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

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Abstract
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.