Temperature dependence of dark-adapted sensitivity and light-adaptation in photoreceptors with A1 visual pigments: A comparison of frog L-cones and rods

Flash responses of L-cones and rods were recorded as ERG mass potentials in the frog retina at different temperatures (2–25 °C). The purpose was to elucidate factors that make cones faster and less sensitive than rods, particularly the possible role of thermal activation of L-cone visual pigment in maintaining a “light-adapted” state even in darkness. Up to ca. 15 °C, cones and rods were desensitized roughly equally by warming (Q10 ≈ 2.2–2.7), retaining a 5-fold sensitivity difference. In this range, the cone/rod difference must depend on factors other than thermal activation of the visual pigment. Above 15 °C, cones showed an additional component of desensitization compared with rods, coupled to accelerated response shut-off. This behavior is consistent with light-adaptation from temperature-dependent intrinsic activity (dark light). The apparent dark light as measured by the minimum background intensities needed to affect sensitivity and/or kinetics increased by ca. 10-fold between 15 and 25 °C, whereas reported increases in visual-pigment activation rates over this range are less than 5-fold. We conclude that the dark state of frog L-cones above 15 °C may be largely set by thermal activation of the phototransduction machinery, but only part of the experimentally determined dark light can be ascribed to the visual pigment.