Cable properties of cultured hippocampal neurons determined from sucrose-evoked miniature EPSCs

1. The passive cable properties of rat hippocampal neurons in dissociated culture were studied using focal application of hypertonic solution to locally elicit miniature excitatory postsynaptic currents (mEPSCs) on the soma and dendrites. Neurons were filled with Lucifer yellow and portions of their dendritic trees were measured. 2. The average mEPSC measured at the soma appeared smaller and slower as the site of sucrose application was made more distal. Normalizing to a 1-micron diam dendrite, the mean mEPSC peak amplitude and charge was reduced e-fold in 170 and 1,000 microns, respectively, and the mean mEPSC decay time constant was increased e-fold in 150 microns. However, for any particular sucrose site, individual mEPSCs varied widely in their amplitudes and time courses. Plots of individual peak amplitudes versus half-width or rise time showed much overlap for mEPSCs originating from sites as much as 100 microns apart. This suggests that use of such plots to estimate the electrotonic location of synaptic currents is highly prone to error. 3. Averaged mEPSCs recorded when applying sucrose at the soma were poorly fitted by an alpha function but were well-described by an equation of the form mxh, where m incorporates a rise-time constant tau 1 and h a decay time constant tau 2. Averaged fits to mean mEPSCs elicited at the somas of five cells gave (mean +/- SE): peak conductance = 832 +/- 126 pS, tau 1 = 0.29 +/- 0.06 ms, tau 2 = 3.03 +/- 0.24 ms, x = 4.7 +/- 0.7. 4. For three cells, the entire dendritic branch to which sucrose was applied was measured and used to construct a passive cable model. The specific membrane resistance (Rm) and intracellular resistivity (Ri) were varied systematically in the model (assuming membrane capacitance Cm = 1 microF/cm2) to search for the best agreement between the mean mEPSCs and the model. Optimal Rm was found to lie in the range 20-30 k omega cm2, Ri in the range 100-200 omega cm. 5. These results confirm those obtained by other methods and emphasize the considerable cable filtering of fast electrical events in cultured hippocampal neurons.