The nucleus of the solitary tract (NTS) is a principal site for coordinating the reflex control of autonomic function. The nucleus receives and organizes primary visceral (sensory) afferent inputs from the great vessels, heart, lung, and gastrointestinal organs. Glutamate, the excitatory neurotransmitter released by the primary afferent fibers, activates non-N-methyl-D-aspartate (non-NMDA) receptors on second-order neurons in the NTS. Still in question is whether NMDA receptors on the second-order neurons are also activated. Accordingly, the purpose of this study was to directly determine whether NMDA receptors contribute to synaptic transmission of primary visceral afferent input to second-order neurons in the NTS. Whole cell patch-clamp recordings were obtained from intermediate and caudal NTS neurons in rat coronal medullary slices. Excitatory postsynaptic currents (EPSCs) were evoked by stimulation of the solitary tract (1-25 V, 0.1 ms, 0.2 or 0.5 Hz) at membrane potentials ranging from -90 to +60 mV. In 28 of 32 neurons in which current-voltage relationships were obtained for solitary-tract-evoked EPSCs, the currents had short onset latencies (3.42 +/- 1.03 ms, mean +/- SD), indicating that they were the result of monosynaptic activation of second-order neurons. Solitary-tract-evoked EPSCs had both a fast and a slow component. The amplitude of the slow component was nonlinearly related to voltage (being revealed only at membrane potentials positive to -45 mV), blocked by the NMDA receptor antagonist DL-2-amino-5-monophosphovaleric acid (APV, 50 microM; n = 12; P = 0.0001), and enhanced in nominally Mg2+-free perfusate at membrane potentials negative to -45 mV (n = 5; P = 0.016), demonstrating that the slow component was mediated by NMDA receptors. The amplitude of the fast component was linearly related to voltage and blocked by the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX, 3 microM; n = 9; P = 0.0014), demonstrating that the fast component was mediated by non-NMDA receptors. The slow component of the EPSCs was not blocked by NBQX (n = 6; P = 0.134), nor was the fast component blocked by APV (n = 12; P = 0.124). These results show that both NMDA and non-NMDA receptors coexist on the same second-order NTS neurons and mediate primary visceral afferent transmission in the NTS. The participation of NMDA receptors suggests that second-order neurons in the NTS may have previously unrecognized integrative capabilities in the reflex control of autonomic function.