A SNARE COMPLEX CONTAINING SYNTAXIN-3 IS PRESENT IN RIBBON SYNAPSES OF THE RETINA

In contrast to conventional synapses, which release neurotransmitter t ransiently, ribbon synapses formed by photoreceptors and bipolar cells of the retina release neurotransmitter continuously and modulate the rate in response to light. Both modes of release are mediated by synap tic vesicles but probably differ in the regulation of docking and fusi on of synaptic vesicles with the plasma membrane. We have found that s yntaxin 1, an essential component of the core fusion complex in conven tional synapses, is absent from ribbon synapses of the retina, raising the possibility that these synapses contain a different type of synta xin or syntaxin-like protein. By immuno-precipitating syntaxin 1-deple ted retina and brain extracts with a SNAP-25 antibody and microsequenc ing the precipitated proteins, syntaxin 3 was detected in retina compl exed with SNAP-25, synaptobrevin, and complexin. Using an antisyntaxin 3 antiserum, syntaxin 3 was demonstrated to be present at high levels in retina compared to brain. Immunofluorescent staining of rat retina sections confirmed that syntaxin 3 is expressed by photoreceptor and bipolar cells in the retina. Thus, in the retina, expression of syntax in 3 is correlated with ribbon synapses and may play a role in the ton ic release of neurotransmitter.