Aging attenuates glucocorticoid negative feedback in rat brain.

Aging is thought to be a risk factor to develop vulnerability of the neuroendocrine system, including the hypothalamic-pituitary-adrenal (HPA) axis, and dysregulation of this axis characterized by dexamethasone (DEX)-mediated negative feedback resistance is sometimes observed in elderly humans and animals. However, the influence of aging on the feedback system including an involvement of the brain is not fully understood. In the present study, we examined the suppressive effects of DEX by the systemic injection or the intracranial infusion into the prefrontal cortex (PFC), hippocampus, and hypothalamus on circulating corticosterone levels, and compared between young (3-month-old) and aged (24-month-old) rats. Moreover, we examined expression levels of glucocorticoid receptors (GRs) and their translocation from the cytoplasm to the nucleus using immunohistochemical and Western immunoblot techniques in the pituitary in addition to three brain regions. When DEX was injected systemically, the suppressive response was significantly enhanced in aged rats, compared with young rats. When DEX was infused into three brain regions, the suppressive response to DEX was abolished in aged rats. The immunohistochemical analysis revealed that the number of GR positive cells in the PFC, hippocampus, and hypothalamus was decreased, but that in the pituitary was increased, in aged rats, compared with young rats. The Western immunoblot analysis confirmed these results. Thus, basal expression levels of GRs in three brain regions were decreased, but those in the pituitary were increased, in aged rats. After the injection or infusion of DEX, the translocation of GRs in three brain regions was reduced, but that in the pituitary was enhanced, in aged rats. These results suggest that aging in rats enhances the feedback ability at the systemic level, which mainly involves the pituitary, but it attenuates the ability in the brain. These mechanisms may underlie the vulnerable neuroendocrine systems associated with aging.

Neuroprotection by propargylamines in Parkinson's disease: intracellular mechanism underlying the anti-apoptotic function and search for clinical markers.

In Parkinson's and other neurodegenerative diseases, a therapeutic strategy has been proposed to halt progressive cell death. Propargylamine derivatives, rasagiline and (-)deprenyl (selegiline), have been confirmed to protect neurons against cell death induced by various insults in cellular and animal models of neurodegenerative disorders. In this paper, the mechanism and the markers of the neuroprotection are reviewed. Propargylamines prevent the mitochondrial permeabilization, membrane potential decline, cytochrome c release, caspase activation and nuclear translocation of glyceraldehyde 3-phosphate dehydrogenase. At the same time, rasagiline induces anti-apoptotic pro-survival proteins, Bcl-2 and glial cell-line derived neurotrophic factor, which is mediated by activated ERK-NF-kappaB signal pathway. DNA array studies indicate that rasagiline increases the expression of the genes coding mitochondrial energy synthesis, inhibitors of apoptosis, transcription factors, kinases and ubiquitin-proteasome system, sequentially in a time-dependent way. Products of cell survival-related gene induced by propargylamines may be applied as markers of neuroprotection in clinical samples.

Involvement of type A monoamine oxidase in neurodegeneration: regulation of mitochondrial signaling leading to cell death or neuroprotection.

In neurodegenerative diseases, including Parkinson's and Alzheimer's diseases, apoptosis is a common type of cell death, and mitochondria emerge as the major organelle to initiate death cascade. Monoamine oxidase (MAO) in the mitochondrial outer membrane produces hydrogen peroxide by oxidation of monoamine substrates, and induces oxidative stress resulting in neuronal degeneration. On the other hand, a series of inhibitors of type B MAO (MAO-B) protect neurons from cell death. These results suggest that MAO may be involved in the cell death process initiated in mitochondria. However, the direct involvement of MAO in the apoptotic signaling has been scarcely reported. In this paper, we present our recent results on the role of MAO in activating and regulating cell death processing in mitochondria. Type A MAO (MAO-A) was found to bind an endogenous dopaminergic neurotoxin, N-methyl(R)salsolinol, and induce apoptosis in dopaminergic SH-SY5Y cells containing only MAO-A. To examine the intervention of MAO-B in apoptotic process, human MAO-B cDNA was transfected to SH-SY5Y cells, but the sensitivity to N-methyl(R)salsolinol was not affected, even though the activity and protein of MAO-B were expressed markedly. MAO-B oxidized dopamine with production of hydrogen peroxide, whereas in control cells expressing only MAO-A, dopamine autoxidation produced superoxide and dopamine-quinone, and induced mitochondrial permeability transition and apoptosis. Rasagiline and other MAO-B inhibitors prevent the activation of apoptotic cascade and induce prosurvival genes, such as bcl-2 and glial cell line-derived neurotrophic factor, in MAO-A-containing cells. These results demonstrate a novel function of MAO-A in the induction and regulation of apoptosis. Future studies will clarify more detailed mechanism behind regulation of mitochondrial death signaling by MAO-A, and bring out new strategies to cure or ameliorate the decline of neurons in neurodegenerative disorders.

