Gamma-atrial natriureric peptide 1-25 is found in bipolar cells in turtle and rat retinas

Immunocytochemistry was used to reveal a population of bipolar cells that c ontain gamma-atrial natriuretic peptide 1-25 (gamma-ANP) in turtle retina. This same antibody was also used in rat retina as a comparative control. Th e retinas were examined by both conventional light microscopy and confocal microscopy with double-labeling to determine whether protein kinase C-alpha -like immunoreactivity (PKC-alpha-LI) was colocalized with the gamma-ANP-LI . Some thick sections of turtle retina immunostained with only the gamma-AN P antibody were also examined by electron microscopy. In rat, a subpopulati on of bipolar cells with axons terminating close to the ganglion cell layer was labeled. Double-labeling experiments indicated that the gamma-ANP-LI a nd PKC-alpha-LI were colocalized in rat retina, and thus all the bipolar ce lls with gamma-ANP-LI were rod bipolar cells. In turtle, the gamma-ANP anti body labeled certain bipolar cells that were characterized by bistratified axon terminals arborizing on the borders of strata S2/3 and S3/4 in the inn er plexiform layer (IPL). Double labeling with PKC-alpha antibody indicated that bipolar cells with gamma-ANP-LI were not the same bipolar cell types with PKC-alpha-LI. Thus, gamma-ANP-LI appears to be a new marker for a dist inct type of bipolar cell in turtle retina. At the ultrastructural level, t he gamma-ANP-LI was visible throughout the cytoplasm of the bipolar cells f rom dendrites to axon terminals. In the outer plexiform layer (OPL), labele d dendrites contacted photoreceptor pedicles almost exclusively at narrow-c left basal junctions, but infrequently formed the central element at a phot oreceptor ribbon synapse. In the IPL, axon terminals with gamma-ANP-LI made ribbon synapses onto a combination of amacrine and ganglion cells. Since n arrow-cleft basal junctions and photoreceptor ribbon-related junctions are known to be associated with ON-center bipolar cells in turtle, and since th e axon terminals of bipolars with gamma-ANP-LI stratify primarily in the ON -strata of the IPL, we suggest that these cells are likely to be ON-center cells. It is possible that the gamma-ANP may be involved in regulating the activity of Na+/K+ ATPase or in the modulation of cGMP levels.