Inhibitory components of retinal bipolar cell receptive fields are differentially modulated by dopamine D1 receptors

Abstract

During adaptation to an increase in environmental luminance, retinal signaling adjustments are mediated by the neuromodulator dopamine. Retinal dopamine is released with light and can affect center-surround receptive fields, the coupling state between neurons, and inhibitory pathways through inhibitory receptors and neurotransmitter release. While the inhibitory receptive field surround of bipolar cells becomes narrower and weaker during light adaptation, it is unknown how dopamine affects bipolar cell surrounds. If dopamine and light have similar effects, it would suggest that dopamine could be a mechanism for light-adapted changes. We tested the hypothesis that dopamine D1 receptor activation is sufficient to elicit the magnitude of light-adapted reductions in inhibitory bipolar cell surrounds. Surrounds were measured from OFF bipolar cells in dark-adapted mouse retinas while stimulating D1 receptors, which are located on bipolar, horizontal, and inhibitory amacrine cells. The D1 agonist SKF-38393 narrowed and weakened OFF bipolar cell inhibitory receptive fields but not to the same extent as with light adaptation. However, the receptive field surround reductions differed between the glycinergic and GABAergic components of the receptive field. GABAergic inhibitory strength was reduced only at the edges of the surround, while glycinergic inhibitory strength was reduced across the whole receptive field. These results expand the role of retinal dopamine to include modulation of bipolar cell receptive field surrounds. Additionally, our results suggest that D1 receptor pathways may be a mechanism for the light-adapted weakening of glycinergic surround inputs and the furthest wide-field GABAergic inputs to bipolar cells. However, remaining differences between light-adapted and D1 receptor-activated inhibition demonstrate that non-D1 receptor mechanisms are necessary to elicit the full effect of light adaptation on inhibitory surrounds.

Keywords: Amacrine cell; Bipolar cell; GABA; Glycine; Luminance.

Figure 1.

Schematic of dopamine D1 receptor locations on major inhibitory inputs to OFF bipolar cells. A. In dark-adapted conditions, AII amacrine cells (AII) are activated by the rod pathway (dark pathway, R = rod photoreceptors, RB = rod bipolar cells) and release glycine onto OFF cone bipolar cells (OFF). AII amacrine cells are coupled to other AII amacrine cells through electrical gap junctions. In light-adapted conditions, other narrow-field glycinergic amacrine cells (Gly) and wide-field GABAergic amacrine cells (GBA) are activated by cone pathways (white pathways, C = cone photoreceptor, ON = ON cone bipolar cell), which provide inhibitory input to OFF bipolar cells. In the inner retina, dopamine is released onto D1 receptors (blue) located on cone bipolar and amacrine terminals. INL = inner nuclear layer, IPL = inner plexiform layer. B. Light adaptation increases dopamine release and decreases OFF bipolar cell receptive field (RF) surround size and strength. However, it is unknown to what extent dopamine, through dopamine D1 receptor activation, replicates the light-adapted effects on OFF bipolar cell surrounds.

Figure 2.

D1 receptor activation weakens the spatial inhibitory input to OFF bipolar cells. A-B. Example L-IPSCs recorded in dark-adapted (black, A) and dark-adapted + SKF (+SKF, blue, B) conditions in response to a light stimulus (1 second, 25 μm bar) presented at −400, 0, and 400 μm from the cell. SKF application reduced L-IPSC strength at all stimulus distances. Light stimulus = light gray bar under example OFF type 1/2 traces. C. Spatial inhibition profiles of response charge transfer normalized to the center bar stimulus in dark-adapted and SKF conditions (n=7). The spatial profile became significantly narrower with SKF. D. Same as in C but for response peak amplitude. The peak amplitude profile was significantly narrower and smaller with light adaptation. E. Spatial inhibition profiles of response charge transfer normalized to the respective dark-adapted center bar stimulus in SKF and light-adapted (n=5, gray) conditions. Light-adapted spatial input was significantly narrower and smaller than with SKF application. F. Same as in E but for normalized response peak amplitude. The peak amplitude profile was significantly smaller in the periphery with light adaptation. Cells included in averages: SKF = 3 OFF type 1/2/4 and 4 OFF type 3; light-adapted = 1 OFF type 1/2/4 and 4 OFF type 3. Light-adapted data was adapted from Mazade and Eggers, 2016 Fig. 2C,F for comparison. Error bars are ±SEM and only show the outer bar for each data point. (*** p<0.001)

Figure 3.

D1 receptor activation affects the edges of GABAergic spatial input to OFF bipolar cells. A-B. Example GABAergic L-IPSCs recorded in dark-adapted (black, A) and dark-adapted + SKF (+SKF, blue, B) conditions. SKF application did not reduce overall response strength 400 μm away from the cell. Light stimulus = light gray disconnected bar under example traces; this OFF type 3 cell responded to the offset of the stimulus so only the end of the light stimulus is shown. C. Spatial inhibition profiles of GABAergic response charge transfer normalized to the center bar stimulus in dark-adapted and SKF conditions (n=4). Inhibition was significantly reduced only at far stimulus distances. D. Same as in C but for GABAergic response peak amplitude. Peak amplitude was only reduced at far stimulus distances. The peak amplitude spatial profile was significantly narrower and smaller with light adaptation. E. Spatial inhibition curves of GABAergic response charge transfer normalized to the respective dark-adapted center bar stimulus in SKF and light-adapted (n=5, gray) conditions. Spatial inhibitory strength was reduced in the near surround more with light adaptation than with SKF. F. Same as in E but for normalized response peak amplitude. The peak amplitude spatial profile was significantly smaller with light adaptation only in the local surround. Cells included in averages: SKF = 1 OFF type 1/2/4 and 3 OFF type 3; light-adapted = 4 OFF type 3. Light-adapted data was adapted from Mazade and Eggers, 2016 Fig. 3C,F for comparison. Error bars are ± SEM and only show the outer bar for each data point. (* p<0.05, ** p<0.01, and *** p<0.001)

Figure 4.

D1 receptor stimulation reduces glycinergic spatial inhibitory strength to light-adapted levels. A-B. Example glycinergic L-IPSCs recorded in dark-adapted (black, A) and dark-adapted + SKF (+SKF, blue, B) conditions. SKF application reduced overall response 400 μm away from the OFF bipolar cell. Light stimulus = light gray disconnected bar under example OFF type 4 traces. The response ended before the end of the light stimulus, so only the initial portion of the light stimulus is shown. C. Spatial inhibition profiles of glycinergic response charge transfer normalized to the center bar stimulus in dark-adapted and SKF conditions (n=5). The spatial profile of inhibitory responses was reduced with D1 receptor activation. D. Same as in C but for glycinergic response peak amplitude. The peak amplitude spatial profile was significantly smaller with SKF activation. E. Spatial inhibition curves of glycinergic response charge transfer normalized to the respective dark-adapted center bar stimulus in SKF and light-adapted (n=4, gray) conditions. Light-adapted spatial input was not different from that with SKF application. F. Same as in E but for normalized glycinergic response peak amplitude. The peak amplitude spatial profile was not significantly different from that with light adaptation. Cells included in averages: SKF = 4 OFF type 1/2/4 and 1 OFF type 3; light-adapted = 3 OFF type 1/2/4 and 1 OFF type 3. Light-adapted data was adapted from Mazade and Eggers, 2016 Fig. 5C,F for comparison. Error bars are ± SEM and only show the outer bar for each data point. (** p<0.01 and *** p<0.001)