Nigrostriatal catecholamine metabolism in guinea pigs is altered by purine enzyme inhibition.

The effect of purine enzyme inhibition on catecholamine metabolism was investigated in guinea pigs. Catecholamine levels were measured in the nigrostriatal brain structures of male guinea pigs following treatment with allopurinol (a xanthine oxidase inhibitor; 250 mg/kg i.p.) or allantoxanimide (a uricase inhibitor; 200 mg/kg i.p.) once a day for 4 days. Tissue was analyzed from the striatum and the substantia nigra. Norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC), ascorbic acid, and uric acid were quantified with electrochemical and ultraviolet detection following separation by liquid chromatography. Allopurinol had no effect on nigrostriatal dopamine levels but decreased DOPAC levels (P<0.05) in the striatum. Allantoxanimide increased norepinephrine levels and decreased DOPAC levels in the striatum (P<0.05). Allopurinol decreased uric acid levels in the striatum and substantia nigra (P<0.05). Allantoxanimide increased uric acid levels in the striatum and the substantia nigra (P<0.05). These results indicate that alterations in purinergic enzyme activity can influence catecholamine metabolism within the nigrostriatal system of the guinea pig.

Effects of uric acid and caffeine on A1 adenosine receptor binding in developing rat brain.

Previous studies have demonstrated that elevated levels of serum uric acid or caffeine are associated with increased locomotor activity in rats and humans. Since uric acid and caffeine are structurally similar, it was hypothesized that these compounds alter locomotor activity through a common neural mechanism, specifically by acting as receptor antagonists at adenosine A1 binding sites. In vitro competition of caffeine and uric acid against the A1 agonist, [3H] cyclohexyladenosine ([3H]CHA), was conducted using homogenates of adult rat forebrain. Caffeine effectively competed for the A1 binding site as previously reported (IC50 = 107 microM), but uric acid failed to compete with [3H]CHA binding at concentrations within a relevant physiological range. Nevertheless, in vivo experiments indicated that chronic elevation of uric acid following allantoxanamide treatment of male rats on days 4-27 of life significantly decreased A1 receptor binding in the striatum, a region traditionally implicated in mammalian locomotion. In contrast, chronic caffeine treatment on days 4-27 of life caused an increase in A1 receptor binding in the cortex similar to increases reported previously in whole brain. These changes in A1 receptor binding following chronic elevation of uric acid or caffeine did not persist in rats that had been withdrawn from allantoxanamide or caffeine treatment for 14 days. Results from in vitro and in vivo experiments indicate that despite a similar molecular structure uric acid does not act by the same mechanism as caffeine to increase locomotor activity in rats.

Hyperuricemia and locomotor activity in developing rats.

This research was motivated by the previous finding that serum uric acid levels correlate with symptoms of hyperactivity in normal children. We attempted, therefore, to investigate this relationship in an animal model. Dose and time relationships between allantoxanamide-induced heightened serum uric acid and locomotor activity were investigated. A dose- and time-dependent relationship was shown between serum uric acid levels and allantoxanamide. Those doses of allantoxanamide which elevated serum uric acid produced time-dependent changes in locomotor activity. In the first two hours following injection, activity increased relative to controls, in the next two-hour block activity decreased, only to rise again above control levels in the third two-hour period. The possible role of uric acid and allantoxanamide are discussed in relation to these complex changes in activity.

Decreasing effect of allantoxanamide, a hyperuricemic agent on renal functions in rats.

Both potassium oxonate and allantoxanamide have been reported as useful reagents for inhibiting urate oxidase, with the hyperuricemic effect of allantoxanamide being longer-lasting than that of oxonate in rats. The present study was done to evaluate the utility of allantoxanamide for investigating problems concerning hyperuricemia using rats. A single intraperitoneal administration of 150 mg/kg allantoxanamide elevated the plasma uric acid level progressively during the experiment for 6 hr, resulting in a much higher level than that maintained by means of repeated dosing with 250 mg/kg potassium oxonate, i.p., at 2-hr intervals. Such a severe and long-lasting hyperuricemia caused by allantoxanamide was due to decreased renal function of uric acid excretion according to its nephrotoxicity in addition to the inhibition of urate oxidase like that by oxonate. Thus, we concluded that allantoxanamide might be a useful reagent for investigating the causes of hyperuricemia with renal failure, but was not a practical agent like oxonate in order to evaluate the response to uricosuric agents.

On the role of dihydroorotate dehydrogenase in growth cessation of Ehrlich ascites tumor cells cultured under oxygen deficiency.

In pyrimidine biosynthesis the oxidation of dihydroorotate is catalyzed by dihydroorotate dehydrogenase, which is linked to the mitochondrial electron transport chain in vertebrates. In order to elucidate the role of this oxygen-dependent anabolic process in the arrest of cell proliferation under anaerobic culture conditions, the effect of the antimetabolite dihydro-5-azaorotic acid on growth, metabolism and cell cycle distribution of Ehrlich ascites tumor cells was studied and compared with the effect of oxygen-free culture conditions. Experiments with cell homogenates confirmed that the dehydrogenation of dihydroorotate is blocked by the inhibitor. In intact cells 2 mM dihydro-5-azaorotic acid inhibited incorporation of dihydro[6-14C]orotate into nucleic acids, and no further increase in RNA and DNA content was observed in its presence for several hours. The cells remained viable; glycolytic activity was normal; respiration was reduced; growth cessation occurred within a few hours. DNA histograms obtained by flow cytometry revealed an accumulation of cells in the G1 phase, which was also observed with anaerobically cultured cells. Substitution of cultures with pyrimidine nucleosides completely sustained cell proliferation in the presence of the inhibitor but not in an anoxic atmosphere. It is concluded that in the absence of oxygen cell proliferation may be arrested by inhibition of the dihydroorotate dehydrogenase alone. However, additional impairments of cell metabolism must play an important role.