Red cones and red rods of the visual system exhibit quite different sensitivities to light, with cones being less sensitive to light and showing faster photoresponse kinetics. These differences may be due to increased spontaneous activity in cones, which results in a desensitization of the cell. One mechanism for the spontaneous activity is that red cone pigment shows a higher rate of thermal isomerization of the chromophore, which activates the intracellular signaling cascade. Kefalov et al. suggest that a second mechanism contributes to the difference between red rods and cones: the reversibility of the bond between the pigment protein opsin and the chromophore 11-cis retinal in red cones. 11-cis retinal was exchanged for 9-cis retinal, leading to a blue shift in the absorption spectrum of red cones, but not rods, in the dark. To confirm that it was the 11-cis retinal that was released and not the isomerized all trans form, which is normally released as part of the response to light, the dark-adapted red cones were treated with an 11-cis retinal binding protein, CRALBP. This produced a decrease in cone sensitivity and response kinetics, and these effects were reversed by addition of 11-cis retinal. Addition of 11-cis retinal actually increased response time beyond that of dark-adapted cones, suggesting that there was free opsin in the cells in dark conditions. Although red rods did not exhibit this effect, blue cones and green rods did. Thus, the reversibility of the opsin-retinal bond appears to be a property of the pigment opsin and not the cellular environment (rod versus cone).
V. J. Kefalov, M. E. Estevez, M. Kono, P. W. Goletz, R. K. Crouch, M. C. Cornwall, K.-W. Yau, Breaking the covalent bond: A pigment property that contributes to desensitization in cones. Neuron 46, 879-890 (2005). [PubMed]