Abstract
Purpose: :
To indentify a crossover inhibition pathway in the retina whereby ON signals inhibit certain OFF ganglion cells via AII amacrine cells. ON cone bipolar signals pass via gap junctions into AII amacrine cells which provide glycinergic inhibition to OFF ganglion cells (Manookin et al., 2008; Münch et al. 2009). This pathway also provides a disinhibitory input at low contrast (Manookin et al., 2008). Two OFF ganglion cell types, OFF α and G9, were identified by a combination of different criteria such as dendritic field size, dye coupling, mosaic properties and stratification depth. Recently, it was shown that α-like ganglion cells in the mouse retina express kinetically fast glycine receptors composed of α1 subunits (Majumdar et al., 2007). Here, we provide anatomical evidence in support of lobular AII contacts via glycine α1 receptors with OFF α ganglion cells and G9 ganglion cells. These triple labeled structures are synaptic contacts.
Methods: :
OFF α ganglion cells and G9 ganglion cells were dye injected with Neurobiotin in wholemount rabbit retina. After fixation, the AII amacrine cell population was labeled with an antibody against calretinin and glycine receptors were marked with an antibody against the α1 subunit. This material was analyzed by triple-label confocal microscopy.
Results: :
OFF α ganglion cells showed typical dendritic morphology. They were stratified in sublamina a below the upper cholinergic band and dye-coupled to other OFF α ganglion cells and unidentified amacrine cells. The lobules of AII amacrine cells made close contacts at many points along the dendrites of individual OFF α ganglion cells. At these potential synaptic sites, we also found punctate labeling for the glycine α1 subunit. G9 ganglion cells, another OFF ganglion cell type, were stratified in sublamina a above the upper cholinergic band and were not coupled to any other cells. The dendritic field size of G9 ganglion cells was smaller than that of OFF α ganglion cells. G9 ganglion cells also received AII contacts mediated via α1 glycine receptors. The number of AII contacts was higher for G9 than for OFF α ganglion cells.
Conclusions: :
The presence of a post-synaptic marker such as the α1 glycine receptor at contact points between AII lobules and OFF α dendrites and G9 ganglion cells supports a direct inhibitory input from AII amacrine cells. This pathway provides for crossover inhibition in the rabbit retina whereby light onset provides an inhibitory signal to OFF α and G9 ganglion cells. Thus, these two OFF ganglion cell types receive a mixed excitatory and inhibitory drive in response to light stimulation.
Keywords: retinal connections, networks, circuitry • retina: proximal (bipolar, amacrine, and ganglion cells) • immunohistochemistry