Morphology and distribution of serotonin-like immunoreactive amacrine cells in the retina of Bufo marinus.

Using an antibody against serotonin (5-hydroxytryptamine, 5-HT), serotonin-like immunoreactive (serotonin-IR) neurons were demonstrated in the retina of adult Bufo marinus. All immunoreactive neurons were identified as amacrine cells (ACs). The dendrites of serotonin-IR ACs branched diffusely and densely throughout all levels of the inner plexiform layer (IPL) of the retina. The great majority of these cell somata were located in the vitread part of the inner nuclear layer (INL) and a few of them (ranging from 9-29 cells) were displaced into the ganglion cell layer (GCL). On the basis of the soma sizes, two populations of serotonin-IR ACs, large (type A) and small (type B), were distinguished. 6-Hydroxydopamine (6-OHDA) injected into the eye abolished immunoreactivity in the recently reported tyrosine hydroxylase (TH)-IR ACs (Zhu & Straznicky, 1990), whereas serotonin-IR ACs remained unaffected. The number of serotonin-IR cells per retina ranged from 23, 750-27, 390, with a ratio of 1:1.6 to 1:1.9 between type A and B cells. Both cell types were distributed nonuniformly across the retina. Cell densities were slightly lower in the peripheral (96 cells/mm2) than in the central (164 cells/mm2) retina. Linear regression analysis confirmed the presence of a decreasing density gradient from the retinal center to the retinal margin for both small and large cell types. The analysis of the nearest neighbor distances showed that the retinal distribution of serotonin-IR ACs was orderly. These results have been taken to indicate that 5-HT-IR cells correspond to a population of serotonin-containing ACs.(ABSTRACT TRUNCATED AT 250 WORDS)