SYNAPSES IN HIPPOCAMPUS OCCUPY ONLY 1-2-PERCENT OF CELL-MEMBRANES ANDARE SPACED LESS-THAN HALF-MICRON APART - A QUANTITATIVE ULTRASTRUCTURAL ANALYSIS WITH DISCUSSION OF PHYSIOLOGICAL IMPLICATIONS

Relatively little information exists regarding the spatial structure o f synaptic neuropil in the brain. The present electron microscopic stu dy employs unbiased stereological techniques and Monte Carlo simulatio ns to characterise quantitatively the spatial organisation of synaptic circuitry in the dentate gyrus of the hippocampus, an area of particu lar importance in mechanisms of learning and the subject of a number o f experimental neurobiological models of synaptic plasticity such as l ong-term potentiation. Firstly, tissue shrinkage/expansion resulting f rom embedding was assessed by imaging 300-mu m thick hippocampal slice s in the course of the entire embedding protocol, giving a value of 94 .3 +/- 1.1% for distance measures and 84.3 +/- 2.8% for volumetric mea sures. Secondly, numeric synaptic density, N-v, was estimated using th e disector. Thirdly, accumulated area of post-synaptic densities (PSDs ) per tissue volume, S-v, and the overall cell membrane area. per tiss ue volume, S-v were assessed using unbiased stereological rules coupl ed with image analysis of single sections. Finally, the mean area of i ndividual PSDs was derived as a ratio S-v/N-v giving: 0.0394 mu m(2) f or axo-spinous PSDs (thus representing similar to 1.3% of total cell m embranes) and 0.0769 mu m(2) for dendritic shaft PSDs (similar to 0.25 % of total cell membranes). From these data, the mean nearest neighbou r distance between synapses was estimated using Monte Carlo simulation s of a random 3D arrangement of synapses constrained by PSD sizes (a t runcated Poisson process), giving a value of 0.48-0.51 mu m. The physi ological importance of the morphometric data obtained is discussed in terms of assessing (i) the role of synaptic environment in modifying s ynaptic efficacy and (ii) the plausibility of cross talk between synap ses in relation to extrasynaptic neurotransmitter diffusion and transi ent depletion of extracellular Ca2+. (C) 1998 Elsevier Science Ltd. Al l rights reserved.