Retinal spreading depression and the extracellular milieu.

We used isolated chick retina in vitro to study the participation of the extracellular milieu in the occurrence and propagation of spreading depression. The propagation was followed by visual observation or microphotometry and the ionic changes in the extracellular compartment were recorded with double-barreled ion-selective microelectrodes. The front of the spreading wave is accompanied by increased light scattering in the tissue and by decrease of Cl-, Na+, and Ca2+, increase of K+, and an alkaline-acid shift in the extracellular space, concomitant with the slow voltage changes characteristic of the wave. As the spread is related to the chemical steady-state of the extracellular milieu, the velocity of propagation is influenced by a balanced interplay of the chemical constituents of the superfusing solution, e.g., K+, HCO-3, and glucose facilitate, while Cl- and Mg2+ hinder the wave. Steady-state alterations induced by physical factors (temperature) or related to experimental conditions (speed and direction of superfusate flow) change markedly the velocity of propagation. Generally the procedures that cause increase of velocity augment the susceptibility of the preparation to the reaction and eventually may trigger it. Propagated spreading depression is considered as a chemical diffusion reaction pervading more intensively the inner plexiform layer of the retina.