Passive electrotonic properties of rat hippocampal CA3 interneurones

1.The linear membrane responses of CA3 interneurones were determined with t he use of whole-cell patch recording methods. The mean input resistance (R- N) for all cells in this study was 526 +/- 16 M Omega and the slowest membr ane time constant (tau(0)) was 73 +/- 3 ms. 2. The three-dimensional morphology of 63 biocytin-labelled neurones was us ed to construct compartmental models. Specific membrane resistivity (R-m) a nd specific membrane capacitance (C-m) were estimated by fitting the linear membrane response. Acceptable fits were obtained for 24 CA3 interneurones. The mean R-m was 61.9 +/- 34.2 Ohm cm(2) and the mean C-m was 0.9 +/- 0.3 mu F cm(-2). Intracellular resistance (R-i) could not be resolved in this s tudy. 3. Examination of voltage attenuation revealed a significantly low synaptic efficiency from most dendritic synaptic input locations to the soma. 4. Simulations of excitatory postsynaptic potentials (EPSPs) were analysed at both the site of synaptic input and at the soma. There was little variab ility in the depolarization at the soma from synaptic inputs placed at diff erent locations along the dendritic tree. The EPSP amplitude at the site of synaptic input was progressively larger with distance from the soma, consi stent with a progressive increase in input impedance. 5. The 'iso-efficiency' of spatially different synaptic inputs arose from t wo opposing factors: an increase in EPSP amplitude at the synapse with dist ance from the soma was opposed by a nearly equivalent increase in voltage a ttenuation. These simulations suggest that, in these particular neurones, t he amplitude of EPSPs measured at the soma will not be significantly affect ed by the location of synaptic inputs.