Rare and spatially segregated release sites mediate a synaptic interaction between two identified network neurons

Abstract

Laser-scanning confocal microscopy (LSCM), electron microcopy (EM), and cellular electrophysiology were used in combination to study the structural basis of an inhibitory synapse between two identified neurons of the same network. To achieve this, we examined the chemical inhibitory synapse between identified neurons belonging to the lobster (Homarus gammarus) pyloric network: the pyloric dilator (PD) and the lateral pyloric (LP) neurons. In order to visualize simultaneously these two neurons, we used intrasomatic injection of Lucifer Yellow (LY) in one and rhodamine/horseradish peroxydase (HRP) in the other. Under LSCM, we found only two zones of close apposition in a restricted part of the neuritic tree of the two network neurons. Then, within these two zones, the synaptic release sites were searched using EM. To this end, photoconversion of LY with immunogold and development of HRP with DAB were performed on the previously observed preparations. Structural evidence was found for only one release site per zone. To confirm this result, and because the zones of contact were always segregated in a restricted part of the dendrites, we used laser photoablation to selectively delete, either pre- or postsynaptically, the branches on which the release sites were located. In both cases, such restrictive ablation completely abolished the functional interaction between these neurons. Our results therefore demonstrate that an inhibitory synapse that is essential for the operation of a neural network relies on only very few sites of contact localized in a highly restricted part of each neuron's dendritic arbor.

Neuronal morphology and neuropil structure in the stomatogastric ganglion of the lobster, Homarus americanus

“…2B). This is consistent with findings in H. gammarus that show extended branching and synaptic contacts of STG neurons into the stn (Cabirol‐Pol et al,2002). Occasionally, we found isolated regions of labeling in the stn, 35 mm anterior to the ganglion (insert in Fig.…”

“…Associated with the somata, as well as distributed throughout the neuropil, were brightly labeled small ellipsoids (gray arrow in inset) that we interpret as either glia cells or glia cell nuclei (French et al,2002). As in P. interruptus (King,1976a), C. borealis (Kilman and Marder,1996), and H. gammarus (Cabirol‐Pol et al,2002), the core of the neuropil in H. americanus contains mostly larger branches (white arrows in middle and lower panel), whereas fine branches make up the peripheral layer of the neuropil (black arrows). Synapsin‐like IR was restricted to the fine neuropil.…”

“…However, this estimate did not take into account the presence of synapses from the terminals of sensory neurons and modulatory projection neurons. Cabirol‐Pol et al (2000,2002) found only a few anatomical appositions on the light‐microscopic level and, at the electron‐microscopic level, only a small number of spatially confined synaptic contacts between the LP and PD neurons in H. gammarus . Our double or triple labels of both chemical and electrical synaptic partners (Fig.…”

“…Even though there have been a number of anatomical investigations of the structure of the STG in several species, they were either restricted to subvolumes of the ganglion or to a subset of cell types, many performed without neuron identification, or, particularly in the case of earlier work, done in limited resolution and detail. These studies include electron microscopic characterization of the neuropil and synaptic contacts (Maynard,1971; King,1976a,b; Kilman and Marder,1996), the cellular localization of ion channels and receptors (Mizrahi et al,2001; French et al,2002; Clark et al,2004; French et al,2004), the spatial distribution of calcium influx (Graubard and Ross,1985; Ross and Graubard,1989; Kloppenburg et al,2000), the location of synaptic sites (King,1976a,b; Cabirol‐Pol et al,2000,2002), the distribution of modulatory inputs within the neuropil (Christie et al,1997), and the organization of neuronal branching patterns within different neuropil regions (Baldwin and Graubard,1995; Wilensky et al,2003).…”

Neuropilar Projections of the Anterior Gastric Receptor Neuron in the Stomatogastric Ganglion of the Jonah Crab, Cancer Borealis

“…It is interesting that the number of individual candidate contact sites between AGR and putative synaptic partners was generally very low, ranging from one to three. In agreement with this, it has been suggested that only few sites of synaptic contact exist between two identified motor neurons of the pyloric network in the European lobster [50]. This pattern of sparse synaptic contact sites supports the idea that dendritic integration in STG neurons is not evenly distributed across the neuropil but rather compartmentalized, as suggested previously in this system by analysis of motor neuron morphology and sub-cellular receptor distribution [15,16,51].…”