Acidification and endosome-like compartments in the presynaptic terminals of frog retinal photoreceptors

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Abstract
By using the ‘acidotropic’ vital dye, Acridine Orange, we have found that the presynaptic terminals of rod and cone photoreceptors in retinas ofRana pipiens maintain a low pH relative to the surrounding medium through an energy dependent mechanism. When this pH is raised, by exposing the retinas to weak bases like ammonium chloride, the terminals exhibit large, membrane-delimited compartments, many of which accumulate endocytic tracers. This effect is partly reversed when the weak bases are removed. We infer that among the acidified structures within the terminals are endocytic compartments with at least some of the characteristics of the endosomes that participate in receptor-mediated endocytosis in other cell types. One role of these structures in the terminals may be in the recycling of synaptic vesicles.