GABA AND GLYCINE IN RETINAL AMACRINE CELLS - COMBINED GOLGI IMPREGNATION AND IMMUNOCYTOCHEMISTRY

Golgi-impregnated amacrine cells in the all-cone lizard retina (Anolis carolinensis) were characterized on the bases of dendritic and somati c criteria. Four major cell categories, comprising 23 types were ident ified: three non-stratified, 13 monostratified, five bistratified, and two tristratified types. Four of the cell types comprised two to four subtypes based on stratification of their dendrites within the inner plexiform layer (IPL). Golgi impregnation strongly favoured monostrati fied amacrine cells with cell bodies at the proximal margin of the inn er nuclear layer. The neurotransmitter content of each of the 23 amacr ine cell types was examined by combined Golgi-immunocytochemistry afte r morphological classification. Putative neurotransmitters examined in cluded gamma-aminobutyric acid (GABA), glycine (GLY) and aspartate (AS P). Seventeen cell types showed GABA-immunoreactivity (IR), three cell types showed GLY-IR, and four cell types showed neither GABA-IR nor G LY-IR. No cell types showed ASP-IR. Each cell type had a characteristi c neurochemical signature, with the exception of one monostratified ce ll type that showed three different neurochemical signatures. Postembe dding immunocytochemistry on conventionally processed retinas confirme d the localization of glutamic acid decarboxylase, the synthetic enzym e for GABA, to cells similar to several of the GABA-IR Golgi-stained t ypes. Postembedding immunocytochemistry for tyrosine hydroxylase (the synthetic enzyme for catecholamines) and GABA on serial sections demon strated colocalization of GABA and a catecholamine, probably dopamine, in a bistratified amacrine cell type. We conclude that GABA-IR amacri ne cell types are more numerous and morphologically heterogeneous than GLY-IR amacrine cells. The morphological heterogeneity and, with one exception, exclusivity of GABA-IR and GLY-IR amacrine cell types indic ate that both neurotransmitters play a variety and different functiona l roles in the lizard inner retina.