Serotonergic and serotonin-synthesizing cells of the Xenopus retina.

It has been shown previously that a subclass of OFF-center bipolar cell in the Xenopus retina contains endogenous serotonin which can be released upon depolarization in a calcium-dependent manner. Based on single- and double labeling immunocytochemical experiments, this study provides evidence that these bipolar cells acquire their "endogenous" serotonin from another retinal source. The primary source in the Xenopus retina are large soma amacrine cells which are the only cellular elements containing the necessary enzymatic pathway for serotonin synthesis.