Visual Neuroscience

Table of Contents - 2006 - Volume 23, Issue 06  


Differences in the pharmacological activation of visual opsins

T.  ISAYAMA  a1 c1 , Y.  CHEN  a2 , M.  KONO  a3 , W.J.  DEGRIP  a4 , J.-X.  MA  a2 , R.K.  CROUCH  a3 and C.L.  MAKINO  a1
a1 Department of Ophthalmology, Massachusetts Eye & Ear Infirmary, Harvard Medical School, Boston, Massachusetts
a2 Department of Cell Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma
a3 Department of Ophthalmology, Storm Eye Research Institute, Medical University of South Carolina, Charleston, South Carolina
a4 Department of Biochemistry, NCMLS, University of Nijmegen, Nijmegen, The Netherlands

Article author query
isayama t   [Google Scholar] 
chen y   [Google Scholar] 
kono m   [Google Scholar] 
degrip wj   [Google Scholar] 
ma j-   [Google Scholar] 
crouch rk   [Google Scholar] 
makino cl   [Google Scholar] 
 

Abstract

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, [beta]-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 [beta]-ionone. To test the two hypotheses, we applied [beta]-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. [beta]-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 [beta]-ionone. Thus opsin structure determines the identity of [beta]-ionone as an agonist or an inverse agonist and in that respect distinguishes the red-sensitive cone opsin from all others.

(Received June 23 2006)
(Accepted September 29 2006)


Key Words: Rod; Cone; Rhodopsin; Transducin; Phototransduction.

Correspondence:
c1 Address correspondence and reprint requests to: Tomoki Isayama, Department of Ophthalmology, Massachusetts Eye & Ear Infirmary, 243 Charles Street, Boston, MA 02114. E-mail: tomoki_isayama@meei.harvard.edu