Passive electrotonic properties of rat hippocampal CA3 interneurones

Abstract

1. The linear membrane responses of CA3 interneurones were determined with the use of whole-cell patch recording methods. The mean input resistance (RN) for all cells in this study was 526 +/- 16 MOmega and the slowest membrane time constant (tau0) was 73 +/- 3 ms. 2. The three-dimensional morphology of 63 biocytin-labelled neurones was used to construct compartmental models. Specific membrane resistivity (Rm) and specific membrane capacitance (Cm) were estimated by fitting the linear membrane response. Acceptable fits were obtained for 24 CA3 interneurones. The mean Rm was 61.9 +/- 34.2 Omega cm2 and the mean Cm was 0.9 +/- 0.3 microF cm-2. Intracellular resistance (Ri) could not be resolved in this study. 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 variability in the depolarization at the soma from synaptic inputs placed at different locations along the dendritic tree. The EPSP amplitude at the site of synaptic input was progressively larger with distance from the soma, consistent with a progressive increase in input impedance. 5. The 'iso-efficiency' of spatially different synaptic inputs arose from two opposing factors: an increase in EPSP amplitude at the synapse with distance from the soma was opposed by a nearly equivalent increase in voltage attenuation. These simulations suggest that, in these particular neurones, the amplitude of EPSPs measured at the soma will not be significantly affected by the location of synaptic inputs.

Figure 1. Linear membrane response of a CA3 interneurone from s. lacunosum-moleculare

A, morphology of cell 970120A. B, voltage-current (V−I) relationship of the steady-state membrane response. The R_N of this cell, determined from the slope of linear regression analysis (continuous line), was 296 MΩ. C, the average of 100 hyperpolarizing transients (continuous line) was best fitted by two exponentials (filled circles). The membrane response and fitted exponentials are superimposed. D, semilogarithmic analysis of the membrane transient. The membrane response (thick trace) was fitted by the two exponentials (thin trace). No significant ‘sag’ in the hyperpolarizing response was observed here or in the V−I relationship (B).

Figure 2. Distribution of the percentage difference between R_N measured experimentally and R_N of the 63 modelled interneurones

The mean of this sample was not significantly different from that predicted by the hypothesis that there was zero difference between modelled and measured R_N.

Figure 3. Effects of varying passive membrane parameters on a simulated membrane response

The membrane response of cell 960210C for various combinations of passive membrane parameters. Varying R_m (A) affected both the transient and steady-state portion of the response while differences in C_m (B) affected only the transient part of the response. When R_i was varied (C) there was less than a 0.5 mV difference in both the steady-state and transient portions of the response.

Figure 5. Examples of rejected and accepted fits

Insets in A−C, reconstructions of cells 970217B, 970212A and 970120A, respectively. Scale bars, 100 μm. A, rejected fit of 970217B. B and C, accepted fits of 970212A and 970120A. Fits were accepted if they were within the 95 % confidence bands. The graphs on the left show the best-fit simulated membrane transient (continuous lines) and the 95 % confidence bands from the grand mean of the membrane responses (dashed lines). The graphs on the right show mean residuals (continuous lines) with the residual 95 % confidence bands (dashed lines). V_m, membrane potential.

Figure 4. Experimental noise does not affect resolution of passive membrane parameters

Experimental worst-case r.m.s. noise was 0.58 mV. White r.m.s. noise was added to the noise-free membrane response (generated with R_m= 49 kΩ cm², C_m= 0.92 μF cm⁻² and R_i= 184 Ω cm) of cell 960210B and averaged (n = 100). Resolved values of R_m, C_m and R_i are shown for each noise level. R_m and C_m, but not R_i, were resolved by the fitting procedure for r.m.s. noise between 0.25 and 1 mV. An acceptable fit was not achieved for 10 mV r.m.s. noise (lower right panel); the ‘true’ values of R_m and C_m were resolved.

Figure 6. Responses of simulated EPSPs at sites of synpatic input and at the soma

A, conductance-change synapses were individually activated at all dendritic locations of cell 960210B (top). A sample of 25 EPSPs at each site of synaptic input and the corresponding EPSPs at the soma are plotted (bottom). Arrows point to the difference in the range of EPSP amplitude at the synapses compared with EPSP amplitudes at the soma. B, EPSP amplitude measured at the site of synaptic input increased by more than 4-fold with distance from the soma. C, depolarization at the soma decreased as the location of synaptic input was progressively further from the soma by only 0.4 mV (≈25 %). D, EPSP rise time at the soma increased as inputs were further from the soma. The increase in rise time with distance from the soma was more than 2-fold.

Figure 7. Relationship between the location dependence of EPSP amplitudes, transfer impedance and input impedance

A, spatial profile of transfer impedance (Z_c) for 20 Hz signals from all dendritic locations to the soma. The range differed across distance from the soma by less than 15 %, indicating that a 20 Hz current signal from the most distal sites will be only 15 % smaller at the soma than at the most proximal locations. Inset, reconstruction of cell 960427A. B, depolarization at the soma in response to synaptic inputs at all locations of the cell. Here the range of EPSP amplitudes was 0.13 mV (≈20 %). C, input impedance (Z_N) calculated for 20 Hz signals increased with distance from the soma. D, consistent with the increase in input impedance with distance from the soma, EPSP amplitudes measured at all sites of synaptic input also progressively increased with distance from the soma. The multiple frequency components of the synaptic conductance change produce the subtle differences between panels A and B and C and D.