Ratio of M/L pigment gene expression decreases with retinal eccentricity

Abstract

Evidence has accumulated to indicate that, on average, there are about 1.5-2.0 long-wave cones in the human fovea for each middle-wave cone. Much less is known about how the ratio of these cone populations might change with retinal eccentricity. We have examined how the ratio of middle- to long-wave cone pigment mRNA changes with eccentricity in individual human retinas. Retinas were examined from seven male eye donors. Patches of retina, 6 mm in diameter were removed using a trephine at three different eccentricities. mRNA was reverse transcribed, photopigment cDNAs were amplified, and the relative amounts of middle- and long-wave pigment mRNA were determined for each patch. For 6 mm patches centred on the fovea, the average ratio of middle- to long-wave mRNA was about 2:3. This value is similar to the average ratio of middle- to long-wave cones previously estimated for the human fovea. There were significant changes in the cone ratios with increasing eccentricities (p = 0.0004). For patches of retina taken from the most peripheral locations (centred 12 mm eccentric from the fovea), the average middle- to long-wave mRNA ratio decreased to about 1:3. These results can be explained by a decrease in the ratio of middle- to long-wave cone populations with increasing eccentricity from central retina to periphery

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