Rafael Martín del Río

Taurine activates GABAA but not GABAB receptors in rat hippocampal CA1 area

We investigated if taurine, an endogenous GABA analog, could mimic both hyperpolarizing and depolarizing GABA(A)-mediated responses as well as pre- and postsynaptic GABA(B)-mediated actions in the CA1 region of rat hippocampal slices. Taurine (10 mM) perfusion induced changes in membrane potential and input resistance that are compatible with GABA(A) receptor activation. Local pressure application of taurine and GABA from a double barrel pipette positioned along the dendritic shaft of pyramidal cells revealed that taurine evoked a very small change of membrane potential and resistance compared with the large changes induced by GABA in these parameters. Moreover, in the presence of GABA(A) antagonists, local application of GABA on the dendrites evoked a GABA(B)-mediated hyperpolarization while taurine did not induce any change. Taurine neither mimicked baclofen inhibitory actions on presynaptic release of glutamate and GABA as judging by the lack of taurine effect on paired-pulse facilitation ratio and slow inhibitory postsynaptic potentials, respectively. These results show that taurine mainly activates GABA(A) receptors located on the cell body, indicating therefore that if taurine has any action on the dendrites it will not be mediated by either GABA(A) or GABA(B) receptors activation.

Presence of γ‐Aminobutyric Acid in Rat Ovary

Abstract: As γ‐aminobutyric acid (GABA) was first discovered as the free acid in the mammalian central nervous system, it has been assumed that GABA is generally to be found in significant amounts only in the brain, in spite of reports of its presence in a number of non‐neuronal tissues. In this study, GABA was detected amongst the free amino acids in most rat tissues that were examined. The highest concentration outside the brain was in the ovary (0.59 μmol/g fresh tissue). It is concluded that the synthesis of the GABA is intragonadal and probably of metabolic importance.

Role of endogenous taurine on the glutamate analogue-induced neurotoxicity in the rat hippocampus in vivo

The glutamate analogues N‐methyl‐d‐aspartate (NMDA), kainic acid (KA), and quisqualic acid (QA), prepared in different hypertonic media, were perfused in vivo in the hippocampal CA1 field of rats using a microdialysis technique. Extracellular taurine levels, estimated after analysis of the taurine content of dialysates, increased during perfusion of all three agonists but varied according to the osmolarity of the medium. The NMDA‐induced increase in extracellular taurine content was only slightly inhibited by perfusion of 150 and 300 mM sucrose. The KA‐evoked increase was partially dependent on extracellular osmolarity, because addition of 50 and 150 mM sucrose caused a dose‐dependent inhibition that was not augmented using higher sucrose concentrations. QA caused a taurine increase that was totally abolished by addition of 50 mM sucrose. These results indicate that the rise in extracellular taurine level elicited by QA and part of the increase elicited by KA are probably due to a release caused by the cellular swelling that these substances evoke, a finding substantiating the previously proposed osmoregulatory role of taurine. However, almost all the increase in extracellular taurine content caused by NMDA and all the osmotically insensitive part of the KA‐evoked rise cannot be explained as release triggered by cell swelling and may reflect a function of taurine other than osmore‐gulation.

Taurine-induced synaptic potentiation: role of calcium and interaction with LTP.

Taurine induces a long-lasting potentiation of excitatory synaptic potentials due to the enhancement of both synaptic efficacy and axon excitability in the CA1 area of rat hippocampal slices. In this study, we characterized the role of Ca2+ in the generation of these long-lasting taurine effects. Taurine perfusion in a free-Ca2+ medium did not induce changes in either field excitatory synaptic potentials (fEPSP) slope or fiber volley (FV) amplitude. Intracellular recordings with a micropipette filled with the Ca2+ chelator BAPTA, prevented the EPSP potentiation induced by taurine in the impaled cell, whereas a long-lasting potentiation of the simultaneously recorded fEPSP was obtained. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM), an inhibitor of endosomal Ca2+-ATPase, transformed the taurine-induced potentiation into a transitory process that declined to basal values after taurine withdrawal. Taurine-induced potentiation was not significantly affected by kynurenate (glutamate receptor antagonist), or nifedipine (high-voltage-activated Ca2+ channel antagonist). But, the presence of nickel (50 microM), an antagonist of low-voltage-activated Ca2+ channel, inhibited the taurine-induced potentiation, indicating that Ca2+ influx through this type of Ca2+ channels could account for the Ca2+ requirement of the taurine-induced potentiation. Occlusion experiments between tetanus-induced long-term potentiation (LTP) and taurine-induced potentiation indicate that both processes share some common mechanisms during the maintenance period.

