Abstract
The role of glutamatergic NMDA receptor channels (NMDARs) in the induction of long‐term potentiation (LTP) has been well established. In contrast, whether or not NMDARs contribute to the expression of LTP has been an issue of debate. In this study, we investigated the contribution of NMDARs to LTP expression in the hippocampal dentate gyrus (DG) by stimulating perforant path afferents with short bursts of pulses delivered at a moderate frequency (40 Hz), instead of using the traditional protocol of a single stimulus at a low frequency (<0.1 Hz). The synaptic summation provided by the “burst” protocol enabled us to measure the NMDAR‐mediated component of synaptic responses (NMDA component), defined as the NMDAR antagonist D‐2‐amino‐5‐phosphonovalerate (APV)‐sensitive component, in the presence of physiological concentrations of Mg^2+^ (1 mM). Intracellular recordings were obtained from DG granule cells of rabbit hippocampal slices, and excitatory postsynaptic potentials (EPSPs) were measured in terms of the integrated area of their profiles. At 40 Hz, frequency facilitation of the evoked EPSPs was observed. The NMDA component gradually increased during the five‐pulse train and frequency facilitation was significantly reduced after the application of APV. We tested the hypothesis that NMDARs undergo potentiation in LTP by comparing the NMDA/non‐NMDA ratio of the synaptic responses in control and LTP groups. An increase in the ratio was observed in the LTP group, strongly suggesting potentiation of NMDARs. To infer changes in conductance at individual synapses based on EPSPs recorded at the soma, we constructed a compartmental model of a morphologically reconstructed DG granule cell. The effect on the NMDA/non‐NMDA ratio of changes in AMPA and NMDA component synaptic conductance, and of differences in the distribution of activated synapses, was studied with computer simulations. The results confirmed that NMDARs are potentiated after the induction of LTP and contribute significantly to the expression of potentiation under physiological conditions. Hippocampus 2002;12:680–688. © 2002 Wiley‐Liss, Inc.