Morphological characterization of substance P-like immunoreactive amacrine cells in the anuran retina

Using substance P immunohistochemistry it was possible to demonstrate a class of morphologically homogeneous group of neurons in the inner nuclear layer (INL) of the retina of two anuran species: Xenopus laevis and Bufo marinus. The number of cells with substance P-like immunoreactivity (SP-LI) was about 250 and 800 in juvenile and 600 and 2500 in adult Xenopus and Bufo, respectively, SP-LI cells had a small soma with one primary dendrite having up to four slender branches, located in the vitreal sublamina of the inner plexiform layer (IPL). Mean dendritic field sizes were 0.12 and 0.30 mm2 in juvenile and 0.29 and 0.65 mm2 in adult Xenopus and Bufo, respectively. The density of SP-LI cells was 40/mm2 in juvenile and 24/mm2 in adult Xenopus compared with 20/mm2 in juvenile and 13/mm2 in adult Bufo. Nearest neighbour distance measurements indicated that SP-LI cells were randomly distributed across the entire retina in both species. The location and the morphology of SP-LI cells indicated that they correspond to a subclass of wide-field amacrine cells, similar to types 20 and 21 described by Golgi techniques in the cat.