Light-dependent delay in the falling phase of the retinal rod photoresponse

Abstract

Using suction electrodes, photocurrent responses to 100-ms saturating flashes were recorded from isolated retinal rods of the larval-stage tiger salamander (Ambystoma tigrinum). The delay period (7" c ) that preceded recovery of the dark current by a criterion amount (3 pA) was analyzed in relation to the flash intensity (If), and to the corresponding fractional bleach (R* 0 /R lol ) of the visual pigment; Rl/R lol was compared with R*/R lol , the fractional bleach at which the peak level of activated transducin approaches saturation. Over an approximately 8 In unit range of If that included the predicted value of R*/R lol , T c increased linearly with In If. Within the linear range, the slope of the function yielded an apparent exponential time constant (T C ) of 1.7 ± 0.2 s (mean ± S.D.). Background light reduced the value of T c measured at a given flash intensity but preserved a range over which T c increased linearly with In If-, the linear-range slope was similar to that measured in the absence of background light. The intensity dependence of T c resembles that of a delay (T d ) seen in light-scattering experiments on bovine retinas, which describes the period of essentially complete activation of transducin following a bright flash; the slope of the function relating T d and In flash intensity is thought to reflect the lifetime of photoactivated visual pigment (/?*) (Pepperberg et al., 1988; Kahlert et al., 1990). The present data suggest that the electrophysiological delay has a similar basis in the deactivation kinetics of / ? ' , and that T C represents T R >, the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the "dark-adapted" value of T R * within the investigated range of background intensity.

Toward a unified model of vertebrate rod phototransduction

“…4a). The T sat function (T sat vs. ln(I); Pepperberg et al, 1992Pepperberg et al, ,1994 derived from the model responses (blue curve, Fig. 4b) matches the empirical T sat function with a slope of 2 s/ln unit reflecting the value of the rate-limiting τ PDE (see Table 1).…”

“…Nakatani & Yau, 1988;Forti et al, 1989;Tamura et al, 1991;Koutalos et al, 1995) and late in the recovery to bright flashes (e.g. Baylor et al, 1974;Torre et al, 1986;Pepperberg et al, 1992Pepperberg et al, ,1994.…”

“…Thus, the presence of an "inactive" product of light absorption might cause weak activation of the transduction cascade, either directly, or perhaps via a slow back-reaction to active rhodopsin (Baylor et al, 1974;Lamb, 1981;Pepperberg et al, 1992;Leibrock et al, 1994Leibrock et al, , 1998. One possible scheme for a back-reaction might be activated and phosphorylated forms of R* reappearing after Arr-capping on rare occasions through reversible reactions and/or side reactions in the retinoid cycle (Lamb, 1981;Leibrock et al, 1998).…”

The Effector Enzyme Regulates the Duration of G Protein Signaling in Vertebrate Photoreceptors by Increasing the Affinity between Transducin and RGS Protein

“…It appears to be not only sufficient but even excessive when compared with the ϳ7 s Ϫ1 rate of the recovery from single-photon responses of dark-adapted mammalian rods (33,34). Although the slowest rate-limiting step in the photoresponse recovery may not be transducin GTPase (35), the rate of 100 s Ϫ1 is too high when compared with the quenching rates of any phototransduction step derived from physiological experiments (36). This implies …”

Functional comparison of RGS9 splice isoforms in a living cell

“…5B), which is determined experimentally from the slope of the relation between response saturation time and the natural log of flash strength (the Pepperberg plot; see refs. 28,29). In normal mouse rods, D is ϳ 200 ms for responses to flashes up to a natural log of the flash strength of ϳ ln i ϭ 9 (ϳ4,000 R*/flash) and reflects the rate of transducin GTPase (21).…”