Spreading Depression of Activity in Amphibian Retina

Abstract

Microelectrode studies on excised amphibian retina have uncovered a phenomenon of spreading retinal depression which in many respects resembles Leão's spreading cortical depression. The phenomenon is characterized by a spontaneous ‘milky’ wave that periodically marches across the surface of the retina and lasts for 2–3 minutes at any one point. There is a concomitant and sudden negative shift in the resting retinal potential of 1–2 mv which gradually returns to its previous level in 3–6 minutes. As the wave front approaches a microelectrode recording ganglion cell activity, there is a marked increase in its spontaneous firing climaxing in an intense discharge as the wave engulfs the electrode. A profound depression follows so that even the most intense photic stimulation becomes unable to elicit a local response. As the color change disappears and the d.c. level returns, the local area of retina becomes reexcitable. Similar changes occur in the local electroretinogram, which always returns slightly before the ganglion cell activity. The process occurs spontaneously but can also be induced by trauma, electrical current, or the application of KCl. It happens less often in bright light or with oxygenation, although neither of these factors can prevent its occurrence. It appears to be entirely reversible.