Light-activation of teleost rod photoreceptor elongation

Rod photoreceptors in the retinas of teleost fish undergo changes in cell length in response to changing ambient light intensities. In the dark rods shorten and in the light rods elongate. These movements are mediated by actin-dependent processes which occur in the ellipsoid and myoid of the inner segment. As an approach to examining the underlying intracellular signaling pathways that link light absorption to actin-dependent motility in the inner segment, we have investigated the quantitative aspects of the light stimulus required to activate elongation in isolated rod inner/outer segments (RIS-ROS) of the green sunfish (Lepomis cyanellus). The intensity thresholds and strength-duration characteristics of the light stimulus required to activate teleost rod elongation were found to differ from those reported to activate vertebrate rod membrane hyperpolarization. In response to brief pulses of light, RIS-ROS elongated in a graded manner, both as a function of increasing light pulse intensity and light pulse duration. Half maximal activation of light-induced RIS-ROS elongation was produced by a stimulus of roughly 6 x 10(15) photons cm-2, which is calculated to bleach approximately 20% of the photopigment molecules in green sunfish rod outer segments. This degree of photopigment bleach is approximately 6-7 orders of magnitude greater than that required to elicit half maximal changes in membrane potential in other vertebrate rod preparations. Furthermore, the reciprocal relationship between light pulse intensity and duration in eliciting an equal elongation response held for relatively long light pulse durations.(ABSTRACT TRUNCATED AT 250 WORDS)