Comparative metabolism and taurine-depleting effects of guanidinoethanesulfonate in cats, mice, and guinea pigs

Taurine: the comeback of a neutraceutical in the prevention of retinal degenerations

Taurine is the most abundant amino acid in the retina. In the 1970s, it was thought to be involved in retinal diseases with photoreceptor degeneration, because cats on a taurine-free diet presented photoreceptor loss. However, with the exception of its introduction into baby milk and parenteral nutrition, taurine has not yet been incorporated into any commercial treatment with the aim of slowing photoreceptor degeneration. Our recent discovery that taurine depletion is involved in the retinal toxicity of the antiepileptic drug vigabatrin has returned taurine to the limelight in the field of neuroprotection. However, although the retinal toxicity of vigabatrin principally involves a deleterious effect on photoreceptors, retinal ganglion cells (RGCs) are also affected. These findings led us to investigate the possible role of taurine depletion in retinal diseases with RGC degeneration, such as glaucoma and diabetic retinopathy. The major antioxidant properties of taurine may influence disease processes. In addition, the efficacy of taurine is dependent on its uptake into retinal cells, microvascular endothelial cells and the retinal pigment epithelium. Disturbances of retinal vascular perfusion in these retinal diseases may therefore affect the retinal uptake of taurine, resulting in local depletion. The low plasma taurine concentrations observed in diabetic patients may further enhance such local decreases in taurine concentration. We here review the evidence for a role of taurine in retinal ganglion cell survival and studies suggesting that this compound may be involved in the pathophysiology of glaucoma or diabetic retinopathy. Along with other antioxidant molecules, taurine should therefore be seriously reconsidered as a potential treatment for such retinal diseases.

Dietary beta-alanine results in taurine depletion and cerebellar damage in adult cats

We have used the taurine analogue, beta-alanine, to perturb the taurine concentrations in taurine-supplemented and taurine-deprived adult cats. By using 5% beta-alanine in the drinking water for 20 weeks, both groups of cats had greatly reduced brain taurine concentrations. Taurine-supplemented cat brain accumulated relatively small amounts of beta-alanine whereas taurine-deprived cats accumulated large amounts of beta-alanine. The cerebellum of cats treated with beta-alanine had a number of pathological changes compared with similar cats drinking water alone. The changes were more severe in the taurine-deprived cats, and included reduced numbers of granule and Purkinje cells, with many of those remaining appearing pyknotic and dying. Long swollen fibers were seen in the white matter, resembling Rosenthal fibers described in some human cerebellar diseases. There was also prominent gliosis. Using antibodies to beta-alanine and taurine, beta-alanine was localized in Purkinje cell soma and dendrites, in Golgi II cells, and in some granule cells, especially in taurine-deprived cats treated with beta-alanine. Taurine appears to have been virtually eliminated from Purkinje and granule cells, and concentrated in Golgi II cells and glia. We conclude that beta-alanine is responsible for these neurotoxic pathological changes.

Guanidinoethane sulfate is neuroprotective towards delayed CA1 neuronal death in gerbils

The potential neuroprotective effects of guanidinoethane sulfate (GES) on delayed neuronal death of hippocampal CA1 neurons were investigated using a gerbil model of forebrain ischemia. Neuronal densities of CA1 neurons in the saline control group (255.1 +/- 11.7 cells/mm) and guanidinoethane sulfate pretreated control group (249.0 +/- 9.4 cells/mm) showed no significant differences. By contrast, in animals subjected to ischemia, CA1 neurons of the guanidinoethane sulfate pretreated group showed a significantly higher number of surviving neurons (61.1 +/- 55.11 cells/mm) compared to the saline group (17.75 +/- 12.73 cells/mm) (p < 0.05, t-test). The study indicated that although partial, guanidinoethane sulfate is neuroprotective towards gerbil hippocampal CA1 neurons against ischemic insult.

