The photoreceptors of dark-adapted skate retinas bathed in a Ringer solution containing horseradish peroxidase (HRP) incorporate the tracer into membrane-bound compartments within the synaptic terminal of the cell; after 1 or 2 h of incubation, approx. 10-38% of the synaptic vesicles were labeled. The receptors appeared to be functioning normally throughout the incubation period, since electrical potentials of normal amplitude could be elicited in response to dimphotic stimuli. However, it was possible to block the uptake of peroxidase by a regimen of light adaptation that effectively suppressed light-induced activity in the electroretinogram. If, during incubation with peroxidase, retinas were exposed at 10-min intervals to an intense 1-ms flash from a xenon discharge tube, the receptor terminals were almost completely devoid of peroxidase; fewer than 2% of the vesicles were labeled. The suppression of HRP uptake could also be achieved in dark-adapted retinas by adding magnesium to the bathing solution, suggesting that calcium is necessary for transmitter release from vesicles in the receptor terminals. These findings are consistent with the view that vertebrate photoreceptors discharge a neurotransmitter in darkness, and that light decreases the release of this substance. It seems likely that the incorporation of peroxidase into vesicles of physiologically active receptor terminals reflects a mechanism for the retrieval of vesicle membrane after exocytosis.

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