The effect of neuromelanin on the proteasome activity in human dopaminergic SH-SY5Y cells.

In Parkinson's disease (PD), the selective depletion of dopamine neurons in the substantia nigra, particular those containing neuromelanin (NM), is the characteristic pathological feature. The role of NM in the cell death of dopamine neurons has been considered either to be neurotoxic or neuroprotective, but the precise mechanism has never been elucidated. In human brain, NM is synthesized by polymerization of dopamine and relating quinones, to which bind heavy metals including iron. The effects of NM prepared from human brain were examined using human dopaminergic SH-SY5Y cells. It was found that NM inhibits 26S proteasome activity through generation of reactive oxygen and nitrogen species from mitochondria. The mitochondrial dysfunction was also induced by oxidative stress mediated by iron released from NM. NM accumulated in dopamine neurons in ageing may determine the selective vulnerability of dopamine neurons in PD.

N-methyl(R)salsolinol produces hydroxyl radicals: involvement to neurotoxicity.

Recently, (R)-1,2-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [N-methyl-(R)salsolinol, NM(R)Sal] and 1,2-dimethyl-6,7-dihydroxyisoquinolinium ion [DiMeDHIQ+] were found to cause a syndrome similar to parkinsonism in rodents. NM(R)Sal is produced in the brain by N-methylation of a naturally occurring catechol isoquinoline, 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [(R)salsolinol, (R)Sal], which is formed from dopamine. The mechanism of NM(R)Sal cytotoxicity to dopamine neurons was examined using in vitro experiments. NM(R)Sal was found to be nonenzymatically oxidized into DiMeDHIQ+, with concomitant formation of hydroxyl radicals. The oxidation and the radical production were completely inhibited by the antioxidants, ascorbic acid and reduced glutathione, and the radical formation was enhanced by Fe(II) and, to a less extent, by Fe(III). The oxidation of NM(R)Sal into DiMeDHIQ+ and the production of hydroxyl radicals may be essential for neurotoxicity to develop in dopamine neurons. The possible involvement of this catechol isoquinoline in the pathogenesis of Parkinson's disease is discussed.

Peroxynitrite induces GADD34, 45, and 153 VIA p38 MAPK in human neuroblastoma SH-SY5Y cells.

Peroxynitrite, one of the most reactive radicals, is produced from superoxide anion and nitric oxide. A peroxynitrite generator, 3-morpholinosydonimine (SIN-1), was found to induce the expression of three different growth arrest and DNA damage-inducible (GADD) mRNA, GADD34, GADD45, and GADD153, at the early phase during cell death in human neuroblastoma SH-SY5Y cells. In addition, peroxynitrite activated p38 MAPK just before induction of three GADD mRNA. A specific inhibitor of p38 MAPK, SB202190, markedly suppressed peroxynitrite-induced expression of three GADD mRNA in SH-SY5Y cells. The expression of three GADD genes and also p38 MAPK phosphorylation were suppressed by treatment with radical scavengers, superoxide dismutase plus catalase and glutathione. Glutathione depletion by L-buthionine-S, R-sulfoximine (BSO), increased the vulnerability of the cells to peroxynitrite. These findings indicate that peroxynitrite-mediated oxidative stress activated p38 MAPK to induce three GADD genes.

Immunohistochemical detection of dityrosine in lipofuscin pigments in the aged human brain.

