MORPHOLOGY OF THE RELEASE SITE OF INHIBITORY SYNAPSES ON THE SOMA ANDDENDRITE OF AN IDENTIFIED NEURON

Synapses are complex arrangements of pre- and postsynaptic differentia tions involved in neural communication. A key element in this synaptic transmission is the presynaptic active zone where the release of neur otransmitter occurs. Active zones can be visualized and analyzed after staining with ethanolic phosphotungstic acid (EPTA) on semithin (0.5 mu m) sections. This staining has been used in association with postem bedding immunogold labeling for the neurotransmitters glycine or GABA, to investigate the organization of chemically defined inhibitory acti ve zones, viewed in their full extent, on different regions of the gol dfish Mauthner (M-) cell. With this approach, a marked variability in size and shape was observed for the release sites contacting the diffe rent parts of the postsynaptic neuron. In the axon cap and on the soma , glycinergic afferent terminals have small presynaptic grids (0.066 /- 0.029 mu m(2), n = 30 and 0.076 +/- 0.037 mu m(2), n = 46, respecti vely). These grids are quite circular and they include 12 to 13 presyn aptic dense projections (PDPs). The situation is different on the late ral dendrite, where glycinergic and GABAergic active zones display a g reater variability in their surface areas n = 125, respectively), and their (mean = 0.147 +/- 0.100 mu m(2),n = 115 and 0.139 +/- 0.080 mu m (2) number of PDPs (mean = 19 +/- 9) per individual grid. Similarly th e shape of the release sites over the dendrite is more complex (annula r, horseshoe-shaped) when compared to those on the soma. These differe nces of dendritic versus somatic release sites could represent a struc tural basis to maximize the shunting effect of glycinergic and GABAerg ic inhibitory junctions, i.e., close to excitatory inputs. We also obs erved 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 zo ne. On the dendrite, their proportion falls to 65.5% for both glycine- and GABA-containing boutons. The remaining inhibitory terminals conta in 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 si gnificance of this observation for the synaptic transmission is discus sed. (C) 1995 Wiley-Liss, Inc.