Identification of sulfur groups in the outer plexiform layer of the newt retina

The staining of newt photoreceptor synaptic vesicles with the zinc iodide-osmium (ZIO) method can be abolished by prior incubation in amphibian Ringer's containing 0.1 M-N-ethyl maleimide which specifically blocks -SH groups. The existence of high concentrations of sulfur groups in the outer plexiform layer of the newt retina has been further demonstrated with mercury orange and a newly synthesized fluorescent thiol probe, DACM. The reason why such high concentrations of -SH groups are present within photoreceptor synaptic vesicles is unclear, however, it is possible that ZIO stains the photoreceptor neurotransmitter itself.