2 µM forskolin increases photoreceptor sensitivity. (A) Responses at various time of incubation in forskolin-containing Ringer’s solution. Response labeled “norm” was recorded in normal Ringer’s solution just before the start of forskolin perfusion; 1–4 were recorded from the same cell after incubation with forskolin during 205, 610, 940, and 1,340 s, respectively. Flash intensity in all records was 0.82 photons · µm⁻² per flash. Responses are normalized to corresponding dark current that was 19 pA in normal solution and 18 pA at the end of perfusion with forskolin. Each trace is the average of eight responses. (B) A single-exponential fit of the falling phase of two photoresponses recorded in normal solution and in the presence of forskolin after 940 s of incubation. Same cell as in A. (C) Changes of rods’ sensitivity during incubation in forskolin-containing Ringer’s solution, in configuration inner segment in (○) and outer segment in (•). The site of the forskolin application has no effect either on the time course or the magnitude of the response. The sensitivity is measured by photoresponses to a weak flash. All individual responses are normalized first to corresponding dark current, and second to corresponding response in normal Ringer’s solution. Initial normalized response amplitudes were ≈0.2. The same flash intensity was used throughout the entire time of incubation for each particular cell in a given recording configuration (an average of four cells in each configuration; mean ± SEM). The heavy smooth line is a parabolic approximation of pooled data for two configurations. It has no mechanistic meaning and is only used to characterize the time course of the effect of forskolin to be compared with the effects of other drugs.