Visual Temporal Sensitivity in Vitamin A–deprived Xenopus

Abstract

Abstract: : Purpose:To investigate both visual sensitivity and rod responses in Vitamin A–deprived Xenopus. Methods:Xenopus laevis tadpoles were fed a custom diet lacking Vitamin A. Their visual sensitivity was determined by assaying their optomotor response to periodic moving patterns (Solessio et al, ARVO 2003). Rod photocurrents were measured with suction electrode techniques. Pigment content in individual rods was measured by microspectrophotometry. Results:Under scotopic conditions, the temporal sensitivity functions of Xenopus tadpoles fed a regular diet have a characteristic low–pass shape and a cut–off frequency at 1 Hz. Vitamin A–deprived tadpoles exhibited a 30–50 fold loss in sensitivity at low frequencies but their high frequency cutoff increased to 2–3 Hz. Spectral sensitivity functions confirmed that despite the large loss in sensitivity, the responses were mediated by rod and not cone photoreceptors. In Vitamin A deficient rods, pigment concentrations were less than 10% of normal values and sensitivity of rod photocurrents diminished more than 10–fold. Vitamin–A deficient rods had significantly faster response kinetics than control rods, reaching peak amplitudes in half their normal times; however, the recovery dominant time constant did not change. Conclusions:Vitamin A is the precursor to 11–cis retinal, the chromophore in visual pigments. Low levels of Vitamin A result in retinal degeneration in mammalian retinas (Fain and Lisman, 1993). In RPE65 –/– mice that produce minimal amounts of 11–cis retinal, constitutive activity from chromphore–free opsin causes photoreceptor cell death (Woodruff et al, 2003). Xenopus retinas do not degenerate when raised with a Vitamin–A deficient diet, although their quantum catch and ERG sensitivity is diminished (Engbretson and Witkovsky, 1978). It is unclear why Xenopus rods do not degenerate when similar conditions prompt mammalian retinas to do so. Our results support the notion of constitutive activation of the phototransduction cascade by chomophore–free opsins. We find that faster rod response kinetics correlate with the increased dynamic range of the behaviorally–measured temporal sensitivity functions in these animals, and is in analogy to the presence of a ‘dark’ adapting light.

Keywords: photoreceptors • electrophysiology: non–clinical • vitamin A deficiency