Abstract
Purpose: :
The receptive field of PV1 (ON alpha) ganglion cell consists of an excitatory center and an inhibitory surround region. However this inhibitory surround is absent in the dark and appears only in the light adapted state. The circuitry underlying the change of the receptive field organization is not fully understood. In this study we show that the activation of surround inhibition exactly overlaps the activation of cone bipolar cells.
Methods: :
We performed targeted patch clamp recordings in the retina of transgenic mice in both dark and light adapted states. Loose cell spike recordings and whole cell voltage clamp recordings were performed in wholemount retinas of PVCre X Thy1Stop-eYFP mice to target PV1 (ON alpha) ganglion cells. We performed whole cell voltage clamp recordings in bipolar cells in retina slices. To target rod bipolar cells we used PCP2-GFP mice. To target ON cone bipolar cells in slice we used Lhx4-GFP mouse line. Perforated patch recordings were performed in the photoreceptor layer. We utilized Chrnb4-GFP line to distinguish between rods and cones.
Results: :
We targeted PV1 (ON alpha) ganglion cells in the wholemount retina and measured light-evoked responses. In the dark-adapted state threshold responses occurred at the light level of 0.0005 photoisomerization (*)/rod/sec and responses peaked at 0.5 */rod/sec. In this range, stimulation evoked no surround inhibition in ganglion cells. The threshold and peak responses observed in ganglion cells correspond well to the activation and peak responses of rod bipolar cells. The intensity-response curve for individual rods is shifted ~1 log unit towards higher intensities indicating powerful integration of rod signals by rod bipolar cells. Inhibitory surround in PV1 (ON alpha) ganglion cells appears at light level of 5 */rod/sec. The appearance of inhibitory surround is instantaneous, not gradual but switch-like and it is mediated by a direct postsynaptic GABA-ergic input. The light level of surround inhibition corresponds to the activation of cones and ON cone bipolar cells. We observed a strong activation of ON cone bipolar cells over the range of 5-500 */rod/sec followed by a gradual decline at higher intensities.
Conclusions: :
Based on the activation and pharmacological profile of the surround inhibition we conclude that it is provided by a widefield amacrine cell. That amacrine cell receives input from a specific subtype of ON cone bipolar cells.
Keywords: ganglion cells • amacrine cells • bipolar cells