Abstract
Purpose :
In the retina, neuromodulators, such as nitric oxide (NO) and endocannabinoid (eCB), were proposed to play a role in adaptation to a constantly changing visual environment. However, the cellular mechanisms and respective signaling pathways are still poorly understood. Here, we hypothesize based on earlier studies that the NO and eCB pathway shape the response of bipolar cells (BCs).
Methods :
We intravitreally injected wild-type mice (both sexes, N=24), with pAAV.hSyn.iGluSnFr.WPRE.SV40 to visualize light-evoked glutamate release at BC axon terminals in dark-adapted retinal explants using two-photon microscopy (n=24). Light stimulus-evoked BC glutamate responses were recorded prior to and after application of NO or eCB agonists or antagonists or as control (no treatment). We clustered BC release sites (n=1676) according to IPL depth and compared their contrast responses between groups using a two-sided Mann-Whitney-Wilcoxon test with Holm-Bonferroni correction.
Results :
When presenting a sine wave ripple stimulus of increasing contrast, we found that the contrast sensitivity (CS) of ON BC terminals was significantly increased upon addition of the NO donor DETA/NO (n=265) compared to control (n=279) (p= 2.199e-08), whereas OFF BC responses were changed to a lesser extent (p= 2.476e-02). With the NO/cGMP pathway inhibitor ODQ (n=444), there was no significant change in CS of OFF BCs (p=2.708e-01) but a significant increase in CS of ON BCs (p=11.309e-12). Upon addition of the eCB receptor agonist (+)-WIN 55,212-2 (n=469), we found no significant decrease in CS in ON (p=9.762e-01), but in OFF BCs (p=2.780e-04). Upon AM251 treatment (n=219), an eCB receptor antagonist, we found a significant increase in CS in both ON BCs (p=1.172e-14) and OFF BCs (p=3.256e-06).
Conclusions :
We found that pharmacological inhibition of the NO pathway increased CS in ON but not in OFF BCs, whereas pathway activation only slightly affected CS in OFF BCs. Contrarily, inhibition of the eCB pathway increases CS in both ON and OFF BCs, while eCB activation decreased CS mainly in OFF BCs. This corroborates our hypothesis that both eCBs and NO play and key role in the retina where they differentially modulate signals at the level of ON and OFF BCs. These findings warrant further studies into retinal signal processing.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.