Detailed passive cable models of layer 2/3 pyramidal cells in rat visual cortex at different temperatures

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Abstract
We present detailed passive cable models of layer 2/3 pyramidal cells based on somatic voltage transients in response to brief current pulses at physiological and room temperatures and demonstrate how cooling alters the shape of postsynaptic responses. Whole cell recordings were made from cells in visual cortical slices from 20- to 22-day-old rats. The cells were filled with biocytin and morphologies were reconstructed from three cells which were representative of the full range of physiological responses. These formed the basis for electrotonic models with four electrical variables, namely membrane capacitance (Cm), membrane resistivity (Rm), cytoplasmic resistivity (Ri) and a somatic shunt conductance (Gsh). Simpler models, with a single value for Rm and no Gsh, did not fit the data adequately. Optimal parameter values were derived by simulating the responses to somatic current pulses, varying the parameters to give the best match to the experimental recordings. Gsh and Rm were badly constrained. In contrast, the total membrane conductance (Gtot) was well constrained, and its reciprocal correlated closely with the slowest membrane time constant (τ0). The models showed close agreement for Cm and Ri (ranges at 36 °C: 0.78–0.94 μF cm−2 and 140–170 Ωcm), but a larger range for Gtot (7.2–18.4 nS). Cooling produced consistent effects in all three model cells; Cm remained constant (Q10 = 0.96), Ri increased (Q10 = 0.80), whilst Gtot dropped (Q10 = 1.98). In terms of whole cell physiology, the predominant effect of cooling is to dramatically lengthen the decay of transient voltage shifts. Simulations suggest that this markedly increases the temporal summation of postsynaptic potentials and we demonstrate this effect in the responses of layer 2/3 cells to tetanic extracellular stimulation in layer 4.
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