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Enrica Strettoi, Rania A. Masri, Ulrike Grunert; AII amacrine cells in the fovea of human and non-human primates. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1871.
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AII amacrine cells are small-field glycinergic neurons which play a crucial role in the rod pathway. They receive input from rod bipolar cells (RBCs) and feed rod-driven information into cone bipolar cells (CBCs) by means of conventional synapses in sublamina a and gap junctions in sublamina b of the inner plexiform layer. Here we addressed the questions (i) whether AII cells are present in the fovea where rods are absent and (ii) how their pattern of connections differs from that found at more peripheral locations.
Two eyes from two adult macaque monkeys (M. fascicularis) were obtained after unrelated electrophysiological experiments. Three post mortem human donor eyes (aged 44 to 53 years) were obtained from the Lions NSW Eye Bank at Sydney Eye Hospital 3 to 9 hours after death. Retinas were fixed in paraformaldehyde and vertical sections were cut across the fovea. Double and triple labeling immunohistochemistry was performed with antibodies against rhodopsin (rods); protein kinase C (RBCs); calretinin (AII cells); GAD (glutamic acid decarboxylase) and glycine transporters (amacrine cells); CtBP2 (synaptic ribbons); connexin 36 (cx36, gap junctions). Amacrine cells were counted using Zen blue edition software (Zeiss). 3-D reconstructions of AII amacrines and maps of CtBP2 and cx36-positive puncta colocalized to their dendritic arbors were obtained and counted with Imaris software (Bitplane).
AII amacrines were identified by their morphology and immunoreactivity to calretinin. Double and triple labeling revealed that most (over 85%) calretinin positive cells in both human (n = 2189 cells) and macaque (n = 904 cells) retinas were also positive for glycine transporter but negative to GAD. In both human and macaque, the first rods appeared at about 100 µm from the center of the fovea, RBCs appeared at about 700 µm and AII cells at about 500 µm. In macaque, colocalization analysis showed that AII amacrines closer to the fovea received a larger fraction of ribbon contacts from presumed CBCs and engaged a higher number of gap junctions with presumptive CB axonal endings compared to AII cells located at higher eccentricities, where contacts with rod bipolar cells prevailed.
AII amacrine cells are present in the fovea and contribute to foveal circuitry, where they maintain connections with cone bipolar cells. The functional consequences on cone bipolar and ganglion cell responses have to be investigated.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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