The effect of taurine transport antagonists on cardiac taurine concentration and the incidence of sudden death syndrome in male broiler chickens

Two experiments were conducted in which 540 day-old male broiler chickens were raised in heated battery brooders to 4 wk of age. Diets contained taurine antagonists to test effects on cardiac taurine content and the incidence of Sudden Death Syndrome (SDS). In Experiment 1 treatments during Weeks 2 to 4 were A) basal diet; B) basal diet supplemented with .25% guanidinoethyl sulfonate (GES) in Week 2 and 1.5% GES in Weeks 3 and 4; and C) basal diet supplemented with .5% GES in Weeks 2 to 4. The taurine content of heart was significantly reduced (P less than .05) with GES supplementation, but no effects on SDS mortality rates were noted. In Experiment 2 birds received diets containing 0. 2.5, or 5% beta-alanine. Supplementation with this compound decreased cardiac taurine concentration to extremely low levels (P less than .05). No significant effects on SDS mortality rates were noted. The results are interpreted as suggesting that taurine does not play a major role in the etiology of SDS.

Increased seizure susceptibility induced by guanidinoethane sulfonate in E1 mice and its relation to glutamatergic neurons

Convulsions and brain levels of amino acids and 5-hydroxytryptamine (5-HT) in E1 mice were examined after oral administration of a 1% guanidinoethane sulfonate (GES) solution. The incidence of convulsions increased 3 days after starting GES administration, and this effect continued throughout the 6 months of drug administration. Glutamate levels were increased in the cerebrum, and glutamine levels were increased in the cerebellum three days after starting GES administration. Brain 5-HT levels were not changed at that time. These results suggest that increased seizure susceptibility induced by GES in E1 mice is related to glutamatergic neurons.

Effect of drug-induced taurine depletion on cardiac contractility and metabolism

Myocardial taurine content was halved in rats maintained for 3 weeks on tap water containing either 1% guanidinoethylsulfonate (GES) or 3% beta-alanine. The decrease in taurine content did not affect myocardial contraction; however, it significantly altered myocardial metabolism. The major effect of the treatment was a significant stimulation in the rate of glycolysis and lactate production. The rise in glycolytic flux was associated with activation of phosphofructokinase, presumably caused by a decrease in tissue citrate levels. GES-treated hearts also contained slightly lower creatine phosphate content, but since this effect was not observed in the beta-alanine-treated hearts, it appeared to be independent of taurine depletion. The consequences of these metabolic changes are discussed.

Cathepsin B and D, and Ca2+-dependent neutral protease activities in the retina of taurine-depleted rats

Rats were treated with guanidinoethanesulfonic acid (GES), a taurine transport inhibitor, which reduces the tissue content of taurine. The quantity of taurine in the rat retinas after GES treatment was reduced by 46% after the first week, 60% after the second week, and 67% after the third week. Activities of cathepsin B and D were not significantly altered when calculated on the basis of either protein or DNA content in the taurine-depleted retinas. However, cytosolic Ca2+-dependent neutral protease activity in retinas from GES-treated rats increased by 55% after the third week.

Modification of the antiepileptic actions of phenobarbital and phenytoin by the taurine transport inhibitor, guanidinoethane sulfonate

We investigated whether chronic administration of guanidinoethane sulfonate, an inhibitor of taurine uptake, could modify the antiepileptic actions of phenobarbital and phenytoin on maximal electroshock seizures in mice. Treatment with 1% guanidinoethane sulfonate decreased the taurine concentration in the brain to 76% of the control value. Under these conditions, neither the severity of tonic convulsions of maximal electroshock seizures nor the threshold for tonic extension caused by electroshock was altered. However, treatment with guanidinoethane sulfonate lessened the antiepileptic actions of phenobarbital and phenytoin on electroshock seizures. The brain concentrations of phenobarbital and phenytoin were unaltered by administration of guanidinoethane sulfonate. The brain concentrations of guanidinoethane sulfonate and total guanidino compounds were unchanged by the injection of either phenobarbital or phenytoin. It is suggested that the observed loss of anticonvulsive potency of phenobarbital and phenytoin may have been related to the decrease in taurine concentration produced by guanidinoethane sulfonate.

