class="enumerated" style="list-style-type:decimal">The effect of dextromethorphan (DM) on the current induced by glycine in acutely dissociated nucleus tractus solitarii (NTS) neurones of guinea-pigs was studied by use of the whole-cell patch clamp technique. The effect of DM on γ-aminobutyric acid (GABA)-induced currents (IGABA) was also examined.DM inhibited 30 μM glycine-induced current (IGly), without affecting the current caused by 30 μM GABA. The action of DM was concentration-dependent, with a maximum effect at 100 μM, and reversible. The half-maximum inhibitory concentration (IC50) of DM was 3.3 μM, about 85 times higher than that of strychnine.DM 3 μM shifted the concentration-response curve for glycine to the right without affecting the maximum value. DM 10 μM shifted the curve even more to the right, although it was not a parallel shift. Strychnine at a concentration of 0.1 μM shifted the curve for glycine in a nearly parallel fashion.The effect of 10 μM DM was slightly weak voltage-dependency, but the lower concentration of DM, 3 μM, inhibited IGly equally at −50 mV and +50 mV. The effect of 3 μM DM on IGly showed no use-dependence. Blockade by strychnine 0.1 μM showed no voltage- or use-dependence.The results indicate that DM inhibits IGly in single neurones of NTS, and further suggest that DM at a low concentration may act on the glycine receptor-ionophore complex, but not on the Cl− channel of the complex. However, a relatively high concentration of DM may at least partly affect the Cl− channel of the complex.
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