Sustained firing is necessary for the persistent activity associated with working memory. The relative contributions of the reverberation of excitation and of the temporal dynamics of the EPSP to the maintenance of activity are difficult to evaluate in classical preparations. We used simplified models of synchronous excitatory networks, hippocampal autapses and pairs, to study the synaptic mechanisms underlying firing at low rates. Calcium imaging and cell attached recordings showed that these neurons spontaneously fired bursts of action potentials that lasted for seconds over a wide range of frequencies. In two week old cells, the median firing frequency was low, 11±8.8 Hz while in three to four weeks old cells, it decreased to a very low value, 2±1.3 Hz. In both cases, we have demonstrated that the slowest synaptic component supported firing. In two weeks old autapses, antagonists of NMDA receptors (NMDA-R) induced rare isolated spikes showing that the NMDA component of the EPSP was essential for bursts at low frequency. In 3-4 weeks old neurons, the very low frequency firing was maintained without the NMDA-R activation. However EGTA-AM or MCPG removed the very slow depolarizing component of the EPSP and prevented the sustained firing at very low rate. A metabotropic glutamate receptor (mGluR) activated calcium sensitive conductance is therefore responsible for a very slow synaptic component associated with firing at very low rate. In addition our observations suggested that the asynchronous release of glutamate might participate also in the recurring bursting.