A comparison by species, age and sex of cysteinesulfinate decarboxylase activity and taurine concentration in liver and brain of animals

Cysteinesulfinate decarboxylase activity and taurine concentration were determined in liver and brain of rats, mice, cats, guinea-pigs and sheep. Values were compared for male and female animals and in some cases measurements were also made in animals of different ages. Cysteinesulfinate decarboxylase activity and taurine concentration were also measured in liver and brain of male and female rat pups during the postnatal period. Hepatic cysteinesulfinate decarboxylase activity increased in both male and female rat pups during the postnatal period and then declined markedly in female rats so that activity in adult males was 16-fold that in adult females. Cysteinesulfinate decarboxylase activity in liver of 5- to 6-week old kittens was 73 times that observed in liver of 15-month old cats. Taurine level in liver of guinea-pigs was much lower than that in liver of any other species studied.

A possible interrelationship between extracellular taurine and phosphoethanolamine in the hippocampus

The effect of guanidinoethane sulfonic acid (GES), an inhibitor of taurine uptake, was examined with respect to endogenous amino acids in the hippocampus of the freely moving rabbit. GES increased the extracellular levels of both taurine and phosphoethanolamine (PEA), other amino acids being unaffected. However, long-term oral administration of GES selectively reduced endogenous taurine levels. The effect of GES on PEA appeared to be a consequence of the elevated extracellular taurine as exogenously administered taurine per se increased PEA levels in the extracellular space. The findings are discussed in conjunction with the proposed membrane-stabilizing effects of taurine.

Free amino acids and the uptake and binding of taurine in the central nervous system of rats treated with guanidinoethanesulphonate

The content of free amino acids in the central nervous system, the uptake of taurine by slices and synaptosomes and taurine binding to synaptic membranes have been studied on adult male Wistar rats after a prolonged administration of 2-guanidinoethanesulphonate with or without a taurine-free diet. The 2-guanidinoethanesulphonate administration both alone and together with a taurine-free diet considerably reduced taurine levels in all regions studied, viz. cerebrum, cerebellum, brain stem and spinal cord, but the contents of methionine and gamma-aminobutyric acid were increased. On the normal diet the maximal taurine loss occurred after a 3-week 2-guanidinoethanesulphonate administration, when also the 2-guanidinoethanesulphonate accumulation in the cerebrum was largest. On the taurine-free diet the taurine levels were still low after the 4-week 2-guanidinoethanesulphonate administration when the taurine content in cerebral synaptosomes was also reduced to almost one half. 2-Guanidinoethanesulphonate treatment modified the uptake and binding processes of taurine in the synaptosomal preparations. The maximal high-affinity taurine uptake was reduced, whereas the affinity of the specific sodium-independent binding sites for taurine increased. The results suggest that the neuromodulator properties of taurine could be affected by the reduced taurine levels and/or by the accumulated 2-guanidinoethanesulphonate.

Effect of guanidinoethane sulfonate on taurine uptake by rat retina

Guanidinoethane sulfonate (GES) markedly decreased 3H-taurine accumulated in the retina by the high-affinity uptake process. The effect of GES was dose-dependent. Analysis of the kinetics of GES effect revealed that it is a competitive inhibitor. The uptake of taurine by GES was less affected in rat cerebral cortex slices, where the inhibition by 1 mM GES was only 28%. Taurine accumulation by tissues of rats treated with GES (0.1% and 1% in the drinking water) was found to be particularly decreased in the retina, although accumulation by heart and liver was also affected by the higher dose. Taurine uptake by cerebellum and cerebral cortex slices was unaffected by GES. Treatment of rats with GES is known to produce an alteration in the structure and function of the retina, but apparently not in other organs. We discuss whether the marked effect of GES on taurine transport by the retina is related to the deleterious effect of the inhibitor in this organ.