Lipofuscin is a yellowish brown fluorescent pigment which is sequestered within cytoplasmic granules during aging. To examine the contribution of protein oxidation to lipofuscin accumulation, we performed immunohistochemical detection of dityrosine, which is considered one of the specific markers for protein oxidation, in lipofuscin in the aged human brain using an antibody specific to dityrosine. By characterization using competitive enzyme-linked immunosorbent assay, the specificity of the antibody to dityrosine was confirmed. None of the other tyrosine-related compounds such as L-tyrosine, 3-nitrotyrosine, 3-chlorotyrosine, or 3,4-dihydroxyphenylalanine cross-reacted with the antibody. The anti-dityrosine antibody reacted with lipofuscin granules in the pyramidal neurons of the aged human brain. The results suggest that protein oxidation by free radicals and/or peroxidases may play an important role in lipofuscin accumulation.

Functional interaction of Fas-associated phosphatase-1 (FAP-1) with p75(NTR) and their effect on NF-kappaB activation.

The common neurotrophin receptor p75(NTR), a member of the tumor necrosis factor (TNF) receptor superfamily, plays an important role in several cellular signaling cascades, including that leading to apoptosis. FAP-1 (Fas-associated phosphatase-1), which binds to the cytoplasmic tail of Fas, was originally identified as a negative regulator of Fas-mediated apoptosis. Here we have shown by co-immunoprecipitation that FAP-1 also binds to the p75(NTR) cytoplasmic domain in vivo through the interaction between the third PDZ domain of FAP-1 and C-terminal Ser-Pro-Val residues of p75(NTR). Furthermore, cells expressing a FAP-1/green fluorescent protein showed intracellular co-localization of FAP-1 and p75(NTR) at the plasma membrane. To elucidate the functional role of this physical interaction, we examined TRAF6 (TNF receptor-associated factor 6)-mediated NF-kappaB activation and tamoxifen-induced apoptosis in 293T cells expressing p75(NTR). The results revealed that TRAF6-mediated NF-kappaB activation was suppressed by p75(NTR) and that the p75(NTR)-mediated NF-kappaB suppression was reduced by FAP-1 expression. Interestingly, a mutant of the p75(NTR) intracellular domain with a single substitution of a Met for Val in its C-terminus, which cannot interact with FAP-1, displayed enhanced pro-apoptotic activity in 293T transfected cells. Thus, similar to Fas, FAP-1 may be involved in suppressing p75(NTR)-mediated pro-apoptotic signaling through its interaction with three C-terminal amino acids (tSPV). Thus, FAP-1 may regulate p75(NTR)-mediated signal transduction by physiological interaction through its third PDZ domain.

Cell death of dopamine neurons in aging and Parkinson's disease.

Dopamine neurons in the substantia nigra of human brain are selectively vulnerable and the number decline by aging at 5-10% per decade. Enzymatic and non-enzymatic oxidation of dopamine generates reactive oxygen species, which induces apoptotic cell death in dopamine neurons. Parkinson's disease (PD) is also caused by selective cell death of dopamine neurons in this brain region. The pathogenesis of Parkinson's disease remains to be an enigma, but it was found that an endogenous MPTP-like neurotoxin, 1(R), 2(N)-dimethyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline [N-methyl(R)salsolinol, NM(R)Sal], may be one of the pathogenic agents of PD. NM(R)Sal increases in cerebrospinal fluid from untreated parkinsonian patients, and two enzymes, a (R)salsolinol synthase and a neutral N-methyltransferase, synthesize this neurotoxin in the nigro-striatum. The activity of a neutral N-methyltransferase is significantly higher in lymphocytes from parkinsonian patients than in control. The mechanism of cell death by this toxin was proved to be by the induction of apoptosis, by use of dopaminergic SH-SY5Y cells. The apoptosis was suppressed by anti-oxidants, suggesting that the generation of reactive oxygen species may initiate cellular death process. These results indicate that in aging and PD oxidative stress induces degeneration of dopamine neurons, and the antioxidant therapy may delay the decline of dopamine neurons in the brain.

Neuroprotection by (-)-deprenyl and related compounds.

