The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40°C. Light flashes evoked transient outward currents that saturated at up to ~ 20 pA. One absorbed photon produced a response of ~ 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kF/F(where kf is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.
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