Morphology of the release site of inhibitory synapses on the soma and dendrite of an identified neuron.

Synapses are complex arrangements of pre- and postsynaptic differentiations involved in neural communication. A key element in this synaptic transmission is the presynaptic active zone where the release of neurotransmitter occurs. Active zones can be visualized and analyzed after staining with ethanolic phosphotungstic acid (EPTA) on semithin (0.5 micron) sections. This staining has been used in association with postembedding immunogold labeling for the neurotransmitters glycine or GABA, to investigate the organization of chemically defined inhibitory active zones, viewed in their full extent, on different regions of the goldfish Mauthner (M-) cell. With this approach, a marked variability in size and shape was observed for the release sites contacting the different parts of the postsynaptic neuron. In the axon cap and on the soma, glycinergic afferent terminals have small presynaptic grids (0.066 +/- 0.029 micron2, n = 30 and 0.076 +/- 0.037 micron2, n = 46, respectively). These grids are quite circular and they include 12 to 13 presynaptic dense projections (PDPs). The situation is different on the lateral dendrite, where glycinergic and GABAergic active zones display a greater variability in their surface areas (mean = 0.147 +/- 0.100 micron2, n = 115 and 0.139 +/- 0.080 micron2, n = 125, respectively), and their number of PDPs (mean = 19 +/- 9) per individual grid. Similarly, the shape of the release sites over the dendrite is more complex (annular, horseshoe-shaped) when compared to those on the soma. These differences of dendritic versus somatic release sites could represent a structural basis to maximize the shunting effect of glycinergic and GABAergic inhibitory junctions, i.e., close to excitatory inputs. We also observed that the proportion of endings containing 1 or more active zones also varies. More precisely, 96% and 82% of glycinergic terminals in the axon cap and on the soma, respectively, display only one active zone. On the dendrite, their proportion falls to 65.5% for both glycine- and GABA-containing boutons. The remaining inhibitory terminals contain 2 (30%) and 3 to 4 (4.5%) presynaptic grids. These results reveal a greater variability of morphology and organization of the inhibitory release sites at dendritic versus somatic locations. The functional significance of this observation for the synaptic transmission is discussed.