DUAL EXPRESSION OF GABA OR SEROTONIN AND DOPAMINE IN XENOPUS AMACRINECELLS IS TRANSIENT AND MAY BE REGULATED BY LAMINAR CUES

Both local cell-cell interactions and lineage bias have roles in deter mining the different retina cell phenotypes. In this study, subpopulat ions of amacrine cells that dually express GABA or serotonin (5-HT) an d dopamine (DA) are identified in the early Xenopus tadpole (stages 42 -48) retina. GABA is first detected by immunocytochemistry in amacrine cells at stage 35/36, 5-HT at stage 39, and DA at stage 41. As the nu mber of these subtypes of amacrine cells increases by differentiation, a subset of them transiently express two neurotransmitters. GABA/DA d ouble-labeled amacrine cells are detected first at stage 42, at which time they constitute 52% of the DA-containing population; this percent age decreases to only 3% by stage 48. 5-HT/DA amacrine cells are detec ted only at stage 44, constituting about 20% of the DA-containing cell s and 4% of the small-dim 5-HT-containing cells. Regional location doe s not differentially affect the differentiation of these three types o f amacrine cells (DA only, GABA/DA, and 5-HT/DA cells); each type is f ound more in the anterior and dorsal than the posterior and ventral qu adrants, and their overall distribution patterns are statistically ind istinguishable. However, these subtypes of amacrine cells reside in di fferent sublamina of the inner nuclear layer. DA-only amacrine cells a re located predominantly in the inner sublayer of the 2-3 cell thick a macrine cell layer, closest to the inner plexiform and the ganglion ce ll layers. Both types of double-labeled cells are located mostly in th e outer sublayer of the amacrine cell layer, closest to other interneu rons in the inner nuclear layer. This distinct sublaminar location of different neurotransmitter phenotypes suggests that local laminar cues influence the coexpression of neurotransmitters in amacrine cells.