There is an increasing number of data by in vitro and in vivo experiments, indicating that (-)-deprenyl is neuroprotective to dopamine neurons, even though detailed mechanism remains to be clarified. In this paper neuroprotection by (-)-deprenyl and structurally related compounds was examined in concern with the suppression of apoptosis induced by a reactive oxygen species, peroxynitrite generated from SIN-1. The apoptotic DNA damage was quantitatively determined using dopaminergic SH-SYSY cells and by a single cell gel electrophoresis (comet) assay. DNA damage induced by peroxynitrite was proved to be apoptotic by prevention of the damage by cycloheximide or actinomycin-D. (-)-Deprenyl and other propargylamines protected the cells from apoptosis in a dose-dependent way. (-)-Deprenyl protected the cells even after it was washed out, suggesting that it may initiate the intracellular process to repress the apoptotic death program. The study on the structure-activity relationship of (-)-deprenyl analogues revealed that a N-propargyl residue with adequate size of hydrophobic structure is essentially required for the anti-apoptotic activity. These results suggest that (-)-deprenyl and related compounds may protect neurons from apoptosis and be applicable to delay the deterioration of neurons during advancing ageing and in neurodegenerative disorders.

Mechanism underlying anti-apoptotic activity of a (-)deprenyl-related propargylamine, rasagiline.

A potent inhibitor of type B monoamine oxidase, (-)deprenyl, is known to protect or rescue dying neurons, independent of inhibition of the enzyme activity. After long term administration to rodents, a propargylamine structurally related to (-)deprenyl, (R)(+)-N-propargyl-1-aminoindan (rasagiline) increased the activities of anti-oxidative enzymes, superoxide dismutase and catalase. Rasagiline protected in vitro dopamine cells from apoptosis induced by oxidative stress or neurotoxins. The mechanism of the anti-apoptotic effect was studied by in vitro experiments using human dopaminergic neuroblastoma, SH-SY5Y cells. Peroxynitrite-generating N-morpholino sydonimine (SIN-1) induced apoptosis in SH-SY5Y cells via disruption of mitochondrial membrane potential (DeltaPsim), followed by caspase 3 activation. Rasagiline prevented the loss of DeltaPsim, the initial step to apoptosis, and also following caspase 3-activation and DNA fragmentation. The results suggest that rasagiline may interact with the specific molecule in the mitochondria and suppress the death signal transduction. By the anti-apoptotic function, rasagiline may rescue or protect declining neurons in aging and neurodegenerative disorders, such as Parkinson's disease.

Inhibition of tryptophan hydroxylase by dopamine and the precursor amino acids.

Effects of dopamine and its precursor amino acids on the activity of tryptophan hydroxylase were examined. They inhibited the enzyme activity prepared from mastocytoma cells in terms of the biopterin cofactor and the substrate L-tryptophan. In relation to the biopterin, tryptophan hydroxylase was found to have two different kinetics, and dopamine inhibited the activity in a non-competitive way to both the components. Dopamine had the highest affinity to the enzyme, followed by L-DOPA and L-tyrosine, while D-tyrosine did not inhibit the activity. In terms of L-tryptophan, L-tyrosine, L-DOPA and dopamine inhibited the enzyme non-competitively and their affinity to the enzyme was in this order. These results indicate that the indoleamine metabolism may be regulated by catecholamines and their related amino acids in the brain.

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.

Inhibition of type A monoamine oxidase by 2(N)-methyl-6,7-dihydroxyisoquinolinium ions.

In the human brain, monoamine-derived 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinolines and 1,2,3,4-tetrahydroisoquinolines have been identified and their enzymatic methylation into N(2)-methylisoquinolines has been also confirmed. N-methylated 6,7-dihydroxyisoquinolines were found to be oxidized into 6,7-dihydroxy-N-methylisoquinolinium ions. The effects of the isoquinolinium ions on type A and B monoamine oxidase were examined, using enzyme samples isolated from human brain synaptosomal mitochondria. 1,2-Dimethyl-6,7-dihydroxyisoquinolinium ion (N-methylsalsolinium ion) and 2-methyl-6,7-dihydroxyisoquinolinium ion (N-methylnorsalsolinium ion), were found to be potent inhibitors of type A monoamine oxidase. The inhibition was competitive to the substrate, while the isoquinolinium ions were much weaker inhibitors of type B and the inhibition was non-competitive to the substrate. Isoquinolinium ions without catechol structure, N(2)-methylisoquinolinium ion and 1,2-dimethylisoquinolinium ion also inhibited both type A and B monoamine oxidase. 1,2-Dimethylisoquinolinium was the most potent inhibitor among examined isoquinolines, followed by the N-methylsalsolinium ion. The activity-structure relationship of the isoquinolines with and without catechol structure was examined in terms of potency and selectivity of inhibition to type A and B monoamine oxidase. Catechol structure was found to increase the selectivity of inhibition to type A, as shown by comparison of N-methylsalsolinium ion with 1,2-dimethylisoquinolinium ion. N-Methylsalsolinium ion inhibited type A MAO more selectively than 1,2-dimethylisoquinolinium ion, which inhibited type A and type B with almost the sam values of the inhibitor constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiapoptotic properties of rasagiline, N-propargylamine-1(R)-aminoindan, and its optical (S)-isomer, TV1022.

