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S.S. Mani, E. Solessio, G. Engbretson, J. Garlow, R. Barlow, B. Knox; Developmental Changes in the Light Responses of Xenopus Rods . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4171.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Investigate the relationship between the growth of Xenopus laevis rods and the sensitivity and kinetics of their light responses. Does the increased probability of photon absorption increase rod sensitivity? Methods: Suction electrode technique recorded changes in circulating current from isolated Xenopus rods in response to brief flashes (20 ms, 520nm). Results: Dark current grows as the outer segment lengthens during tadpole development. Also rod responses speed up and the time to peak shortens during development. Rod outer segments (ROS) lengthen from 22.9+/- 2.5 µm in stage 48 tadpoles to 68.6 +/- 9.0 mm in juvenile froglets. Dark currents were proportionally larger: 6.7 +/- 2.4 pA in tadpoles to 21.1 +/- 6.6 pA in juveniles. They correspond to an increase in collecting area from 7.8+/-2.7 in tadpoles to 19.2+/- 5.3 photons/µm2 in juveniles. Despite the difference in collecting area (and thus rhodopsin content), the light sensitivity of rods did not differ (~4 photons/ µm2 at half maximum). Estimated single photon amplitudes also did not differ between tadpoles and juveniles (0.31 pA/ R*). However, the kinetics of responses to dim flashes differed: time to peak was 2.1 +/-0.5 s for stage 48 rods and just 1.2 +/- 0.2 s in juvenile rods. The rising phases of normalized dim flash responses coincided indicating that the amplification of phototransduction cascade was similar for both tadpoles and juveniles (A= 0.055 s-2). We tested whether the recovery of the light responses also changed during development by measuring the recovery time of the dark current to saturating flashes. We found that the dominant time constant did not change; however, a non-dominant component decreased in juveniles as reflected by a faster (5-7 s) recovery from saturation. Conclusions: Despite nearly a 3-fold increase in rhodopsin content the sensitivity of rods do not change with development in Xenopus. The faster kinetics of rods in juveniles suggests a tradeoff between sensitivity and time resolution.
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