Basal concentration and evoked changes of extracellular taurine in the rat hippocampus in vivo

Inhibitory influence of ginsenoside Rb3 on activation of strychnine-sensitive glycine receptors in hippocampal neurons of rat

Whole-cell patch-clamp technique was used to investigate the effect of ginsenoside Rb(3) (Rb(3)), an active constituent of Panax ginseng, on glycine receptor activity in immature hippocampal neurons, which were dissociated acutely from hippocampal CA(1) area in Sprague-Dawley rats aging 10-14 days using the method of enzyme digestion with mechanical dissociation. As a result, glycine elicited an inward current (I(gly)) in a concentration-dependent manner in approximately 86% of those isolated neurons tested. This current was strychnine-sensitive. Rb(3) itself did not elicit any membrane currents. However, coapplication of Rb(3) inhibited peak current of I(gly). This depressant effect of Rb(3) varied with its concentrations. At a concentration of 0.1 micromol/L, ginsenoside Rb(3) had the most significant inhibition, with a net reduction of 31% in average. Moreover, the inhibition of I(gly) by Rb(3) did not depend on the membrane potential. Rb(3) (0.1 micromol/L) presented inhibitory effect on I(gly) mainly at higher glycine concentrations (>100 micromol/L), and decreased maximal glycine efficacy. This effect was the same as that of a non-competitive antagonist of glycine receptors. Finally, we found that Rb(3) prolonged the time constant of activation of I(gly). It is therefore suggested that ginsenoside Rb(3), possibly as a non-competitive antagonist, could inhibit strychnine-sensitive glycine current at a dose-dependent manner in acutely dissociated hippocampal CA(1) neurons of young rats, and decrease of affinity of glycine to receptors and delay of receptor activation may be involved in this inhibition. Inhibitory effect of ginsenoside Rb(3) on I(gly) is possibly one of the bases of many pharmacological actions of Panax ginseng.

Pharmacological characterization of glycine-gated chloride currents recorded in rat hippocampal slices

An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus has been overlooked, largely due to the misconception that GlyR expression ceases early during development and to few functional studies demonstrating their presence. As a result, little is known regarding the physiological and pharmacological properties of native GlyRs expressed by hippocampal neurons. In this study, we used pharmacological tools and whole cell patch-clamp recordings of CA1 pyramidal cells and interneurons in acutely prepared hippocampal slices from 3- to 4-wk old rats to characterize these understudied receptors. We show that glycine application to recorded pyramidal cells and interneurons elicited strychnine-sensitive chloride-mediated currents (I(gly)) that did not completely desensitize in the continued presence of agonist but reached a steady state at 45-60% of the peak amplitude. Additionally, the inhibitory amino acid, taurine, which has been shown to activate GlyRs in other systems, activated GlyRs expressed by both pyramidal cells and interneurons, although with much less potency than glycine, having an EC(50) 10-fold higher. To examine the potential subunit composition of hippocampal GlyRs, we tested the effect of the GABA(A) receptor antagonist, picrotoxin, on I(gly) recorded from both cell types. At low micromolar concentrations of picrotoxin (< or =100 microM), which selectively block alpha homomeric GlyRs, I(gly) was partially attenuated in both cell types, indicating that alpha homomeric receptors are expressed by pyramidal cells and interneurons. At picrotoxin concentrations < or =1 mM, approximately 10-20% of the whole cell current remained, suggesting that alphabeta heteromeric GlyRs are also expressed because this subtype of GlyR is relatively resistant to picrotoxin antagonism. Finally, we examined whether hippocampal GlyRs are modulated by zinc. Consistent with previous reports in other preparations, zinc elicited a bidirectional modulation of GlyRs, with physiological zinc concentrations (1-100 microM) increasing whole cell currents and concentrations >100 microM depressing them. Furthermore, the same concentration of zinc that potentiates I(gly) suppressed currents mediated by the N-methyl-D-aspartate subtype of the glutamate receptor. Thus we provide a pharmacological characterization of native GlyRs expressed by both major neuron types in hippocampus and show that these receptors can be activated by taurine, an amino acid that is highly concentrated in hippocampus. Furthermore, our data suggest that at least two GlyR subtypes are present in hippocampus and that GlyR-mediated currents can be potentiated by zinc at concentrations that suppress glutamate-mediated excitability.

Effect of ICV taurine on the impairment of learning, convulsions and death caused by hypoxia

The effect of the intracerebroventricular (ICV) administration of taurine on amnesia, convulsions and death caused by hypoxia was investigated in mice. Taurine in doses of 80-100 micrograms/mouse impaired acquisition of a single trial in passive avoidance performance, but protected mice from the learning impairment induced by hypoxia. Neither beta-alanine nor saccharose were able to mimic the effects of taurine. Taurine had no effect on amnesia induced by scopolamine injected intraperitoneally. Taurine protected against the onset of convulsions induced by hypoxia, while convulsions induced by pentylenetetrazole (PTZ) and hyperbaric oxygen were unaffected. The survival time of mice exposed to hypoxia was significantly increased by taurine treatment. These data suggest that taurine may play a role as an antihypoxic agent.