Rasagiline and structurally related propargylamines protected dopaminergic SH-SY5Y cells from apoptosis induced by 6-OHDA and peroxynitrite-generating SIN-1. It was suggested that the intracellular mechanism of the neuroprotection is related to the stabilization of mitochondrial membrane potential, as indicated by use of a fluorescent indicator, JC-1. The opening of the permeability transition pore (PTP) was prevented by rasagiline, even in isolated mitochondria. The activation of apoptotic cascade by the oxidative stress and neurotoxins, such as activation of caspase 3 and DNA fragmentation, was also inhibited by pretreatment with rasagiline. These propargylamines may prevent or rescue declining neurons induced by mitochondrial apoptotic cascade and may be applicable as "neuroprotective agents" in aging and age-related neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases.

Do antioxidant strategies work against aging and age-associated disorders? Propargylamines: a possible antioxidant strategy.

The free radical theory of aging was initially proposed by Harman half a century ago primarily to explain biological aging processes. Although administration of so-called antioxidant chemicals, which have been tested in the past for several decades, turned out to be mostly ineffective in prolonging the life spans of animals, the same theory of age-associated diseases appears to be increasingly supported in the last two decades. Despite these difficulties, the success in extending life span of 4 different animal species (mice, rats, hamsters, and dogs) with (-)deprenyl (including a study of our group) indicates that there might exist another type of antioxidant strategy in addition to a simple administration of antioxidant chemicals. (-)Deprenyl has also been shown to increase superoxide dismutase (SOD) and catalase (CAT) activities selectively in brain dopaminergic tissues. Interestingly, we have recently shown that another propargylamine, rasagiline not only increases antioxidant enzyme activities (CAT and SOD) in brain dopaminergic regions as (-)deprenyl does, but also increases CAT and SOD activities in extrabrain catecholaminergic systems such as the heart and kidneys as well. These recent observations coupled with previous observations on the life span of animals with (-)deprenyl suggest that pharmacological modulation of endogenous antioxidant enzyme activities could be one potential antioxidant strategy against aging and age-associated disorders. If the causal relationship between the two effects of (-)deprenyl exists as we hypothesized, we might be able to advance the elucidation of mechanism(s) of aging based on the free radical theory of aging.

Characterization of the in vivo action of (R)-salsolinol, an endogenous metabolite of alcohol, on serotonin and dopamine metabolism: a microdialysis study.

Using a microdialysis-HPLC technique in conscious rats, we examined the action of (R)-1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, (R)-salsolinol (R-Sal), a possible endogenous metabolite of alcohol, on serotonin (5-HT) and dopamine (DA) metabolism in four regions of the brain: the striatum, the substantia nigra, the hippocampus and the hypothalamus. Following 1 mM R-Sal perfusion, the dialysate level of 5-HT in the striatum markedly increased from non-detectable levels to 4259.2 +/- 617.5 nM, while DA increased from 3.4 +/- 0.9 nM to 206.0 +/- 56.5 nM. This increase was one order of magnitude larger in 5-HT than in DA. Conversely, the output of 5-hydroxyindoleacetic acid decreased markedly to non-detectable levels, while 3,4-dihydroxyphenylacetic acid and homovanillic acid outputs decreased below 40% of basal levels. These effects were dose-related to R-Sal (1 microM to 1 mM) and were confirmed also in 3 other brain regions. The R-Sal-induced responses in the striatum were observed even after pretreatment of 2 microM tetrodotoxin, a blocker of nerve-firing activity, via the dialysis membrane. The repetitive perfusion with 1 mM R-Sal into the striatum induced the reproducible response of 5-HT and DA. Furthermore, the potencies of 1 mM R-Sal to increase the output of 5-HT and DA were approximately 783.0-fold and 2.6-fold stronger, respectively, than those of the same dose of methamphetamine. The results suggest that R-Sal acts to stimulate a release of monoamines, 5-HT preferentially, with inhibition of monoamine oxidase and catechol-O-methyltransferase activities.

