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

• Published in: Visual neuroscience 1992-01, Vol.8 (1), p.9-18
• Main Authors: Pepperberg, David R., Cornwall, M. Carter, Kahlert, Martina, Hofmann, Klaus Peter, Jin, Jing, Jones, Gregor J., Ripps, Harris
• Format: Article
• Language: English
• Subjects: Ambystoma; Animals; Dark Adaptation; Electrophysiology; Light; Mathematics; Photic Stimulation; Photoreceptor; Photoreceptor Cells - physiology; R deactivation; Rod photocurrent; Scattering, Radiation; Signal Transduction - physiology; Transducin; Transducin - physiology; Visual pigment
• ISSN: 0952-5238; 1469-8714
• DOI: 10.1017/S0952523800006441

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 (Te) 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/Rtot) of the visual pigment; R*0/Rtot was compared with R*s/Rtot 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*s/Rtot, Te increased linearly with In If. Within the linear range, the slope of the function yielded an apparent exponential time constant (TC) of 1.7 ± 0.2 s (mean ± S.D.). Background light reduced the value of Tc measured at a given flash intensity but preserved a range over which Tc increased linearly with In If; the linear-range slope was similar to that measured in the absence of background light. The intensity dependence of Tc resembles that of a delay (Td) 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 Td and In flash intensity is thought to reflect the lifetime of photoactivated visual pigment (R*) (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 R*, and that Tc represents TR* the lifetime of R* in the phototransduction process. The results furthermore suggest a preservation of the “dark-adapted” value of TR* within the investigated range of background intensity.