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S. Kim, S.-S. Paik, H.-L. Kim, M.-H. Chun, I.-B. Kim; Interesting Synapses Formed in ipRGC Dendrites at Stratum 1 of the Inner Plexiform Layer in the Rabbit Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):667.
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Melanopsin-expressing intrinsically-photosensitive retinal ganglion cells (ipRGCs) are known to be involved in various non-image-forming and image-forming visual processing. Their main dendritic plexus is formed at stratum 1, OFF sublayer of the inner plexiform layer (IPL), but physiologically they show light-driven ON response. Recently, anatomical basis of this paradoxical functional influence has proven by two research groups at light microscopic level. Further, using electron microscopy, we examined connections of ipRGC at stratum 1 of the IPL in the rabbit retina.
For preembedding immuno-electron microscopy, rabbit retinas were dissected and fixed in 4% paraformaldehyde and 0.5% glutaraldehyde. The tissues were then immuno-labeled with antibodies against melanopsin or Cx45. After postfixation, the tissues were embedded in Epon. Serial ultrathin sections were made and examined by electron microscope.
In vertical ultrathin sections of the rabbit retina, somata of the displaced type of ipRGCs and their dendrites were identified in the inner nuclear layer close to the IPL and at stratum 1 of the IPL respectively, by the electron-dense reaction product precipitated on cytoplasmic membrane. Dendrites of ipRGCs received synaptic inputs from amacrine and bipolar cells. Among these input synapses, two interesting phenomena were found. One was the somatodendritic synapse that was formed between soma of a certain type of amacrine cell and a labeled ipRGC dendrite. At the synapse, synaptic vesicles were closely associated with cytoplasmic membrane of the soma. The other was the ribbon synapse formed between the axon of a certain type of bipolar cell and a labeled ipRGC dendrite. At this type of ribbon synapse, ipRGC dendrite was sole output target, that is, the postsynaptic element appeared as monad.
Our results demonstrate that ipRGCs receive synaptic influences from amacrine and bipolar cells in various and specific manners, and suggest that amacrine and bipolar cells may have influence on postsynaptic target at stratum 1 of the IPL through another mode of synapse.
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