Abstract
Purpose :
In the retina, ON cells depolarize to light stimulation, while OFF cells hyperpolarize to light. This fundamental rule has been occasionally violated, termed “polarity switch,” which is reported in some ganglion or amacrine cell types. However, the mechanism of the polarity switch remains elusive. We recorded from starburst amacrine cells (SACs) and bipolar cells (BCs) in wildtype (WT), cone-knockout, and rod-knockout (KO) mice to examine the contribution of rod and cone signals to the polarity switch.
Methods :
Whole-cell patch clamp recordings were conducted from SACs and BCs in the wholemount retina of C57 WT, cone-KO (Cgna3-/-), or rod-KO (Gnat1-/-). We performed voltage recordings using a spot of green LED step light (diameter 150 µm) with three different luminance conditions ranging from mesopic to photopic conditions. Cell types were identified by intracellular staining with sulforhodamine dye and Neurobiotin.
Results :
In WT mice, approximately half of ON SACs exhibited the polarity switch in mesopic conditions (n=50). Blocking inhibition through GABA-A, GABA-B, GABA-C, and glycine receptors in the bath or potassium channel blocker in the recording pipettes did not prevent the polarity switch. Blocking feedback from horizontal cells via the HEPES application corrected the ON SAC response in some cases but was inconsistent. Silencing ON pathway signaling via LAP4, a mGluR6 agonist, abolished the light response but did not change the polarity switch. However, the polarity switch was reliably corrected at photopic light levels. Surprisingly, the polarity switch occurred in BCs (n=28). As rods and cones are both functional at mesopic light levels, we used mutant mice to examine their contributions. A subset of BCs in the cone-KO mice showed a polarity switch in mesopic conditions but was corrected in photopic conditions (n=24). In rod-KO mice, light responses were not recorded at the dim condition, and the polarity switch was not observed in photopic conditions (n=15).
Conclusions :
We found that the polarity switch occurs in BCs, advancing the origin at least to second-order neurons. This explains the polarity switch results in downstream SACs. Interestingly, the polarity switch still occurred in BCs in cone-KO but not in rod-KO mice, indicating that the rod-signaling pathway causes the polarity switch.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.