Abstract
Purpose :
Starburst amacrine cells have been studied intensively in the rabbit and mouse retina where it has been shown that their dendrites respond preferentially to centrifugal radial motion. Starburst cells are present in the primate retina but have never been targeted for physiological study. Our purpose was to locate starburst cells, determine their basic light response properties and use 2-photon calcium imaging of dendrites to test for direction selectivity.
Methods :
In the light-adapted choroid attached whole-mount in vitro macaque monkey retina we identified ON starburst cells in the ganglion cell layer by their small spherical cell bodies (~8 µm diam). Starbursts were targeted with patch electrodes filled with a K-based internal solution, Oregon Green BAPTA (OGB) 488 (150 µM) and Alexa 568 (200 µM). Stimuli included spots, annuli and radially drifting ‘bullseye’ grating stimuli modulated around a mean photopic luminance. Light evoked changes in dendritic calcium were imaged using 2-photon excitation of OGB at 935 nm.
Results :
In current clamp, spot stimuli evoked a transient ON depolarization and a receptive field diam similar to the starburst cell's dendritic field diam as originally described in rabbit and mouse retina. Annular stimuli (outer diam 800 µm; inner diam 10-600 µm) revealed an additional large and sustained OFF depolarization. The OFF field was about twice the diam of the ON field and persisted in the presence of GABAergic and glycinergic receptor antagonists. Under voltage clamp the OFF depolarization was confirmed as an excitatory synaptic current. Radial centrifugal stimuli elicted a large ON depolarization whereas centipetal motion elicited a smaller response showing ON and OFF components. Dendritic calcium signals were also directionally selective showing similar large centrifugal ON vs small centipetal ON-OFF responses. The directional dependence of the calcium response was also unaltered by inhibitory receptor antagonists.
Conclusions :
In the macaque retina starburst dendritic calcium signals show profound directional preference for centrifugal over centripetal motion. Destructive interference between the transient ON and sustained OFF depolarizations may underlie the response reduction to centripetal motion seen in both the somatic voltage and dendritic calcium recordings. The origin of this distinctive OFF excitatory input remains to be determined.
This is a 2020 ARVO Annual Meeting abstract.