The effect of 6-aminonicotinamide blockade of the pentose phosphate pathway on catecholamines in the rat adrenal medulla, superior cervical ganglion, hypothalamus and synaptosome fractions

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.

Sympathetic nervous system and experimental diabetes: role of adrenal medullary hormones

The sympathetic nervous system is of major importance in the regulation of various physiological functions. The present review discusses the mechanisms which control glucose homeostasis and the role of the sympathetic nervous system in experimental diabetes with special emphasis on the role of adrenal medullary hormones, over-activity of the sympathetic nervous system and its relationship to hypertension in the diabetic state and the effect of stress. The chapter also reviews the ability of various drugs and pharmacological agents to produce hyperglycemia in experimental animals and how this information can be used in screening new chemical entities and in differentiating the mode of action of these agents.

Neurotoxin 6-aminonicotinamide affects levels of soluble proteins and enzyme activities in various tissues of golden hamsters

The effects of neurotoxin 6-aminonicotinamide (6-AN) on the levels of soluble proteins and enzyme activities in various tissues of golden hamsters were investigated. SDS-polyacrylamide gel electrophoresis showed that a soluble spinal cord protein with molecular mass 75.0 kDa was present at a higher concentration in the treated group compared to that in the control while that of a molecular mass 64.8 kDa appeared to be missing. However, there were no noticeable differences in protein concentrations observed with the cerebrum, brain stem, and cerebellum. Similarly, treatment with 6-AN decreased the concentration of a soluble protein in pectoral muscle having molecular mass 97.2 kDa and increased those having molecular masses 207.4 and 32.1 kDa. In the kidney, soluble proteins with molecular masses 176.6 kDa was missing and those of molecular masses 97.6, 49, 43.3, and 33.8 kDa were decreased whereas those of molecular masses 64.7 and 33.1 kDa were increased. In the testis the soluble proteins with molecular masses 125.4, 88.7, 69.0, 31.2, 19.1, and 17.4 kDa were missing and those of molecular masses 97.0, 51.3, 42.0, 33.0, 27.2, and 22.6 kDa were present in lower amounts whereas those of molecular masses 311.5, 75.0, 64.0, 54.1, and 53.2 kDa were present in higher amounts. The specific activity of 6-phosphogluconate dehydrogenase was markedly increased in the liver but that of other tissues was not affected. Acetylcholinesterase activity was markedly reduced in the spleen but was enhanced in the intestine. Monoamine oxidase activity was markedly reduced in the brain stem, cerebrum, kidney, and liver. The results suggest that the changes in levels of soluble proteins and enzyme activities shown with golden hamster tissues by 6-AN administration were quite different from those shown with quail tissues.

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)

Effect of 6-aminonicotinamide on monoamine oxidase and Na+K+ATPase activity in different regions of rat brain

The monoamine oxidase activity in the cerebral hemispheres decreased significantly after 2, 4, 8 and 16 hr of 6-amino-nicotinamide (35 mg/kg body weight, i.p.) administration. In the cerebellum, the MAO activity was not affected significantly. In the brain stem, however, a significant increase was observed after 2 hr of drug administration followed by a gradual decrease at later time intervals. The Na+K+ATPase activity in the cerebral hemispheres was increased significantly at 2 and 4 hr of 6-aminonicotinamide administration. This was followed by a gradual decrease at later time intervals. In the cerebellum, like monoamine oxidase, the Na+K+ATPase did not change significantly. The brain stem showed a decrease at 2 hr of drug administration, followed by a significant increase at 4 hr and then a gradual decrease to near control values.

Participation of adrenal medulla in 6-amino-nicotinamide-induced hyperglycaemia in the rat

In fasted rats 6-aminonicotinamide (6-AN) produced delayed hyperglycaemia, the peak effect being seen by 7 h. Fasting plasma insulin concentrations were not significantly altered but liver glycogen concentrations were decreased following treatment with 6-AN. Adrenalectomy, demedullation and pretreatment with reserpine, phentolamine, nicotinamide and nicotinic acid completely blocked the hyperglycaemic response whereas guanethidine and propranolol or oxprenolol were ineffective. Catecholamine concentrations in the adrenal venous plasma were markedly increased by treatment with 6-AN, the peak effect being seen by 5 h. It is concluded that adrenal medullary release, which is slow in onset, is mainly responsible for the development of sustained hyperglycaemia in the rat.