Electrotonic profile and passive propagation of synaptic potentials in three subpopulations of hippocampal CA1 interneurons.

To elucidate the role of dendritic morphology in signal transfer, the passive propagation of somatic and dendritic potentials was compared in multi-compartment models of three interneuron subpopulations in the CA1 region. Nine calbindin-, 15 calretinin- and 10 parvalbumin-containing cells were modelled incorporating the detailed geometry, the currents of the action potentials in the soma, and the AMPA, N-methyl-D-aspartate and GABA-B receptor-mediated postsynaptic currents in the dendrites. The cable properties show characteristic differences among the subpopulations. The morphotonic length of calbindin and calretinin cell dendrites is larger than of parvalbumin cells. Thus parvalbumin cells are more compact than calbindin or calretinin cells unless the ratio of their axial and membrane resistivities exceeds the ratios of the other two cell types by more than 33%. In calbindin cells, the distal parts of the extremely long dendrites that invade the alveus are virtually isolated from the soma for passively propagating signals. The synaptic potentials evoked at a given morphotonic distance from the soma show larger differences locally on the dendrites than on the soma in all subpopulations. Both the somatic and dendritic amplitude ratios are the smallest in PV cells. In calbindin cells the somatic amplitude of synaptic potentials evoked at the same morphotonic distance from the soma is similar regardless of the number of branchpoints along their path. In calretinin and parvalbumin cells, from dendrites with long primary segments synaptic potentials reach the soma with larger amplitude than from dendrites that are branching close to the soma. The dendrites with the larger impact on somatic membrane potential are usually the dendrites that enter the stratum lacunosum-moleculare. These results indicate that dendritic morphology plays a role in changing the effectiveness of synaptic potentials evoked at different dendritic locations, and in this way is likely to be an important factor in determining the integrative properties of the different neuron populations.