M-wave of the toad electroretinogram.

Area of Science:

• Neuroscience
• Retinal Physiology
• Electrophysiology

Background:

• Two light-evoked potassium (K+) increases occur in the retina: distal in the outer plexiform layer (OPL) and proximal in the inner plexiform layer (IPL).
• The distal K+ increase generates the electroretinogram (ERG) b-wave, but the proximal K+ increase's ERG contribution is less understood.

Purpose of the Study:

• To investigate the proximal K+ increase and its contribution to the ERG.
• To identify and characterize a novel ERG component, the M-wave, in the vitreal ERG.

Main Methods:

• Studied proximal K+ increase and ERG M-wave in isolated toad retinas (Bufo marinus).
• Utilized K(+)-sensitive microelectrodes and conventional intra-/extracellular microelectrodes.
• Recorded K+ changes, local ERG (LERG), vitreal ERG, and Müller cell responses.

Main Results:

• Observed an M-wave in the LERG, generated by Müller cell response to proximal K+ increase, with spatial tuning.
• Used N-methyl-DL-aspartate (NMA) to reveal a small, positive M-wave in the vitreal ERG, previously obscured by b-wave and PIII components.
• Picrotoxin (PTX) enhanced proximal K+ increase and M-wave, suggesting GABAergic inhibition's role in inner retinal processing.
• Found threshold ERG primarily composed of M-wave, indicating proximal retina drives amphibian threshold ERG.

Conclusions:

• The proximal K+ increase significantly contributes to the ERG via the M-wave.
• The M-wave is a distinct ERG component observable in the vitreal ERG under specific conditions.
• Inner retinal GABAergic inhibition plays a crucial role in retinal processing.
• The amphibian threshold ERG is driven by proximal retinal activity.