The excitatory amino-acid-induced currents in nucleus tractus solitarii neurons freshly isolated from rats were investigated in a whole-cell recording mode using a conventional patch-clamp technique. At a holding potential of -70 mV, L-glutamate (Glu), N-methyl-D-aspartate (NMDA) with 10(-9) M glycine, kainate (KA), quisqualate (QA) and L-aspartate (Asp) evoked inward currents. The currents increased in a sigmoidal fashion with increasing agonists concentration. The half-maximum concentration (EC50) values were 5 x 10(-5) M for Glu, 10(-6) M for QA, 10(-4) M for KA, 6 x 10(-5) M for NMDA and 5 x 10(-5) M for Asp. The Hill coefficients of the Glu-, QA-, KA-, NMDA- and Asp-induced responses were 1.0, 1.3, 1.1, 1.3 and 1.1, respectively. The Glu-, QA-, NMDA- and Asp-induced currents consisted of a transient initial peak and a successive steady-state component showing no desensitization. These currents had the same reversal potential near +5 mV. In the current-voltage (I-V) relationships for the Glu-, NMDA- and Asp-induced currents, slight outward rectifications were observed in Mg2(+)-free external solution at membrane potentials negative to 0 mV. In the presence of extracellular Mg2+, the currents induced by Glu, NMDA and Asp were suppressed at negative membrane potentials, but the suppression was less for the Glu response. The I-V relationships for QA- and KA-induced responses were almost linear at a membrane potential between -90 and +50 mV with or without the presence of Mg2+.