Vasoactive intestinal polypeptide‐containing cells in the rabbit retina: Immunohistochemical localization and quantitative analysis

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Abstract
Vasoactive intestinal polypeptide (VIP) possesses neuroactive properties in the nervous system. In this study we characterized VIP immunoreactive neurons in the rabbit retina to provide a basis for a better understanding of the role of this peptide in retinal functions and to further define the morphology of wide‐field amacrine cells. VIP immunoreactivity was demonstrated in colchicine‐treated retinas. Immunolabeling was observed in amacrine cells located in the proximal inner nuclear layer and, occasionally, in the ganglion cell layer and inner plexiform layer (IPL). Varicose fibers were distributed in laminae 1, 3, and 5 of the IPL. The distribution of VIP immunoreactive cells showed a peak of ∼ 50 cells/mm2 in the visual streak and minimum values of ∼ 12 cells/mm2 in the peripheral retina. The total number of VIP immunopositive neurons was estimated to be about 11,000. Cell body diameters in the visual streak (8–9 μm) were slightly smaller than those measured in the dorsal or in the ventral retina (9–10 μm). The distribution of nearest neighbor distances (∼ 109 μm in the visual streak and ∼ 99 μm in the peripheral retina) showed that VIP immunoreactive neurons were nonrandomly spaced. Labeled neurons emitted one to three thick primary processes, arborizing in secondary processes and collaterals rich in varicosities; these processes often crossed among different IPL laminae. Arborization fields of individual cells overlapped extensively. In the dorsal retina, estimated areas of single arborization fields were larger and processes had lower branching frequency than in the visual streak and in the ventral retina. On the whole, VIP immunoreactive amacrine cells gave rise to a loose meshwork of fibers in the IPL. These characteristics indicate VIP is contained in a class of wide‐field amacrine cells and is likely to be involved in widespread regulatory or modulatory functions rather than in the direct transmission of visual information through the retina.