Morphology and connectivity of the small bistratified A8 amacrine cell in the mouse retina

Abstract

Amacrine cells comprise ~30 morphological types in the mammalian retina. The synaptic connectivity and function of a few GABAergic wide-field amacrine cells have recently been studied, however, with the exception of the rod pathway-specific AII amacrine cell the connectivity of glycinergic small-field amacrine cells has not been investigated in the mouse retina. Here, we studied the morphology and connectivity pattern of the small-field A8 amacrine cell. A8 cells in mouse retina are bistratified with lobular processes in the ON sublamina and arboreal dendrites in the OFF sublamina of the inner plexiform layer. The distinct bistratified morphology was first visible at postnatal day 8, reaching the adult shape at P13, around eye opening. The connectivity of A8 cells to bipolar cells and ganglion cells was studied by double and triple immunolabeling experiments using various cell markers combined with synaptic markers. Our data suggest that A8 amacrine cells receive glutamatergic input from both OFF and ON cone bipolar cells. Furthermore, A8 cells are coupled to ON cone bipolar cells by gap junctions, and provide inhibitory input via glycine receptor (GlyR) subunit α1 to OFF cone bipolar cells and to ON A-type ganglion cells. Measurements of spontaneous glycinergic postsynaptic currents and GlyR immunolabeling revealed that A8 cells express GlyRs containing the α2 subunit. Taken together, the bistratified A8 cell makes very similar synaptic contacts with cone bipolar cells as the rod pathway-specific AII amacrine cell. However, unlike AII cells, A8 amacrine cells provide glycinergic input to ON A-type ganglion cells.

Nonlinear Spatiotemporal Integration by Electrical and Chemical Synapses in the Retina

“…However, the experiment in Figure 3B–D cannot rule out additional contributions from direct coupling between bipolar cells (Arai et al, 2010; Cohen and Sterling, 1990) or coupling between bipolar cells and other amacrine cell types (Lee et al, 2015). …”

Multiple cell types form the VIP amacrine cell population

“…Cell field sizes were calculated from the average of the major and minor axis of the field fitted to an ellipse. We defined narrow‐field cells as having a field diameter of <125 µm, medium‐field cells as having a field between 125 and 500 µm and wide‐field cells as having a field >500 µm based on previous reports in the mouse retina (Badea & Nathans, ; Pérez de Sevilla Müller et al, ; Dedek et al, ; Grimes et al, ; Pang, Gao, & Wu, ; Helmstaedter et al, ; Lee et al, ; Lee et al, ). These values are consistent with the definition of narrow‐, medium‐, and wide‐field cells in the rabbit retina (MacNeil & Masland, ).…”

Glycine receptor alpha subunit (GlyRa) specific inhibition contributes to ganglion cell signaling in mouse retina.

“…There are two explanations for this discrepancy: 1) either the OFF RGCs that are under the GlyRα2-mediated modulation are not A-type, or 2) GlyRα2 indirectly mediates the RF center response of these OFF RGCs via other ACs, instead of a direct On-to-Off crossover circuit. One type of bistratified ACs (A8) was recently found to express GlyRα2 and make synaptic contacts with OFF BCs (Lee et al, 2015), suggesting a candidate cell to mediate On-to-Off crossover inhibition onto the OFF RGCs and receive GlyRα2-mediated inhibition which indirectly modulates RF center responses. To continue to enhance our understanding we need to identify the exact RGC types that are subject to GlyRα2/GlyRα3-mediated modulation and responsible AC circuits.…”

“…Taken On the other hand, the composition of GlyRs is more complex in other retinal cells; some express more than one type of GlyR α subunit. For example, type 5, 6, 7 and A8 ACs express both GlyRα2 and GlyRα4 subunits (Lee et al, 2015;Weiss et al, 2008). Although ON SACs receive GlyRα4-predominant glycinergic synaptic input, the absence of GlyRα2 expression reduced half of their glycine-induced current, suggesting a co-expression of GlyRα2 extrasynaptically .…”

“…Similarly, some PV RGCs (PV0, 4 and 7) express multiple GlyR α subunits, as their glycinergic sIPSCs could not be eliminated in any single GlyR α subunit KO mouse. PV0 RGCs might express both GlyRα2 and GlyRα4, which is the same case for several types of ACs (Lee et al, 2015;Majumdar et al, 2009;Weiss et al, 2008). PV4 RGCs are likely to express both GlyRα1 and GlyRα3…”