CALRETININ IN THE CAT RETINA - COLOCALIZATIONS WITH OTHER CALCIUM-BINDING PROTEINS, GABA AND GLYCINE

Immunocytochemical techniques were used to determine the distribution of the calcium-binding protein calretinin in the cat retina. Compariso ns were made with parvalbumin and calbindin as well as with the inhibi tory neurotransmitters GABA and glycine. Calretinin immunoreactivity w as seen in horizontal cells and multiple subpopulations of amacrine an d ganglion cells. Cone outer segments were also stained. Calbindin imm unoreactivity was present in cone photoreceptors, horizontal cells, at least two subtypes of cone bipolar cell, numerous amacrine cells, and cells residing in the ganglion cell layer. Heavy staining for parvalb umin was found in both A- and B-type horizontal cells, distinct subpop ulations of amacrine and ganglion cells, and a small population of con e photoreceptor cells. To confirm the identity of cone photoreceptors, comparisons were made with retinas stained for opsins specific for re d/green or blue cones (Szel et al., 1986). The localization of parvalb umin corresponded with that of blue-type cones only whereas calretinin and calbindin staining showed the same distribution as both red/green and blue cones. Double-label immunofluorescence studies revealed colo calization of all three of the calcium-binding proteins in a number of neurons including horizontal cells and AII amacrine cells. To assess a possible transmitter-specific relationship for calretinin, double-la bel studies were carried out with GABA and glycine. However, the stain ing patterns for each of these inhibitory amino acids differed substan tially from that of calretinin. The possibility remains that calretini n and other calcium-binding proteins may play a role in neurotransmiss ion through interactions with receptors or second-messenger agents.