REGIONAL TOPOGRAPHY OF ROD AND IMMUNOCYTOCHEMICALLY CHARACTERIZED BLUE AND GREEN CONE PHOTORECEPTORS IN RABBIT RETINA

Evidence from several sources indicates that the photoreceptors of rab bit retina include rods, green cones and blue cones, and that blue-gre en color opponency occurs in select retinal ganglion cells. One of us (Famiglietti) has identified wide-field cone bipolar cells as probable blue-cone-selective bipolars, and type C horizontal cells as possibly connected to blue cones. We wished to extend the analysis of blue con e pathways in rabbit retina and to characterize the topographic distri bution of blue and green cones. Two monoclonal antibodies raised again st chicken visual pigments are reported to label medium- and long-wave length cones (COS-1) and short-wavelength cones (OS-2) in all mammalia n retinas studied thus far (Szel and colleagues). Using selective labe ling with these two antibodies and a nonselective method in nasal and temporal halves of the same retinas, we have found that densities of p hotoreceptors vary systematically, depending upon the size of the eye and age of the animal. In 'standard' New Zealand rabbits of 2-3 kg (2- 3 months old), rods reached a peak density of about 300,000/mm(2) just dorsal to the visual streak, while cones exhibit peak density at mid- visual streak of about 18,000/mm(2). Published measurements of visual acuity in rabbit are less than predicted by this calculation. The rati o of cones to rods is significantly higher in ventral retina, where th e density of cones declines to a plateau of 10,000-12,000/mm(2), when compared to dorsal retina, where cones are uniformly distributed at a density of about 7000/mm(2). The density of OS-2 labeled (presumably ' 'blue'') cones is uniformly low, 1000-1500/mm(2) in a wide expanse tha t includes dorsal retina, the visual streak, and much of ventral retin a, except for a region of higher density along the vertical midline. W e confirm that there is a far ventral horizontal region near the perim eter that is populated exclusively by a high density (about 13,000/mm( 2)) of OS-2-positive cones (Juliusson and colleagues). This region doe s not extend to the ventral retinal margin, however, where cone densit y drops precipitously. Transitional zones between COS-1 and OS-2 label ing, in a region of relatively high and uniform cone density, where su ms of COS-1 and OS-2 labeling are higher than expected and in which we akly and strongly labeled cones are intermixed, raise questions about the identities of the visual pigment epitopes, the possibility of doub le labeling, and therefore the possibility of dual expression of pigme nts in single cones. The ''inverted- T-shaped'' topography of higher d ensity OS-2 labeling raises doubts about the significance of a ventral concentration of blue cones for visual function in rabbit retina.