The cytoskeleton-associated neuronal calcium-binding protein caldendrin isexpressed in a subset of amacrine, bipolar and ganglion cells of the rat retina

Caldendrin is a novel calcium-binding protein confined to the somatodendrit ic compartment of neurons. Here we have studied the expression pattern of c aldendrin in the rat retina. First we assessed the distribution of caldendr in transcripts in the adult and developing retina by in situ hybridization. In the adult retina, transcripts are expressed mainly in the inner half of the inner nuclear layer (INL) and to a lesser extent in the ganglion cell layer (GCL). During development labeling of the inner part of the cytoblast layer, where amacrine cells reside, is already present at postnatal day 1 (P1). The intensity of hybridization signal in this sublamina of the develo ping INL increases up to P8, whereas significant labeling in the GCL was fi rst found at P14, coinciding with eye opening. Immunodetection with a polyc lonal antibody revealed intensive staining of cells in the inner retina, wh ich are presumably mainly amacrine and significantly fewer bipolar and gang lion cells. All parvalbumin-containing AII amacrines were immunopositive fo r caldendrin. Colocalization with calbindlin was found in cone bipolar cell s, the majority of AII amacrines, and calbindin-positive cells in the GCL. In the GCL, caldendrin was also colocalized with calretinin-immunopositive cells. Most caldendrin-positive amacrine cells in the adult rat retina were glycinergic and only a few were GABAergic. In retinal flat mounts, it was confirmed that less than 10% of retrogradely labeled retinal ganglion cells (RGC) are caldendrin-positive. Caldendrin immunoreactivity does not coloca lize with tyrosine hydroxylase, VIP, substance P and somatostatin immunorea ctivity. In summary, caldendrin expression is regulated differentially in r etinal cell types during development and is restricted to a subpopulation o f amacrine, bipolar, and ganglion cells, suggesting specific functions in t he developing and mature retina.