Structural studies of condensation products of biogenic amines as inhibitors of tryptophan hydroxylase.

The effects of condensation products of dopamine and indoleamines on the activity of tryptophan hydroxylase (TPH) were evaluated to determine the structures associated with modulation of this enzyme activity. The compounds having a catechol structure, such as 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline, markedly inhibited the activity of the enzyme prepared from the rat brain. The inhibition was non-competitive in terms of both the biopterin cofactor and the substrate L-tryptophan. Substitution on the one or two positions of catechol isoquinolines did not affect the inhibitory activity towards TPH. Among these compounds, a charged substance, 1,2[N]-dimethyl-6,7-dihydroxy-isoquinolinium ion, was an extremely potent inhibitor; the Ki values were 0.88 +/- 0.17 and 0.64 +/- 0.08 microM (mean +/- S.D.) in terms of the substrate and cofactor, respectively. By contrast, the condensation products of tryptophan and tryptamine with acetaldehyde scarcely affected TPH activities. 1-Methyl-1,2,3,4-tetrahydroisoquinoline, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ion (MPP+) were almost inactive. These results indicated that the catechol structure recognized and combined with TPH at a binding site different from that of the substrate or cofactor and the positive charge on the dopamine-derived substance enhanced the affinity to TPH. The selective inhibition of TPH by dopamine-derived catechol isoquinolines was discussed in relationship to the interactions between catecholamines, indoleamines and their metabolites in the brain under physiological and pathological conditions.

Dopamine-derived endogenous 1(R),2(N)-dimethyl-6,7-dihydroxy- 1,2,3,4-tetrahydroisoquinoline, N-methyl-(R)-salsolinol, induced parkinsonism in rat: biochemical, pathological and behavioral studies.

Dopamine-derived 1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (salsolinol, Sal) and related compounds were examined for their selective neurotoxicity to dopamine neurons by injection into the rat striatum. Among salsolinol analogs examined, only N-methyl-(R)- salsolinol (NM(R)Sal) induced behavioral changes very similar to those in Parkinson's disease: hypokinesia, stiff tail, limb twitching at rest and postural abnormality. Biochemical analysis showed that after NM(R)Sal injection, NM(R)Sal itself and its oxidation product, 1-2-dimethyl-6,7-dihydroxyisoquinolinium ion (DMDHIQ+) accumulated in the striatum, and also in the substantia nigra definite amount of DMDHIQ+ was detected. Dopamine and noradrenaline were reduced in the striatum and more markedly in the substantia nigra, whereas serotonin and its metabolite were not affected. Morphological analysis revealed selective reduction of tyrosine hydroxylase (TH)-containing neurons in the substantia nigra after continuous NM(R)Sal administration in the striatum. These results demonstrate the selective cytotoxicity of NM(R)Sal to the dopamine neurons in the substantia nigra, and the possible involvement of this 6,7-dihydroxy-isoquinoline in the pathogenesis of Parkinson's disease is discussed.

Binding of 1,2(N)-dimethyl-6,7-dihydroxy-isoquinolinium ion to melanin: effects of ferrous and ferric ion on the binding.

1,2(N)-Dimethyl-6,7-dihydroxy-isoquinolinium ion (N-methylsalsolinium ion) was found to bind to melanin, while N-methyl(R)salsolinol and (R)salsolinol did not. Two components with different affinity to melanin were found; the dissociation constant and maximal binding capacity were obtained as 2.1 and 79.4 nM, and 103 and 1336 pmol/mg melanin, respectively. The binding was enhanced by ferrous ion markedly and inhibited by 1-methyl-4-phenylpyridinium ion and ferric ion. N-Methylsalsolinium ion may be accumulated in dopamine neurons as melanin conjugate, which is regulated by ferric and ferrous ions.