CA1 PYRAMID-PYRAMID CONNECTIONS IN RAT HIPPOCAMPUS IN-VITRO - DUAL INTRACELLULAR-RECORDINGS WITH BIOCYTIN FILLING

In adult rat hippocampus, simultaneous intracellular recordings from 9 89 pairs of CAl pyramidal cells revealed nine monosynaptic, excitatory connections. Six of these pairs were sufficiently stable for electrop hysiological analysis. Mean excitatory postsynaptic potential amplitud e recorded at a postsynaptic membrane potential between -67 and -70 mV was 0.7 +/- 0.5 mV (0.17-1.5 mV), mean 10-90% rise time was 2.7 +/- 0 .9 ms (1.5-3.8 ms) and mean width at half-amplitude was 16.8 +/- 4.1 m s (11.6-25 ms). Cells were labelled with biocytin and identified histo logically. For one pair that was fully reconstructed morphologically, excitatory postsynaptic potential average amplitude was 1.5 mV, 10-90% rise time 2.8 ms and width at half-amplitude 11.6 ms (at -67 mV). In this pair, correlated light and electron microscopy revealed that the presynaptic axon formed two synaptic contacts with third-order basal d endrites of the postsynaptic pyramid, one with a dendritic spine, the other with a dendritic shaft. In the four pairs tested, postsynaptic d epolarization increased excitatory postsynaptic potential amplitude an d duration. In two, D-2-amino-5-phosphonovalerate (50 mu M) reduced th e amplitude and duration of the excitatory postsynaptic potential. The remainder of the excitatory postsynaptic potential now increased with postsynaptic hyperpolarization and was abolished by 20 mu M 6-cyano-7 -nitroquinoxaline-2,3-dione (n=1). Paired-pulse depression was evident in the four excitatory postsynaptic potentials tested. This depressio n decreased with increasing inter-spike interval. These results provid e the first combined electrophysiological and morphological illustrati on of synaptic contacts between pyramidal neurons in the hippocampus a nd confirm that connections between CAI pyramidal neurons are mediated by both N-methyl-D-aspartate and o-3-hydroxy-5-methyl-4-isoxazoleprop ionate/kainate receptors. Copyright (C) 1996 IBRO. Published by Elsevi er Science Ltd.