Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells

The integrative properties of neurons depend strongly on the number, propor tions and distribution of excitatory and inhibitory synaptic inputs they re ceive. In this study the three-dimensional geometry of dendritic trees and the density of symmetrical and asymmetrical synapses on different cellular compartments of rat hippocampal CA1 area pyramidal cells was measured to ca lculate the total number and distribution of excitatory and inhibitory inpu ts on a single cell. A single pyramidal cell has similar to 12,000 mum dendrites and receives ar ound 30,000 excitatory and 1700 inhibitory inputs, of which 40% are concent rated in the perisomatic region and 20% on dendrites in the stratum lacunos um-moleculare. The pre and post-synaptic features suggest that CA 1 pyramid al cell dendrites are heterogeneous. Strata radiatum and oriens dendrites a re similar and differ from stratum lacunosum-moleculare dendrites. Proximal apical and basal strata radiatum and oriens dendrites are spine-free or sp arsely spiny. Distal strata radiatum and oriens dendrites (forming 68.5% of the pyramidal cells ' dendrite tree) are densely spiny; their excitatory i nputs terminate exclusively on dendritic spines, while inhibitory inputs ta rget only dendritic shafts. The proportion of inhibitory inputs on distal s piny strata radiatum and oriens dendrites is low (similar to3%). In contras t, proximal dendritic segments receive mostly (70-100%) inhibitory inputs. Only inhibitory inputs innervate the somata (77-103 per cell) and axon init ial segments. Dendrites in the stratum lacunosum-moleculare possess moderat e to small amounts of spines. Excitatory synapses: on stratum lacunosum-mol eculare dendrites are larger than the synapses in other layers, are frequen tly perforated (similar to 40%) and can be located on dendritic shafts. Inh ibitory inputs, whose percentage is relatively high (similar to 14-17%), al so terminate on dendritic spines. Our results indicate that: (i) the highly convergent excitation arriving on to the distal dendrites of pyramidal cells is primarily controlled by proxi mally located inhibition; (ii) the organization of excitatory and inhibitor y inputs in layers receiving Schaffer collateral input (radiatum/oriens) ve rsus perforant path input (lacunosum-moleculare) is significantly different . (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.