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H. Hoshi, W.-L. Liu, S. C. Massey, S. L. Mills; ON Cone Bipolar Inputs Which Break the Stratification Rules of the Inner Plexiform Layer. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5683.
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The functional division of the inner plexiform layer (IPL) into ON and OFF layers is one of the most fundamental and enduring tenets of retinal morphology. Yet dopaminergic amacrine cells, whose light-evoked release is blocked by the ON channel agonist APB, ramify principally in sublamina a and the identity of their excitatory inputs have never been established. In this study, we show that synaptic ribbons contained in descending axons of a cone bipolar cell of the rabbit retina make synapses with the arbors of two types of ON ganglion cell that send dendrites to sublamina a and also with the dopaminergic amacrine cell.
In rabbit retina, cells were labeled using either immunostaining or intracellular dye injection of Neurobiotin. An ON bipolar cell type was visualized with anti-calbindin (Cb+ ON-BC). Ribbon synapses were identified with an antibody to RIBEYE.
The descending axons of Cb+ ON-CBs regularly contained ribbons as they descended through stratum 1. The spines of both bistratified diving ganglion cells and melanopsin-containing ganglion cells (ipRGCs) were closely examined and were found to appose ribbons from these Cb+ ON-CBs in stratum 1 in addition to those contributed by the axon terminal in stratum 5 . The axons of ON-CBs also made ribbon synapses onto processes of the dopaminergic amacrine cell in stratum 1. The existence of functional synapses made by the descending axons of the Cb+ ON-CB was further verified by the presence of GluR4-immunoreactivity apposed to the ribbons.
We found ON cone bipolar cell ribbon synapses that regularly occur in the OFF sublamina. These ribbons are contained in the axons of calbindin-positive bipolar cells and contact at least three different post-synaptic targets, ipRGCs, bistratified diving ganglion cells and dopaminergic amacrine cells. These results suggest that the light response of these postsynaptic cells does not depend exclusively on the stratification pattern of the ganglion cell/amacrine cell dendrites in the IPL and that ON cone bipolar inputs can break the stratification rules of the IPL. Further, a source of ON input to the dopaminergic amacrine cell plexus has now been positively identified.
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