)-desmethyldeprenyl, which seems to be the active principle, and further to (-)-amphetamine and (-)-methamphetamine, which antagonize its rescuing effects. These complications may explain the limited neurorescuing potential of (-)-deprenyl observed clinically. CGP 3466 (dibenzo[b,f]oxepin-10-ylmethyl-methyl-prop-2-ynyl-amine), structurally related to (-)-deprenyl, exhibits virtually no MAO-B nor MAO-A inhibiting properties and is not metabolized to amphetamines. It was shown to bind to glyceraldehyde-3-phosphate dehydrogenase, a glycolytic enzyme with multiple other functions including an involvement in apoptosis, and shows neurorescuing properties qualitatively similar to, but about 100-fold more potent than those of (-)-deprenyl in several in vitro and in vivo paradigms. In concentrations ranging from 10(-13)-10(-5) M, it rescues partially differentiated PC12 cells from apoptosis induced by trophic withdrawal, cerebellar granule cells from apoptosis induced by cytosine arabinoside, rat embryonic mesencephalic dopaminergic cells from death caused by MPP+, and PAJU human neuroblastoma cells from death caused by rotenone. However, it did not affect apoptosis elicited by a variety of agents in rapidly proliferating cells from thymus or skin or in liver or kidney cells. In vivo, it rescued facial motor neuron cell bodies in rat pups after axotomy, rat hippocampal CA1 neurons after transient ischemia/hypoxia, and mouse nigral dopaminergic cell bodies from death induced by MPTP, in doses ranging between 0.0003 and 0.1 mg/kg p.o. or s.c., depending on the model. It also partially prevented the loss of tyrosine hydroxylase immunoreactivity in the substantia nigra of 6-OHDA-lesioned rats and improved motor function in these animals. Moreover, it prolonged the life-span of progressive motor neuronopathy (pmn) mice (a model for ALS), preserved their body weight and improved their motor performance. This was accompanied by a decreased loss of motor neurons and motor neuron fibers, and protection of mitochondria. The active concentration- or dose-ranges in the different in vitro and in vivo paradigms were remarkably similar. In several paradigms, bell-shaped dose-response curves were observed, the rescuing effect being lost above about 1 mg/kg, a fact that must be considered in clinical investigations.

Neurotrophins and their receptors in nerve injury and repair

Cytokines are a heterogenous group of polypeptide mediators that have been associated with activation of numerous functions, including the immune system and inflammatory responses. The cytokine families include, but are not limited to, interleukins (IL-I alpha, IL-I beta, ILIra and IL-2-IL-15), chemokines (IL-8/ NAP-I, NAP-2, MIP-I alpha and beta, MCAF/MCP-1, MGSA and RANTES), tumor necrosis factors (TNF-alpha and TNF-beta), interferons (INF-alpha, beta and gamma), colony stimulating factors (G-CSF, M-CSF, GM-CSF, IL-3 and some of the other ILs), growth factors (EGF, FGF, PDGF, TGF alpha, TGF beta and ECGF), neuropoietins (LIF, CNTF, OM and IL-6), and neurotrophins (BDNF, NGF, NT-3-NT-6 and GDNF). The neurotrophins represent a family of survival and differentiation factors that exert profound effects in the central and peripheral nervous system (PNS). The neurotrophins are currently under investigation as therapeutic agents for the treatment of neurodegenerative disorders and nerve injury either individually or in combination with other trophic factors such as ciliary neurotrophic factor (CNTF) or fibroblast growth factor (FGF). Responsiveness of neurons to a given neurotrophin is governed by the expression of two classes of cell surface receptor. For nerve growth factor (NGF), these are p75NTR (p75) and p140trk (referred to as trk or trkA), which binds both BDNF and neurotrophin (NT)-4/5, and trkC receptor, which binds only NT-3. After binding ligand, the neurotrophin-receptor complex is internalized and retrogradely transported in the axon to the soma. Both receptors undergo ligand-induced dimerization, which activates multiple signal transduction pathways. These include the ras-dependent pathway utilized by trk to mediate neurotrophin effects such as survival and differentiation. Indeed, cellular diversity in the nervous system evolves from the concerted processes of cell proliferation, differentiation, migration, survival, and synapse formation. Neural adhesion and extracellular matrix molecules have been shown to play crucial roles in axonal migration, guidance, and growth cone targeting. Proinflammatory cytokines, released by activated macrophages and monocytes during infection, can act on neural targets that control thermogenesis, behavior, and mood. In addition to induction of fever, cytokines induce other biological functions associated with the acute phase response, including hypophagia and sleep. Cytokine production has been detected within the central nervous system as a result of brain injury, following stab wound to the brain, during viral and bacterial infections (AIDS and meningitis), and in neurodegenerative processes (multiple sclerosis and Alzheimer's disease). Novel cytokine therapies, such as anticytokine antibodies or specific receptor antagonists acting on the cytokine network may provide an optimistic feature for treatment of multiple sclerosis and other diseases in which cytokines have been implicated.