Membrane current of retinal rods of Caudiverbera caudiverbera (Amphibia: Leptodactylidae): Dark noise, spectral and absolute light sensitivity

Abstract

We investigated the photocurrents from isolated rods of the South American anuran, Caudiverbera caudiverbera. Rod outer segments were on average 66.4 ± 11.2 μm (mean ± S.D., n = 104) in length and 6.6 ± 0.9 μm (mean ± S.D.) in diameter; 40 ± 22 photoisomerizations (mean ± S.D., range 10–99, n = 16) were required for eliciting a half-saturating photocurrent response. The time-to-peak was 911 ± 217 ms (mean ± S.D., n = 14, 20°C) in the linear range of the response and the integration time of the current response was 1744 ± 451 ms (mean ± S.D., n = 14). The time-to-peak appears to be slower and the integration time shorter in Caudiverbera than in Ambystoma tigrinum, Rana pipiens or Xenopus laevis rods under similar experimental conditions. The α-band of rod spectral sensitivity has a λ_max at 520 ± 2.1 nm (mean ± S.D., range 516–525 nm, n = 24) and the bandwidth fits a porphyropsin visual pigment. The single-event response amplitude ranges from 0.31–0.51 pA, depending on the calculation method. The intrinsic dark current (variance at dark minus variance under bright light) was 0.045 ± 0.040 pA² (mean ± S.D., n = 24). Our results support the presence of a dark-noise component below 1 Hz, with kinetics similar to the single-photon evoked response and a rate of 0.006 events s⁻¹ (n = 9).