A REEVALUATION OF EXCITATORY AMINO ACID-MEDIATED SYNAPTIC TRANSMISSION IN RAT DENTATE GYRUS

1. Intracellular recordings were made from granule cells in combined slices of the hippocampus and parahippocampal cortex from adult Wistar rats. The neurons had a mean resting membrane potential (E(M)) of -85.1 ± 0.7 (SE) mV, input resistance (R(in)) of 30.9 ± 1.5 MΩ and action potential (AP) amplitude of 79.9 ± 1.06 mV measured from the threshold potential. The threshold for AP generation was ~40 mV positive to E(M). 2. The passive current-voltage relationship showed anomalous rectification, with R(in) increasing by 34% on average at modest depolarizations. With large excursions of the E(M) (by ± 30 mV or more), there was a marked fall in R(in). 3. With extracellular recording, a monophasic, positive-going field potential of 5-15 mV was evoked by stimulation of the perforant path (PP). Intracellularly, an excitatory postsynaptic potential (EPSP) of up to 40 mV in size was recorded. It was unusual to evoke an AP on orthodromic stimulation. Perfusion with picrotoxin (PTX, up to 20 μM) had small and variable effects on the EPSP, which implies that GABAergic inhibition does not play a major role. 4. Tonic depolarization reduced the EPSP. Hyperpolarization either had no effect or again decreased the EPSP. 5. The role of excitatory amino acid (EAA) receptor subtypes in mediation of the EPSP was investigated. Perfusion with the non-N-methyl-D-aspartate (NMDA) receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) caused a dose-dependent reduction of the EPSP with a shift to the right of the input-output relationship. The ED50 for this effect was ~2 μM. 6. In the presence of 5-10 μM CNQX a small component of the EPSP usually remained, which could be blocked by the NMDA receptor antagonist ±2-amino-5-phosphonovaleric acid (APV, 20-50 μM). This depolarizing component was markedly enhanced during perfusion with Mg2+-free medium. It increased in size and duration when the membrane was depolarized and decreased with hyperpolarization. These properties are consistent with the mediation of this potential via NMDA receptors. 7. These results indicate that NMDA receptors contribute to transmission at the synapse between the PP and the granule cell. This was confirmed by demonstrating that APV caused a small reduction in the size of the untreated EPSP and a shortening of the recovery phase. Furthermore, perfusion with a Mg2+-free medium enhanced the untreated EPSP. 8. It is concluded that the EPSP at the synapse between PP axons and dentate granule cells is predominantly mediated by non-NMDA type receptors. NMDA receptors also make a contribution that can be detected at normal levels of resting E(M) and in the presence of Mg2+ ions. It is proposed that the size and kinetics of the NMDA receptor-mediated component is accounted for by a combination of the pattern of channel activation, a high affinity of the NMDA receptor for the natural transmitter, and possibly spillover of the transmitter to extrasynaptic receptors.