Immunohistochemical Localization of Taurine in the Male Reproductive Organs of the Rat

The amino acid taurine has been implicated in several aspects of reproductive system physiology. However, its localization in these organs has not been previously analyzed. The aim of this study was to characterize its distribution in male rat reproductive organs by immunohistochemical methods. Taurine was localized in the smooth muscle cells of the tissues studied and in the skeletal fibers of the cremaster muscle. In the testis, taurine was found in Leydig cells, vascular endothelial cells, and other interstitial cells. No immunoreactivity was observed in the cells of the seminiferous tubules, either in germ cells at all spermatogenic stages or in Sertoli cells. However, peritubular myoid cells were immunostained. Most epithelial cells of the efferent ducts were immunolabeled, whereas the epithelial cells of the rete testis (extratesticular segments), epididymis (caput, corpus, and cauda regions), and ductus deferens were unstained. However, most epithelial cells from the intratesticular segments of the rete were immunopositive. Some cells identified as intraepithelial macrophages and lymphocytes, apical cells, and narrow cells were intensely immunolabeled. Regional differences in the distribution of these cell types along the ducts studied were also noted. The possible functional roles for taurine in these cells are discussed.

Conversion into GABA (gamma-aminobutyric acid) may reduce the capacity of L-glutamine as an insulin secretagogue.

We have carried out a detailed examination of L-glutamine metabolism in rat islets in order to elucidate the paradoxical failure of L-glutamine to stimulate insulin secretion. L-Glutamine was converted by isolated islets into GABA (gamma-aminobutyric acid), L-aspartate and L-glutamate. Saturation of the intracellular concentrations of all of these amino acids occurred at approx. 10 mmol/l L-glutamine, and their half-maximal values were attained at progressively increasing concentrations of L-glutamine (0.3 mmol/l for GABA; 0.5 and 1.0 mmol/l for Asp and Glu respectively). GABA accumulation accounted for most of the 14CO2 produced at various L-[U-14C]glutamine concentrations. Potentiation by L-glutamine of L-leucine-induced insulin secretion in perifused islets was suppressed by malonic acid dimethyl ester, was accompanied by a significant decrease in islet GABA accumulation, and was not modified in the presence of GABA receptor antagonists [50 micromol/l saclofen or 10 micromol/l (+)-bicuculline]. L-Leucine activated islet glutamate dehydrogenase activity, but had no effect on either glutamate decarboxylase or GABA transaminase activity, in islet homogenates. We conclude that (i) L-glutamine is metabolized preferentially to GABA and L-aspartate, which accumulate in islets, thus preventing its complete oxidation in the Krebs cycle, which accounts for its failure to stimulate insulin secretion; (ii) potentiation by L-glutamine of L-leucine-induced insulin secretion involves increased metabolism of L-glutamate and GABA via the Krebs cycle (glutamate dehydrogenase activation) and the GABA shunt (2-oxoglutarate availability for GABA transaminase) respectively, and (iii) islet release of GABA does not seem to play an important role in the modulation of the islet secretory response to the combination of L-leucine and L-glutamine.

γ-Aminobutyric Acid Greatly Increases the In Vivo Extracellular Taurine in the Rat Hippocampus

Abstract: The effects of γ‐aminobutyric acid (GABA) on the extracellular levels of taurine and on excitability in the dentate gyrus were studied in anesthetized rats by the dialytrode technique. The dentate gyrus was perfused by means of a dialytrode with Krebs‐Ringer‐bicarbonate or GAB A solutions. Amino acid contents in perfusates and dentate field potentials evoked by electrical stimulation of the perforant pathway were evaluated. GAB A drastically elevated the levels of extracellular taurine in a dose‐dependent manner, decreasing the amplitude of the population spike. This result indicates that GABA stimulates taurine release, probably by a counter‐transport process. It is suggested that in physiological conditions an increase in extracellular taurine may be produced by synaptically released GABA.

