Effects of L-glutamate, AMPA, NMDA and NPY on the discharge activity of neurons located in the ventral subdivision of the suprachiasmatic nucleus were examined in submerged coronal slices of the rat hypothalamus. All substances were bath applied. Application of L-glutamate (14 neurons examined) induced an excitatory response in 8 suprachiasmatic neurons (+248.9 +/- 122.24%, mean +/- S.E.M.; P < 0.001). A biphasic response, i.e. an initial transient excitation (+54.3 +/- 8.21%; P < 0.001) succeeded by an inhibition (-66.2 +/- 9.31%; P < 0.001), was observed in 6 neurons. Application of AMPA (36 neurons examined) resulted in an excitation of 31 neurons (+209.2 +/- 58.58%; P < 0.0001). Application of NMDA (57 neurons examined) induced an excitation in 34 neurons (+253.8 +/- 91.18%; P < 0.0001), but an inhibition in 8 neurons (-757 +/- 6.52; P < 0.0001). Biphasic effects of NMDA with an excitatory component (+58.7 +/- 9.94%; P < 0.0001) succeeded by an inhibitory component (-62.0 +/- 8.07%; P < 0.0001) were observed in 13 neurons. In 5 of 13 examined cases, the inhibitory component of neuronal responses to NMDA was significantly attenuated by the simultaneous application of strychnine (attenuation was 56%; P < 0.05). The application of NPY (40 neurons examined) induced significant effects on the discharge rate of 29 suprachiasmatic neurons. 18 of these neurons were inhibited (-59.3 +/- 6.39%; P < 0.0001) whereas 11 neurons were excited (+156.6 +/- 107.222%; P < 0.001) by NPY. In 8 of 11 neurons examined, the NPY-induced inhibition was significantly attenuated by 92% during simultaneous application of strychnine (P < 0.001). In 23 NPY-sensitive neurons, the discharge activity was also affected by NMDA. Neurons excited by NPY were also excited by NMDA (8 cells). In neurons inhibited by NPY, application of NMDA induced either an inhibition (3 cells) an excitation (5 cells) or a biphasic effect (7 cells). Results suggest a direct excitatory effect of AMPA, NMDA and NPY on suprachiasmatic neurons. In contrast, inhibitory actions of NMDA and NPY are considered induced by an activation of inhibitory interneurons. Antagonistic effects of strychnine suggest an involvement of glycinergic interneurons in a subpopulation of neurons inhibited by NMDA and in most neurons inhibited by NPY. The involvement of inhibitory mechanisms in photic entrainment of the circadian system is discussed. An integrative model of excitatory and inhibitory actions of EAA and NPY on suprachiasmatic neurons is proposed.