Sodium-dependent and sodium-independent binding of taurine to rat brain synaptosomes

Taurine Supplementation as a Neuroprotective Strategy upon Brain Dysfunction in Metabolic Syndrome and Diabetes

Obesity, type 2 diabetes, and their associated comorbidities impact brain metabolism and function and constitute risk factors for cognitive impairment. Alterations to taurine homeostasis can impact a number of biological processes, such as osmolarity control, calcium homeostasis, and inhibitory neurotransmission, and have been reported in both metabolic and neurodegenerative disorders. Models of neurodegenerative disorders show reduced brain taurine concentrations. On the other hand, models of insulin-dependent diabetes, insulin resistance, and diet-induced obesity display taurine accumulation in the hippocampus. Given the possible cytoprotective actions of taurine, such cerebral accumulation of taurine might constitute a compensatory mechanism that attempts to prevent neurodegeneration. The present article provides an overview of brain taurine homeostasis and reviews the mechanisms by which taurine can afford neuroprotection in individuals with obesity and diabetes. We conclude that further research is needed for understanding taurine homeostasis in metabolic disorders with an impact on brain function.

Taurine allosterically inhibits binding of [35S]-t-butylbicyclophosphorothionate (TBPS) to rat brain synaptic membranes

The modulatory effects of taurine on [35S]-t-butylbicyclophosphorothionate (TBPS) binding to rat brain synaptic membranes were evaluated and compared with that of GABA. Taurine allosterically inhibited TBPS binding by interacting with a bicuculline-sensitive site, similar to GABA. Taurine was as effective as GABA but less potent. The potency of taurine inhibition of TBPS binding varied among brain regions with cerebellum > olfactory bulb > cortex, similar to that of GABA. Inhibition of TBPS binding to cortical membranes measured under nonequilibrium conditions yielded a dynamic biphasic inhibition curve that was similarly shaped for GABA and taurine. The effect of taurine on TBPS binding was pharmacologically specific in that beta-alanine and guanadinoethanesulfonate were as effective as taurine, while hypotaurine and alpha-aminoethylhydrogen sulfate were only partially effective at high concentrations, and isethionic acid was without effect. Taurine, similar to GABA, enhanced the effects of pentobarbital on TBPS binding when present at concentrations that were otherwise ineffective on their own. The results of these studies support the notion that taurine interacts with the GABA recognition site of the GABAA receptor complex.

Taurine allosterically modulates flunitrazepam binding to synaptic membranes

Taurine is hypothesized to exert its inhibitory neuromodulatory effects, in part, by interaction with the GABAA receptor. Although taurine displaces GABA agonist binding to synaptic membranes, its allosteric effects on the benzodiazepine recognition site of the GABAA receptor complex is unsettled. We determined the effects of taurine on [3H]flunitrazepam (Flu) binding to well-washed, frozen-thawed synaptic membranes prepared from rat cortex. Comparative binding studies were conducted at 37 degrees C and on ice (0-4 degrees C). At 37 degrees C taurine increased Flu binding in a concentration dependent way by interaction with a bicuculline sensitive site, similar to GABA. Taurine increased Flu binding by causing a decrease in KD. The maximal effectiveness of taurine on Flu binding could not be increased further by addition of GABA. In contrast, the maximal stimulation of Flu binding by GABA was decreased by addition of taurine to the level attained by taurine alone. These mixed agonist/antagonist effects of taurine are pharmacologically specific and qualify taurine as a partial GABA agonist in this type of allosteric interaction. However, taurine causes opposite effects on Flu binding when measured at 0-4 degrees C: taurine interacts with a bicuculline insensitive site to inhibit Flu binding by increasing the KD. Taurine inhibition of Flu binding is not overcome by increasing concentrations of GABA. Although the mechanism of taurine inhibition of Flu binding at 0-4 degrees C is unclear, it may be an indirect effect of taurine interaction with membrane phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Taurine acts on a subclass of GABAA receptors in mammalian brain in vitro

Taurine, an inhibitory amino acid, potently acts on a subclass of gamma-aminobutyric acid type A (GABAA) receptors. Taurine competitively inhibits [3H]muscimol binding to purified GABAA receptors with an average IC50 value of 50 microM, and enhances [3H]flunitrazepam binding to GABAA-linked benzodiazepine receptors with an EC50 of about 10 microM and with maximal extent lower than that for GABA. Taurine shows variable affinities (low micromolar to near millimolar) for muscimol binding sites in different brain regions as measured by autoradiography. The taurine-sensitive GABA sites are enriched in dentate gyrus, substantia nigra, cerebellum molecular layer, median thalamic nuclei, and hippocampal field CA3; these areas are also enriched in mRNA for the GABA-binding beta 2 subunit subtype. Taurine shows differential affinities for the multiple muscimol-GABA binding polypeptides present in purified GABAA receptors, notably a higher affinity for a beta 55 than a beta 58 polypeptide; these probably represent beta 2 and beta 3 clones, respectively. This work defines a significant target of taurine's inhibitory activity as some GABAA receptor GABA sites, lending support to the hypothesis that this endogenous substance may have a physiological action.

Isoproterenol selectively releases endogenous and [14C]-labelled taurine from a single cytosolic compartment in astroglial cells

Taurine is stored in and released from astroglial cells. We have investigated whether taurine is stored in multiple subcellular compartments and whether taurine is released from a distinct pool. Taurine compartmentation was examined by determining the subcellular distribution of labelled taurine and by comparing the specific radioactivities of taurine in cells with that of the taurine released into the superfusion medium during receptor stimulation. Three observations indicate that taurine is found in a single subcellular pool. First, labelled taurine was localized exclusively within a freely exchangeable pool and not sequestered in membrane-bound compartments. Second, endogenous and newly acquired [14C]taurine was released identically by the beta-adrenergic agonist isoproterenol (IPR). Third, measurements of the specific radioactivity of taurine indicate that accumulated [14C]taurine mixes homogeneously with endogenous taurine and both are simultaneously released from the same pool. Chemical analysis revealed net uptake of taurine from the medium during loading and a net loss during IPR-stimulated release. Taurine was selectively released by IPR; five other amino acids found in the superfusion medium were unaffected. One possible mechanism capable of explaining these results is that receptor-mediated taurine release from astroglia occurs by a selective transporter moving taurine from a single cytoplasmic pool to the extracellular space.

Taurine receptors in membranes from retinal pigment epithelium cells in culture

[3H]Taurine-specific binding to membranes from retinal pigment epithelium was demonstrated. A single saturable system was found, with KB = 237 nM and Bmax = 2.8 pmol/mg protein. Binding to freshly prepared membranes showed partial Na(+)-dependence while in frozen/thawed membranes, binding remained unchanged in the absence or presence of this ion. A 30-40% increase in binding was observed at physiological temperature (37 degrees C) compared to 4 degrees C in fresh but not in frozen membranes. Accumulation of taurine was followed during differentiation in vitro; results showed that changes in uptake and receptor binding to frozen membranes are not parallel, discarding the possibility of an interaction with uptake sites. Pharmacology of these binding sites suggests that they could be common to other amino acids, since displacement experiments showed that glycine, beta-alanine and strychnine were as potent as taurine itself in displacing [3H]taurine. Our data open the possibility of taurine being involved in the communication between the retina and the retinal pigment epithelium through an interaction with specific receptors.