Morphological and pharmacological analysis of putative serotonergic bipolar and amacrine cells in the retina of a turtle, Pseudemys scripta elegans

Using immunocytochemical methods, we have been able to demonstrate serotonin-like immunoreactivity (SLI) in amacrine and bipolar cells of the turtle retina. Inhibition of monoamine oxidase with pargyline drastically increases the amount of 5-hydroxytryptamine within both cell types. The indoleamine 6-hydroxytryptamine is taken up by both cell types and both types are destroyed within 10 days following intraocular injection of 5,7-dihydroxyryptamine. Increasing the external potassium concentration induces release of serotonin in both cell types. Our data support the idea that these neurons use serotonin during neuronal processing. Morphologically, amacrine and bipolar cells with SLI can be subdivided into two and three subclasses, respectively, based on their ramification pattern within the inner plexiform layer. A comparison of the morphological data with those of intracellularly stained amacrine and bipolar cells suggests that all bipolar cells with SLI are center-hyperpolarizing cells and all amacrine cells center-depolarizing cells.