Dendritic attenuation of synaptic potentials in the CA1 region of rat hippocampal slices detected with an optical method.

We directly measured fast excitatory postsynaptic potentials (EPSPs) along the dendrites of hippocampal CA1 pyramidal neurons by employing an optical method to study how synaptic potentials spread along the dendrites. Rat hippocampal slices were stained with a fluorescent voltage-sensitive dye JPW1114 and optical signals were monitored with a 16 x 16 photodiode array system. A stimulating electrode was placed either at stratum lacunosum moleculare to activate perforant fibers that make synaptic contacts to the distal apical dendrites or at stratum oriens to induce EPSPs at the basal dendrites of CA1 pyramidal cells. CNQX-sensitive components of the optical signals, which were assumed to be population EPSPs, were isolated. Propagation and attenuation of the CNQX components were successfully observed with the optical method. At the cell body layer, the peak of the CNQX-sensitive component was delayed by 17.08 +/- 1.64 ms from the input sites. Additionally we performed a simulation study to estimate the passive membrane parameters of the apical dendrites. Estimated apparent specific internal axial resistance (Ri) following stratum lacunosum moleculare stimulation was 76.0 +/- 4.2 Omega.cm and apparent specific membrane resistance (Rm) was 27.8 +/- 2.1 kOmega.cm2 (assuming the specific membrane capacitance of dendrites Cm = 1.6 microF/cm2). These values are comparable to those previously reported. When synaptic inputs were applied at stratum oriens, these apparent passive membrane parameters were different (high Ri and low Rm), suggesting that nonuniform dendritic membrane conductance or voltage-dependent conductances which are active near the resting potential may contribute to the measured passive membrane properties.