Renewal of normal and degenerating photoreceptor outer segments in the Ozark cave salamander

Abstract

To determine whether photoreceptor degeneration in the Ozark cave salamander is associated with cessation or changes in the kinetics of outer segment (OS) renewal, an autoradiographic study of 3H-leucine incorporation in photoreceptors was carried out. Six days after isotope injection rods and cones showed labeling in both inner and outer segments. Cone OS were diffusely labeled whereas rods contained a band of radioactivity at the base of the OS. At 13 and 21 days the radioactive band in rods was located progressively nearer the distal tip of the OS. The rate of rod OS renewal ranged from 0.30 to 0.38 mu of OS length per day at 18 degrees C. L-thyroxin induced metamorphosis and light increased the renewal rate compared to larvae in darkness, and adults with photoreceptors in an early stage of degeneration had a slightly higher renewal rate than larvae. Light and electron microscope autoradiographs of degenerate photoreceptors revealed that even in the final stages of degeneration when OS are reduced to small, irregular whorls of membrane, 3H-leucine labeling was present in inner segments and OS membranes. These observations demonstrate that OS renewal occurs in both larvae and adults, and suggest that photoreceptor degeneration may be due to disruption of some aspect of the OS disposal process.