Voltage-clamp analysis of taurine-induced suppression of excitatory postsynaptic potentials in frog spinal motoneurons

1. The depressant actions of taurine applications on lumbar motoneurons in the isolated frog spinal cord were studied using conventional intracellular recordings and the two-electrode voltage-clamp technique. 2. With microelectrodes containing K+-acetate, 0.75-2 mM taurine mostly induced a hyperpolarization that often faded or turned into depolarization during the continuous application. A higher concentration (5-7.5 mM) depolarized a majority of cells. The effects on the membrane potential were associated with an increase in input conductance (approximately 285%). 3. The reversal potential of the taurine-induced currents was approximately -70 mV, with microelectrodes containing K+-acetate. In recordings using KCl-filled electrodes, taurine (less than or equal to 2 mM) produced a large depolarization (greater than or equal to 20 mV) at resting potentials near -50 mV, thereby indicating that the reversal potential was positively shifted by loading the cell with Cl-. These results suggest that the taurine potentials were mediated predominantly by an increased Cl- permeability. 4. Voltage-dependent relaxations of taurine currents were observed in 10 of 14 neurons. 5. A linear relation was found between the input conductance and the amount of current required to generate a 1-mV increment in EPSP at resting potential. 6. Polysynaptic excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) were more susceptible to taurine than the monosynaptic responses. Taurine (less than 1 mM) seemed to suppress the interneurons mediating polysynaptic pathways. 7. Monosynaptic EPSPs and EPSCs were decreased with higher concentrations of taurine (greater than 1 mM). The percent reduction of EPSPs and that of the corresponding EPSCs had a positive correlation (r = 0.95), whereas, there was no significant correlation between changes in EPSPs and in input conductance, and between changes in EPSCs and in input conductance. The amount of current required to produce a 1-mV increment of EPSP was increased in the presence of taurine, in association with the increased input conductance. 8. Taurine suppressed synaptic potentiation of EPSPs evoked by paired stimuli, at an interval of 60-180 ms. Gamma-D-glutamylglycine, an antagonist of receptors for excitatory amino acids, greatly reduced the amplitude of EPSPs, but had little effect on synaptic potentiation. 9. Taurine suppressed glutamate currents evoked at membrane potentials, clamped near rest in low Ca2+, high Mg2+ solution. 10. These findings suggest that the taurine-induced reduction of EPSPs is due mainly to suppression of EPSCs, through both presynaptic and postsynaptic mechanisms.(ABSTRACT TRUNCATED AT 400 WORDS)