The responses of amacrine cells to light and intracellularly applied currents

1. Intracellular responses to light were recorded from amacrine cells in the retina of the turtle Pseudemys scripta elegans. 2. The recorded responses were identified on the basis of physiological criteria reported previously (Marchiafava, 1976). Amacrine cells produced transient 'on' and 'off' depolarizing responses irrespective of the retinal area illuminated and of wavelength. 3. The transient depolarizing responses increased by enlarging the illuminated circle up to 120 micrometer in radius. Circles covering larger areas, up to 200 micrometer, produced a relative decrease of the response amplitude. Thus, amacrine cells' receptive fields appear as a central 'excitatory' area of about 120 micrometer radius, surrounded by a 'suppressor' area. 4. Amacrine cells' photoresponses were associated with an increase in membrane conductance. The responses to illumination of central or peripheral areas of the receptive field, however, showed different reversal potentials. The responses to peripheral illumination reversed at about 15 mV above resting potential, while the equilibrium potential of the centre-photoresponses was indicated by extrapolation at about +30 mV. No conductance chance was detectable during steady lights. 5. Repetitive stimulation of the optic nerve invariably reduced amacrine cells' photoresponses, but not those recorded from bipolar cells. It follows then that only ganglion cell photoresponses originating from amacrines' input would be depressed by the nerve stimulation, which thus becomes a reliable test to discriminate whether ganglion cell photoresponses originate from amacrine or bipolar inputs.