A subpopulation of displaced ganglion cells of the pigeon retina exhibits substance P-like immunoreactivity

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Immunohistochemical and retrograde tracing techniques were combined to demonstrate the occurrence of displaced ganglion cells (DGCs) exhibiting substance P-like immunoreactivity (SP-LI) in the pigeon retina. Following injections of rhodamine-labeled latex microspheres into the nucleus of the basal optic root (accessory optic system), about 5200 DGCs were observed to contain rhodamine fluorescence in the contralateral retina. Approximately 26% of the retrogradely labeled DGCs also contained SP-LI. The soma sizes of the doubly labeled DGCs ranged from 12 to 24 μm, and their distribution mirrored the overall distribution of DGCs projecting to the nucleus of the basal optic root. The density of doubly labeled DGCs ranged from 2 to 15 cells/mm 2, with density peaks occurring in the superior-nasal and inferior-temporal retinal quadrants. Larger DGCs projecting to the nBOR (25–32 μm) were never seen to contain SP-LI. Together with previous results of enucleation experiments, these data indicate the existence of a subpopulation of SP-LI DGCs which are connected with the accessory optic system in the pigeon. The present results also contribute information on the heterogeneity of retinal ganglion cells transmitters and modulators.