Abstract
Purpose :
Small bistratified ganglion cells are proposed to carry S-ON signals required for color vision. However, to the contrary, the ON signals from cones to bipolar cells including S-cone bipolar cell input to small bistratified cells are interrupted in patients with mutations in the gene encoding mGluR6 yet they have normal color vision and cone vision is little disturbed in general (Dryja et al. 2005, PNAS, 102, p. 4884). This suggests that there is a second principle ON pathway whereby signals including those involved in conscious color vision are relayed to bipolar cells. To begin testing the hypothesis that it’s based on GABA mediated feedforward via horizontal cells, we have used an ex vivo ERG system in mice which allows recording of signals transretinally in the presence and absence of GABA antagonists.
Methods :
Pieces of mouse retina from defined locations were placed in an ex vivo ERG chamber and perfused with Ames' solution at 37°C. ERG responses were recorded transretinally to on-off stimuli. Light arriving from the photoreceptor side was from a Gooch & Housego OL 490 Agile Light Source that has an incorporated diffraction grating and Digital Light Processor; it was controlled by custom ERG hardware and software to deliver programmable spectral output capable of producing cone isolating stimuli and narrowband lights for measuring spectral sensitivity functions.
Results :
High signal-to-noise ratio responses could be recorded for several hours from a single preparation. We compared responses before and after application of the glutamate agonist, L-AP4, and GABA antagonists, gabazine and TPMPA; recordings before and after washout of the drugs were reproducible. ERG waveforms in the presence of gabazine and TPMPA could be subtracted from the control to reveal a GABA mediated waveform or “GABAwave”. This was compared to responses mediated by glutamate signaling that were obtained by subtracting the waveform after application of L-AP4 from the control ERG.
Conclusions :
The ERG ON waveform attributable to GABA signaling had a faster onset than the ON response mediated by the G-protein-coupled receptor mGluR6 which was blocked by application of L-AP4, demonstrating the outer retina as the source of the GABAwave. The GABAwave constituted a major component of the ERG b-wave indicating that GABA mediated feedforward is a major ON signaling pathway in the outer retina.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.