Action of dopamine agonists on parkinson-like muscle rigidity induced by 6-aminonicotinamide

Neurotoxin-induced impairment of biopterin synthesis and function: Initial stage of a Parkinson-like dopamine deficiency syndrome

Disorders of the function of the tyrosine hydroxylase play an important role in the occurrence of the Parkinson syndrome. The enzyme that catalyses the first, rate-limiting step in the biosynthesis to dopamine requires the cofactor tetrahydrobiopterin. This compound supplies the reduction equivalent for activation of molecular oxygen. Binding of the cofactor to the enzyme is affected by phosphorylation or dephosphorylation of the enzyme protein and, thereby, influences the activity. Nerve and chromaffin cells that synthesize dopamine, noradrenaline and serotonin are able to synthesize the cofactor tetrahydrobiopterin de novo from guanosine-triphosphate as a precursor. In patients suffering from Parkinson's disease a remarkable decrease in biopterin content was found in the brain. The function of the dopaminergic system was studied with an experimental Parkinson model. The antimetabolite 6-aminonicotinamide induces a dopamine deficit in the striatum with a significant slowdown in the utilization of this transmitter. The abolition of the 6-aminonicotinamide-induced muscular rigidity by l-DOPA and dopamine agonists implies that the antimetabolite produces a Parkinson-like syndrome in rats. There are reports on the molecular basis of this effect which are also important for understanding possible disturbances of the synthesis of biopterins. The effector 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP), which blocks the pentose phosphate pathway, is formed by an enzymatic neurotoxic synthesis. The clonal cell line PC-12 was used to study the molecular basis of the disturbances occurring in the dopaminergic system. These cells contain all the enzymes for catecholamine synthesis, including those for the synthesis of the cofactor tetrahydrobiopterin. Addition of 6-aminonicotinamide to the culture medium resulted in the synthesis of the neurotoxic agent, 6-ANADP, by a glycohydrolase localized in the endoplasmic reticulum. The synthesis of biopterin was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin resulted in reduced DOPA production. The decreased content of biopterin cofactor synthesis was compensated for by the addition of the precursor sepiapterin, indicating that the NADPH-dependent reductases in biopterin synthesis were not inhibited by the antimetabolic nucleotide 6-ANADP. DOPA production was not fully normalized by sepiapterin. Addition of NADH to the medium resulted in a further increase of DOPA production, probably by activation of the recycling pathway. The first step in the synthesis of biopterin from GTP to 7,8-neopterin-triphosphate seems to be particularly sensitive to the action of exogenous neurotoxins. A further sensitive site of action in synthesis to the cofactor BH(4) concerns the function of the dihydropteridin-reductase, which recycles qBH(2) to BH(4). Neurotoxin-induced impairment of biopterin synthesis is probably a pathogenetically important disorder at the initial stage of Parkinson's disease.

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

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

Inhibition of biopterin synthesis and DOPA production in PC-12 pheochromocytoma cells induced by 6-aminonicotinamide

Pheochromocytoma cells (clone PC-12) were treated with 6-aminonicotinamide. Tetrahydrobiopterin content and DOPA production of the cells were determined by reverse-phase HPLC and subsequent electrochemical detection. The same chromatographic system was used to determine total biopterin (tetrahydrobiopterin, dihydrobiopterin and quinoide dihydrobiopterin) by fluorescence detection. Tetrahydrobiopterin plays a decisive role as cofactor of tyrosine hydroxylase for the biosynthesis of DOPA and dopamine. Addition of 6-aminonicotinamide to the culture medium resulted in the accumulation of 6-phosphogluconate, suggesting that PC-12 cells synthesize 6-aminonicotinamide-adenine-dinucleotide-phosphate (6-ANADP) by a glycohydrolase localized in the endoplasmic reticulum. This substance is known to be a strong inhibitor of 6-phosphogluconate dehydrogenase and leads to a blockade of the pentose phosphate pathway. In our experiments, the synthesis of biopterins was depressed after application of 6-aminonicotinamide. The decrease of intracellular tetrahydrobiopterin and total biopterin by 6-aminonicotinamide at different concentrations was strongly correlated with a reduced cellular DOPA production. The decreased content of biopterin cofactor was compensated by addition of the precursor sepiapterin, indicating that the NADPH2-dependent reductases in biopterin synthesis are not inhibited by the antimetabolite. However, DOPA production remained suppressed at the same time. After application of NADH2, we observed an increased DOPA production though the decreased biopterin levels remained almost unchanged. The results imply that the first step in the synthesis of biopterin from GTP as well as the recycling pathways of the oxidized cofactor might be the site of action of the antimetabolite.(ABSTRACT TRUNCATED AT 250 WORDS)

Lisuride in de novo parkinsonian patients: a four-year follow-up

Lisuride at a mean daily dose of 3.2 mg was given to 15 untreated idiopathic Parkinson's disease patients. There were 10 dropouts, due mainly to inefficacy in the first months of therapy. The parkinsonian pattern in the patients who remained in the study for the full 4 years showed distinct improvement, which was maintained for less than 2 years. The patients did not develop "on-off" phenomena or abnormal involuntary movements during follow-up.

[Nerve cell clonal lines in culture--models for studying the molecular basis of neuropharmacological actions]

Nerve cell lines with stable properties are isolated from neuroblastomas, glioblastomas or pheochromocytomas by periodic cloning using defined culture media. After the action of different drugs, these cells show all morphological and biochemical signs of differentiation and maturation. Depending on the origin of the clone, the cell lines synthesise typical neurotransmitters, which are stored in vesicles. It is demonstrated on cell lines which synthesise catecholamines that noradrenergic and dopaminergic clones are particularly suitable test objects for basic research in neuropharmacology.

Mouse neuroblastoma clone N1E-115: a suitable model for studying the action of dopamine agonists of tyrosine hydroxylase activity

The DOPA-content in neuroblastoma clone N1E-115 is higher than the dopamine or noradrenaline content. Blockade of tyrosine hydroxylase by alpha-methyl-p-tyrosine (1 X 10(-3) M) resulted in a decrease of cellular DOPA-content to 24.9% after 4 hr. The accumulation of DOPA in these cells which is probably due to limited activity of l-aromatic amino acid decarboxylase led us to use DOPA-content as a measure of tyrosine hydroxylase (TH) activity. Dopamine and especially apomorphine were effective at low concentrations (dopamine IC50 1 X 10(-5) M, apomorphine 2 X 10(-7) M); lisuride had no effect on TH-activity. The low effective dose of apomorphine and the failure of lisuride to influence TH-activity are comparable to the observations in striatal synaptosomal preparations and make the N1E-115 clone a suitable model for studying the mechanism of TH-regulation. However, since haloperidol (1 X 10(-5) M) did not reverse the apomorphine-induced blockade of TH, a receptor-mediated blockade of TH seems to be improbable.