Characterization and quantification of peptidergic amacrine cells in the turtle retina: Enkephalin, neurotensin, and glucagon

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Abstract
Immunocytochemical methods were used for selective labeling and characterization of amacrine cells of the turtle (Pseudemys scripta elegans) retina which contained neuropeptide-like immunoreactivity (leu-enkephalin, glucagon, and neurotensin). Processes of amacrine cells arborized in specific strata of the inner plexiform layer (IPL). Different strata were defined in relation to the boundaries of the IPL. Zero represented the strata nearest the inner nuclear layer and 100 represented the strata nearest the ganglion cells. Antisera directed against leu-enkephalin labeled approximately 7,300 amacrine cells in a single turtle retina which were concentrated in the region of the visual streak and decreased in density toward the periphery. In retinal regions outside the visual streak the labeled neurons were similar in size and shape with dendritic arbors which lacked a particular orientation. In contrast, in the visual streak, there were particular neurons which were labeled with enkephalin antiserum which had elongated dendritic arbors that ran parallel to the streak. Both types of amacrine cells with enkephalinergic immunoreactivity sent their dendrites into the 0–20 region and the 65–100 region of the IPL. Antisera directed against glucagon labeled approximately 2,500 amacrine cells in a single turtle retina. These cells were concentrated in areas near the visual streak. Amacrine cells labeled with glucagon antiserum had dendritic arbors which were asymmetrically skewed toward one end of the cell and ramified in the 0–20 strata with sparse projections at the 40 and 80 strata of the IPL. Antisera directed against neurotensin labeled 12,800 amacrine cells in a single turtle retina These cells were concentrated in the region of the visual streak. Two distinct amacrine cell types were labeled selectively. One type had a large, vertically oriented cell body (10 × 14 μm) which gave rise to a single 2-μm-thick process that branched and ramified within the 45–70 strata. The other amacrine cell type with neurotensin-like immunoreactivity had a smaller cell body (8 μm) that sent numerous thin dendrites into the same 45–70 strata. The present results indicated that various neuropeptides were present in amacrine cells of the turtle retina and that a specific neuropeptide could be found in more than one anatomical type of amacrine cell. Each anatomical type of amacrine cell had a unique dendritic arborization which ramified within particular strata within the IPL.