Opsins, like many other G-protein-coupled receptors, sustain
constitutive activity in the absence of ligand. In partially bleached rods
and cones, opsin's activity closes cGMP-gated channels and produces a
state of “pigment adaptation” with reduced sensitivity to
light and accelerated flash response kinetics. The truncated retinal
analogue, β-ionone, further desensitizes partially bleached
green-sensitive salamander rods, but enables partially bleached
red-sensitive cones to recover dark-adapted physiology. Structural
differences between rod and cone opsins were proposed to explain the
effect. Rods and cones, however, also contain different transducins,
raising the possibility that G-protein type determines the
photoreceptor-specific effects of β-ionone. To test the two
hypotheses, we applied β-ionone to partially bleached blue-sensitive
rods and cones of salamander, two cells that couple the same cone-like
opsin to either rod or cone transducin, respectively. Immunocytochemistry
confirmed that all salamander rods contain one form of transducin, whereas
all cones contain another. β-Ionone enhanced pigment adaptation in
blue-sensitive rods, but it also did so in blue- and UV-sensitive cones.
Furthermore, all recombinant salamander rod and cone opsins, with the
exception of the red-sensitive cone opsin, activated rod transducin upon
the addition of β-ionone. Thus opsin structure determines the identity
of β-ionone as an agonist or an inverse agonist and in that respect
distinguishes the red-sensitive cone opsin from all others.