GABA and tyrosine hydroxylase immunocytochemistry reveal different patterns of colocalization in retinal neurons of various vertebrates

Colocalization of GABA- and tyrosine hydroxylase-like immunoreactivity was studied in the retinae of various vertebrate species in order to ascertain whether the presumed coexistence of GABA and dopamine, reported earlier for mammals (Kosaka et al.: Exp. Brain Res. 66:191-210, '87: Wässle and Chun: J. Neurosci. 8:3383-3394,'88) is a common phenomenon. GABA-immunopositive cells constituted a separate population from tyrosine hydroxylase-positive cells in fish and amphibians, whilst in higher--i.e., amniote--vertebrates, such as reptiles, birds, and mammals, all dopaminergic cells contained GABA-like immunoreactivity. No clear correlation was found between the type of dopaminergic cell (amacrine/interplexiform) and the presence or absence of colocalization.