Abstract Caldendrin is a novel calcium-binding protein confined to the somatodendritic compartment of neu- rons. Here we have studied the expression pattern of caldendrin in the rat retina. First we assessed the distri- bution of caldendrin transcripts in the adult and develop- ing 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 developing INL increases up to P8, whereas signifi- cant labeling in the GCL was first found at P14, coincid- ing with eye opening. Immunodetection with a polyclo- nal antibody revealed intensive staining of cells in the in- ner retina, which are presumably mainly amacrine and significantly fewer bipolar and ganglion cells. All parv- albumin-containing AII amacrines were immunopositive for caldendrin. Colocalization with calbindin was found in cone bipolar cells, the majority of AII amacrines, and calbindin-positive cells in the GCL. In the GCL, calden- drin was also colocalized with calretinin-immunoposi- tive 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 immu- noreactivity does not colocalize with tyrosine hydroxyl- ase, VIP, substance P and somatostatin immunoreactivi- ty. In summary, caldendrin expression is regulated differ- entially in retinal cell types during development and is restricted to a subpopulation of amacrine, bipolar, and ganglion cells, suggesting specific functions in the de- veloping and mature retina. Key words Development · Colocalization · Calretinin · Calbindin · Parvalbumin · EF-hand · Rat Introduction The calcium ion (Ca 2+ ) is an important second messen- ger involved in a large variety of signaling processes within neurons. Principally calcium signals are mediated by specific Ca 2+ -binding proteins (CaBPs). Intracellular CaBPs are thought to serve two general functions, i.e., they may have buffering functions to limit the intracellu- lar free Ca 2+ concentration and Ca 2+ -sensing functions to modulate activities of enzymes, ion channels, or cell sur- face receptors (Ikura 1996). In the latter case, CaBPs change their conformation upon Ca 2+ -binding triggering the target interaction. The prototype of an ubiquitously expressed Ca 2+ -sensing protein is calmodulin (Persechini et al. 1989). Numerous intracellular CaBPs belong to the EF-hand superfamily (Kretsinger 1987; Heizmann and Hunziker 1991; Nakayama and Kretsinger 1994). These proteins are characterized by the occurrence of one or more EF-hand motifs as the high-affinity Ca 2+ -binding site(s) (Persechini et al. 1989; Ikura 1996). Most EF-hand proteins are expressed in a tissue- or cell type-specific fashion and therefore are thought to be involved in specialized cellular functions (Heizmann and Hunziker 1991; Andressen et al. 1993). A large variety of EF-hand CaBPs have been identified and localized in This work was supported by the Deutsche Forschungsgemein- schaft (Kr1879/1-1), the Fonds der Chemischen Industrie, and a Young Investigator Award of the Otto-von-Guericke University to C.S. N. Menger Department of Neuroanatomy, MPI for Brain Research, D-60528 Frankfurt, Germany C.I. Seidenbecher · E.D. Gundelfinger · M.R. Kreutz ( ✉ ) “AG Molecular Mechanisms of Plasticity,” Department of Neurochemistry/Molecular Biology, Leibniz Institute for Neurobiology, Brenneckestr. 6, D-39118 Magdeburg, Germany e-mail: Kreutz@ifn-magdeburg.de; Tel: +49-391-6263518; Fax: +49-391-6263229 C. I. Seidenbecher AG Mol. & Cell. Neurobiology, Institute for Medical Psychology, Otto-von-Guericke University, D-39120 Magdeburg, Germany Cell Tissue Res (1999) 298:21–32 © Springer-Verlag 1999 Digital Object Identifier (DOI) 10.1007/s004419900060 REGULAR ARTICLE N. Menger · C.I. Seidenbecher · E.D. Gundelfinger M.R. Kreutz The cytoskeleton-associated neuronal calcium-binding protein caldendrin is expressed in a subset of amacrine, bipolar and ganglion cells of the rat retina Received: 1 December 1998 / Accepted: 5 May 1999 / Published online: 2 August 1999