Characteristics and ionic processes involved in feedback spikes of turtle cones

Abstract

In about 20% of the cones of untreated retinas of turtles, bright flash illumination of the periphery of their receptive field evokes a spike through the feedback mechanism from the L-horizontal cell. Such feedback spikes, never observed with central stimulation, are labile, but after they have disappeared they can be regained by depolarizing the cone. Feedback spikes are actual regenerative responses, since they show a critical threshold potential, are facilitated by cone depolarization and are blocked by hyperpolarization. They are associated with a membrane resistance decrease; tetrodotoxin (10$^{-5}$ M) does not block them. High Ca$^{2+}$ media facilitate their appearance, but their effect is transient because of the cone hyperpolarization and the light response block that Ca$^{2+}$ ions induce. Sr$^{2+}$ ions (4-10 mM) facilitate the discharge of feedback spikes in response to peripheral illumination in every cone, whether or not it has previously shown feedback effects. In Sr$^{2+}$ media, feedback spikes are stable and can be evoked by dim lights. Ba$^{2+}$ (2-6 mM) also facilitates and stabilizes the discharge of feedback spikes. Co$^{2+}$ and D-600 block the feedback spikes. Pharmacological agents that depolarize the L-horizontal cells, such as GABA, glutamate or nicotine, also block the feedback spikes. Both Sr$^{2+}$ and Ba$^{2+}$ also induce the appearance of spontaneous and off spikes, which are also blocked by Co$^{2+}$, but these are not related to the feedback mechanism. These results strongly suggest that every turtle cone receives a feedback input from the L-horizontal cells, which would be able to induce an increase of the cone Ca$^{2+}$ conductance, which may become regenerative.