Comparison of transretinal mouse rod ERG with local ERG across photoreceptor outer segments

Abstract

 

Purpose

 

Pharmacologically isolated transretinal electroretinogram (TERG) is increasingly used for studies of photoreceptor function. While TERG is believed to mainly reflect changes in the circulating “dark” current, it potentially contains components originating in the photoreceptor inner segments or even in the inner retina. Our aim was to examine how reliably TERG reflects the changes in the outer segment (OS) current by comparing TERG recordings with the ERG signal received locally across the photoreceptor outer segments (LERG).

 

Methods

 

Dark-adapted ERG responses to green LED light flashes were recorded from C57BL/6N mice. Light stimuli arrived from the photoreceptor side to the isolated retina perfused with modified Ringer’s solution at 37±1°C. Synaptic transmission was blocked with 2mM aspartate at the distal side and the glial component was abolished with 10mM BaCl₂ at the proximal side of the retina. ERG responses were simultaneously recorded 1) with macroelectrodes on both sides of the retina and 2) across the OS layer with one microelectrode at the level of cilium and a reference electrode at the tip of the rods. In some cases, the microelectrode was passed to the rod synaptic region.

 

Results

 

In LERG recordings across the OS layer the saturated responses settled to a plateau. The fast “nose” component present in the activation phase of the TERG response was not observed in the LERG recorded across the OS layer but it appeared when the recording electrode was advanced into the inner segment layer. There was no significant difference in the time to peak values of dim flash responses between the methods and the τ_D values were equivalent. The final recovery was slower and τ_rec was ∼30 % larger in the TERG responses. Fractional responses to dim flashes were somewhat larger in LERG. When the activation phases of the LERG responses were scaled to match those of TERG responses, the plateau levels of the saturated TERG and LERG responses matched well.

 

Conclusions

 

Pharmacologically isolated TERG responses correspond reasonably well to the LERG responses recorded across the OS layer. The main differences are the extra nose component in the early phase of responses to strong stimuli and the slower final decay of TERG responses, both of which have their origin in the inner segment.

 

 

Simultaneous TERG (black) and LERG (red) recordings. Flash strengths 10-2400Rh*/rod.

 

 

Saturated LERG responses at different depths.

 

Keywords: 510 electroretinography: non-clinical • 688 retina • 648 photoreceptors