Spectral sensitivity of single cones in rainbow trout (Oncorhynchus mykiss): A whole-cell voltage clamp study

The UVS cone mechanism is known to light adapt at low intensities in comparison to other cones. We were interested in whether this property was related to higher sensitivity in UVS cones or to network adjustments in sensitivity. We investigated spectral sensitivity of 107 individual cone photoreceptors in rainbow trout (Oncorhynchus mykiss) using a whole-cell voltage clamp technique. Mean time-to-peak response was 339 ± 90 ms and flash sensitivity for a 100 ms flash was 4.37 × 10−3 ± 2.50 × 10−3 pA photons−1 μm2, with no significant differences between the UVS, SWS, MWS and LWS cone classes. The spectral sensitivity of the UVS, SWS and LWS cones conformed to the expression of SWS1, SWS2 and LWS opsin genes. The spectral sensitivity of MWS cones, however, showed clear evidence of co-expression of RH2a and RH2b opsin pigments. The fish used in this study bridged the ontogenetic stage where the MWS cones shift their expression from RH2b to RH2a.