Visual pigment coexpression in Guinea pig cones: a microspectrophotometric study.
PURPOSE: To determine the visual pigment content of the rods and cones of the guinea pig (Cavia porcellus) and to quantify the level of coexpression of pigments within individual cones. METHODS: Microspectrophotometry was used to measure the absorbance spectrum of visual pigments in individual rods and cones from three retinal regions: dorsal, ventral, and a subequatorial transition zone. Partial bleaching was used to establish whether two spectrally distinct visual pigments were present within a single cone. RESULTS: Rods possessed a pigment with a wavelength of maximum absorbance (lambda(max)) close to 500 nm. A population of middle-wave-sensitive cones (M cones) contained a pigment with lambda(max) at approximately 530 nm, and a short-wave-sensitive cone population (S cones) contained a pigment with lambda(max) close to 400 nm. The majority of cones in all regions were M cones. Approximately 10% of cones in the transition region were found to coexpress the M and S cone pigments in a ratio of approximately 4:1. Coexpression was not detected in S cones. CONCLUSIONS: In C. porcellus, coexpression of cone pigments occurs in a small number of cells but is biased in favor of the M pigment. Given the relatively low level of coexpression, detectable in only approximately 10% of the cones in the transition region, it is unlikely to cause any significant detriment to dichromatic color vision.