Tracer uptake by photoreceptor synaptic terminals. I. Dark-mediated effects.

This electron microscope study examines the sites of uptake and subsequent redistribution of the extracellularly applied tracer, horseradish peroxidase (HRP), in photoreceptor synaptic terminals during dark adaptation. First, chicks are sacrificed by perfusion fixation at various intervals after the addition of tracer to in situ eyecups. It is demonstrated that when terminals contain only one tracer-labeled organelle, this may be either a synaptic or a coated vesicle. At later times vacuoles in terminals become labeled. Some of these are seen as involutions of terminal membrane and some are observed in the cell body. Therefore, it seems likely that some involutions of terminal membrane pinch off and are transported retrogradely. Second, chicks are given intraocular injections of HRP at various intervals in a 12-hr dark period and sacrificed 1 hr later. Terminals of chicks sacrificed at 1 hr into the dark period contain predominantly tracer-labeled synaptic vesicles and vacuoles, whereas terminals of chicks sacrificed at 3 hr into the dark period contain tracer-labeled coated vesicles also. Diverticular membrane, thought to be the result of synaptic vesicle fusion and coalescence with the presynaptic membrane, waxes during the first hour of darkness and then wanes over a period of several hours (N. Cooper and B. McLaughlin, 1982, J. Ultrastruc. Res. 79, 58-73). These observations, together with those presented here, suggest that not all synaptic vesicles coalesce to form the diverticular membrane and that during the waxing stage, some synaptic vesicles are recycled rapidly without passing through a coated vesicle intermediate stage. This is discussed in the context of current models of localized synaptic vesicle membrane recycling.