Abstract
Purpose :
In vertebrate retinas, cone photoreceptors use glutamate to signal cone bipolar cells and cone horizontal cells (cHCs) at triad synapses. cHCs use GABA to signal cone bipolar cells following light adaptation (Chaffiol et al, 2017), but whether and how cHCs signal cones and/or other cHCs following light adaptation is unresolved. cHC dendrites in goldfish retina form ‘finger-like’ evaginations or spinules into cone membranes following light adaptation but spinules are absent following dark adaptation. It has been suggested that spinules mediate feedback from cHCs to cones, but no direct evidence has been reported.
Methods :
To understand the physiological role of spinules in cHC signaling, we performed immunolabeling for GABAa receptor (GABAaR) expression in light- and dark-adapted goldfish retinas in the day and used transmission electron microscopy to obtain high magnification images of cone pedicles. Because cHC dendrites express GABAaRs and NKCC, a chloride cotransporter that mediates GABA excitation, we studied the effects of gabazine (50 μM; selective GABAaR antagonist) and bumetanide (10 μM; selective NKCC inhibitor) on cones and cHCs following light- and dark-adaptation in the day.
Results :
cHC spinules were observed significantly more often in light-adapted compared to dark-adapted retinas. Intense GABAaR expression was observed on cHC spinules and dendrites following light adaptation compared to dark adaptation. In contrast, cone plasma membranes at triad synapses exhibited minimal GABAaR immunolabeling in the day following light or dark adaptation. Gabazine and bumetanide hyperpolarized cHCs by ~10 mV following light adaptation but had little effect following maintained darkness. Gabazine had minimal effect on cones following light or dark adaptation in the day.
Conclusions :
These results suggest that GABAaRs on cHC spinules and dendrites following light adaptation in the day mediate excitatory signaling between cHCs. GABAaRs are not expressed on cone plasma membranes at triad synapses which suggests that GABA-mediated feedback from cHCs to cones is minimal in the day following both light and dark adaptation. The appearance of cHC spinules following light adaptation may thus increase the lateral extent of GABA-mediated cHC to cHC excitatory signaling.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.