Julián Bustamante

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.

Application of a low-cost web-based simulation to improve students’ practical skills in medical education

BACKGROUND Practical sessions in undergraduate medical education are often costly and have to face constraints in terms of available laboratory time and practice materials (e.g. blood samples from animals). This makes it difficult to increase the time each student spends at the laboratory. We consider that it would be possible to improve the effectiveness of the laboratory time by providing the students with computer-based simulations for prior rehearsal. However, this approach still presents issues in terms of development costs and distribution to the students. OBJECTIVE This study investigates the employment of low-cost simulation to allow medical students to rehearse practical exercises through a web-based e-learning environment. The aim is to maximize the efficiency of laboratory time and resources allocated by letting students become familiarized with the equipment and the procedures before they attend a laboratory session, but without requiring large-scale investment. Moreover, students can access the simulation via the Internet and rehearse at their own pace. We have studied the effects of such a simulation in terms of impact on the laboratory session, learning outcomes and student satisfaction. METHODS We created a simulation that covers the steps of a practical exercise in a Physiology course (measuring hematocrit in a blood sample). An experimental group (EG, n=66) played the simulation 1 week before the laboratory session. A control group (CG, n=77) attended the laboratory session without playing the simulation. After the session, all students completed a survey about their perception of the difficulty of the exercise on a scale of 1-10 and the HCT final value that they obtained. The students in the EG also completed a survey about their satisfaction with the experience. RESULTS After the laboratory session, the perceived difficulty of the procedure was lower on average in the EG compared to the CG (3.52 vs. 4.39, 95% CI: 0.16-1.57, P=.016). There was no significant difference in terms of perceived difficulty using the equipment. The HCT measures reported by the EG group also presented a much lower dispersion, meaning a higher reliability, in determining the HCT value (3.10 vs. 26.94, SD; variances significantly different, P<.001, F: 75.25, Dfd: 68.19 for EG and CG). In the satisfaction test, the majority of the students in the EG reported that the experience was positive or very positive (80.7%) and reported that it had helped them to identify and use the equipment (78%) and to perform the exercise (66%). CONCLUSIONS The simulation was well received by students in the EG, who felt more comfortable during the laboratory session, and it helped them to perform the exercise better, obtaining more accurate results, which indicates more effective training. EG students perceived the procedure as easier to perform, but did not report an improvement in the perceived difficulty in using the equipment. The increased reliability demonstrates that low-cost simulations are a good complement to the laboratory sessions.

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.

Taurine-induced synaptic potentiation and the late phase of long-term potentiation are related mechanistically.

The application of taurine (2-aminoethanesulfonic acid) induces a long-lasting increase of synaptic efficacy and axon excitability (LLP-TAU) in rat hippocampal CA1 area. After taurine withdrawal, LLP-TAU lasted at least 3 h. This fact prompted us to assess whether the mechanisms involved in the maintenance of this particular potentiation were similar to those implicated in the late phase of long-term potentiation (L-LTP). In the presence of KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase, taurine perfusion (10 mM, 30 min) did not affect the induction of LLP-TAU. However, LLP-TAU maintenance was completely suppressed by KT5720, an inhibitor of the cAMP-dependent protein kinase (PKA). Moreover, the late phase of LLP-TAU was blocked by inhibiting protein synthesis with anisomycin. In addition, taurine perfusion increased the phosphorylation of cAMP response element-binding protein (CREB), although did not affect cAMP levels. These features of LLP-TAU do not appear to be caused by the activation of D1/D5 dopamine receptors, as taurine also induced synaptic potentiation in the presence of SCH23390, an antagonist of this type of receptors. Finally, the late phase of both L-LTP and LLP-TAU occluded mutually. These results suggest that taurine triggers the sequence of some of the molecular events involved in the induction of L-LTP.

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.

Taurine Levels and Localization in Pancreatic Islets

Taurine is present in most mammalian tissues including the pancreas. In this organ, taurine appears to be specifically concentrated in the islets as was reported 25 years ago using semi-quantitative techniques2. Experiments confirming these data or studies on the implications of taurine on pancreatic physiology have been scarce. It has been shown, for instance, that taurine administration strongly suppresses glucose-stimulated secretion of insulin from isolated mice islets15, and also that intraperitoneal injection of taurine inhibits the increase in serum insulin induced by glucose administration8. On the contrary, it has been reported very recently that taurine enhances glucose-stimulated insulin release by cultured rat fetal islets4. Clinical studies linking taurine and pancreatic pathological situations are almost absent. It has been shown, however, that taurine plasma levels are low in diabetic patients and that supplementation with this amino acid reduces the increased tendency towards platelet aggregation in these patients5. These data indicate possible functional roles of taurine in the pancreas, and thus, a re-evaluation of its presence and cellular distribution in the islet needs to be carried out. The aim of this study was to determine taurine levels in the pancreatic islets by using a quantitative biochemical method, and also to localize the taurine distribution among the different cell types of the rat pancreas by immunohistochemical techniques.

A New Neuromodulatory Action of Taurine: Long-Lasting Increase of Synaptic Potentials

The physiological role of taurine, one of the most abundant free amino acids in the mammalian brain, is still poorly understood. A solid body of electrophysiological studies has demonstrated that taurine application causes neuroinhibitory actions in different regions of the CNS. In this sense, taurine has been shown to reduce the spontaneous neuronal firing, to hyperpolarize the resting membrane potential, to diminish the membrane input resistance and to increase the membrane Cl- conductance in different CNS neurons2, 4, 7, 19, 23, 24. On the basis of these observations, taurine has been proposed as a putative neuroinhibitory transmitter in the CNS. However, because of the lack of selective taurine antagonists, this possibility has not been unambiguously demonstrated so far. In fact, many of the taurine-in-duced neuroinhibitory effects have been shown to be blocked by GABA and/or glycine receptor antagonists4, 7, 20.

Taurine-Induced Potentiation is Partially Reversed by Low-Frequency Synaptic Stimlation

Neuroinhibitory actions of taurine in the brain are well known to be mediated by the activation of GABAA receptors11. We have recently reported a new neuromodulatory taurine action in the hippocampus independent of this GABA receptor type8,9. The new taurine action consists of a long-lasting potentiation of synaptic transmission, that is induced when taurine is applied at a concentration of 5–10 mM over a 10–30 min period. Excitatory postsynaptic potentials (EPSP) increase during taurine application, remaining elevated long after taurine withdrawal (at least three hours). This new taurine action could have important relevance to brain function, since persistent changes in the synaptic strength are envisaged as the cellular substrate of learning and memory4. Taurine-induced potentiation, unlike long-term potentiation (LTP) induced by high frequency synaptic stimulation of afferent fibers, is independent of NMDA receptor activation8. Nevertheless, taurine-induced potentiation has some points in common with LTP, primarily because its induction requires a rise in intracellular calcium, and both potentiation phenomena occlude mutually (Del Olmo et al., in preparation).