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Ala Morshedian, Gordon Fain; Evolution of Adaptation in Vertebrate Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2017;58(8):1023.
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Lamprey are cyclostomes, which diverged from jawed vertebrates (fishes, amphibians, mammals) in the Cambrian perhaps 500 Mya. We previously showed that adult Petromyzon marinus has a duplex retina: rods respond to single photons, have a longer integration time, and are 80 times more sensitive than cones, much as in other vertebrates. Do lamprey photoreceptors also have mechanisms of light and dark adaptation like jawed vertebrates?
Single-cell measurements were made from rods and cones of Petromyzon marinus with suction recording as previously (Current Biology 25: 484–487, 2015).
Responses to maintained steps of light decay as in other vertebrates with two time constants (taus in rods of 8s and 26s, in cones 800ms and 7.8s). Flash responses superimposed on steady backgrounds show decreases in sensitivity and changes in waveform in both rods and cones, also typical of other vertebrates. Backgrounds produce a decrease in maximum flash-response amplitude and an increase in the flash intensity necessary to produce a detectable response, with characteristic shifts of response-intensity curves along the intensity axis. Increasing background light had little effect on response onset but monotonically increased the rate of response decay. Sensitivity as a function of background intensity decreased by Weber’s Law in both rods and cones; rods show incremental saturation, and cones begin to adapt near the intensity at which rod saturation occurs. Bright bleaching light produces an equivalent background, with opsin in rods 7.5 x 10-6 times as effective in stimulating the cascade as Rh* (2 x 10-5 in mouse rods). The decreases in sensitivity and acceleration of response decay in stably bleached photoreceptors can be nearly completely reversed with exogenous 11-cis retinal.
Lamprey rods and cones adapt to backgrounds and bleaches with a phenomenology nearly identical to that of other vertebrates including mammals. Our experiments taken together with previous results show that primitive vertebrates before the divergence of jawed from jawless vertebrates had a duplex retina with rods and cones responding and adapting much like photoreceptors of other vertebrates. Nearly all of the mechanisms of transduction and sensitivity regulation seem to have evolved a relatively short time after the emergence of chordates.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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