Unique properties of NMDA receptors enhance synaptic excitation of radiatum giant cells in rat hippocampus

In the hippocampus, fast excitatory synaptic transmission of principal proj ection neurons is mediated by non-NMDA glutamate receptors, whereas NMDA gl utamate receptors serve a slower modulatory role. We used the whole-cell pa tch-clamp technique in adult hippocampal slices to assess the role of NMDA receptors in synaptic excitation of a recently discovered excitatory projec tion neuron, the CA1 radiatum giant cell (RGC). Glutamatergic synaptic acti vation, even after blocking non-NMDA receptors, fired an NMDA receptor-depe ndent burst of action potentials in RGCs. In contrast, the contribution of NMDA receptors to synaptic activation of pyramidal cells (PCs) was minimal. Stimulation of the same synaptic inputs evoked greater than threefold larg er EPSCs in RGCs than in PCs. Isolated NMDA receptor-mediated EPSCs were si gnificantly less sensitive to blockade by extracellular Mg2+ and had slower decay kinetics in RGCs than in PCs. Thus, unique properties of synaptic NM DA receptors underlie enhanced synaptic excitability in a newly discovered excitatory hippocampal projection neuron.