Immunohistochemical localization of calbindin D28k and calretinin in the retina of two lungfishes, Protopterus dolloi and Neoceratodus forsteri: Colocalization with choline acetyltransferase and tyrosine hydroxylase

There are no previous studies of the distribution of calbindin-D28k (CB) and calretinin (CR) in retinal neurons in lungfishes. The present study aimed to characterize the distribution of these neurons in a lepidosirenid lungfish (Protopterus dolloi) and a ceratodontid lungfish (Neoceratodus forsteri) using the same immunohistochemical protocol, with specific poly- and monoclonal antibodies, used in a previous comparative study of amphibians. The pattern for both proteins was similar in the two lungfishes studied, but some differences were also found. CB was found in photoreceptors, in subpopulations of bipolar and amacrine cells, and in horizontal and ganglion cells. Photoreceptors were negative for CR, whereas subpopulations of horizontal, bipolar, and amacrine cells contained CR, as did cells in the ganglion cell layer. Colocalization of CB/CR occurred in some cells of the inner nuclear layer. Tyrosine hydroxylase (TH) immunoreactivity colocalized with CB in some amacrine cells and, less frequently, with CR, but colocalization of TH with CB or CR in cholinergic cells was never found. Our data suggest a specific pattern for CB and CR distribution in the retinal neurons of lungfishes, a pattern that is highly comparable to that seen in amphibians, with unique features not observed in other vertebrates.

Research Highlights
► Detailed distribution of calbindin and calretinin in retinal neurons in lungfishes.
► CB and CR distribution in the retina is highly comparable to that seen in amphibians.
► CB in photoreceptors and in bipolar, amacrine and putative ganglion cells.
► CR in horizontal, bipolar, amacrine and ganglion cells. Little CB/CR colocalization.
► Scarce CR and CR colocalization with ChAT and TH in amacrine cells.