Differential expression of the presynaptic cytomatrix protein bassoon among ribbon synapses in the mammalian retina

Bassoon is a 420-kDa presynaptic protein which is highly concentrated at th e active zones of nerve terminals of conventional synapses, both excitatory glutamatergic and inhibitory GABAergic, in rat brain. It is thought to be involved in the organization of the cytomatrix at the site of neurotransmit ter release. In the retina, there are two structurally and functionally dis tinct types of synapses: ribbon and conventional synapses Antibodies agains t bassoon were applied to sections of rat and rabbit retina. Strong punctat e immunofluorescence was found in the outer and inner plexiform layers. Usi ng pre- and post-embedding immunostaining and electron microscopy, bassoon was localized in the outer plexiform layer at ribbon synapses formed by rod s and cones but was absent from basal synaptic contacts formed by cones. In the inner plexiform layer a different picture emerged. Ads in the brain, b assoon was found at conventional inhibitory GABAergic synapses, made by ama crine cells, but it was absent from the bipolar cell ribbon synapses. These data demonstrate differences in the molecular composition of the presynapt ic apparatuses of outer and inner plexiform layer ribbon synapses. Thus, di fferential equipment with cytomatrix proteins may account for the functiona l differences observed between the two types of ribbon synapses in the reti na.