Activation of NMDA Receptors in Rat Dentate Gyrus Granule Cells by Spontaneous and Evoked Transmitter Release

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Abstract
Activation of N-methyl-d-aspartate (NMDA) receptors by synaptically released glutamate in the nervous system is usually studied using evoked events mediated by a complex mixture of AMPA, kainate, and NMDA receptors. Here we have characterized pharmacologically isolated spontaneous NMDA receptor-mediated synaptic events and compared them to stimulus evoked excitatory postsynaptic currents (EPSCs) in the same cell to distinguish between various modes of activation of NMDA receptors. Spontaneous NMDA receptor-mediated EPSCs recorded at 34°C in dentate gyrus granule cells (DGGC) have a frequency of 2.5 ± 0.3 Hz and an average peak amplitude of 13.2 ± 0.8 pA, a 10–90% rise time of 5.4 ± 0.3 ms, and a decay time constant of 42.1 ± 2.1 ms. The single-channel conductance estimated by nonstationary fluctuation analysis was 60 ± 5 pS. The amplitudes (46.5 ± 6.4 pA) and 10–90% rise times (18 ± 2.3 ms) of EPSCs evoked from the entorhinal cortex/subiculum border are significantly larger than the same parameters for spontaneous events (paired t-test, P < 0.05, n = 17). Perfusion of 50 μM d(–)-2-amino-5-phosphonopentanoic acid blocked all spontaneous activity and caused a significant baseline current shift of 18.8 ± 3.0 pA, thus identifying a tonic conductance mediated by NMDA receptors. The NR2B antagonist ifenprodil (10 μM) significantly reduced the frequency of spontaneous events but had no effect on their kinetics or on the baseline current or variance. At the same time, the peak current and charge of stimulus-evoked events were significantly diminished by ifenprodil. Thus spontaneous NMDA receptor-mediated events in DGGC are predominantly mediated by NR2A or possibly NR2A/NR2B receptors while the activation of NR2B receptors reduces the excitability of entorhinal afferents either directly or through an effect on the entorhinal cells.