DENDRITIC MORPHOLOGY AND ITS EFFECTS ON THE AMPLITUDE AND RISE-TIME OF SYNAPTIC SIGNALS IN HIPPOCAMPAL CA3 PYRAMIDAL CELLS

Detailed anatomical analysis and compartmental modeling techniques wer e used to study the impact of CA3b pyramidal cell dendritic morphology and hippocampal anatomy on the amplitude and time course of dendritic synaptic signals. We have used computer-aided tracing methods to obta in accurate three-dimensional representations of 8 CA3b pyramidal cell s. The average total dendritic length was 6,332 +/- 1,029 mu m and 5,0 62 +/- 1,397 mu m for the apical and basilar arbors, respectively. The se cells also exhibited a rough symmetry in their maximal transverse a nd septotemporal extents (311 +/- 84 mu m and 269 +/- 106 mu m). From the calculated volume of influence (the volume of the neuropil from wh ich the dendritic structures can receive input), it was found that the se cells show a limited symmetry between their proximal apical and bas ilar dendrites (2.1 +/- 1.2 x 10(6) mu m(3) and 3.5 +/- 1.1 x 10(6) mu m(3), respectively). Based upon these data, we propose that the geome try of these cells can be approximated by a combination of two cones f or the apical arbor and a single cone for the basilar arbor. The recon structed cells were used to build compartmental models and investigate the extent to which the cellular anatomy determines the efficiency wi th which dendritic synaptic signals are transferred to the soma, We fo und that slow, long lasting signals show only approximately a 50% atte nuation when they occur in the most distal apical dendrites. However, synaptic transients similar to those seen in fast glutamatergic transm ission are transferred much less efficiently, showing up to a 95% atte nuation. The relationship between the distance along the dendrites and the observed attenuation for a transient is described simply by singl e exponential functions with parameters of 195 and 147 mu m for the ap ical and basilar arbors respectively. In contrast, there is no simple relation that describes how a transient is attenuated with respect to these cells' stratified inputs. This lack of a simple relationship ari ses from the radial orientation of the proximal apical and basilar den drites. When combined, the anatomical and modeling data suggest that a CA3b cell can be approximated in three dimensions as the combination of three cones. The amplitude and time-course for a synaptic transient can then be predicted using two simple equations. (C) 1996 Wiley-Liss , Inc.