Properties of the 'neutral zone' explain polarity of retinal spreading depression

Retinal spreading depression was evoked using low Cl- Ringer's solution and the concomitant field potentials (spreading depression potential; SDP) were recorded. The polarity of the transretinally recorded SDPs was not consistent among animal species. The SDPs recorded from carp and frog were receptor side negative, while chick and cat induced receptor side positive SDPs. According to the K+ hypothesis, the retinal SDP is generated by Müller cells responding to an increase in the extracellular K+ concentration in the inner plexiform layer. In order to clarify the relationship between the K+ increase and the polarity of the SDP, a high-K+ solution was injected at various retinal depths and the evoked potential was recorded transretinally. The neutral zone within the retina, where a change in the extracellular K+ concentration produces no net potential difference, was revealed to be near the proximal end of the retina in carp and frog, while it was located distal to the inner plexiform layer in the chick and cat. These results support the Müller cell K+ hypothesis and explain the polarity of SDPs. We conclude that the concept of the neutral zone is valuable for the investigation of the mechanism and polarity of transretinal field potentials.