Reduction of plasma membrane glutamate transport potentiates insulin but not glucagon secretion in pancreatic islet cells

Glutamate is generated during nutrient stimulation of pancreatic islets and has been proposed to act both as an intra- and extra-cellular messenger molecule. We demonstrate that glutamate is not co-secreted with the hormones from intact islets or purified α- and β-cells. Fractional glutamate release was 5-50 times higher than hormone secretion. Furthermore, various hormone secretagogues did not elicit glutamate efflux. Interestingly, epinephrine even decreased glutamate release while increasing glucagon secretion. Rather than being co-secreted with hormones, we show that glutamate is mainly released via plasma membrane excitatory amino acid transporters (EAAT) by uptake reversal. Transcripts for EAAT1, 2 and 3 were present in both rat α- and β-cells. Inhibition of EAATs by L-trans-pyrrolidine-2,4-dicarboxylate augmented intra-cellular glutamate and α-ketoglutarate contents and potentiated glucose-stimulated insulin secretion from islets and purified β-cells without affecting glucagon secretion from α-cells. In conclusion, intra-cellular glutamate-derived metabolite pools are linked to glucose-stimulated insulin but not glucagon secretion.

Immunocytochemical localization of taurine in different muscle cell types of the dog and rat.

The presence and distribution of the amino acid taurine in different muscle cell types of the dog and rat was examined by immunocytochemical methods. The light microscope study revealed that smooth muscle cells were similarly immunoreactive for taurine, whereas skeletal muscle fibres showed wide differences in taurine immunoreactivity among individual cells. Some skeletal fibres were strongly immunoreactive whereas others did not display immunolabelling. Mononucleated satellite cells, found adjacent to skeletal fibres in a quiescent stage, were also immunostained. Other myoid cells, such as testicular peritubular cells showed a cytoplasmic and a nuclear pool of taurine. By means of electron microscope immunolabelling, the subcellular localization of taurine was studied in vascular and visceral smooth muscle cells. Taurine was present in most subcellular compartments and frequently appeared randomly distributed. Taurine was localized on myofilaments, dense bodies, mitochondria, the plasma membrane and the cell nucleus. Moreover, the labelling density within individual smooth muscle cells was variable and depended on the state of contraction of each single fibre. Contracted cells showed a higher density of gold particles than relaxed cells. Unmyelinated nerve fibres, found adjacent to smooth muscle cells from the muscularis mucosae and the lamina propria of the stomach, were unstained or poorly stained.

Role of taurine uptake on the induction of long‐term synaptic potentiation

Taurine application in the CA1 area of rat hippocampal slices induces a long‐lasting potentiation of excitatory synaptic transmission that has some mechanistic similitude with the late phase of long‐term potentiation (L‐LTP). Previous indirect evidence such as temperature and sodium dependence indicated that taurine uptake is one of the primary steps leading to the taurine‐induced synaptic potentiation. We show that taurine‐induced potentiation is not related to the intracellular accumulation of taurine and is not impaired by 2‐guanidinoethanesulphonic acid, a taurine transport inhibitor that is a substrate of taurine transporter. We have found that taurine uptake in hippocampal synaptosomes was inhibited by SKF 89976A, a GABA uptake blocker that is not transportable by GABA transporters. SKF 89976A prevents the induction of synaptic potentiation by taurine application. This effect is neither mimicked by nipecotic acid, a broad inhibitor of GABA transporters that does not affect taurine uptake, nor by NO‐711, a specific and potent inhibitor of GABA transporter GAT‐1. In addition, L‐LTP induced by trains of high‐frequency stimulation is also inhibited by SKF 89976A, and taurine, at a concentration that does not change basal synaptic transmission, overcomes such inhibition. We conclude that taurine induces synaptic potentiation through the activation of a system transporting taurine and that taurine uptake is required for the induction of synaptic plasticity phenomena such as L‐LTP.