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SI Firth, DW Marshak; Cholecystokinin Immunoreactive Processes in the Rat Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2763.
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Purpose: High levels of endogenous cholecystokinin (CCK) are present in the rat retina (Eskay and Beinfeld, 1982), but the cellular localization of CCK in the rat retina is uncertain. Our goals were to identify the retinal cells containing CCK in the rat retina. Method: Rat eyes were immersion fixed with 4% paraformaldehyde in 0.1M phosphate buffer for 1 hour. Cryostat sections and whole mounts of the retina were labeled with antibody to gastrin-CCK using standard immunofluorescence techniques and were examined using confocal microscopy. Double labeling was done with antibodies to recoverin, protein kinase C a (PKC), glutamate decarboxylase (GAD) and glycine transporter-1 (Glyt-1). Results: CCK immunoreactive (-IR) axons were seen in sublamina 5 of the inner plexiform layer of the rat retina, as described previously by Eriksen and Larson (1981). These axons were evenly distributed throughout the retina. However, the CCK-IR cell bodies were detected in the ganglion cell layer only in the peripheral retina. These CCK-IR somata also contained GAD but not Glyt-1 immunoreactivity. In the vicinity of the somata, sparse CCK-IR dendrites were also seen in the outer layers of the IPL. CCK-IR axons contacted both the PKC-IR rod bipolar cells and recoverin-IR ON-cone bipolar cells in sublamina 5. Based on their diameters, the CCK-IR processes contacting the rod bipolar cells are likely to be amacrine cells making feed forward synapses described by Strettoi et al., 1990. However, there are many other CCK-IR varicosities that do not contact these ON-bipolar cells. The CCK-IR cells are unlikely to be contacting the AII amacrine cell, as its inputs are from large amacrine processes, and the A17 amacrine cell is not postsynaptic in this layer of the IPL. Therefore, the CCK-IR amacrine cells may also contact other amacrine cells or ganglion cells in this layer. Conclusion: CCK is present in a displaced wide-field amacrine cell with its perikaryon in the peripheral retina that is likely to contribute to both the rod and cone pathways. This is consistent with the physiological experiments in cat demonstrating that CCK-8 is inhibitory to all brisk ganglion cells in the cat retina (Thier and Bolz, 1985).
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