Cyclic AMP has no effect on the generation, recovery, or background adaptation of light responses in functionally intact rod outer segments: with implications about the function of phosducin.

In retinal rods, light exposure decreases the total outer segment content of both cGMP and cAMP by about 50%. The functional role of the light-evoked change in cAMP is not known. It is postulated to trigger changes in the phosphorylation state of phosducin, a phosphoprotein that is phosphorylated in the dark by cAMP-dependent protein kinase (PKA) and dephosphorylated by basal phosphatase activity when PKA is inhibited by the light-evoked drop in cAMP. In biochemical studies, dephosphorylated phosducin binds to free beta gamma dimer of transducin (Tbeta gamma) and prevents the regeneration of heterotrimeric transducin by blocking the re-association of the beta gamma and alpha subunits. Phosducin's interaction with Tbeta gamma is blocked when it is phosphorylated on a single residue by PKA. To evaluate the effect of the light-evoked fall in cAMP, functionally intact isolated lizard rod outer segments were dialyzed in whole-cell voltage clamp with a standard internal solution and electrical light responses were recorded with and without adding cAMP to the dialysis solution. Since the total outer segment content of cAMP in darkness is approximately 5 microM, internal dialysis with solution containing a much higher concentration (100 microM) of cAMP (or 8-bromo-cAMP) will overcome the effects of a light-evoked decrease in its concentration by keeping cAMP-dependent processes fully activated. Neither cyclic nucleotide had any influence on the generation, light sensitivity, recovery, or background adaptation of the flash response. These results also argue against the participation of phosducin in the sequence of events that are responsible for these aspects of rod function. This does not exclude the possibility of phosducin being involved in adaptation caused by higher light levels than used in the present study, that is, bleaching adaptation, or in light-dependent processes other than phototransduction.

Effect of rhodopsin C-terminal peptide on photoresponses in functionally intact rod outer segments.

The protein-protein interactions that underlie shut-off of the light-activated rhodopsin were studied using synthetic peptides derived from C-terminal region of the rhodopsin. The photoresponses were recorded in whole-cell voltage clamp from rod outer segments (ROS) that were internally dialyzed with an intracellular solution containing the synthetic peptides. This was the first time that synthetic peptides have been used in functionally intact ROS. None of the tested peptides promoted the shut-off of the photolyzed rhodopsin (R) by stimulating the binding of an activated arrestin to non-phosphorylated R, contrary to what was expected from in vitro experiments (Puig et al. FEBS Lett. 362: 185-188, 1995).

Protein kinase C and IP3 in photoresponses of functionally intact rod outer segments: constraints about their role.

Protein kinase C and polyphosphoinositide metabolism are reported to affect light-activated processes in cell free systems. To investigate their role in phototransduction under more physiological conditions the effects of nonhydrolyzable inositol trisphosphate (IP3) analogs as well as of protein kinase C and phospholipase C inhibitors on the characteristics of the electrical light response were studied. Rod outer segments were dialyzed in whole-cell voltage clamp and photoresponses in the presence and absence of the tested compounds were compared. None of the compounds influenced the light responses suggesting that neither IP3 nor protein kinase C participate in the phototransduction cascade. A number of different proposals about the participation of protein kinase C and inositol trisphosphate (IP3) in the phototransduction process based on a wide variety of in vitro experiments should therefore be reevaluated.

Vertebrate phototransduction: activation, recovery, and adaptation.

Vision is a fascinating example of the interaction of a biological system with the outside world. The first step of translating electromagnetic energy into a biologically recognizable signal involves the phototransduction cascade in retinal photoreceptor cells. Phototransduction is the best studied example of a GTP binding protein (G protein)-coupled signal transduction pathway. A great body of knowledge about phototransduction has been established in the past several decades but there are still many unanswered questions, particularly about photoresponse recovery and adaptation. The purpose of this review is to outline the events following photon absorption by vertebrate photoreceptors, to demonstrate the great complexity of the phototransduction cascade mechanisms, and to point out some of the controversies arising from recent findings in the field of visual transduction.