DETAILED PASSIVE CABLE MODELS OF WHOLE-CELL RECORDED CA3 PYRAMIDAL NEURONS IN RAT HIPPOCAMPAL SLICES

Tight-seal whole-cell recordings were made from cleaned somata of CA3 pyramidal cells deep in hippocampal slices from 19-21-d-old rats. The cells were filled with biocytin, and their voltage responses to short current pulses were recorded. After washout of initial sag, responses scaled linearly with injected current and were stable over time. The d endritic and axonal arbors of four cells were reconstructed and measur ed using light microscopy. Dendritic spines and axonal boutons were co unted and the additional membrane area was incorporated into the relev ant segments. The morphology of each neuron was converted into a detai led branching cable model by assuming values for specific membrane cap acitance C-m and resistance R(m), and cytoplasmic resistivity R(i). Th ese parameters were optimized for each cell by directly matching the m odel's response to that of the real cell by means of a modified weight ed least-squares fitting procedure. By comparing the deviations betwee n model and experimental responses to control noise recordings, approx imate 95% confidence intervals were established for each parameter. If a somatic shunt was allowed, a wide range of possible R(m) values pro duced acceptable fits. With zero shunt, C-m was 0.7-0.8 mu Fcm(-2), R( i) was 170-340 Omega cm, and R(m) ranged between 120 and 200 Omega cm( 2). The electrotonic lengths of the basal and oblique dendrites were 0 .2-0.3 space constants, and those of the apical tufts were 0.4-0.7 spa ce constants. The steady-state electrical geometry of these cells was therefore compact; average dendritic tip/soma relative synaptic effica cies were >93% for the basal and oblique dendrites, and >81% for the t ufts. With fast transient synaptic inputs, however, the models produce d a wide range of postsynaptic potential shapes and marked filtering o f voltage-clamp currents.