The effects of specific excitatory amino acid (EAA) antagonists on evoked excitatory synaptic responses were studied in the hypothalamic paraventricular nucleus (PVN) of the guinea pig, by the use of the in vitro slice preparation. Intracellular recordings were obtained from paraventricular neurons, and excitatory postsynaptic potentials (EPSPs) and currents (EPSCs) were induced by perifornical electrical stimulation. To reduce the influence of a potential gamma-aminobutyric acidA (GABAA) inhibitory component on the synaptic responses, all experiments were performed in the presence of 50 microM picrotoxin. 2. Of 20 cells tested, 13 had electrophysiological characteristics similar to magnocellular neuropeptidergic cells (MNCs) and 7 displayed low-threshold Ca2+ spikes (LTSs). No difference was detected in the effect of the antagonists on the synaptic responses of cells with or without LTS potentials. 3. The broad-spectrum EAA antagonist kynurenic acid decreased the amplitude of the EPSPs and EPSCs in a dose-dependent manner: the mean decrease was 5% for 100 microM, 43% for 300 microM, and 70% for 1 mM. 4. The quisqualate/kainate-receptor-selective antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX) induced a dose-dependent decrease of the EPSPs and EPSCs: 1 microM had no detectable effect, 3 and 10 microM caused 30 and 70% decreases, respectively, and 30 microM blocked the response almost completely. This effect was not accompanied by a change in resting membrane potential or input resistance and was slowly reversible. 5. The N-methyl-D-aspartate (NMDA)-receptor-selective antagonist DL-2-amino-5-phosphonopentanoic acid (AP5), applied at 30 and 300 microM, reduced slightly the amplitude of the decay phase of the EPSP but did not significantly affect the peak amplitude. In some cells, the current-voltage relationship of the decay phase of the EPSC revealed a region of negative slope conductance between -70 and -40 mV. 6. These results suggest that 1) glutamate or a related EAA is responsible for the fast excitatory input to magnocellular and parvocellular neurons in the PVN and probably also for cells around PVN, 2) a quisqualate/kainate receptor type is responsible for the rising phase and peak amplitude of the synaptic current, and 3) an NMDA receptor contributes to the late part of the synaptic response.
