Visual pigment composition in zebrafish: Evidence for a rhodopsin–porphyropsin interchange system
Numerous reports have concluded that zebrafish (Danio rerio) possesses A1-based visual pigments in their rod and cone photoreceptors. In the present study, we investigated the possibility that zebrafish have a paired visual pigment system. We measured the spectral absorption characteristics of photoreceptors from zebrafish maintained in different temperature regimes and those treated with exogenous thyroid hormone using CCD-based microspectrophotometry. Rods from fish housed at 15°C and 28°C were not significantly different, having [lambda]max values of 503 ± 5 nm (n = 106) and 504 ± 6 nm (n = 88), respectively. Thyroid hormone treatment (held at 28°C), however, significantly shifted the [lambda]max of rods from 503 ± 5 nm (n = 194) to 527 ± 8 nm (n = 212). Cone photoreceptors in fish housed at 28°C (without thyroid hormone treatment) had [lambda]max values of 361 ± 3 nm (n = 2) for ultraviolet-, 411 ± 5 nm (n = 18) for short-, 482 ± 6 nm (n = 9) for medium-, and 565 ± 10 nm (n = 14) for long-wavelength sensitive cones. Thyroid hormone treatment of fish held at 28°C significantly shifted the [lambda]max of long-wavelength sensitive cones to 613 ± 11 nm (n = 20), substantially beyond that of the [lambda]max of the longest possible A1-based visual pigment ([similar]580 nm). Thyroid hormone treatment produced smaller shifts of [lambda]max in other cone types and increased the half-band width. All shifts in photoreceptor [lambda]max values resulting from thyroid hormone treatment matched predictions for an A1- to A2-based visual pigment system. We therefore conclude that zebrafish possess a rhodopsin–porphyropsin interchange system that functions to spectrally tune rod and cone photoreceptors. We believe that these observations should be carefully considered during analysis of zebrafish spectral sensitivity.(Received July 26 2004)
(Accepted October 5 2004)
Key Words: Retina; Teleost fish; Giant danio; Chromophore; Thyroxine.
c1 Address correspondence and reprint requests to: Craig W. Hawryshyn, Department of Biology, University of Victoria, P.O. Box 3020 Stn. CSC, Victoria, British Columbia, Canada, V8W 3N5. E-mail firstname.lastname@example.org
a The authors are in alphabetical order.