Rod Sensitivity During Xenopus Development

We have measured the sensitivity of rod photoreceptors from overnight-dark–adapted Xenopus laevis through developmental stages 46–66 into adulthood by using suction-pipette recording. The dark current increased gradually from ∼5 pA at stage 46 to ∼20 pA at stage 57, compared with an adult (metamorphosed) current of ∼35 pA. This increase in dark current largely paralleled the progressive increase in length and diameter of the rod outer segment (ROS). Throughout stages 46–66, the dark current increased approximately linearly with ROS surface area. At stage 53, there was a steep (∼10-fold) increase in the rod flash sensitivity, accompanied by a steep increase in the time-to-peak of the half-saturated flash response. This covariance of sensitivity and time-to-peak suggested a change in the state of adaptation of rods at stage 53 and thereafter. When the isolated retina was preincubated with 11-cis-retinal, the flash sensitivity and the response time-to-peak of rods before stage 53 became similar to those at or after stage 53, suggesting that the presence of free opsin (i.e., visual pigment without chromophore) in rods before stage 53 was responsible for the adapted state (low sensitivity and short time-to-peak). By comparing the response sensitivity before stage 53 to the sensitivity at/after stage 53 measured from rods that had been subjected to various known bleaches, we estimated that 22–28% of rod opsin in stage 50–52 tadpoles (i.e., before stage 53) was devoid of chromophore despite overnight dark-adaptation. When continuously dark adapted for 7 d or longer, however, even tadpoles before stage 53 yielded rods with similar flash sensitivity and response time-to-peak as those of later-stage animals. In conclusion, it appears that chromophore regeneration is very slow in tadpoles before stage 53, but this regeneration becomes much more efficient at stage 53. A similar delay in the maturity of chromophore regeneration may partially underlie the low sensitivity of rods observed in newborn mammals, including human infants.