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In-Beom Kim, Hong-Lim Kim, Ji Hyun Jeon, Tae Hyung Koo, Myung-Hoon Chun, Jung-Il Moon; Axonal Synapses of Calbindin ON Bipolar Cells in the OFF Layer of the Rabbit Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3028. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Bipolar and ganglion cells whose axon terminals and dendrites stratify in sublamina a of the inner plexiform layer (IPL) show OFF responses at light stimuli and those that stratify in sublamina b show ON responses. This stunning relationship between anatomy and physiology is a key principle of retinal organization. However, there are at least 3 types of inner retinal neurons, dopaminergic amacrine cells, intrinsically photosensitive retinal ganglion cells, and bistratified diving ganglion cells which violate this principle. Their main dendrites stratify in sublamina a of the IPL, the OFF sublamina, but physiologically, they have light-driven ON responses. Recently, anatomical studies showed that some ON cone bipolar cells provide ON inputs to these inner retinal neurons in the OFF sublamina (Hoshi et al., 2009; Dumitrescu et al., 2009). In this study, we examined the synaptic connections of the calbindin-immunoreactive ON cone bipolar cell in sublamina a of the IPL in the rabbit retina.
Retinas were isolated from New Zealand white rabbits. Using serial ultrathin sections and electron microscopy, eight calbindin labeled ON cone bipolar cell axons were reconstructed.
All calbindin ON cone bipolar cells (n=8) examined in this study made ribbon synapses onto amacrine and ganglion dendrites in sublamina a of the IPL. Most of these ribbon synapses were found at the border between the inner nuclear layer and the IPL. In total, 24 synapses onto postsynaptic targets were observed, and thus, a calbindin ON cone bipolar cell axon makes, on average, 3 ribbon synapses in sublamina a. At these 24 axonal ribbon synapses, a total of 44 synaptic ribbons were observed and thus, on average, each axonal synapse contained 1.83 ribbons. Interestingly, all these ribbon synapses appeared as monads (see also Hokoc and Mariani, 1987) and 62.5% contained two or more ribbons from the same calbindin bipolar axon.
These findings suggest that some ON cone bipolar cells provide ON signals to certain amacrine and ganglion cells whose dendrites stratify in the OFF sublamina. The presence of a monadic ribbon synapse may be a diagnostic feature of ON cone bipolar output in sublamina a.
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