INTEGRITY OF PERFORANT PATH FIBERS AND THE FREQUENCY OF ACTION-POTENTIAL INDEPENDENT EXCITATORY AND INHIBITORY SYNAPTIC EVENTS IN DENTATE GYRUS GRANULE CELLS

Whole-cell voltage clamp recordings in 400-mu-m thick hippocampal slices revealed discrete excitatory and inhibitory postsynaptic currents which persisted at synapses on granule cells following abolition of action potentials with 1-mu-M tetrodotoxin (TTX). The conductances associated with excitatory amino acid and GABA(A) receptor mediated events had mean peaks of 200 and 800 pS, and decayed monoexponentially with time constants of 5.6 and 5.3 ms. At a holding potential close to the normal resting membrane potential of granule cells (-80 to -90 mV), the frequency of glutamate/aspartate mediated spontaneous excitatory postsynaptic currents (sEPSCs) was decreased from 2.04 Hz in slices cut parallel to the plane of the perforant path to 0.87 Hz in slices cut in a plane that disrupted the distal perforant path fibres, suggesting that presynaptic integrity influences the rate of action potential independent neurotransmitter release. The orientation of the slicing had no effect on the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs).