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 (λ_max) close to 500 nm. A population of middle-wave-sensitive cones (M cones) contained a pigment with λ_max at approximately 530 nm, and a short-wave-sensitive cone population (S cones) contained a pigment with λ_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.