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C. Davenport, O.S. Packer, P.B. Detwiler, D.M. Dacey; Calcium Imaging in Primate A1 Amacrine Cells Reveals Evidence for Functional Polarity of Dendritic and Axon–Like Components . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2671.
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© ARVO (1962-2015); The Authors (2016-present)
The A1 cell is an axon–bearing amacrine cell with a diffusely stratified, moderately branched dendritic tree (∼400 micron diameter). Axon–like processes arise from proximal dendrites giving rise to a second concentric, larger arborization of thin, bouton–laden processes (>4 mm diameter). A1 cells respond at light onset and offset with transient depolarizations and large, fast action potentials (Stafford and Dacey, 1997). It was hypothesized that A1 cells receive local input to their dendrites, with action potentials serving to propagate output via the axons across the retina, thereby serving a global inhibitory function in the retina. Here we explore this basic hypothesis by mapping the spatial receptive field structure and by imaging light evoked Ca2+ transients in the dendrites and axons directly.
An in vitro preparation of the intact macaque monkey retina was used to record intracellularly from anatomically identified A1 cells. Receptive field structure was mapped using spots and annuli. A1 cells were filled with the Ca2+ sensitive dye Oregon Green BAPTA–1 and a two photon laser scanning microscope was used to image fluorescence transients in isolated segments of morphologically identified dendritic and axon–like components in response to full field light flashes. Both receptive field mapping and Ca2+ imaging were done before and after application of tetrodotoxin (TTX) to block sodium spikes.
A1 cells have an excitatory receptive field center approximately the same size as the dendritic field and an antagonistic surround that extends beyond the dendrites but is far smaller than the axonal field. TTX blocks A1 action potentials but has minimal effect on center–surround receptive field organization. We observed light induced Ca2+ transients in A1 dendrites and axons. TTX abolishes the Ca2+ transient in the axons but not in the dendrites.
Ca2+ imaging reveals an input–dependent Ca2+ transient in A1 dendrites and an action potential–dependent Ca2+ transient in the axons. TTX block suggests that action potentials are not involved in receiving and integrating inputs. Spatial receptive field size is correlated with the dendritic field size and not the axonal field. These results support the current model of A1 function, with the dendrites receiving inputs that elicit action potentials that propagate away from the soma via the axons across the retina.
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