Spectral properties of turtle cones

Microelectrodes were used to record from red and green cones of the turtle Pseudemys scripta elegans. The purpose of this study was to determine the action spectra of the red and green cone photopigments, and to look closely for direct interactions between the two cone classes. An isolated retina preparation was employed so that cones could be stimulated from the outer segment side, thereby avoiding the oil droplets that reside in the inner segments of many cones and normally filter incident light. In agreement with some previous electrophysiological studies, we found little evidence for significant direct connections between red and green cones. Exceptions to this rule are noted and discussed. Measurements indicate that this result does not appear to be due to a general loss of cone connectivity in the isolated retina preparation. Action spectra of the cone photopigments differed markedly from action spectra reported for cones in the eyecup preparation. In contrast to cones in the eyecup, cones in the isolated retina showed higher short-wavelength sensitivity and had action spectra that were adequately described by photopigment nomograms. A model of cone optical properties suggests that in the eyecup up to about 40% of the light that reaches a cone outer segment may do so without first passing through an oil droplet.